United States
Environmental Protection
Agency
Robert S. Kerr Environmental
Research Laboratory
Ada OK 74820
Research and Development
EPA-600/S2-84-163 Dec. 1984
Project Summary
Overland Flow Treatment of
Municipal Wastewater in
Florida
Allen Overman and Thomas Schanze
The physical and biological processes
of overland flow treatment of municipal
wastewater were studied. A 5 ha spray
field and four plots were employed
along with an activated sludge treatment
plant to determine relations of treatment
efficiency, residence time and mass
transfer with application rate for pri-
mary and secondary wastewaters.
Mathematical models were presented
and used to describe relations between
dispersion, mass transfer, treatment
efficiency and application rate. Correla-
tions were found for dispersion coeffi-
cient and mass transfer coefficient with
velocity, and of residence time with ap-
plication rate.
It was shown that for the application
rates of the plots used, the 1) residence
time followed a power function of
application rate, 2) mass transfer was
diffusion limited rather than reaction
limited, 3} dispersion was not adversely
affected by increasing flow depth, and
4) plots were not loaded to a point
where efficiency changed with applica-
tion rate.
Also, the field study indicated that,
for most parameters except phosphorus,
runoff did not degrade the quality of the
receiving waters, and overland flow
treatment provided an excellent polishing
unit for wastewater treatment plant
effluent.
This Project Summary was developed
by EPA's Robert S. Kerr Environmental
Research Laboratory. Ada, OK. to
announce key findings of the research
project that is fully documented in a
separate report of the same title (see
Project Report ordering information at
back).
Introduction
This project was developed to evaluate
the impact of effluent from a secondary
treatment plant on runoff quality from a
spray field and on quality of the receiving
stream, determine the suitability of
several grasses on treatment efficiency
with both secondary and primary waste-
water, quantify the transport processes for
suspended solids, BOD, turbidity and
organic nitrogen, and determine seasonal
changes in treatment efficiency for a
warm, humid climate. Measurements
were made on several parameters to
evaluate performance of the treatment
plant as well as the overland flow units.
All the field work was done at Gretna,
Florida, in the panhandle.
Treatment Plant
The plant used the activated sludge
process. Design capacity was 450 mVday
(120,000 gallons/day) with average daily
flow of 225 mVday (60,000 gallons/day).
Effluent flowed into a holding pond.
Water was pumped to a 5 ha (12 acre)
spray field through impact sprinklers at
an application rate of approximately 5
cm/week.
Plots were designed to receive both
secondary and primary wastewater.
Secondary was pumped from the holding
pond through open impeller pumps onto
four plots during 1982. In late 1982 two
plots began receiving primary wastewater.
Raw sewage was pumped through an
open impeller pump into a clarifier with
one hour mean detention. A portion of the
overflow was routed to the plots through
positive displacement pumps with variable
speed drives. All pumps operated from
timers 5 hours/day and 7 days/week. A
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tlistribution, systerrfwas developed*
i red&edT clogging problem^ atid
|inimized pumping energy. r
.,
>pray Field
A rectangular spray field had been
constructed on a Plummer sand soil with
a two percent slope. The vegetative cover
was coastal bermuda grass. Due to a
combination of rainfall, slope, and clay
below the surface horizon, the field
produced a lot of runoff. Discharge flowed
to a small receiving stream that drains an
agricultural area. During the period 4/82
- 8/82 samples were collected of raw
sewage, pond, runoff, upstream and
downstream. Measurements were made
of several parameters, including dissolved
oxygen, to evaluate treatment efficiencies
and impact on the stream.
Overland Flow Plots
Four plots were constructed on an
average slope of 1.7 percent. Approximate
widths and lengths were 12m and 34m,
respectively. Each plot was surrounded
by a berm. Small berms were constructed
up and down slope on a 1.2m spacing to
aid in flow control. Water was applied
through drop tubes on a 1.2m spacing at
an average pumping rate of 49 liters/min
(13 gallons/min) and loading rate of 25
cm/week (10 inches/week). Runoff was
collected at the bottom center of each plot
through a 7.5cm (3 inch) invert and
routed through trapezoidal flumes, into a
common sump, and to the lift station of
the treatment plant. Vegetation included
bahia grass (2 plots), common bermuda
grass and St. Augustine grass. Bahia and
bermuda were seeded while St. Augustine
was sodded. Vegetation was mowed
monthly with a flail mower. Grass was
not removed from the plots.
Conclusions
Results are divided into (1) field and
stream study, (2) plot studies, (3) rate
processes, and (4) quality assurance.
Each phase contributed significantly to
the total project.
Field and Stream Study
A summary of results for the field study
is presented in Table 1. Column 2 of Table
1 shows the ratio of pond concentration
to raw concentration. For total suspended
solids (TSS), this value was 0.25,
meaning that the treatment plant reduced
TSS by 75%. The ratio was high due to
presence of algae in the pond water.
Across the field, the TSS ratio was 0.19,
meaning that the spray field reduced the
applied TSS concentration by 81%. The
Table 1. System Performance for Field
Study
Parameter Pond Runoff Runoff
Raw
Pond Raw
TSS
BOD
TKN
TP
0.25
0.088
0.18
0.74
0.19
0.22
0.33
0.61
0.047
0.020
0.059
0.46
Sample Period: 4/82 - 8/82
field was effective in removing algae at
the loading rate of 5 cm/week. The
overall ratio was 0.047, for an overall
reduction of 95%. Overall reductions of
biological oxygen demand (BOD), total
kjeldahl nitrogen (TKN), and total phos-
phorus (TP) were 98%, 94% and 54%,
respectively. With the exception of TP, the
spray field served as an excellent
polishing unit. Corresponding runoff
concentrations were: TSS=6 mg/l, BOD=3
mg/l, TKN=2 mg/l and TP=4 mg/l.
Maintenance mowing of the coastal
bermuda grass twice per year was
adequate for weed control.
Stream data showed no significant
degradation of stream concentrations of
BOD and TKN. There was an increase in
TP. Dissolved oxygen (DO) in the stream
showed no correlation with the runoff.
The seasonal sag in DO appears to relate
to decomposition of tree litter in the
swamp. Chloride measurements showed
that runoff constitutes 14% of downstream
flow, ranging from 5% in wet periods to
28% in dry periods.
Plot Studies
A summary of results for the plots is
shown in Table 2. Reductions for secon-
dary are for runoff compared to raw. For
primary, reductions are for runoff compared
to primary. For TSS and TP, the plots
Table 2. Average Reductions for Plots
Parameter
Secondary Primary
TSS
BOD
TKN
TP
93%
95
92
35
93%
89
62
35
provided equal treatment to the treatment
plant plus plots — 93% and 35% respective-
ly. Treatment plant plus plots was
superior to plots only for BOD and TKN.
DO data show that the plots receiving
primary should be longer (say 50m) to
provide more complete decomposition of
carbon and nitrification.
The grasses functioned quite well for
wastewater application. Bahia grass
receiving primary was eventually displaced
..j,
:ment
by penr
provide*
grass grev
height of.c
The srrd
provide adequate
tribution system' u
vides adequate f low
trol. Both are critical to hig,
performance with overland flbi
No seasonal change in treatment
efficiency for TSS, BOD and TKN was
apparent.
Rate Processes
A range of pumping rates with primary
wastewater used on plot 4 (St. Augustine
grass) provided additional insight into
various rate processes. Other research
shows that the concentration of several
constituents (including TSS, BOD, and
organic nitrogen) decreases exponentially
down the slope. The essential rate
processes controlling transport down the
slope include convection (bulk fluid flow),
dispersion (hydrodynamic mixing) and
mass transfer (exchange between solution
and solid phases). It is common practice
in mass transfer studies to relate
dispersion and mass transfer coefficients
to average fluid velocity. Results for this
phase are shown in Table 3. Mean
velocity is obtained from mean residence
time using salt tracer curves. Increases of
U,D and d with q are as observed in other
studies. The d values are greater than
sometimes observed due to the density of
the turf used.
Table 3. Rate Coefficients for Transport
Study
q
m /hr/m
0.12
0.17
0.23
0.27
0.37
U
cm/min
0.135
0.172
O.223
0.252
0.325
D
mz/min
0.52
0.64
0.76
0.79
0.80
d
cm
1.48
1.59
1.72
1.78
1.91
q = pumping rate/width
U = mean velocity
D = dispersion coefficient
d - mean flow depth (calculated)
Mass transfer coefficients for TSS,
BOD and organic nitrogen vary approxi-
mately linearly with mean velocity. For
this range of velocity, mass transfer is
diffusion limited rather than reaction
limited. We conclude that treatment
efficiency is approximately independent
of pumping rate and much more sensitive
to slope length. Of course, if pumping rate
is increased to a high enough level,
reduction becomes reaction limited.
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y Assurance
'•&*'• -••'<*• ~';~,p.:^ >' ••• ">,:
Qcedufeii" far; quality assurance
\£ an integral part of the project,
iciuded sample handling, analytical
^procedures, data management, and
statistical analysis. With each batch of
samples, measurements were made on
blanks, standards, spikes, and duplicates.
Results are reported as % recovery for
standards and spikes. Table 4 gives data
on standards for several parameters. The
coefficient of variation is defined as the
ratio of standard deviation to the average
(also shown in Table 4). Corresponding
values for a set of EPA quality assurance
standards are: TSS = 0.07, BOD = 0.13,
TKN = 0.07, and TP=0.04. Resultsforthis
project compare closely with those of the
EPA quality assurance program.
Table 4. Statistics for Q/A Standards
Parameter Avg. Std. Dev./Avg.
TSS
BOD
TKN
TP
100
101
1O5
100
0.01
0.06
0.09
0.01
Allen Overman and f^'^^s^^amAj^ymm'vdr^fjry^/fSnda, Gainesville,
K 32611. .
LowellE. Leach is theff^^o/^OffiCer (sib below);
The complete report, entitled "Overlandftow Treatment of Municipal Wastewater
in Florida," (Order No. PB 55- 7/5 798; Cost: $ 17.50, subject to change) will be
available only from: .,
National Technical Information Service
5285 Port Royal Road
Springfield, VA22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Robert S. Kerr Environmental Research Laboratory
U.S. Environmental Protection Agency
Ada, OK 74820
US GOVERNMENT PRINTING OFFICE; 559-016/7871
Avg. - average recovery, %
Std. Dev. = standard deviation
Recommendations
Studies should be conducted at greater
hydraulic loading rates to more nearly
approach reaction limit. In most work to
date, mass transfer was diffusion limited.
Data on mean residence time, pumping
rate and input/output concentrations of
TSS, BOD, DO, and nitrogen species
should be collected.
More data are needed with secondary
wastewater low in algae, since the
efficiency of overland flow as a polishing
unit appears to be limited by algae.
Shorter detention time before field
application should be provided.
More information on phase-in time
after a down period is needed for warm,
humid climates. It appears to be in the
range of one to three days, based on the
rate of decline in turbidity of runoff after
startup.
Field measurements on slopes less
than two percent are needed. Smaller
slopes would reduce cut and fill costs and
erosion potential. The success of such a
system would greatly depend upon
uniformity of grade and flow control.
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United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
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