019468
OVERLAND FLOW OF OXIDATION POND EFFLUENT
AT DAVIS, CALIFORNIA
by
Sch
and R.J . Stenquist
D.L. Tucker,7 E.D. Schroeder,2 D.B. Pelz,3
prepared for the
Environmental Protection Agency
Technology Transfer Program
January 1977
Vroject Engineer, Brown and Caldwetl. Walnut Creek, California
2Chairperson, Department of Civil Engineering, University of California, Davis,
California
3Director of Public Works, City of Davis, California
^Project Engineer, Brown and Caldwell, Walnut Creek, California
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OVERLAND FLOW OF OXIDATION POND EFFLUENT
AT DAVIS, CALIFORNIA
Oxidation ponds are one of the most commonly employed secondary wastewater
treatment systems in the United States for systems with average dry weather flows of
approximately 0.2 cu m/sec (5 mgd) and less. Oxidation pond systems are generally
easy and inexpensive to operate and maintain but require larger land areas than
conventional systems. Thus, they are cost-effective for smaller communities where
land is usually less costly than in urban areas.
Oxidation ponds are an effective method of waste stabilization except for the
large concentrations of algae carryover in the effluent. This algae is the source of
high suspended solids measurements, and decomposition results in oxygen demand
in the receiving waters. The EPA definition of secondary treatment requires a
30-day average value of 30 mg/1 or less for suspended solids and 6005. A process
for algae removal will enable oxidation pond systems to meet secondary treatment
requirements.
This paper reviews the analysis of algae removal alternatives examined for Davis,
California, and describes pilot studies of overland flow as a new approach to algae
removal. The pilot study was a crucial part of the selection process leading to
recommendation of an alternative.
BACKGROUND
The original sewerage system in Davis discharged effluent, after treatment in an
Imhoff tank and rock filter, to the old Putah Creek channel approximately 600 m (2,000
ft) south of the city. In 1950 trunk sewers and pumping stations were constructed
and a new plant was constructed about three kilometers (two miles) north of Davis.
This plant had primary facilities for domestic flow with a capacity of 0.048 cu m/sec
(1.1 mgd) and a total of approximately nine hectares (22 acres) of oxidation ponds.
The primary facilities consisted of preaeration and sedimentation tanks, a sludge
digester and digested sludge drying beds.
In 1958 separate industrial waste treatment facilities were constructed at this
same location to handle tomato and peach processing wastes generated by a local
cannery, Hunt-Wesson. These facilities included a 0.6-m (24-in.) industrial waste
sewer from the cannery to the treatment plant, a separate industrial waste pumping
station, and 50 hectares (122 acres) of waste stabilization ponds.
A 1961 survey of the sewerage system by Brown and Caldwell indicated that
the domestic wastewater treatment facilities had reached capacity. In addition to
a series of trunk sewer system improvements, a staged expansion of the treatment
plant was recommended. However, these improvements were not undertaken.
In 1968 a new sewerage study was prepared by Brown and Caldwell for the City
of Davis.z In the interim period, both the domestic facilities and the cannery stabi-
lization ponds of the existing 77-hectare (186-acre) site had become significantly
overloaded. Because of disadvantages of the existing site and the inability to
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incorporate existing facilities in alternative treatment plans, the 1968 report recom-
mended a site about eight kilometers (five miles) northeast of downtown Davis.
At the time of the 1968 study, consideration was being given to consolidation of
sewerage from the El Macero District, located to the southeast, with that of Davis.
Analysis indicated that it would be less costly for both Davis and El Macero to treat
their combined flow at this eastern site.
Soon after the 1968 report was issued, the only major industrial contributor,
Hunt's Cannery, elected to provide their own separate treatment facilities near the
location of the recommended site for the Davis facility. The new Hunt's facility
provides screening of raw wastewater at the cannery. Screened waste is conveyed
to a 91-hectare (220-acre) overland flow site and applied directly through 1.6 cm
(5/8-in.) spray irrigation nozzles. Overland flow effluent from the Hunt's facilities
is collected in open ditches at the bottom of terraces and discharged to the Willow
Slough Bypass. This facility operates about four months per year. Well water is
used for irrigation in other months to maintain the grass. The Hunt's facility has
experienced difficulty in meeting effluent limitations during the initial part of their
yearly operation because of inherent start-up problems in the overland flow scheme.
Design data for the present Davis treatment plant, completed in 1974, are given
in Table 1 and the plant layout is shown in Figure 1. The plant was constructed to
accommodate a population of 45,000 with an initial average dry weather flow of
0.22 cu m/sec (5 mgd) and a maximum dry weather flow of 0.44 cu m/sec (10 mgd) .
The plant is designed for a peak wet weather flow of 0.88 cu m/sec (20 mgd) .
Provision was envisioned during the 1970 design studies for the site to be able to
eventually accommodate a design population of 240,000 by replacing the oxidation
ponds with an activated sludge secondary treatment facility.
The initial treatment facilities include coarse screening, prechlorination, influent
pumping, comminution, preaeration and grit removal and primary sedimentation.
Secondary treatment is provided by three oxidation ponds operated in parallel. Final
effluent disposal is through an outfall to Willow Slough Bypass after the oxidation
pond effluent is chlorinated. Sludge digestion, with dewatering and disposal in
holding basins, is provided for sludge from the primary sedimentation basins.
Pumping units are provided for effluent pumping during extreme flooding in Willow
Slough Bypass, and postchlorination facilities are available if required.
The three-pond system of 50 hectares (120 acres) includes circulation channels
to orovide for load distribution as well as initial mixing. Return flow through the ponds
of UD to six times the influent volume can be attained. With the influent discharged
to the channel ahead of the recirculation pumps, intimate mixing is immediately
achieved Both pond inlets from the channel and outlets to the channel are through
graed culvert type ports operating partly full so that scum does not accumulate.
Outlet ports are downwind so that scum is effectively prevented from forming on the
pond and any that does form is redispersed in the circulating water by the pumps.
Provision is made so that effluent can be discharged either from the return channel
or from the final pond if the ponds are operated in series. The ponds provide a
minimum of 39 days detention for design flow conditions.
Water level in the ponds is controlled by a weir in the plant effluent control
structure Effluent spills over the weir and drops about eight feet where it
discharges through the outfall to Willow Slough Bypass. Discharging in this
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Table 1. Design Data for Existing Facilities
Design Factor
Deslun Flow
Average dry weather, mgd
Maximum dry weather, myd
Peak wet wather, mgd
Design
Design population, thousands
Biochemical oxygen demand, 10UO lb/djy
Suspended solids, 1000 lb/ddy
Raw Sewage Pumps
Number
Capacity, each, mgd
Total dynamic head.PWWF, feet
Rated horsepower per unit
Raw Sewage Screening Equipment
Commlnutors, number
Channel width, feet
Aerated Grit Tanks (preaeratlon t.mks)
Number
Detention time at avg, dry weather flow, hrs
Air supplied, cfm to tanks
Air supplied, cfm to channels
Chlorlndtors
Number
Capacity, each, Ib/day
Plant Outfall Sewer
Number
Size, Inch diameter
Emergency Generator
Number
Generator rating, kw
Engine horsepower
Sludge Digestion Facilities
Digesters
Number
Total volume, 1000 cu ft
Loading, 1000 Ibs dry solids per day
Detention at 4 percent solids, day
Gas produced, 1000 cu ft per day
Assumed solids reduction, percent
Digested sludge, 1000 Ibs dry
solids/day
Value
S
10
20
4S
11
11
2
20
AO
200
1
0.48
300
360
2
2000
1
60
1
300
335
1
78.5
7.15
27
45
40
4.3
Design Factor
Value
Sludge Holding Basins
Number 2
Net area , acres 2
Total volume, 10 ft depth, mil cu ft 0.85
Solids loading
Million Ib per year l.SS
Ik per cu ft 18.5
Capacity, solids, mil Ib IS
Primary Sedimentation Tanks
Number 2
Detention time at avg dry weather flow, hrs l.S
Overflow rate at avg dry weather flow, gal/iq ft/day ^060
Mean forward velocity, fpm 1.36
Maximum hydraulic capacity, mgd 10
Raw sludge pumps, number 2
Scum ejectors, number 1
Assumed Primary Treatment Efficiency
BOD removal, percent 3$
Suspended solids removal, percent 65
Oxidation Ponds
Number 3
Total area, acres 120
Average depth, feet 5
Total volume, mil gal 196
Detention at avg dry weather flow, days 39
Pond Circulation Pumps
Number 2
Capacity, each, mgd 15
Total dynamic head, feet 3.S
Rated horsepower per unit 15
Chlorine Contact Tank
Total volume, 1000 cu ft 9
Contact time at avg dry weather flow, minutes 20
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SITE PLAN
Figure 1 Layout of Existing Davis Treatment Plant
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manner is intended to aid photosynthesis in the ponds and to maintain a near-
saturation DO in the effluent at all times. The effluent flows east in Willow Slough
Bypass a distance of about two miles to Yolo Bypass. The Willow Slough Bypass
does not receive flow from Willow Slough during dry weather. It is a shallow
ditch during dry weather receiving some agricultural runoff. Dissolved oxygen
downstream from the Davis discharges is generally not depleted but tends to remain
at near-saturation levels . The waste discharge requirements in effect at the time of
the existing treatment facilities design were adopted by the Central Valley Regional
Water Quality Control Board on December 1, 1969, by Resolution No. 70-104. The
applicable provisions include a receiving water limitation stating that discharge
shall not depress dissolved oxygen content of Yolo Bypass waters below 5 mg/1 at
any time.
The federal definition of secondary treatment was taken into consideration by the
Regional Board in establishing 1977 discharge requirements for Davis. The require-
ments include a limitation of BOD5 and suspended solids to a 30-day average of 30 mg/1.
The 30-day median total coliform organism concentration is limited to 23 MPN/100 ml.
Average daily dry weather discharge is limited to 18,700 cu m (five million gallons)
and pH must be between 6.5 and 8.5. Davis is required to limit mineralization to no
more than a reasonable increment and not to cause dissolved oxygen concentration in
Willow Slough Bypass to fall below 5.0 mg/1.
In 1975 the Davis pond effluent average suspended solids concentration was
74 mg/1. The average for the highest month (April) was 93 mg/1 and the low monthly
average (December) was 56 mg/1. Because a significant portion of the effluent BODc
from oxidation ponds is made up of biodegradable cell solids, the effluent BODc values
during 1975 were high, although they were still generally below the 30-mg/l level.
Average 6005 for the maximum month was 27 mg/1. Average 1975 BOD5 was 19 mg/1
for the pond effluent.
For analysis of alternatives to meet the new discharge requirements, estimates of
primary treatment efficiency were based on the design assumptions for the existing
facilities, as developed in the 1970 design study. Estimated primary BODs removal
efficiency is 35 percent. Estimated primary suspended solids removal efficiency is
65 percent. For design conditions, the average peak month outflow at the Davis
facilities is estimated to be 0.21 cu m/sec (4.75 mgd), taking into consideration
oxidation pond evaporation. Under present conditions of an average dry weather
inflow of 0.12 cu m/sec (2.73 mgd), oxidation pond evaporation during much of the
late summer and fall allows zero discharge. At design inflow conditions, discharge
would occur year-round.
An operations staff of eight people is located at the treatment plant. All waste-
water management functions are the responsibility of the director of public works.
Total operation and maintenance cost for the 1975/76 fiscal year was $342,300, which
included costs for the collection system, treatment facilities, and general administra-
tion . Based on an average flow of 0.12 cu m/sec (2.73 mgd) , this is a cost of 9.0 cents/
cu m (34 cents/1000 gallons). For the treatment facilities only, average operation and
maintenance cost for the 1975/76 fiscal year was 5.8 cents/cu m (22 cents/1000 gallons).
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ALTERNATIVES FOR IMPROVED TREATMENT
The principal objective of the planned facilities improvement at Davis is to provide
treatment which will produce an effluent suspended solids levels which meets the new
discharge requirements . Alternatives were considered under several categories
including replacement of existing treatment processes and additions to the existing
system.
Alternatives considered for implementation at Davis were compared in several
ways. A principal consideration was cost-effectiveness, which includes a monetary
cost analysis and an environmental and social impact analysis. Other important
factors may also affect an alternative's suitability. Two important categories of
criteria are "engineering effectiveness" and "conformance with identified constraints."
Engineering effectiveness concerns the ability of an alternative to perform as
planned and to be free of mechanical breakdowns. Identified constraints include
relevant local, state, and federal laws; administrative guidelines and regulations;
and the opinions and goals of the affected community.
Preliminary estimates of construction costs for the alternatives were developed
principally from the experience of Brown and Caldwell in designing similar facilities.
Supplemental cost information on overland flow and intermittent sand filtration was
taken, respectively, from "Costs of Wastewater Treatment by Land Application,"*
and "Intermittent Sand Filtration to Upgrade Existing Wastewater Treatment
Facilities."4
Construction cost estimates used in the Davis analysis were based on an ENR
Construction Cost Index of 3200, a value expected to be applicable in September,
1977 and which is equal to 97 percent of the San Francisco ENR Index projected for
September, 1977. Cost data given herein can be related to current price levels at
any time by applying the ratio of the ENR Index prevailing at the time to 3200.
A contingency allowance was also made for uncertainties unavoidably associated
with preliminary designs. Such factors as changes in design criteria, necessity for
special construction methods, or unusual foundation conditions may increase
construction costs, and some allowance must be made in preliminary design
estimates. The allowances used for construction contingencies and engineering
together were 30 percent of the basic construction cost for categories A and C
(described below), and 35 percent for category B alternatives, because less historic
cost data was available.
Annual operation and maintenance includes all costs for labor, power, chemical
supplies, laboratory control and monitoring, administration, and incidental costs
chargeable to various components of the system improvements. Estimates of annual
labor requirements for the various alternatives were based primarily on the
experience of Brown and Caldwell, with some information for the overland flow
alternatives taken from "Costs of Wastewater Treatment by Land Application." It
was assumed that the effective annual labor contribution of one man is 1,450 hr, or
6.5 hr per day, with 38 days off for vacation, sick leave, and holidays. The annual
cost of one employee was taken as $16,000 per year, including fringe benefits and
overhead. Electrical power costs were taken as $0.02 per kwh. Chemical costs
were based on current estimates escalated to projected 1978 values.
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The feasible alternatives considered for the Davis analysis fall into three main
categories. The substitution of conventional secondary treatment processes was
considered under the first (category A) . The second category (B) considered addi-
tional treatment to polish oxidation pond effluent. Land disposal was considered as a
third category (C). The remainder of this paper is divided into sections dealing with
each category and a concluding section summarizing the comparison of alternatives.
Secondary Treatment Replacement
Under the first set of alternatives (category A), abandoning the ponds and
substituting a new secondary treatment process, fall those solutions which can be
classified as "traditional" or "standard" approaches to waste water treatment. These
are secondary biological treatment processes which usually follow primary sedi-
mentation; the present primary sedimentation tanks at Davis would be retained for
use in the treatment scheme. To fully meet all the discharge requirements at Davis,
chlorine disinfection and dechlorination for toxicity removal would follow the
biological treatment process. An important characteristic of these processes is
that most of them produce an additonal quantity of wastewater solids which must be
treated and disposed of. Solids removed in the primary sedimentation tanks at
Davis are presently treated by anaerobic digestion and then stored in sludge lagoons
before final disposal to land. Because present solids loadings at Davis are lower
than anticipated at the time of design, it is believed that the increased solids
production resulting from the addition of a biological treatment process could be
accommodated by the existing digester. However, to provide for the slightly
increased solids loading, a third sludge lagoon would be added.
Three alternatives were considered under category A. These were the conven-
tional activated sludge process, trickling filtration, and the extended aeration varia-
tion of the activated sludge process. Because a portion of the existing ponds could
be used for aeration basins, and because the extended aeration process produces a
very small quantity of biological solids, it was initially believed that this alternative
would show up well in the comparison.
Alternative A-1: Activated Sludge. The analysis was based on an aeration
tank volume of 3,220 cu m (115,000 cu ft) which would provide a volumetric loading
of 0.64 kg BODs/cu m/day (40 lb/1,000 cu ft per day) and an organic loading of
0.5 kg BODs/kg MLVSS per day at an MLSS concentration of 1,500 mg/1. Use of a
portion of the existing oxidation ponds for emergency and peak wet weather flow
storage would allow use of a single two-pass aeration tank and a single secondary
clarifier with an ADWF overflow rate of 21.4 cu m/day/sq m (525 gpd per sq ft) .
Avoiding duplicate, parallel units would reduce the costs for this portion of the
plant. It is anticipated that the storage basins could be used whenever the plant
flow exceeds 0.44 cu m/sec (10 mgd) or whenever a portion of the secondary
treatment process must be shut down.
A third sludge lagoon would be required in order to receive the increased
quantity of digested sludge. Capital and operating costs would be reduced sub-
stantially by avoiding construction of a second digester.
Estimated capital costs for Alternative A-l were $3.54 million. The capital cost
for the activated sludge process was the highest of the seven alternatives. Additional
operation and maintenance (above that required for the existing plant) cost was
estimated at $172,000 per year.
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Alternative A-2: Trickling Filtration. The trickling filter system would replace
the oxidation ponds as the secondary treatment step and would consist of a trickling
filter and clarifier with clarifier underflow pumped to the digester. The existing
oxidation ponds would be retained to provide wet weather peak flow and emergency
storage of primary effluent. A plastic media trickling filter, sized for the average
design flow rate of 5 mgd with a 1: 1 recirculation, would be designed for an organic
loading rate of 1.0 kg BODs/cu m/day (60 Ib per 1,000 cu ft per day) . The filter
would require a media volume of 3,640 cu m (130,000 cu ft); cost analysis was
based on a center shaft rotating distributor system.
The clarifier would be sized for an average flow of 0.22 cu m/sec (5 mgd) , with
an average overflow rate of 40.7 cu m/day/sq m (1,000 gpd per sq ft) . Capital
costs for the trickling filtration alternative were estimated at $2.91 million. Addi-
tional operation and maintenance costs were estimated at $90,000 per year.
Alternative A-3: Extended Aeration (Pond Modification-Aerated Lagoons) .
Oxidation pond No. 1 would be modified to contain three extended aeration basins.
The mid-depth area of each basin would be 3,260 sq m (35,000 sq ft) in order to
provide a total volume of 23,400 cu m (835,000 cu ft) . Aeration basins would be
situated in the oxidation pond so as to utilize a portion of the existing pond levee.
A portion of oxidation pond No. 1 would be used as a storage basin to regulate
peak wet weather flows and to provide emergency storage. Activated sludge would
be returned from the clarifier to the aeration basins to maintain desired mixed liquor
suspended solids concentration. Aeration and mixing in the ponds would be pro-
vided by floating surface aerators anchored to concrete pads in the basins. The
analysis was based on the assumption of a clarifier designed for an average flow of
0.22 cu m/sec (5 mgd) , with a peak overflow rate of 60 cu m/day/sq m (1,400 gpd
per sq ft) at 0.44 cu m/sec (10 mgd) . Because solids production from the extended
aeration process is low, additional anaerobic digestion capacity would not be pro-
vided. Sludge lagoon capacity would be increased to receive increased loadings.
Capital costs for Alternative A-3 were estimated at $2.38 million. Operation
and maintenance costs were estimated at $170,000 per year. An important component
of operating costs is for power to operate the mechanical aerators. In addition
to power for oxygenation, sufficient energy must be expended to prevent particles
from settling in the relatively large basins. This added requirement increases
power costs considerably.
Analysis. The category A alternatives would generally involve construction
within the boundaries of the existing treatment plant site, so that environmental
impact is localized. The major environmental impact of the alternatives in the A
category was a potential negative impact resulting from the reduction in a valuable
waterfowl sanctuary created by the oxidation ponds. Maintenance of pond capacity
for storm flows was a potential mitigating measure for winter conditions, but the
ponds might have had to be drained in the summer to prevent conditions conducive
to wild fowl botulism. These alternatives would have continued to support wildlife
habitat in Willow Slough Bypass through continued discharge to the bypass.
In terms of reliability to perform as planned, the group A alternatives were
rated highest. The activated sludge process, trickling filtration, and extended
aeration in aerated lagoons are all well-known conventional treatment processes for
which design criteria and operating procedures are well-established. They can be
expected to consistently meet the discharge requirements at Davis.
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The most likely future change in surface discharge requirements is the imposi-
tion of a requirement calling for nitrogen removal. This would occur if future studies
indicate that water quality in the Delta-Suisun Bay area is being impaired by high
nitrogen levels and that improvements would result from limiting municipal nitrogen
discharges. Although such a change may occur, it was considered improbable at
the time that the alternative analysis was undertaken.
Of the three group A alternatives, extended aeration/aerated lagoons (A-3) is
the one which can accommodate nitrogen removal with the least modification. Because
the extended aeration process normally produces a nitrified effluent (nitrogen in the
nitrate form) , only a denitrification step (conversion of nitrate to nitrogen gas) need
be added. For the activated sludge and trickling filtration alternatives, a nitrifica-
tion step (conversion of ammonia to nitrate) preceding denitrification would also need
to be added.
Upgrading of Oxidation Pond Effluent
The second set of alternatives (category B) considered for Davis involves the
reduction of the suspended solids level of the oxidation pond effluent. Most oxidation
ponds have difficulty meeting the 30-mg/l. 30-day average suspended solids require-
ment because of the presence of algal cells . Many techniques for algae removal have
been proposed. None has a long history of full-scale application, and some are still
in the early stages of experimental investigation and development. Nonetheless, in
situations where these techniques can be used effectively to reduce suspended solids
levels, their capital and operating costs may be significantly lower than for conven-
tional processes.
In category B, three processes were chosen for analysis: coagulation-flocculation-
sedimentation, overland flow, and intermittent sand filtration. Several additional
processes were evaluated before final selection of these three alternatives for
detailed analysis. An example was coagulation-dissolved air flotation. This is
similar in concept to coagulation-flocculation-sedimentation, except that finely
dispersed air bubbles are used to raise the algae-chemical floe to the water surface,
from where it is removed by skimming. Dissolved air flotation usually involves
lower capital costs because of shorter detention times and the resulting smaller
tanks. Because complicated air dissolution equipment is required, however, opera-
tion is more difficult, and this makes air flotation less suitable than sedimentation
for use in small and medium size communities. The high chemical costs associated
with coagulation-flocculation-sedimentation are also present with dissolved air
flotation, making total operation and maintenance costs for this process very high.
Other algae removal processes which have not been studied sufficiently or which
have proved unsatisfactory are submerged rock filtration, centrifugation, and
microstraining. In-pond removal systems which have been studied include series
pond arrangements, series ponds with intermediate chlorination, intermittent
discharge lagoons with chemical addition, and aquaculture, which is the use of an
ecological food chain that produces a useful product in the form of fish as opposed
to a material requiring further disposal. These in-pond systems also have not
been developed sufficiently or suffer from some defect in their operation which
would preclude their use at Davis.
Maximum performance and operational ease can be expected from algae removal
processes if influent (pond effluent) algae concentrations are minimized. The
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original plant design allowed simple conversion from parallel to series operation of
the oxidation ponds . Series arrangement of the ponds minimizes short-circuiting,
and provides about as good a prototype situation for minimizing effluent suspended
solids as can be expected.
Over the two-month period of the pilot study (9/9/75 to 11/7/75) discussed in
the overland flow section, conversion was made to series operation. However, no
evidence of significant autoflocculation was found. In addition, there was not a
significant difference in the suspended solids concentrations of the three ponds .
The conclusion reached was that autoflocculation would not be a viable process,
and that there was no advantage to other (tertiary) treatment processes in using
the ponds in series .
Alternative B-l: Coagulation-Flocculation-Sedimentation. In order to reduce
the size of the sedimentation tank required for the coagulation-clarification alterna-
tive, it was assumed that tube settlers would be utilized which allow a detention
time of one hour to be used in design. This reduces capital costs significantly.
Alum (A12 (SO4)o ' 14H2O) would be used as the coagulant chemical. Adjust-
ment of pH through the addition of sulfuric acid (H2SO4) would be used to maximize
alum-algae floe precipitation. Conclusions reached from jar tests conducted in the
spring of 1976 were that an alum dose of approximately 125 mg/1, with acid addition
of 5 meq/1, would produce the greatest effuent clarity.
Main components of this system would be an influent pumping station, alum and
acid storage facilities, a flash mix unit for the addition of alum, flocculation com-
partments with a detention time of 20 min at design flow, and tube settling basin
with a detention time of 60 min. Chlorine disinfection in the existing facility and
dechlorination with sulfur dioxide would follow the coagulation-clarification process .
Sludge produced by the process would be returned to the oxidation ponds. It
is anticipated this would be the most cost-effective solids disposal method. Steps
would have to be taken to prevent autoflotation and to ensure that the sludge is
distributed fairly evenly throughout the pond area. Otherwise, no serious
operational problems would be anticipated.
Capital and annual operating costs were estimated at $1.51 million and $278,000
per year, respectively. A major fraction of the operating costs would be for
chemicals.
Alternative B-2: Overland Flow. For the overland flow alternative, approxi-
materly 83 hectares (200 acres) of land would be required. There are several
tenative sites close to the treatment plant. The 83 hectares (200 acres) include a
net application area of 66 hectares (158 acres) based on a loading rate of 2,700 cu
m/hectare/day (30,000 gallons per acre per day or gpad) plus additional land to
provide for roads, hay drying and buffer zones. A preliminary layout developed
for the most likely site would have 14 terraces 44 m (145 ft) wide by 1,070 m
(3,500 ft) long. The slope of each terrace would be 2 .5 percent. Seven maintenance
roads and eight drainage ditches would be included in the layout, with connecting
roads at each end and a main drainage ditch at the southern end. Orientation of the
terraces in a north-south direction would prevent the prevailing north wind from
blowing spray across the roads.
10
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The collection sump would be located at the southwest corner of the plot to
receive runoff prior to its discharge to Willow Slough Bypass. Provision would
be made to pump runoff back to the oxidation ponds when discharge requirements
were not being met. This could occur during start-up following grass harvest.
Costs were based on the assumption that oxidation pond effluent would be sprayed
through nozzles from a fixed sprinkler system located along a line near the top of
each terrace. Sprinkler spacings of 24 m (80 ft) with discharge pressures of
400,000 to 550,000 Pa (60 to 80 psi) and nozzle flows of 0.0025 cu m/sec (40 gpm)
would be typical operating parameters. This would allow for four hours operation
per sprinkler each day under design conditions of an application rate of 2,700 cu
m/hectare/day (30,000 gad), with the entire system designed to operate over a
24-hr period. A spray radius for the 180 degree nozzles of 152 m (50 ft) would
allow overlap and maximum terrace coverage. As a result of the analysis discussed
in this paper, overland flow was selected as the recommended project for meeting
the new requirements at Davis. When this project reaches the design stage, alter-
native delivery systems will be considered in more detail. The appendix to the
Davis project report contains a discussion by Donald M. Parmalee and Vaughan
Sparham of surface delivery systems for overland flow treatment based on experi-
ence in Australia and England .5 This experience will be taken into consideration
in optimizing the delivery system design.
Care in design and operation of the overland flow system will be required to
minimize conditions allowing mosquito propagation. For example, a thorough
dryout will be required prior to harvesting in order to prevent equipment ruts
which may become small breeding pools. Gambusia (mosquito fish) would be
planted in the runoff collection channels.
Costs for the overland flow alternative are presented in Table 2. Total costs
for this alternative were the lowest of the seven studied in detail.
Pilot Studies - As part of the concept development for improving wastewater
treatment at Davis, pilot studies of the overland flow process were undertaken.
The overland flow studies began one month after seeding of test plots located at
the Davis treatment plant. Data collection began on November 11, 1975, and
continued through March 27, 1976. The purpose of running the experiments during
the winter months was to develop data for the period of the year with the worst
operating conditions (maximum flow, lowest temperatures, and greatest precipita-
tion) ; for all practical purposes it does not rain in Davis during late spring to
early fall. Unfortunately, the year was a record drought and no effects of
precipitation were developed. This may be a reason for conservatism in scaling
up pilot data for design. Consideration of use of the ponds as temporary storage
tanks may eliminate the need for oversizing the system, however.
Three test plots, each 15 m (50 ft) wide and 31 m (100 ft) long with a 0.61-m
(2-ft) drop were constructed for the overland flow studies. Preparation of the plots
included grading, rototilling, flooding with digester supernatant and seeding with
annual rye grass . Annual rye grass was chosen because of the speed of germination
since plots were not seeded until late, October 1, 1975. Supernatant application
was not uniform, and the grass development was both faster and better on plots 1,
and 2 than on plot 3. During the five months of applying oxidation pond effluent to
the plots the grass grew continually, eventually reaching a height of 25 to 30 cm
(10 to 12 in.) .
11
-------
Table 2 . Estimated Capital and Operating
Costs: Overland Flow
Alternative
Item
Capital costs3
Gravity Line to Sump
Distribution and Runoff
Collection Sump
Terrace Construction
Distribution System
Distribution Pumping
Runoff Collection
Electrical
Service Roads
Fencing
Subtotal
Engineering and
contingencies, 35%
Land (200 acres @
$l/800/acre)
Total capital cost
Operation and maintenance
costs"
Labor
Materials
Power
Total operation and
maintenance costs
Cost
$ 55,000
45,000
250,000
290,000
290,000
30,000
45,000
70,000
120,000
$1,195,000
420,000
360,000
$1,975,000
$ 48,000
per year
10,000
30,000
$ 88,000
per year
ENR Index = 3200
Vor additional facilities only
12
-------
Pond effluent was supplied from the chlorination basin effluent line at a nominal
pressure of 550,000 Pa (80 psig) . A separate pressure regulator and solenoid
valve was used to control the application rate to each plot. Five shrub type (two 90-
degree and three 180-degree) spray nozzles were installed on 0.6-m (24-in.) risers
at each plot. Initially the sprays were located on the upper edge of the plots, but
were later moved about three meters (10 ft) from the edge because a considerable
amount of spray was being blown behind the plots. Windy days are common in
Davis with the most common wind direction being from the north. On some days
most of the spray was directed off the plots. During these same periods dust was
blown off of the access road running below the plots into the sample containers.
This factor accounted for several very high suspended solids readings during
December, January, and February.
Although the spray heads were designed for use with tap water, very few
plugging problems occurred. Those problems that did occur were easily and
quickly handled.
From November 7 to February 7 the loading rates and application times were as
listed in Table 3. After the first week in February the loading times, and consequently
the hydraulic loading rates, were increased on plots 2 and 3. This change was in
part because of a desire to determine the effects of higher loading rates and in
part because of the lack of rainfall. Some method of estimating the probable
effect of rain was needed. Considerable difficulty accompanied this change because
of the coincidental plugging of the pressure regulators of plots 2 and 3. Although
the application periods were increased during this period the pressures were
greatly reduced. This situation was not fully corrected until February 27th.
Application rates for the period February 27th to March 28th are given in Table 4.
Plot effluent characteristics are summarized on a monthly basis in Figures 2
and 3. The monthly averages are useful in noting how the effluent quality changed
with season and with some of the operating parameter changes. Requirements set
on the discharge by the Regional Board are based on running 7- and 30-day
averages, however. These values were calculated in the pilot study.
Several samples exceeded the 90-mg/l maximum set by the Regional Board. All
of these samples were taken on extremely windy days. In several cases it was noted
on the raw data sheet that there was considerable silt in the sample. Finally, if the
true suspended solids reading were high the BODs value should be correspondingly
high. This was not the case. It can therefore be concluded that wind-blown dust
was the cause of these extremely high suspended solids readings.
Limited nitrogen data was developed in the study. Some nitrogen was taken up
by the grass, but there is no clear evidence of the quantity. Because the 1977 dis-
charge requirements do not include any nitrogen limitation, it was decided to defer
those studies until such time as a limitation was imposed. Then, studies for optimum
nitrogen removal could be conducted using the full-scale system.
From the pilot studies, it was concluded that:
1. Loading rates up to 290 cu m/hectare/day (32,000 gal/acre/day) (3.00 cm/
day or 1.18 in./day) are suitable for process design. The data from
the pilot studies appear to be good, and support the above value for
design use.
13
-------
Table 3. Pilot Study Application Rates, 11/7/75 through 2/7/76
Plot
Time of Application, hrs.
morning
afternoon
Application Rate
gal/acre/hr
Average Daily Flow
gal/acre dayb
in/dayc
1
3
3
3476
20,856
0.77
2
2
2
3350
13,400
0.49
3
1
1
3716
7430
0.27
gal/acre/hr x 0.0091 = cu m/hectare/hr
gal/acre/day x 0.0091 = cu m/hectare/day
*
'in/day x 2. 54 = cm/day
Table 4. Pilot Study Application Rates, 2/27/76 through 3/28/76
Plot
Time of Application, hrs.
morning
afternoon
Application Rate
gal/acre/hr
Average Daily Flow
ga I/acre/day b
in/day0
1
3
3
5530
32,000
1.18
2
4
4
5500
44,000
1.62
3
12
4630
56,000
2.07
agal/acre/hr x 0.0091 = cu m/hectare /hr
gal/acre/day x 0.0091 = cu m/hectare/day
c,
'in/day x 2.54 = cm/day
14
-------
CO
o
o
LLJ
O
z
UJ
a.
CO
CO
50
40
30
20
10
NOV
DEC
JAN
FEB
MAR
Figure 2 Effluent Suspended Solids Concentrations from Study Plots
50
40
30
a
§ 20
10
NOV
A
•
DEC
JAN
FEB
MAR
Figure 3 Effluent BOD5 Concentrations from Study Plots
15
-------
2. Loading rates studies above 290 cu m/hectare/day (32,000 gal/acre/day)
resulting in effluent quality very close to the minimum required. Using
these values would leave little margin (perhaps none) for natural process
variation.
3. Rye grass would be a suitable cover for prototype land treatment systems.
This conclusion was based upon the observations made in this study and
those reported in the literature.6
4. Application times up to 12 hours per day resulted in excellent grass growth.
Twelve hours was the maximum application period and it is not known if
longer periods would be satisfactory also.
5. Meeting BOD§ requirements with an overland flow treatment process de-
signed for suspended solids removal will not be a problem.
6. Chlorinated effluent will not damage the grass.
7. Data developed in the study provide conservative estimates of prototype
operation. As the grass root zone develops, effluent quality may continue
to improve and stabilize.
8. The fact that grass grows well in Davis during many winters may provide
a method of year-round nutrient removal from wastewater. Additional
studies need to be made year-round to determine the extent of possible
nutrient removal.
9. The effect of precipitation on the effectiveness of a prototype process
could not be predicted from the results of the study because there was
almost no precipitation during the study period. The response of the
plots to very large increases in loading during March was sluggish. This
leads to the conclusion that precipitation will not dramatically affect
effluent quality from the prototype system unless the storm is very
intense.
Alternative B-3: Intermittent Sand Filtration. For the intermittent sand filter
alternative, a design application rate of 5,500 cu m/hectare/day (0.6 million gallons
per day or mgad) was assumed. Sixteen 0.21-hectare (0.5-acre) filter basins
would be required. It was assumed that each basin would be 46 m (150 ft) square,
and the basins would be located in a double row in order to provide access for
cleaning. An area of approximately 10 hectares (25 acres), located along the east
side of the present treatment plant site, would be required if this alternative were
implemented.
In determining the intermittent sand filter design requirements, it was assumed
that a filter run would be 28 days. With 16 basins, one basin could be cleaned each
day during a regular five-day work week, with four days during each four-week
period allotted to other maintenance items. The influent supply line and main drain
would run along the spine between the rows of basins. The influent line would be
sized for hydraulic loading of each filter for two hours with a maximum of four
basins being loaded at a time. Loading would be rotated automatically by a timer-
controlled valve. Under design conditions it would be possible to load all the
basins within an eight-hour period. When the total amount of applied effluent
16
-------
fails to drain through the filter within 22 hr, the filter would be considered
plugged and would require cleaning.
It was assumed that the filters would be contained by soil embankments paved
with asphalt along the sides to facilitate cleaning. The basin tile drains would be
covered with about one-third meter (one foot) of graded gravel from 0.6 cm (0.25 in.)
minimum diameter to a maximum diameter of 4.0 cm (1.5 in.) . The filter medium placed
on top of the gravel would consist of one meter (three feet) of sand with an effective
size approximately 0.50 to 0.75 mm. Cleaning would be accomplished by removing
the top two to five centimeters (one to two inches) of sand and replacing with clean
sand. Sand would be washed and reused. Sand wash water, after passing through
a sand sedimentation basin, would be returned to the oxidation pond. Effluent from
the intermittent sand filter basins would be drained to a sump and then pumped to
the chlorine contact tank before discharge to Willow Slough Bypass.
Capital costs for intermittent sand filtration were estimated at $3.52 million.
Operation and maintenance costs were estimated at $79,000 per year. The chief
uncertainty in the cost estimate was the amount of labor required for sand cleaning.
This results directly from the lack of adequate information for designing intermittent
sand filters for wastewater characteristics and effluent quality requirements similar
to those at Davis.
Analysis. All the alternative methods of effluent improvement have similar
environmental impact on the service area. There are significant differences in the
immediate vicinity of the treatment plant. The overland flow alternative has a
favorable environmental impact. Wildlife habitats in the vicinity of Davis include
croplands, pasture, riparian and water surfaces. Rodents, small mammals and
birds use these habitats. Water surfaces are heavily used by both resident and
migratory waterfowl. Varying amounts of riparian habitat exist along the Yolo
Bypass, Willow Slough Bypass, Putah Creek and other stream and slough banks.
Riparian vegetation, considered one of the most valuable wildlife habitat types, is
a concentration point for a variety of game and nongame species and provides
excellent feed and cover. The Davis Audubon Society has identified the Hunt's
Cannery overland flow treatment site, located about two kilometers (one mile) east
of the central treatment plant, as one of the best wildlife habitats in the Davis area
and considers that the recommended overland flow project will significantly enhance
the wildlife habitat. A wildlife inventory from the vicinity of the treatment plant is
discussed in the summary section.
Because the group B alternatives all involve algae removal, all provide some
removal of nitrogen, which is incorporated into algal cells in the oxidation ponds.
Of the three group B alternatives, overland flow can be expected to provide the
greatest removal of unassimilated nitrogen. The principal mechanisms are crop
uptake and nitrification-denitrification in the soil.
Reclamation/Irrigation
The third major alternative investigated (category C) was to apply the effluent
to land, either as a reclamation program for crop irrigation or simply as a method
of disposal. Advantages to this approach are, in general, that crop irrigation is
a beneficial use (if reclamation is practiced) , less costly treatment is required,
and the possibility of future upgrading being required to meet more stringent
discharge requirements is less likely. Disadvantages are that large tracts of land
17
-------
are often required, wastewater storage during wet portions of the year is usually
required, and care must be taken to prevent degradation of groundwater aquifers.
Reuse of reclaimed wastewater for industrial purposes may be undertaken in some
situations, but the lack of industrial demand at Davis made this infeasible.
The wastewater effluent from the Davis facility has a sodium adsorption ratio
of 10. This high concentration of sodium in an irrigation supply can reduce soil
permeability by causing clay minerals to swell. The SAR value of 10 classifies
the plant effluent as potentially leading to severe soil permeability problems.
Using effluent as the source of water on soils around the treatment plant where
permeability is already low without any mitigating measures would, in time,
produce this sealing effect. This is unsuitable for irrigation but ideal for over-
land flow where water infiltration into the soil profile beyond the grass root zone
is not desirable. Due to the high SAR value, the wastewater must be blended at
a ratio of 1:1 with local irrigation water for long-term irrigation. It is estimated
that an application area of 400 to 600 hectares (1,000 to 1,500 acres) would be
needed.
Because irrigation is a seasonal use at Davis, storage of effluent would be
required during the winter months. It was determined that a storage reservoir
with a capacity of approximately five million cubic meters (4,000 acre-ft) would
be required. A 200-hectare (500-acre) parcel, probably located to the east of the
existing plant site, would be purchased and used as the storage reservoir site.
Flow from the storage reservoir and existing ponds would be delivered by pressure
pipeline to the irrigation systems of local farmers.
In order to assure a reuse demand for the Davis effluent, it would be necessary
to develop long-term (e.g., 15-year) agreements with local farmers to receive the
effluent. An alternative would be to purchase land and lease it to farmers on the
condition that they use the effluent for irrigation. This would add $2,000,000 to
$2,500,000 to the capital cost. Resulting savings in other areas might result from
reduced storage and conveyance costs. Operating costs would be reduced by the
rent payments received for the leased land.
An important aspect of irrigation reuse is control of subsequent runoff from
irrigated fields. A probable Regional Board requirement would be that such drain-
age not reach surface waters (unless it has been treated to meet surface discharge
requirements) . This would require collection of drainage waters and return either
to the treatment plant or to the irrigation system. This could also add to the esti-
mated costs. Estimated capital costs were $3.24 million. Operation and maintenance
costs were estimated at $40,000 per year for the additional facilities.
Summary of Alternatives Analysig
Cost-effectiveness, including environmental and social impact analysis, and
project suitability were considered for the seven alternatives. Alternatives were
compared on the basis of equivalent annual cost, using a discount rate of 7 percent
and a 20-year planning period, conforming to state and federal guidelines for
cost-effectiveness analyses. The annual cost is computed by applying a capital
recovery factor, available from standard interest tables, after first taking into
account any facilities salvage value at the end of the 20-year planning period. No
depreciation was assumed for the value of land, so that capital recovery factor used
for land was equal to the interest rate. The results of the monetary cost analysis
is depicted graphically in Figure 4.
18
-------
cr
<
in
>
cc
Ul
o.
O
a
o
i
vt
O
o
z
600
500
400
300
200
100
OPERATION AND MAINTENANCE COSTS
CAPITAL COSTS (AMORTIZED
@7%AND20YR)
A-1
A-2
A-3 B-1
ALTERNATIVE
B-2
B-3
C-1
Figure 4 Monetary Cost Analysis for Project Alternatives
-------
The most notable aspect of the costs is range of values. The most costly
alternative, the actived sludge process (A-l), is nearly 90 percent higher than
overland flow (B-2) . Overland flow and reclamation/irrigation (C-l) are the two
lowest-cost alternatives, and are significantly less expensive than the others.
In terms of operation and maintenance costs, which must all be borne locally,
reclamation/irrigation has the lowest cost, $40,000 per year, and trickling filtra-
tion (A-2) , overland flow (B-2), and intermittent sand filtration (B-3) are in the
$48,000 to $90,000 per year range. The remaining three alternatives have much
higher operating costs . Activated sludge (A-l) has high labor and power costs.
The extended aeration alternative (A-3) has lower labor and materials cost, but
requires more power to keep all the particulate material in suspension in the
aerated lagoons . Coagulation-flocculation-sedimentation has the highest operating
costs; this results from the large quantities of alum and sulfuric acid which must
be used.
The alternatives have significantly different environmental impacts. Due to
the general dryness of the area and the absence of major water bodies, aquatic
resources are limited within the study area. Water quality in Willow Slough Bypass
is poor, consisting in the summer of agricultural return flows and treated waste-
water effluent. Water quality in the fall, between the end of the irrigation season
and the beginning of the rainy season, is especially poor. Nutrients contained in
the agricultural runoff support excessive algae growth in the Bypass drainage;
algae decompositon consumes the oxygen supply. Most warm water fish species
are absent. California Department of Fish and Game personnel indicate that some
bluegill and catfish may inhabit the water course.
The major value of the Willow Slough Bypass is the riparian habitat it provides.
Vegetation in the vicinity of the treatment plant is sparce, reflecting the dry
climate. Most of the area is under cultivation. Dense primary riparian vegetation
along the Willow Slough Bypass provides food, nesting, dens and escape sites for
muskrats and other water-related mammals, amphibians, and ducks. Bottom lands
of the bypass provide habitat for rodents and upland birds such as pheasant. These
species attract and supply food to hawks and other raptors. Category A and B
surface water disposal alternatives would support the continuation of this habitat.
The oxidation ponds are one of the most valuable wildlife habitats in the study
area. These ponds provide a sanctuary for several species of waterfowl and shore
birds. The treatment area is surrounded by a high chain link fence, and the opera-
tion of the oxidation ponds is automated so that the waterfowl are well isolated from
man's activities. These ponds are always occupied by large numbers of several
species of waterfowl. Category B and C alternatives would maintain this valuable
habitat.
The reclamation alternative, C, would involve irrigation during the summer
and storage of effluent during the winter. There would be no discharge to Willow
Slough Bypass . Storage would create 170 hectares (400 acres) of additional water-
fowl habitat from land currently devoted to irrigated agriculture.
The overland flow alternative would create 80 hectares (200 acres) of habitat
very favorable to wildlife, as indicated by the use of the Hunt's overland flow area.
Table 5 shows a wildlife inventory in the vicinity of the Davis treatment facility.
20
-------
Table 5. Wildlife Inventory for Treatment Plant Vicinity
N>
Hunt-Wesson
overland flow site
Birds
Mallard (tlC-AY)
Turkey vulture (C-S,W,F)
White-tailed kite (C-AY)
Fed-tailed hawk (C-AY)
Swainsons hawk (UC-Su)
Rough legged hawk (UC-W)
Marsh hawk (C-AY)
Prairie falcon (Rare-W)
Sparrow hawk (C-AY)
•Ringnecked pheasant (C-AY)
•Killdeer (C-AY)
Long-billed curlew (C-AY)
Whimbrel (Rare-W)
Mourning dove (C-AY)
Bam owl (UC-AY)
Burrowing owl (UC-AY)
*Short-eared owl (UC-AY)
Western kingbird (C-Su)
Homed lark (C-F,W,Sp)
Tree swallow (C-Su)
Bam swallow (C-Su)
Cliff swallow (C-Su)
Common crow (C-AY)
Water pipet (C-W)
Loggerhead shrike (C-AY)
'Western meadowlark (C-AY)
Brewers blackbird (C-AY)
Brown-headed cowbird (C-AY)
House finch (C-AY)
American goldfinch (C-AY)
Lesser goldfinch (C-AY)
Savannah sparrow (C-F,w,Sp)
Mammals
Omate shrew
California mole
Black-tailed jackrabblt
Auduoon cottontail
California ground squirrel
Valley pocket gopher
Deer mouse
Meadow mouse
Striped skunk
Reptiles
Coach whips
Long-nosed snakes
Common king snake
Gopher snake
Western terrestrial garter snake
*• Western fence lizard
Treatment plant
oxidation pond
Birds
* Pied-billed grebe (C-AY)
Whistling swan (C-W)
Canada goose (C-W)
White-fronted goose (C-W)
Snow goose (C-W)
•Mallard (C-AY)
Gadwall (UC-W)
Pintail (C-W)
Greenwinged teal (UC-W)
Cinnamon teal (C-W)
American widgeon (C-W)
Shoveler (C-W)
Canvasback (C-W)
Ruddy duck (C-W)
•American coot (C-AY)
Wilsons phalarope (UC-Su)
Northern phalarope (UC-Su)
Herring gull (C-W)
California gull (C-AY)
Ring-billed gull (C-AY)
Blacktem (UC-Su)
Lesser nighthawk (UC-Su)
Tree swallow (C-Su)
Bam swallow (C-Su)
Cliff swallow (C-Su)
Riparian area
Birds
Great blue heron (UC-AY)
Green heron (UC-AY)
•White-tailed kite (UC-AY)
•Sparrowhawk (C-AY)
•Marsh hawk (UC-AY)
•Ring-necked pheasant (C-AY)
** Virginia rail
••Sora rail
•Killdeer (C-AY)
•American Arocet (C-Su)
•Black-necked stilt (C-Su)
•Mourning dove (C-AY)
• Brown owl (C-AY)
•Long-eared owl (Rare-AY)
•Western kingbird (C-Su)
•Mockingbird (C-AY)
Water pipet (C-AY)
•Loggerhead shrike (C-AY)
•Red-winged blackbird (C-W)
Yellow throat
** ,*Long-billed marsh wren
•Brewers blackbird (C-AY)
Brown-headed cowbird (C-AY)
Mammals
Omate shrew
"Racoon
California vol
Muskrat
Reptiles and amphibians
Common garter snake
Western water garter snake
Western toad
California newts
Agricultural areas
Rice
Birds
Great blue heron
American bittern
Mallard
White-tailed kite
Red-tailed hawk
Marsh hawk
Sparrowhawk
Ring-necked pheasant
Killdeer
American arocet
Black-necked stilt
Black tern
Cliff swallow
Bam swallow
Tree swallow
Brewers blackbird
Mammals
Muskrat
Reptiles
Garter snake
Wheat, Com, Hay
Birds
Sparrowhawk
Swainson's hawk
Ring-necked pheasant
White-tailed kite
Rough-legged hawk
Common crow
Greater yellowlegs
California gull
Herring gull
Ring billed gull
Brewers blackbird
Cow bird
LEGEND
* - nesting
** - Identification is
uncertain
C - common
UC - uncommon
AY - afl year
Sp - spring
Su - summer
F - fall
W - winter
-------
The relative abilities of alternatives to meet engineering effectiveness criteria
and to conform to identified constraints may strongly affect selection of the recom-
mended plan. The effectiveness criteria include reliability, flexibility, flood
protection and bypass prevention. Constraints include ability to meet discharge
requirements, conformance with the basin plan, compatibility with "best practicable
treatment" requirements, reclamation potential, ability to implement, and public
acceptability. All of the alternatives were judged to be fairly reliable. None would
be affected greatly by power outages or process shutdowns. Retention of the
oxidation ponds, as emergency storage for the A alternatives and as part of the
treatment process for the B and C alternatives, would provide adequate storage
capacity for several days in the event of a process shutdown. Emergency power
for influent pumping is provided by gas engines. Resource commitments for the
various alternatives are summarized in Table 6.
Alternatives B-2 (overland flow) and C-l (reclamation/irrigation) were rated
as being the most flexible. They would be least affected by a future nitrogen
limitation, and both have low capital costs, a portion of which is for land, which
does not depreciate as structural or mechanical components do. In addition, the
reclamation/irrigation alternative would not be affected by any future requirement
mandating land disposal (although this was considered quite unlikely) . The
activated sludge (A-l) and trickling filtration (A-2) alternatives were rated the
least flexible: a high capital investment would be required, and major additions
would be needed to provide nitrogen removal.
As a result of the analysis, an overland flow system was recommended. A
summary of the project alternative analysis, taken from the Environmental Impact
Report,' is given in Table 7. A more detailed discussion of the alternatives is
contained in the EIR and in the Project Report.5
The consideration of alternatives at Davis may be generally applicable to small
communities which have existing oxidation pond systems. This may be especially
true where existing capacity is sufficient to meet anticipated growth for the inter-
mediate future, but where upgrading of effluent quality is required. As more
experience is gained in the United States with overland flow systems, it is expected
that the positive environmental aspects will be recognized.
22
-------
Table 6. Estimated Resources Commitments of Alternatives
ro
00
A-l Activated sludge
A-2 Trickling filtration
A-3 Extended aeration
(aerated lagoon)
B-l Coagulation
B-2 Overland flow
B-3 Intermittent sand
filtration
C-l Reclamation
0
0
0
0
170
15
450
0
0
0
0
30
10
50
0
0
0
0
0
0
1,200
2,100,000
900,000
3.300,000
500,000
1.500,000
350,000
500,000
175,000
75,000
275.000
50,000
150,000
35,000
65,000
0
0
0
950
0
0
0
0
0
0
2,000
0
0
0
40
40
40
40
40
40
55
16
16
16
16
0
16
0
3.5
2.9
2.4
1.5
2.0
3.5
3.2
172
90
170
278
88
79
40
7.800
6.500
13.400
3,300
6,700
11.700
10.700
2,200
1.100
1,825
1,100
1,100
1,100
550
-------
Table 7. Comprehensive Evaluation of Project Alternatives
Con s idero tion s
ENGINEERING/INSTITUTIONAL
Regulatory compliance
Implementation
Reliability
Flexibility
Flood protection
SOCIAL - ECONOMIC
Public Health
Archeologica I/Historical
Project cost
Annual operation.
maintenance cost
Cost per household/month
Public acceptance
ENVIRONMENTAL
Surface water supply
Surface water quality
Groundwater quality
Soils
Wildlife habitat
Air quality
Environmental rating
(1 is highest)
OVERALL RATING
A-l
Good
Marginal
Acceptable
Marginal
Good
Enhance
None
3.540,000
172.000
4.29
Unacceptable
Enhance
Enhance
None
None
Moderate
Slight
g
8
A-2
Good
Marginal
Acceptable
Marginal
Good
Enhance
None
2,910.000
90.000
3.48
Unacceptable
Enhance
Enhance
None
None
Moderate
Slight
£
6
A-3
Good
Marginal
Acceptable
Marginal
Good
Enhance
None
2,380,000
170.000
4.15
Unacceptable
Enhance
Enhance
None
None
Moderate
Slight
£
7
B-l
Good
Marginal
Acceptable
Marginal
Good
Enhance
None
1,510,000
278,000
5.03
^Unacceptable
Slight
Enhance
None
None
None
Slight
2
4
B-2
Good
Good
Good
Good
Acceptable
Enhance
Insignificant
1,975,000
86,000
3.35
Good
Slight
Enhance
Insignificant
Slight
Enhance
Slight
1
1
B-3
Good
Marginal
Marginal
Marginal
Good
Enhance
Insignificant
3,515,000
79.000
3.48
Acceptable
Slight
Enhance
None
None
None
Slight
3
2
C-l
Acceptable
Marginal
Acceptable
Good
Good
Enhance
Insignificant
3.235,000
40,000
3.76
Acceptable
Moderate
Slight
Slight
Moderate
Slight
Slight
4
3
No project
Unacceptable
Unacceptable
-
Good
-
Slight
None
-
2.35
Marginal
Slight
Slight
None
None
None
Insignificant
5
5
to
tfe.
ENVIRONMENTAL EVALUATION
Severe
Moderate
Slight
Insignificant
None
Enhance
ENGINEERING/INSTITUTIONAL EVALUATION
Unacceptable
Marginal
Acceptable
Good
-------
REFERENCES
1. Brown and Caldwell, City of Davis Sewerage Survey, December 1961.
2. Brown and Caldwell, City of Davis Sewerage Study, December 1968.
3. Pound, C.E., D.W. Crites and D.A. Griffes, Costs of Wastewater Treat-
ment by Land Application, U .S. EPA, Office of Water Program Operations,
Technical Report No. EPA-430/9-75-003, June 1975.
4. Marshall, G.R. and EJ. Middlebrooks, Intermittent Sand Filtration to
Upgrade Existing Wastewater Treatment Facilities, Utah State University,
College of Engineering, Utah Water Research Laboratory, PRJEW115-2,
February 1974.
5. Brown and Caldwell, City of Davis, Project Report, Algae Removal
Facilities, February 1977.
6. Overman, A.R. and H.C. Ku, Effluent Irrigation of Rye andRyegrass,
Proceedings ASCE, JEED, Vol. 102, 475 (April 1976).
7. Brown and Caldwell, City of Davis, Draft Environmental Impact Report,
Algae Removal Facilities, February 1977.
25
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