LRWRP_Design_Report_Chapter_4.pdf

CHAPTER 4

SECONDARY TREATMENT (TM 2)

Introduction

Brown and Caldwell held a brainstorming meeting with City staff on September 16,2004, to discuss liquid train treatment processes for the Lompoc Regional Wastewater Reclamation Plant (LRWRP) upgrade project. The purpose of the meeting was to further define concepts and criteria for liquid treatment processes, including secondary treatment, to be incorporated into the upgrade project. A copy of the meeting minutes is attached.

The Lompoc Regional Wastewater Reclamation Plant Master Plan (Kennedy/Jenks Consultants, September 2002) included a number of secondary treatment alternatives to accomodate projected growth within the plant service area and meet more stringent nitrogen and phosphorus discharge limits. Seven nitrogen removal alternatives were discussed. Six of these alternatives incorporated fixed film biological treatment (using the existing trickling filter and a new trickling filter) upstream of suspended-growth biological treatment (i.e., activated sludge). The one alternative based on suspended growth biological treatment without upstream fixed film biological treatment was dropped from further consideration because the Master Plan concluded it had "the poorest capability for process flexibility." The recommended alternative included two trickling filters operated in series upstream of an activated sludge system with staged anoxic and aerobic zones.

This alternative included rehabihtation of Primary Clarifier No. 1 and construction of a second trickling filter and second aeration tank for redundancy.

Future modifications to the recommended alternative were identified to provide additional biological nutrient removal to meet lower effluent nitrogen and/or phosphorus discharge limits if imposed.

These modifications included a primary sludge fermenter to generate volatile fatty acids (VFAs) necessary for enhanced biological phosphorus removal (EBPR) as part of the activated sludge system.

Brown and Caldwell reviewed the 2002 Master Plan as part of our proposal for the current upgrade project. Given the significant costs associated with rehabihtation of Primary Clarifier No. 1, construction of a second, redundant trickling filter, and construction of a second, redundant aeration tank, we recommended an oxidation ditch-based secondary treatment system to replace the existing treatment facilities. This approach eliminates construction costs for rehabilitation to the existing primary clarifiers and for providing redundancy for existing facilities, while providmg a secondary treatment system to accommodate planned growth and to meet current effluent discharge limits. In the future, the recommended secondary treatment system can be modified easily to meet more stringent nitrogen and phosphorus limits. This approach reduces the complexity of the liquid treatment train presented in the 2002 Master Plan, reducing the number of individual unit processes to be operated and maintained.

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Secondary Treatment (TM 2) 4-2

Objectives

Brown and Caldwell recommends an oxidation ditch-based secondary treatment process for the LRWRP upgrade project based on our evaluation of the secondary treatment recommendations and costs presented in the 2002 Master Plan. The recommended process accomplishes the following objectives:

Meets the provisions in the current NPDES permit that the City "wdl take all available steps toward nitrate reduction to the current Basin Plan limit of 10 mg/L (as N)".

Provides flexibility to meet hture effluent nutrient levels that will be established following completion of a TMDL study for the Santa Ynez River, scheduled to be completed in 2007.

Simplifies the proposed secondary treatment recommendations in the 2002 Master Plan and eliminates the existing dgester gas-dnven aeration air blowers if feasible.

The recommended system conserves influent concentrations of the soluble readily biodegradable BOD needed for biological nutrient removal (BNR) processes. These readily biodegradable constituents would have been removed if a trickling filter were used upstream of the activated sludge process. As described in the 2002 Master Plan, an additional unit process (primary sludge ferrnenter) or an external source of these constituents would be needed for BNR if a trickling filter was used. The recommended oxidation ditch-based secondary treatment system uses these soluble constituents for BNR while providmg an aeration credit, reducing the aeration system size, and recovering some of the alkalinity consumed through nitrification. Additionally, the recommended process avoids capital costs for rehabilitation of existing treatment facilities and to provide unit process redundancy if the existing primary clarifiers, trickling filter, aeration tank, and secondary clarifiers were to be used.

T h ~ s technical memorandum summarizes development of design criteria for the recommended secondary treatment system, provides some additional detaiI about system configuration and operation, and presents a prehminary construction cost estimate.

Existing Facilities

The existing treatment facilities include primary clarifiers for suspended solids and particulate BOD removal, a trickling flter for carbonaceous BOD removal, and an aeration tank and three secondary clarifiers for nitrification. These facilities were modified or constructed as part of the 1974 Lompoc Regional Wastewater Reclamation Plant project, whch upgraded the treatment plant to meet more stringent BOD and TSS discharge h i t s and new ammonia dtscharge limits.

As much as possible, Brown and Caldwell recommends that existing tankage be used for the same or new purpose and, when t h s is not possible, that existing tankage be abandoned as part of the LRWRP upgrade project, as discussed above. Accordingly, we did not analyze performance of the existing facilities.

Secondaly Treatment (TM 2) 4-3

We did obtain historical flow and concentration data from the past five years for the influent and headworks sampling locations to develop design loading conditions at the ultimate 5.5 mgd average dry weather flow (ADWF). These data were analyzed to develop influent conditions for subsequent process simulation.

4.3.1 Influent Flows and Loads

Historical headworks and final effluent flows are shown on Figure 4.3.1. The final effluent flow is measured using the depth of flow over a Cipoletti weir, while the headworks flow is a calculated parameter based on measured effluent flow and measured and estimated internal plant recycle flows.

The influent flows have been equalized historically, as indicated by the low variation in h a 1 effluent flows shown on the figure. The ADWF averaged 3.54 mgd over the past five years.

7/24/98 711 9/99 7/13/W 7/8/01 7\3/02 6/26/03 6/22/04 W17105 r Headwork Final effluent (monthly report)

Figure 4.3.1 Histo~~'cal Influent and Emuent Flow (curves hdicate 30-day movitg averages)

A typical weekday and weekend influent flow pattern was developed from flow data collected in September 2000 and January 2001. The data were collected using a temporary open channel flow meter installed on each of the three interceptors from the City, Vandenburg Air Force Base, and Vandenburg Vdlage Community Services District. The normalized flow patterns are shown on Figure 4.3.2, and represent the ratio between the instantaneous flow and the daily average flow. The weekend flow pattern shows a higher peak occurring later in the day than the corresponding weekday peak flow. This difference is typical for a predominantly residential service area as the population gets up later on the weekend but generally follows a similar activity pattern as that followed on a weekday. The maximum daily, or diurnal, flow is approximately 1.5 times the average daily flow.

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Secondary Treatment (I'M 2) 4-4

1z:uu

Time

Figure 4.3.2 Diurnal Influent Flow Pattern

Figures 4.3.3,4.3.4, and 4.3.5 show historical influent and headworks chemical oxygen demand (COD), total suspended solids (TSS), and volatile suspended solids (VSS) concentrations.

Carbonaceous BOD (cBOD) concentrations also were measured over &.IS period; however, nitrification inhibitors used to suppress nitrogenous oxygen demand in raw sewage samples can cause inaccurate BOD measurements by suppressing consumption of the carbonaceous BOD. It appears this is the case at the LRWRP based on an atypically high influent average C0D:cBOD ratio of 2.96. The raw wastewater C0D:cBOD ratio for a typical municipal wastewater is approximately

2.05. These ratios imply that the "true" cBOD concentrations are underreported by approximately 30 percent because of the nitrification inhibitors, so the "true" raw sewage cBOD concentrations were calculated from the COD concentrations using the typical C0D:cBOD ratio of 2.05. (Note that the plant laboratory analyzed a plant influent composite sample for BOD, with and without inhibitors, on October 8. The measured BOD and cBOD concentrations were 280 mg/L and 213 mg/L, respectively. The cB0D:BOD ratio for this single comparative analysis is 0.76. We recommend that additional raw wastewater comparative analyses be conducted to verify our assumption about the historical cBOD data, as discussed in Section 4.3.3 below.)

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Secondary Treatment 2) 4-5

-a- Influent 4 HeadwoRs

Figure 4.3.3 Historical Influent Chemical Oxygen Demand (COD)

-€-Influent -R- Headworks

Figure 4.3.4 Histon'cal Influent Total Suspended Solids (TSS)

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Secondary Treatment (TM 2) 4-6

7124198 711 9199 7IlWOO 7/8/01 713102 6/28/03 WW04 8117105

-e-lnfiuent -e- Headwork6

F.ipure 4.3.5 Historical Influent Volatile Suspended Solids (VSS)

These concentrations were multiplied by the corresponding final effluent flow to calculate equalized influent loads. These equalized influent loads were analyzed to estimate peak month and peak day peakmg factors for the subsequent process simulation. The average dry weather and peak equalized influent loadmg criteria are summarized in Table 4.3-1.

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Secondary Treatment (TM 2) 4-7

Table 4.3-1. Secondary Treatment Design Criteria

Flow Average dry weather (ADWF) Peak dry weather (PDWF) Equalized peak wet weather flow (EPWWF) Unequalized peak wet weather flow (PWWF)

Biochemical Oxygen Demand (BOD) Dry weather concentration, mg/L Dry weather load, lb/d Peak month load, lb/d Peak day load, lb/d

Total Suspended Solids (TSS) Dry weather concentration, mg/L Dm weather load. lb/d Peak month load. lb/d Peak day load, lb/d

Volatile Suspended Solids (VSS) Dry weather concentration, mg/L Dry weather load, lb/d Peak month load, lb/d Peak day load lb/d

Total Kjeldahl Nitrogen (TKN) Dry weather concentration, mg/L Dry weather load, lb/d Peak month load, lb/d Peak day load lb/d

Ammonia Nitrogen Dry weather concentration, mg/L Dry weather load, lb/d Peak month load, lb/d Peak dav load lb /d

Total Phosphorus Dry weather concentration, mg/L Dry weather load, lb/d Peak month load, lb/d Peak day load lb/d

Alkalinity, mg/L as CaC03 pH, standard units Temperature, degrees C

Maximum Minimum

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Secondary Treatment (TM 2)

4.3.2 Other Parameters

Wastewater temperature and influent pH design criteria were determined based on historical temperature and pH data shown on Figures 4.3.6 and 4.3.7, respectively. Minimum and maximum temperatures of 15.0 degrees C and 25.1 degrees C were determined based on Figure 4.3.6. The maximum and minimum temperatures are needed to determine biological kinetics and oxygen solubilities. An average influent pH of 7.3 was determined based on Figure 4.3.7.

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Secondary Treatment (TM 2) 4-9

4.3.3 Wastewater Characterization

Historical influent data from the LRWRP include COD, TSS, and VSS as presented above.

Collection of additional influent data was recommended as part of this initial study to characterize the various COD, nitrogen, and phosphorus fractions - particulate versus soluble, biodegradable versus unbiodegradable, etc. - for subsequent process analysis using an activated sludge process simulator. Collection and analysis of a week of influent composite samples and one day of grab samples was recommended to quantify these wastewater fractions and determine diurnal loading patterns.

The results of the daily composite and diurnal wastewater sample analyses are attached.

Unfortunately, the outside analytical laboratory did not prepare the samples properly (the samples were not mixed before aliquots were removed and solids settled to the bottom of the sample bottle), so the COD, BOD, TKN, and phosphorus data are suspect. Accordingly, default wastewater characteristics and an average measured influent alkalinity of 200 mg/L as CaC03 were used for subsequent process analysis.

We recommend that the plant lab conduct additional influent wastewater characterization over the next several months to confirm the default wastewater characteristics used. These confirmation samples do not need to be collected over a single seven-day period, but could be collected for two days per week over several weeks so they could be worked into the laboratory schedule. Each day of the week should be represented by an influent composite sample. Addtionally, diurnal grab samples should be collected on one day to confirm the diurnal loading pattern used. Care should be taken to obtain representative sample aliquots for "total" constituent analyses. (This was the failing of the outside analytical laboratory. Dr. David Lewis, the LRWRP Laboratory Director, indcated that representative sample preparation is a standard procedure for the plant laboratory.) The various soluble constituent analyses should also be conducted, so we have a concurrent characterization data set.

Process Interactions

4.4.1 Upstream Processes

The screened influent wastewater to the LRWRP wdl be routed through aerated grit tanks for grit removal. The degritted raw sewage will be equalized to limit the maximum wet weather flow to

9.5 mgd. Based on the typical diurnal flow pattern shown on Figure 4.3.2, this limit will pass influent flows up to the peak dry weather flows to the secondary treatment processes.

4.4.2 Downstream Processes

Waste sludge from the secondary treatment process will be thickened using new dissolved air flotation thickeners @AFTs) and w d be stabilized using existing anaerobic digesters. Table 4.4-1 summarizes the anticipated average day, peak month, and peak day sludge production at the 5.5 mgd ADWF design conditions.

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Secondary Treatment (TM 2) 4-1 0

Table 4.4-1. Projected Sludge Production

The digested sludge will be stored in the existing sludge lagoons for further stabilization and pathogen reduction. Sludge lagoon supernatant w d be metered and returned to the secondary treatment process or the DAFTs.

A portion of the alkahty consumed for nitrification will be recovered in the new secondary treatment sy.stem through denitrification. This alkalinity recovery will likely increase the historical median effluent pH of 6.9 and will help offset downstream alkalinity consumption if ferric chloride (or alum) is added as part of the tertiary treatment facilities.

Average month WML Peak month WML Peak dan WML

Process Recommendations

Design

The recommended secondary treatment facilities will be constructed in two stages to meet current and future discharge requirements. The Stage 1 improvements, which are part of the LRWRP upgrade project, include secondary facilities for nitrification and denitrification. The location of the recommended oxidation ditch-based system is in the southwest comer of the plant site as shown on Figure 1.12.1. This location allows the new facihties to be constructed without impacting operation of existing treatment facilities.

Startup (approximately)

The recommended facilities include a sufficient number of process units for redundancy and reliabihty. Process simulations and clarifier state-point analyses show that one oxidation ditch or one secondary clarifier can be taken out of service for maintenance during average loading periods in the summer. (Polymer addition and/or reduction of the activated sludge solids residence time may be necessary to take one oxidation ditch out of service based on sludge settleability to avoid overloading the three secondary clarifiers that would be in service.) The ability to take units out of service allows staged preventive maintenance of the secondary treatment facilities throughout the summer, so all process units are available during peak loading conditions. The sizing of the recommended facilities assumes that both oxidation ditches and all secondary clarifiers are available during peak loading periods.

VS

73% 73% 73%

TS

(lb / d)

10,064 13,385 14,593

4.5.1 Stage 1 Improvements

VS

("/o)

73% 73% 7 3 '10

VS

(lb /d)

7,356 9,783 10,666

TS

(lb/d)

6,862 9,126 9,950

4.5.1.1 Oxidation Ditches. Figure 4.5.1 shows a process schematic for the two oxidation ditch trains for the LRWRP Upgrade Project. These two trains will provide staged biological treatment for carbonaceous BOD removal, biological oxidation of ammonia to nitrate (i.e., nitrification), and biological reduction of nitrate to nitrogen gas (i.e., denitrification). Overall nitrogen removal will occur when the nitrogen gas is stripped from the mixed liquor into the atmosphere.

VS

(lb/d)

5,015 6,671 7,272

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LEGEND:

ML MIXED LIQUOR

RAS RETURN ACTIVATED SLUDGE

SE SECONDARY EFFLUENT

WML WASTE MIXED LIQUOR

WW WASTEWATER

RE-AERATION

BASIN (FUTURE)

OXIDATION - F DITCH 2

FIRST ANOXIC

Y ' I

ANAEROBIC BASINS

6" WML I I

I

DISTR~UTION

BOX

SECONDARY

CLAlFlERS (3) Figure 4.5.1. - SECONDARY TREATMENT

PROCESS SCHEMATIC

Secondary Treatment (TM 2) 4-12

Aerated grit tank effluent, equalized to a maximum flow of 9.5 mgd, will flow by gravity to the oxidation ditch inlet box, where it will blend with return activated sludge @AS). The inlet box w d include two gates to divide the mixed liquor between the two trains and to isolate one of the trains should it need to be taken out of surface. The inlet gates will provide a free water surface between the inlet box and the anoxic zones to prevent trapping of scum or foam.

Provisions will be included in the inlet box design for future gates to divide and route the mixed liquor to future anaerobic zones for biological phosphorus removal, as described in the following section on Stage 2 improvements.

The mixed Liquor will flow into an anoxic zone for nitrogen removal. This zone will not be aerated, so nitrate recycled from the downstream aerobic zone is denitrified. The anoxic zone also wilI help maximize mixed liquor settleability by consuming the soluble, readily biodegradable portion of the influent o~ganic load in the denitrification process. The lack of soluble, readily biodegradable COD will suppress the growth of filamentous organisms in the downstream aerobic zone. Submersible mixers will be provided to keep the degritted raw sewage solids and mixed liquor suspended. Again, a free water surface between the anoric and aerobic zone will be maintained to prevent trapping of scum or foam.

Mixed liquor w d flow from the anoxic zone into the aerobic zone, or oxidation ditch proper. Each oxidation ditch will consist of an aerated, continuous-loop tank that is interconnected to the upstream anoxic zone. Mixlng will be provided to maintain an average minimum velocity of 1 foot per second (ft/s) to keep degritted raw sewage solids and mixed liquor suspended. The oxidation ditch can be configured as a single loop or in a "folded" configuration as shown schematically on Figure 4.5.1. This folded configuration minimizes overall length.

Aeration will be provided at specific points in the oxidation ditch so there is a dissolved oxygen concentration gradient as the mixed liquor travels around the d~tch. Sufficient aeration will be provided to maintain carbonaceous BOD removal, nitrification, and provide sufficient dissolved oxygen (DO) to suppress the growth of low-DO filaments. The D O gradient will promote some denitrification and enhanced biological phosphorus removal w i t h the ditch as localized DO concentrations drop. The concentration gradtent also will minimize recycle of significant quantities of D O into the anoxic zone, whlch would reduce the denitrification rate and, potentially, promote filament growth.

Mixing and aeration can be provided together with mechanical surface aerators or separately with jet aeration supplemented with submersed mechanical mixers. If mechanical mixers are used, a concrete deck is required to span the oxidation &tch channels and support the aerator shaft and impeller. Replacement of the gear box or aerator impeller requires draining the oxidation dltch so that the aerator shaft and impeller can be lowered to the bottom of the channel. A jet aeration system includes a fixed header mounted on the channel floor and aeration blowers and recirculation pumps, which can be located adjacent to the oxidation ditch structure at grade.

Further evaluation of these two aeration alternatives and supplemental mixing requirements and an equipment recommendation for the upgrade project will be made during prelimnary design. The recommended aeration system wdl include the capabihty to control oxygen transfer with seasonal variation of influent loads and as influent flows and loads increase through the life of the project.

February 2005

Secondary Treatment (TM 2) 4-13

Two dssolved oxygen probes in each &tch wdl be included to provide feedback control at each of the aeration points.

Mixed liquor flow from the oxidation &tch to the anoxic zone will be controlled by a manually adjusted butterfly gate; recirculation pumps are not needed with this process configuration. The butterfly gate position is set during startup and typically does not need to be adjusted.

&fixed liquor wasting is used for solids residence time (SRT) control in the existing nitrifying activated sludge system and is recommended for the new oxidation &tch system as it sipficantly simplifies operations. Mixed liquor wasting is simple to operate, is compatible with the recommended DAFTs, and can help control oxidation ditch foam and scum buildup. This approach eliminates the need to measure mixed liquor and RAS solids concentrations in order to calculate and adjust the waste activated sludge flow rate. The target waste mixed liquor (WML) flow rate is simply the aerobic oxidation ditch volume divided by the target SRT. Because the target SRT and WML flow rate are constant throughout the year, the mixed liquor suspended solids concentration will vary seasonally as loadings and temperature changes occur and wlll increase throughout the project life as influent flows and loads increase. Continual adjustment of the WML flow rate to maintain a given mixed liquor suspended solids concentration w d not be needed. The impact of the resulting mixed liquor solids concentration variations has been accounted for in the overall process design.

Figure 4.5.1 shows that a WML pumping station will be located adjacent to each oxidation ditch.

Each WML pumping station will include two variable speed submersible pumps to provide adequate capacity with both trains in service or one train out of service. The flow of mixed liquor into the WML pumping stations wdl be controlled with multiple motor-operated downward opening slide gates so that foam w d be removed in the wasting operation. The gate position wdl be adjusted to maintain a set-point wet-well water depth. An angled surface baffle wdl be constructed across the channel to dn-ect any scum and foam into the pumping stations to prevent proliferation of foam-causing microorganisms. The WML pumping station wet wells w d be pumped down periodically (by temporarily increasing the pump speed to maximum) to transfer any scum and foam to the dmolved air flotation tbckeners (DAFTs).

Oxidation ditch outlet gates will be located imrnedmtely downstream from the WlML pumping station. These gates wdl be manually operated to adjust impeller submergence (i.e., oxygen transfer) if mechanical aerators are used.

4.5.1.2 Secondary Clarifiers. Three secondary clarifiers will be included in the LRWKP upgrade project for process flexibility as shown on Figure 4.5.1. A splitter structure with cutthroat flumes wlll be used to split the mixed liquor from the ditches among the on-lme clarifiers with minimal headloss. Manual gates w d be provided to take a clarifier out of service.

Each clarifier wdl include an energy dissipating dstribution tub and flocculation baffle to evenly distribute and flocculate the influent suspended solids. The clarifiers wdl have a flat bottom with rotating suction arms for rapid sludge withdrawal (i.e., a TowBro-type clarifier mechanism). The RXS pumps for each clarifier are connected hect ly to the suction arms through an opening in the clarifier floor to an annular ring at the base of the center column that rotates with the suction arms.

This configuration provides positive sludge removal over the entire clarifier floor. The clarifiers wd

Secondary Treatment (TM 2) 4-1 4 have an inboard launder with rectangular notch weirs for clarified effluent removal. A full-width scum trough will be included for scum removal. These features are typical of our secondary clarifier design approach and are shown schematically on Figure 4.5.2. The final clarifiers at the San Luis Obispo W W P , designed by Brown and Caldwell, are examples with these features.

Two VTSH pumps will be included for each clarifier. The pumps will be located in a wet well adjacent to each clarifier. Each wet well is directly connected to the sludge suction arms. The RAS pump flow rate will be flow-paced to minimize the clarifier sludge blanket depth throughout the day.

Secondary effluent will be collected from the three secondary clarifiers and flows by gravity to the existing aeration tank, which is being converted into a diurnal equalization tank as described in subsequent technical memoranda.

4.5.2 Stage 2 Improvements

The two oxidation ditch trains included in the Stage 1 Improvements will include provisions to modify the anoxic/aerobic biological treatment system into a five-stage Bardenpho process should future discharge permits contain more stringent effluent nitrogen and/or phosphorus limits. The three additional process stages include an anaerobic zone upstream of the anoxic zone, a second anoxic zone downstream of the aerobic zone, and a second aerobic zone. These future facilities are shown schematically on Figure 4.5.1.

New gates will be added to the oxidation ditch inlet box to split the mixed liquor between two sets of anaerobic zones rather than the two anoxic zones. Three unaerated, mechanically mixed reactors in series will comprise each anaerobic zone. This configuration uses VFAs, a component of the soluble readily biodegradable COD, to promote the growth of phosphorus accumulating organisms to achieve EBPR. VFAs occur typically in raw wastewater and additional VFAs are produced through fermentation in the anaerobic zone.

The second anoxic zone will provide additional denitrification to meet lower future effluent nitrogen limits. The second aerobic zone will provide mixed liquor reaeration to strip residual dissolved nitrogen upstream of the secondary clarifiers.

The elevations of the Stage 1 Improvements oxidation ditch facilities, mixed liquor splitter structure, and secondary clarifiers wlll be set to accommodate additional hydraulic losses from the future Stage 2 Improvements.

Increased, relatively inexpensive capacity gain can be realized, if and when needed, by constructing primary clarifiers. Primary clarifiers will reduce the organic and nitrogen loads to the oxidation ditches by approximately 30 percent. T h s reduction, along with an approximately 65 percent reduction in inert suspended solids (i.e., the difference between total suspended solids and volade suspended solids), will allow one oxidation to be taken out of service if primary clarifiers are added in the future. Hydraulic provision for primary clarifiers will be accommodated in the Stage 1 design.

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Inboard Launder

\ Energy Dissipating Inlet

T:~ction ~ r m s I' \ VTSH RAS Pump8

Mked Liquor

RAS

Figure 4.5.2. Secondary Clarifier Design Featurn

Secondary Treatment (TM 2) 4-1 6

4.6 Design Criteria

Design criteria for the recommended secondary treatment facdities for the LRWRP upgrade project are summarized in Table 4.6-1. These design criteria are based on process simulations using BioWin 2.1, a PC-based suspended growth simulator, and equalized influent loading conditions described above. An SRT of 8 days is recommended for year-round process control. This value was selected based on process simulation under maximum anticipated influent loading conditions and a minimum wastewater temperature of 15.0 degrees C. The mixed liquor suspended solids concentration will vary throughout the year with a constant SRT as influent loads and wastewater temperature change, but will not affect process performance.

Table 4.6-1. Secondaq Treatment Design Criteria

Oxidation Ditch Anoxic Zone

Number Volume, hlG each Side water depth (SWD), ft Anoxic hydraulic residence time (HRT), hr Internal mixed liquor recycle, percent of oxidation ditch influent flow (Q) Mixer type Number Mixer HP, each

Aerobic Zone Number Volume, MG each SWD, ft Channel width, ft Straight run length, ft Aerobic hydraulic residence time (HRT), hr Aerobic solids residence time, days Aerators

Supplemental mixer type Number Mixer HP

Waste Mixed Liquor Pumps TYPe Number Capacity, gpm, each

Secondary Clarifiers Type Number Diameter, ft SWD, ft Return Activated Sludge Pumps

Type Number

Submersible

13.1

2@200 HP each 2@150 HP each

Submersible

18.2

Submersible Screw Centrifugal 2 duty12 standby

Circular Rapid Sludge 3 Withdrawal

16.0

Vertical Turbine Solids Handling

I 3 duty13 standby 17C. Capacity, gpm (each) L. I -, Secondary Treatment (TM 2) 4-1 7

4.7 Preliminary Cost Estimate

The preliminary estimate of probable construction cost for the secondary treatment facilities is $17,120,128 as indicated for TM 02 in Table 2.1-2. A detailed breakdown of fIvs estimate is provided in Appendix B.

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