United Stales
•onmental Protc
Agency
Municipal f
Laboratory
Cincinnati OH 45268
earch
Research and Development
oEPA
Sewage Disposal
by Evaporation-
Transpiration
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RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development. U.S. Environmental
Protection Agency, have been grouped into nine series. These nine broad cate-
gories were established to facilitate further development and application of en-
vironmental technology. Elimination of traditional grouping was consciously
planned to foster technology transfer and a maximum interface in related fields.
The nine series are:
1. Environmental Health Effects Research
2. Environmental Protection Technology
3. Ecological Research
4. Environmental Monitoring
5. Socioeconomic Environmental Studies
6. Scientific and Technical Assessment Reports (STAR)
7. Interagency Energy-Environment Research and Development
8. "Special" Reports
9. Miscellaneous Reports
This report has been assigned to the ENVIRONMENTAL PROTECTION TECH-
NOLOGY series. This series describes research performed to develop and dem-
onstrate instrumentation, equipment, and methodology to repair or prevent en-
vironmental degradation from point and non-point sources of pollution. This work
provides the new or improved technology required for the control and treatment
of pollution sources to meet environmental quality standards.
This document is available to the public through the National Technical Informa-
tion Service, Springfield, Virginia 22161.
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EPA-600/2-78-163
September 1978
SEWAGE DISPOSAL BY EVAPORATION-TRANSPIRATION
by
Edwin R. Bennett
K. Daniel Linstedt
Department of Civil, Environmental and
Architectural Engineering
University of Colorado
Boulder, Colorado 80309
Grant No. R 803871-01-0
Project Officer
James F. Kreissl
Wastewater Research Division
Municipal Environmental Research Laboratory
Cincinnati, Ohio 45268
MUNICIPAL ENVIRONMENTAL RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
CINCINNATI, OHIO 45268
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DISCLAIMER
This report has been reviewed by the Municipal Environmen-
tal Research Laboratory, U.S. Environmental Protection Agency,
and approved for publication. Approval does not signify that
the contents necessarily reflect the views and policies of the
U.S. Environmental Protection Agency, nor does mention of trade
names or commercial products constitute endorsement or recommen-
dation for use.
11
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FOREWORD
The U.S. Environmental Protection Agency was created because
of increasing public and government concern about the dangers of
pollution to the health and welfare of the American people.
Noxious air, foul water, and spoiled land are tragic testimony
to the deterioration of our natural environment. The complexity
of that environment and the interplay between its components re-
quire a concentrated and integrated attack on the problem.
Research and development is that necessary first step in
problem solution and it involves defining the problem, measuring
its impact, and searching for solutions. The Municipal Environ-
mental Research Laboratory develops new and improved technology
and systems for the prevention, treatment, and management of
wastewater and solid and hazardous waste pollutant discharges
from municipal and community sources, for the preservation and
treatment of public drinking water supplies and to minimize the
adverse economic, social, health, and aesthetic effects of pol-
lution. This publication is one of the products of that research;
a most vital communications link between the researcher and the
user community.
In the role of developing alternative wastewater technologies
for use in rural communities, freedom from the limitations of
soils has long been sought in order to permit the optimum devel-
opment of the Nation's land resources. One such non-dependent
solution which has been employed for individual households in
recent years is the evaporation-transpiration disposal of waste-
water. This report details the results of a scientific investi-
gation of the relevant parameters affecting the performance of
evaporation and evapotranspiration systems for non-sewered rural
communities. It serves as a basis upon which the feasibility of
these types of alternative systems can be determined.
Francis T. Mayo
Director
Municipal Environmental
Research Laboratory
111
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ABSTRACT
One of the methods for on-site disposal of wastewater from
individual homes is by evaporation. Two types of evaporative
disposal systems have been investigated in this study; evapo-
transpiration beds and mechanical evaporation units.
Evapotranspiration beds can be designed for completely
evaporative disposal with no discharge to the adjacent soil or
as combination beds utilizing seepage as well as evaporation
disposal. The non-discharging type of ET bed was investigated
in this study but the results can also be interpreted to des-
cribe the evaporative portion of the combination, ET seepage
bed. Twenty nine test lysimeters of 0.22 cubic meters volume
each were utilized to evaluate the effect of design and opera-
tional parameters for ET beds. The variables studies were waste-
water loading rate, effect of the weather variables of evaporation
and rainfall, ET sand size, evaporation rate as a function of the
water saturation depth, and the transpiration contribution of
surface vegetation. A design method is presented along with cost
data and an analysis of the national application potential of
this type of system.
The evaporation of wastewater using mechanical systems was
studied using a pilot scale unit, constructed as part of the
project. Two types of evaporation designs were evaluated. One
unit utilized a row of circular, vertically mounted disks,
rotated about a horizontal shaft with a portion of the disk
submerged in wastewater in reservoirs. The wetted area of the
disks exposed to the atmosphere provided the evaporation surface.
The second unit was similar, except that a cylinder was made up
of concentric wraps of burlap cloth as the evaporation surface.
The burlap became wetted as the partially submerged cylinder
turned in the vat of wastewater. Ambient air was forced through
the center shaft and moved through the wetted wraps producing
evaporation of the wastewater. The variables studied for the two
units were: the wastewater loading rate, ambient air evaporation
potential, wind speed, amount of submergence and disk spacing.
Design equations were established for both units. Cost data and
analysis of national application potential is also presented.
This report is submitted in fulfillment of Contract Number
R 803871-01-0 by the University of Colorado under the sponsorship
of the U.S. Environmental Protection Agency. The report covers
a period of July 1, 1975 through April 30, 1978 and work was
completed on May 31, 1978.
iv
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CONTENTS
Page
Foreword iii
Abstract iv
Figures x
Tables xv
Abbreviations and Symbols xvi
Acknowledgement xviii
1. Introduction 1
2. Conclusions 8
Evaporation transportation beds 8
Mechanical evaporation systems 11
3. Recommendations 14
ET bed 14
Mechanical evaporation systems 14
4. Background 15
Systems for individual homes 17
Background and use of ET beds 17
ET bed application in the US 18
Purpose of the study 21
Characteristics of home wastewaters .... 21
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CONTENTS
Page
5. Theory , , . 28
Aerodynamic theory* .......,.,,, 28
Combination equation method ,,.,,,., 29
Correlation method. . 31
Energy budget method ,,.,., 31
Application of theory to ET systems . , . . 34
Application of theory to mechanical systems 35
Psychrometric charts and wet bulb theory. . 36
6. Experimental Procedures 40
Lysimeter studies 46
Mechanical evaporation units 52
Series and purpose of tests 56
Rotating disk evaporator 56
Concentric cylinder evaporator. ... 58
Weather measurements 59
7. Results 61
ET Lysimeter data analysis 61
Loading parameter 68
Depth of gravity water 73
Sunlight and rainfall effects 76
Rainfall runoff and surface slope. ... 77
vi
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CONTENTS
Page
ET sand evaluation ...,.,,.,,. 81
Wastewater temperature effects 84
Effluent type ,...,..,. 84
Surface cover and vegetation ,..,.. 86
Salt build-up , 92
Field observations of ET beds. 92
Results of mechanical evaporation
unit studies 93
Rotating disk evaporator 93
Liquid mixture temperature 93
Adiabatic humidity deficit 94
Wetted disk area 94
Wind speed and direction 98
Study of machine variables 98
Rotational speed 103
Disk spacing 103
Disk materials and color 106
Effect of direct sunlight radiation . 108
Comparison of laboratory and field
data. HO
Comparison of evaporation equations . Ill
vii
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CONTENTS
Page
Cold weather operation of the
rotating disk evaporator ...,,.., 114
Continuous operation of the
rotating disk evaporator .,.,,... 118
Comparison of actual and
computed evaporation ... 119
Investigation of health hazards
and odors ,.....,..,. 120
Concentric cylinder evaporator ( CCE J . , . . 122
Mixture temperature , 123
Air flow rate 123
Adiabatic humidity deficit 123
Effect of wind on evaporation from CCE . 127
Effects of ambient air humidity deficit. 127
Effects of solar radiation 129
Machine variables, , 132
Cold weather effects 132
Operational problems 132
Equations for evaporation rate from
the CCE. . 135
Vlll
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CONTENTS
Page
8. Design Methods, National Applications and Costs. . 136
Evapotransporation bed design 136
National application of ET systems 143
Cost analysis for ET systems 150
Mechanical system design 154
National applications of mechanical
evaporation systems 158
Cost analysis for mechanical evaporation
systems 161
Concentric cylinder evaporator 163
References 166
Bibliography . . 169
IX
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FIGURES
Number
1 Typical ET bed cross-section 3
2 Lysimeter cross-section 4
3 Rotating disk test unit 5
4 Concentric cylinder test unit 6
5 Hourly COD profile 25
6 Distribution of flow and pollutional sources .... 26
7 Psychrometric chart for Boulder Colorado
elevation 39
8 Gradation analysis for three samples of ET sand. . . 42
9 Capillary rise test results (1Q hour) 43
10 Capillary rise test results (long term) 44
11 Moisture content of sand column as a function
of height above free water surface 45
12 Apparatus used to relate water table and saturation. 47
13 Moisture content of sand as a function of the
height of free standing water 48
14 Test lysimeters with clear and opaque covers .... 50
15 Drawing of rotating disk evaporator 53
16 Drawing of concentric cylinder evaporator 54
17 Effect of wastewater loading of 1.6 mm/d
(0.04 gpd/ft )on lysimeter gravity water level ... 69
18 Effect of wastewater loading of 3.2 mm/d
(0.08 gpd/ft ) on lysimeter gravity water level. . . 71
x
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FIGURES
Number Page
19 Gravity water level pattern during critical
months for six loading conditions 72
20 Monthly evaporation rate as a function of the
depth of the gravity water surface in the lysimeter 74
21 Lysimeter evaporation rate as a function of
gravity water depth 75
22 Monthly evaporation rate for clear and opaque
covered lysimeters 78
23 Effect of clear and opaque covers on
lysimeter gravity water level 79
24 Runoff as a function of rainfall, slope
and soil condition 80
25 Gradation analysis for Golden Sand and for
fractions greater and less than 100 mesh 82
26 Lysimeter gravity water level as a function of
evaporation and water loading for different
sized ET sands 83
27 Effect of elevated wastewater temperatures on
lysimeter gravity water level 85
28 Effect of topsoil cover on lysimeter
gravity water level 87
29 Monthly evaporation rate for different
vegetation cover 88
30 Effect of vegetation cover on lysimeter
gravity water level 90
31 Rotating disk unit bulk mixture temperature,
T ,, as a function of adiabatic saturation
temperature of the ambient air, T 95
S
xi
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FIGURES
Number Page
32 Evaporation as a function of adiabatic
humidity deficit for aluminum disk unit
in wind tunnel 96
33 Evaporation as a function of disk submergence
for the multi-diameter plywood disk unit in
the laboratory 97
34 Evaporation as a function of wind speed for
the aluminum disk unit in the laboratory wind
tunnel at various wind directions 101
35 Circumferential variation in evaporation with
wind direction relative to 90° 102
36 Evaporation as a function of wind speed for
various rotational speeds 104
37 Evaporation as a function of wind speed for
various disk spacings 105
38 Evaporation as a function of adiabatic humidity
deficit for disks of different colors 107
39 Evaporation as a function of adiabatic humidity
deficit for the aluminum disk unit operating
under field ambient air conditions 109
40 Comparison of actual evaporation to calculated
evaporation for the aluminum disk unit
receiving solar radiation 112
41 Comparison of RDE evaporation equation with
field and lake evaporation equations 113
42 (T -T ,) as a function of power consumed for
the aluminum disk unit using a tank heater 115
43 Evaporation as a function of wind speed at
various tank mixture temperatures. Data taken
from 36.0 inch dia. plywood disk unit 117
xii
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FIGURES
Number Page
44 Evaporation as a function of pan evaporation
for the aluminum disk unit 121
45 Concentric cylinder unit bulk mixture temper-
ature as a function of the adiabatic saturation
temperature of the supply air 124
46 Evaporation as a function of air flow rate at
different supply air humidity deficits for
CCE in the laboratory 125
47 Evaporation as a function of adiabatic humidity
deficit of supply air for CCE in the laboratory. . . 126
48 Evaporation as a function of wind speed for CCE
unit in the laboratory wind tunnel 128
49 Evaporation as a function of piped air flow rate
for a covered and an open CCE operating
in the laboratory 130
50 Evaporation as a function of piped air flow rate
for the CCE operating under field ambient
conditions 131
51 CCE evaporation as a function of submergence .... 133
52 Evaporation as a function of air flow rate at
different rotational speeds for the CCE
operating in the laboratory 134
53 Curve for establishing permanent home loading
rate for Boulder, Colorado based on
winter data, 1976-1977 139
54 Ten year evaporation and precipitation pattern
for Boulder, Colorado 141
55 Curve for establishing summer home loading rate
for Boulder, Colorado based on critical year
data of 1967 142
xiii
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FIGURES
Number Page
56 Precipitation - evaporation plots for eastern
US locations 144
57 Precipitation - evaporation plots for
mid-west locations 145
58 Precipitation - evaporation plots for
Rocky Mountain locations 146
59 Precipitation - evaporation plots for
Inter Mountain locations 147
60 Precipitation - evaporation plots for
Arizona locations 148
61 Precipitation - evaporation plots for
Western US locations 149
62 Areas of potential ET bed use in the US 151
63 RDE loading rate curve considering evaporation
potential and freezing weather periods for
Boulder, Colorado 157
64 RDE loading rate curve considering evaporation
potential for Hialeah, Florida 159
65 RDE loading curve considering evaporation potential
and freezing weather periods for Canton, 4SE,New York 160
xiv
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TABLES
Number Page
1 Home Wastewater Systems in the US .......... 16
2 ET Bed Applications in the US ............ 20
3 Sources of Wastewater Flows ............. 22
4 Average Characteristics of Home Wastewater Sources. . 23
5 Design Flow Comparison with Reduction and Recycling . 27
6 Weather Summary for the Test Area .......... 62
7 Lysimeter Study Conditions ......... ..... 65
8 Evaporation from Three Different Diameter Plywood
Disk Units under Controlled Identical Conditions. . . 99
9 24 Hour Evaporation Rate as a Function of
Disk Color and Material ............... 106
10 Total Solids and Total Volatile Solids
of Tank Mixture ...................
11 Comparison Between Actual and Computed Evaporation
using Mean Monthly and Mean Weekly Water Parameters . 119
12 Results of Modified Total Coliform Membrane
Filter Tests ..................... 122
13 Typical Costs for Constructing ET Systems ...... 153
14 Calculated Evaporation Rates for Boulder, Colorado. . 156
15 Cost Estimates for Rotating Disk Evaporator ..... 162
16 Vault Storage Costs ................. 161
17 RDE System Costs ................... 164
xv
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LIST OF SYMBOLS
A — interfacial area of evaporation
A — exposed wetted surfae area of CCE
AS — Vertical projected area of the set of disks
vp of the RDE
A — wetted surface area of the RDE
B$f — horse power required
C — ratio of cloudless hours to total sunshine
hours available
C — heat of the entering air
Evap — evaporation rate
G — heat from the ground
G — mass velocity of the unsaturated air
" per unit area of wetted surface
H -T- humidity of air
Ha — humidity of ambient air
Hi — humidity of entering air
Ho — humidity of exiting air
Hv — heat of vaporization
Hw — humidity of wet bulb temperature
AH, — humidity deficit of ambient air with
respect to saturation
AH2 — humidity deficit of piped air with
respect to saturation in CCE
K — von Karman constant
Kg — mass transfer coefficient
K1 — constant
Ma — molecular weight of air
Mv — molecular weight of water vapor
P — atmospheric pressure
Q — volumetric flow rate of air
R — gas constant
R — sunlight radiation
T — temperature
Ta — air temperature
Tmxd — reservoir liquid temperature, concentric cylinder
and rotating disk evaporator
T — air temperature at adxabatic saturation
TS , — adiabatic saturation temperature of piped air in CCE
Tws — temperature of wet bulb
W — wind speed
Wa — air flow rate
Ww — mass of water vaporized with time
xvi
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LIST OF SYMBOLS
a and b — empirical constants
b, — compressed air mass transport coefficient
bi — coefficient for effectiveness of
capture of solar radiation
e — vapor pressure of ambient air
e3. — efficiency fraction of a compressor
e — vapor pressure at air saturation
e° — vapor pressure at wet bulb temperature
fw — cloudiness factor
hg — heat transfer film coefficient
k, — ambient air mass transport coefficient
m — emissivity
n — compression constant for air
q — rate of heat flow
r — reflectance coefficient
w — mass rate of evaporation
z — elevation above ground of wind speed measurement
z — roughness length
y —psychrometric constant
A —slope of the saturated vapor pressure-
temperature curve
e —energy
—total short wave solar radiation reaching an
evaporation surface
—energy utilized in water evaporation
fl t —reflected solar radiation
re ect —net iongwave radiation exchange between
ongwave atmosphere and wetted surface
e , —energy lost or gained from the earth
eboun arT —energy stored or lost by the materials of the
internal evaporation system
etemnerature loss ~~sensible heat loss to the atmosphere
Ae d t d —energy brought in by water entering the system
e a vec e —total potential solar radiation reaching the
R earth
A. —latent heat of vaporization at saturation
s humidity
XTW —latent heat of vaporization
a —Stefan-Boltzman coefficient at wet-bulb
temperature
p —density of air
xvii
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ACKNOWLEDGMENTS
The support of several individuals within the Department of
Civil,Environmental and Architectural Engineering, University of
Colorado, is recognized. Four graduate students; John S. Nemcik,
Phillip W. Butterfield, John F. Steighner and Bruce T. Lytle did
much of the work in constructing the experimental units, taking
and interpreting the experimental data and literature search and
review as a part of the thesis project for the Master of Science
degree that each has earned.
Appreciation is extended to Dr. J. Ernest Flack for consul-
tation and advice regarding the hydraulic concepts.
XVlll
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SECTION 1
INTRODUCTION
A significant number of individual homes in the United
States are not connected to any form of central sanitary
sewerage and utilize individual home, on-site systems for
wastewater disposal. Based on information from the 1970 census,
approximately twenty-nine percent of the nation, or over fifty
million people, utilize this type of wastewater control for
their residences. This is an important consideration for health
authorities since the number of people served by on-site systems
is greater than the total people living in all cities of greater
than 100,000 population.
On-site disposal consists primarily of the use of a septic
tank and a soil absorption system, which is commonly termed a
leaching field. Approximately one-fourth of the homes in the
United States utilize this system. .The remaining four percent
of dwelling units, housing about nine million persons, utilize
other methods such as aerobic treatment, evaporation disposal,
sand filters and the more primitive forms such as cesspools and
privies.
The septic tank-soil absorption system (ST-SAS) usually
provides the most economic alternative for individual home
disposal where conditions favorable for leaching fields exist.
Wenk (1971) has shown that geological limitations for the use of
leaching fields exist in a portion of every state in the nation.
The limitations result from the following geologic conditions:
very shallow soil mantle, high groundwater tables, tight soils
or fractured rock which will not permit adequate filtration
purification of the wastewater before reaching the groundwater
reservoir. Failure of ST-SAS systems is often observed as
chemical or bacterial pollution of groundwaters. In most cases,
it is indicated by appearance of effluents at the ground surface.
The proper application of individual systems for new homes
is generally controlled by local health authorities. Where the
use of ST-SAS is proposed, a soil percolation test and geologic
survey are used to establish suitability. If any of the
limitations preclude the use of ST-SAS, a specially designed
system must be used. Such methods include evapo-transpiration
disposal (ST-ET), import fill, mounded disposal units,
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intermittent sand filters, or special mechanical treatment
providing a high degree of wastewater purification.
The evapo-transpiration (ST-ET) method is unique in that
it can be utilized in such a way as to be non-discharging to the
surface water or groundwater reservoir. This precludes the
potential pollution of drinking water sources. Most of the
specially designed systems involve a higher construction cost
than ST-SAS. Under the proper circumstances the ST-ET method
may be the most economical of the specially designed systems.
Evaporative methods may be used in several different ways.
Sealed evaporation-transpiration beds composed of water tight
liners and specifically selected sands are presently in use. A
somewhat similar appearing unit uses an unsealed bed which has
evaporation-transpiration as the chief disposal mechanism but
percolation is also utilized. The use of mechanical evaporative
devices for wastewater is in the development stage. Evaporative
mechanical systems can be designed to require only a very small
amount of electrical energy and have a very low maintenance
requirement, two desirable features of any individual home
wastewater disposal concept.
This study was initiated to investigate the application of
the evaporation disposal concept under the condition that no
water is discharged to surface streams or groundwater. Two
general types of units have been evaluated at laboratory scale,
evaporation-transpiration beds and mechanical evaporation
systems.
A typical ET bed installation is shown in Figure 1. Septic
tank effluent flows into the lower portion of the bed as it is
generated. The water is raised to the top portion of the bed by
capillary action in the fine sand. Evaporation takes place at
or near the surface of the bed. Plant life may be used to raise
the water from the area of the root zone to the leaves to be
removed by transpiration.
In this study, twenty-nine small scale ET bed lysimeters
were used to provide concurrent evaluation of several design and
operational parameters. This permitted the study of parametric
variations under the same ambient conditions. A drawing of the
lysimeter unit is shown in Figure 2.
Two types of mechanical evaporation units were studied
(see Figures 3 and 4). A rotating disk mechanical evaporation
unit is shown in Figure 3. The disks turn slowly and the
moisture on the wetted surface is continuously transferred to
the ambient air moving over the unit. The concentric cylinder
mechanical evaporation unit of Figure 4 utilizes forced air
entering at the center of the cylinder and moving outward
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impervious
membrane
V//AW///
Figure 1. Typical ET bed cross-section.
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~ feed column, 2.5 cm (1")
PVC pipe
double 6 mil /
seamless liner
21"
Figure 2. Lysimeter cross-section.
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Figure 3. Rotating disk test unit
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Figure 4. Concentric cylinder test unit.
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through the wetted cloth wraps for vapor transport. Both
mechanical units require a septic tank for pretreatment and a
storage vault as a part of the total disposal system. Each of
the test units shown was capable of evaporating about twenty-five
liters per day (6-7 gal/day) under average conditions.
The purpose of the study was the evaluation of design and
operational parameters for each type of unit, correlation of
laboratory data with field units for ET beds and the development
of cost criteria for the full scale installation of both types
of units.
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SECTION 2
CONCLUSIONS
Nearly twenty million homes in the U.S., housing approxi-
mately twenty-nine percent of the population, are in unsewered
areas and utilize individual home treatment and disposal tech-
niques for wastewater discharge. The septic tank and soil
absorption system is the most common individual home wastewater
disposal method because of the generally lower cost involved.
Many areas of the U.S. are not suited for soil absorption systems
and alternate methods must be used.
Disposal methods utilizing evaporation processes have been
adapted to wastewater disposal techniques. This method can be
utilized with permanent homes in certain portions of the country
and may be applicable for summer homes, outdoor recreation areas,
highway rest stops and similar installations.
The rate of evaporation occurring in these processes is
directly related to the vapor pressure or humidity deficit of
the ambient air and is markedly increased at higher temperatures.
For this reason, evaporation processes are more feasible for use
in the southern portions of the country and also lend themselves
to applications for summer homes.
The ET bed and mechanical units used for individual homes
are exposed to ambient air and are subject to rain and snowfall.
As a result, the precipitation pattern of a specific location
has a significant effect on the efficacy of the application of
the evaporation concept.
The size requirements of an evaporation unit are directly
related to the amount of wastewater generated. Accurate
estimates of flow must be made in each design to prevent failure
due to undersizing or excessive cost due to flow quantity over-
estimation. The use of water saving plumbing devices and
appliances will result in a direct cost savings in the construc-
tion of an evaporative system.
EVAPORATION-TRANSPIRATION BEDS
Two major concepts for ET beds are used; the non-discharging
unit where all wastewater is evaporated to the atmosphere and the
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combination concept where seepage into the adjacent earth
materials is utilized in conjunction with evaporation for liquid
disposal. Another design consideration is whether some surface
runoff is to be permitted during precipitation events. The
consideration of these different concepts of design criteria has
been termed the acceptance criteria, and the standards for per-
formance are usually set by state or local health authorities.
A survey was made to determine the present use character-
istics of ET beds. Approximately four thousand to five thousand
ET beds are now in use in the U.S. About three-quarters of these
units are of the combination type.
The results of this study are from investigations of the
non-discharging type of ET bed. The studies were made using
twenty-nine ET bed lysimeters, 53 cm (21 in.) in diameter and
79 cm (31 in.) deep.
Analysis of the results of the lysimeter studies showed
that ET beds have only limited capacity for storage of waste-
water and cannot store a large portion of the winter loading
for later evaporation in the summer. Non-discharging units
should be designed at a loading rate equal to the evaporation
minus precipitation for the critical year in a ten year span.
In order to meet these conditions, the weather conditions in the
design area must be such that the evaporation rate substantially
exceeds the precipitation rate for.every month of the year.
This condition exists in a portion of the states of Texas, New
Mexico, Arizona, California, Nevada, Utah and Colorado. When
the concept is used for summer home applications, the areas that
have acceptable climatological conditions can be expanded to
include parts of the ten additional states of Oregon, Washington,
Idaho, Montana, Wyoming, North Dakota, South Dakota, Nebraska,
Kansas and Oklahoma, as shown in Figure 62.
The most reasonable approach to the design of ET beds is
with the use of the National Oceanic and Atmospheric Administra-
tion (NOAA) weather data for pan evaporation and precipitation.
It was found that winter evaporation from nearly full ET beds
was approximately the same as measured or estimated for winter
pan evaporation. Summer evaporation from nearly full ET beds
was about seventy percent of the measured pan value. The mode
of operation of ET beds for permanent homes is such that the
units have no saturated pore gravity water during long periods
in the summer and a low moisture content is held as interstitial
capillary water. Summer ET bed evaporation rates are as low as
twenty percent of the pan evaporation rate due to the fact that
tightly held interstitial capillary water does not rise to the
surface and therefore is not evaporated at the same rate as the
saturated pore water. It was found in these studies that the
moisture content of the bed adjusts itself in such a way that
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the evaporation rate is fairly constant throughout the year and
is about the same as the winter pan evaporation rate.
The type and size of sand used in an ET bed is very impor-
tant. The sand should be a clean, uniform sand in the size range
of DCJQ (fifty percent by weight smaller than) equal 0.1 mm. This
type of sand is available in many areas from the settling ponds
of gravel washing operations. Capillary rise height and rate
tests should be made on ET sand sources before they are selected
for use.
The characteristics of the home wastewaters had little
effect on the functioning of the ET beds. All units must be
preceded with a properly functioning septic tank. Variations in
wastewater temperature within the ranges to be expected in the
field had no measurable effect on the performance of the system.
The wastewater type had no measurable effect on evaporation rate.
Primary treated effluent, secondary effluent, and tap water gave
essentially the same results when used with the lysimeters.
Approximately seventy-one percent of the annual evaporation
from an ET bed was found to be due to the movement of unsaturated
air over the surface of the bed. The remaining twenty-nine
percent was due to direct sunlight radiation striking the wet
sand surface. Selecting surface vegetation for an ET bed to
increase ET rates and improve appearance is a difficult task.
The very moist, high saturated pore water conditions of the
winter and the very dry conditions of the bed in the late summer
requires that any vegetation planted on the surface of the bed
must have a very high tolerance for wide variations in soil
moisture conditions. Most trees and plants, including lawn
grass, could not tolerate the moisture extremes. Juniper shrubs
and weeds were the only plants that survived the annual soil
moisture cycle. In the design of ET beds for summer home appli-
cations, the use of fast growing, highly transpiring plants such
as alfalfa can substantially increase ET rates.
The appropriate loading rates to be used in the design of
ET beds for permanent homes range from 1.2 mm/d (0.03 gpd/ft2)
for areas of eastern Colorado or northern Nevada to 3.2 mm/d
(0.08 gpd/ft2) for southwestern Arizona. Higher loading rates
have been reported in the literature but these are usually
associated with systems relying on evapo-transpiration plus soil
absorption, or are coupled with very high estimates of wastewater
generation rates. Loading rates for summer home application
range from 4 mm/d (0.10 gpd/ft2) to 8 mm/d (0.20 gpd/ft2) or
higher.
The health hazards associated with non-discharging ET beds
appear to be minimal. Salt build-up in the bed is not a major
problem affecting the functioning of the unit, although it may
have an adverse effect on surface vegetation after a long period
10
-------
of use. Salt concentrations build up at the surface of the bed
during dry summer periods but are redistributed in the pore
water throughout the bed after a rainfall event.
The cost of lined, non-discharging ET beds with imported
ET sand materials is in the range of $10.00-$15.00 per square
meter of surface area ($1.00-$1.50/ft^). Typical non-discharging
ET bed installations range in cost from $3,000 to $10,000 for
permanent, individual homes.
MECHANICAL EVAPORATION SYSTEMS
Two types of mechanical evaporation devices were fabricated
in small scale and evaluated. Most of the studies were conducted
on a multiple rotating disk evaporator (RDE) and a portion of
the work involved a rotating cylinder cylinder evaporator (CCE)
with multiple wraps of burlap cloth forming the concentric
cylinder and with piped air entering the center of the unit.
The advantage of the mechanical unit as compared to an ET bed
is that a large evaporation surface is created within a very
small area for rainfall catchment. This type of unit can be
used for evaporation disposal in areas having high annual pre-
cipitation.
The mechanical evaporation unit concept can be applied in
most locations in the U.S. The unit is preceded by a septic
tank and in most areas, a storage vault must also be used.
When ambient temperatures drop below 4°C (40°F), freezing of the
water on the disks occurs and the unit must be stopped. The
wastewater inflow must be held in the vault during low tempera-
ture periods. Large vault storage adds significantly to the
cost of the system; consequently the most economical applica-
tions are in the southern regions where freezing conditions do
not occur.
Equations for the design of RDE units have been developed
from the small test unit. The major factors in the equations
are wind speed and humidity deficit of the ambient air. A minor
factor is the solar radiation intensity. These parameters can
be established from NOAA weather data for different areas of the
country on a monthly basis. Correlation of machine evaporation
rate with measured or estimated pan evaporation rate was also
made and can be used to estimate unit size for different loca-
tions. Sizing of the CCE is dependent on the piped air mass
flow rate and humidity deficit and, to a small extent, on
ambient air wind speed and humidity deficit as well as solar
radiation intensity.
Wind movement over the disks of the RDE maintains the
humidity deficit driving force for evaporation. A wind rose
(a scaled graphical presentation of surface wind data in terms
of speed and direction) should be established for a design
11
-------
location and the unit should be oriented with the disks parallel
to the prevailing wind. Wind movement of the unsaturated air is
responsible for about 95 percent of the evaporation from the
unit. Because of the small size of the unit, direct sunlight
striking the disks accounts for only about five percent of the
evaporation rate.
The optimization of the machine variables for this type of
unit will be a continuing process. Studies on the test unit
resulted in some initial findings.
a. The rotational speed of the disks does not have much
effect on evaporation rate as long as the speed is
great enough to keep the total evaporation surface of
the disks wet at all times.
b. The evaporation rate per unit of wetted area is essen-
tially a constant for any set of ambient conditions.
The maximum wetted exposed surface area for the RDE
exists when the disks are submerged to a depth equal
to 0.71 R, where R is the disk radius. This submergence
level results in a 75 percent exposed wetted surface and
produces the maximum evaporation rate.
c. Optimum disk spacing was found to be at 2.5 cm (1 inch)
between centers for the 3 mm (1/8 inch) thickness. At
larger spacing, the unit costs increase without pro-
viding additional evaporation capacity. At closer
spacing the wind movement between disks is retarded and
evaporation rate is reduced.
d. Using different disk materials did not affect evapora-
tion rate except when styrofoam was used. Apparently
the poor heat conducting properties of styrofoam cause
a slightly reduced evaporation rate. The color of the
disk had only a minor effect on evaporation. White
disks reflected more direct solar radiation and tended
to reduce evaporation slightly. The strength charac-
teristics and materials cost should be the important
considerations in selecting disk materials.
A brief test was used to indicate the health hazard
associated with bacterially contaminated aerosols being carried
by the wind from the unit. No fecal coliform bacteria were
found at distances greater than 0.15 m (0.5 ft) downwind from
the disks. Salt accumulation will occur in the liquid in the
storage vault and in the reservoir of the unit. This will
require the removal and disposal of some liquid every few years
in order to prevent excessive build-up of salts and sludge.
A preliminary cost estimate for an RDE constructed with
plastic disks resulted in a total cost of $6300 for the situation
12
-------
of a southern climate where no freezing conditions would exist,
to values of $15,000 and higher for northern application where
very large vault storage would be required due to extended
periods of freezing weather. The cost for summer home applica-
tions would be in the range of $4000.
Evaporation systems for wastewater disposal are relatively
expensive, especially when compared with soil absorption systems,
For design situations in certain parts of the country where
alternate systems are required, they may be a feasible option.
The cost analysis for the concentric cylinder evaporator
resulted in the conclusion that this type of unit is not eco-
nomically feasible due to the power costs associated with
pumping of the air. The capitol cost of the evaporator is in
the same range as that of the rotating disk evaporator but the
power cost would be in the order of $0.32/1000 liters ($1.20/
1000 gal). The total annual cost would be much higher than for
the RDE and it would be unacceptable for nearly all conceivable
applications.
13
-------
SECTION 3
RECOMMENDATIONS
As a result of this study, several recommendations are made
to gain improved understanding of the application of the
evaporation concept.
ET BED
1. There is a need for studies that would define more
clearly the considerations involved in the setting of
acceptance criteria for the different types of ET beds.
2. Many individual tests of ET bed systems are being made
throughout the U.S. It would be useful for local health
authorities if all studies were analyzed, correlated
and reported by one national agency.
3. It would be useful if several carefully sealed test
units were installed at different locations throughout
the U.S. and evaluated under uniform monitoring condi-
tions.
MECHANICAL EVAPORATION SYSTEMS
1. Further work should be encouraged to improve and
optimize the design of the RDE. This should include
the selection of materials of construction that would
reduce the overall cost of the system.
2. Several full scale units should be evaluated at dif-
ferent locations in the U.S.
14
-------
SECTION 4
BACKGROUND
During the last three decades, strong emphasis has been
placed on community sewerage systems and centralized wastewater
treatment plants. Much less concern has been shown for the
development of new technology for individual home wastewater
systems. According to the figures from the 1970 census, twenty-
nine percent of the dwelling units in the U.S. are not connected
to a central sewerage system and utilize individual home systems.
The average number of persons per dwelling unit in the U.S. is
almost exactly three people based on the 1970 census data for
population and housing units. Using this average figure, a total
of approximately 58.5 million people can be considered as the
population living in homes that are not connected to a central
sewer system. As a comparison, there were 154 cities in the U.S.
in 1970 that had populations of over 100,000. The total popula-
tion of these 154 cities was 55.5 million people. It is apparent
that new developments in individual home systems have not kept
pace with the rapidly expanding technology for large city
sewage treatment systems.
A tabulation of the 1970 census figures by states indicating
the number of homes not connected to public sewer systems is
shown in Table 1. The data are arranged in descending order,
beginning with New York and Pennsylvania which had over a million
homes of this type, to the District of Columbia which had only
1325. The bracketed numbers are the percentages of unsewered
dwelling units in each state.
Several rurally oriented states in the Southeast and New
England have high percentages of population on individual systems.
States with over fifty percent reliance on individual systems
include Mississippi, North Carolina, South Carolina, Maine, New
Hampshire and Vermont. Over one-half of the total number of
individual systems in the U.S. are located in the states of New
York, Pennsylvania, Michigan, Ohio, Indiana, Illinois, California,
Texas and Florida. This indicates that individual systems are
used extensively in suburban areas as well as for rural applica-
tions. A significant number of individual home units exist in
every state. The only exception is totally urbanized Washington,
D.C.
15
-------
TABLE 1. HOME WASTEWATER SYSTEMS IN THE U.S. (1970 CENSUS)
Number of Homes not
State on Public Sewer
New York 1
Pennsylvania 1
Florida
Michigan
California
North Carolina
Ohio
Texas
Indiana
Tennessee
Illinois
Georgia
Virginia
Alabama
Kentucky
Massachusetts
Missouri
South Carolina
Wisconsin
Washington
New Jersey
Louisiana
Connecticut
Mississippi
Minnesota
Arkansas
Iowa
West Virginia
Oregon
Maryland
Oklahoma
Kansas
Maine
Arizona
Colorado
Nebraska
New Hampshire
Rhode Island
Idaho
New Mexico
Montana
South Dakota
Vermont
North Dakota
Hawaii
Utah
Delaware
Alaska
Wyoming
Nevada
District of Columbia
U.S.A. 19
,334
,081
981
897
891
885
882
819
650
625
619
618
578
548
524
499
491
441
421
418
414
367
360
358
354
317
288
288
286
281
251
192
169
132
129
125
116
109
100
91
86
81
77
71
54
53
44
33
27
23
1
,506
/
r
r
t
r
r
t
t
t
t
t
i
i
r
r
t
t
r
r
r
f
r
t
9
1
r
r
r
r
r
r
t
r
r
r
r
r
r
r
r
r
r
r
f
r
r
t
r
r
r
r
r
136
516
095
942
337
429
076
223
855
680
683
109
793
484
184
485
895
206
116
373
364
726
218
629
561
286
718
628
503
000
587
726
726
446
979
586
246
387
412
508
172
336
580
531
402
255
730
052
566
939
325
167
(22%)
(28%)
(39%)
(32%)
(13%)
(55%)
(26%)
(22%)
(38%)
(48%)
(17%)
(42%)
(39%)
(49%)
(49%)
(27%)
(30%)
(55%)
(30%)
(35%)
(18%)
(32%)
(37%)
(51%)
(29%)
(47%)
(31%)
(49%)
(39%)
(23%)
(27%)
(24%)
(50%)
(23%)
(18%)
(25%)
(47%)
(36%)
(42%)
(28%)
(36%)
(36%)
(52%)
(36%)
(24%)
(17%)
(26%)
(37%)
(24%)
(14%)
<
-------
SYSTEMS FOR INDIVIDUAL HOMES
The septic tank, soil absorption system (ST-SAS) is used
extensively in the U.S. Over eighty-seven percent of the
individual systems are of this type. This system has been
utilized with few modifications since it was patented by Mouras
in 1881. It is a relatively low cost method of disposal that
has proven to be satisfactory for many applications. The
limitations that may occur with ST-SAS are well known. Soil
absorption systems cannot be installed where highly permeable
coarse soils or highly impermeable clay soils exist, in areas of
shallow soils over fractured rock, steeply sloping topography or
high groundwater conditions. Some of these limitations are
present in a significant portion of all areas of the country.
Where limitations exist, alternate means of disposal are needed.
The evaporative disposal techniques provide a means of approach-
ing the problem that can result in no discharge to the ground or
surface water environment.
BACKGROUND AND USE OF ET BEDS
The concept of the evaporation-transpiration bed for
disposal of home wastewaters developed from studies on the
functioning of seepage beds. Studies by Dr. A.P. Bernhart (1964,
1972, 1973, 1974) at the University of Toronto defined the con-
tribution of evaporation in predicting the disposal rate for
shallow seepage beds. Design equations have been presented in
which the evaporation rate from shallow seepage beds was related
to water depth below ground surface, type of soil and vegetation
cover on the bed.
Consulting engineers from western states adapted the
evaporation data from Bernhart's studies to develop the concept
of a total evaporation disposal system for use in areas where
severe limitations precluded the use of leaching fields. With
the use of an impermeable liner and carefully selected sand,
systems have been designed and put into operation that are
totally evaporative and non-discharging.
At the present time, the ET bed concept is being used in
the design of individual home systems in several states, either
on an experimental basis or as an accepted design practice for
specialized applications.
Two types of ET bed concepts are in use: (1) the totally
non-discharging type and (2) the combination type where both
seepage and ET are utilized as the means of disposal.
The first type is utilized where protection of groundwater
quality is essential. A continuous liner is placed so as to
ensure that evaporation is the only means of water leaving the
bed. A carefully selected ET sand is used so that capillary
17
-------
action will raise the water to the surface for evaporation.
Most of the rainfall and snowmelt occurring on the unit will
drain into the bed and become a part of the water volume to be
evaporated. The successful operation of this type of ET system
requires that it be applied in areas where meteorological con-
ditions are such that evaporation potential exceeds the preci-
pitation during every month of the year.
The second type/ the combination ET-soil absorption system,
is designed on the basis that limited percolation will assist
evaporation as the water removal mechanism. Combination systems
are unlined or partially lined and the water level in the unit
is maintained near the ground surface. Precipitation tends to
run off of the bed due to the high water level in the soil. This
is the type of system studied by Bernhart. His work provided
quantitative information on the evaporation portion of the dis-
posal mechanism. Many experimental tests of both types of units
are under study in several states by local health authorities.
A published paper by Tanner and Bouma (1975) has reviewed
the potential for use of the non-discharging system in Wisconsin.
Utilizing the energy balance technique to establish potential
evaporation and the annual rainfall pattern in Madison, Wisconsin,
they have shown that the system is not feasible in that area
because rainfall exceeds evaporative potential. They have also
shown with calculations that the use of small trees or shrubs to
increase water vaporization by transpiration would not result in
enough additional water loss to make the concept feasible in
their area. They have also shown with calculations that the
biological energy available from the aerobic decomposition of
organic matter in the wastewater is insignificant when considered
in the evaporation calculations.
Research studies at the University of Wyoming by Hasfurther,
et al. (1976) have shown that non-discharging ET beds are feasible
in that area for summer use applications. Loadings between 2.6
mm/day (0.06 gallons/day/ft2) and 10.4 mm/day (0.24 gallons/day/
ft2) were used on a short term, May through July, study.
ET BED APPLICATIONS IN THE U.S.
In order to assess the extent of application of ET beds in
the U.S., a questionnaire was sent to the health departments of
all 50 states and five Districts and Territories. The survey
asked for estimates of the number of ET bed installations of each
type and for information on design criteria and legal restric-
tions on the use of this disposal method.
Forty-three responses were received. The results are shown
in four groupings in Table 2. Nineteen state agencies replied
that ET beds were not used. All of the states in this category,
except Utah, have annual rainfall rates that exceed evaporation
18
-------
potential.
Seven states replied that a relatively small number of
units were installed. In most cases it was indicated that the
units were experimental in nature. The responses were mostly
from eastern states and indicated that agencies within these
states are evaluating the concept, using primarily the non-
discharging method.
In the regions of the country where ET beds are used more
extensively, it is difficult for state health authorities to
accurately estimate the number of installed units. Permits for
the installation of individual home wastewater systems are
normally handled on the county or city level and this information
is generally not compiled on a state-wide basis. It can be
concluded from the replies in this category that the combination
system is used in several eastern states, while both types of
units are more common in the semi-arid, western U.S.
In Colorado, the authors estimate that several hundred
units of each type have been installed. In Boulder County,
which has about five percent of the state's population, over one
hundred permits for ET systems have been issued, with approxi-
mately one-half of the total for each type of system. The first
units were of the non-discharging type and were installed in
1972.
Based on the approximate figures of Table 2, it can be
estimated that the total number of ET systems in use in the
United States is probably in the range of 4000 to 5000 units,
with about three-quarters of the total being of the combination
type.
That portion of the questionnaire relating to design cri-
teria for ET systems revealed that this is usually set by local
agencies or the design engineer for each system. The criteria
developed by Bernhart are used extensively with modifications
based on experience gained in local areas. New criteria for
design are beginning to appear. The State of Washington has
developed very complete guidelines for non-discharging systems
that are based on a month by month water balance utilizing
localized, ten-year evaporation and precipitation data.
The question relating to legal limitations on ET system use
was included in the questionnaire because of a special situation
that exists in Colorado. Most states require that the design
meet local health authority requirements and conform to general
state regulations. In Colorado, a law has recently been passed
that creates a class of water wells where the water cannot be
used in a completely consumptive manner; that is, some of the
effluent must be returned to the ground water or surface water
resource. This would exclude the use of a totally non-
19
-------
TABLE 2. ET BED APPLICATIONS IN THE U.S.
(Health Department Replies)
Group I - ET beds not used.
Alabama
Florida
Hawaii
Illinois
Iowa
Michigan
Minnesota
Mississippi
Missouri
New Hampshire
Group II - Small number of applications.
North Dakota
Oklahoma
Utah
Virginia
Wisconsin
Canal Zone
Puerto Rico
Trust Territory
of the Pacific
Virgin Islands
Connecticut
Delaware
Kentucky
Maine
Nebraska
New Jersey
South Carolina
Non-discharging
1
1
1
6
3
2
1
Combination
15
2
0
0
0
0
0
Group III - ET beds used but estimate of number not available.
Arizona, California, South Dakota
Group IV - ET beds used more extensively.
Non-discharging
Colorado
Idaho
Montana
Nevada
New Mexico
New York
North Carolina
Ohio
Oregon
Rhode Island
Texas
Washington
West Virginia
Wyoming
Combination
40
15
20
0
70
0
1
0
16
0
300
10
12
30
30
unknown
500
50
200
500
100
400
200
300-400 total
600
80
12
20
-------
discharging ET system for homes where this class of water wells
is used.
PURPOSE OF THE STUDY
This study was initiated in response to the growing interest
in evaporative methods for home wastewater disposal. The
purpose was to evaluate the functioning of non-discharging ET
beds and mechanical evaporation systems, and establish the
relationship between ambient weather conditions and operational
and design variables. Design criteria have been developed for
ET beds operating under the ambient conditions found in a semi-
arid region, where evaporation rate exceeds rainfall values on
an annual and monthly basis. In this study, seepage was com-
pletely eliminated so that criteria for total evaporative
system could be established. Rainfall, snowmelt and freezing
weather effects were assessed on a daily and seasonal basis.
One of the limitations of ET bed systems is that the large
surface area required is exposed to precipitation. The rainfall
and snowmelt entering the unit must be included in the loading
placed on the evaporative process. As a result, the feasible
application of non-discharging ET beds is restricted to areas
where the ET rate substantially exceeds that of precipitation.
The mechanical evaporation unit concept was devisQd to
provide a large surface for evaporation and at the same time
have a minimum catchment area for precipitation. The objective
was to extend the potential application of the evaporative
method to areas of higher rainfall. Two types of mechanical
units were evaluated under controlled laboratory conditions and
under variable outdoor ambient weather conditions. The rotating
disk and the concentric cylinder units were designed to provide
a large exposed wetted area for the size of precipitation
catchment area, and have a mechanically uncomplicated method.of
maintaining the wetted surface. The effect of several of the
major design variables on the evaporation rate was studied.
CHARACTERISTICS OF HOME WASTEWATERS
The quantity and composition of home wastewater flows are
major design parameters for all types of disposal systems.
Evaporation systems and seepage methods are sized in direct
proportion to the average daily flow. Underestimating the flow
will cause failure of the designed system. Overestimating flow
will add unnecessary cost and affect feasibility considerations.
Since system cost is nearly proportional to flow rate, water
saving devices are particularly encouraged in individual homes
utilizing evaporative wastewater disposal. Flow reduction must
be considered on an individual fixture basis; therefore detailed
water use information is valuable. Strength characteristics of
home wastes provide information on nuisance and odor potential
21
-------
and can be used for considerations of flow segregation and
separate treatment for different waste sources.
The results of several recent studies have shown that the
average per capita in-home use of water is approximately 170
liters/day (45 gallons/day). The breakdown of water use by
appliance is given in Table 3. It has also been shown in these
reports that large families use less water per capita than small
families. Bennett and Linstedt (1975) have proposed the formula
Q U/house/day) = 225 U) + 150 (A/adult resident)
+ 75 (5,/child resident)
Q (gal/house/day = 60 (gal) + 40 (gal/adult resident)
+ 20 (gal/child resident)
TABLE 3. SOURCES OF WASTEWATER FLOWS
(gallons/person/day)
Source
Bennett, Cohen, Witt, Ligman, Range Average
et al. et al. et al. et al.
Toilet
Laundry
Bath- total
tub , shower
sink
Kitchen-total
sink
dishwasher
garbage disp.
Misc.
Total
15
12
11
(9)
(2)
7
(5)
(1)
(1)
0
44.5
13
8
5
13
5
43.8
9
11
10
5
8
42.6
20
10
12
4
2
0
47.5
9-20
8-12
5-12
5-13
0-8
42.6-47.5
14
10
10
(9)
(2)
8
(5)
(3)
(1)
3
44.6
32
22
22
18
6
100
On this basis, the average home consumption for design
purposes would only rarely exceed 850 H (225 gallons) per
dwelling unit per day. This would be equivalent to a family of
three adults and two children or two adults and four children.
Basing system design on the number of bedrooms and flows of
375 H to 567 5, (100 to 150 gallons) per bedroom, as has been
done in the past, is unrealistic when viewed in terms of recent
water use and family size trends.
Two recent studies by Witt, et al. (1974) and Bennett and
Linstedt (1975) have reported similar results for pollutional
strength characteristics of home wastewaters. The results of
the second study are summarized in Table 4. The average hourly
distribution of flow and pollutional strength as measured by COD
22
-------
TABLE 4. AVERAGE CHARACTERISTICS OF HOME WASTEWATER SOURCES
Flow/Use COD
(gal) mg/l
Sink
Kitchen
Bathroom
Average
Bath- Shower
Toilet
Feces (gm/CD)
Urine (gm/CD)
Paper (gm/CD)
Average
Washing Machine
Cycle 1
Cycle 2
Cycle 3
Average
Garbage Disp.
Dishwasher
Cycle 1
Cycle 2
Cycle 3
Cycle 4
Cycle 5
Average
Combined (mg/i)
Average (t/CD)
Witt (#/CD)
1.0
2.0
27.2
4.1
17.2
4.2
17.2
38.6
2.1
2.0
1.0
2.0
1.0
1.0
7.0
1652
495
850
220
(34)
(13)
(18)
1180
1050
185
130
550
11780
620
210
44
35
11
225
930
0.34
BOD
mg/Jl
1082
261
533
100
(4.7)
(2.2)
(0)
124
400
46
39
200
4065
363
99
23
6
2
125
278
0.11
0.13
TOC
mg/X,
370
21
174
4
10
82
137
47
18
5
0
50
TS
mg/J,
1328
480
760
339
(22)
(40)
(14.5)
1400
1185
421
.527
760
10748
2207
1083
412
75
73
920
1080
0.40
0.31
%
Org.
71
23
53
6
78
65
95
73
17
6
20
16
68
20
16
3
0
0
16
55
0.22
0.19
SS
mg/fc
209
228
215
27
(22)
(0)
(14.5)
650
128
0
46
78
6672
83
16
0
0
0
26
396
0.15
0.11
%
Org.
100
100
100
100
78
0
95
85
95
95
100
90
0.14
0.09
MBAS
mg/J,
_
6
-
53
—
77
4
2
35
-
0.3
0.1
0
0
0
0.1
15
0.005
PO4
mg/fc
1
0
1
0
(2.2)*
(0.9)
55
30
3
1
14
24
15
1
0
0
0
4
22
0.008
0.009
Total N
mg/Jl
114
1
38
0
(1.7)*
(3.5)
0
95
8
1
1
5
285
0
0
0
0
0
0
44
0.015
0.015
Temp
°C pH
43
20
28
38
21
55
39
55
53
30
58
58
58
58
90
62
36
6.6
7.9
7.2
8.2
5.6
8.5
7.9
7.0
7.8
6.4
10.9
8.3
7.0
6.9
7.0
7.6
7.4
to
CO
Literature values
-------
is shown in Figure 5.
A schematic drawing of the flow and pollutional strength
proportion from each fixture is shown in Figure 6. It can be
noted that black waters (wastes from the toilet and garbage
grinder) contribute about three-fourths of the pollutants but
comprise only about one-third of the flow. The remainder is
contributed by gray waters (wastes from sinks, bathing, clothes
washing and dish washing). The idea of separate treatment or
recycling of gray waters coupled with evaporation disposal for
the black waters could be investigated.
Special fixtures and appliances designed for lowering water
use in the home can be very beneficial in reducing the required
size of individual home wastewater treatment units. The major
water using appliances in the home are toilets, washing machines,
bath-showers and sinks. The fixtures presently in use in most
homes are designed to provide maximum convenience to the user.
Flow reduction could be accomplished in the design of appliances
and fixtures without substantially altering the convenience
level.
Modern toilets use approximately 15 liters (4 gallons) per
flush and the float can be adjusted so that 13 liters (3.5
gallons) are used. Redesigning the tanks and bowl so that 10
liters (2.7 gallons) are required per flush can reduce daily
household water use by 90 liters (25 gallons) each day. Other
approaches such as dual cycle toilets may reduce water use even
further.
Washing machine water use could be reduced by washing only
full loads or using the load size selector. The use of a suds
saver, where rinse water from previous loads is used as wash
water in following loads, is generally not convenient and results
in lower wash water temperatures. Modern washing machines have
a permanent press cycle that uses about 50 liters (13 gallons)
more water than the normal load cycle, 150 liters (40 gallons).
From recent studies by Bennett and Linstedt (1975), it was found
that washing machine water saving techniques, except for the suds
saver, are presently being used in washing machine design. As
a result, further water saving with the washing machine does not
seem to be available.
Bath and sink water use may be lowered significantly by
reducing plumbing pipe and valve size, coupled with smaller
bathroom sink and tub sizes. Plumbing pipe and valves are
presently of 5/8-inch (1.6-cm) diameter. This size is capable
of delivering about 40 liters/min (10 gal/min). It has been
found that with a controlled shower head, 4 liters per minute
(1 gallon/minute) is adequate for bathing (Bennett and Linstedt,
1975). Smaller piping and valves are required for proper
blending of hot and cold water to obtain desired bathing water
24
-------
shower
bath
X 24 000
disposal
u 16 000
washing
machine
6 10N246810M
Time of Day
Figure 5. Hourly COD profile.
25
-------
Washing
Machine
Bath
Dish
Waahing
Machin
PLOW
SUSPENDED SOLIDS
COD
Figure 6. Distribution of flow and pollutional sources.
-------
temperature when the lower flow rate is used. Reduction of
bathroom sink and shower plumbing to 0.6 cm (1/4 inch) size could
lower water use in these fixtures from 300 liters (80 gallons)
to 130 liters (35 gallons) per family per day.
Application of toilet, sink and shower water reduction
devices could lower water use by 260 liters (70 gallons) per
family per day, based on the normal flow of 850 liters/day (225
gal/day). This represents a 31 percent reduction in wastewater
flow. If the concept of flow segregation is used with bath and
washing machine wastes being recycled for toilet flushing, the
flow can be further reduced by about one-third resulting in less
than one-half of the original design flow. The total for these
modifications are shown in Table 5.
TABLE 5. DESIGN FLOW COMPARISON WITH REDUCTION AND RECYCLING
Design Flow
without Flow Reduction
Design Flow
with Flow Reduction
£/house/day gal/house/day £/house/day gal/house/day
Toilet
Sinks
Garbage Disp.
Bath- Shower
Dishwasher
Washing Machine
TOTAL
275
142
15
162
19
218
830
73
38
4
43
5
58
220
185
87
15
43
19
218
567
48
23
4
12
5
58
150
Total with gray
water recycle
555
147
382
102
27
-------
SECTION 5
THEORY
Evaporation is a process governed by the principles of heat
and mass transfer. The application of evaporation theory to the
understanding and qualitative definition of specific evaporative
processes can take several forms. 1) An isolated system can be
defined in terms of a constant rate of air movement over a
wetted surface. Heat is transferred from the air to the water
producing vaporization of the water and transfer of the vapor to
the air stream. This approach is termed the mass transport or
aerodynamic theory. 2) A system similar to the one above may be
subject to external forms of energy such as sunlight radiation.
The evaporation due to the supplementary energy source is con-
sidered as a separate additive term established on an empirical
basis. This approach is termed the combination equation method.
3) Evaporation occurring in an experimental system can be cor-
related with known or systematically measured evaporation
influenced by the same energy sources. Correlations can be made
with measured and reported class A pan values or with formulas
established by others for evaporation rates established under
similar circumstances. The procedure is empirical in nature and
is termed the correlation method. 4) Evaporative systems
influenced completely by ambient conditions can be formulated
using the energy budget theory. An energy balance is formulated
based on sunlight radiation, advected energy and water vaporiza-
tion.
AERODYNAMIC THEORY
The aerodynamic or mass transfer approach is based on the
thermodynamic properties of the moving air, those of the wetted
surface and the rate controlling conditions at the air-water
interface. It is an expression of Dalton's Law in relating
evaporation rate, Evap, to a transfer coefficient, k, and the
vapor pressure deficit, e -e , is in the form,
o a
E = k(e -e )
vap o a
The driving force for evaporation is the vapor pressure
deficit of the moving air, which is the difference between the
vapor pressure (ea) of the ambient air and the vapor pressure
(e ) of the air at the saturation temperature. Humidity deficit,
28
-------
AH, may be used in place of the vapor pressure deficit since they
are related through the molecular weights of the air (Ma) and
water (Mv) and the total pressure, P, of the air water vapor
mixture. The relationship between humidity and vapor pressure
is given by,
H Mv f e ) 18
" — n—
Ma lP~eJ 29
For ambient air temperatures and pressures, e is very small with
respect to P, and H = e(0.622/P).
The mass transfer coefficient, k, is dependent upon wind
speed, air density and pressure and on the roughness of the air-
water interfacial surface. Evaporation involves the transport
of vapor across a boundary layer. Increase in wind speed de-
creases the boundary layer thickness and increases the transport
rate.
The height used in the measurement of wind speed must be
defined. Because of the drag forces produced at the ground
surface, wind speed varies in a generally parabolic fashion with
height of measurement. Wind speeds at different elevations can
be estimated using power law equations based on height.
The mass transfer equation is well suited for formulation
of short term measurements of hourly or daily evaporation where
humidity deficit and wind speed can be considered as constants
throughout the test period. This approach has been used in
describing seasonal evaporation but the precision of the cal-
culated evaporation rate is affected by the long term averaging
of the highly variable quantities of wind speed and humidity
deficit. An early study by Fitzgerald (1886) related field
evaporation rate, Evap, to wind speed, W, and vapor pressure
deficit, eo-ea, in deriving the equation,
Evap(in/d) - 0.0166(1 + |
Many refinements to this basic form of the equation have been
developed since that time.
COMBINATION EQUATION METHOD
The combination equation method is a variation of the
aerodynamic theory where direct sunlight effects are included.
It is an approximate approach in that it includes an empirical
term for the direct solar heating of the water as well as the
evaporation from the action of the advected ambient air on the
water surface. The method allows for short term evaporation
estimates under ambient conditions since it is based on air
measurements at any time interval. The equation can be
29
-------
expressed as,
Evap = klAH + k2Rs
where k, and k~ = constants
R = sunlight radiation
AH = humidity deficit of the ambient air (Ae and
AH are interchangeable if the constants are
adjusted)
Some of the most frequently used field and lake evaporation
formulas are of the combination type.
Penman (1963) - Field
Evap =
-------
p = air density (gin/cm )
P = atm pressure (mb)
K = von Karman constant
CORRELATION METHOD
An approach that is useful for studying the effect of
variation in design parameters under controlled conditions is
the comparison of empirical evaporation rate with that of the
standard evaporation pan. In this way, the effect of each para-
meter can be judged against a baseline evaporation resulting at
the same site and under the same meteorological conditions. The
technique is useful for long term studies where measurement of
actual pan evaporation is more representative than values
calculated from equations using long term averages of weather
parameters.
Pan evaporation data are also useful for determining poten-
tial application of evaporative disposal systems for different
locations in the country. Standard pan evaporation data are
available throughout the nation and provide an excellent means
of comparison, based on actual evaporation measurements. The
National Oceanic and Atmospheric Administration (NOAA) publishes
pan evaporation data for more than 100 stations throughout the
United States. The validity of the formulations derived from
the results of other research techniques is often based on com-
parison to recorded pan data. Where pan data are not available,
they can be estimated from the formulations resulting from
research studies. These types of equations are summarized in
technical publications such as ASCE Technical Committee (1974)
and Gray (1970). Evaporation from lakes and saturated- soils has
been found to be in the range of seventy to eighty percent of
the pan evaporation measurement.
ENERGY BUDGET THEORY
The energy budget method defines evaporation rate as a
function of the energy available from sunlight and may be written
to include energy advected to the system with the water. The
total energy that the sunlight imparts can be written as,
ST ~ £evap reflect longwave ex boundary
einternal temp loss advected H^O
where e = total solar radiation reaching the evaporation
surface
31
-------
evap
reflect
longwave ex
boundary
internal
'temperature loss
eadvected
= energy utilized in water evaporation
= reflected solar radiation
= net longwave radiation exchange
between atmosphere and wetted surface
= energy lost or gained from the earth
= energy stored or lost by the materials
of the evaporation system
= sensible heat loss to the atmosphere
= energy brought in by water entering
the system
'total solar
'reflect
'longwave ex
'temperature loss
evap
Ae
advected
boundary
For most analyses, Ae±n ftl and ebound are very small
and difficult to estimate and are therefore not included in the
balance. The advected energy, Aeadv' involves a liquid materials
balance and for most problems, it can be considered an insigni-
ficant value. Advected energy is important for large water
bodies, such as with lake evaporation, but is a very small value
for ET beds or mechanical evaporation units.
The total short wave solar radiation (eg-p) reaching an area
on the earth can be estimated by determining the total potential
radiation and applying an empirical cloudiness coefficient. The
total potential radiation (eR) reaching the earth is known and
32
-------
is tabulated in many sources as a function of latitude. The
relationship between eST and eR as a function of the cloudiness
factor has been estimated by Penman (1948) and others to be of
the form eST = e^Ca + bC). The empirical constants have been
found to be in the range of a = 0.2 and b = 0.55. The function,
C, is the ratio of cloudless hours to total sunshine hours
available.
A portion of the sunlight reaching the earth is reflected
instead of being absorbed. The fraction reflected, expressed as
a reflectance coefficient (r), is dependent on the surface cover.
The value of e f, . can be estimated from,
ereflect = £STr
Reflectance coefficients (albedo) are unique for each surface
but are in the range of 0.15 for soils and planted fields, 0.1
for water surfaces, 0.5 for ice and 0.8 for snow cover.
The earth emits longwave radiation, and much of this inter-
acts with the atmosphere and returns to the earth. The longwave
or black body radiation escaping the atmosphere can be estimated
from the product of the Stefan-Boltzman coefficient and the
earth surface temperature raised to the fourth power. This must
be modified by the emissivity, m, which is the ratio of recap-
tured radiation to total longwave emission, a different cloudi-
ness function, f.
e, = a T4(m-l)f'
longwave exchange
where a = 1.17 x 10~7 cal/cm2/°K4/day
m = a + b/vapor pressure air(mb at 2 m)
a & b = empirical coefficients
f = 0.97 (for H20) to 1.0
The energy from water temperature loss to the atmosphere is
very difficult to measure. An approach has been presented by
Bowen (1926) with the following expression,
e. . = e (const = 0.61) x
temp to a tin evap
Temp H20 - Temp air ( C) UpreSsure (mb) ]
ksat vap press - vap press air(mb)J [ 1000 J
Once all of the values in the energy budget have been
estimated and a sum produced, the remaining energy can be equated
33
-------
to the potential evaporation for a given volume of water by
dividing by the heat of vaporization of water. The heat of
vaporization can be expressed as Hv = 596 - 0.52 (temp of
in °C) or approximately 585 to 590 cal/gm. Evaporation can occur
directly from ice but the rate is reduced and the heat of vapori-
zation is in the range of 680 cal/gm (Tanner and Bouma, 1975).
An example of the calculation of potential evaporation using the
energy budget approach has been presented in the article refer-
enced above.
The energy budget method is theoretically sound but the
application involves empirical coefficients and estimation of
many factors. For this reason and because of the difficulty of
establishing the small variations in parameter values with time
that are needed for correlation with research measurements, the
method is applicable to general types of problems but difficult
to use for the correlation of research measurements.
APPLICATION OF THEORY TO ET SYSTEMS
Research studies on the functioning of ET beds involve long
term evaluations. The changes in evaporation rate on a seasonal
basis and analysis of the performance over the complete annual
weather cycle are important. Detailed studies of design vari-
ables can be made for each month of the year. Measured pan
evaporation data are used for the research and design correlation
analyses because they provide the degree of sensitivity required
for long term studies at a specific location. The use of
measured pan evaporation data in research studies provides a
reference evaporation rate that is determined under identical
weather conditions and one that can be used to assess the effect
of each variable being studied. In design, the use of pan
evaporation data provides a means of comparison between different
locations throughout the country and also allows for an assess-
ment of the ranges of evaporation rates that have occurred at a
location over a period of many years. It would be very difficult,
using the empirical equations or the energy balance method, to
obtain similar reliability for the reference data because of the
long term averaging of meteorological data and the assumptions
that must be used with those techniques.
All of the weather stations that provide pan evaporation
measurements reported in the NOAA summary also record rainfall
data. The weekly or monthly rainfall amounts can be used in the
design analysis of an ET system.
In this report, correlations of research data were made with
pan evaporation and rainfall measurements obtained at the research
site. The design analyses for different locations in the country
are based on measured values from weather stations reported in
the NOAA monthly summaries.
34
-------
APPLICATION OF THEORY TO MECHANICAL SYSTEMS
The research studies with the mechanical evaporation units
involved detailed analyses of each of the design parameters.
For each design variable, a series of tests was run under con-
trolled conditions to establish an equation or functional
relationship for different values of that parameter. Each of
the test periods was of relatively short duration, two to
twenty- four hours. This time period was used so that essentially
steady state conditions, with respect to the weather variables,
could be maintained during each test.
The functioning of the mechanical units is basically the
same in theoretical concepts as that of the aerodynamic and
combination equation evaporation theories. Evaporation results
from the movement of unsaturated air over a wetted surface. The
driving force for the vapor transport is the adiabatic humidity
deficit of the air and the rate of evaporation can be defined
with a film transfer coefficient and the principles of psychro-
metry. A psychrometric chart is used to establish the humidity
deficit based on the temperature and relative humidity of the
air moving through the evaporator.
The aerodynamic theory was used in the analysis of the data
for the rotating disk evaporator. Some of the studies were made
in the outdoors where direct sunlight effects were present and
therefore the results were analyzed in ,a combination form equa-
tion of the following type,
Evap = klAH + k2Rs
The transport coefficient, k, , includes the effects of wind
speed and includes the effects of geometric variables of the
unit design. The coefficient, k~, relates the effectiveness of
capture of the direct sunlight energy.
The operation of the concentric cylinder evaporator can be
defined with a similar basic theory except that the effects of
the compressed air introduced to the unit are considered in
addition to the ambient air and sunlight parameters. A com-
bination equation of the following form is used,
Evap = klAHl + klAH2 + k2Rs
The term AH, represents the humidity deficit of the ambient
air, and AH2 for the compressed air. The transport coefficient,
k, was evaluated for the effects of wind speed and machine design
variables, kJ , includes a compressed air mass flow rate term and
k~r is the direct sunlight proportionality term.
35
-------
The design concepts for both types of mechanical evaporators
involve the application of the combination equations and pan
evaporation correlations.
PSYCHROMETRIC CHARTS AND WET BULB THEORY
Evaporation is a process of simultaneous heat and mass
transfer. At the air-water interface the temperature adjusts
such that, at steady state/ the rate of heat transfer required
for the phase change of the water is balanced by the rate of
moisture vapor mass transfer. Evaporation produces a lowering
of the sensible heat of the water causing heat flow from the air
to the water. When equilibrium is attained/ the temperature of
the water is the wet bulb temperature. The transfer of heat as
water vapor from the water to the air can be expressed as/
= XTWWw
where q = rate of heat flow
W = mass of water vaporization with time
X = latent heat of vaporization at wet-bulb temperature
The rate of mass transfer of water vapor across the interfacial
boundary is,
where A = interfacial area
K = mass transfer coefficient
g
M = molecular weight of water vapor
e = vapor pressure at wet bulb temperature
e = vapor pressure of air
a
At ambient temperatures where vapor pressure is small compared
to total pressure of the gas phase,
where M = molecular weight of air
cl
P = total pressure of air
Ha = humidity of the air
36
-------
H = humidity at wet bulb temperature
Vv
and the equations combine to,
q = KgAMvATW(ew-ea) =
The rate at which heat flows to the water is,
* = v(vv
where h = heat transfer film coefficient
T = temperature of air
a
T = temperature of wet bulb
At equilibrium the two heat transfers are equal and
W A " MaPKg 1 ATW
In the adiabatic process the temperature of the water in
contact with ambient unsaturated air will drop causing heat flow
from the air to the water and vapor transport to the air. If
the contact time is relatively long and equilibrium exists, the
temperature of the water surface will become equal to the tem-
perature of the saturated air. Unsaturated air moving across
the wet surface will drop in temperature and gain humidity as
the evaporation process takes place. The driving force for the
transfer will be the humidity deficit of the entering air. A
heat balance for the adiabatic cooling of the air is
WW + GsVs = GGHsAs
where G_ = mass velocity of the unsaturated air per unit area
of wetted surface
C = heat of the entering air
s
X = latent heat of vaporization at saturation humidity
5
T = air temperature at saturation
o
Rearranging the above equation gives
C T -T
H - H = S * S
s a X
37
-------
By combining the two expressions for AH,
VTs
The expression on the right side is the psychrometric
constant and has been found to have a value near unity (Rich,
1961) . This allows the use of adiabatic saturation temperature
to be determined directly from the wet bulb reading. The
psychrometric chart is a plot of saturation humidity as a
function of temperature. A family of lines is drawn at per-
centages of saturation. Lines indicating temperature and
humidity change for an adiabatic process can be constructed as
straight lines with the slope of C /A . Charts are slightly
different for high altitude areas Because pressure is a function
in the psychrometric constant. A chart for the elevation of
Boulder, Colorado (1700 meters) is shown in Figure 7.
Entering the chart with ambient temperature and relative
humidity (T = 25°C (77.5°F), RH = 10% in the example), the
humidity of the air can be determined. The adiabatic saturation
humidity (2.2 gm/kg) is found by following the adiabatic line to
the saturation line and reading the humidity (8.7 gm/kg). The
humidity deficit is the difference of the two readings.
38
-------
40 50 60 70 80 90 100 110 120 130
Air temperature (°F)
o
o
o
§
8
Figure 7. Psvchranetric chart for Boulder, Colorado elevation
39
-------
SECTION 6
EXPERIMENTAL PROCEDURES
The purpose of the experimental studies was to define and
evaluate the variables affecting the design of evaporative
wastewater disposal systems of the two types described.
The studies relating to ET beds were made primarily with
lysimeters of the type shown in Figure 2 and these results were
correlated with five full scale units in use at homes in the
Boulder, Colorado area.
Twenty lysimeters were constructed originally and nine were
added during the study. The main containers were water tight
55 gallon (208 liter) drums buried in the ground to within 7.5 cm
(3 in) of their top lip. A thin layer of sand was placed in the
bottom to prevent puncturing of the liner. A double layer of
0.15 mm (6 mil) polyvinyl liner was placed in such a way that no
opening or seams existed within the unit. A small amount of
gravel, 6 to 12 mm (1/4 to 1/2 inch) size was placed in the
bottom of each unit and the ell shaped, 18 mm (3/4 inch) diameter
filling tube was put in place. The gravel was mounded over the
discharge end of the tube. All units were carefully filled with
selected ET sand. Drain troughs were constructed on the outer
perimeter of some of the units for the collection and measurement
of runoff water during rain and snow events. Surface slopes of
the ET sand were shaped for each unit and vegetation cover was
planted on some of the units.
Before construction of the lysimeters, several locally
available ET sands were located and analyzed. The physical and
capillary rise properties were determined in order to select the
best sand for use in the lysimeter studies. Sieve analyses,
capillary rise tests, and specific gravity analyses were made
for each of three different sands available from local suppliers.
Two of the sands were produced from the settlement of wash-
water used to clean mortar sand at local sand and gravel sup-
pliers. The third was from a natural deposit of wind eroded
materials mined in the local area. It is referred to as "wind-
bio" .
40
-------
Sieve analyses, consisting of mechanical shaking of a 500
gm sample of sand that had been dried and separated into indi-
vidual particles was accomplished with ten minute shaking
through a stack of U.S. standard sieves of number 40, 60, 80,
100, 140 and 200 mesh. The particle size distributions deter-
mined are shown in Figure 8 for the sands from the Golden Sand
and Gravel Company, C-M Sand and Gravel Company and the wind-bio
deposit. Two of the sands had a 050 (mean particle diameter) in
the desired range of 0.1 to 0.15 mm. The C-M was a slightly
finer, more poorly graded sand. The wind-bio was a more coarse
and more uniform sand.
Samples of each sand that had been dried and separated into
individual particles with a mortar and pestle were used in a
capillary rise test. Clear plastic vertical tubes of 2.54 cm
(1 inch) inside diameter were filled with each of the sands.
The lower ends of the columns were in a pan that was filled with
water during testing. Six different samples were tested with
tap water and with primary effluent from a local wastewater
treatment plant. The six samples consisted of each of the three
sands and fractions of the Golden and C-M sands. Two of the
fractions consisted of the C-M and Golden sands with all par-
ticles of less than 75 u (200 mesh) removed. The sixth sample
was the combined materials of less than 200 mesh for the C-M and
Golden sands.
The water was added to the pan at the base of the columns
and the height of the wetted water level was recorded with time
for a period of ten days. The results of the capillary testing
are shown in Figures 9 and 10. It was found that the capillary
rise rate for tapwater and primary effluent were identical.
After the water movement had reached equilibrium at ten
days, the moisture content as a function of height of rise was
determined for the C-M and Golden sands by carefully removing a
series of five centimeter portions from the top of the columns
and measuring the weight of water and weight of dry sand. A
drying and weighing procedure was used. The results of this
analysis are shown in Figure 11.
In order to determine the void ratio and percent saturation
of the water in the lysimeters, the specific gravity of the sand
particles was measured by using the procedure described by Lambe
(1951). The specific gravities of the sand particle solids were
found to be nearly identical at a value of 2.69.
The sand from the Golden Sand and Gravel Company was
selected for the lysimeter studies. The capillary rise tests
resulted_in a height of rise slightly greater than the depth of
the bed used in the lysimeters and in field units. The beds
were 0.7 meter (28 in.) deep. Golden sand was used in most of
the in-place beds selected for the field study correlation.
41
-------
0.5
0.4 _
0.3
0.2
Grain
size
(mm)
0.1
0.09
0.08.
0.07
0.06
0.05
0.04
Windblo -
\ I I I I I I I I
10 20 30 40 50 60 70 80 90
Percent Finer by Weight
Figure 8. Gradation analysis for three samples of ET sand.
42
-------
01 23 45 6789 10
all finer than
200 mesh, CM
or Golden.
greater than
200 mesh, CM
or Golden.
Figure 9. Capillary rise test results (10 hour)
43
-------
1.0 ,
40
all finer than 200 mesh,
/ CM and Golden
0.75_--
CO
M
0)
4J
0
S
4J
Cr>
Q)
CO
all coarser than 200 mesh,
CM and Golden
120
150
180
210
240
Time Hours
Figure 10. Capillary rise test results (long term)
-------
l.OJ
0)
TO
-H
PS
0.5.
3 Cm)
0)
re
40
30
20
(in)
10
C-M
Golden o
\
l
20
0 10 20 30 40
Moisture Content % (wt H20/wt dry soil)
Figure 11. Moisture content of sand column as a function of
height above free water surface.
45
-------
One further test of the sand was made prior to filling the
lysimeters. This was done to establish the relationship between
the water table level, as measured by the height of standing
water in the lysimeter filling tube, and the degree of saturation
of water in the sand. The apparatus used is detailed in Figure
12. The sand was tapped into place and the water level in the
opposite column was measured after the water movement had come
to equilibrium. Two water levels were used and the moisture
content was measured at 5-cm levels in a similar manner to
previous tests. A measured volume of water was added to the
left column of the apparatus. The volume of water to fill the
tubing and the left column to the measured height was calculated
and subtracted from the total water added. The remaining water
added occupied the sand pores. The percent saturation was cal-
culated based on the pore water volume and the volume of void,
as shown. The results are shown in Figure 13. It is apparent
that when any water was standing in the filler column, the
lysimeter sand would be more than 80 percent saturated. The
depth of water in the filler column can be related to the avail-
able remaining storage capacity in the unit. The results pre-
sented in the figure also show that the surface moisture content
is reduced when the free water surface within the unit is lowered.
The moisture content at the surface affects the rate of evapora-
tion.
LYSIMETER STUDIES
Twenty-nine lysimeters were used to study the design and
operational parameters. This allowed for testing of several
values for each variable simultaneously under identical weather
conditions. The lysimeters were loaded in one of two basic ways.
1. Constant loading rate - In this mode, a predetermined
amount of feed water was added to a unit each day. The
free water surface in the filler tube was monitored
with a graduated dip stick to establish the depth of
the water table and to note the times when the unit was
full. For a totally non-discharging system, the full
condition represented overloading and failure of the
unit for that design parameter.
2. Constant water level - In order to compare relative
evaporation rates of different types of surface cover,
water table depth, etc., a variable amount of feed
water was added to a unit each day in such a way as to
maintain a nearly constant standing water level in the
filler tube. The amount of water added was a direct
measure of the amount of water evaporated over the
previous period.
The lysimeter units were operated for twenty months. During
that period, parametric studies of several variables were
46
-------
1 in. dia.
feed column
standing
datum
=O<[=
gate /
valve
V
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•.'»••
'*V
••"• '.
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fa'
'. •
.//
"• '
-J
«* J *
•*..-.'
••••>»
c»,»vH
>
^- ET sand
,^- 3 in. dia.
^""^ plastic
ti±>e
X" pea gravel
"^ /^ plastic hose
X 1/2" dia.
Figure 12. Apparatus used to relate water table and saturation,
47
-------
70
60
50 .
1 40
-P
.c
•H
w 30
CO
c
H
O
U 20 .
10
-28 \g <
\
26 \ ,
n \ '
. 24 - \
M \
0) \
-221 \ '
i Run 1 \
- 20 V
\
. 18 li
•^ \
\
-12 A
Vol. \
8 added Stand. \
Run (ml) % Sat Ht.
0
.
o
>
1
6 1
1 572 81.9 0" ol
4 2 622 89.1 18" '
Run 2
Col Vol.= 1442 ml
Dry wt.sand=2000 g
S.G.= 2.69
V = 744 ml
Time of equilibrium
= 24 hrs. Q
Temperature= 20 C
10
20
30
40
(M.C. =
Moisture Content, %.
wt. of H00
wt. of dry soil
x 100%)
Figure 13. Moisture content of sand as a function of the height
of free standing water.
48
-------
completed. Some of the studies were made for the full time
period while others were shorter term. For all studies (except
for the one noted), the wastewater utilized was primary effluent
from the Boulder sewage treatment plant. The wastewater was
stored for four days prior to use. The wastewater was a readily
available source that simulated the quality of septic tank
effluent.
The major considerations affecting the design of ET beds
are the type and size distribution of the ET sand and the
loading rate. The other parameters evaluated provided insight
to the functioning of the units and were used to explore the
effects of construction features. The variables in the studies
were as follows:
1. Wastewater loading rate - The wastewater feed rate was
varied from 0.00 mm/day to 8.15 mm/day (0.20 gal/day/ft2
or 8.15 liter/day/m2) for different units. Each unit
had a constant loading rate. The loading rates used
were 0.00, 0.41, 0.82, 1.23, 1.63, 2.05, 2.45, 3.26,
4.07, 6.52, 8.15 mm/day which are equivalent to 0.00,
0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.08, 0.10, 0.16
and 0.20 gal/day/ft . Golden ET sand was used in all
units. Each had a bare, unplanted, sand surface with
a slope of 1:6 (vertical:horizontal). Wastewater
temperature was 30°-37°C, the temperature of combined
wastewater generated in the individual home.
2. Depth of free water surface - The moisture content at
the surface of a unit has been shown to be greater
when the standing water level is high and less when the
water level is reduced. Six static water level lysi-
meters were used to evaluate the effect on evaporation
rate as a function of standing water depth in the unit.
The levels used, as measured from the ground surface
downward were 0, 12.7, 22.9, 33.0, 45.7, and 58.4 cm
(0, 5, 9, 13, 18, and 23 inches). The total unit depth
was 68.6 cm (27 inches). The lysimeters were identical
to those described in #1 except that the water level
was determined each day and a sufficient volume of water
was added to maintain a nearly constant level on a day
to day basis. The standing water level and the volume
added were measured and recorded daily.
3. Rainfall and sunshine effects - Special covers were
constructed over four of the units. They are shown in
Figure 14. The top surfaces on two of the covers were
made of clear lucite, to allow sunlight to reach the
bed surface but to preclude rainfall. The tops of the
other two covers were made of plywood to eliminate sun-
light and rainfall. The sides of the covered structures
were open and wind movement over the lysimeter surface
49
-------
Figure 14. Test lysiraeters with clear and opaque covers.
50
-------
was not inhibited. The unit construction was identical
to that described in #1.
One of the lucite covered and one of the plywood covered
units were loaded at 1.63 mm/day (0.04 gal/day/ft2) so
that the results could be compared with the uncovered
units described in #1 in order to establish the effect
of sunlight and rainfall. The second lucite and plywood
units were kept at a constant free water level of 22.9
cm (9 in.) below the bed surface so that comparative
evaporation rates could be established with respect to
the uncovered units described in #2.
4. Surface slope runoff effects - A portion of the rainfall
landing on an ET bed will run off and not become a part
of the liquid loading. The amount of runoff from the
lysimeters was evaluated by using three different
surface slopes: flat, 1:6 and 1:2. The units were
identical to those in #1 except for surface slope.
Each unit was provided with a small, aluminum gutter
along the periphery that discharged into a collection
bottle. After each rain, the amount of runoff collected
was measured volumetrically.
5. Soil grain size - Two lysimeter units were filled with
ET sands that were sieved to produce different capillary
characteristics. Golden ET sand was dry sieved through
a 100 mesh (0.15 mm) screen. One unit was filled with
the material whose grain sizes were all greater than
100 mesh and the other unit was filled with all material
less than 100 mesh. Evaporation rates were measured
under the conditions of a constant static water level.
Two different levels were used at different times in the
study: 22.9 cm (9 inches) and 58.4 cm (23 inches) below
the bed level.
6. Wastewater temperature - One lysimeter constructed to
be identical to those in #1 was fed wastewater at an
elevated temperature of 50°C. and a constant rate of
1.63 mm/day (0.04 gal/day/ft2). This unit was compared
with a similarly loaded lysimeter with wastewater tem-
perature of 30°-37°C.
7. Type of wastewater - Near the end of the study, six
lysimeters were used to establish if different per-
formance could be expected from influent wastewaters
which had received different degrees of treatment. All
units were identical to those of #1 except that two
units were loaded with primary effluent, two with well
treated secondary effluent and two with tap water. With
each pair, one was held at a static water level of 22.9
mm (9 inches) and one at 58.4 mm (23 inches) below the
51
-------
sand surface. The free water surfaces were maintained
at a relatively constant level, and therefore the volume
of water added each day was equal to the evaporation
rate. The evaporation from all units was compared and
related to the measured pan evaporation rate. The
average characteristics of the waters used based on the
records of the Boulder water and wastewater utility for
the study period were as follows:
Ave. Suspended Ave. Total
Ave. BOD Solids Solids
Primary effluent 117 mg/£ 80 mg/ 310
Secondary effluent 27 mg/fc 22 mg/ 250
Tap water - - 50
8. Surface vegetation - An initial assessment of the effect
of vegetative transpiration on the operation of ET units
was made. It was not intended to be a thorough study
of this complex subject. Eleven units were evaluated
over different time spans, several as constant water
level lysimeters for the measurement of evaporation rate
and others at constant loading rate to assess the effect
of summer drought, winter freezing and year-round per-
formance. Two units each were planted with Kentucky
bluegrass, pfitzer (Juniper) bush, cottonwood (Populus
candicans) sapling and Lombardy poplar (Populus nigra
italica). In each case one unit was studied at a con-
stant level of 22.9 cm (9 inches) below surface and one
unit with constant loading, 16.3 mm/day (0.04 gal/day/
ft^). The other three units were planted with alfalfa,
pussy willow (Salix discolor) and salt cedar (Tamarak).
These units were observed under both loading conditions
at different time intervals.
MECHANICAL EVAPORATION UNITS
The mechanical evaporation units were a development of the,
concept of designing a device that would provide a maximum of
evaporative surface in a small vertically-projected area, utilize
the energy of ambient air, be relatively maintenance free and
have a low commercial energy requirement. Two types of units
were tested: the Rotating Disk Evaporator (RDE) and Concentric
Cylinder Evaporator (CCE) units. Schematic drawings of the
units are shown in Figures 15 and 16. Photographs of the units
were presented in Section 1.
The RDE and CCE units were fabricated to be identical except
for the evaporation surfaces. The materials of construction were
selected for durability and ease of fabrication. The wastewater
reservoir was made from a 208 liter (55 gallon) drum cut
52
-------
belts and pulley;
2.5 cm (1 in)
rotating ,§haf tn
bearing
air supply-r*-"^r
swivel coupling'
k HP variable
speed motor
0.3 cm (1/8 in)
aluminum disks
26 cm
(10 h in)
waste water reservoir
one half of 55 gal drum
bearing
Figure 15. Drawing of rotating disk evaporator,
53
-------
71 cm
belts and pulleys
2.5 cm (1 in)
rotating shaft
air supply"" \
swivel coupling
k HP variable
speed motor —
waste water reservoir
one half of 55 gal drum
1
1
wood frame
burlap
wrap's
; »•: v •«. -~ ,~:
bearing
Figure 16. Drawing of concentric cylinder evaporator.
54
-------
lengthwise. An adjustable hook gage was bolted to the inside of
the reservoir so that evaporated water volumes could be easily
accounted. The support structure was fabricated from aluminum
angle and channel sections. The center shaft of the rotating
section was one inch (2.5 cm) diameter hollow T6064 aluminum
pipe. The disks for the main test unit were cut from 1/8 inch
(0.3 cm) T6064 aluminum sheet. The shaft collars, sheaves and
sealed ball bearing pillow blocks were made of iron. The motor
was an AC-DC universal type rated at 0.5 amps and supplied with
rheostat control and reduction gearing. The unit shaft rota-
tional speed was further reduced by use of a V-belt drive
mechanism. Shaft rotational speeds could be set between 0.35
and 5 rpm. The fractional depth of wetting of the disks or
cylinder could be set between 0.0 and 0.9. The CCE unit was
identical except that the cylinder was constructed with aluminum
disk end pieces to which three wraps of burlap cloth were
attached. The diameter of the three cylinders were 15, 30, and
46 cm (6, 12, and 18 inches). Compressed air entered through
drilled holes in the hollow center shaft and moved outward
through the wetted cloth wraps. The compressed air was unsat-
urated upon entering and nearly saturated when leaving the outer
cylinder wrap. Temperatures of the entering air and the water
bath were recorded as well as compressed air pressure at the
unit and air flow rate.
The method of operation of the units was to set a value for
each of the machine variables and to fill the reservoir with
wastewater to the level of the point of the hook gage. The unit
was operated for a recorded period of time, usually two to twenty-
four hours. The time period of a run was selected to be long
enough so that several liters of wastewater was evaporated. In
this way the inaccuracies of reading a hook gage were held within
one percent. At the end of each run the unit was stopped and the
amount of water evaporated was determined from the amount of
wastewater required to bring the level back up to the point on
the hook gage.
Two types of studies were made with each unit. Ambient air
data was determined while the units were operated on the roof of
the Engineering Center. This provided data under conditions
that would be similar to those of a prototype unit. A small
weather station was set up near the unit where continuously
recorded data for ambient air temperature, pressure, relative
humidity, wind speed, and solar radiation were taken. Standard
class A pan evaporation and rainfall amounts were also measured.
For the study of some of the machine variables, it was
found that more precision could be accomplished by moving the
unit into the hydraulics laboratory where constant air tempera-
tures and relative humidity existed. A plywood wind tunnel
utilizing a fan was constructed for the laboratory studies so
55
-------
that wind speed could be accurately controlled, measured and
reproduced.
For some of the studies of machine variables for the RDE
unit, such as disk spacing or disk type, a reservoir was fabri-
cated with water tight dividers and multiple hook gages so that
comparative evaporation data for several values of the design
parameters could be taken simultaneously with identical condi-
tions for temperature, wind speed and relative humidity.
Series and Purpose of Tests
Rotating Disk Evaporator—
Three major parameters are involved in the equation
describing the operation of the RDE unit: wind speed, humidity
deficit and solar radiation. These parameters were evaluated
separately and in combination. Since freezing weather requires
special considerations for the sizing of the unit and storage
vault, this parameter was also studied. Optimization of the
design of a unit was approached with a series of tests involving
each of the machine variables. The experimental methods and
conditions were as follows:
1. Wind speed - Two types of testing were used to establish
the wind speed function. The unit was placed in the
plywood wind tunnel in the laboratory. Under conditions
of constant temperature and relative humidity, measure-
ments of evaporation rate were made for different wind
speeds by adjusting the speed of the fan in the tunnel
with a voltage regulator. The equation developed under
the controlled laboratory conditions was compared to the
results obtained with ambient air flow when the unit
was operated outdoors. Ambient measurements were made
on the roof of the University of Colorado Engineering
Center, with and without the use of the wind tunnel.
Wind speed measurements were made with a three-cup
anemometer.
2. Wind direction - Using the laboratory wind tunnel con-
ditions the RDE unit was tested at four different wind
angles ranging from perpendicular to parallel to the
rotating shaft. Evaporation measurements were made to
establish the effect of wind direction.
3. Solar radiation - The contribution of solar radiation
was established under ambient conditions by using a
reservoir divided into two sections. One-half of the
disks were shaded, using a wooden cover well above the
disks so as not to affect wind movement. The evapora-
tion rate of each group of disks was measured and the
56
-------
difference was correlated with solar radiation measure-
ments of the on-site pyranograph.
4. Humidity deficit - The degree of undersaturation of the
air in contact with the evaporation surface is a major
factor in the evaporation process. The function of
this parameter was established from analysis of opera-
tional data recorded under different weather conditions
during the winter and summer periods of outdoor opera-
tion. The average humidity deficit for each operational
period was established from recorded temperature and
relative humidity charts from the meteorograph located
near the unit. Using a psychrometric chart, the
average adiabatic humidity deficiency was calculated
for each test condition.
5. Freezing conditions - An important consideration in the
planning of a total evaporation system for year-round
application is the period of the year when freezing
conditions will prevent the use of an RDE unit or will
require modification and commercial energy to prevent
freezing. A brief study was made to determine the
minimum temperature that would allow the unit to func-
tion. This was done under ambient winter conditions.
When freezing occurred, a 3000 watt electric immersion
heater was used in the reservoir of the unit. Elec-
trical power requirements were measured with a meter
and correlated with reservoir temperature, air tempera-
ture and evaporation rate.
6. Machine variables - The design of an RDE unit requires
the optimization of mechanical components. The RDE
unit was constructed in such a way that several machine
adjustments could be made so that the relationship
between evaporation rate and machine variables could be
studied. This was done under both ambient and labora-
tory wind tunnel conditions. One variable was studied
at a time, while keeping other parameters fixed.
Evaporation rate was measured for different settings
for each component variable. Those evaluated were:
a. Rotational speed. A variable speed drive on the
shaft motor allowed for rotational speeds of 0.3 to
5 rpm. The study was made in the laboratory to
provide constant conditions for each setting.
b. Submergence. By adjusting the hook gage in the
reservoir, the depth of wetting of the rotating
disks could be varied from zero to 0.9 of the full
radius, measured from the outside edge. A reservoir
with dividers was used so that five different sub-
mergence values could be studied simultaneously.
57
-------
c. Disk spacing. The disks were adjustable on the
shaft so that the spacing between disks could be
varied from 1.27 to 7.62 cm (h to 3 inches). The
divided reservoir was used and several disk spacings
were evaluated at the same time to determine evapor-
ation rate per wetted disk area.
d. Disk color and material. Aluminum, plain plywood,
plywood painted white, black plywood and white
styrofoam disks all of 51 cm (20.5 inches) diameter
were tested at the same time with the five section
reservoir to study the effect of color and type of
material used in the disks on the evaporation rate.
The study was made under ambient conditions in
bright sunlight.
e. Disk diameter. In order to utilize the results of
the test unit for prediction of full scale prototype
units, a larger three section reservoir was con-
structed and disks of three diameters were evaluated
simultaneously. The disks were fabricated from
plywood and had diameters of 0.51, 0.92 and 1.21
m (20.5 in., 3 ft and 4 ft).
7. Potential health hazard - The possibility of wind drift
of water droplets from the units was evaluated in the
laboratory by placing culture dishes directly upwind
and downwind from the units at distances of 0.15, 1.5
and 3 m (0.5, 5 and 10 ft). The dishes contained
coliform bacteria test media and milipore filters. The
unit was operated for two hours and then the dishes
were removed, incubated and tested for coliform bacteria
in accordance with Standard Methods, 13th Ed. (1975).
Concentric Cylinder Evaporator—
The operational parameters of the CCE unit were quite
similar to those of the RDE except that the mass flow and
humidity deficit of the compressed air were the major factors
controlling evaporation rate. The experimental methods and
conditions were as follows:
1. Mass flow rate and AH - The compressed air entering the
unit was controlled with a valve and had pressure gages
at each end of the inlet line. The pressure drop
through the inlet line was used to determine the flow
rate. The relationship between flow rate and pressure
drop was calibrated with a precalibrated rotometer and
checked with a volume displacement technique. Evapora-
tion rate was measured by use of a hook gage in the
liquid reservoir in a manner similar to that with the
RDE unit. The humidity deficit was determined from a
58
-------
psychrometric chart. A thermometer was fixed into the
air inflow line and a tee was placed in the line so
that wet and dry bulb temperatures of the compressed
air could be measured for each experimental test using
an electronically aspirated Bendix psychron instrument.
2. Wind speed - The CCE unit was tested in the laboratory
wind tunnel and under ambient conditions on the roof of
the engineering building. The velocity of the air
passing over the outside of the drum was measured with
a three-cup anemometer in a manner similar to that with
the RDE unit.
3. Solar radiation — The effect of solar radiation was
measured from tests in which the unit was operated in
the sunlight for a period during the middle of the day
and then under shaded conditions for another period of
nearly identical ambient parameters. Sunlight intensity
was measured with a recording pyranograph.
4. Machine variables - Only three machine variables were
evaluated: compressed air mass flow rate, rotational
speed of the drum and submergence of the drum. The
compressed air mass flow rate measurements have been
discussed. The methods of making variations in rota-
tional speed and cylinder submergence were identical
to those of the RDE unit for rotational speed and disk
submergence.
WEATHER MEASUREMENTS
For all outside field investigations, the temperature,
relative humidity and barometric pressure were measured using a
Weather Measure Corp. Model M701 meteorograph. The temperature,
relative humidity, and barometric pressure were simultaneously
recorded on a seven-day chart mounted on a clock driven drum.
The meteorograph utilized a bimetallic strip for temperature
measurement, a human hair bundle for relative humidity measure-
ments, and an aneroid for pressure measurements.
Solar and diffuse sky short-wave radiation were measured
and recorded using a Weather Measure Corp. Model R401 mechanical
pyranograph. The instrument utilized bimetallic strips to
determine solar and diffuse sky short-wave radiation on a
horizontal plane. Radiation was recorded on a seven-day chart
mounted on a clock driven drum.
The rainfall was recorded using a Weather Measure Corp.
Model P501-I remote recording rain gage which was connected to
a Model P521 event recorder. The rain gage was of the tipping
bucket type. Each bucket tip sent an electric impulse to the
59
-------
event recorder which recorded the impulse on a seven-day chart
mounted on a clock driven drum.
Wind was measured using a Weather Measure Corp. Model
W164-B/M contact anemometer. The three-cup anemometer measured
kilometers of wind at a height of 40 cm (15 in.). The instru-
ment was calibrated in the low velocity wind tunnel at the NOAA
Boulder test facility.
Pan evaporation was measured using a standard U.S. Weather
Bureau class A pan. The pan was of stainless steel construction,
122 cm (4 ft) in diameter and 25.4 cm (10 in.) deep. The pan
was placed on a wood frame to allow air to circulate beneath it.
A hook gage placed in a stilling well was used to measure the
drop in the water surface over a definite time period. The
screw-type hook gage had a precision of 0.025 mm (0.001 in.).
Pan evaporation was computed as the difference between observed
levels minus the measured precipitation.
The meteorograph and event recorder were housed in a
screened instrument shelter at a height of 1.2 m (4 ft). The
rain gage was placed through the roof of the shelter. The
pyranograph was attached to the roof of the shelter. The
evaporation pan and anemometer were placed near the instrument
shelter at the ground level.
Mean values of temperature, relative humidity and solar
radiation were determined by integrating the area under the line
made by the trace on the recording chart. The mean wind speed
was determined by dividing the total kilometers of wind during
a data period by the time interval. Only the total depth of
rainfall over a data period was required and this could be read
directly from the event recorder's chart. The mean humidity
deficit over a data period was determined using the mean tempera-
ture of the air, the mean relative humidity, and a psychrometric
chart corrected for the altitude of Boulder, Colorado; barometric
pressure of 84 kPa (0.83 atm) Hg.
In the portion of the report dealing with design and cost
consideration, meteorologic data from NOAA was used for inter-
pretation of applicability to Boulder and other locations. The
individual readings taken at the research location correlated
closely with the NOAA readings for the same period.
60
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SECTION 7
RESULTS
The testing of the ET lysimeters and mechanical units was
conducted at the Engineering Center of the Boulder Campus of the
University of Colorado.
Twenty lysimeters were constructed in the fall of 1975.
After initial equilibration, testing was begun in January of
1976. Nine additional lysimeters were added in September of
1976. Testing was continued through September of 1977. The
lysimeters were buried in the earth in an area adjacent to the
southeast corner of the building. The units were in an area
protected by a concrete wall. The building and the wall were
such a distance as to provide no restrictions to sunshine or
rainfall and relatively little effect on air movement.
The two mechanical evaporation units were fabricated in
the laboratory shop in the winter of 1975 and spring of 1976.
Testing was initiated in June of 1976 and continued until August
of 1977. The ambient testing of the mechanical units was con-
ducted on the roof of the three story civil engineering wing of
the building. This provided an essentially unobstructed loca-
tion. The weather station used to gather ambient air readings
was at the same location. The testing of the units under con-
trolled windspeed and temperature conditions was done in the
fluid mechanics laboratory.
The test site is located in a semi-arid region of the U.S.
in the rain shield of the front range of the Rocky Mountains.
Boulder is located at an elevation of 1700 meters (5600 feet)
above sea level, immediately adjacent to the eastern edge of the
mountains. The monthly weather summary for the test period and
the normal values are shown in Table 6. Boulder has a normal
average precipitation rate of 412 mm (18.57 inches) per year,
snowfall of 2057 mm (81 inches) and total evaporation of approxi-
mately 1500 mm (60 inches). The study period was characterized
by drought conditions with moisture about twenty percent below
normal and snowfall at approximately one-half of normal.
ET LYSIMETER DATA ANALYSIS
The first series of tests with parametric variations was
initiated in January 1976. The loading patterns were changed at
61
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TABLE 6. WEATHER SUMMARY FOR THE TEST AREA
NJ
January
February
March
April
May
June
July
August
September
October
November
December
Mean
Total
Mean
1976
32.5
41.4
38.5
49.2
56.4
66.3
73.2
69.7
60.8
49.4
39.3
36.6
51.1
Temp °F
1977 Norm
28.2 32.7
39.0 34.7
39.5 39.4
49.5 48.7
56.9 57.4
71.2 67.5
73.6
72.1
64.4
53.8
41.4
35.8
51.7
Max.
of
1976
65
65
69
74
83
92
99
92
91
80
70
63
65
Temp
1977
56
68
69
78
86
95
96
87
92
Min.
Of
1976
-2
-1
0
26
31
43
52
48
32
18
— 7
i
37
Temp
1977
-10
12
14
8
33
47
50
48
31
Days with
Min. Temp
Below 32°F
1976 1977
25
11
24
2
1
0
0
0
0
15
20
23
121
31
22
25
7
0
0
0
0
1
Cloud
Clear
1976
13
7
10
5
8
12
7
8
11
14
8
16
119
Cover
Partly
1976
17
20
19
21
18
18
24
22
14
16
18
14
221
Days
Cloudy
1976
1
2
2
4
5
0
0
1
5
1
1
1
16
Moisture in
1976 1977 Norm
0.42
0.34
1.19
1.99
2.14
1.25
1.62
1.43
2.73
1.02
0.21
0.32
14.66
0.08 0.72
0.45 0.83
0.53 1.75
3.32 2.75
0.93 3.36
0.66 1.89
3.24 1.41
2.38 1.64
0.16 1.25
1.39
0.96
0.62
18.57
(Continued)
-------
TABLE 6 (Continued)
U)
January
February
March
April
Kay
June
July
August
September
October
November
December
Mean
Total
Snow
1976 1977
8.3
6.2
4.5
0
0
0
0
0
0
2.1
1.0
7.0
4.0
7.4
5.9
12.6
0
0
0
0
0
in
Norm
10.9
10.8
17.3
14.3
2.8
0.1
0
0
1.0
4.3
11.5
8.0
Pan Evap.
in
1976 1977
2.36* 2.36*
3,78 2.36*
4.84 3.77
5.24 6.29
5.65 9.76
10.65 11.56
10.32 10.23
8.38. 9.45
5.01
4.13
2.36*
2.48*
Relative
Humidity
51
56
55
50
52
58
53
53
55
54
54
54
Wind,
mph
9.6
9.7
10.3
10.6
9.8
9.3
8.7
8.5
8.4
8.3
8.9
9.3
AH
gm/kg
0.29
0.71
1.00
1.43
2.14
2.21
2.79
2.71
2.29
1.86
1.00
0.86
Sunlight
ly/hr
10
12
18
22
25
25
28
23
18
15
10
7
41.0
81.0
65.2
Estimated from lysimeter studies.
-------
specific intervals during the course of the study in order to
investigate a wide range of conditions. The loading conditions
are shown in Table 7. It can be noted that some of the units
were utilized for several different parametric tests. This is
indicated with the letter designation (a) through (f) with the
unit number. Tests on units with the letter (a) were initiated
at the beginning of the study. The loading rates of three of
the units were changed on July 14, 1976. These units were
designated with the letter (b) at that point. Eight additional
units were added on October 1, 1977. For the new lysimeter, and
for the existing units where the loading rate was changed,
designation with the letter (c) is used. Further variations
were made on January 1, 1977 and designated with (d). During
the last two months of the testing, short term tests were made
with transpiration rate measurements of cottonwood and poplar
trees using the unit designated with (e). These tests were
initiated on August 1, 1977. In order to assess evaporation
rate differences due to the type of feed water involved, primary
effluent, secondary effluent and tap water evaporation rates
were measured in the units labeled (f), commencing on September
1, 1977.
ET units respond rather slowly to changes in loading
variables. For some of the variables studied, it was necessary
to observe the units for more than the full year cycle. For a
few units, loadings were unchanged over the complete study
period. Other parameters could be evaluated in a shorter period
of time and loading parameters were changed on a more frequent
interval in order to study a wider variety of conditions. In
all, fifty-seven different test conditions were evaluated.
The two major types of loading methods are indicated in the
table. Under one type of loading condition, a constant amount
of wastewater was added each day, such as 1.6 mm/day (0.04
gallons per square foot of surface area) and the water level
within the unit was allowed to be variable, changing with the
evaporative conditions present. The other type of loading
involved feeding a variable amount of wastewater each day in
such a way that the gravity water level within the lysimeter
remained nearly constant. The first type of loading generally
requires long term testing and water level changes within the
lysimeter are observed through different seasons of the year.
The maximum loading that would allow for year round operation
without overflowing was established from units operated in this
manner. The second type of loading allows for continuous eva-
poration rate measurements. The amount of water added to
maintain a constant level represents the amount evaporated and
can be correlated to the parameters being studied.
64
-------
TABLE 7. LYSIMETER STUDY CONDITIONS
Lysimeter Study
Number Period
la
Ib
Ic
2a
3a
4a
4c
4f
5a
5f
6a
7a
7e
8a
8c
8d
8f
9a
lOa
lOd
lla
12a
12e
13a
1-1-76/7-14-76
7-14-76/10-1-76
10-1-76/8-30-77
1-1-76/8-30-77
1-1-76/8-30-77
1-1-76/10-1-76
10-1-76/8-30-77
9-1-77/9-22-77
1-1-76/8-30-77
9-1-77/9-22-77
1-1-76/8-30-77
1-1-76/8-1-77
8-1-77/S-22-77
1-1-76/10-1-76
10-1-76/1-1-77
1-1-77/8-30-77
9-1-77/9-22-77
1-1-76/8-30-77
1-1-76/1-1-77
2-1-77/8-30-77
1-1-76/8-30-77
1-1-76/8-1-77
8-1-77/9-22-77
3-1-76/10-1-76
Loading Rate
mm/day
1.6
1.6
3.2
0.4
0.8
1.6
1.6
1.6
8.0
4.0
1.2
1.6
1.6
2.0
1.6
1.6
(0.04 gpd/ft2)
variable
variable
(0.04 gpd/ft!?)
(0.08 gpd/ft*)
(0.01 gpd/ft*)
(0.02 gpd/ft2)
variable
(0.04 gpd/ft2)
variable
(0.04 gpd/ft2)
variable
(0.04 gpd/ft2)
•)
(0.20 gpd/ft*)
(0.10 gpd/ft2)
(0.03 gpd/ft )
variable
(0.04 gpd/ft2)
•)
(0.04 gpd/ft*)
9
(0.05 gpd/ftz)
O
(0.04 gpd/ft*)
variable
variable
f\
(0.04 gpd/ft*)
Gravity
H20 Depth
from Surface
variable
0.58 m (23 in)
0.23 m (9 in)
variable
variable
variable
variable
0.23 m (9 in)
variable
0.23 m (9 in)
variable
0.58 m (23 in)
variable
variable
variable
variable
0.23 m (9 in)
variable
variable
variable
variable
0.46 m (18 in)
0.23 m (9 in)
variable
Special Conditions
Alfalfa planted on
surface
Alfalfa planted on
surface
Alfalfa planted on
surface
*
*
*
*
Secondary effluent
feed @ 20°C (68°F)
Primary effluent
feed @ 50°C (122 F)
Tap water feed at
20°C (68°F)'
Opaque cover excludes
sunlight & precip.
*
Cottonwood tree
planted
Has gutter to
measure runoff
*
*
Primary effluent
feed @ 20°C (68°F)
Clear plastic cover
excludes precip.
Has gutter to
measure runoff,
surface slope
1:2, 2" top soil
Surface slope
changed to 1:6,
top soil removed
Sod grass cover.
has gutter to
measure runoff
*
Cottonwood tree
planted
Special finer ET
sand, Golden Sand
w/all larger than
100 mesh removed
(Continued)
65
-------
TABLE 7 (Continued)
Lysimeter
Number
Study
Period
Loading Rate
mm/day
Gravity
H2O Depth
from Surface
Special Conditions
13c
13d
14a
14b
14c
10-1-76/1-1-77
1-1-77/8-30-77
variable
variable
0.23 m (9 in)
0.58 m (23 in)
1-1-76/7-14-76 1.6 (0.04 gpd/ft"4) variable
7-14-76/10-1-76
10-1-76/8-30-77
variable
variable
0.58 m (23 in)
0.23 m (9 in)
Special finer ET
sand. Golden Sand
w/all larger than
100 mesh removed
Special finer ET
sand, Golden Sand
w/all larger than
100 mesh removed
Has gutter to
measure runoff,
surface slope
flat
Clear plastic cover
precludes sunlight
& precipitation
Clear plastic cover
precludes sunlight
& precipitation
15a
15c
16a
16c
17a
17c
17d
18a
18c
18e
19a
19b
19c
20a
1-1-76/10-1-76 6.4
10-1-76/8-30-77 2.4
1-1-76/10-1-76
10-1-76/8-30-77
3-1-76/10-1-76 1.6
10-1-76/1-1-77
1-1-77/8-30-77
1-1-76/10-1-76 1.6
10-1-76/8-1-77
8-1-77/9-22-77
1-1-76/7-14-76 0.0
7-14-76/10-1-76
10-1-76/8-30-77
1-1-76/8-30-77
(0.16 gpd/ft,)
(0.06 gpd/ft^)
variable
variable -
(0.04 gpd/ft )
variable
variable
(0.04 gpd/ft2)
variable
variable ,
(0.00 gpd/ft^)
variable
variable
variable
variable
variable
0.23 m (9 in)
0.23 m (9 in)
variable
0.23 m (9 in)
0.58 m (23 in)
variable
0.23 m (9 in)
0.23 m (9 in)
variable
0.58 m (23 in)
0.23 m (9 in)
0 mm (full)
*
*
*
Sod grass cover
Special coarse ET
sand, Golden Sand
w/all smaller than
100 mesh removed
Special coarse ET
sand. Golden Sand
w/all smaller than
100 mesh removed
Special coarse sand
sand, Golden Sand
w/all smaller than
100 mesh removed
Has gutter to
measure runoff
Pussy willow
planted
Poplar tree planted
Has gutter to
measure runoff
Opaque cover added,
precludes sunlight
& precipitation
*
Has gutter to
measure runoff
(Continued)
66
-------
TABLE 7. (Continued)
Lysimeter
Number
21c
21f
22c
22d
23c
23e
24c
24d
25c
25f
26c
27c
28c
28f
Study
Period
10-1-76/8-30-77
9-1-77/9-22-77
10-9-76/8-1-77
8-1-77/9-22-77
10-1-76/8-1-77
'8-1-77/9-22-77
10-1-76/1-1-77
1-1-77/9-22-77
10-1-76/9-1-77
9-1-77/9-22-77
10-1-76/9-22-77
10-1-76/9-22-77
10-1-76/9-1-77
9-1-77/9-22-77
Loading Rate
nun/day
0.0 (0.00 gpd/ft2)
variable
variable _
1.6 (0.04 gpd/ft )
variable
variable
variable _
1.6 (0.02 gpd/ft,)
1.6 (0.02 gpd/ftz)
variable
variable
variable
variable
variable
Gravity
H2) Depth
from Surface
variable
0.58 m (23 in)
0.32 m (15 in)
variable
0.18 m (7 in)
0.23 m (9 in)
0.23 m (9 in)
variable
variable
0.58 m (23 in)
0.23 m (9 in)
0.23 m (9 in)
0.23 m (9 in)
0.58 m (23 in)
Special Conditions
*
Secondary effluent
feed, 20°C (68°F)
*
Poplar tree planted
*
Poplar tree planted
Pfitzer planted
Pfitzer planted
Surface slope flat
Primary effluent
feed @ 20°C (68°F)
Pfitzer planted
Salt cedar tree
planted
*
Tap water feed
r r. _ n
pan (29) 1-1-76/9-22-77
variable
@ 20UC (68UF)
pointer Filled each day to
point gage
Standard lysimeter conditions - All units had Golden ET sand with bare
surface at a slope of 1:6 and primary effluent feed water at a
temperature of 30°-37°C except as noted under special conditions.
67
-------
Loading Parameter
Ten different constant loading rates were used during
several periods of the study to provide an understanding of how
the units functioned and to establish the maximum year round
loading for the weather conditions that existed. The objective
of this portion of the research was to correlate performance of
ET beds with weather variables in order to establish parameters
for design for other locations and weather conditions. The ten
loadings utilized were:
Loading _
mm/d gal/day.ft Lysimeter No. Length of Testing
0 0.00 19a & 21c 20 months
0.4 0.01 4a 8
0.8 0.02 4c 12
1.2 0.03 8d 8 "
1.6 0.04 2a 20
2.0 0.05 lOd 7
2.4 0.06 15c 12
3.2 0.08 3a 20
4.0 0.10 8c 4 "
6.4 0.16 15a 8
8.0 0.20 8a 8
A plot of the results of one of the long term test units
(#2a, 1.6 mm/d or 0.04 gpd/ft2) is shown in Figure 17. The
lower portion of the figure shows the loading due to rainfall
and wastewater as well as the evaporation rate. Loadings and
measurements were made each day. The data has been plotted on
the basis of weekly averages in order to provide the detail
necessary for understanding the functioning of the units. The
upper portion of the figure shows the variation in depth of the
gravity water surface for the unit over the twenty month test
period.
The unit was constructed with moist ET sand delivered by a
local supplier. It can be noted that in February and March of
1976 two snow storms provided loadings that were in excess of
the evaporation rate and the unit began to fill with gravity
water. In late April and May two large rainstorms caused rapid
filling of the unit, almost to overflowing. During June, July
and August, the high evaporation rate resulted in rapid drying
of the sand and no gravity water was present, although the unit
did contain capillary-held soil moisture. In the fall of 1976,
when rainfall exceeded evaporation, the unit began to fill again.
A large storm in April caused the unit to fill nearly to the top
but it was possible to load the lysimeter in the normal fashion
without causing overflowing. May of 1977 was an unusually dry
month and the gravity water level disappeared and did not return
for the remainder of the study. It was possible to load the unit
68
-------
top JFM.AMJJASONDJFMAMJJAS
(m)
mm/d
0.1 -
0.2 -
0.3 -
0.4 -
0 . 5 _
0.6 -
s~~
/
/
I
h
A
\
\
\
\
\
\
\
1 \
/-
. /
\, A/
S sat. I /\ I "X/VV
Gravity
14 _|
12 -
10 -
8 -
6 -
4 -
2 .
0 -
yN,
7 N
A
A/\
V\
\
\
i
/
/
/
/
/
sat. \ /
Ly #2
. 4
• 8
12
(in)
- ID
• 20
- 24
water level
x£ Evaporation
f l\ ,-A h
Rainfall^ A
IK 1r
•~
'V
'Jk
-,
\
.'V
f
A
/ i
/
-rr
i
\
Wastewater loading
^
Tl
1.6
\
f
/
-
'I
i A/ I
I/ ? \
\
i Y
T rl
•
~
i
I
f
\
\ r
I A
-n
,A
/
A ^XV ,
,/ w
/•*
/vV
/\ h-
/S
W^x
U ,_, ^
mm/d (0.04 gpd/ft )
I
nV
/
/
/
'flj]7]
j
\
I
1
t
/
1
ctillfkn-
"
r
r
-
I
|A
' 1\^
-0.3
- 0.2
o
gpd/ft:
-0.1
JFMAMJJASON. DJF
1976
M A M J J A S
1977
Figure 17. Effect of wastewater loading of 1.6 mm/d (0.04 gpd/ft ) on lysimeter
gravity water level.
-------
without overflowing throughout the length of the study period.
There was a brief period in January and February when the filler
pipe was frozen and daily loading was not possible. This problem
was due to the way the lysimeters were constructed and would not
occur with an actual home system where the piping would be
underground.
The loading diagram and water level profile for unit #3
with a sewage (primary effluent) application rate of 3.2 mm/day
(0.08 gpd/ft2) are shown in Figure 18. At this loading rate the
unit was full many times during the test period. When the full
condition occurred, the feed rate was reduced instead of per-
mitting the unit to overflow. As a result, all those portions
of the lower curve where the feed rate was reduced represent
periods of failure of the system at the test loading condition.
It can be noted that failure of the unit was frequent during the
winter and spring.
The lysimeters that were tested with low feed rates, _
0.00 mm/day, 0.4 mm/day and 0.8 mm/day (0.00, 0.01, 0.02 gpd/ft )
did not fill enough to produce a gravity water level in the
lysimeter during any portion of the study. These loading values
are well below the critical value and all water was held as
capillary pore water. The units tested with very high feed
rates: 6.4 mm/d and 8.0 mm/d (0.16 and 0.20 gpd/ft2), were full
continuously throughout the test period and could not be loaded
at the prescribed rate without overflowing during any season of
the year. The unit loaded at 4.0 mm/d (0.10 gpd/ft2), #15c, was
operated for the four winter months of September through December.
During that period the unit was full most of the time and could
not be fed at the prescribed rate.
It became apparent after the first year of study that the
appropriate loading for year round operation for the climatic
conditions of Boulder was in the range of 1.6 mm/d (0.04 gpd/ft ).
During the second winter of the study three units were brought
to a water level similar to that of the 1.6 mm/d lysimeter and _
were then loaded at 1.2, 2.0, 2.4 mm/d (0.03, 0.05, 0.06 gpd/ft )
(#8d, lOd, 15c). The results of these loadings as well as from
the 0.8 mm/d, 1.6 mm/d and 3.2 mm/d loadings are shown in Figure
19. It can be noted that #8d at 1.2 mm/d (0.03 gpd/ft2) never
completely filled and that #10d and #15c, loaded at 2.0 and 2.4
mm/d/ respectively, filled to the point of overflowing several
times during the winter and spring months and the prescribed
loading rates could not be sustained.
Two conclusions can be reached from this portion of the
study: (1) the appropriate maximum loading rate for the test
conditions was 1.6 mm/d (0.04 gpd/ft2) and (2) the units per-
formed in a very similar manner and only through daily measure-
ment was it possible to determine the point of maximum allowable
loading. If observations could be made of field installed units
70
-------
t0p J 1F if! | *.|.»..l J ,J IA Is f° .K,|.D , J ,,F ,?..,'.' M . J i J ,A ,3
0,1 -
0.2 ,
»S:3:
0.5 -
0.6 _
~ • j i—'—T-
77V
j Ly #3
sat.
. 4
- B
- 13
- 16
- 20
_ 24
(in)
Gravity Water Level
mra/d
14 n
12 •
10 -
a -
6 .
4 -
a -
0
rt
J
' \
A /V
' ^ ^
f V
(, ^ ^-Evaporation ''
l\ \' \ .A
Rainfall
IP* A
n ! ^'
^J^KJy
'--' l/^H
1 v V' \ n
Art n rrTI tf,
tip nl
MhJil ^
f^i^t
III
MR /^ (TL/'Jif
u^flv^ 'vpf^1^
^
/
^
inillf^nJ
-
-
- I
A
»1
^
|
ffastewater loading 3.2 mre/d (0.08 gpd/fP) ^ n^
J ' T ' M A
» J T-J ^ A ^S1
197G
O'S'D'J'F'M'A'M'J'J'A'S
. 0.3
- O.2
gpd/ft2
- 0,1
1977
Figure 18.
of 3.2 ™/d (0.08 gpa/ft2) on lysi.eter
-------
3.2 mm/d (0.08 gpd/ft )
J , F . M . A M
2.4 mm/d (0.06 gpd/ft )
M
o
2.0 mm/d (0.05 gpd/ft2) 1.6 mm/d (0.04 gpd/ft )
_ m- * » f
top
1.2 mm/d (0.03 gpd/ft2) 0.8 mm/d (0.02 gpd/ft )
J F M A M
top J
M
M
0.1
0.2
0.3
0.4
0.5
0.6
Figure 19. Gravity water level pattern during critical months
for six loading conditions.
72
-------
with loadings between 1.2 and 2.4 mm/d (0.03 and 0.06 gpd/ft2),
it would be very difficult to define differences in their per-
formances by noting surface moisture conditions.
Depth of Gravity Water
A greater understanding of the performance of an ET bed can
be gained from studies of evaporation rate as a function of
gravity water depth in the bed. Six lysimeters operated at
constant gravity water levels were used to establish this func-
tion. The test conditions are given below.
m
0
0.17
0.22
0.37
0.44
0.58
Inches Fraction Lysimeter # Length of Testing
0
7
9
15
18
23
full 20a
1/4 23c
1/3 16a & 28c
1/2 22c
2/3 12a
1/7 7a
20 months
10
20
10
20
20
Total depth = 0.67 m, 27 inches
The evaporation rates for each month are shown graphically
in Figure 20. The evaporation rate in each case was calculated
as the sum of the sewage effluent added to the unit and the
recorded rainfall. In Figure 21 the average evaporation rate
corresponding to each gravity water level was calculated and
plotted as a fraction of the evaporation rate measured from the
class A pan at the site. The evaporation rate is reduced with
the depth of gravity water because of the reduced moisture
content at the surface of the sand bed. This has been illus-
trated in Figure 11. It has been shown in Figure 17 (ly #2)
that a gravity water surface did not exist in the summer period
and all water was held as interstitial capillary water. Under
this condition, the evaporation rate would be even a lower
fraction of the pan evaporation rate than the minimum value shown
in Figure 21.
The average evaporation rate for the unit #2a that was
loaded at 1.6 mm/day (0.04 gpd/ft ) was calculated to be 62
percent of the measured evaporation of the unit maintained in
the saturated condition (#20a), and 58 percent of the pan eva-
poration. Since the evaporation rate in the eight winter-spring
months of October through May was about 73 percent of pan
evaporation, the summer month rate was only about 25 percent of
pan evaporation. From this it can be concluded that the moisture
content of the unit fluctuates in such a manner that the actual
evaporation rate from the unit is nearly constant throughout the
year. The actual evaporation rate increases slightly in late
May and June as the gravity water level decreases and disappears
73
-------
pan
18"
1976
Figure 20. Monthly evaporation rate as a function of the depth of
the gravity water surface in the lysimeter.
74
-------
I
nJ
I
55
o
I
]00
75 -
50 -
25
[ 1
0 (full) 0.18 0.23
—I 1
0.38 0.46
T
0.58 (meters)
0 79 15 18 23 (inches)
Gravity water depth below sand surface
Figure 21. Lysimeter evaporation rate as a function of
gravity water depth.
75
-------
and is decreased slightly in the fall when the gravity water
reappears, but it is much more constant over the year than the
pan evaporation rate.
The high pan evaporation rates occurring at mid-summer,
during the period when moisture content of the unit is low, have
little effect on the functioning of the system. The ET units
studied did not provide enough water storage capacity to hold
the winter inflow for evaporation in the summer. Increasing the
depth of the unit would necessitate the use of very small
particle size materials (silts) to provide the necessary capil-
lary rise potential. This type of media has a very low hydraulic
conductivity and is subject to clogging with sewage solids.
Therefore, ET beds should have a depth of 0.5 m to 0.7 m (20 in.
to 30 in.) and should be designed based on the weather conditions
during the critical months of December through May and not on
yearly evaporation rates, nor on the basis that the unit acts as
a storage reservoir during periods of low evaporation and has
accelerated evaporation rates in the summer. It can be noted in
Figure 17 that the lysimeter was near saturation in early
October and remained that way until May. Non-discharging ET
beds designed for year round application can only be used in
locations where evaporation rate substantially exceeds precipi-
tation during every month of the year. This consideration can
be applied to combination ET-seepage systems to indicate that
evaporation will provide a minimal contribution during those
months where evaporation does not substantially exceed precipi-
tation.
Sunlight and Rainfall Effects
Four covered lysimeters were used to evaluate the effect
of rainfall and direct sunlight. The units had low open covers
as pictured in Figure 14. Two of the units had clear plastic
surfaces that prevented rainfall or snow from entering the unit
but permitted direct sunlight to strike the ET sand surface.
The clear plastic surface was replaced twice during the study
period to ensure a maximum transmittance of sunlight. The other
two units had opaque plywood covers that eliminated rain, snow
and sunlight. The units were tested 1) in the constant loading
mode at 1.6 mm/day (0.04 gpd/ft2) and 2) at a constant gravity
water level, 0.23 m (9 in) from the surface.
Unit Cover Loading Period
14 clear variable at 0.23 m 12 mo
19 opaque variable at 0.23 m ' 12 mo
6a clear 1.6 mra/d (0.04 gpd/ft^) 20 mo
9a opaque 1.6 mm/d (0.04 gpd/ft^) 20 mo
The results of the variable loading units with the water
level held at 0.23 m (9 in. ) below the ET sand surface are
76
-------
shown in Figure 22. It can be noted that the opaque covered
unit produced less evaporation, especially in the summer period
when sunlight intensities were high. The average evaporation
rate for the 12 months was 4.2 mm/d (0.105 gpd/ft2) for the
clear covered unit and 3.0 mm/d (0.075 gpd/ft2) for the opaque
unit. Direct sunlight was responsible for an average of twenty-
nine percent of the annual evaporation rate with ranges from
thirteen percent in the winter months of November, December and
January to thirty-seven percent in June, July and August. The
major portion of the evaporation at all times of the year
resulted from the wind movement over the ET surface.
The evaporation rate from the clear covered unit (#14) was
nearly the same as from the uncovered units held at the same
water level (#16a, 28c on Figure 20). This is one indication
that very little rainfall runoff occurred and that calculations
of total monthly evaporation rates on the basis of wastewater
loading plus precipitation are valid for this investigation.
The results for the covered units loaded continuously at
1.6 mm/d (0.04 gpd/ft^) are shown in Figure 23. Superimposed
on the upper graph is the water level pattern of the open unit
(#2a) with the same loading. During the first nine months of
testing, the results show the general pattern of the opaque
covered unit to be similar to the open unit except for the
absence of the rainfall spikes. The open unit received a slightly
higher loading because of the rainfall but it also received
direct sunlight. The clear covered unit received the lower
loading without rainfall and received direct sunlight. The
effect of direct sunlight is quite apparent in June and July
when comparing the opaque covered unit with the others. The
second year of the study produced a similar pattern except that,
after the first summer, the clear plastic cover began to discolor
and it was decided not to replace it. As a result, the per-
formance became quite similar to that of the opaque unit.
Rainfall Runoff and Surface Slope
Seven of the lysimeters were equipped with small aluminum
gutters around the circumference of the unit to collect runoff
during precipitation events. The gutters drained into covered
plastic containers and the collected water was measured volu-
metrically after each rainfall or snowmelt. The measurement of
runoff was made for each storm during the first eight months of
the study. Three different surface slopes were used: 26° (1:2),
9° (1:6), and 0° (flat). The 9° (1:6) slope was tested under
three conditions: (1) with the ET soil near saturation at the
time of rainfall, (2) with the ET soil undersaturated during
rainfall, and (3) with a grass-sod growing on the surface. The
results are shown in Figure 24. The curves rise sharply for
precipitation of 25 mm (1 inch) but in general the runoff coef-
ficient (volumetric ratio of runoff to precipitation) was less
77
-------
10-,
9-
8-
7 -
ca
E
o 5-
•r-l
4J
CO
a
CO
4-
3-
1 -
Clear
Cover
x» Opaque
Cover
S1 ' 'Dfji T I I
1976 1977
Figure 22. Monthly evaporation rate for clear and opaque
covered lysimeters.
78
-------
MA,M,J,J.A.S.O.N.D.JF.M A M J J A S
Cm)
J-00 -J S M A M O . «J A S . U IN L) J *
C°P I | 1 I | 1 | ! I t I I I I L
0.1 -
0.2 -
0.3 -
0.4 -
0.5 -
0.6 -
Open Ly #2
--'"x-». Opaque cover
••"'., "V,Ly #6
Clear..., t \
cover Ly'''i#"9
-4
8
12
L16
20
- 24
(in)
Gravity Water Level
mm/d
14 -
12 -
10 -
8 -
6 -
4 -
2 -
0 -
l K ^Evaporation
P /\ ^ A
)\ \* V
Rainfall loading | \, V A/ \.
for open unit r
only — .
' + T n i I
* A /
I ' \ 1
•' V
t \ \ . J
, V * \ .
An U^
r*i i <
x v n-H * ' I /v /-,
fj /
TT T 1,1 I: Lrt , Wv N: „ I
Wastewater loading 1,6 mm/d (0.04 gpd/ft2^ *"] [fl
• ' • — i— —\ • -- '•— i'i«™ — .1. . • . i. ' i' . i i. i . i— • — i • « . i _ , „ ,»i^,-, t 1 i
/
'
1
]
'frl>l
\
^
,A
A M' i
,/ ^ ^
r
/vV
^ /
i
'
,
~
r^fka
r~
"
-
nA
•r-.
- — i i i • t i '
.0.3
- 0.2
0
gpd/ft^
^•0.1
J F M
MJJ
1976
1977
Figure 23. Effect of clear and opaque covers on lysimeter gravity water level.
-------
0.25
0.20.
0.15
e
u
o
c
3
o.io
0.05
0.1
Saturated
Saturated (1:6)
Unsaturated (1:6)
Sod covered (1:6)
0 0.5 1.0 1.5 2.0 2.5
Rainfall (cm) .
Figure 24. Runoff as a function of rainfall, slope and soil
condition.
80
-------
than 0.1 and was in the range of 0.02 for the bare, unsaturated
ET sand surface. Although the units had a very small surface
area for making runoff studies, the results show that runoff is
a minimal fraction of precipitation. The assumption that all
precipitation entered the beds and became a part of the loading
to be evaporated was therefore justified.
ET Sand Evaluation
The effect of sand size on evaporation performance was
evaluated in a limited study. A portion of the Golden ET Sand
was passed through a #100 sieve. The finer fraction (<100 mesh)
Was used in lysimeter #13 and the coarser fraction in lysimeter
#17. The size distribution of the two fractions as well as the
unaltered Golden ET Sand is shown in Figure 25. The D5Q value
of the unaltered ET sand was 0.13 mm, the finer fraction
DCQ = 0.075 mm and the coarser fraction D5Q = 0.26 mm.
Three test conditions were used to compare the finer and
coarser sands with the unaltered ET sand. These were: (1) con-
stant loading at 1.6 mm/day (0.04 gpd/ft2), (2) constant water
level at 0.23 m (9 in.) below the sand surface, and (3) constant
water level at 0.58 m (23 in.) below the sand surface.
During the period of March through August of 1976 the units
were loaded at a constant rate of 1.6 mm/day (0.04 gpd/ft2).
The results can be compared to those of the unaltered sand in
lysimeter #2. The plot of gravity water level for the three
units is shown in Figure 26. The graphs are quite similar
except that the rate of evaporation was somewhat less for the
coarser sand in June and July, when the gravity water level was
low. This reduction in rate was due to the smaller capillary
rise potential of the coarser sand.
During the months of October, November and December, 1976,
the lysimeters were operated at a constant gravity water level
of 0.23 m (9 in.) below the sand surface. The comparable
unaltered ET sand lysimeter was #28c. The evaporation rates
from the three units were:
finer ET sand = 2.00 mm/day
coarser ET sand = 2.20 mm/day
unaltered ET sand = 2.08 mm/day
These results are quite similar and indicate that for high
gravity water levels where all sands had adequate capillary rise
potential, the evaporation rate was not affected by sand size.
During the period January through July of 1977 the water
level in the units was held at 0.58 m (23 in.) below the sand
surface and the evaporation rate was measured and compared with
81
-------
0.5 -.
0.4 ,
0.3
0.2
Grain
size
(nun)
0.1
0.09
0.08
0.07
0.06
0.05
0.04
100 mesh + all
I I
100 mesh -
10
20 30 40 50 60 70 80 90
Percent Finer by Weight.
Figure 25. Gradation analysis for Golden sand and for fractions
greater and less than 100 mesh.
82
-------
Depth
(m)
surface
0.1
0.2
0.3
0.4
0.5
0.6
F M A
M
J A
Ly #12v
#17a
. 4
- 8
12
- 16 (in)
20
24
Gravity Water Level
Key to graph above
Solid line - normal ET sand, Ly #12
Dashed line - fine fraction <200 mesh, Ly #13a
Dotted line - coarse fraction *200 mesh, Ly #17a
Loading
(mm/d)
10 4
8
6
4 -
2 .
0
, Evaporation
V /\
\ i
'\/
1 V
Rainfall /\
./A/
jvtr
'
-* • /
y V
''Jk
/\
/
rr
V
1
j1
Wastewater loading
1 — — i 1
-r Tt **• T*
i i
* * nr
\,' \
\,
p.
.!_-.
f
''i /\ '
' ' v
V
T
1.6 mm/d
i •
1
0.2
gpd/ft2
. 0.1
0
1976
Figure 26.
Lysimeter gravity water level as a function of
evaporation and water loading for different
sized ET sands.
83
-------
unit #7a containing unaltered ET sand. The measured rates for
the three units were:
finer ET sand =1.32 mm/day
coarser ET sand = 1.20 mm/day
unaltered ET sand = 3.76 mm/day
The results of this limited study indicate that the
selection of ET sand is a consideration as it affects the
raising of water from deep in the ET bed during the summer.
Coarser sands do not provide adequate capillary rise potential
and very fine sands or silts may restrict water movement. It
has been shown that the summer is not the critical operating
period for an ET bed. Therefore, the overall performance, at
the nearly constant loading that is characteristic of home
wastewaters, would be less affected than these values indicate.
However, ET sand selection is important in bed design to allow
for maximum loading and ensure optimal performance.
Wastewater Temperature Effect
One of the lysimeters (#5a) in the study was loaded at a
rate of 1.6 mm/day (0.04 gpd/ft2) using wastewater at a tempera-
ture of 50°C. The results of the elevated temperature can be
compared directly with those of unit #2a, which had identical
loading with water at 30°-37°C. The results of the two units
are compared in Figure 27. It can be noted that the performance
was quite similar for the two units, with the unit receiving
wastewater of higher temperature performing slightly better in
the winter months. The added heat from the 50°C wastewater would
account for a maximum of
20 cal/cc (wastewater heat difference)
600 cal/cc (approximate heat of evaporation)
or approximately three percent increase in evaporation rate.
This is a relatively insignificant quantity, but it is identi-
fiable in the long term studies. The higher temperature feed
water did not prevent freezing of the surface of the unit in the
winter. For most design purposes, wastewater temperature would
not be a consideration. This was also concluded by Tanner and
Bouma (1975).
Effluent Type
Three types of water were fed to the lysimeters during a
brief study in September 1975. The three feed waters were
primary effluent and secondary effluent (both from a local
sewage treatment plant) and tap water. Six lysimeters were
studied in the constant gravity water mode; three with the water
level maintained at 0.23 m (9 in.) below ground surface and
three with the gravity water level 0.58 m (23 in.) below the sand
surface. After a period of operation to achieve equilibration,
84
-------
0.1.
0.2-
0.3-
0.4-
0.5-
0.6-
JFMAMJJAS 0 NDJFMAMJJAS
Ly #
32° C
. 4
- 8
- 12
_ 16
• 20
- 24
(In)
Gravity Water Level
CO
Ul
mm/d
14 -
12 -
10 -
8-
6-
4 -
2 -
0 -
i h * ^Evaporation
• J v , \ L A
\' ^ / ' I
r
Rainfall-^ A
f.
x
l/v
i n--
r!W
i , V M
<'V n
' 1 nfl A
-rrLiiil L:
1
1
\
\
\ I
-Tl
M
I
r~\
\ A !-^/ 1
L^T^.^^ V
1 ' ' ' ' ^ ' — — . t L J U J
Wastewater loading 1.6 mm/d (0.04 gpd/ft ) jf]
X
/
^HM
\
\
"Iv
"1
(A
/\
n f yV '
_/ " '
i"
/v V
/
/
i
r^Pk^n-
H
r-
-
- V\
— . — , — , — . — _
-0.3
- 0.2
gpd/ft2
- 0.1
J F M A M J J A
1976
1977
Figure 27. Effect of elevated wastewater temperatures on lysimeter gravity
water level.
-------
the evaporation rate was measured for a period of two weeks.
The results were as follows:
Depth of
Lysimeter Gravity Water Evaporation
Number Feed Water Below Surface Rate, mm/day
8f Primary 0.23 m (9 in) 5.60
4f Secondary 0.23 m (9 in) 5.46
5f Tap 0.23 m (9 in) 5.87
25f Primary 0.58 m (23 in) 4.38
21f Secondary 0.58 m (23 in) 4.24
28f Tap 0.58 m (23 in) 4.38
It has been concluded that the degree of treatment of waste-
waters did not affect their evaporation rate in the ET lysimeters.
Surface Cover and Vegetation
Initially, in this study, two inches of top soil was placed
on the surface of each of the lysimeters. Early observations
indicated that the top soil was reducing the evaporation rate.
The top soil was removed from all units except unit #10a. The
unit was loaded at 1.6 mm/d (0.04 gpd/ft^) for a period of one
year. A comparison of this unit with a similarly loaded unit
without top soil (#2) is shown in Figure 28. In this study, a
thin layer of top soil was found to be detrimental to the
optimum functioning of an ET bed.
Several types of vegetation cover were utilized in order
to assess the effect of plant transpiration on the functioning
of an ET bed. Two of the shrubs used, pussy willow (#18a) and
salt cedar (#27c) (tamarack) did not survive the high water
level and freezing conditions of winter operation and resulting
evaporation data from these units was essentially the same as
that from units with bare soil cover.
Three types of vegetation cover were evaluated in long term
studies. These were Kentucky blue grass sod (#lla, #16c),
alfalfa (#1), and the juniper shrub, pfitzer (#24c, #26c). The
sod and pfitzers were studied using two lysimeters each, one
loaded at a constant rate of 1.6 mm/d (0.04 gpd/ft ) and the
other at a constant gravity water level of 0.23 m (9 in.) below
the surface. The alfalfa planted unit was operated initially as
a constant loading lysimeter at 1.6 mm/d (0.04 gpd/ft ), and in
the second year as a constant water level unit with the water
level at 0.23 m (9 in.) below the sand surface.
The results of the constant level tests where evaporation
rate was measured directly are shown in Figure 29. It can be
noted that alfalfa produces a very high evaporation rate in the
summer but that, in the critical winter period, the evaporation
rate was considerably below that of bare soil due to the shading
86
-------
II
0.1-
0.2-
0.3.
(m) 0.4-
0.5-
0.6.
|
tli i J
f
X. x
X-'
i
A '
Ai
f M
X w
f\(
1 1 /• A Ly *10a
Vi ,'" • / >
S,.ithtop ; "" "•', /,,
i
i
i i t 11
,
end j, /
ilr.1.1 - . v r
sat. 1 WUho«tst,j . 1 -VVV
/N
J
V
/
/
/
f
Ly #2
. 4
. 8
• 12
: 11 «->
- 24
Gravity Water Level
14-i
12 -
CO
-o 10-
8-
mm/d
6-
4-
2-
0
Rainfall , A
• fkC 1
S
;7V/
in
OJ
H
H
o
CO
I \ A O.
j\ 'V \ A Evaporation ^
i \. \S\ A, \'
/""" '
/vV
\ /
V
/
I
n
rJkn-
p
•
-
11 ^
- '/»
»
- 0.3
- 0.2
O
gpd/ff*
- 0.1
- — i i i i i
A M J J A S
1977
Figure 28. Effect of topsoil cover on lysimeter gravity water level.
-------
CO
•o
\
s
£
e
o
-1-1
-1-J
ca
SH
O
a
CO
20-
15 -
10.-
5 -
0
bare soil and
sod covered
O ' N I D I J ' F ' M
1976 1977
M ' J ' J
Figure 29. Monthly evaporation rate for different vegetation
cover.
88
-------
and snow holding characteristics of the alfalfa. Sod grass
cover produced nearly identical evaporation rates as the bare
soil except that it was slightly lower in the important months
of March and April. The pfitzer bush produced greater evapora-
tion rates than bare soil throughout the year.
The effects of the evaporation rates on the year round
functioning of the lysimeters is shown in Figure 30. These
results indicate that the use of pfitzers can be beneficial to
the operation of the beds and that grass cover will slightly
reduce evaporation rates in the winter and spring.
Another observation that was made during the constant
loading studies was that the very dry conditions of the lysi-
meters in mid-summer were a problem for maintaining vegetation.
The alfalfa dried out to the point that it was completely brown
in mid-summer, although it did become green again with new
growth in the fall when the moisture content of the unit
increased.
The sod grass unit, operated at constant loading, was
severely affected by the low moisture conditions of summer. The
grass gradually died out and gave way to weeds. Although some
of the grass remained, it was apparent that maintaining grass on
an ET unit would be very difficult with the normal pattern of
household wastewater loading and prevailing evaporation condi-
tions of the study area.
The pfitzers did not seem to be adversely affected by the
dry conditions of summer or the high water level and freezing
conditions of winter. This evergreen showed a high tolerance
for soil moisture variations and produced improved evapotranspira-
tion conditions throughout the year.
Near the end of the study/ during August and September of
1977, two species of deciduous populus trees, cottonwood and
Lombardy poplar, were planted in five of the lysimeters. Three
of the units were operated at a constant gravity water level of
0.23 m (9 in.) below the surface in order to measure evaporation
rates. One tree of each type was evaluated at a constant
loading of 1.6 mm/d (0.04 gpd/ft ) of wastewater to provide
information on the survival of this type of tree under summer
conditions. The lysimeters used were as follows:
89
-------
top J F M
J J
D J F M A M J
0.1-
0.2-
, x °'3-
(m) 0.4-
0.5-
0.6-
fv
t^-
14 -j
12 -
10
o 10-
8-
mm/d
6-
4-
2-
\
\ Solid -
\ bashed
\ Dotted
l\
* X*
- bare soil
- sod
- alfalfa
•
*
i
Gravity
\
i
* *
yi
. * » / /i
v ; /I
>WV-i/'
;\./\
/Mv'
/, i*
/.
i
1
\
^/
/
II 1 1 1
' Solid - bare soil
. Dashed - sod
;i Dotted - pfitzer
''\
•\
:\
. 4
• 8
. 12
- 16
: ;: «•>
Water Level
f Evaporation
Rainfall, A
n
jMT
1 1
~
/
J 1 f
!\
'Jd
\
n
I
\>
\!
A
/ i
/
-rr
1
i
V ^
V i
>/ 1
i
rr
A
f\' \
* 1
/ \
\
\
rl~
II'
-\
i \
\ /
>
A",
V v/ V
rry IT M
I
riy
/
i
i
i
/
'fl|h
/i
/ 1
A ^ '
I
\
_,\ '
V
1
/
n-,fLa
•
c
-
/I
• y»
v»
- 0.3
- 0.2
gpd/ft2
- 0.1
JFMA MJJA SONDJFMA MJJAS
1976
1977
Figure 30. Effect of vegetation cover on lysimeter gravity water level.
-------
Lysimeter
Number Tree Type
12e
18e
23e
7e
22e
cottonwood
(2m)
poplar
(1.5m)
poplar
(1.5m)
cottonwood
(2m)
poplar
dm)
Leaf Area
(one side)
1.0 m2
(400 leaves)
0.007 m2
(200 leaves)
0.12 m2
(350 leaves)
0.75 m2
(300 leaves) (0
0.08 m2
(250 leaves)(0
Loading
variable
variable
variable
1.6 mm/d _
.04 gpd/ft2)
1.6 mm/d ~
.04 gpd/ft )
Gravity
Water Evaporation
Level Rate
0.23 m 12.8 mm/d
(9 in.)
0.23m 7.25 mm/d
(9 in.)
0.23 m 11.07 mm/d
(9 in.)
variable
variable
The cottonwood trees had leaves that were triangular in shape
with an average base and height measurement of 7 cm (2-3/4 in.).
The leaves of the poplar trees were smaller, having base and
height measurements of 1.3 cm (1/2 in.).
The bare soil evaporation rate for this testing period was
5.8 mm/day. The use of small deciduous trees increased the
evapotranspiration rate by as much as one hundred percent in
August and September compared with bare soil. However, the two
trees that received a constant wastewater loading of 1.6 mm/day
(4.0 mm/day including rainfall) showed severe signs of wilting,
especially the larger cottonwood tree (#7e).
Deciduous trees have a large water requirement in the
summer growing season and this is the period when an ET bed is
subject to its driest conditions. In addition, this type of tree
provides negligible transpiration in the dormant winter months
which is the critical time when extra evapotranspiration poten-
tial is needed for an ET bed. The type of ET beds evaluated in
this study do not provide enough storage so that winter flows
can be held for evaporation during the subsequent summer period.
Since deciduous trees do not aid in winter disposal and because
their water requirements are high in the summer, they are not
suited to be used as an evapotranspiration adjunct for the type
of ET bed studied. A completely different type of bed could be
considered, designed with enough depth to provide for storage of
wastewater generated throughout the dormant season and disposal
primarily by transpiration in the summer. This type of system
was not a part of this study.
For the type of ET bed evaluated in this study, small
evergreens seem to provide the practical choice of vegetation in
conjunction with bare soil or with grass or weed surface cover-
ings. The evergreens provide some transpiration in the winter
which aids disposal to a small extent and they seem to tolerate
91
-------
the dry soil conditions present in the summer period.
The design of ET systems for summer homes could utilize
highly transpiring plants such as alfalfa, resulting in higher
loading rates.
Salt Build-Up
Salt build-up in a non-discharging ET bed will have a long
term effect on surface vegetation. The total dissolved salts in
home wastewater have been found to be in the range of 400 to 500
mg/L (Bennett, Linstedt, 1975). For a loading rate of 1.6 mm/day
(590 liters/yr-m2) (0.04 gpd/ft2) the salt increase would be
about 270 gm/yr-m2 (25 gm/yr/ft2). A two foot deep bed with a
porosity of 0.35 would contain 20,000 grams of water at satura-
tion and 100,000 grams of ET soil. The average salt build-up
would be approximately 1250 mg/£ per year in the water or 0.25
mg/yr/gm of soil. The salts tend to concentrate at the evapora-
tive surface. A measurement was made of the elutriate of the
surface sand of a home ET bed system that had been in operation
for five years. The total salt level was found to be 11.5 mg/gm
of soil with 0.91 mg of chloride/gm of soil. At soil water
saturation conditions, this would represent a total salt concen-
tration of 50,000 mg/Jl and a chloride concentration of 4500 mg/£.
The build-up of salts over a period of years in a non-discharging
ET bed would be an important factor in the design consideration
of utilizing plant transpiration to increase the ET rate.
FIELD OBSERVATIONS OF ET BEDS
Five ET units installed and in use at individual homes were
observed every few weeks. The water use rate of the homes was
not measured and the loading rates were estimated based on the
size of the ET bed and the number of occupants of the home.
Using a value of 170 liters per person per day (45 gal/per/day)
for the estimation, the wastewater loading rates varied from
0.8 mm/day to 4.0 mm/day (0.02 to 0.10 gpd/ft2). The units with
loadings of 0.8 to 2.0 mm/day (0.02 to 0.05 gpd/ft2) appeared to
perform satisfactorily during the study period. Each of the
units had a center stand pipe for observing the water level in
the bed. None of the units within this loading range became
completely filled during the period of observation.
Two ET beds had estimated loading rates of 2.4 mm/day and
4 mm/day (0.06 and 0.10 gpd/ft2). The bed with the highest
loading experienced frequent saturation with subsequent ponding
and runoff. The other ET bed filled at times and the owners
stated that during those periods they had reduced bed loadings
by washing their clothes at a laundromat. Another unit in the
area, which was not monitored during this study, had an estimated
loading of 3.2 mm/day (0.08 gpd/ft2). Due to frequent satura-
tion of the bed, a second bed of equal size was installed to
92
-------
reduce the estimated loading to 1.6 iran/d (0.04 gpd/ft^). The
field observations provided information that the maximum satis-
factory loading rate was approximately 1.6 mm/d (0.04 gpd/ft^),
the same as that determined in the lysimeter study.
RESULTS OF MECHANICAL EVAPORATION UNIT STUDIES
Studies of the two types of mechanical units were made and
the results presented in a sequence for each unit involving the
basic operational parameters followed by tests relating to
optimization of machine design variables.
Rotating Disk Evaporator (RDE)
It has been shown that the rate of evaporation from a
wetted surface can be described using a mass transfer approach
where the major parameters are the humidity deficit of the air
in contact with the wetted disks and direct solar radiation.
The general equation can be written as
E = k1(AH) + k2Rg
Since contribution of direct solar radiation is relatively
small, a major portion of the study was involved in defining the
parameters affecting the mass transport coefficient, k^. In the
study of a device of this type, it is important to isolate each
parameter while maintaining the others at constant values. Many
of the results were obtained during operation of the unit in the
laboratory under conditions of constant air temperature and
humidity deficit. This approach was used to study the effects
of wind speed and direction, disk size and area, rotational
speed and disk spacing. Studies were made under field conditions
with the unit operated on the roof of the Engineering Center to
establish the effect of a wide variation in temperature and
humidity deficit as well as the contribution from direct solar
radiation. The data obtained under field conditions show more
variation or scatter than those from the laboratory studies. In
order to obtain an accurate measure of evaporation rate, the unit
was operated for a minimum of two hours. During this period,
temperature and humidity variations occurred in the ambient air.
Average values of temperature and humidity were obtained from the
recorder charts and used in the data analysis. The somewhat
lower precision for field data was due to this variation in
ambient conditions.
Liquid Mixture Temperature—
As evaporation proceeds in an adiabatic process, an equi-
librium will be established in the liquid where the gain in
sensible heat from the air will equal the loss in latent heat
due to evaporation. At equilibrium the temperature of the water
will equal the adiabatic saturation temperature of the air.
93
-------
Figure 31 shows the bulk mixture temperature as a function of
the adiabatic saturation temperature of the ambient air under
laboratory and field conditions for the rotating disk unit.
This figure shows that the bulk mixture temperature correlated
reasonably well with the adiabatic saturation temperature of the
ambient air. The value of the adiabatic saturation temperature,
Ts, was computed using the psychrometric chart and measured
values of the air temperature and relative humidity at the time
the mixture temperature was recorded. The air temperature in
all cases was at least 5°C above the mixture temperature. From
these results, it can be concluded that the liquid temperature
is approximately the same as the adiabatic saturation tempera-
ture of the ambient air and that the measured humidity deficit
will represent the adiabatic humidity deficit for evaporation
from the disk surface.
Adiabatic Humidity Deficit—
The equation for evaporation rate is based on a linear
relationship between adiabatic humidity deficit and evaporation
rate in the absence of direct solar radiation. In order to
study this relationship under a wide range of temperature and
humidity conditions while holding other major parameters con-
stant, studies were made in the laboratory and outdoors using
the wind tunnel. This produced a nearly constant wind speed and
excluded sunlight.
The results of these studies are shown in Figure 32. The
wind direction was at 90° angles to the shaft (parallel with the
disks) and the disk rotational speed was 0.5 rpm. It can be
concluded from these data that a linear relationship exists
between evaporation rate and humidity deficit of the ambient
air. Studies at lower wind speeds were attempted outdoors but
the ambient wind changes made it impossible to maintain a con-
stant air velocity across the disks for the period of measure-
ment.
Wetted Disk Area—
For mass transport operations, the usual assumption is that
the rate of the process is directly proportional to the inter-
facial area involved. The exposed surface area of the multiple
disk evaporator can be varied by changing the submergence (depth
of water in the reservoir) or by using a different diameter disk.
The segmented reservoir was used in laboratory studies to
study four different submergence levels simultaneously under the
same ambient conditions. Four different tests were conducted
using the wind tunnel at various wind speeds. The results are
shown in Figure 33. It can be noted that the evaporation rate
per unit of wetted disk area was essentially constant at all
submergences. The maximum wetted area occurs at a submergence
of approximately seventy-one percent of the radius measured from
the outer edge and this value should be used in prototype units.
94
-------
X
e
Q)
^
3
-P
0)
a
0)
EH
(U
H
P
-P
X
•H
S
•H
O
0)
(0
0)
20
15
o field data
o lab data
10 .
000
10
15
20
Air Saturation Temperature, T , c.
Figure 31.
Rotating disk unit bulk mixture temperature, T ,,
as a function of adiabatic saturation
temperature of the ambient air, T .
95
-------
CN
G
O
5-1
O
cu
(0
0.3
01
•H 0.2-
-p
(C
0.1
-0.8
CN
-P
14-1
O
O
_0.4
fO
0.2
w
O 0.00 kmph (0 mph)
A 2.80 kmph (1.75 mph)
a 6.88 kmph (4.30 mph)
o 10.96 kmph (6.85 mph)
solid points - field data
open points - lab data
0.7 submergence
2.5 cm spacing
0.5 rpm
4.0
5.0
Humidity Deficit (AH) gm/kg ( Ib/lOOQlb)
Figure 32. Evaporation as a function of adiabatic humidity
deficit for aluminum disk unit in wind tunnel
96
-------
CM
hr
iters
0
*
u>
0
•
ro
Evaporation Rate
0
•
.0.8
W
O Q.QO kmph ( Q mph)
A 2.80 kmph (1.75 mph)
o 6.88 kmph (4.30 mph)
Q 10.96 kmph (6.85 mph)
CN
_ 0.6 £
o
o
0.2
y-v
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
Submergence (fraction of radius submerged)
Figure 33.
Evaporation as a function of disk submergence for
the multi-diameter plywood disk unit in the
laboratory.
97
-------
A larger, three segment reservoir was used to evaluate the
effect of disk size. Plywood disks of 0.52 m, 0.91 m, and 1.22
m (20.4 in.f 36 in. and 48 in.) were evaluated at a submergence
of 0.7, spacing of 2.5 cm (1 in.) and rotational speed of 0.5
rpm. The testing was accomplished in the laboratory using the
wind tunnel under three different wind speed conditions. The
results are presented in Table 8. It can be seen that the
evaporation rate per unit area was essentially constant and that
evaporation rate can be considered as proportional to wetted
area. The results from the small test unit can be used for the
design of a prototype unit, based on wetted area.
Wind Speed and Direction—
Wind movement across the wetted disk surface provides a
mechanism to bring sensible heat to the water surface for eva-
poration and to remove the water vapor from between the disk
surfaces. Since the position of the disks is fixed with respect
to the wind, the direction of the wind is an important variable
in determining how much air moves between the disks and how fast
the air moves across the disk surface. In order to investigate
the effects of wind speed, the aluminum disk unit was operated
in the laboratory using the wind tunnel. The test unit was
varied with respect to orientation of the air flow in the wind
tunnel to assess the effect of wind direction. The unit was
operated at a constant disk submergence of 0.71 of the radius as
measured from the outer edge, a rotational speed of 0.5 rpm, and
a disk spacing of 2.5 cm (1 in.). The adiabatic humidity deficit
of the air in the laboratory was nearly constant at 4.45 gm/kg
The results of the tests are shown in Figure 34. It can be
noted that evaporation rate can be expressed as a linear function
of wind speed.
The results of the studies of wind direction with respect
to the orientation of the RDE drive shaft are shown in Figure 34
and the circumferential variation of evaporation with wind
direction is shown in Figure 35. All points are relative to the
maximum evaporation which occurs at wind directions of 90° with
the shaft. It is important that a field installation should be
oriented with the disks parallel to the major prevailing wind
direction. Most of the laboratory studies were made with wind
movement perpendicular (6 = 90°) to the shaft.
Study of Machine Variables—
The major machine variables that were varied and evaluated
in this study were: disk submergence, rotational speed of the
98
-------
TABLE 8. EVAPORATION FROM 3 DIFFERENT DIAMETER PLYWOOD DISK UNITS
UNDER CONTROLLED IDENTICAL CONDITIONS (English Units)
Date
2/22/77
2/22
2/23
2/23
2/24
2/24
At
hr
21.50
4.00
19.00
3.75
19.00
7.00
El
gal/hr
100 ft2
0.38
0.50
0.18
0.54
0.17
0.32
E2
gal/hr
100 ft2
0.36
0.52
0.16
0.53
0.15
0.33
E3
gal/hr
100 ft2
0.38
0.56
0.19
0.54
0.18
0.35
W
raph
4.78
7.61
1.95
7.61
1.95
4.78
AH
Ib
1000 Ib
5.21
4.93
5.00
5.00
5.00
5.00
Disk
Spacing
(X)
in.
1.0
1.0
1.0
1.0
1.0
1.0
Rd
rpm
0.5
0.5
0.5
0.5
0.5
0.5
Diameter - D, =
D2 =
D, =
20.4 in.
36.0 in,
0 = 90
w
48.0 in.
o
-------
TABLE 8, EVAPORATION FROM 3 DIFFERENT DIAMETER PLYWOOD DISK UNITS
UNDER CONTROLLED IDENTICAL CONDITIONS (S.I. UNITS)
Date
2/22/77
2/22
2/23
2/23
2/24
2/24
At
Hr
21.50
4.00
19.00
3.75
19.00
7.00
E1(0.52 m)
£/hr . m2
0.15
0.21
0.073
0.22
0.070
0.131
E2(0.91 m)
Vhr.m2
0.15
0.21
0.065
0.23
0.062
0.135
E3(1.22 m)
Vhr-m2
0.16
0.23
0.08
0.22
0.073
0.143
W
kmph
6.88
10.96
2.80
10.96
2.80
6.88
AH
gm
1000 kg
5.21
4.93
5.00
5.00
5.00
5.00
Disk
Spacing
(X)
cm
2.5
2.5
2.5
2.5
2.5
2.5
Rd
rpm
0.5
0.5
0.5
0.5
0.5
0.5
.
o
o
Diameter - D, = 0/52 m
D0 = 0.91 m
D „
8 = 90
w
1.22 m
o
-------
= 4.45 gm/kg
CO
M
Q)
4J
Q)
-P
<0
Pi
O
•H
5-1
O
04
0-3 .
0.2
0.1 .
wind angle
with shaft
0°
Wind Speed (W),(kmph)
Figure 34.
Evaporation as a function of wind speed for the
aluminum disk unit in the laboratory wind
tunnel at various wind directions.
101
-------
Maximum**] .62xAve.
Wind Direction (6w)
5°
Figure 35.
Circumferential variation in evaporation with wind
direction relative to 90 .
102
-------
disks, spacing between the disks and type of material and color
of the disks. The results relating to submergence were presented
in the section on interfacial area. The rate of evaporation was
found to be proportional to exposed wetted surface area and the
geometry of the unit is such that a submergence of seven-tenths
of the radius results in the maximum evaporation rate.
Rotational speed—The rotational speed of the multi-diameter
plywood disk unit was varied from 0.5 to 2.0 revolutions per
minute at different wind speeds to study the effect of rotational
speed on evaporation. Figure 36 shows evaporation as a function
of wind speed for rotational speeds of 0.5, 1.0, and 2.0 rpm.
The submergence was constant at 0.7 and the disk spacing was
constant at 5 cm (2 in.). The points shown in Figure 36 repre-
sent the mean of the data from the three different diameter
units, and also represent the mean of at least three separate
determinations of evaporation at any one wind speed. The eva-
poration rate was found to be independent of rotational speed at
rotational speeds above 0.5 rpm. The minimum rotational speed
must be great enough to keep the disk surface wet during its
exposure to the ambient air, but increased rotational speed
beyond that point does not increase evaporation. Greater rota-
tional speeds will meet the wetting requirement, but may tend to
cause more power consumption and more wear on mechanical parts.
The optimum rotational speed was found to be the slowest speed
necessary to keep the disks wet. This was found to be approxi-
mately 0.5 to 1.0 rpm.
Disk spacing—Wind speed and wind direction have been shown
to be important parameters in the evaporation of wastewater using
the rotating disk unit. Closely associated with wind speed and
direction is horizontal disk spacing on the drive shaft. As
disk spacing increases, the wind will be able to move between
the disks more easily. An increase in wind movement will in
turn increase the transfer of sensible heat from the air to the
water and the transfer of water vapor from the water surface to
the overlying air. Large disk spacings will enhance heat and
mass transfer per unit of wetted disk area up to some point, yet
large disk spacings will also increase the unit size and capital
costs. The multi-diameter plywood disk unit was operated at a
constant rotational speed of 0.5 rpm and a constant disk sub-
mergence of 0.7 in the wind tunnel in order to investigate the
effect of disk spacing on evaporation. Disk spacings of 1.25,
2.5 and 5 cm (0.5, 1.0, and 2.0 in.) were used during the test.
Figure 37 shows the results of the study on disk spacing.
The points shown in the figure represent the mean value
obtained from the three different diameter units and the mean
from at least two separate determinations of evaporation at a
single wind speed. The wind direction was 90° with respect to
the drive shaft in all cases. There was an increase in evapora-
tion with disk spacing at the wind speeds obtained in the wind
103
-------
CM
,B
£1
\
cn
a)
-P
•H
0)
-p
o
•H
-p
(0
J-l
o
0,
-------
to
^
-------
tunnel. There was a substantial increase in evaporation when
the disk spacing was increased from 1.25 to 2.5 cm (0.5 to 1.0
in.), especially at the greater wind speeds. The difference in
evaporation rate between disk spacings of 2.5 and 5.0 cm (1.0
and 2.0 in.) was smaller, indicating that a limiting value of
evaporation was influencing this variable. Optimum disk spacing
could be determined from a cost analysis comparing a larger
number of disks at close spacing versus a larger vat and shaft
for a smaller number of disks. The optimum spacing is probably
in the range of three centimeters.
Disk materials and color—Two testing sequences were used
to evaluate different disk materials and color. In the first
series, the sectioned reservoir was used so that disks of several
different types could be tested simultaneously under identical
ambient conditions. The disks were identical in size and the
same spacing was used with each group. The materials used were
aluminum (gray, well oxidized surface), white styrofoam, black
plywood and natural plywood. The evaporation rate was measured
under ambient outdoor conditions for a period of five days. The
results are shown in Table 9.
TABLE 9. 24 HOUR EVAPORATION RATE AS A FUNCTION
OF DISK COLOR AND MATERIAL
Material
Evap 5,/hr
Gray
Aluminum
0.101
White
Styrofoam
0.079
Black
Wood
0.097
Natural
Wood
0.100
Most of the materials produced essentially the same evapora-
tion rate. The only exception was the white styrofoam. In order
to gain an insight into the cause of the reduced evaporation
with the styrofoam disks, an experiment was initiated to evaluate
the effect of disk color. The two-chambered reservoir unit was
used and plywood disks painted white were compared with disks
painted black. Testing was done under ambient conditions with
evaporation rates measured during the middle of the day with
maximum sunlight, and at night. The results of these studies
are shown in Figure 38. Without sunlight the evaporation rates
were essentially the same. During daytime operation the black
plywood disks absorbed more direct sunlight radiation and pro-
duced a greater evaporation rate. During the daytime, the eva-
poration from the white plywood disks was about seventy-nine
percent of that of the black disks. The data in Table 9 was
obtained on 24 hr operation and the solar radiation was less
than one-half of the value during daytime testing. If color
were the only factor involved, it would be expected that the
white disks would evaporate about 90 percent as much as the black
on a 24 hr basis. Since the white styrofoam evaporated about 80
percent as much as the black plywood on a 24 hr basis, it can be
106
-------
§
JG
W
J-l
(1)
4J
•H
O
•H
-P
td
W
0.20
0.15.
0.10
0.05
_0.5
-0.4
CN
0.3
o
o
n
U.
0.1
black in
sunlight
*white in
sunlight
black or white
without sunlight
W = 3.36 kmph (2.1 mph)
R = 0.66 ly /min
s
0.5 rpm
0.7 submergence
2.5 cm(l in)spacing
f i i i i
1.0 2.0 3.0 4.0 5.0
6.0
Humidity Deficit (AH),gin/kg (lb/1000 Ib)
Figure 38. Evaporation as a function of adiabatic humidity
deficit for disks of different colors.
107
-------
concluded that a portion of the reduced evaporation of the styro-
foam was due to the white color and a significant portion was
due to the nature of the material. Duffie and Beckman (1974)
have shown that the absorbance of solar energy was 98 percent
for parson's black colored surfaces as compared to 26 percent
for acrytic white paint. The results in Figure 38 show that the
sunlight contribution to evaporation of the black disk was about
four times that of the white disks/ approximately the same range
as the reported value.
Effect of Direct Sunlight Radiation—
The effect of direct sunlight radiation has been assessed
using data from the long term operation of the unit under ambient
conditions. Data was selected with similar wind speed and varia-
tions in humidity deficit for daytime and nighttime measurements.
The results are shown in Figure 39. The differences in evapora-
tion rate due to direct sunlight presented in this figure
represent a maximum since they were developed from the data in
mid-summer during the portion of the day with the greatest
measured solar radiation.
The average evaporation attributable to direct sunlight as
determined from the figure is 0.044 £/hr/m2 (0.11 gal/hr/100 ft2)
of wetted disk surface for each Langley per hour, ly/hr, of
measured sunlight radiation. The above evaporation rate has
been expressed as a function of wetted area of the disk to
correlate with the aerodynamic (due to wind movement) evaporation.
The energy received by the unit from direct sunlight was some-
what variable throughout the day due to the changing angle of
the sun. In order to derive an expression for the sunlight
contribution to evaporation, it was assumed that the sunlight
energy available for evaporation was proportional to the vertical
projection area of the group of disks on the shaft. The ratio
of vertical projecting area to wetted surface area for the test
unit was approximately five percent as calculated below:
23 disks x 2.5 cm spacing x 0.52 m diameter _ Q Q51
23 disks x 0.6 frac. wetted x f (0.52 m) x 2 sides
2
The measured evaporation of 0.044 £/hr/m or 0.00073
Vmin/m2 of wetted disk area for an intensity of one Langley
represents a rate of 0.0146 JL/m2 of vertically projecting area.
One Langley is one gm cal/cm2 or 10,000 gm cal/nr of projected
area. Using these values, the sunlight energy required to
evaporate one gm of water is 10,000 gm cal/14.6 gm = 685 cal.
The theoretical heat of vaporization of water at the test con-
ditions is 588 cal/gm. The apparent efficiency of solar radia-
tion capture for the unit is approximately eighty-six percent.
although one would assume that the wetted, oxidized aluminum
disks would have a reflectance greater than 14 percent, the close
spacing of the disks causes much of the reflected energy to be
108
-------
CN
O
IO
03
}-)
(1)
-P
•rH
o>
.p
rd
O
•rH
-P
(0
n
O
ft
W
0.3.
R
Iy7min
0.71
0.60
0.62
0.80
0.00
0.00
7.0
8.0
Humidity Deficit (DH), gm/kg (lb/1000 lb).
Figure 39, Evaporation as a function of adiabatic humidity deficit for the aluminum
disk unit operating under field ambient air conditions.
-------
captured by adjacent disks and the liquid in the reservoir,
thus producing the high rate of solar energy capture.
Under the maximum sunlight conditions used in this experi-
ment, the direct solar radiation accounted for approximately ten
percent of the total evaporation. Considering full 24 hour
operation where sunlight is absent for approximately one-half of
the day, the effect of direct solar radiation is considered to
be relatively minor, less than five percent of the total evapora-
tion capacity of the test unit.
The solar radiation contribution to total evaporation would
be considerably less for full scale, prototype units. As the
disk diameter is increased, the area of the wetted surface which
controls aerodynamic evaporation is increased by the square of
the diameter. The projected vertical area controlling the solar
radiation capture increases with the first power of the diameter.
Prototype units would be approximately two meters (6.5 ft) in
disk diameter or approximately four times as great as the test
unit. The number of disks used in a prototype unit would also
be three or four times that used in the test unit. Increasing
the number of disks does not change the solar energy contribu-
tion with respect to aerodynamic evaporation. Increasing the
disk diameter and the number of disks in the prototype unit would
result in a reduction in the solar energy contribution to a value
in the range of two or three percent of the total evaporation.
Direct solar radiation is a relatively minor consideration in
the design of a rotating disk unit.
Comparison of Laboratory and Field Data—
Equations have been developed defining the evaporation rate
for the rotating disk unit, based on laboratory studies using
the wind tunnel (see Figures 33 and 34). The test conditions
included a 2.5 cm (1 in.) disk spacing and wind at 90° to the
shaft.
E(liter/hr.m2) = <0'0074 + °-0046 W(kmph)) AH(gm/kg)
or
E(gal/hr.lOO ft2) = (0'018 + 0'018 W(mph)) AH(lbs/1000 Iba)
Addition of the solar energy term results in the following equa-
tions :
E(liter/hr.m2) = (0.0074 + 0.0046 W(kmph)) AH + 0.88 Rg
or
A
E,__, ~_ ,nn *<_->, = (0.018 + 0.018 W, , .) AH + 2.1 R
(gal/hr-100 ft2) = ^Uie + u'u±0 w(mph)' *" + *••>• «s AVS
110
-------
The solar energy term utilizes the area relationship, Avp
(vertical projecting area) divided by Aws (disk wetted surface
area). This ratio was 0.05 for the test unit. The solar energy
term R0 is in ly/min.
t>
A comparison of the calculated results with data collected
for the unit operating under field ambient conditions is shown
in Figure 40. The values calculated from the equations developed
from laboratory measurement were slightly lower than the values
measured under field conditions. The reason for this was
probably due to the nature of the wind velocity. The three-cup
anemometer that was used measures only the horizontal component
of the wind. In the laboratory wind tunnel the wind direction
was essentially horizontal, but under field conditions, some
vertical movement of the wind was normal. The coefficients for
the evaporation equation adjusted to describe the field ambient
measurements are:
(liters/hr.m2) = (0.0074
°-005 W(kmPh))AH(gm/kg)
+ 0.88 R
S A
ws
or
E(gal/hr.lOO ft2) =
°'02 W
+ 2.1
(mph)'L
A
"sr2
ws
(lbs/1000 Ibs)
Comparison with Evaporation Equations—
Several empirical equations developed for field and lake
evaporation have the same general form as those presented for
the rotating disk unit. A comparison of the equations is shown
below and presented in graphical form in Figure 41 based on the
conditions of this study.
Assume: (AH = 5 gm/kg and total pressure = 84 kPa)
(AH = 5 lb/1000 Ib and total pressure = 0.83 atm)
This study - rotating disk evaporator
E(liter/hr-m2) ' (0-0074 + °-005 W(kn,Ph> >AH (gm/kg)
A
+ 0.88 R
S A
ws
111
-------
CN
cn
M
QJ
4-1
•H
0)
-P
00
O
•H
•P
rfl
w
.0.8
0.3-
. 0.6
0.2
0.1
-u
4-1
O
O
_0.4 «
. 0.2
a
o
V
w
2
4
1
0
4.5
7.7
mph
1.3
2.5
2.8
4.8
-s
ly/miri
0.71
0.60
0.62
0.80
filed ambient
calculated, E
7 J7 kmph
4.Okmph
2.1 kmph
= (Oi074+0.0046 W) AH+0.88
—I—
5.0
1.0
2.0
3.0
4.0
6.0
7.0
8.0
Humidity Deficit (AH), gm/kg (lb/1000 Ib).
Figure 40. Comparison of actual evaporation to calculated evaporation for the
aluminum disk unit receiving solar radiation.
-------
to
H
0)
-p
a)
-p
o
•H
-P
(fl
SH
O
P,
0.3_
0,2.
O.:L
0
. 0.8 .
(N
rotating disk
evaporator
Wind Speed fW), kmph,
Figure 41. Comparison of RDE evaporation equation with field
and lake evaporation equations.
113
-------
Rohwer (1931) - lakes
E(liter/hr.m2) = <°-0154 + °-0015 W(kmPh)'AH(gm/kg)
Penman (1948) - small tanks
E(liter/hr.m2) = "-0094 + 0.0014 W(kmph))AH(gm/kg)
Manciano and Harbeck (1952) - Lake Hefner
E/T-^. /, 2\ = 0.0012 W,, ,.AH, ., .
(liter/hr.m^) (kmph) (gm/kg)
The magnitude of the evaporation rate for the normal
ambient wind speeds of 0 to 5 kmph are generally the same using
the Rohwer, Penman and the RDE equations. The wind speed func-
tion for the rotating disk unit has a slope approximately two
times greater than that of the other equations. The reason for
this relates to the measurement of wind velocity. The measure-
ment for the rotating disk unit was the actual velocity at the
wetted surface while the other equations were based on wind
velocities measured at some distance above the air-water inter-
face, usually two to four meters. Since air velocity increases
with height above a lake surface, the coefficient of wind
velocity is lower in order to compensate for the higher wind
mea s ur emen t.
Cold Weather Operation of the Rotating Disk Evaporator—
The aluminum disk RDE was placed on the roof of the
Engineering Center during the month of February to study the
effects of cold weather on its operation. The unit was put into
operation and allowed to come to equilibrium. As the tempera-
ture began to drop during the late afternoon the unit was
constantly observed so as to determine when freezing on the
disks began to occur. At an air temperature of 4°C (39°F) the
mixture temperature reached 0°C (32°F) and ice began to form on
the surface of the disks. When allowed to continue operation
under freezing conditions, ice completely filled the one inch
space between the disks, forming a large, cylindrical ice mass.
The thawing of this ice mass took until late afternoon of the
following day with high sunlight intensities and temperatures
above 10°C (50°F).
An electric submersible tank heater was placed in the tank
of the aluminum disk unit to study the efficacy of heating the
tank mixture to prevent disk freezing under ambient winter con-
ditions. The 3000 watt heater was equipped with a thermostat
control to maintain a constant temperature in the tank. An
electric meter was attached to the heater to record total energy
consumption. The tank mixture was kept at temperatures of 4°C
(39°F) or above, to prevent freezing. The minimum air tempera-
ture that occurred during the study was -6 C (21°F). Figure 42
114
-------
x
e
EH
I
5-1
•H
(13
rt
•H
4J
C
(D
H
OJ
•H
Q
(1)
5-1
a
e
(!)
EH
10
5.
0
-5
-10
-15
.-10
tank volume = 0.1m
W = 4.65 kmph (2.9 mph)
i
0.2
0.4
0.6
0.8
Heating Power Requirement (kilowatts)
Figure 42. (T -T ) as a function of power consumed for the
aaluMnum disk unit using a tank heater.
115
-------
shows the difference in mean air temperature and mixture tempera-
ture versus the total power consumed in kilowatts. The figure
shows total power consumed for operation of the unit at 0.5 rpm
and at 0.0 rpm. A power savings of 0.31 kilowatts was obtained
by terminating disk rotation during periods when the air tempera-
ture was below 4°C, while using the heater to prevent freezing
of the tank mixture. This allowed normal operation of the unit
during periods when the air temperature was above 4°C (39°F)
without use of the heater and without any delay to allow for
thawing.
A 3000 watt, thermostatically controlled submersible
electric tank heater was placed in the tank of the 0.91 m (36
in.) diameter plywood disk unit in the laboratory wind tunnel
in order to study the effect of tank mixture temperature on
evaporation. Figure 43 represents the results of this study by
showing evaporation as a function of wind speed at different
mixture temperatures. As the temperature of the tank mixture
was increased, the evaporation rate increased. In addition, the
slope of the evaporation versus wind speed line was greater with
the use of the heater. This was due to the ability of the heater
to supply more power at higher wind speeds to meet the increased
potential for evaporation. The equation for rate of evaporation
as a function of temperature in the liquid reservoir can be
approximated with the equation
E(liter/hr.m2) = (°-01 + °-00075 W(kmph)) T(°C)
The data for Figure 42 were taken at a wind speed of 4.65 kmph
(2.9 mph). Approximately 0.054 KW was required for each one
degree centigrade rise in reservoir temperature for the unit
which had a wetted surface area of 6.2 meters2. Using these
values, the heat required for evaporation of the water has been
calculated as follows:
0 0542kw/°C X (0-01 + 0.00075 x 4.65)1°C = 1.57 liters/KWH
= 550 cal/gm
The energy requirement for evaporation of wastewater with
commercial heat sources was found to be approximately equal to
the heat of vaporization of the water when determined under
laboratory conditions where heat losses were at a minimum. For
outdoor ambient conditions, the energy requirement would be
greater.
Using an electrical cost of 4C/KWH, the cost of wastewater
disposal by this method would be 2.5C/liter (IOC/gal) or more,
an unacceptable value. Reservoir heating during freezing weather
with the rotating disk unit is not an economically feasible con-
sideration. Heating of the air passing over the disks would
also be economically unsound. It appears that the best method
116
-------
1.0
air temperature = 19.1°C (66.4°F)
AH = 4.5 gm/kg (lb/1000 Ib)
10
15
Wind Speed (W), kmph.
Figure 43.
Evaporation as a function of wind speed at various
tank mixture temperatures. Data taken from 36.0
inch dia. plywood disk unit.
117
-------
of operation would be to terminate operation and drain the unit
when the ambient air temperature drops below 4°C (39°F) and to
fill and operate the unit when the temperature rises again. This
operation will require a storage vault capable of holding the
wastewater flow through the maximum freezing weather period.
Continuous Operation of the Rotating Disk Evaporator—
The RDE with aluminum disks pictured in Figure 3 was
operated during June and July under outdoor ambient conditions
to assess the correlation of long term operational data with
recorded weather data, biological growth development on the disks
and possible problems with continuous operation.
Algae growth began to cover the end disks within the first
week and all disks were covered with algae within six weeks.
This was the only visible growth occurring on the disks. Exten-
sive algae growth also occurred in the reservoir. An interrup-
tion in the continuous operation of the unit, allowing the disk
to dry, eliminated algae growth on the disks.
During the period of operation, using primary clarified
wastewater, no odors from the unit were detected. The rotation
of the disks aerated the tank mixture and kept it aerobic, thus
preventing odors. The total solids and total volatile solids
were measured after 30 and 37 days of operation and the results
are shown in Table 10. The increase in total solids and total
volatile solids was approximately 1000 mg/£ per week and 100 mg/£
per week, respectively. A large fraction of the total solids
and total volatile solids was algae that had developed in the
tank.
TABLE 10. TOTAL AND TOTAL VOLATILE SOLIDS OF TANK MIXTURE
Sample
Primary effluent
Tank wastewater mixture
Tank Wastewater mixture
Days of
Operation
30
37
TS
mg/i
329
4008
5038
TVS
mg/i
127
918
1022
Comparison of Actual and Computed Evaporation—
The design of a rotating disk evaporator for a specific
location involves applying values of mean weekly or monthly
weather parameters in the evaporation equation. To determine
the validity of using mean monthly or weekly values, a comparison
between actual evaporation and computed evaporation was made for
data collected during the operation of the aluminum disk unit
under field conditions. The resllts are shown in Table 11. It
can be seen that the actual evaporation from the unit was
118
-------
TABLE 11. COMPARISON BETWEEN ACTUAL AND COMPUTED EVAPORATION USING
MEAN MONTHLY AND MEAN WEEKLY WEATHER PARAMETERS
From
5/31
5/31
6/30
To
7/5
6/7
7/5
At
hr
819.75
175.75
131.00
¥d
£
689.01
163.26
132.00
Ta
°C
25.3
26.1
26.5
r.h.
%
36.7
29.4
36.0 .
AH
gin/kg
4.10
4.85
4.49
W
kmph
3.7
3.2
4.0
Rs
ly/min
0.333
0.357
0.344
actual
mm/hr
0.13
0.14
0.16
E
comp
mm/hr
0.12
0.13
0.14
E
pan
mm/hr
0.29
0.32
0.34
vo
2
wetted area = 6.6 m A
E = computed evaporation = (0.0074 + 0.005 W)AH + 0.88 R
comp r r s
¥, = volume evaporated over the time period
-------
slightly greater than the value computed from the equation. The
evaporation rate per unit of wetted area for the unit was found
to be about 0.44 times that of the rate measured using the
standard class A pan at the same location.
The comparison of disk evaporation per unit area with pan
evaporation was made for many of the short term tests that were
used in the study of operating variables. This correlation is
shown in Figure 44. The effect of sunlight is greater in pan
evaporation because solar radiation strikes the total wetted
surface. The effect of wind speed is also different for air
moving between the disks and air moving over the water surface
in the pan. In spite of these differences, the correlation in
the figure is relatively good and gives a value for disk evapora-
tion per unit wetted area of 0.42 times the pan evaporation, a
figure very similar to that derived from long term data.
Investigation of Health Hazards and Odors--
No objectionable odors were detected in the air exiting the
wind tunnel during operation of the rotating disk evaporators in
the laboratory wind tunnel. Primary effluent was used during
these studies. The rotation of the disks sufficiently aerated
the wastewater to prevent it from becoming anaerobic and pro-
ducing odors. When the unit was stopped for more than 24 hours,
odors began to develop. This problem can be eliminated by
draining the unit when not in use or by operating the drive
mechanism for at least a short period each day.
The possibility that wind blowing over ihe rotating disk
may contain bacterial aerosols which would present a health
hazard was recognized and a brief evaluation was made. Two
types of tests for coliforms were made. In one test, a membrane
filter apparatus was set up at several distances, 0.15 m, 1.5 m
and 3 m (0.5 ft, 5 ft and 10 ft) from the downwind edge of the
rotating disks. Air was drawn through the filter while the unit
was operating. After a period of operation, the membrane filters
were applied to total coliform media and incubated. The second
test involved the use of membrane filters in dishes containing
total coliform media placed at the same distances from the unit.
During operation, aerosol falling from the air passing over the
disks could drop onto the filters. After designated periods of
operation, the filters were incubated and counted.
No coliforms were measured at any of the distances using
the first technique. The second method produced coliform
measurements immediately adjacent to the unit at 0.15 m (0.5 ft)
but not at greater distances. The results are shown in Table 12.
From this preliminary testing, it can be concluded that
bacterial transport in aerosols is not a serious health problem.
120
-------
o
•H
-P
id
5-1
O
Q
0.25.
0.20.
0.15.
0.10.
0.05
0
CM
0.6 §
0.4 g,
gal /hr/100 £t .
0.6 0.8 1.0
0.2
0
f
0.1
0.2 0.3 0.4 0.5
0.6
Pan Evaporation Rate, mm/hr.
Figure 44. Evaporation as a function of pan evaporation for the
aluminum disk unit.
121
-------
TABLE 12.
RESULTS OF MODIFIED TOTAL COLIFORM
MEMBRANE FILTER TESTS
Total Coliforms*
At
hr
W
kmph i Blank 0.15m 1.5m 3m
: i.o
2.5
2.0
2.25
2.0
1.0
3.1
3.1
3.1
7.6
12.2
7.6
0
0
0
0
0
0
5
27
20
2
0
4
-
0
0
0
0
-
0
0
0
0
Total coliforms - modified membrane filter test,
petri dish with media pad, agar and filter exposed
to wind at tank level. Dishes incubated for 24
hours at 37°C.
Concentric Cylinder Evaporator (CCE)
The functioning of the concentric cylinder unit involves
the movement of pressurized, unsaturated air through layers of
wetted burlap. The operation results in adiabatic humidification
and cooling of the air with heat transfer to the liquid producing
mass transfer of the evaporated water to the air stream. During
the adiabatic process, the air in contact with the water surface
will exit in a condition near saturation and the water surface
temperature will reach an equilibrium near that of the adiabatic
saturation temperature of the air entering the system. A mass
balance for the system will yield a relationship for the amount
of water transferred from the water surface to the air.
m H. = m H - w
i o
where m = Mass flow rate of air
H.,H = Humidity of air in and out of system, respectively
w = Mass rate of evaporation
For the adiabatic process, the mass rate of evaporation can be
expressed as
w = m AH2
This equation can be rearranged to the form:
E = K'QpAH
122
-------
where EC = Evaporation from CCE, £/hr or gal/hr
K1 = Conversion constant
Q = Volumetric flow rate of air, £/hr or ft3/hr
p = Density of air, kg/ft or Ib/ft
AH2 = Adiabatic humidity deficit of air entering unit,
gm/kg or lb/1000 Ib
The equations are based on an isolated system with the only
heat entering or leaving the system being associated with the
piped air flow. In the operation of the test unit, two other
sources, the ambient air flowing over the cylinder and solar
radiation on the cylinder, had a small but significant effect.
These sources were accounted for by adding terms to the evapora-
tion equation.
Mixture Temperature—
The relationship between the bulk mixture temperature and
the adiabatic saturation temperature of the supply air for the
concentric cylinder unit operating under both field and labora-
tory conditions is shown in Figure 45. The mixture temperature
was within 3°C of the adiabatic saturation temperature of the
supply air for most cases and therefore the adiabatic humidity
deficit accurately describes the water vapor gradient above the
water surface.
Air Flow Rate—
The evaporation rate from the concentric cylinder unit was
directly proportional to the mass or volumetric flow rate of the
supply air entering the unit. The relationship between evapora-
tion from the concentric cylinder unit operating under laboratory
conditions and the air flow rate at different adiabatic humidity
deficits of the air supply is shown in Figure 46. Since the unit
was exposed to the ambient air of the laboratory, there was a
small evaporation rate from the wetted fabric surface at zero
air flow rate.
Adiabatic Humidity Deficit—
The mass balance equation presented previously was based o*.
the assumption that, as the air passed through the saturated
fabric, it was saturated with water vapor. Evaporation was
therefore assumed to be directly proportional to adiabatic
humidity deficit. The evaporation rate as a function of adia-
batic humidity deficit for the concentric cylinder unit operated
at four different air flow rates in the laboratory is shown in
Figure 47. The evaporation rate is shown as a linear function
of adiabatic humidity deficit. The intercept of the curve is
slightly above the origin due to a small effect of the ambient
air passing over the surface of the cylinder.
123
-------
o
§10
E-i
V
0)
rd
dJ
P*
CD
5-
55
o laboratory data, T = 20 C
a field data, T = variable
50
-40
cP
50
55
I
0
10
15
Supply Air Saturation Temperature, Tssif°c.
Figure 45. Concentric cylinder unit bulk mixture temperature
as a function of the adiabatic saturation
temperature of the supply air.
124
-------
W
n
0)
-P
-H
(D
4J
nj
PS
C
O
-H
0
0,
(0
Air Flow Rate (Q) , m /rain,
Figure 46.
Evaporation as a function of air flow rate at
different supply air himidity deficits for
CCE in the laboratory.
125
-------
M
^
OJ
-P
•H
Q)
4J
C
o
•H
4J
(C
(0
>
w
_0.4
.0.3
l.Q-
0.5
en
1-0.2
-0.1
o
o
a
A
Q
2.7 nu/min (91 cfm)
2.2 m /min (72 cfm)
1.3 m3/min (43 cfm)
0.9m /min (29 cfm)
0
10
Humidity Deficit of Supply Air, AH2, gm/kg(lb/1000 Ib)
Figure 47. Evaporation as a function of adiabatic humidity
deficit of supply air for CCE in the laboratory.
126
-------
The major variables influencing evaporation from the con-
centric cylinder unit have been shown to be the flow rate and
humidity deficit of the air piped into the system. The equations
relating these variables were established from the slope of the
lines in Figures 46 and 47.
(S.I. units) Ec U/hr) = 0.0684 Q (m3/min) ^2 (gin/kg)
(Engl. units) EC (gal/hr) = 0.00055 Q (cfm) AH,, (lb/100(J lb)
The effect of the other factor influencing evaporation
rate, the ambient air speed, humidity deficit, and direct solar
radiation were investigated in order to provide a more complete
equation describing system performance.
Effect of Wind on Evaporation from CCE—
The concentric cylinder unit was operated in the laboratory
wind tunnel at three different piped air flow rates, three
different ambient air wind speeds, and a single adiabatic
humidity deficit of the supply air in order to determine the
effect of ambient wind speed on evaporation. The relationship
between evaporation and wind speed is shown in Figure 48. The
increase in evaporation was the same for each air flow rate at
a constant AH-. The increase can be expressed as
AE/W = 0.0165 liters/hr-kmph, for the unit
AE/W = 0.007 gal/hr-mph, for the unit
at a AH2 = 5.49 gm/kg (lb/1000 Ib). The exposed wetted fabric
area was 0.7 m2 (7.2 ft2), making the ratio of E/W = 0.024
Vhr.m2.kmph (0.001 gal/hr.ft2-mph). The assumption has been
made that the effect of wind speed is directly proportional to
AH]^. This was found to be true with the rotating disk unit and
is incorporated in the field evaporation formulas presented in
the previous sections. On this basis, the effect of wind speed
becomes, AE = 0.005 AHW when using SI units, and AE = 0.0002 AHW
for English units.
Effects of Ambient Air Humidity Deficit—
The adiabatic humidity deficit of the surrounding air will
affect evaporation from the surface of the outer cylinder. The
mixing of the saturated supply air and the ambient air at the
surface of the outer cylinder causes an increase in the water
vapor gradient at the surface which affects evaporation.
To show the effects of the surrounding air under constant
operating conditions, the cylinder was fitted with a cover con-
structed from one-half of a 55 gallon drum. The cover matched
the reservoir and fitted snugly over the unit except for a slot
that allowed the air to escape. The cover prevented ambient air
127
-------
M
0)
-P
•H
0)
4-)
C
O
•H
-P
O
CU
rd
>
W
1.5--0-4
_Q.3
1.0
0.5_
0
fd
Cn
LQ.2
Q.I
Q
A 2.6 m3/min (87 cfm)
a 2.1 itu/min (70 cfm)
o 1.0m /min (32 cfm)
AH-,= 4.85 gm/kg
A Hr= 5.49 gm/kg
mph
6
10
T
10
Wind Speed (W), km/hr.
Figure 48.
Evaporation as a function of wind speed for CCE
unit in the laboratory wind tunnel.
128
-------
from contacting the cylinder. Data from an open unit and a
covered unit were plotted as shown in Figure 49. The evaporation
from the open unit was consistently higher than evaporation from
the covered unit. The amount of increased evaporation from the
open concentric cylinder unit appeared to be a constant under
laboratory conditions.
Ambient wind movement in the laboratory studies was less
than one km/hr and therefore the increase in evaporation with
the uncovered unit is considered to be due to the humidity
deficit of the ambient air. It has been shown, for the piped
air of the concentric cylinder unit and the ambient air of the
disk unit, that evaporation is proportional to humidity deficit.
The average evaporation at zero wind speed was approximately 0.1
£/hr.unit at an ambient air humidity deficit of 4.4 gm/kg. This
results in a value of 0.032 £/hr.m2-AH or (0.008 gal/hr•ft2-AH)
to be used in the evaporation equation.
Effect of Solar Radiation—
Direct solar radiation absorbed by the moist outer surface
of the cylinder adds energy to the system that results in
increased evaporation. Several determinations of evaporation
rate were made outdoors under field conditions during daylight
hours in the summer. Data for two different solar radiation
intensities were selected where all other operating variables
were essentially constant. Solar radiation was measured with
a recording pyranometer and the units of measurement are
Langleys/min (cal/cm2.min). A plot of the data is shown in
Figure 50. The dashed lines are from calculated data without
the effect of solar radiation.
(S.I. units) E,, = Ac(0.032 + 0.005 W)AH, + 0.0684 QAH0
Co -L £•
(English units) £„ = Ac(0.0008 + 0.0002 W)AH, + 0.00055 QAH0
Co X ^
2
where A_ = exposed, wetted surface area, 0.7 m for the test unit.
The evaporation due to direct solar radiation measured from
the curve is 0.46 H/hr at an RS of 0.69 Ly/min and 0.10 £/hr at
0.14 Ly/min or
(S.I. units) AE = 0.67 Rc = 0.96 A^R-
vap o o D
(English units) AETa = 0.18 Rc = 0.025 AqRQ
vap o o t>
The unit evaporated an average of 460 gm of water due to a
solar radiation of 0.69 ly/min x 60 min/hr x 0.7 m2 x 104 cm2/m2
= 289,800 calories, or 630 cal/gm of H2O. The water temperature
was 12°C. At that temperature, the heat of vaporization of water
129
-------
0)
-p
(0
G
o
o
a
flj
>
W
2.0_
1.5
1.0 _
0.5
H2(gm/kg)
9.33
4.07
8.66
3.91
covered 36.6 C ( 97.4°F)
covered 13.7 C ( 56.6°F)
33.4°c ' ~~ "°
13.5°C ( 56.2~F)
4.3 gm/kg
40 cfm
I
Air Flow Rate (Q), m /min,
Figure 49.
Evaporation as a function of piped air flow rate for
a covered and an open CCE operating in the
laboratory.
130
-------
•c
x.
CO
^
0)
.p
•H
0)
-P
(ti
o
•^
-P
n3
M
O
Oi
nfl
>
W
2.0-
1.5
1.0
0.5.
-0.5
.0.4
\
rH
(fl
0.3
0.2
0.1
0
a
A
I, (gm/kg} R (ly/min)
~i ~^o
4.59
2.74
0.69
0.14
AH2= 6.27 gm/kg (lb/1000 Ib)
W = 3.0 kmph (1.9 mph)
^
X
calculated with AH,,=4.59 gm/kg
/^calculated with AH =2.74 gm/kg,R = 0
-L S
cfm
40 60
80
Air Flow Rate (Q), m /min.
Figure 50.
Evaporation as a function of piped air flow rate for
the CCE operating under field ambient conditions.
131
-------
is approximately 590 calories/gm. The reflected solar radiation
was apparently about six percent which is within the range to be
expected for a dark colored, rough fabric surface.
From Figure 50, it appears that solar radiation is a signi-
ficant source of energy for evaporation with this type of unit.
The data for the figure was obtained during the sunny portion of
a summer day. On a full day, year round basis, direct solar
radiation is not as significant and the capacity of the unit is
related primarily to the flow rate and saturation deficit of the
piped air.
Machine Variables—
Two machine variables were evaluated, the depth of submer-
gence of the drum and the drum rotational speed. Maximum
evaporation rate occurred when submergence was great enough to
wet all three layers of wraps in the cylinder. This is shown in
the results presented in Figure 51. The rotational speed of the
drum had a small effect on the evaporation rate as shown in the
results in Figure 52. Higher rotational speed kept the cloth
slightly wetter and increased vapor transfer a small amount.
Cold Weather Effects
Operation in cold weather is dependent to a large extent on
the temperature of the piped air. If ambient air is piped into
the cylinder, the minimum operating temperature will be reached
when the adiabatic saturation temperature of the piped air
reaches freezing. This condition is similar to that described
for the rotating disk unit and freezing will occur when the air
temperature reaches 4° to 5°C. The test unit utilized pressurized
air from the laboratory at a temperature of 20°C. No freezing
problems were encountered with operation at an ambient air
temperature of 1°C. Lower ambient temperatures could probably
be used with the unit, but 1°C was the lowest ambient temperature
encountered during this portion of the study.
Operational Problems—
No odors were detected from the concentric cylinder unit
during operation under laboratory or field conditions. Primary
clarified municipal wastewater was used during all studies. A
dark mold developed on the surface of the outer burlap cloth
cylinder. The cloth easily decayed and lost its strength. Any
attempt to clean the outer cloth was unsuccessful and led to its
replacement. No pore clogging problems were encountered with
respect to the inner cylinders. The inner cloth did begin to
decay, but not as rapidly as the cloth on the outer surface. For
a prototype unit, a fabric would be required which can resist
corrosion and be easily cleaned. A stainless steel or synthetic
fiber fabric could be used.
132
-------
(U
•p
•H
rH
o
•H
-P
rd
>-t
O
CU
fti
l.Q,
0.8.
0.6.
0.4.
0.2.
0
0.2
Cn
0.1
0.2
0.4
0.6
0.8
1.0
Submergence (fraction of radius under water in vat)
Figure 51. CCE evaporation as a function of submergence.
133
-------
1.5-f
- 0.3
= 4.55 gm/Kg
AH2= 5.43 gm/Kg
20
40
cfm
60
80
1
Q
1 I
i
1
l I
i
2
3
Air Flow Rate (Q), m^/min
Figure 52.
Evaporation as a function of air flow rate at differ-
ent rotational speeds for the CCE operating in
the laboratory.
134
-------
Equations for Evaporation Rate from the CCE—
The machine variables have a very small effect on the
performance of the unit. For a cylinder submergence great
enough to wet the inner fabric wraps and a rotational speed of
about one or two RPM, the expression developed in this section
based on air flow rate Q (m3/min/ cfs), supply air humidity
deficiency AH2 (gm/kg, lb/1000 Ib) , ambient air humidity
deficiency AHjL (gm/kg, lb/1000 Ib) and solar radiation Rg
(ly/min), adequately describe the process.
(S.I. units) E,
(English units) E
U/hr) = A (0.032 + 0.005 W)
+ 0.0684 QAH2 + 0.96
(gal/hr) = Ag (0.0008 + 0.0002 W)
+ 0.00055 QAH2 + 0.025 A
It should be noted that the evaporation rate equations
above describe the evaporation rate of the total unit. If these
relationships are divided by the area of the outer burlap surface
covering, AS/ an expression similar to that of the RDE equations
results and evaporation, Evap, is expressed as a function of a
unit area of the burlap surface.
(S.I. units) E (£/hr-m) = (0.032 + 0.005 W(kmph) )
n
0.
0.96
Q
/_v
(m )
(English units) EV& (gal/hr. 100 ft) = (0.08 + 0.02 ( h) )
+ 0.055 a
+ 2.5 Rs
Only one size concentric cylinder evaporator was used in
the studies and therefore the use of the above equations for
designing a larger prototype unit, with a larger value of Ag,
was not experimentally verified.
135
-------
SECTION 8
DESIGN METHODS, NATIONAL APPLICATIONS AND COSTS
EVAPOTRANSPIRATION BED DESIGN
The design criteria for an ET bed are highly site specific
and take into account as major considerations the water use
pattern in the home, acceptance criteria, and local weather
parameters.
The water use pattern can be estimated based on the type
of facility. For a single family home, a flow of 170 liters
(45 gallons) per person per day has been established, resulting
in a design flow rate per dwelling unit, based on five occupants
per home, of 850 liters (225 gallons) per day. Although some
minor daily and seasonal variations occur, this flow rate can be
assumed to be constant throughout the year. The use of water
saving appliances and plumbing devices can reduce the design
flow by one-third. Because of the direct cost savings realized
with reduced ET bed size, these devices should be considered in
the design of any ET system. Use of water saving devices result
in a reduced design flow of 600 liters (150 gallons) per day for
a house. Flow estimates based on the number of bedrooms in a
house tend to result in overestimates for flows from large homes.
Apparently, the number of bedrooms in a home and the number of
potential occupants is not closely correlated.
An attractive application of evaporative systems is for
installations where flow rates are higher during the summer
months. These systems take advantage of the higher evaporative
potential available in warm weather. Wastewater disposal for
summer homes, many types of recreational areas, and highway rest
areas are some of the examples of this application. Generally,
summer home flow rates are similar to those of permanent homes
except that the period of use is limited to the summer months.
Other applications are based on the specific conditions of the
site use; consequently the flow rate must be established for
each individual installation.
Variations in acceptance criteria produce a wide range of
design values and sometimes makes it difficult to compare design
loading rates for different areas. If the acceptance criteria
are based on a totally evaporative, non-discharging system, the
loading rate must be low enough that the bed never completely
136
-------
fills, assuming that all precipitation enters the unit and does
not run off. This is the criteria that was used in the evalua-
tion of loading rate for the lysimeter studies in this report.
The acceptance criteria may allow for lateral or vertical seepage
in conjunction with evaporation. The design loading rate for the
combination concept can be considerably higher and the incremen-
tal increase is dependent upon the allowable seepage rate. The
acceptance criteria may allow occasional surface runoff during
precipitation events as long as nuisance conditions do not result.
This approach will generally result in increases in loading rate
of fifty to one hundred percent above that for the non-discharging
system. The acceptance criteria are usually set by local health
authorities to accommodate the needs of a particular area. The
requirements to be met for an isolated farm home in a rural area
may be quite different from those for homes in suburban subdivi-
sions. The non-discharging criteria may be required in the latter
case where the homes are in relatively close proximity, expecially
if well water supplies are involved. Higher loading rates and
less demanding criteria may be acceptable for the more rural
applications.
The weather parameters are major considerations in the
design of an ET system. These include precipitation and evapora-
tion rate data. Rainfall and snowfall measurements are available
from NOAA for thousands of weather stations throughout the
country. Many local agencies also maintain records. A critical
wet year should be used for design based on at least ten years
of records.
Establishing evaporation data at a specific location can be
a more difficult problem. Measurements of class A pan evapora-
tion are reported for all of the states by NOAA in the publica-
tion, "Climatological Data", U.S. Department of Commerce available
in repository libraries for government documents at major univer-
sities in each state. Pan evaporation measurements are made at
a few (five to thirty) weather stations in each state. Data for
the winter months is often omitted because this method cannot be
used under freezing weather conditions. The critical period of
the year for design of ET units for permanent homes is in the
winter. Establishing representative winter evaporation data is
probably the most difficult part of ET bed design analysis. The
application of ET beds is most favorable in the warm dry climates
of the southwestern U.S. For these areas, pan evaporation data
are available for the complete year. The analysis of evaporative
potential for cooler, semi-arid regions such as eastern Washington
and Oregon, Utah, Colorado and similar areas requires that winter
data be established by means other than pan evaporation measure-
ments since these data are generally not available.
One method for establishing representative winter evapora-
tion data is to take measurements on buried lysimeters in a
manner similar to that used in this study. Another method is to
137
-------
use the empirical formulations such as the Penman formula or
others. Most of the empirical formulas were developed for cal-
culating evaporation during the growing season and may not apply
directly to winter circumstances. It was found in this study
that the Penman formula did give results comparable to the
measured winter values. A graphical method has been reported by
Dalinsky (1975) that utilizes the sinusoidal nature of solar
radiation to give monthly evaporation patterns. Based on studies
in Israel, he noted that the mean evaporation for the year was of
the same magnitude as that measured for the months of April or
October and that the monthly variations form a sine curve with a
maximum in July and a minimum in January. For most locations,
data is available for the months of April, July and October.
Using the April/October data as the annual average, the addi-
tional amplitude in July can be subtracted from the average to
establish the minimum value in January. If the April/October
evaporation rate is 5 mm/day and the July rate is 8 mm/day, the
estimated January value would be 5 - (8-5) =2 mm/day. Ward
(1977) has illustrated the use of this technique for Denver,
Colorado. In general, the method gives reasonably good correla-
tions for the portion of the U.S. that is east of the Rocky
Mountains but it does not appear to be applicable in western
states where weather is influenced by the mountainous topography.
After the precipitation and evaporation curves for an area
have been established, the design is based on an allowable liquid
loading rate per unit of surface area. Based on the data
established during the study period, the allowable loading rate
for year round operation of a non-discharging system in the study
area was found to be 1.6 mm/day (0.04 gpd/ft^). This is shown in
the shaded portion of Figure 53, which was constructed from the
data in Table 6. This is the same value as that established in
the more detailed experimental study. This analysis was pre-
sented to show that, in this case, the use of monthly weather
data provides results that concur with those of the detailed
experimental lysimeter studies. The allowable loading value was
established, based on the proven assumption that an ET system
has very limited storage capacity, and therefore the evaporation
rate in the winter months must be great enough to remove all
entering wastewater and precipitation. In addition, winter
evaporation rates from an ET bed are nearly equal to the measured
evaporation values from pan or lysimeter measurements.
Based on the evaporation rates established in this way, the
size of ET bed required for a non-discharging system in a per-
manent home without flow reduction devices would be (850x 10" 3
m3/day)/(1.6 x 10~3 m/d) =530 m2 or 5700 ft2. This is approxi-
mately the size of the home ET beds in use in the Boulder area.
The value of 1.6 mm/d was established on the basis of a single
winter of study during a period when a drought condition existed.
The amount and distribution of precipitation are quite variable
throughout a long period in any given area. In order to assess
138
-------
Boulder, Colorado
10
ran/day .
evap.
precip.
Figure 53. Curve for establishing permanent home loading rate
for Boulder, Colorado based on winter data
1976-1977.
139
-------
the influence of changing weather conditions, the monthly rain-
fall patterns should be evaluated for at least the previous ten
years. This analysis for the Boulder/ Colorado study area is
shown in Figure 54. It would be desirable to have measured
evaporation values for each month of each year. Evaporation
data is so difficult to establish, especially in the winter
months, that this was not possible. The measured values for the
1976 study were used for each of the years in the figure. In
general, the reported pan evaporation values for most stations
listed in "Climatological Data" are much more constant from year
to year than are the corresponding precipitation data. For this
reason, the use of one year of representative evaporation values
appears to be justified.
It can be noted from the figure that during 1976 Boulder
was in a drought cycle and the data does not represent a critical
year for design purposes. The late spring storms in 1969, 1970
and 1971 provide the most restrictive period in the ten year
sequence. In April of 1971, precipitation equaled evaporation
and an ET bed of any practical size would have produced some
runoff during that period. Although ET beds have been used
successfully in the area for the past five years, this location
must be considered as highly questionable for truly non-
discharging systems in the long term application.
This raises an important question regarding the concept of
ET bed design and acceptance criteria. Very large precipitation
events can occur at very infrequent intervals at any location.
ET beds are designed for use over a period of decades and there-
fore will encounter some unusually large rainfall or snowfall
occurrences. For this reason, a fail safe, truly non-discharging
system is probably not possible anywhere in the U.S. A precise
definition of acceptance criteria is difficult to establish.
Failure of a system may involve contamination of groundwater
from seepage, the health hazard or nuisance potential of surface
runoff, or the inconvenience to the homeowner when the system
cannot accept the amount of wastewater normally generated. The
relationship between these conditions and the weather variables
used in design can only be established with long experience with
actual systems.
The use of ET systems for summer homes is a very attractive
application in the western states of the U.S. A much smaller
bed is required and the cost becomes more competitive with
leaching fields and other alternatives. It can be determined
from Figure 54 that 1967 is the critical year of the ten year
period for summer evaporative conditions. A graph of these data,
as shown in Figure 55, is used to establish a design wastewater
evaporative rate of 4 mm/d (0.10 gpd/ft) for the case of summer
home design where only the months of June, July and August are
considered. ET bed evaporation rate has been shown to be only
about 75-80 percent of the pan evaporation rate in the summer
140
-------
ram/d
1967
1968
1969
1970
1971
Boulder, Colorado
10 J.
nrn/d
5 J
' - i *
- K/\A,...,; y
' \ A
v-
1 1—
1972 1973 1974
1975
1976
Figure 54. Ten year evaporation and precipitation pattern for
Boulder, Colorado.
141
-------
10 j
mm/d
5 -
Boulder, Colorado
4 ntn/d
evap.
precip.
J J
1967
D
Figure 55.
Curve for establishing summer home loading rate for
Boulder, Colorado based on critical year
data of 1967.
142
-------
months. On this basis, the required bed size for summer home
application in the study area would be (850 x 10~^ m-^/d)/(4 x .8
x 10~3 m/d) = 265 m2 or 2850 ft^ without the use of water saving
appliances, or 175 m^ (1900 ft^) or less if water conservation
is practiced.
NATIONAL APPLICATIONS OF ET SYSTEMS
The use of ET beds for year round application for permanent
homes can be assessed from plots of the weather variables for
different locations in the nation. This provides only a general
overview and individual analyses must be made at each potential
site based on localized weather data and acceptance criteria.
Several locations were selected throughout the country and plots
are presented that represent the critical precipitation year from
a ten year span from 1967 to 1976. Data for the figures were
obtained for each state from the NOAA publication "Climatological
Data".
The data for three locations in the eastern U.S., southern
Florida, northern Georgia and upstate New York are shown in
Figure 56. It can be noted that in all of these locations,
there were several months when precipitation exceeded evaporation.
The use of non-discharging ET systems would not be acceptable for
either permanent homes or summer residences at these locations.
Combination seepage and evaporation systems would function
primarily as seepage units with only a small fraction of the
water disposal due to evaporation. A similar situation exists
in the midwestern states as shown in Figure 57.
The application of ET systems in the intermountain valleys
between the western coastal mountains and the eastern front range
of the Rockies is much more favorable. The high plains adjacent
to the eastern edge of the Rocky Mountains is an area with eva-
poration exceeding precipitation in all months of the year as
shown in Figure 58. The concept of using ET units for permanent
homes is applicable for some limited areas of Wyoming, Colorado
and New Mexico, although the required size of beds may be quite
large. The use of ET systems for summer home installations would
be acceptable. The weather data for several other eastern areas
are shown in Figures 59, 60 and 61. El Paso, Texas is one of
the most arid places in the U.S., but it is subject to quite
infrequent, very high intensity storms. One such storm occurred
during the analysis period in 1974. If it were not for the
September 1974 rainfall, the analysis would show that this area
would allow one of the highest loading rates for ET systems in
the country. This is an example of the considerations that must
be taken into account with acceptance criteria. Saltair, Utah
shows very marginal conditions for December, January and February
of the critical precipitation year. Fallon, Nevada is an example
of another very arid region of the U.S.
143
-------
15-
10
mm/d
0
-n
Hialeah, Florida
Evap
Precip
:i—>—'—r
J J
1962
D
Univ of Georgia
Plant Science Farm
5 -
mm/d
mm/d
N
\
Canton 45E
New York
Precip
Evap
J J J
1972
D
Figure 56.
Precipitation-evaporation plots for eastern
US locations.
144
-------
10 _
mm/d
5 -
0
Arlington Univ Farm
Wisconsin
--d
J V ' '
1973
D
10 —,
mm/d
5 _
0
Martin Dam
Alabama
precip
evap
D
10 -i
5 _
mm/d
J ' ' ' 'J'J
Austin WSD AP
Texas
D
1973
Figure 57. Precipitation-evaporation plots for midwest locations,
145
-------
10 -^
mm/d
5 _
0
mm/d
10-,
mm/d
5_
Pathfinder Dam
Wyoming
D
1973
i—i—i—i——i—i—i—«
Alamosa WB Airport
Colorado
Evap
Precip
D
1969
Clovis
New Mexico
D
1969
Figure 58. Precipitation-evaporation plots for Rocky Mountain
locations.
146
-------
10 -4
mm/d
5_
0
10 -,
mm/d
5 -
0
Vsleta (El Paso)
Texas
Evap
Precip
Saltalr Salt Plant
Utah
Evap
Precip
D
1968
mm/d
10 -
5_
0 ._
Fallen Exp. Station
Nevada
Figure 59.
Precipitation-evaporation plots for inter-
mountain locations.
147
-------
10 -
rrm/d
0
J J
1972
Yurna Citrus Station
Arizona
evap.
(iprecip.
D
10 _
irm/d
5 _
0
10 _<
5 _
Tucson, Univ. of
Arizona
evap.
precip.
Citrus Station
Arizona
evap.
precip.
Figure 60.
Precipitation-evaporation plots for Arizona locations,
148
-------
mm/d
mm/d
10 -
0
mm/d
Precip
Puyallup
Washington
Evap
Prosser
Washington
Evap
Precip
D
San Luis Dam
California
1969
Figure 61. Precipitation-evaporation plots for western US locations
149
-------
Yuma and Tucson have very high winter evaporation rates.
They represent an area that would have a maximum loading rate for
the U.S. For most years, a loading rate of 3.2 mm/d (0.08
gpd/ft2) could be used. The data for the critical year of 1972
show an unusual storm in October. Tempe, Arizona which is in
the same geographical area, has much lower winter evaporation
rates and greater precipitation and thus would be a very marginal
area for the use of non-discharging ET beds. This points out
the importance of individual analysis for any particular location.
The winter evaporation rate in California is generally too
small to allow for the use of ET beds for permanent homes. San
Luis Dam in the central portion of the state has extremely high
temperatures and evaporation rates in the summer, but winter
precipitation exceeds the evaporation rate.
Two stations in the State of Washington are shown for
contrast. Puyallup, near Seattle, has very high winter preci-
pitation conditions while Prosser, on the eastern plains, has
conditions of evaporation exceeding precipitation for all months.
The available excess evaporation capacity in the winter months
at Prosser is limited and the use of non-discharging ET beds
would be marginal.
A generalized summary of the national applications of non-
discharging ET bed systems is shown in Figure 62. Although each
location should be evaluated on an individual basis, the map
gives an indication of the regions where application may be
feasible. The areas on the west coast and all of the eastern
U.S. have weather conditions where precipitation exceeds evapora-
tion during many months of the year, making the system infeasible.
The southern intermountain region (cross-hatched) has conditions
that may be favorable for the use of the ET concept. Another
area, separating the two zones (single hatched) has conditions
that are favorable for summer home applications and marginal for
permanent home units.
It can be noted that the favorable areas are a significant
part of the land area of the country but that the area contains
only a small percentage of the unsewered homes of the nation.
Another consideration in the overall feasibility of ET systems
is that they are applicable in semi-arid and arid regions where
water has a high value. Disposing of wastewater in a totally
consumptive manner as is done with ET beds may not be a desirable
alternative from a total water resource standpoint.
COST ANALYSIS FOR ET SYSTEMS
The cost of construction of an ET bed is almost directly
proportional to its surface area, which is a function of the
design loading rate. The loading rate for non-discharging, per-
manent home units constructed in areas where the concept is
150
-------
( I!
Summer hone applications;
marginal for permanent homes
Permanent and summer home
applications
Figure 62. Areas of potential use of non-discharging ET beds in the U.S
-------
feasible range from 1.0 mm/d (0.025 gpd/ft2) to 3.0 mm/d (0.075
gpd/ft2). Using a wastewater generation rate of 850 H/d (225
gal/day) for a home, ET bed size requirements would range from
280 m2 (3QOO ft2) in southern Arizona to 840 m2 (9000 ft2) in
more northern regions such as Colorado and Utah. The use of
water saving plumbing devices can reduce these requirements by
approximately one-third.
The design loadings for summer home applications are
generally about four times greater than that for permanent homes
and can range from 1.0 mm/d (0.025 gpd/ft2) to 12 mm/d (0.3
gpd/ft2) with corresponding bed sizes of 840 m2 (9000 ft2) to
70 m2 (750 ft-*) . Water saving appliances could reduce the bed
area requirements by one-third.
The cost of an ET system is highly dependent on the avail-
ability of a suitably sized sand. The ET sand must have a
capillary rise potential slightly greater than the depth of the
bed. The earth materials removed in the excavation of the bed
are very often not suitable for the bed media and imported ET
sand obtained from gravel washing operations must be used. The
cost of imported ET sand is dependent on availability and the
distance it must be hauled. The cost for ET sand shown in this
analysis is based on actual charges for materials purchased on
this project.
Another major cost factor in ET bed construction is the
type and thickness of liner used. Many of the installations in
Colorado have utilized a 10 mil thickness, PVC liner. The cost
is relatively low at $0.37/m2 ($0.04/ft2). There are questions
as to whether it is possible to provide a complete water tight
seal with this material that will last through the life of the
bed. The liners used in the lysimeters for this study remained
water tight for the two year project period. Whether this con-
dition will exist for field installed units, especially at the
point where the inlet pipe passes through the liner, is still a
question. Thicker liners or materials that provide potentially
tighter sealing can increase the cost by as much as $10/m2
($1.00/ft2).
A generalized estimate of costs for a unit of 465 m2
(5000 ft2) is shown in Table 13. The basis for estimation is
given at the bottom of the table. A cost breakdown for an
installed bed in the study area was not available but local
contractors have stated that installed costs for actual systems
are in the range of $1.00 to $1.50 per ft2 of surface area. If
the cost of the ET sand in a local area is greater than that used
in the estimate or if a more expensive liner is used, the cost
could be significantly higher than that shown.
152
-------
TABLE 13. TYPICAL COSTS FOR CONSTRUCTING ET SYSTEMS
Unit Costs
; Item
1
1. ET sand, 20 miles haul
and spread
2. Plastic liner, 10 mil,
PVC, 1 layer
3. ET bed distribution
Pipes, 4" diameter
4. Gravel, in place
5 . Excavation
Material
Unit & Equip.
yd3 3.50a $
n
ft2 0.04a
PVC ft 0.50°
yd3 3.50a
yd3 0.27b
6. House drain pipe, 4" VCP K
inst.
7. Septic tank, in place
ft 1.80"
ea 160C
Inst. Total
1.00e $ 4.50
0.04e 0.08
0.40e 0.90
1.00e 4.50
0.38b 0.65
v.
2.20 4.00
120° 280
Estimate for Typical Bed
(5,000
Item
1. ET sand
2. Plastic liner
; 3. Distribution pipe
; 4 . Gravel
5 . Excavation
6. House drain
j 7. Septic tank
i
Engineering; permits;
Unit cost to 5,000 ft2
ft2 x 2 ft deep)
Unit Price Quantity
4.50/yd3 340
0.08/ft2 5000
0.90/ft 625
4.50/yd3 38
0.65/yd3 375
4.00/ft 100
280 each 1
Total Construction
and contractor profit
Total System Cost
= $1.08/ft2 ($11.60/m
Total
$ 530
400
560
170
260
400
280
$3,600
~ l,800d
$5,400
2>
Actual charges for materials purchased on this project.
"'Means Building Construction Cost Data., 34th Ed. (1976) .
'Data from local contractor and local engineer.
Data not available, value estimated.
2Based on estimated time for use of machine and operator or
laborer, 105 HP dozer and op. @ $42/hr, labor @ $7/hr total.
153
-------
MECHANICAL SYSTEM DESIGN
The sizing and costs of mechanical evaporation units are
based on the relationships and equations developed in this study.
No prototype units have been constructed. Further development
of the concept could result in significant reductions in the size
and cost requirements of a unit.
The design of a mechanical evaporation unit for operation
under field ambient weather conditions requires the application
of local climatological data to determine the rate of evaporation
and an estimation of the number of days during which freezing
will occur. When the wastewater flow rate exceeds the evapora-
tion rate from the unit, storage facilities for the excess
wastewater must be provided. When ambient air temperatures are
below 4°C (40°F), the unit must be shut down and drained to the
storage reservoir, allowing no evaporation to take place.
Two approaches can be used in calculating the approximate
required size of a unit. These include: the use of the equa-
tions developed in the Results Section of this report, and the
relationship between measured pan evaporation and actual evapora-
tion that has been presented. One of the problems common to
using the equations and reported data from the NOAA publication
"Climatological Data" is the interpretation of wind speed measure-
ments. It was shown in the experimental work that wind direction
had a significant effect on the evaporation rate with the rotating
disk evaporator (RDE). Weather bureau measurements of windspeed
are made in the direction of the wind. By orienting the RDE in
the direction of the prevailing wind, maximum evaporation rates
can be attained. However, when using the mean monthly wind
speed as reported in "Climatological Data", care must be exer-
cised because of the height and conditions under which the wind
speed was measured. The standard measurement height is 20 ft
(6.1 m), however, this may vary and is reported with the data.
The equations presented in the Theory Section of this report can
be used to adjust the wind velocity to apply to the elevation of
the disks.
Utilizing the weather data in Table 6 and the equation
developed in the previous chapter for evaporation from the disks
of the RDE, the values shown in Table 14 were calculated. Wind
speed values were adjusted to an elevation of four ft (1.2 m)
above the ground using the following equation:
.1/7
W2Q = 0.8 W20
The evaporation rate equation utilized was:
E( Vhr.m2, = (0.0074 + 0.005 W(kmph) VH (gm/kg) + 0.88 R
ws
154
-------
The second approach involves the correlation of disk
evaporation rate with pan evaporation. It has been shown that:
Edisk(Vhr-m2) = °*42 Epan (5,/hr.m2)
The monthly evaporation rates for Boulder, Colorado are
given in the table. A further consideration is that the unit
cannot be operated when ambient temperatures are below 4°C
(40°F). An analysis of the temperature data in Table 6 shows
that the unit could be operated only a very small percentage of
the time during the months of December, January and February
when average temperatures are well below 4°C (40°F). During
March and November the average temperature is near 4 C (40°F)
and it has been assumed that the unit would operate fifty per-
cent of the time. During the other months the unit is assumed
to operate continuously. When these non-freezing utilization
factors are applied to the evaporation data, the RDE evaporation
rate values are obtained.
It can be noted that the values in Table 14 for RDE evapora-
tion rate using the two methods are similar but not identical.
The reason for this is that the term AH in the equation is highly
sensitive to temperature and relative humidity variations. A
change of only a few degrees in the monthly average temperature
produces a marked change in the AH value. For this reason, the
curve obtained must be considered as approximate. The curve in
Figure 63 represents the calculated RDE evaporation values using
the equation.
The maximum average loading rate for an RDE unit in Boulder
is about 0.1 £/hr.m2. This is the value from Figure 63 whereby
the sum of the storage areas, indicated as Q) , is equal to the
excess evaporation area, indicated as QT). Operation of the
unit would be terminated during January and February and the
storage vault would be filling. During March the unit would
operate about one-half time, and since the loading rate is
greater than the evaporation rate, the vault would continue to
fill slowly. Near the first of April, the vault would be full
and from that point the evaporation rate would exceed the
incoming wastewater flow rate, and the vault would begin to
empty. Near the end of October the vault would be empty.
Through November and December the vault would fill again and
continue filling through the cycle to April.
The volume of vault storage required can be calculated from
the inflow rate and the sum of the hatched areas designated by
(l). For this case, the storage requirement is 100,000 liters
(27,500 gal, 3500 ft2). It is apparent that a large storage
vault is required with this system in northern climates.
The required size of the RDE can be calculated from the
yearly average loading rate of 0.1 £/hr.m2. If a two meter
155
-------
TABLE 14. CALCULATED EVAPORATION RATES U/hr*m ) FOR BOULDER, COLORADO
en
January
February
March
April
May
June
July
August
September
October
November
December
Equation
0.027
0.058
0.086
0.123
0.167
0.165
0.196
0.184
0.153
0.124
0.072
0.063
Utilization RDE Pan
Factor Evap. Correlation
0.0a
o.oa
0.5b
1.0
1.0
1.0
1.0
1.0
1.0
1.0
0.5b
o.oa
0.000
0.000
0.043
0.123
0.167
0.165
0.196
0.184
0.153
0.124
0.036
0.000
0.037
0.048
0.067
0.090
0.121
0.174
0.161
0.140
0.078
0.065
0.037
0.037
Utilization RDE
Factor Evap.
o.oa
0.0a
0.5b
1.0
1.0
1.0
1.0
1.0
1.0
1.0
0.5b
o.oa
0.000
0.000
0.033
0.090
0.121
0.174
0.161
0.140
0.078
0.065
0.019
0.000
Assumes unit not operational during these months due to freezing conditions,
Assumes unit operational 50% of the time due to intermittant freezing
conditions.
-------
0.2 J
Boulder, Colorado
maximum ^Loading, rate \
storage
wastewater
evaporation
I I 1 I I I III
JFMAMJJ
SOND
Figure 63. RDE loading rate curve considering evaporation
potential and freezing weather periods for
Boulder, Colorado.
157
-------
(6.56 ft) diameter disk unit is used with a 0.71 submergence
(fraction of the radius measured from the outside edge), an
exposed area of 4.3 m2 (46.3 ft2) per disk is available. Using
a wastewater flow rate of 850 liters per day, an average loading
of 0.1 Vhr.m2 and 4.3 m2/disk, a total of 83 disks would be
required. A safety factor of 1.25 to 1.5 should be used to com-
pensate for the uncertainties of the evaporation calculations
and the variation in evaporation potential for different years.
If 120 disks were used with a spacing of 2.5 cm (1 in.) between
centers of disks, the total length of the disk pack would be
three meters (10 ft). The RDE would be approximately the size
of an automobile. Increasing the size of the RDE will not
significantly decrease the size of the storage vault required
in northern climates because the storage must hold the volume
produced during slightly more than three months of cold weather.
Precipitation entering the unit was not included in the
volume to be evaporated. The catchment area of the reservoir is
small, approximately 2mx3m=6m2. With an annual precipi-
tation of 0.5 m (19.6 in.) per year, the volume of water added
due to precipitation is 3 m3 (3000 liters) per year. This is
less than one percent of the wastewater inflow rate and can be
omitted from the calculation without appreciable error.
NATIONAL APPLICATION OF MECHANICAL EVAPORATION SYSTEMS
It is readily apparent that the size of the unit and the
volume of the storage vault could be greatly reduced in southern
climates where freezing problems are not encountered. The RDE
evaporation curve for Hialeah, Florida is shown in Figure 64.
No freezing periods are encountered in this area which allows
for year round operation at a rate of 0.09 A/hr.m2. The size of
unit required would be ten percent larger than that for Boulder,
or 132 disks with a length of 3.3 meters. The storage require-
ment would be only 40,000 liters (10,600 gallons or 1500 ft3).
It can also be noted from the curve that in southern climates
the use of a larger unit can reduce storage requirements. If a
loading rate of 0.05 £/hr.m2 were used, the unit could keep up
with wastewater flow in the winter and run intermittently in the
summer. Under these conditions, no storage vat would be required.
However, an RDE unit twice the size of the one designed for
Boulder would be required for this condition. Similar design
requirements would exist in all southern climates where freezing
weather is not encountered.
For northern climates such as Canton, New York, the annual
loading rate is approximately 0.05 Jl/hr-m2 as shown in Figure 65.
This would require an RDE unit about twice the size of the one
for Boulder, Colorado (2 m diameter x 3 m long). The storage
requirement would be 130,000 liters (34,000 gal or 4600 ft3).
158
-------
Hialeah, Florida
0.1 —
RDE
evap
rate
(£/hr.m2)
maximum loading rate «\_
e
wastewater
evaporation
ii i 1 1 1
JFMAMJJASOND
Figure 64. RDE loading rate curve considering evaporation
potential for Hialeah, Florida.
159
-------
0.1 _
Canton 4SE New York
RDE
evap
rate
(1/hr-m2)
0
naximum loading _rate _\ .
storage
wastewater
evaporation
I 1 j I
MAM
A S O N D
Figure 65.
RDE loading curve considering evaporation potential
and freezing weather periods for Canton,
4 SE, New York.
160
-------
The RDE concept is applicable for permanent homes in any
part of the U.S., except Alaska, but the vault storage volume
will be very large in northern climates. The use of water
saving devices can reduce the RDE and storage size by one-third.
Summer home applications can be used throughout the U.S.
Loading rates will be approximately fifty percent higher than
for permanent homes (RDE ,about two-thirds size) and no storage
volume will be required. In all cases, the RDE, with or without
storage vault, must be preceded with a septic tank.
COST ANALYSIS FOR MECHANICAL EVAPORATION SYSTEMS
It is difficult to estimate the cost of a proposed mechani-
cal system. The approach that was used was to scale up the
actual costs of the model RDE used in the testing. Further
development and optimization of the RDE concept and the use of
mass production techniques could reduce unit costs significantly.
The cost of the rotating disk unit has been estimated with
the option of two different disk materials, aluminum and plastic.
A detailed cost listing is presented in Table 15. The fixed
costs for a unit including assembly labor, motors, pumps, con-
trols, drive shaft and other total to $2700. The variable costs
for different sized units include disk material at $1.53/ft2
($16.46/m2) for aluminum or $0.68/ft2 ($7.32/m2) if plastic is
to be used. The steel vat reservoir is estimated at $1.21/ft2
($13.00/m2). When these values are combined the following
equations result:
Cost
(A1) = 52700 + $1.53/ft^(alsk) + ?1.21/ft^(vat)
+ $1.21/ft2(vat)
Cost(plastic) = *2700 + *°
The cost of the storage vault was based on the values in
Table 16 and results in the expression:
Cost
(vault)
= $300 + $1.94/ff
(storage)
These values are based on buried concrete vault storage. Less
expensive means of storage might be desired to reduce this cost.
TABLE 16. VAULT STORAGE COSTS
I Unit Unit Cost !
Excavation
Hauling
Concrete (complete)
Pumps and Controls
Total Storage
ya3
yd3
yd3
ea
ft3
$ 1.50
1.50
170.00
300.00
1.94
($1.96/m3) I
($1.96/m3)
($221/m3) i
i
i
($20.87/m3) i
161
-------
TABLE 15. COST ESTIMATES FOR ROTATING DISK EVAPORATOR
Item
ROTATING DISK UNIT
Aluminum disks - 1/16"
(includes material waste)
Disk fabrication (Al)
6 $20/hr
Drive Unit -
Bearings
Large pulleys
Sheaves
Belts
Motor w/speed control
Drive shaft
Shaft and motor supports
! (including fabrication)
• Steel Tank -
3/16" A36 plate
Epoxy finish
Fabrication
Controls
Unit fabrication
Tank supports
Concrete pad for unit
Hauling to site
Electrician
Installation
Design and inspection
Fixed Costs
Variable Disk Costs (Al)
Variable Tank Costs
Plastic Disk - 1/16"
polypropylene (includes
material waste)
Disk Fabrication (plastic)
@ $20/hr
Variable Disk Costs
(plastic)
Unit
2
ft2
•-)
ft2
ea.
ea.
ea.
ea.
ea.
ea.
lb-,
ft2
ft2
hr
ea.
hr
ft
cu yd
hr
day
ea.
unit
ft2
ft2
9
ft2
o
ft2
ft2
Unit
Cost
$
1.36
0.17
8.00
8.00
2.00
2.00
225.00
12.00
0.26
0.96
0.25
20.00
55.00
20.00
4.60
100.00
20.00
120.00
500.00
1.53
1.21
0.53
0.15
0.68
Quantity
8
2
2
2
1
1
16
1
24
50
1.9
4
2
1
1
Total
Cost
$
64.00
16.00
4.00
4.00
225.00
12.00
240.00
320.00
55.00
480.00
230.00
190.00
40.00
80.00
240.00
500.00
2700.00
162
-------
Using the cost basis shown, the system cost for the examples
cited are presented in Table 17. A value of $350 was added to
include the cost of a septic tank complete and installed. The
cost per 1000 gallons of wastewater disposed is presented, based
on a 30 year unit life and 8.5 percent interest. The maintenance
and operation costs were assumed to be very small and a figure of
$25 per year was used.
It can be concluded that for the use of plastic disk RDE's
with water saving devices for permanent homes in areas where
freezing weather is not a problem, the system cost should be in
the range of six thousand dollars. For more northern climates,
the cost rises sharply and can exceed ten thousand dollars.
Summer home installations, under the above conditions, should
result in a system cost of approximately four thousand dollars.
These costs are generally higher than those for a septic tank
and leaching field. The RDE is a totally non-discharging system
and in mild climate areas where subsurface discharge is not
feasible, this system is probably cost competitive with many
other alternatives.
The use of commercial heat in the form of a liquid or air
heater to eliminate the need for the storage vault in cold
weather applications is not feasible. The minimum heat required
to evaporate the water under cold weather conditions is 600
calories per gram. It has been shown previously that the cost
involved is approximately $100/1000 gallons. The use of a
storage vault for winter conditions results in a cost well
below this value.
CONCENTRIC CYLINDER EVAPORATOR
The concentric cylinder unit was devised to test the concept
of evaporative disposal for the condition where a source of
heated air was available, such as from a stack or chimney. The
design of a unit of this type is specific to the conditions of
the air that is available for use. The equations derived in the
results chapter are specific for the geometry of the device
tested and may not be directly applicable to a larger prototype
unit, but the equations should provide an approximation of the
performance of a larger unit.
The national applications of the CCE concept are very
limited and relate to the specialized condition where a warm air
source exists. Another application could be for winter condi-
tions where air moving through the unit would be heated to pro-
vide continuous operation. It has been shown that the use of
commercial heat to evaporate wastewater is a very costly
operation and would be used only for very specialized conditions.
Another large drawback to the use of this concept from an
economics standpoint is the very high cost of pumping air.
163
-------
TABLE 17. RDE SYSTEM COSTS
Boulder, Colorado
RDE aluminum
storage & septic
Total
RPE plastic
storage & septic
Total
Hialeah, Florida
With storage
RDE aluminum
storage & septic
Total
RDE plastic
storage & septic
Total
Without storage
RDE aluminum
Total
RDE plastic
Total
Canton, New York
RDE aluminum
storage & septic
Total
RDE plastic
storage & septic
Total
Summer Home -
Boulder, Colorado
RDE aluminum
Total
RDE plastic
Total
i
r
Normal
$ 8,721
7,440
$16,161
5,444
7,440
$12,884
9,325
3,560
$12,885
5,718
3,560
$ 9,278
$14,742.
$ 8,188
14,742
9,574
$24,316
8,188
9,574
$17,762
$ 6,714
$ 4,530
rotal $/1000 gal
Water Sav* Normal Water Sav*
$ 6,715
4,960
$11,675 18.16 13.12
4,528
4,960
$ 9,488 14.48 11.76
7,115
2,373
$ 9,488 14.48 11.76
4,712
2,373
$ 7,085 10.43 7.96
$10,730 16.57 12.06
$ 6,356 9.2 7.14
10,730
6,600
$17,330 27.33 19.48
6,356
6,600
$12,956 19.97 14.56
i
$ 5,376 30.60 24.51
$ 3,920 20.66 17.87
*
Assumes 33 percent reduction in home water use with water
saving devices.
164
-------
Metcalf and Eddy, 1972, gives an expression for estimating the
power requirement for pumping air as:
W RT
BHP = 55lTEef
where W = air flow rate (Ib/s)
a
R = gas constant
T = temperature °K
n = compression constant, 0.283 for air
e = efficiency fraction
The minimum evaporation rate required for a CCU is 225 gal
x 8.34 Ib/gal x 1/24 x 60 x 60 = 0.022 Ib/sec of water evaporated.
If the piped air entering the compressor has a temperature of
20 C (68°F), an outlet pressure of 4 psig and an outlet humidity
deficit AH = 10 lb/1000 Ib, the air flow required would be 2.2
Ib/sec. The horsepower required for 70 percent efficiency is:
2.2 x 53.5 x 528
550 x 0.283 x 0.70
or 30 kilowatts. Thirty kilowatt hours at $0.04 each will
evaporate (0.022 x 3600)/7.5 = 10.5 gal of water, resulting in a
cost of $120/1000 gal of water evaporated, an unacceptable cost
for nearly all conceivable conditions.
It can be concluded that using commercial energy to enhance
wastewater evaporation is very expensive. The rotating disk unit,
Which uses the heat and wind movement of ambient air, is a far
more economic approach.
165
-------
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Bibliography - cont'd
Yamaoka, Yoshita. 1958. Experimental Studies on the Relation
Between Transpiration Data and Meteorological Elements.
Trans, of Amer. Geophys. Union, 39 (2).
Young, A.A. 1947. Evaporation from Water Surfaces in California,
U.S. Dept. of Agriculture, SCS Bulletin, No. 54.
175
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
i. REPORT NO.
EPA-600/2-78-163
3. RECIPIENT'S ACCESSION NO.
4. TITLE AND SUBTITLE
SEWAGE DISPOSAL BY EVAPORATION-TRANSPIRATION
5. REPORT DATE
September 1978 (Issuing Date)
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
Edwin R. Bennett
K. Daniel Linstedt
8. PERFORMING ORGANIZATION REPORT NO.
and
9. PERFORMING ORGANIZATION NAME AND ADDRESS
Dept. of Civil, Environmental and
Architectural Engineering
University of Colorado
Boulder, Colorado 80309
10. PROGRAM ELEMENT NO.
C611B
11. CONTRACT/GRANT NO.
R 803871
12. SPONSORING AGENCY NAME AND ADDRESS
Municipal Environmental Research Laboratory—Cin.,OH
Office of Research and Development
U.S. Environmental Protection Agency
Cincinnati, Ohio 45268
13. TYPE OF REPORT AND PERIOD COVERED
Final - 7/75 - 5/78
14. SPONSORING AGENCY CODE
EPA/600/14
15. SUPPLEMENTARY NOTES
Project Officer: James F. Kreissl (513) 684-7614
16. ABSTRACT
One of the methods for on-site disposal of wastewater from individual homes is by
evaporation. Two types of evaporative disposal systems have been investigated in this
study; evapo-transpiration beds and mechanical evaporation units.
Twenty nine test lysimeters of 0.22 cubic meters volume each were utilized to
evaluate the effect of design and operational parameters for ET beds. The variables
studies were wastewater loading rate, effect of the weather variables of evaporation
and rainfall, ET sand size, evaporation rate as a function of the water saturation
depth, and the transpiration contribution of surface vegetation. A design method is
presented along with cost data and an analysis of the national application potential
of this type of system.
The evaporation of wastewater using mechanical systems was studied using a pilot
scale unit, constructed as part of the project. Two types of evaporation designs were
evaluated. Design equations were established for both units. Cost data and analysis
of national application potential is also presented.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS C. COS AT I Field/Group
Sewage disposal
Evaporation
Tranpsiration
On-site sewage disposal
Non-sewered area
Parametric evaluation
13B
18. DISTRIBUTION STATEMENT
Release to Public
19. SECURITY CLASS (ThisReport)
Unclassified
21. NO. OF PAGES
194
20. SECURITY CLASS (Thispage)
Unclassified
22. PRICE
EPA Form 2220-1 (Rev. 4-77)
176
4 U.S. GOVEMMBfTntMnKOfFICE: 1971— 657-060/1472
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