Subject: Upgrade Alternatives for the Air Flotation Tanks (AFT) at the Sunnyvale WPCP

Transcription

1 201 N. Civic Drive, #115 Walnut Creek, CA Tel: Fax: Prepared fr: City f Sunnyvale, Sunnyvale, CA Prject Title: Sunnyvale Strategic Infrastructure Plan fr the WPCP Prject N: Subject: Upgrade Alternatives fr the Air Fltatin Tanks (AFT) at the Sunnyvale WPCP Date: April 24, 2009 T: Frm: Lrrie B. Gervin, P.E., City f Sunnyvale Llyd Slezak, Prject Manager, Brwn and Caldwell Prepared by: Jhn Bratby, P.E., Executive Engineer, CO PE N Exp 06/30/2009 Reviewed by: Denny Parker, Ph.D., P.E., Senir Vice President, N. C24965 Exp.12/31/2011

2 EXECUTIVE SUMMARY This (TM) evaluates alternatives t remve algae frm the effluent frm the Sunnyvale Water Pllutin Cntrl Plant (WPCP). The fllwing alternatives were evaluated t remve and thicken the algae frm the xidatin pnds: 1. Maintain existing AFTs. 2. Upgrade existing AFTs. 3. Maintain fltatin as the algae remval prcess, but replace system. 4. Replace fltatin technlgy with sedimentatin and separate thickening. 5. Replace fltatin technlgy with ballasted sedimentatin and separate thickening. 6. Replace fltatin technlgy with membrane separatin and separate thickening. Of these alternatives, nly tw, alternatives 2 and 3 were maintained fr further cnsideratin. The ther alternatives were rejected fr the fllwing reasns: Alternative 1 requires a similar investment fr structural and electr-mechanical imprvements, but withut the benefit f the technlgy upgrades in Alternative 2. Alternative 4 requires deaeratin f the xygen supersaturatin with uncertain results. It als requires additinal thickening which is likely t be DAF thickening, with an area requirement similar t Alternatives 2 and 3. Alternative 5 has n knwn track recrd fr algae remval and als requires the same additinal thickening step as Alternative 4. Alternative 6 has n knwn track recrd fr algae remval frm xidatin pnds and als requires the same additinal thickening step as Alternatives 4 and 5. The analysis shws that fr Alternative 2, nly tw f the fur existing air fltatin tanks (AFT) need be rehabilitated and upgraded. The expected flat cncentratin is 4 t 6%. Because very little f the existing AFT system is anticipated t be utilized, Alternative 3 abandns the existing equipment and infrastructure and replaces the AFTs with a new DAF system. This alternative uses rectangular disslved air fltatin (DAF) tanks. The required prcess area is apprximately 33-percent the existing AFT area. Fur DAF tanks 37 ft lng and 25 ft wide wuld be required. A preliminary site layut wuld be with the new DAF tanks cnstructed ver AFTs 3 and 4. AFT 4 wuld first be demlished and DAFs 1 and 2 cnstructed. Then AFT 3 wuld be demlished, and DAFs 3 and 4 cnstructed. AFTs 1 and 2 wuld be abandned. The expected flat cncentratin is 4 t 6%. Selectin f Alternative 3 may allw higher lading rates, with even smaller DAF prcess areas. Hwever this wuld require n-site pilt tests. The recmmendatin in this TM is t carry ut cst evaluatins f Alternatives 2 and 3 t decide which alternative shuld be carried thrugh t final design and implementatin. 2

3 1. INTRODUCTION This (TM) evaluates alternatives t remve algae frm the effluent frm the Sunnyvale Water Pllutin Cntrl Plant (WPCP). There are fur existing air fltatin tanks (AFT) that use disslved air fltatin (DAF) t remve and recver the algae that grws in the xidatin pnds at the WPCP. Remval f the algae frm the effluent is required t maintain effluent quality within permitted limits. The fur AFTs were cnstructed in the mid-1970s and early 1980s and, therefre, are in need f extensive mechanical and electrical imprvements t cntinue peratin in the future. This TM examines the current system f disslved air fltatin, and cmpares this with ther alternative appraches fr algae remval frm the effluent frm the xidatin pnd effluent. 2. ALTERNATIVES FOR ALGAE REMOVAL AT THE WPCP The prjected maximum mnth flw fr the WPCP is 22.4 mgd in The crrespnding peak week and peak hur flws are 24.3 and 50 mgd, respectively, althugh peratin f the lagn system allws attenuatin f peak flws. Therefre, the algae remval system shuld be designed t receive up t the maximum mnth design flw, assuming that peak flws will be cntained in the lagn system. The fllwing list prvides a number f appraches fr remving algae at the WPCP. These alternatives have been shrt-listed frm a wider spectrum f alternatives t cnsider nly thse that appear t be viable fr the Sunnyvale WPCP. Each f the alternatives will be evaluated based n the prjected flws. Alternative Table 1. Algae Remval Alternatives Descriptin 1 Maintain existing AFTs Rehabilitate existing AFT equipment in kind. Prcess remains the same, but mechanical equipment replaced as needed. Maintain existing filtratin system. 2 Upgrade existing AFTs Upgrade AFTs t imprve energy efficiency and prvide enhanced thickening capability within the AFTs. Maintain existing filtratin system. 3 Maintain fltatin as the algae remval prcess, but replace system 4 Replace fltatin technlgy with sedimentatin and separate thickening 5 Replace fltatin technlgy with ballasted sedimentatin and separate thickening 6 Replace fltatin technlgy with membrane separatin and separate thickening Replace AFTs with newer technlgy. Maintain existing filtratin system. Prvide deaeratin t relieve xygen supersaturatin, t allw sedimentatin. Maintain existing filtratin system. Deaeratin is nt required fr sedimentatin. Maintain existing filtratin system. Cagulant may nt be required; r at much reduced dsage. Existing filtratin system nt required. 3

4 2.1 Alternative 1 Maintain Existing AFTs Figure 1 shws a schematic f ne f the AFTs in the existing system. The recent RMC Imprvements Alternatives (December 29, 2008) pinted ut that extensive structural, mechanical, electrical and I&C imprvements are required with the existing installatins t cntinue peratin int the future. There are fur AFTs, each with a diameter f 60-feet. Based n histrical perfrmance, the flat cncentratin expected is apprximately 3 t 4 % under prjected peak week lads. Since this alternative requires extensive structural, mechanical, electrical and I&C imprvements, as delineated in the December 2008 RMC TM, it appears bvius that Alternative 2, which builds n Alternative 1 t upgrade the system and incrprate the latest technlgy is the preferred alternative. Therefre, Alternative 1 is nt cnsidered further in this TM. Figure 1. Prcess schematic f a single AFT 2.2 Alternative 2 Upgrade Existing AFTs The difference between this alternative and Alternative 1, is that in additin t the structural, mechanical, electrical and I&C upgrades required, design changes wuld be made t the existing system in terms f the saturatrs, chemical additin, the scrapers and flat beach assemblies. In ther wrds, there wuld be a re- 4

5 design and replacement f much f the basic equipment within the AFTs. Figure 2 shws sme f the ptential prpsed imprvements t the riginal AFT design. In additin t these imprvements, changes t the way cagulating chemicals are fed t the AFTs, and the saturatr design, wuld be prpsed. POL Plymer Batch Tank Flat hpper design t prevent flat degradatin SA Air Cmpressr Cmpressed Air PF Replace verflw weirs with water level cntrller POL Plymer Feed Pumps POL Air Receiver Saturatin Tanks DAFT Tank SNT SNT T SPS Flat cnveyed by PD pump instead f pneumatic ejectr WML/SSC Drain Drain DAFT Feed Bx Feed t AFT MS TWAS T Digesters Thickened TWAS Sludge Pumps Figure 2. Prcess Schematic f AFT Imprvements In summary, the prpsed imprvements wuld be: Replace verflw weirs with submerged clarified water cllectin and water level cntrller. The benefit is that drainage and thickening f the flat can be ptimized. Replace flat hpper with a design that prevents rtatin f the flat mass and encurages flat dewatering. The benefit is that thickening f the flat wuld be ptimized. T reslve maintenance and peratinal issues with the existing pneumatic ejectrs, these wuld be replaced with psitive displacement pumps. Replace saturatr system including pumps. The benefit here is that the existing system culd have a saturatin efficiency f apprximately 65% based n measurements n ther systems f this generatin. Mre recent Brwn and Caldwell designs have prven efficiencies f apprximately 90%. This means that energy csts are significantly reduced fr a given air/slids rati. The hydraulic and slids ladings t the AFTs wuld be the same as fr Alternative 1. Because f imprvements t the thickening functin, the flat cncentratin expected is apprximately 4 t 6% under prjected peak week lads. 5

6 2.3 Alternative 3 Maintain Fltatin as the Algae Remval Prcess, but Replace System In this alternative, the existing AFT system wuld be abandned, and replaced with a cmpletely new system. The new system culd take varius frms: The first ptin culd be the suspended air fltatin (SAF) system marketed by Hern Innvatrs, shwn schematically in Figure 3. In this system, the SAF system injects a chemical emulsin presaturated with cmpressed air int the center well f the AFT. The chemical used is prprietary but it appears that it may essentially cmprise a catinic detergent. The catinic charge facilitates attachment f the detergent mlecule t bubbles and algae particles, and stabilizes the air suspensin. The SAF system des nt utilize pressurizatin pumps and the cmpressed air utilized has a pressure f nly apprximately 20 psi. Plymer wuld likely still be used t ptimize cllectin f the algae and maximize clarificatin f the effluent. Figure 3. Hern s SAF Fltatin System Pssible disadvantages with this system include: This system des require prcess prving. Therefre, befre cnsidering this ptin, pilt scale prcess trials shuld be cnducted at the plant alngside the existing AFTs. Catinic detergents are generally txic, and are ften used fr disinfectin purpses. Therefre, there may be an undesirable effect n effluent WET tests, which needs t be demnstrated. It appears that this system has heretfre been used n relatively small systems. Therefre there may be issues with scale-up in the case f applying the technlgy t the WPCP. Because f these disadvantages, this ptin is nt cnsidered further in this TM. If the City des cnduct successful pilt trials then this ptin culd be re-examined in the future if required. The secnd ptin is als a DAF prcess, but different t the existing system by the methd f air disslutin. In this case special centrifugal pumps allw air t be injected t the suctin side f the pump as shwn in Figure 4. N saturatin vessels are required. One manufacturer, Wrld Water Wrks claims that the system is much simpler t cntrl and perate than a DAF system that includes pressurizatin pumps, saturatin vessels, cmpressrs and level cntrls. 6

7 Figure 4. Pump Injectin Disslutin System There are ptential capital cst savings with this alternative, since saturatin vessels are nt required. Hwever, disadvantages are that there is a limit t the amunt f air injectin pssible befre air binding ccurs, and higher energy is required because f the reduced pump efficiency. On-site measurements at existing full-scale installatins are required t determine the actual amunt f disslved air prduced by this system, and the energy expended in prducing the disslved air. Because f the higher energy csts that wuld be assciated with this system, it is nt cnsidered further in this TM. The third ptin culd be the rectangular DAF prcess marketed by Infilc r ITT/Lepld. Such systems have been able t prcess water at rates f apprximately 20 gpm/ft 2 in water treatment applicatins. An integral feature f this system is a full flr cverage f effluent drawff by nzzles r cllectin headers, as shwn in Figure 4. DAF TANKS SKIMMER NOTE: FLOCCULATION TANKS SHOWN IN THIS PICTURE WOULD NOT BE INCLUDED FOR THE SUNNYVALE WPCP. CHEMICAL WOULD BE INJECTED VIA IN- LINE RAPID MIXERS THEN INTRODUCED DIRECTLY TO THE DAF TANKS. BOTTOM DRAWOFF LATERALS SATURATORS PRESSURE RELEASE NOZZLES Figure 4. High Rate DAF System (ITT/Lepld Clari-DAF System) The benefit f the rectangular system is that area requirements wuld be significantly less, with verall space savings. 7

8 Since there is lng-term experience with this type f fltatin device, particularly applied t water treatment, backwash water treatment, secndary effluent plishing, and with algae recvery, this alternative shuld be cnsidered further. 2.4 Alternative 4 Replace Fltatin Technlgy with Sedimentatin and Separate Thickening In this alternative, the existing AFTs wuld be abandned and replaced with sedimentatin tanks. The advantage wuld be that the sedimentatin system wuld be simpler than the AFTs withut pressurizatin pumps, saturatin vessels, cmpressrs r level cntrls. Hwever, the main disadvantage is that the high degree f xygen supersaturatin develped in the pnds wuld seriusly hinder settling. Therefre, befre settling, a deaeratin tank wuld be required t strip the supersaturated gases frm the water. In additin, cagulatin and flcculatin tanks in series wuld be required befre the sedimentatin tanks. The required surface verflw rate fr the sedimentatin tanks, even after deaeratin is estimated at apprximately 0.5 gpm/ft2. The prcess area required, based n maximum mnth flw wuld be 31,100 ft 2, r almst 3-times the area f the existing AFTs. It is expected that the thickened sludge cncentratin remved frm the bttm f the sedimentatin tanks wuld be apprximately 1% r less. Therefre, an additinal thickening step wuld be required befre the anaerbic digesters. Ptentially there are a number f thickening devices that culd be used, including DAF thickeners and gravity belt thickeners. Of these, the prven track recrd is with DAF thickeners. The area requirement fr the DAF thickeners is dependent n the applied slids lading rate t achieve the required degree f thickening. Since the slids lading rates in fltatin alternatives 1 thugh 3 wuld be similar t that required in the thickener, the area fr the DAF thickener wuld be similar t the AFT r DAF tank areas in Alternatives 1 thrugh 3. Therefre, there appears t be little advantage in having a separate thickening stage, when bth clarificatin and thickening can be carried ut simultaneusly in alternatives 1 thrugh 3. In additin t the lw settled sludge cncentratin and the separate thickening issue, the large area required fr the sedimentatin tanks is a significant disadvantage. Fr these reasns, this alternative is nt cnsidered further in this TM. 2.5 Alternative 5 Replace Fltatin Technlgy with Ballasted Sedimentatin and Separate Thickening In this alternative, the need fr deaeratin is replaced by the additin f a weighting agent t the algae. This allws a higher sedimentatin rate and cnsequently smaller tankage. Tw alternative weighting agents are sand, used in the Actifl prcess, and magnetite, used in the CMag prcess, shwn in Figure 5. There are sme advantages t using magnetite, including the higher rate f sedimentatin pssible given the higher specific gravity f the magnetite, and the pssibility f using either the existing filters, r a magnetic filter fr final effluent plishing. 8

9 Figure 5. Schematic f the CMag Prcess. Assuming a surface verflw rate f apprximately 7.5 gpm/ft 2 fr this applicatin, the prcess area requirement wuld be apprximately 2,100 ft 2. Despite the additin f the ballasting agent, the settled sludge cncentratin after magnetite recvery wuld still likely be apprximately 2% r less. Therefre, similarly t Alternative 4 an additinal thickening step wuld be required befre digestin. Because f the issues identified with separate thickening, and that there is n knwn track recrd with this alternative fr algae separatin, this alternative is nt cnsidered further in this TM. 2.6 Alternative 6 Replace Fltatin Technlgy with Membrane Separatin and Separate Thickening This alternative wuld represent the mst cmpact ptin fr the WPCP. There are n knwn applicatins f micrfiltratin (MF) r ultrafiltratin (UF) fr separatin f algae frm xidatin pnds, althugh the applicatin f membranes in water treatment with algae in the raw water has been reprted. Generally membranes have experienced serius flux declines due t the presence f algae in the raw water, but at least ne applicatin using submerged aerated membranes (GE-Zenn UF membranes) was successful, althugh cagulatin and sedimentatin did precede the membranes in that applicatin. Despite the pssible applicatin f UF fr algae separatin at the WPCP, a majr disadvantage is that the algae cncentratin t be cnveyed fr further prcessing wuld be apprximately 1% r less. This wuld require an additinal thickening step prbably DAF thickening, as in Alternatives 4 and 5. Fr this reasn, and the pssible fuling and flux limiting cnditins, this alternative is nt cnsidered further in this TM. 3. COMPARISON OF SHORT-LISTED ALTERNATIVES The previus sectin cnsidered six alternatives. Of these, Alternative 2 and a sub-alternative in Alternative 3 were shwn t be wrthy f further cnsideratin. Specific features f these tw shrt-listed alternatives are as fllws: 3.1 Alternative 2 Upgrade Existing AFTs In this alternative, the existing AFTs wuld be essentially stripped and almst all f the electr-mechanical equipment replaced with updated equipment designed fr prcess ptimizatin in terms f bth clarificatin and thickening. The main features f this alternative are as fllws: Maintain existing AFT cncrete structures 9

10 Carry ut recmmended structural imprvements t prevent rebar crrsin and leakage, and t bring the structure up t seismic and structural cde requirements. The existing structure has been fund t be cmprmised due t past seismic events, accrding t the December 2008 RMC TM. Replace mst f existing electr-mechanical equipment, including: Sludge pumps and check valves Saturatr system including pressure cylinders and pressurizatin pumps Center drive mechanisms Flat scraper assemblies Replace perimeter baffle with submerged launder Flat cllectin trughs Replace MCC and ther electrical equipment as needed Replace and install instrumentatin as needed The plant layut fr the AFT cmplex will remain essentially the same as the existing installatins. A key design criterin is the amunt f air supplied fr fltatin. Design will be based n the greater f tw parameters: an air/slids rati f 0.03, and at least 8 mg air per liter pnd effluent treated. The reasn why the air/slids rati is nt cnsidered a sufficient design parameter in this case, is the need t prvide adequate cntact pprtunity between the clud f precipitated bubbles, and the relatively lw TSS exiting the pnds at the WPCP. Frm the EOA Memrandum (Octber 5, 2005) the mnthly average pnd effluent TSS, fr the perid 1990 thrugh 2005 varies frm a lw f 59 mg/l (typically in January-February) t 130 mg/l (typically in July). Table 2 presents the calculated air and required saturatr recycle flw, based n 2035 flws. Assuming a saturatr pressure f 70 psig, the ttal required saturatr recycle flw is calculated t be 1.5 mgd. The amunt f air supplied wuld be 8 mg air per liter pnd effluent flw. The crrespnding air/slids rati under design maximum mnth cnditins wuld be 6%. 10

11 Flw a s 1 Assumed Assumed Sat. Press. Table 2. Air Calculatins Precipitated air Required Saturatr Recycle basis TSS 2 air 3 mg/l basis Basis mgd g/g mg/l psig mg/l Rati mgd mg/l mg/l mgd mg/l g/g MMF a s = air/slids rati (mass/mass basis) 2 Frm EOA Mem Oct.5, Highest mnth average pnd TSS in July = mg/l 3 Recmmended 8-10 mg air/l wastewater flw fr adequate cntact at lw TSS Calc. air 3 n a s basis Required Required recycle Calc. air 3 n Anther key design criterin is the area prvided fr clarificatin and thickening in the AFTs. Figure 6 shws the results frm pilt studies n DAF applied t separate algae frm xidatin pnd effluents. Frm these results, the maximum air/slids rati applied t separate algae frm the xidatin pnds in thse studies was g/g. The maximum limiting dwnflw rate at that air/slids rati, befre slids were drawn dwn t the effluent frm the pilt DAF unit, was 7.4 gpm/ft 2. Therefre, this represents the maximum knwn hydraulic lading applied t algae separatin frm xidatin pnds. Applying a factr f 1.25 t this limiting lading, a maximum design hydraulic lading f apprximately 6 gpm/ft 2 is determined. It is pertinent t nte that this hydraulic lading may nt be the maximum that culd be applied successfully fr algae separatin it merely represents the limit f past reprted experience. Figure 6 als presents results frm the same pilt studies n DAF thickening f the separated algae. Frm these results, and assuming a design maximum mnth thickened flat cncentratin f apprximately 5%, the prpsed maximum applied slids lading fr design maximum mnth cnditins is 10 lb/ft 2.d. Calc. a s Limiting dwnflw rate (gpm/ft 2 ) Air/slids rati (%) Flat cncentratin (%) Slids lading rate (lb/ft2.d) Figure 6. Oxidatin Pnd Algae Separatin and Thickening Results Using Ferric Chlride as Sle Cagulant at 50 mg/l as Fe. (Frm Bratby J., Filtratin & Separatin, Nv/Dec, 1974, pp ) Table 3 presents calculated hydraulic and slids ladings t the existing AFTs. Calculatins are presented fr all fur, three, tw r ne AFTs in service. Based n the hydraulic and slids lading criteria presented abve, prjected design flws culd be treated in ne f the fur existing AFTs. Hwever, in terms f redundancy, at least tw units wuld be required. Therefre, nly tw f the existing fur AFTs need t be upgraded. 11

12 2 Frm air calculatins Table 3. Hydraulic and Slids Lading Calculatins Flw Number Air/ Ttal Calc. Calc. Assumed Saturatr f AFTs slids hydraulic TSS 1 rati 2 recycle 2 AFT slids in area lading lading service Basis mgd mg/l g/g mgd - ft2 lb/ft 2.d gpm/ft 2 MMF , MMF , MMF , MMF , Frm EOA Mem Oct.5, Highest mnth average pnd TSS in July = mg/l 3.2 Alternative 3 Maintain Fltatin as the Algae Remval Prcess, but Replace System Because very little f the existing AFT system is anticipated t be utilized in Alternative 2, this alternative f cmpletely replacing the existing AFT system with a new DAF system is a viable alternative. The frmat prpsed fr the replacement system is rectangular. The new DAF tanks will be cnstructed with in-line rapid mix facilities and reciprcating r chain and flight flat remval scrapers. Figure 6 shws examples f such systems. Figure 6. Examples f Rectangular DAF Tanks 12

13 Table 4 presents calculated DAF area requirements t cmply with the prpsed design hydraulic and slids lading criteria. Flw Applied slids lading 1 Table 4. DAF Tank Area Calculatins Basis mgd lb/ft 2.d mg/l ft 2 mgd gpm/ft 2 g/g gpm/ft 2 ft 2 lb/ft 2.d MMF , , estimated t achieve 5% thickened flat 2 Frm EOA Mem Oct.5, Highest mnth average pnd TSS in July = mg/l 3 Frm air calculatins 4 Maximum value frm pilt wrk (Bratby, 1974) and 1.25 factr applied t limiting dwnflw rate. 5 Based n largest calculated area Assumed TSS 2 Calc. area based n slids lad Saturatr recycle 3 Calc. hydraulic lading Air/ slids rati 3 Maximum acceptable hydraulic lading 4 Calc. area based n hydraulic lad Calc. slids lading 5 Table 4 shws that the required ttal area fr the DAF tanks is principally dependent n the hydraulic lading limit f 6 gpm/ft 2. On this basis, the applied slids lading under 2035 design cnditins is 8.8 lb/ft2.d under maximum mnth cnditins. Assuming a cmmn length:width rati f apprximately 1.5:1 fr this type f applicatin, and a maximum area f each DAF tank f apprximately 1,200 ft 2, then fur DAF tanks, each with length 37 ft and width 25 ft wuld be required. The furth tank wuld be prvided fr redundancy. Figure 7 presents a preliminary suggestin f hw the new rectangular DAF tanks culd be implemented. In this case the cnstructin sequence culd be t: 1. Deactivate and demlish AFT-4; 2. Cnstruct and put int service DAFs 1 and 2; 3. Deactivate and demlish AFT-3; 4. Cnstruct and put int service DAFs 3 and 4; 5. Deactivate and abandn AFTs 1 and 2. 13

14 New rectangular DAFs DAF-4 DAF-3 DAF-2 DAF-1 Figure 7. Preliminary Site Layut fr New Rectangular DAF Tanks It was nted previusly that the maximum hydraulic lading adpted fr design may nt be the maximum that culd be applied successfully fr algae separatin it merely represents the limit f past reprted experience. Therefre, this suggests that there wuld be a benefit t cnducting pilt trials t test whether higher hydraulic and slids ladings can be applied fr the design f the new facilities. One apprach is t pre-select an equipment supplier and, as part f the pre-purchase agreement, cnduct n-site pilt tests using the manufacturer s pilt equipment. 4. RECOMMENDATIONS Tw alternatives are recmmended fr further cnsideratin: Alternative 2 retains the existing AFT structure, but requires extensive structural rehabilitatin and cmplete electr-mechanical equipment replacement. Alternative 3 abandns the existing AFT system and installs a new DAF system in its place. Cst estimates f Alternatives 2 and 3 shuld be develped t decide which alternative shuld be carried thrugh t final design and implementatin. The new rectangular system prpsed fr Alternative 3 may allw higher lading rates than assumed in this TM. On-site pilt tests are recmmended t establish this. 14