EPA R2-73-223
MAY 1973 Environmental Protection Technology Series
Management of Recycled
Waste-Process Water Ponds
Office of Research and Monitoring
U.S. Environmental Protection Agency
Washington, D.C. 20460
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RESEARCH REPORTING SERIES
Research reports of the Office of Research and
Monitoring, Environmental Protection Agency, have
been grouped into five series. These five broad
categories were established to facilitate further
development and application of environmental
technology. Elimination of traditional grouping
was consciously planned to foster technology
transfer and a maximum interface in related
fields. The five series are:
1. Environmental Health Effects Research
2. Environmental Protection Technology
3. Ecological Research
4. Environmental Monitoring
5. Socioeconomic Environmental Studies
This report has been assigned to the ENVIRONMENTAL
PROTECTION TECHNOLOGY series. This series
describes research performed to develop and
demonstrate instrumentation, equipment and
methodology to repair or prevent environmental
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.
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EPA-R2-73-223
May 1973
MANAGEMENT OF RECYCLED WASTE-PROCESS WATER PONDS
By
Charles E. Renn
Project WPD-117
Project Officer
Dr. Herbert Skovronek
Edison Water Quality Research Laboratory
Edison, New Jersey 08817
Prepared for
OFFICE OF RESEARCH AND MONITORING
U.S. ENVIRONMENTAL PROTECTION AGENCY
WASHINGTON, D.C. 20460
For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402
Price 86 cents domestic postpaid or 60 cents QPO Bookstore
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EPA Review Notice
This report has been reviewed by the
Environmental Protection Agency and approved
for publication. Approval does not signify
that the contents necessarily reflect the
views and policies of the Environmental
Protection Agency, nor does mention of trade
names or commercial products constitute
endorsement or recommendation for use.
ii
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ABSTRACT
This study describes the successful operation of a storage pond
used to collect treated wastewaters and runoff for recycle to
manufacturing operations under conditions of drought and severe
water shortages. Treated sewage and cafeteria wastes are stored
in an air sparger mixed pond and are returned to the manufacturing
plant to provide water for evaporative cooling and a variety of
production processes. By applying long term storage, air sparger
agitation, and controlled stratification during the summer, it has
been possible to increase the effectiveness of limited well supplies
from six to fifteen times.
The efficiency of the pond depends in large part upon biological
processes that go on in the comparatively shallow areas of the
system. These act to capture phosphorus and to stabilize algal
organics generated in the pond itself.
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CONTENTS
Section - Page
I Conclusions 1
II Recommendations 3
HI Introduction 5
17 Early Biological Problems of Wastewater Storage 7
V Description of the Hampstead Plant Treated Waste- 9
Water Storage and Recycle System
VI Performance of Hampstead Plant's Storage-nBeuse 15
Pond; BOD-COD
VII Performance of Storage-Reuse Pond — Nitrogen 19
and Phosphorus
VIII Performance of Storage-Reuse Pond — Thermal 25
Stratification and Sparger Mixing — The
Oxygen Crop
IX Stabilization of Converted Organics in the Pond — 35
Role of Grazing Organisms
X Leaf Wash, Drift, Storms and Filling of Shallows 39
XI Quality of Water in Process Water Pond 43
XII Acknowledgements 47
XITT Bibliography 49
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FIGURES
PAGE
1 PLANT WATER SUPPLY AND PROCESS WATER RECYCLE SYSTEM 10
2 VOLUME VS DEPTH OF PROCESS WATER POND 11
3 SPARGER MIXING OF POND 12
4 DEVELOPMENT AND BREAKDOWN OF THERMAL STRATIFICATION
IN STORAGE POND 28
5 VERTICAL DISTRIBUTION OF DISSOLVED OXYGEN AND PH
DURING THERMAL STRATIFICATION AND BREAKDOWN 30
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TABLES
No. Page
1 BOD-COD of Recycled Water, June, 1966 Series 16
2 BOD-COD of Recycled Water, September, 196?'Series 17
3 Analyses of Bottom Algal Mat and Bloom Materials 23
V13.
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SECTION I
CONCLUSIONS
1. It is possible and practical to use stored, secondary treated
domestic wastewaters for a variety of industrial purposes, particularly
for air conditioning heat exchange, cooling towers, and a variety of
machine tool cutting .operations.
2. Successful application of this supplementary water source requires
that industrial operations be modified to accept water containing
varying concentrations of organics, color, suspended materials and
particulate stuffs generated during storage or in transmission.
It also requires that the storage pond provide residence time and
conditions necessary to stabilize organics delivered in the treated
wastewater streams and generated by algal production in the pond
itself.
3« Storage and reuse of treated wastewaters during drought periods
is a practical method of extending the available water supply for
some industries and communities. It is also a device for achieving
zero effluent during critical, low stream flow periods. Ultimately,
however, the storage system must be recharged.
4. A recycled waste-process water pond with relatively large volumes
stored in shallow areas will produce water of greater stability and
lower organic content than a deep pond of the same volume. Sequences
of biological activities that take place on the illuminated shallow
bottom — two to three feet deep — favor the production of stable,
nutrient—low water.
5. While not of the technological character usually associated with
closed loop systems, the approach described does, in fact, succeed
in achieving a high level of water reuse, particularly when water is
in short supply.
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SECTION II
RECOMMENDATIONS
This study was carried out at a time when there was widespread
general concern with the availability of industrial water sources
during drought. At that time there was considerable interest in
the development and expansion of industries in foothill areas and
other portential sites where water is normally limited after spring
flood runoff. The project itself was designed to discover "why"
the wastewater storage source used at Black and Decker's Hampstead
plant worked, with the expectation that the findings might be used
to develop similar reserve and water conservation systems when and
where necessary.
Since weather, and especially long term weather, is unpredictable,
it would appear desirable to review the applicability of a drought
period water conservation system such as that described in this
study under new existing and pending state and federal regulations.
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SECTION III
INTRODUCTION
The Black and Decker.Manufacturing Company's electric hand tool plant
at Hampstead, Maryland, is the largest manufacturing facility of its
kind in the world. The growth of the plant, and its continued operation,
has depended upon the recycling of stored, treated wastewaters and a
program of internal water conservation and management. By rigorous
engineering control of a limited primary well supply and recycled waste-
waters, the plant has managed to expand the effective usefulness of
its available primary water supply about ten times, and this during
a sequence of record drought years in which the plant's working
population doubled.
The Hampstead plant is located about twenty miles north of Baltimore
on farmland just south of Hampstead, a Carroll County community midway
between Baltimore, Maryland, and Hanover, Pennsylvania, It borders
the east side of Maryland Route 30. Geologically, the general area
of Hampstead is an unpromising aquifer. The soil burden of this
ancient mountain plain is shallow; there is little higher ground,
and most streams originate in and drain down from Hampstead.
Initially, during the plant's early history, the well system was
adequate for the general requirements of slowly expanding operations.
But a spectacular failure of the supply early in the summer of 1955
demonstrated the need for a substantial emergency reserve of water.
The uncertainties of yields from local aquifers, water rights,
storage capabilities, anticipated plant growth and changing processes,
and especially, the rising requirements for air conditioning led the
company's water engineering consultants to recommend the development
of a large water storage pond to capture treated sewage, manufacturing
wastewaters, storm runoff from roofs, parking lots, and the pond's
watershed. A recycle pumping system was designed to deliver stored
water back to the plant for uses admitting a lower quality of water,
principally for cooling operations. At the same time, a systematic
program of internal water conservation was established to secure the
most efficient uses of the higher quality primary supplies.
A storage pond and recycling pump system were build and placed in
service in the fall of 1956. For the following five years, stored
treated plant wastewaters were used to supplement the well supplies.
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In May, 1962, the Project Director was called to the Hampstead plant
as a consultant to investigate a series of biological problems
associated with the sudden development of anaerobic septic conditions
in the pond and the appearance of hydrogen sulfide in the recycled
water. An atmosphere of hydrogen sulfide about evaporative cooling
towers near the fresh air intake to the plant's ventilating system
is a cause of understandable concern where manufacturing processes
require mass production of precision shafts, bearings, and other
reactive metal parts.
Interim control of hydrogen sulfide production by heavy chlorination
was secured, but a quick review of the economics and physical require-
ments of this treatment or any other then recognized system made it
clear that hydrogen sulfide removal after formation would not be
practical.
Fortunately, the causes of the condition were determined withing a
few hours after the first visit. Two unanticipated factors operated
to produce an anaerobic pond and sulfate reduction. First, the high
rate trickling filter of the wastewater treatment plant was made
ineffective by altered raw wastewaters from the plant. These bore
slugs of strongly alkaline salts that raised the pH of wastewaters
sprayed over the filter surfaces to pH 10.0 or higher, sometimes
above pH 11.0. The stone surfaces of the filter were literally
scoured free of active biological films. Reduction in organic load
of the wastewater passing through treatment was limited to settling
of solids in primary Imhoff tanks.
Second, the storage pond receiving the ineffectively treated wastewaters
had become thermally stratified. A warm water surface layer, eight
to ten inches deep, generated by solar radiation and atmospheric
warming during windless late spring, effectively sealed off the deeper
water from atmospheric exchange with the water mass.
Two recommendations were made and followed. Automatic acid neutralization
was established in the raw wastewater sewer. A fixed air sparger was
placed in the deeper section of the pond to break up thermal stratifi-
cation and to prevent redevelopment in intervals of warm quiet weather.
The compressed air sparger was placed in service in June, 1962. With
the exception of short intervals required for maintenance, it has
remained in service under varying diurnal schedules through the warm
months since that date. The device has been completely effective in
preventing troublesome thermal stratification 2nd hydrogen sulfide
production. (A full description of the sparger and its operation will
be given in another section.)
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SECTION IV
EARLY BIOLOGICAL PROBLEMS OF TREATED WASTEWATER STORAGE
Although the institution of sparged air vertical mixing to break
thermal stratification of the storage pond and the correction of
pH of wastewater fed to the biological treatment system eliminated
further incidences of hydrogen sulfide in recycled water from the
pond, other biological problems appeared in the wake of the changes.
Several heavy infestations of dead and living snails delivered in
the recycled stream clogged screens and orifices in cooling lines
of presses and other manufacturing equipment. Masses of Chironomus
larvae (midge-redworms) and tubificid worms appeared in some
process water streams. Sloughed bacterial slimes and mats of
decomposing algae accumulated in dead ends of water lines, sumps,
and traps. At intervals recycled water in sumps developed offensive
odors. Heavy blankets of blue-green algae accumulated in pockets
of heat exchangers developed biological reduction cells that produced
pockets of corrosion.
Over the three years prior to the initiation of the study described
in this report, practical solutions were found to most of these
problems. For example, dead ends, pockets and other quiet zones in
which fine organic solids might accumulate and condense were
eliminated; where this was not possible, cleaning access and routine
cleaning was established. Apertures were enlarged, protective screens
were made more readily cleared, sumps were automatically drained or
were eliminated, and other plant changes were made to adapt operations
to water that might carry solids.
With changing manufacturing processes a continuous review of the
possible uses of the two separate water supplies in early planning
stages was established in plant engineering and plant maintenance.
Water at the Hampstead plant was treated as a chemical plant might
regard basic solvents and recoverable reagents. At all times, however,
basic aesthetics were preserved; recycled water was not used where
health hazards might be involved or where the appearance of hazard
existed. It was not used where it would be offensive "to users.
For example, high quality well water was used for toilet flushing.
Cooling and quenching processes in manufacturing operations were
modified to reduce splash and mists. Plant workers, a large proportion
of them women, were fully aware that recycled water was used in
various plant processes.
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Following the institution of sparger vertical mixing in the pond,
a number of field experiments were carried out in efforts to
control algal blooms, plagues of worms and snails, weeds, leaves,
and other causes of "biological incidents," These included attempts
to suppress water weeds in shallow margins with chemical weed killers j
control of snails and small, crusteaceans with insecticides; and
ultimately, control of filamentous algal growth with nigrosin dye
additions. Through all of these trials, one unresolved variable
always emerged. What was the effect of sparger mixing and
mixing schedules upon, the various blooms and swarms that developed?
It was true that the current nuisances did not exist when the
greater problems associated with the anaerobic stratified pond were
present, so there seemed to be a causal relationship between mixing
and infestations. There remained the hope that some simple change
in the mixing schedule might be found that would minimize or
prevent biological incidents. It was clear that we needed to know
what processes were going on in the pond itself and how the
sparger nrioriTig program affected them.
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SECTION V
DESCRIPTION OF THE HAMPSTEAD PLANT TREATED WASTEWATER STORAGE AND
RECYCLE SYSTEM
Fresh water is pimped on controlled schedules based on drawdown
plots from three to five gravel packed wells on the ^plant's property.
The pimping rates employed during the period of this study varied
from 100 gpm to 150 gpm through five and half days of the week.
The water, like other well waters of the region, is soft and corrosive.
It is treated with soda lime to pH 7»2 to pH 8.0 and hypochlorinated
to flash residuals of 0.2 mg free chlorine per liter. The treated
water is pumped to the plant and to an elevated surge and emergency
supply tank.
Approximately two thirds of the water taken from the wells eventually
passes as wastewater through the biological treatment plant to the
storage and recycle pond. This loading arises from the demands of
the washrooms and toilets, potable water supplies, and cafeterial
uses. The remaining 35% to 40$ is about evenly divided between
special process uses in the plant and evaporative losses. Thus,
between 17$ and 20$ of the well water passes through manufacturing
and appears as untreated wastewater discharged to the pond.
Sanitary, washroom, and cafeteria wastewaters are collected into a
common gravity sewer that carries them through a comminutor chamber
(and acid neutralization in the latter course of the study) to a
divider box that distributes the flow to two parallel Imhoff tanks
that provide primary settling and digestion of settled solids from
primary and secondary steps. Effluent from the tanks flows to a
dual sump and pumping controls from which it is delivered to a
high rate trickling filter. The trickling filter is 22 ft in
diameter and & ft deep, and is equipped with a single arm rotary
distributor. Effluent from the filter underdrains passes through
a box type secondary clarifier; the settled sludges are pumped
to the Imhoff tank—digesters, and the supernatant moves by gravity
to a baffled chlorine contact chamber* The contact chamber is
designed to provide a minimum of 20 minutes contact with available
chlorine residuals of 0.5 mg/1 at discharge to the sewer leading
to the pond.
The pond itself was constructed in the lowest available area of
the plant property. It was developed by excavating and lining a
swampy creek bed that drained the north-western section of the
site. Excavation was carried to firm soil and the basin sealed
with a one foot thick membrane of compacted clay. A wide riprapped
earthen dam, approximately 23,000 cubic yards in volume, closes
the basin and secures the impoundment.
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A general schematic of the flow plan of fresh water, treated waste-
waters, and recycled process water is given below in Figure 1,
"Plant Water Supply and Process Water Recycle System."
Well Field - Potable Water Source
Q P Q Q P .
Elevated Tank - Potable and
Fire
Overflow
Reservoir
to Fire Loop t
Manufacturing Plant
water
Treatment
System
Drainage from
Watershed
from ParMng
Lots
Pumphouse
Figure. 1 — PLANT WATER SUPPLY AND PROCESS WATER RECYCLE SYSTEM
10
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In recycle service water is taken through a submerged intake located
2.5 ft from the bottom in the deep end of the pond and drawn through
10 mil conical automatic backwashing screens to dual pressure pumps
located in a service building on the border of the pond. The pumps
deliver water at rates required by operations and weather — from low
rates of 288,000 gal/day to 2,600,000 gal/day, with average recycle
returns of 1,150,000 gal/day. No treatment other than screening is
applied at the pump-house.
Depending on the rainfall pattern, the pond receives variable flows
of storm water from roofs, the watershed, and parking lots. These
flows during storm conditions can be very heavy and may carry loads
of eroded soil and solids. At the time of this study the roof of
the main plant was 17 acres. In addition, the impervious surfaces of
roads and parking lots represented 13 acres. A flash rain of 0.5 in
yields 416,000 gallons, roughly 2.3 times the daily well yield. The
contribution from the pond's pervious watershed of approximately
120 acres will vary with the history of rainfall, but, in very long
wet periods, it may represent three or four million gallons per day.
The process water storage pond differs substantially from conventional
oxidation pond design in that its major volumes are held in broad
shallows. A graphic plot of the pond's volume vs depth is given in
Figure 2, "Volume vs Depth of Process Water Pond.*1
Millions gal Volume
Figure 2 — VOLUME VS DEPTH OF PROCESS WATER POND
The volume of the pond, filled to overflowing at the spillway, is
about 16,0 million gallons. At this level, the pond has an area of
approximately 8.3 acres.
11
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The pond's greatest depth is 12.5 ft. When filled, about 40$ of
the pond's volume is held in depths of three feet or less, and
about two thirds of the total volume exists at five feet and above.
The deep waters are contained in a sump area; less than 10$ of
the pond's volume exists below 10 ft.
The air sparger installed in 1961 consisted of a float-manifold
of 8 in diameter pipe, 30 ft long, anchored in the deep end of the
pond. From the float-manifold two 8 ft air lines extend downward
to a lateral distributor carrying sixteen evenly spaced 2 ft
wrapped aerator candles, eight on each side of the distributor.
A 5 ft Rootes blower, .operating at 2.5 hp capacity, supplied air
at 20 lb/in2 through a floating rubberized hose.
In service,the sparger acts as a massive low level air lift pump.
The action is shown in Figure 3, "Sparger Air Mixing of Pond,"
below. Fine and coarse air bubbles discharged at the candles
form a relatively light air-water dispersion. This low density
dispersion is .pushed upward by the surrounding denser water.
At the surface, the light dispersion spreads outward in all
directions. When the bubbles break, the cold water "rains" downward
through the warmer water below and mixes with it to form water
of intermediate temperatures. At the same time cold deep water
is being pushed into the rising air water dispersion column.
Eventually, the pond's water are mixed to uniform temperature.
o o
o
0
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It should be pointed out that the air sparged, recycled wastewater
pond is neither an "oxidation pond" nor an "aerated lagoon," though
it has superficial features of both. A wide range of planktonic and
sessile algae and changing populations of zooplankton predators are
present in the plant's storage pond, as in conventional oxidation
ponds, but vertical zonation is absent. The process water storage
pond has no anaerobic hydrolytic phase of accumulated digesting solids
as oxidation ponds commonly do.
On the other hand, the aeration capabilities of the air sparger mixing
system are small compared to the demand of the pond. Spargers of
this type powered at the level of 2.5 hp may be expected to supply
not more than 60 Ib of oxygen per day to an oxygen deficient system.
The oxygen demand of the treated wastewater is in the order of 200
to 250 Ib oxygen per day. The greater fraction of this requirement
is met by photosynthetic production of oxygen and the conversion
of wastewater organics to more stable organic stuffs by the complex
of predators.
The retention time of the pond at full volume and influent rates
of about 144»000 gal/day is approximately 114 days, but the exchange
time with recycle at ten times this rate is around 11 days. At
low levels, as in the summer of 1966, with the pond containing
11.5 million gallons and high recycle rates of 2,600,000 gal/day,
the exchange time was reduced to 4»4 days.
Average retention time affects b.iological processes by controlling
the degree of completion of serial processes. At full volume and
low recycle rates, the pond has the characteristics of a storage
system with very long residence time. The BOD loadings under these
conditions are about 20 to 25 Ib BOD per surface acre per day, which
places the loading in the low range of conventional oxidation ponds.
13
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SECTION VI
PERFORMANCE OF HAMPSTEAD PLANT'S STORAGE-REUSE POND; BOD-COD
The most striking characteristics of the water furnished for recycled
use from the process water pond were (a) low BOD values, (b) relatively
high COD/BOD ratios, and (c) low 20 day/5 day BOD'ratios. All three
characteristics indicate high biological stability of persisting
organics in the water.
Two extended series of tests established these properties. The
first series was run during an extended low water, drought period
in the month of June,1966, when treatment plant efficiency was
poor. The second was carried out in the middle weeks of September,
1967, during a period when the wastewater treatment plant was
operating within design efficiencies of 70% to BOfo BOD removal.
The data of these two series are presented in Table 1, "BOD-COD
of Recycled Water, June, 1966," and Table 2, "BOD-COD of Recycled
Water, September, 196?.
Samples analysed in both series represent 2l± hour continuous, collec-
tions from the waste treatment plant outfall and from the barge
sampler (described in a later section) — the first representing
the feed to the pond; the second, stored water fed back to the plant.
It is to be noted that both series show substantial reductions in
BOD concentrations during residence in the pond. Pond BOD values
are commonly less than 10$ of the treated wastewater BOD values,
and usually less than 5$ of the 20 day BOD values. Day to day
changes in influent BOD and COD going to the pond may be expected
to have little effect on pond values since dilution by mixing with
the pond water is more than a hundred fold.
The greater biological stability of organics in the recycled water
is evident in both series. This shows itself in two ways; in the
spread of 5 day BOD/COD ratios of treated wastewater fed to the
pond and that of the water in the pond itself; and in the relatively
smaller differences in 5 day BOD and 20 day BOD values of the pond
water. In the series of Table 1, representing inefficient waste-
water treatment conditions, COD values of effluents are commonly
more than twice the 5 day BOD values; in general the 20 day BOD
values are slightly more than one and a half times the 5 day BOD
values, which is within anticipated ranges. The pond waters, on
the 6ther hand, show COD values ranging from six to. ten times the
pond 5 day BOD values, and 20 day BOD values that vary from 1.2
to 1.3 times the 5 day BOD's.
15
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Table 1 — BOD-COD OF RECYCLED WATER, JUNE, 1966 SERIES
Date
(1266)
6/3
6/6
6/8
6/10
6/13
6/15
6/17
6/20
6/22
6/25
6/2?
6/29
7/1
Day
Fri
Mpn
Wed
Fri
Mon
Wed
Fri
Mon
Wed
Fri
Moh
Wed
Fri
Feed BOD
5 day
(mg/l)
85
70
90
78
65
88
110
72
82,
96
60
75
92
Feed BOD
20 day
(«ig/l)
122
141
101
108
161
125
138
108
Pond BOD
5 day
(mg/l)
4.2
3.8
3.4
3.0
3.8
4.1
3.4
3.2
4.1
3.3
3.6
4.0
3.8
Pond BOD
20 day
(mg/l)
5-0
4.4
4.8
4.8
4.1
5.0
4.4
4.4
Feed COD
(mg/D
170
170
185
165
152
158
218
151
171
178
141
149
175
Pond COD
(mg/l)
30
28
30
28
32
36
28
30
32
30
32
36
34
Feed
BOD/
COD
0.50
0.41
0.48
0.47
0.42
0.56
0.50
0.47
0.49
0.51
0.42
0.50
0.52
Pond
BOD/
COD
0.14
0.14
0.11
0.11
0.12
0.11
0.12
0.11
0.13
0.11
0.11
0.11
0.11
Averages 82 125 3.4 4.6 167 31 0.49 0.12
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Table 2 — BOD-COD OF RECYCLED WATER, SEPTEMBER, 1967 SERIES
Date
(1967)
9/6
9/7
9/8
9/9
9/10
9/11
9/12
9/14
9/15
9/16
9/17
9/ia
9/19
Day
Wed
Thur
Fri
Sat
Sun
Mon
Tue
Thur
Fri
Sat
Sun
Mon
Tue
Feed BOD
5 day
(mg/l)
40
60
52
35
20
35
35
60
58
52
28
—
41
Feed BOD
20 day
(mg/l)
98
58
- 55
90
—
70
Pond BOD
5 day
(mg/l)
3.6
6.2
4.6
2.8
4.2
2.8
4.0
3.8
3.6
2.8
4.0
3.6
3.0
Pond BOD
20 day
(mg/l)
4-3
3.6
3.6
4.7
4.7
4.1
Feed COD
(mg/l)
75
92
90
75
45
68
65
101
100
98
65
-_
75
Pond COD
(mg/l)
47
42
40
25
30
24
30
28
28
22
33
30
27
Feed
BOD/
COD
0.53
0.65
0.57
0.47
0.44
0.51
0.54
0.60
0.58
0.53
0.43
0.55
Pond
BOD/
COD
0.08
0.10
0.12
0.11
0.14
0.12
0.13
0.14
0.14
0.13
0.12
0.12
0.11.
Averages 43 74 3.8 4.2 79 31 0.53 0.12
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In the June, 1966 series, when wastewater treatment operated
inefficiently, the COD of the stabilized water in the pond was
approximately IS. 5$ of the treated feed. During the September,
1967 series, when markedly better treatment operated, the COD
of the stabilized pond water was about 39.2$ of the treated
wastewater flowing to the pond. It is interesting to note that
average COD of the pond water ranged about the same values —
an average of 31 mg COD/1 — in both series. This suggests that
the prevailing pond COD represents residuals from near -terminal
biological processes.
A striking and consistent feature of-the BOD measurements made
at irregular intervals through the three summers of the project
was their low values. The highest single measurement,: 9»0 mg BOD/1,
was found after an autumn storm in October, 1968. Values were
consistently so low that Hach respirometers proved unsatisfactory
for routine measurements and were abandoned in analytical work.
Undiluted pond.samples rarely fell outside the 60$ oxygen depletion
levels in BOD bottles. There were no evidences of inhibitory
substances in the pond water, and routine measurements of BOD were
taken with undiluted preparations.
18
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SECTION VII
PERFORMANCE OF STORAGE-REUSE POND — NITROGEN AND PHOSPHORUS
The summer of 1966 was uniquely suited for studies of the nitrogen
and phosphorus balance in the Hampstead plant's treated wastewater
storage-recycle pond. From June to early September, a period of
eighty days, no rain fell, and earlier showers had been light. The
weather was bright and clear with little cloud cover. No water
flowed over the spillway of the dam during this period, and in July
and August the volume of the pond was down to minimum levels of
11.5 million gallons. Conditions were excellent for establishing
"steady state" values for the distribution of nitrogen and phosphorus
in the system. The wastewater treatment plant was operating poorly
at that time, and relatively small fractions of these nutrients were
being removed in sludges and lost to the accounting. In normal
operation, from 35$ to 45$ of the nitrogen and phosphorus would
have been disposed of off-site in the biological sludge generated in
treatment.
Although automatic sampling equipment had not yet been installed,
it was possible to collect vertical profile samples during the
stratified daylight period and during the mixed night and early
morning period, and to make rough estimates of "primary productivity."
It was found, however, that losses of total nitrogen and phosphorus
well in excess of sampling variation and error were occurring in
the pond. Only 20$ to 25$ of the anticipated total nitrogen could
be found in the water phase, including suspended materials, and
nitrite and nitrate nitrogen commonly exceeded the organic fraction.
Maximum total nitrogen found in mixed pond water receiving treated
sewage containing 15 to 25 mg total N/l ranged from about 4»0 to
5.0 mg.N/1, more than 80$ of these concentrations as nitrite and
nitrate combined. Ammonia was found only in trace concentrations.
The phosphorus budget also showed unanticipated shortages in the
mixed water phase; treated wastewater effluent samples ranged from
5 mg P/l to 12 mg P/l, with most values falling around 7 mg P/l.
A few fine filter separations indicated that most of the phosphorus
was present as fine particles.
Unfortunately, September storms brought the work to a close before
we stumbled upon the clues to the mechanism of nitrogen and phosphorus
losses. These came later in the review of dissolved oxygen/thermal
stratification data -and the analyses of bottom sediment samples
picked up early in the summer.
19
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During a series of temperature-dissolved oxygen profile measurements
in July, 1966, using improvised theinistor and dissolved oxygen probe
equipment, we noted intervals of 4 to 6 hours in which no dissolved
oxygen could be measured in depths below 5 feet — commonly from
early afternoon until an hour or more after air sparger mixing was
started at 6:00 pm. Although no oxygen was indicated by the probe,
hydrogen sulfide was not formed in the oxygen deficient layers.
This was confirmed by lowering panels painted with white lead paint
into the anaerobic layers. (Many oxygen deficiencies were noted in
the lower strata during the summer of 196? when automatic vertical
sampling and analyses were used.)
The peculiarities in the nitrogen balance may be explained by assuming
the following reasonable sequences. The relationships must remain
as conjectures, however, since critical experimental checks were not
made. First, it may be anticipated that ammonia nitrogen from
wastewater treatment and from degradation of organic nitrogen added
to the pond will be biologically oxidized to nitrite and nitrate.
The long holding periods, the pH range, the ammonia concentrations
and temperatures are all favorable for this conversion. Oxygen is
available at intervals from photosynthetic processes. Second, it
is possible that nitrate and nitrite are reduced biologically during
intervals of zero oxygen. The reduction of nitrate and nitrite
precedes the reduction of sulfate in polluted waters. Reduction of
nitrate and nitrite yields nitrogen gas which may be lost to the
atmosphere during mixing-equilibration. Unfortunately, we did not
set up gasometric analyses to check swings in dissolved nitrogen in
the pond water.
Phosphorus losses are supported by somewhat better evidence. During
the summer of 1966, as water levels in the pond receded and the
algae-coated freshly exposed shoreline dried and caked, a number of
samples of the cake were taken for calcium carbonate analyses. The
white cake that appeared in some places suggested marl formation.
Samples of the chips and cake were dried and taken to the laboratory
for analysis during the winter. In the initial acid treatment of
this material, there was a vigorous effervescence with carbon dioxide
evolution. Analyses for PQif3 yielded concentrations ranging from
16,600 mg P/kg to 26,600 mg P/kg dry weight of material. The COa
evolved was measured on some specimens and was found to represent
carbonate concentrations ranging from 12,000 mg ;C03~2/kg to
13,500 mg C03"2/kg. Calculated as calcium carbonate, the values
range from 2% to 3.8
20
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We did not at first recognize the significance of these analyses.
Initially, we attributed the high phosphate concentrations to the
activities of flocks of tame mallard ducks that lived on the pond,
along with visiting migratory wild fowl, gulls, and other transient
water birds. The ducks were particularly active in the shallows
and along shore where they fed on the bottom stock and performed
the useful function of keeping down emergent weeds. We credited
the ducks with a phosphorus contribution that belonged elsewhere.
The role of the shallow bottom processes of the pond will be
treated in another section, but a summary of our observations of
marl formation should be entered at this point to clarify the
mechanisms by which phosphorus is removed from the water and
concentrated in the shallow bottom solids. The history of our
late concern with the pondTs shallow areas arises from a "prejudice"
favoring thermal stratification and mixing schedules as factors
in the troublesome algal blooms and floating algal mats reported
at various times before the study was begun. Until May of 196S,
however, no blooms were observed by the study group, and the older
notions of their production continued to dominate our thinking.
However, on the early afternoon of May 30 of that year we were
surprised to find a classic bloom in progresse
We discovered this first as a heavy carpet of brown, greasy, puffy,
algal stuffs that had accumulated as a band fifteen to twenty feet
wide along the northern shore of the pond. At the shallow embayed
end of the pond, near the entering return flow from the plant,
small pads of algae were still rising from the bottom, floating
together into mats, and drifting with the very light south wind
to join the pack along the north shore. The accumulation was slow
but steady.
It was possible to observe the process itself in shallows & inches
or less in depth. The loose carpet of mixed algae that grew on
the shallow bottom — principally Oscillatoria and naviculoid diatoms,
together with entangled small predators ranging from armored amoebae
to rotifers and fly larvae — was covered with fine gas bubbles.
Here and there flakes of material would tear loose and be buoyed to
the surface to join the drift. When the bottom was disturbed gently,
the carpet broke up and detached readily from the muddy clay bottom,
floated to the surface in fragments, and merged with neighboring
floating stuffs. The floating, consolidated drift packed along shore
into a "blanket an inch or more thick and developed pressure folds
where there was gentle wave action. When stirred, the blanket broke
into a watery mass, bubbles were released with an audible rustling ,
and much of the stirred material settled into the water.
21
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The flotated bloom material was watery and loose; a mass scooped
by hand slipped through the fingers like light oil. Fourteen mesh
kitchen strainers used for collecting larger benthic organisms
retained only a few large fly larvae and worm tubes from the mass.
The material broke into a coarse suspension and settled into
density fractions — an oily residual film formed at the surfaces
of buckets holding the screens. The bloom material and liquors
showed surfactancy, in the form of a slight spreading film, when
placed on top of clean water.
The oxygen gas in the flotated blanket was identified by the
classical method of Lavoisier. Bubbles stirred loose from the
coated bottom and collected through a plastic funnel in a test
tube by water displacement were tested with a red hot iron wire;
the wire sparkled in a lively fashion and the tube became coated
with a thin film of iron oxide.
The water over the shallow bottoms showed high pH values ranging
from pH 9.2 to pH 10.0. These high values were anticipated from
observations taken with the continuous pH recorder from the upper
strata of the deep sampling area during bright weather thermal
profile studies. When the pH probe was lowered into the bottom
algal mats themselves, the pH values rose even higher; rises of
from 0.3 to 0.5 pH units prevailed through the bottom inch of
mat material. Higher pH values were also found in the floating
mat stuffs.
A number of samples of floating bloom and bottom mat stuffs collected
from the bottom in shallows five to eight inches deep at the inflow
end of the pond were taken. These were dried and analyzed for
phosphate, carbonate, and total nitrogen. The data from these
analyses are given in Table 3, "Analysis of Bottom Algal Mat and
Bloom Materials" on the following page.
The relatively high ratios of phosphorus to nitrogen in the mat
materials, roughly 1.3 P/1.0 N, suggest that the phosphate exists
in a precipitated inorganic form rather than as cellular stuffs.
Since the primary well water itself is soft, with calcium hardness
falling below 40 mg CaCOo/1, the marl carbonate and phosphate must
represent calcium and phosphorus contributed by domestic sewage,
cafeteria wastes, and a limited amount of phosphate detergent used
in parts washing. The binding of phosphorus at the bottom of the
pondfs shallows is an exaggerated version of the phosphorus trapping
that goes on in large lakes. The high hydroxyl ion concentrations
generated by phot©synthetic stripping of carbon dioxide and bicarbon-
ate force calcium carbonate and phosphate out of solution to
precipitate on or in the biological mat structure.
22.
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Table 3 — ANALYSES OF BOTTOM ALGAL MAT AND BLOOM MATERIALS
Sample # Phosphorus (a) Carbonate (b) Nitrogen (c)
(rag/kg)(mg/kg)(rag/kg)
1 - 5/30/68 15,700 22,100 , 12,500
2 - " 20,500 18,600 12,800
3 - " 17,200 10,900 16,400
4 - " 14,400 18,300 —
5 - " 15,500 12,700 11,200
6 - " 17,800 18,100 13,000
7 - " 14,900 16,200 12,100
8 - " 16,600 14,100 11,900
9 - " 19,300 12,800 12,700
A - 7/18/66 16,600 14,400
B - " 26,600 23,500
C - " 20,400 18,200
D - " 19,800 18,700
(a) measured as P0,~*j (b) as CO ~2; (c) Kjeldahl N
Samples 1-6 bottom mat; 7-9 floating mat; A - D, shore cake
Mud takaidirectly below the bottom mat, 6/6/68, one week after the
bloom, showed 25,600 mg P/kg. This material dried to a hard, gray
marl-like material. Other shallow mud samples from the bloom area
yielded from 640 mg P/kg to 16,000 mg P/kg. Variations in the
mud samples followed wide differences in the sand and organic
content of the materials. It was not possible to take satisfactory
cores in these delta—like shallows.
If we take conservative values of ifo phosphorus in the bottom mud of
the shallows and assume the mud to be 10^& dry solids, a one inch layer
over one acre will contain approximately 435 pounds of phosphorus.
Againj assuming an initial concentration of 7 mg/1 phosphorus in the
treated sewage, and an average wastewater flow of 120 gal/min, each
acre inch of mud would have captured 360 days'contribution of sewage
phosphorus — roughly a year's total contribution.
23
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SECTION VIII
PERFORMANCE OF STORAGE-REUSE POND
THERMAL STRATIFICATION AND SPARGER MIXING
THE OXYGEN CROP
During the late spring of 196? automatic sampling-analytical equip-
ment designed to follow changes associated with the development and
breaking of thermal stratification in the pond was completed and
tested. A small covered steel raft was built to support the sampling
apparatus. This device, developed by Black and Decker's maintenance
shop, consisted essentially of a continuous worm-type pump coupled
with a variable depth intake and programmed to deliver water for
five minutes from vertical profile stations 6 in. apart. The water
was pumped from the raft to the shore laboratory trailer through a
350 ft, 1 in. ID, rubber hose and delivered to a common sensor-sampling
trough established at one end of the trailer. The raft sampler was
placed in the deepest section of the pond, in the 12.5 ft bowl, and
adjusted to take samples from 6 in. below the surface to 12 ft below.
A small Genevamatic movement winch stepped the sampling — 24 samples
over 2 hrs in the lowering cycle, 5 minutes rewind to the 6. in. level,
and repeat. The sampler-analyzer examined a vertical profile once
each 125 minutes.
Electrodes, probes, and continuous pressure tape recorders were
established in the system to sense and record temperature, dissolved
oxygen concentration, pH, and conductivity. It was also possible to
take grab samples of up to 3 gallons from any selected depth. In
practice, 3 minutes of flow were allowed to waste in order to clear
sample mix—through or drastic temperature changes.
A continuous sampling pump was placed at the treatment plant outfall.
This pumped treated wastewaters through a buried line 600 ft long to
the sampling system at the trailer laboratory when desired; a remote
switching relay system controlled this pump.
This relatively elaborate sampling system was designed to give
detailed information for the design of an optimum sparger mixing
program. The expectation was that such a program would minimize
troublesome blooms and infestations experienced in the past.
Continuous turbidity and light transmittance measuring equipment
were originally included in the monitoring system. These units were
taken out of the study early in the program because of maintenance
difficulties.
-------
Prior to the 1968 observations that established the role of the
bottom biological processes in the development of algal blooms,
the sparger mixing schedule was considered to be the most probable
cause for the development of algal blooms and for their failure to
appear. Mixing determined the efficiency of light utilization by
suspended algal masses and the mixing schedule adopted was based on
reducing primary production to a minimum.
Several mixing schedules were possible.
A. The pond could be mixed continuously — 24 hrs a day.
B. The pond could be mixed intermittently through the day
and night to prevent any stratification whatever.
C. The pond could be mixed during the day to prevent thermal
stratification and allowed to rest during the night.
D. The pond could be mixed at night and allowed to stratify
during the bright daylight hours.
Schedule D, mixing during the late evening, night and early morning,
and permitting stratification during the bright warm daylight
interval was chosen as the system most likely to limit algal growth.
This choice was based on the expectation that "self shielding" of
the stable upper strata would limit phot©synthetic processes in the
deeper water. Field studies of light transmittance during the
summer of 1966 indicated varying degrees of absorption in upper
layers of the pond — commonly more than 90$ of visible bands
within the upper 3 ft, and 95$ to 99$ within the upper 5 ft. The
3 ft to 5 ft band commonly showed dissolved oxygen gains from
photosynthesis, and the "zone of compensation" as determined by
the hanging bottle technique appeared to be somewhere between 5 ft
and 7 ft from the surface on bright August days.
From these data, and the "logic" of the self shielding concept, the
mixing schedule was fixed to fit the practical shift schedules of
those in charge of waste disposal operations. The sparger was
turned on in the evening at 6:00 pm (DST) and turned off auto-
matically at 6:00 am (DST).
The summers of 196? and 1968 differed from the classical drought
summers of the previous five years. The clear, dry, warm periods
were broken with numerous showers, winds, and heavy storms. The
longest "undisturbed" record of summer thermal stratification that
we were able to secure in 1967 and 1968 was fifteen days*
26
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The patterns of vertical stratification and breakdown vary with
weather. Wind patterns are especially important. The pond lies in
a shallow protected bowl. East-west winds move along its long
axis and push warm water against the dam. North-south winds cross
the pond's narrow section. In the late afternoon there is usually
a gentle flow of air down the northern slope of the basin that
forces warm water toward the south shore. Saturated air fills the
basin on quiet evenings. On clear nights with gentle air movement,
the bowl begins to cool after 10:00 pm to 11:00 pm and heavy dew
forms on the slopes. Thermal stratification appears in late April
or early May, following a succession of quiet nights. Stratification
commonly disappears in the first or second week of October. Thus,
the need for cooling water for air conditioning coincides with
the normal period of thermal stratification in the pond.
Day to day differences in the thermal input and loss patterns, wind,
weather, and storm surges cause wide variations in the building and
breakdown of vertical temperature profiles. Figure 4»"Development
and Breakdown of Thermal Stratification in Storage Pond," on the
following page, presents a representative sequence of changing
profiles during a relatively quiet warm period — June 11 and 12,
1968.
The profiles were sampled over 125 min period; each 2 hrs and 5 min
the automatic profile sampling barge pumped samples to the shore
laboratory from which the temperature at 6 in. increments in depth
were taken. To simplify the presentation in Figure 4> the profiles
given by alternate lowerings are presented.
The day begins in the figure at 6:00 am DST, at the time that the
air sparger mixing was automatically turned off. It will be seen
that the temperatures are uniform from surface"down to the 10 ft
level. This is the level at which the intake to the pumphouse is
located. Below the 10 ft level a puddle of cooler water extending
to the bottom prevails. This cool pool is not available to the
plant; it is a static water mass that is mixed into the overlying
water only during the natural autumn thermal overturn of the pond.
At the finish of the 10:10 am lowering, a layer of warm water
more than 3°C above the mixed column temperature has been formed
through the first foot of depth; this has mixed down through the
lower water to a depth of 2.5 ft. Below this depth, the column
is identical to that which existed four'hours earlier.
At 2:20 pm warm water has been developed down to 5 ft below the
surface. A thin layer of 30°C water exists at the surface, and
there is a sequence of less warm bands below this layer extending
to the mixed column below 5 ft.
27
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20°C
Surface
6 in .
25°C
30°C
35°C
5 ft =
10 ft-
12.5 ft
Figure 4 — DEVELOPMENT AND BREAKDOWN OF THERMAL
STRATIFICATION IN STORAGE POND
(A) 6:00 am 6/11; (B) 10:10 am 6/11j(C) 2:20 pm 6/11;
(D) 6:30 pm 6/11; (E) 10:40 pm 6/11;(F) 2:50 am 6/12
-------
At 6:30 pm a further increment of heat has been absorbed in the
upper bands of water; the sparger mixer has been in service for
half an hour. It will be seen in profile (D) that the upper 6 in.
of the pond has warmed to 35°C and that the water down to 6 ft
had accumulated heat. The waves in the long broken line indicate
the effects of variable light winds in mixing down warm water into
the column. Practically, the pond continues to gain total heat
well after sundown, but surface temperatures drop rapidly after
mixing begins.
By 10:40 pm, 6/11, the vertical temperature profile (E) shows
that surface water has been thoroughly mixed through the top
two feet and the slope of temperature below that level indicates
the upward mixing of cool water into the reservoir of warm water
developed during the bright day. The air sparger hangs 8 ft
below the surface and the mixing produced is most marked above
this level. Mixing does occur, however, down to the level of
the pumphouse intake at 10 ft from the surface.
The temperature profile (F) ending at 2:50 am on 6/12 shows mixing
downward of warmed water to the 7 ft level. The difference between
this column and that existing on the morning of 6/11 is approximately
1°C. Below the 7 ft level mixing in has not yet occurred. The
full record extending through 7:00 am on 6/12 shows a gain of
approximately 2.0°C through the mixed column down to 10 ft. There
has been a slight total gain in heat uptake.
Under normal undisturbed bright weather in June, a band of water
about five feet deep, 5°C to 7°C warmer than the underlying bands,
persists through the night. This stable warm layer of water
effectively isolates the lower water and blocks exchange of dissolved
gases with the atmosphere. The degree of thermal stratification
developed on 7/H/6& would have required strong storm winds over
several hours duration to mix and break up this stable seal.
The pond's capacity as a heat sink was studied during weather
stable periods in July, 196S. Net gain and loss of heat energy
can be estimated from temperature data given by vertical sampling
of the sort developed by the automatic depth sampling system.
The findings were not wholly satisfactory as a heat budget study,
though it was clearly shown that the heat added by the return of
warm process water from the plant was negligible compared to that
added from the sun and air mass.
29
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The schedule of daytime stratification and nighttime mixing
captures and makes available a larger part of the oxygen that
is produced by photosynthesis in the illuminated upper layers
of the pond. This represents a much larger "oxygen crop" than
that delivered by mechanical aeration or by direct transfer from
the overlying atmosphere. The sequences of production and transfer
are outlined in the figure below, Figure 5» "Vertical Distribution
of Dissolved Oxygen and pH during Thermal Stratification and
Breakdown."
0 mg DO/1
Surface
10
15
20
pH 8.0
pH 9.0
pH 10
pH 11.0
Figure 5 — VERTICAL DISTRIBUTION OF DISSOLVED OXYGEN AND PH
DURING THERMAL STRATIFICATION AND BREAKDOWN
(6:30 pm, 7/11/68 stratification — 7:00 am, 7/12 breakdown)
30
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It may be seen that at the close of the unmixed illuminated day
(l) oxygen concentrations in excess of 15 mg DO/1 exist through
the top two feet of depth, (2) DO concentrations in excess of
11 mg DO/1 extend to the 5 ft level, and (3) the dissolved oxygen
concentrations drop off sharply to essentially zero at 7«5 ft
and below. At temperatures prevailing during these runs, the upper
two feet show slightly more than 100$ super saturation with DO,
and the water between two and five feet shows between 30$ and
supersaturation. These levels of DO supersaturation indicate
active photosynthetic production beyond respiratory and BOD demands.
The oxygen load carried in the upper two feet of the pond is in
excess of 590 Ib; that in the next three feet is in excess of 560 lb;
the total oxygen down to five foot levels represents a daily product
of more than 1150 lb. (These estimates are low since the recorder
did not register values exceeding 15 mg DO/1.)
To evaluate the oxygen contribution from photosynthesis on 7/11/68,
some rough approximations of equivalent mechanical aeration require-
ments may be made. At 2 lb D0»hp»hr for a mechanical surface
aerator, the roughly 1200 lb DO load found represents the work of
two, twenty five horsepower floating aerators operating in the pond
for 12 hours, or 25 hp^days aeration.
Not all of the oxygen generated by photosynthesis is available to
the system, however. During nighttime mixing, some is lost by
"desaturation" or reequilibration of the supersaturated surface
layers with the atmosphere. The load of oxygen held in the water
down to the 9 ft level at ?:00 am on the morning of 7/12 is slightly
more than 500 lb. Approximately 650 lb DO has been lost during the
night — some by desaturation and some by respiratory uptake and
the satisfaction of chemical oxygen demand of the deep water. It is
to be noted that the surface 18 in. of water is about 25$ below
saturation in the morning. This indicates mixing in of deficient
deep Water.
Photosynthetic processes during the daytime are marked by the
development of high pH in surface waters as well as by oxygen super-
saturation. As pointed out earlier, the relatively high hydroxyl
ion concentrations associated with the photosynthetic reduction of
dissolved carbon dioxide and bicarbonate ion are associated with
the formation of phosphate bearing marl over the shallows. Figure 5
shows the wide swings in pH that occur through the vertical water
column during daytime stratification and nighttime mixing. The
extreme ranges are represented by the 6:30 pm 7/H/68 and the 7:00 am
7/12/68 samplings. On the evening of 7/11, the uppermost 18"in.of
the pond showed pH values about pH 10.5; the next morning this
depth showed readings of pH 8.4, suggesting that the changes in
carbon dioxide concentrations associated with photosynthesis and
respiration may be in the order of 100 times.
31
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Changes in pH due to photosynthesis found in the stratified pond
are much like those observed in very hard pond water or in sea water
where photosynthetic processes are active. The 'pH values rise with
rising oxygen and fall with declining oxygen concentrations. Below
levels of sparger mixing in the pond, at & to 10 ft below the surface,
the day and night range of pH values is narrow, commonly between
pH 7.4 and pH 7.6, like that of water recycled to the plant. These
lower pH values represent low rates of loss of biologically generated
carbon dioxide from the less completely equilibrated deep water
reservoir.
About Q.6 Ib of primary organics will be produced per pound of oxygen
yielded by photosynthesis in a rich pond like the Hampstead plant's
process water storage system. The daily production of 1150 Ib DO
means that about 690 Ib of organic stuffs is being formed. This
represents an incremental concentration of about 96 mg/l/day, an
extremely high rate of primary plankton production. It is known
that oxidation ponds frequently yield effluents bearing much higher
concentrations of total organics than are fed them. If photosynthesis
were the only biological process controlling organics in the process
pond, it is obvious that organics would ultimately crowd out the water.
The organics produced by photoreduction in the process water pond
include a wide range of biodegradable stuffs. A fraction undergoes
biological oxidation and shows respiratory demand at a continuous,
relatively high ratee This is represented in the daily oxygen
demand indicated by nighttime losses. The residue, which contains
the more stable fractions, breaks down at a lower rate and either
moves out of the pond in overflow, "flies or crawls out of the pond"
as predatory animals, birds, or insects, or accumulates in the
bottom as detritus. About half of the organics , around 350 Ibs,
appears to be oxidized during the dark periods; another half is
metabolized at lower rates. In the steady state condition, of
course, the oxygen demand and oxygen production rates approach one
another. The process water pond is essentially a system that trades
relatively unstable organics in the treated wastewater feed streams
for a lesser quantity of comparatively stable stuffs. The net yield
of algae, predators, and detritus is ultimately set by the available
accessory nutrients, principally phosphorus and nitrogen. Both of
these are kept in short supply in the Hampstead Pond — phosphorus
is removed by marl formation, nitrogen is removed by denitrification
of nitrates.
Evidences of the production of soluble stable organic fractions in
the pond are given by the relatively high COD/BOD ratios of the
pond water — approximately 8/1 — compared to the ratios in the
treated wastewaters going to the pond — around 2/1. See Tables
Nos. 1 and 2.
32
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As will be noted later, the organics leaving the pond on wing,
as insects, well fed ducks, and migratory water birds is an
appreciable fraction of that generated. The resident residue
of slowly degrading organics in detritus, cell fragments, casts
of invertebrates, and dead bits accumulate on the bottom. Such
a pond system ultimately becomes a bog. However, heavy storm
flows with strong wind mixing prolong the process water storage
pond's life by flushing out part of the settleable solids and
carrying away the organic-rich suspension as storm runoff.
Without floods, rivers themselves become sequences of channels,
pools, and bogs.
33
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SECTION IX
STABILIZATION OF CONVERTED ORGANICS IN THE POND
ROLE OF THE GRAZING ORGANISMS
At times prior to the start of these studies, engineers at the Hamp-
stead plant observed that the color and turbidity of water in the
process water storage pond changed markedly from day to day. An
interval of greenish turbidity would be followed by a day or more
in which the water was decidedly less turbid but rusty. Secci disc
readings confirmed the wide day to day swings in turbidity.
In attempts to produce a running record of the color and turbidity
changes, daily samples of recycle water were drawn through Millipore
filters and the adhering solids preserved with acetic acid and
copper sulfate fixing solution. This produced a chart record
running through several summer months. The membranes mounted in
calender fashion showed sequences of striking color shifts —
from green and copper green to straws, brown, to straw and green.
No color persisted for more than two or three days, and no fixed
sequence of colors appeared.
Microscopic examination of the filter-captured materials showed a
dominance of planktonic diatoms and green or blue-green planktonic
algae — usually motile flagellates — in the greenish preparations.
The rusty preparations showed relatively few diatoms or algal
masses, but yielded numbers of small crustaceans: Limnocalanus,
Gypris,-Bosmina, and sometimes Daphnia.
In the late summer of 196?, some observations made by accident on
the behavior of large batch samples brought into the trailer
laboratory gave this earlier set of observations new meanings
and altered the direction of our studies of the pond. A series
of one gallon samples taken for laboratory studies were stored in
aquaria on the counter in the trailer and left over the week-end.
On Monday morning it was -discovered that the water in the aquaria,
initially green and turbid, had cleared to a light straw color and
now contained some hundreds of small Bosmina flipping freely about.
- • -I
The bottoms of the aquaria were covered with a light film of detritus
and fecal pellets, together with moults of Bosmina. By the end
of the working day, most of the Bosmina were dead and on the bottom.
Those that swam were pale and empty. s
We left these aquaria in the light for five days to see if plankton
algae would redevelop. This did not occur.
35
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Instead, very light films of attached blue-green algae grew on the
walls of the aquaria, and oily bacterial films bearing packs of
armoured amoeba appeared on the water surfaces.
This grazing experiment was repeated four times with variations.
Whenever heavy growths of green or blue green algae appeared in
the pond, gallon aquarium samples would be taken and stored in the
trailer laboratory for close observation. Usually, noticeable
changes in color and turbidity would take place within four or
five hours, and always overnight. The direction was the same in
all cases; the shift would be from turbid greenish color to
clear straw or yellowish. Phacus and Euglena mixed with diatoms
and occasional blooms of Pandorina would change to suspensions
of swimming Cladocerans which starved and died. The sequence time
at 220C was four days.
These closed aquaria systems differ from the process pond in that
they have no steady input of nutrients. Full recycle in the aquaria
requires regeneration of all nutrients from the decomposing debris.
In the pond slow regeneration from the debris undoubtedly occurs,
but there is, in addition, a daily input of fresh nutrients that serves
to maintain a more nearly steady state condition.
Following observations on the building of the algal bloom in May,
1968, a series of walled cells were set up in the shallows. These
consisted of "bottomless boxes" — 24" long, 12" wide, and 12" deep —
made of acrylic plastic. These compartments were set on the bloom
producing bottom in water about 8" deep, maintaining 4" freeboard.
The cells effectively limited flow and mixing exchange of water
over the enclosed bottoms but did not appreciably shade the enclosed
water. Materials that floated up from the bottom could not float
away.
In two days it became apparent that the grazing organisms and other
predators had taken over on the sheltered bottom. The water itself
had cleared up and the carpet on the bottom of the cells was riddled
with holes produced by snails, worms, and fly larvae. Many snails
were visible and stringy slime tracks covered the loose settled
detritus j hundreds of fly pupae and casts had accumulated around the
upper margins of the cells, gnat larvae, mosquito larvae, and worms
were swimming freely in the overlying water. The algal carpet which
still existed over large patches of the open shallows was reduced
to frayed networks within the cells.
The grazing invertebrates continued to dominate the biota of the
cells for five more days. The cells were then lifted and moved to
another area in the shallows. Within three days the old grazed over
areas occupied by the cells were covered with fresh carpets of algae.
36
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This simple experiment demonstrated (a) that the presence of heavy
growths of bottom blue greens and other algae in the bottom carpet
of the shallows — from which the floating algal blooms were derived —
required a continuous supply of nutrients from the overlying water,
(b) that the regeneration of nutrients from the bottom itself is
slow, and (c) that air sparger mixing schedules applied to the deeper
waters of the pond had nothing to do with the incidence of blooms.
When the same grazing breakup of the shallow carpets in the newly
located cells began to appear in 2 to 4 days, the cells were covered
with transparent plastic panels. On the following morning all
covered cells were examined. Massive emergences of Chironomid
gnats and small Tabanid flies were found on the upper side-walls
and undersides of the covers, most trapped in droplets of water
condensate. Numbers of Chironomids exceeded 500 in two of the
cells, and 700 in two others. The Tabanids trapped ranged from
15 to 7&. Culex mosquitoes were found in only one cell — 10 adults
trapped in condensate.
After inspection, the cover plates and free sidewalls of the cells
were wiped free of adhering insects and the covers replaced. The
cells were left in their original positions. After three days, the
systems were again examined for continued emergences of insects.
This time the cover plates were literally felted with Chironomus
gnats, standing room only. The insects were trapped in the condensate
and matted into a thick film by entomophagus fungus hyphae; a count
and identification of the mass was too laborious to consider. The
density of gnats on the surface plate exceeded 10/in , indicating
emergences of over 1500 per square foot of bottom. A continuous
film of pupal casts and other floating debris covered the enclosed
water surfaces of the cells. Microscopic examination of this oily
film showed a zoogleal bacterial structure and a wide range of large
grazing ciliates, clusters of amoebae, and nematode worms. Water
mites were also abundant.
It would appear that the quiet surfaces provided by the sheltering
cells and cover favored the successful emergence of the large gnat
population. At intervals, spiraling swarms of gnats were noted
about the edges of the pond, but these did not suggest the high
densities and activities of larvae in the coated shallow bottom.
It would also appear that night emergences are common. Casts of
gnat pupae commonly appeared in surface films swept along shore
by gentle winds, but these gave no indication of the large populations
active in the shallows of the pond.
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SECTION X
LEAF WASH, DRIFT, STORMS AND FILLING OF SHALLOWS
The useful life of a storage pond is determined by the rate at which
it fills with silt or other solids and by the accumulation of materials
that limit useful water quality. The processes involved are like
those that take place in all natural lakes and ponds. Feeder streams,
storm flows, and winds carry soil and organics into the pool and
they can be moved out only by flood carryover. Flushing is always
incomplete, and there is a net accumulation of water displaced
stuffs in the basin.
In small natural lakes, the sequence of filling — from lake, to
pond, to marsh, to swamp <— is accelerated by the growth of emergent
weeds, shrubs, and trees in the shallows and around the borders.
These contribute seasonal loadings of dead plant stuffs, seeds and
pollen, leaves, twigs, fallen limbs and trunks to build a border that
encroaches upon the deeper waters. This border buildup also captures
storm-borne solids. The net effect is the projecting of a swamplike
shoreline into the deeper areas. The deep areas fill last. As
waterlogged materials, leaves, and fines collect in deep areas, a
false bottom of anaerobic, slowly decomposing organics builds up.
The lower oxygen-free strata of collected organics in the false
bottom are esisentially "pickled." Organics are lost relatively
slowly -by anaerobic fermentation from these boglike bottoms; methane
and a variety of organic acids are lost to the overlying water by
slow diffusion or gasification.
The Hampstead plant process water storage pond would normally evolve
into a swamp with no storage Value if left alone. A number of routine
maintenance operations operate to forstall progress to this end.
First, the margins have been kept clear of weeds and woody plants
by mowing and other landscaping operations. Second, air-sparging
of the deeper pond has prevented the accumulation of fine debris
as a false bottom. Third, ducks and visiting waterfowl reduce the
emergent and floating weed populations in the shallows. In addition,
the high densities of small invertebrates — insect larvae, snails,
beetles, and worms — break down leaf materials and other plant
tissue that drifts into the shallows where they are active. At
various times after the sparger system was installed, Black and
Decker f's maintenance engineers made studies of the pond*s bottom
in the'vicinity of the pumphouse intake. A specially built illumi-
nated water periscope permitted direct viewing of the bottom 12 ft
deep. In this area, the bottom was found to be a clean clay surface
free of any organic sludges or recognizable fragments.
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The drought of the summer of 1965 was so extreme that a deep trench
was dug around the western border of the plant's property to capture
all surface runoff that might be generated by providential rains and
to carry this to the pond. There were no rains, but during the fall
the trench served an unanticipated function} it collected a large
fraction of the hardwood leaf fall from some acres of forested slope,
stored them, and then discharged the whole load into the pond with
the first storms of early winter.
This incident was cause for concern. A wet leaf is an appreciable
object in a process water stream. Up to this washout, however, no
clogging of lines, orifices, or screens by recognizable leaves or
stems had occurred, though leaves undoubtedly reached the pond on
earlier storms. The leaf load delivered from the emergency runoff
trench represented an unprecedented slug loading, and nothing whatever
was known about the fate of leaves in the pond.
A series of battery jar and BOD bottle tests were set up in the
University laboratory to see if the leaf wash might be significant
in the recycle from the pond during the next summer. The results
were not reassuring. Layers of leaves mixed with pond soil and
stored under 10 in. of water at 8°C showed occasional bubbles of gas,
the water became stained, and nothing else seemed to happen over a
six month period. Loss of organic carbon was approximately 3% per
month. At 4°C it could not be measured. BOD tests of diluted leaf
suspensions indicated that &fo to 10$ loss per month in aerobic
water might be expected at 8°C. It appeared from this that leaves
might be a problem in water recycle. They had been washed into the
pond, and some interval of neutral suspension of the waterlogged
stuffs was inevitable.
In May, 1966, small snapper samples from the bottom muds of the
general bottom were examined for identifiable stems, leaf fragments,
and other evidences of the heavy, charge pushed in from the trench
during the previous winter. These mud samples taken from the 3 ft,
5 ft, and 10 ft levels showed only occasional fragments of bark and
rotted twigs, but nothing recognizable as leaf material. The bottom
at 10 ft to 12 ft depth was free of organic mud.
During the late August, 1968, studies of the grazing activities of
insect larvae and other invertebrates in the shallows, dry leaves from
the woods were placed in two of the cells in the 8 in.shallows and
lightly stirred into the surface mud. The wetted leaves were examined
over the following two months. Within ten days the leaves were
reticulated by small beetles and fly larvae; within a month the heavy
vein structures of oak and hickory leaves that remained fell apart
on handling. Microscopic examination showed limited fungus attack
of the heavier tissue; the general pattern indicated mechanical
attrition by cutting and chewing.
40
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Numerous fecal pellets containing globules of plant resins were
mixed through the mud film.
The relatively large shallow areas of the Hampstead plant's process
water pond appear to be effective sites for invertebrate grazing of
leaf materials that would normally accumulate in the unstirred deeper
waters of wooded ponds where these organisms may not be so effective.
For reasons not clear, whole leaves were not picked up by the pump
intake system. Observations of the shallow pool at the discharge of
the cone seive backwash line failed to show any leaves or stems at
any time during the summers of 196? and 196£.
Shallow borders are more efficient settling systems than deep pools
are. This is demonstrated in Hampstead plant's storage pond where
delta formation at the mouths of channels and sloughs feeding the
pond has taken place following heavy floods. First, the distance
that heavy solids travel to meet the bottom is shorter in the shallows.
Second, winds push floating stuffs to the shore where they waterlog
and settle.
Following torrential storms in September, 196?, which gullied sections
of the feed trench and undermined parts of the plant's parking lot,
much heavy material, fragments of masonry, rocks, pebbles, sand
and debris was swept into the shallow influent neck of the pond to
build a substantial bar and to extend the delta that encroaches on
the shallows. At lowering water levels, new, firm land stood above
water, and the barely wet submerged areas grew the start of a water
weed crop that developed in the following May as a dense weedy patch
of Persecaria, fox tail, water willow, and emergent grasses.
Approximately half an acre of shallow water surface was lost in this
changeover. A gravel spit developed beyond the sluice carrying
water from the western plant boundary.
Rooted vegetation stabilizes deltas and islands formed in this way
and limits the loss of solids with storm exchanges through the pond.
Inevitably, over a long period and many floods, the pond will lose
its useful volume and this occurs most rapidly in the shallows.
Dredging is the only practical method of recovering these biologically
useful areas.
41
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SECTION XI
QUALITY OF WATER IN PROCESS WATER POND
The most distinctive feature of the water in the process water pond
during low rainfall summer months, such as those of 1965 and 1966,
when there was almost no dilution with runoff water, was its straw
to yellowish-green color. This color was only slightly reduced when
the water was filtered through 0.47 mu Millipore membranes; the tint
itself persisted. Color densities ranging to 20 units on the standard
chloroplatinate were usual-for the filtered samples. This color was
not changed by changes in pH over the practical pH adjustment range —
pH 7«0 to pH 8.2. It decreased slowly in samples stored in Saran
bags exposed to bright sunlight. Hypochlorination to 20 mg flash
residual chlorine/1 reduced the color from 20 units to less than
5 units. This persistent color limited the usefulness for such
otherwise possible applications as toilet flushing.
The high pH values in water encountered in shallows and surface
strata during bright days has been mentioned earlier. The water
appears to be saturated in respect to calcium ion at these high
al&alinities. This limited the use of recycled water for plant
applications where flash heating over surfaces might deposit calcium
carbonate crusts as in the cooling of rectifier tubes and hollow
qored welding operations. Lime deposit was not significant for
massive cooling and quenching steps. The apparent high calcium
hardness obtained from surface water of the pond — exceeding the
solubility of calcium carbonate at pH 9.6 to pH 10.0 — seems to
be related to local concentrations of colloidal marl. Calcium
carbonate hardness determined by EDTA titration gave high values of
800 mg CaC03/l and median values of 400 mg CaC03/l during the very
dry, summer period of 1966. Hardness of the treated wastewater inflow
during this period ranged from 350 mg CaC03/l to 425 mg CaC03/l.
The odor of water from the pond, during summer storage varied.
A variety of opinions may -be expected from any large group aware
of the sources of water used in manufacturing processes in which
they are involved. Over the period of the study there were very
few complaints. The best point for judging odor quality objectively
is the outdoor area in the vicinity of the evaporative cooling tower.
Here the odors have been described as "woody," "fishy," and "grassy."
In the vicinity of the pond itself and around the sampling sink of
the trailer laboratory, the odors were predominantly woody and
grassy. There were occasional intervals of light kerosene-like
odors in this area. It was not possible to determine the sources
and no "slicks" were noted.
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Only at the inlet feeder end of the pond was there any suggestion
of a phenolic-sewage odor. Judging the odor of the pond itself was
complicated by the considerable areas of mowed lawn around the
basin.
Total bacterial counts were highly variable and subject to the technique
used. Agar plate counts of recycled water ranged from high values
exceeding 50,000/100 ml to less than 1000/100 ml. Common soil
spreaders were abundant and confused total counts badly. Coliform
values ranged from more than 1000/100 ml to zero with most values
falling between 100 - 500/100 ml. The coliforms, determined by
MF Endo techniques, did not demonstrate the high rates of acetaldehyde
production characteristic of fecal strains. Essentially, the bacterial
characteristic of the process water pond were much like those of the
farm water conservation pond.
Metals were surprisingly low in the recycled water —• iron was present
in traces, below 0..05 mg total iron per liter; total zinc and copper
ranged below 0.1 mg/1; and total chromium below 0.02 mg/1. Cyanide
could not be found and detergent concentrations (as anionics) fell
below 0.5 mg MBAS/1 in the recycled water.
Ether soluble oils gave maximum values of 40 mg/1, and ranged about
20 mg/1. (The plant removes and recovers cutting oils and coolants
in an internal separation-extraction-recycle system.)
Chlorides during the summer of 1966 ranged about 80 mg Cl~/l, with
maximum values of 110 mg Cl~/l. These concentrations occurred during
a period in which the pond lost volume through evaporation.
The recycled water showed no unusual foaming tendencies, though surf-
active substances, indicated both by spreading force tests and
DuNuoy balance tests, were present. At various process points in
the plant^ some foam accumulation was observed, but this condition
could not be separated from additions at these points.
Since the principal and important function of the recycled water was
to remove heat from the plant and plant processes •— by direct and
by evaporative cooling exchange — and secondarily, to wash and
cool metal surfaces in production processes, potable water standards
are inapplicable in judging the usefulness of the water. The heat
adsorbing and heat exchanging capabilities of recycled waters are
only affected by quality changes that produce scaling, corrosion,
sliming, foaming, clogging or which limit heat exchange and washing.
As a rough estimate, approximately 60" of water was evaporated from
the pond during the long dry summer of 1966 when no rain fell.
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In that interval, about 20 million gallons of water was pumped from
the wells — slightly more than the total volume of the pond. During
peak cooling, the evaporative losses were estimated to be about
equal to the well production. This water loss by evaporation increased
the concentration of stable components from the treated sewage
approximately 200$. That increase in mineral content and stable
substances was less than that developed by evaporation in creeks
and rivers in a number of high water use areas of the East coast.
45
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SECTION XII
ACKNOWLEDC
The Project Director wishes to acknowledge the cooperation and
assistance given to this investigation by Black and Decker
Manufacturing Company's Hampstead and Towson, Maryland, officers
and managers, for providing access and facilities and developing
necessary accessory information. He wishes also to thank the
Edison Electric Institute Cooling Water Project of the Johns
Hopkins University for developing special meteorological data.
Mr. James Woodward, Mr. Philip Pedone, Mr. John Preston, Mr. Donald
Hertz, Mr. George Burck, Mr. Herbert Allen, Mr. LaVere Grimes, and
Mr. Edward Orndorff, of Black and Decker's engineering staff provided
a wide range of day to day technical assistance and maintenance
service at the Hampstead plant and made the practical operation of
the study possible. Mr. Woodward was especially helpful in getting
the work organized and in developing historical information.
Special appreciation is due to Mr. James E. Taylor, retired, of
Black and Decker's engineering department, who designed and built
the sequential sampling apparatus used to study stratification and
mixing in the process water pond.
Dr. Imgard Wintner, Mrs. Mary Barada, and Dr. and Mrs. Loren Jensen
provided invaluable analytical and field liminological services;
Mr. Edward Herricks and Mr. Jay Sculley developed a series of unique
data in the course of their graduate studies in the program.
Mr. Herricks1 observations on the sequences of production and
predation were especially helpful.
This project was supported jointly as Demonstration Grant WPD 117
by the Office of Research and Monitoring, U. S. Environmental
Protection Agency, and by the Black and Decker Manufacturing Company.
The Project Director wishes to thank Dr. H. S. Skovronek, EPA Grant
Project Officer, for his patient encouragement.
47
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SECTION XIII
BIBLIOGRAPHY
A number of presentations of various phases of this study were made
before professional groups during the investigations. The following
papers derived from the project were published.
1. Renn, Charles E.; Squeezing More Out of Water
Proceedings of the llth Annual Conference; Water for Texas,
Texas A. and M. University, Nov. 21-22, 1966 — pp 9 - 12
2. Renn, Charles E.; Serendipity at Hampstead, a Study in Water
Management
Industrial Water Engineering; ^ (9); September, 196?f
pp 25 - 28
3« Jensen, Loren D. and Charles E. Renn; Use of Tertiary Treated
Sewage as Industrial Process Waters
Water and Sewage Works; April, 1968, pp 38 - 41
4. Renn, Charles E.; Experience in the Treatment and Re-Use of
Industrial Waste Waters
Proceedings of the 24th Industrial Waste Conference;
Purdue University, May 6-8, 1969, PP 961 - 968
5. Sculley, J. R.; An Energy Budget Study of a Small Industrial
Process Water Pond
Master's Essay, The Johns Hopkins University, Department of
Engineering Science, May, 1970 — (Jointly with Edison
Electric Institute)
49
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SELECTED WATER
RESOURCES ABSTRACTS
INPUT TRANSACTION FORM
1, • Repart ffo.
3. A ccession No.
w
4. Title
Management of Recycled Waste-Process Water Ponds
7. A uthor(s)
Renn, Charles E.
6* ' •
S, -Performing Organization
Report ffo. •
9. Organization
The Johns Hopkins University
1 Dept. of Environmental Engineering Science
Baltimore, Maryland
10. Project No.
WPD 117
11. Contract/Grant No.
13. Typeo* Report and.
Period Covered
2. ^.Sponsoring Organization
IS. Supplementary Notes
Environmental Protection Agency report number,
EPA-R2-73-223, May 1973.
16. Abstract
This study describes the successful operation of a storage pond used to
collect treated wastewaters and runoff for recycle to manufacturing operations
under conditions of drought and severe water shortages. Treated sewage and
cafeteria wastes Mire stored in an air sparger mixed pond and are returned to
the manufacturing plant to provide water for evaporative cooling and a variety
of production processes. By applying long term storage, air sparger agitation,
and controlled stratification during the summer, it has been possible to
increase the effectiveness of limited well supplies from six to fifteen times.
The efficiency of the pond depends in larger part upon biological processes
that go on in the comparatively shallow areas of the system. These act to
capture phosphorus and to stabilize algal organics generated in the pond itself.
17s. Descriptors
*Water Reuse, ^Biological Treatment, *Self Purification
17b. Identifiers
*Lagoon, Aeration, Waste Assimilative Capacity, Industrial Wastes
17c. COWRR Field & Group
18. Availability
79..- *S6earity Class. s
(Report)
20. 'Secofity Class.
--•.- , (fage> . .
21. fffhOt
.Pages
22.*, Price
Send To:
WATER RESOURCES SCIENTIFIC INFORMATION CENTER
U.S. DEPARTMENT OF THE INTERIOR
WASHINGTON, D. C. 2O24O
Abstractor Charles E. RTTI ' Inst'tut'on The Johns Hookins Unlversitv
WRS1C 1 O2 (REV. JUNE 1971)
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