Side By Side By Side - The Evaluation of Three High Rate

Side By Side By Side The Evaluation of Three High Rate Process Technologies for Wet Weather Treatment

David A. Frank, P.E., ARCADIS, and Thomas F. Smith III, P.E., City of Akron

ABSTRACT Most large combined sewer communities face significant financial investment to address wet weather compliance issues. Innovative, cost-effective methods of dealing with wet weather flows is a must to prevent spiraling rate increases for rate payers. High rate clarification and high rate filtration systems are becoming the technology of choice for treating wet weather flows. The City of Akron, Ohio selected three high rate treatment technologies to pilot test and evaluate for incorporation into their wastewater treatment plant as a Secondary Bypass process treatment train. The technologies were evaluated based on their ability to reduce pollutant parameters contained in their NPDES Permit. The pilot units were operated at unit flow rates commensurate with full-scale units. Treated effluent was also tested for potential impacts to ultraviolet disinfection systems. The results of these evaluations establish a baseline expectation for full-scale performance of each process technology if applied for treating Secondary Bypass flow at the Akron Water Pollution Control Station. KEYWORDS Secondary Bypass, Ballasted Flocculation, ACTIFLO, DensaDeg, CMF INTRODUCTION The purpose of this Study is to evaluate high rate process technologies for treatment of wet weather flows at either the Akron Water Pollution Control Station (WPCS) for treatment of Secondary Bypass flow or in Akron’s collection system for treatment of combined sewer overflow (CSO). To achieve this objective, three different pilot-scale process units were operated at the Akron WPCS in three phases. They included the Krüger ACTIFLO process, the Infilco Degremont, Inc. (IDI) DensaDeg process and, the Wet Weather Engineering & Technology, LLC (WWETCO) compressed media filter (CMF) process. Other pilot studies for each technology were also reviewed, and existing full-scale processes in the United States were assessed. Evaluation of process treatment performance for treating primary treated municipal wastewater flow (Secondary Bypass) was emphasized in the evaluations. Background The Akron WPCS screening and grit removal systems have a maximum capacity (with all units in service) of 280 MGD. Akron’s primary settling tanks (PSTs) have a capacity of 150 MGD while the secondary treatment process has a capacity of 110 MGD. Wastewater flows exceeding the PSTs’ capacity are directed to a Storm Retention Tank (SRT). Once the SRT is full, it overflows (up to a 130 MGD rate) and the overflow combines with PST effluent that exceeds the secondary treatment capacity (up to a 40 MGD rate). Subsequently, during the heaviest rain events, up to 170 MGD can flow through Secondary Bypass piping once the plant’s 60 MG SRT

6723

WEFTEC®.06

Copyright 2006 Water Environment Foundation. All Rights Reserved©

has been filled. The Secondary Bypass flow is typically of primary effluent quality, having been screened and degritted and settled in the PSTs or SRT. It is disinfected during summer months prior to being discharged into the Cuyahoga River. The WPCS currently operates under a 1998 NPDES permit, which establishes separate limits for the final effluent flow versus the Secondary Bypass flow during wet weather events. However, it is not known whether, upon renewal of this permit, limits will change to become more stringent for the Secondary Bypass limits or if the “split permit” will be replaced with one blended final effluent permit that will need to be met even during wet weather events. To address this concern, the city could: 1) Construct an equalization basin to store wet weather flow for secondary treatment once the storm event subsides, 2) Construct additional capacity in their secondary treatment process, or 3) Treat the Secondary Bypass with a high rate process. This Study is limited to evaluating the high rate process alternative. The City of Akron has studied their combined sewer collection system and is planning construction of storage basins at several CSO locations. A high rate treatment process could be added to the overflow of such basins in the future. While there are currently no specific effluent quality limits established for combined sewer overflows, the Ohio Environmental Protection Agency (OEPA) has established a “primary treatment” level as a general benchmark with the possible additional requirement of disinfection. A high rate treatment process may be a viable treatment alternative at CSOs. This Study also relates pilot test results to potential use at Akron CSO locations. The Krüger ACTIFLO Process ACTIFLO is a micro-sand ballasted clarification process that may be used to treat water or wastewater. By adding ballast to a chemical mixing and flocculation process, wastewater flow can be treated in high rate sedimentation tanks that are a fraction of the footprint and costs of conventionally designed settling tanks. The process, shown in Figure 1, begins with the addition of a coagulant to destabilize suspended solids. The flow enters the coagulation tank for flash mixing to allow the coagulant to take effect, then overflows into the injection tank where micro-sand is added. The micro-sand serves as a “seed” for floc formation, providing a large surface area for suspended solids to bond to, and is the key to ACTIFLO. It allows solids to settle out more quickly, thereby requiring a smaller footprint than conventional clarification. Polymer may either be added in the injection tank or at the next step, the maturation tank. Mixing is slower in the maturation tank, allowing the polymer to help bond the micro-sand to the destabilized suspended solids. Finally, the settling tank effectively removes the floc with help from plate settlers, allowing the tank size to be further reduced. Clarified water exits the process by overflowing weirs above the plate settlers. The sand and sludge mixture is collected at the bottom of the settling tank with a conventional scraper system and pumped to a hydrocyclone, located above the injection tank. The hydrocyclone converts the pumping energy into centrifugal forces to separate the higher-density sand from the lower-density sludge. The sludge is discharged out of the top of the hydrocyclone while the sand is recycled back into the ACTIFLO process for further use. Screening is required upstream of ACTIFLO so that particles larger than 3 to 6 mm do not clog the hydrocyclone.

6724

WEFTEC®.06


Figure 1 - The ACTIFLO Process. (Source: Krüger/Krüger)

The IDI DensaDeg Process DensaDeg is a sludge ballasted clarification process that may be used to treat wastewater. The process, shown in Figure 2, begins with the addition of a coagulant to destabilize suspended solids. The flow enters the rapid mix tank for flash mixing to allow the coagulant to take effect, then overflows into the reactor tank where sludge and polymer are added. A draft tube and mixer in the reactor tank allow for thorough mixing of the wastewater with the recirculated sludge and added chemicals. The sludge serves as a “seed” for floc formation providing a large surface area for suspended solids to bond to, and is the key to DensaDeg, allowing solids to settle out more quickly, thereby requiring a smaller footprint than conventional clarification. Wastewater flows over a weir from the reactor tank through a transition zone before entering the clarifier. The clarifier effectively removes the flow with help from settling tubes, allowing the tank size to be further reduced. Clarified water exits the process by overflowing weirs above the settling tubes. Sludge is collected at the bottom of the clarifier with a conventional scraper system and recirculated back to the reactor tank. Periodically, a separate sludge pump energizes and wastes a small portion of the sludge from the system. Scum is removed from the process at the top of the transition zone by a cylindrical collector that automatically rotates periodically.

6725

WEFTEC®.06


Figure 2 - The DensaDeg Process (Source: Infilco Degremont, Inc.)

The WWETCO CMF Process The WWETCO Compressed Media Filter (CMF) is a high-rate, passively-operated, compressible media filter that requires no mechanical devices or chemicals to filter wastewater. No additional hydraulic (driving) head is necessary because the filter operates on the incoming wet weather hydraulic gradient using differential pressure to create a compressed lower media zone and an uncompressed upper zone resulting in removal performance for particles down to the 10 to 20 micron range. Filterable solids are removed by the synthetic media as the wastewater passes through the layer of individual ‘fuzzy’ balls. Collected solids are removed from the filter balls during the backwashing process. During backwash the filter uses a central airlift to scrub and separate solids minimizing backwash water requirements. This CMF technology has reportedly removed up to 90% of sewer overflow or storm water total suspended solids (TSS) and is cleaned with minimal backwash. The basic configuration of the WWETCO CMF technology for wet weather flow control and treatment, as pilot tested, is shown in Figures 3 and 4. There are two WWETCO CMF standard, full-scale configurations: 1. WWETCO’s Filter Pod is a 6 ft. x 6 ft. surface area filter similar to the pilot unit

configuration having a molded flexible membrane with a donut shaped backwash trough around a central air lift and backwash system. Multiple Filter Pods can be placed into an open basin to make up a filter cell. Each filter cell would typically have 1-influent, 1-effluent and 1-backwash valve. Each filter pod would have its own air piping for backwashing.

2. WWETCO’s Strip Filter is a 6 ft. wide by multiple 6 ft. length configuration using flexible

sheet membrane, longitudinal air diffuser and dual parallel troughs. The WWETCO Strip Filter is more compact, requires fewer metal parts, is less costly and can be supplied in pre-cast and steel tankage as well as cast-in-place concrete. Multiple strip filters can be placed in a cast-in-place concrete basin.

6726

WEFTEC®.06


6’-1/4” Skid Width

10’ Skid Length

2” Drain Valve

5’ Diameter Filter Pod

Backwash Trough

Lower Chamber ManwayControl Panel

Backwash Air Piping

8” Backwash

Blower

8” Effluent

8” Influent

12” Wide Site Glass

Perforated Upper Plates

Plan View

6’ Diameter Tank

11’ 1/2” Tank Height

Backwash Trough

5’ Diameter Filter Pod

Upper Perforated Plates

Lower Perforated Plate

Air Diffuser

8” Backwash8” Influent

8” Effluent

6-1/4”7.5”x 2.5” Forklift Slots, 40” OC

Coupling

4” Air Piping

Blower

Filter Media

72” Tank OD

Section View

Lifting Lugs

10”

3’

Flanges rotated for vertical location

5’

Top Media

6’ Diameter Tank

Figure 3 - CMF Pilot Unit Plan View (Source: WWETCO, LLC) Figure 4 - CMF Pilot Unit Section View (Source: WWETCO, LLC)

6727

WEFTEC®.06


METHODOLOGY Treatment Objectives Currently, there are no established water quality limits for discharging CSO. Conversely, the Akron WPCS has a National Pollutant Discharge Elimination System (NPDES) permit with limits established for Secondary Bypass flow, which they consistently meet (daily TSS 157 mg/l and daily CBOD 91 mg/l). However, it is anticipated that future NPDES permits could be more stringent, although quantitative limits are unknown. Therefore, the plant’s current NPDES limit for final effluent was used as a goal for the evaluation of the three technologies. The water quality parameters considered, and their associated NPDES permit numerical limits, are as follows:

• Total Suspended Solids (TSS) – 30 day 15 mg/l • Total Phosphorus (P) – 30 day 1.0 mg/l • CBOD – 30 day 10 mg/l • pH 6.5 to 9.0 • Fecal Coliform – 30 day 1000/100 ml (summer)

Each high rate treatment pilot unit was evaluated on its ability to meet the above parameters either by itself or with disinfection as an aid. TSS and Total P were tested for all trials. CBOD tests were run for a select number of trials. Wastewater Characteristics Implementation of a high rate treatment process is being considered at the Akron WPCS to treat Secondary Bypass and in the collection system to treat combined sewer overflow (CSO) following a storage basin. Both wastewater flows are considered to be of primary effluent characteristic and strength based on historical sampling data. And, as such, it was deemed appropriate to use PST effluent for the pilot test. Table 1 presents historical TSS and BOD (or CBOD) values for Akron WPCS wastewater conditions and locations. As shown in Table 1, WPCS primary effluent quality is similar to Secondary Bypass wastewater characteristics (sample station 603) based on historical trends. The WPCS relationship between BOD and CBOD is typically 0.7 BOD = CBOD. While past CSO data indicates that it is a stronger wastewater than typical plant primary effluent, the CSO that would be treated using the high rate process will pass through a storage basin, which will act similar to a primary settling tank, prior to entering the high rate process. Subsequently, a lower strength wastewater than typical Akron CSO is anticipated. WPCS primary effluent was the wastewater used for most of the pilot protocol based on the above. Once all relevant tests had been performed using primary effluent wastewater, primary influent wastewater was treated by the pilot unit to serve as a comparison and an indicator for potential direct treatment of CSO without a storage basin.

6728

WEFTEC®.06


LOCATIONPILOT UNIT

AERATIONBASIN NO. 6

TANKSPRIMARY

Table 1 – Akron WPCS Wastewater Characteristics

100-150 MGD Plant Primary Influent

150-200 MGD Plant Primary Influent

100-150 MGD Plant Primary Effluent

150-200 MGD Plant Primary Effluent

Sample Station 603: Secondary Bypass

Sample Station 602: Secondary Effluent

Primary Effluent CSO

TSS (mg/l) 146 147 51 60 47 7.2 48 182 BOD (mg/l) 100 62 56 44 59 4.1 63 74 CBOD (mg/l) 62 43 39 31 41 1.8 44 52 Italicized values calculated based on historical plant correlation (0.7 BOD = CBOD) Pilot Unit Set-up Each of the three pilot units was set-up at the Akron WPCS and operated for a three to four-week pilot test. The pilot units were located next to Aeration Basin No. 6, as shown below in Figure 5. Figure 5 - Pilot Unit Location at Akron WPCS.

6729

WEFTEC®.06


Photo 1-ACTIFLO Pilot Photo 2-Densadeg Pilot Photo 3-CMF Pilot The primary goal of the pilot tests was to evaluate process treatment removal efficiency for total suspended solids (TSS) and carbonaceous biochemical oxygen demand (CBOD) in a primary treated wastewater flow. Each process technology that proved successful will be considered for full scale treatment of primary settling tank (PST) effluent that exceeds the WPCS secondary treatment system capacity, and treatment of overflow from the WPCS Storm Retention Tanks (SRTs), which wastewater strength and characteristic is comparable to primary settled wastewater. If the pilot test schedule allowed for each unit, it was also operated to process screened wastewater (PST influent) to further evaluate the process capabilities if used at CSO locations. The focus of the data and results presented in this paper is the treatment of primary effluent. When PST effluent was being treated, the pilot unit influent was pumped from the Aeration Basin No. 6 influent channel. The pump was suspended above the bottom of the channel to reduce intake of any settled material. Pilot unit effluent, sludge, and chemical tank and sink drains from the pilot unit discharged into the first pass of Aeration Basin No. 6. This arrangement ensured that the pilot unit influent would not be biased from any waste streams. When primary influent wastewater was being treated, a City-provided pump transported the wastewater from a PST influent channel to the pilot unit. Electrical power, potable water, and coagulants used during the study were provided by the City. Laboratory analysis of TSS, Total P, COD, CBOD, and sludge percent solids was performed by the City with some contract laboratory assistance. ACTIFLO Test Protocol Wastewater was pumped into the pilot unit through a flow meter, a valve that was used to throttle influent flow, and an in-line mixer. The mixer was not operated for this project because it is only needed when coagulant is added upstream of the trailer to increase mixing and contact time. Two injection ports are located in the influent piping prior to the tanks. The first port was only used for experiments involving supplemental chemical addition, such as the Sodium bisulfite and defoamer experiment. The coagulant was added to the wastewater through the second port. The flow then passed through an in-line static mixer before entering the bottom of the coagulation tank. From there the flow would overflow the coagulation tank weir and pass through a screen into the injection tank. The detention time for each of these two tanks at a 60 gpm/s.f. rise rate is one minute, and the mixers in each tank provided a G value of 300 s- to push the water the

6730

WEFTEC®.06


opposite direction of its flow to prevent short-circuiting. The hydrocyclone typically discharged 6 g/l of sand into the injection tank. With sand added, the flow passed under a baffle into the maturation tank, where polymer was added. The maturation tank had a retention time of five minutes at 60 gpm/s.f. and a mixer that provided a G value of 200 s-, also “pumping” in the opposite direction of flow. Here the mixer not only prevented short-circuiting but also encouraged particles to collide and maintain formed floc in suspension. The wastewater then flows over a baffle into the final clarifier tank and upward through 3-inch plate settlers before overflowing the effluent weirs into the effluent channel. A diagram of the ACTIFLO pilot unit layout is presented in Figure 6. Figure 6 - ACTIFLO Pilot Unit Configuration. (Source: Krüger/Krüger) The suspended sludge-sand mixture settles into a hopper in the clarifier bottom where it is removed by a rubber-lined sand recirculation pump. This pump conveyed the sludge-sand mixture to the hydrocyclone at a rate of about 17 gpm when the ACTIFLO unit was operating at 280 gpm (approximately 6% of the total flow). The hydrocyclone apex size was just over one-half inch for this Study. The sludge comprised approximately 80% of this mixture and was separated in the hydrocyclone and drained back to the aeration basin. The remaining 20% was sand and water and was injected back into the process. Actual primary effluent wastewater used for ACTIFLO influent ranged from 31 mg/l to 69 mg/l TSS and averaged 51 mg/l TSS. Primary influent wastewater ranged from 92 mg/l to 190 mg/l TSS and averaged 135 mg/l TSS. The ACTIFLO testing protocols for this Study were developed with input from the City and Krüger to evaluate the effectiveness of ACTIFLO for treating wet weather flow and to determine design recommendations for such treatment. A prioritization of trials was provided by the City prior to testing. System optimization was most important, followed by start-up trials, loss simulations, and finally, steady-state performance. To optimize the time available for testing,

6731

WEFTEC®.06


field adjustments and quick analysis of previous test results was necessary. If inconclusive results were received from a previous test, the possible cause of such results was investigated and a trial was adjusted and run again, as necessary. Several trials for each of the following protocol were completed:

• Optimum polymer dose determination • Coagulant dose curve development using alum • Coagulant dose curve development using sodium aluminate • Rise rate curve • Wet start-up • Dry start-up • Loss of coagulant • Loss of sand • Effects of sodium bisulfite on treatment • Effects of defoamer on treatment • Treatment analysis for ferric chloride and alum at extreme dosages

DensaDeg Test Protocol Wastewater was first pumped upstream of the pilot unit’s mechanically cleaned bar screen. A pump equipped with a VFD to control influent flow then transported screened wastewater to the rapid mix tank through piping that included a flow meter and coagulant injection port. The mixer in this tank provided a G value of 100 s- (full scale would provide 250 s-). After a retention time of two minutes at a 40 gpm/s.f. rise rate, the wastewater would flow down a pipe into the bottom of the reactor tank. The wastewater flowed upward in the reaction tank through a draft tube with a mixer used to pump this flow as well as the recirculated sludge added to this tank for 5.4 minutes at 40 gpm/s.f. The mixer in this tank provided a G value of 30 s- (full scale would be similar, but is adjusted based on influent wastewater strength). Following this reaction time, flow passed through a transition zone and under a baffle to the clarifier to ensure collected scum did not enter this final tank. Treated wastewater flowed upward in the clarifier through tube settlers before overflowing the effluent weirs. At 40 gpm/s.f., the clarifier’s retention time was 10.6 minutes, bringing the total unit’s retention time to 18 minutes at this rise rate. Sludge was collected in a hopper at the bottom of the clarifier with a scraper mechanism and pumped back to the reactor tank. Sludge recirculation began ten minutes after the unit is started up and pumps at approximately 6% of the influent flow rate until wastewater overflowed the effluent weirs at which time the rate reduces to 5% of the influent flow rate. Polymer was dosed to the recirculated sludge. Sludge was wasted from the system at frequencies ranging from every 15 minutes to every two hours using a pump sized the same as the sludge recirculation pump for a duration of two minutes each occasion. The sludge wasting frequency was adjusted to maintain a sludge depth of 6 to 12 inches in the clarifier. Grit and scum were removed from the system at two locations in the rapid mix tank and from the top of the transition zone by automatic valves opening for approximately 20 seconds every hour. DensaDeg influent samples were taken from the bar screen’s wet well prior to coagulant addition. DensaDeg effluent samples were taken from a sample tap in the effluent piping. DensaDeg sludge samples were taken from a sample tap in the sludge recirculation pump

6732

WEFTEC®.06


discharge piping before it entered the reactor tank. The DensaDeg process was controlled from a touch screen PLC unit. Actual primary effluent wastewater used for DensaDeg influent ranged from 30 mg/l to 73 mg/l TSS and averaged 49 mg/l TSS. Primary influent wastewater ranged from 96 mg/l to 292 mg/l TSS and averaged 161 mg/l TSS. One rain event occurred during the study on Friday, September 17. This resulted in a 175 MGD flow rate through the plant during the morning of testing. All wastewater received primary treatment and flow in excess of 110 MGD was bypassed around secondary treatment. Although the setup to the pilot unit was not changed, the pilot unit was treating actual Secondary Bypass quality wastewater for that trial. The testing protocols for this Study were developed with input from the City and Infilco Degremont, Inc. (IDI) to evaluate the effectiveness of DensaDeg for treating wet weather flow and to determine design recommendations for such treatment. Trials for the DensaDeg pilot unit were kept similar to those performed for the Krüger ACTIFLO pilot so the processes could be compared. The same prioritization of trials was established. The final protocol and tests performed were comparable to the ACTIFLO protocol noted above, with exception of the trials involving the loss of sand, which is not applicable to the DensaDeg process. CMF Test Protocol Compressed Media Filter (CMF) tests began with a clean filter and empty tank. Filter influent and effluent sample tap valves were opened and sampler suction lines were attached. A strap-on flow meter was installed prior to testing and always enabled. The sampling interval and volume collected varied based on test flow rates and solids concentrations. In order to provide the sample volume required for laboratory testing, sample volumes were adjusted according to flow rate and type of wastewater tested. The influent pump was turned on and discharged to the effluent channel, by-passing the filter. The influent pump piping was flushed until wastewater quality was visually consistent and the desired flow rate was steady. The tank influent valve was then opened and adjusted to maintain constant flow during the test. Pump throttle and/or valve adjustments were made to achieve the desired flow rate. The outer pilot tank then filled, wastewater spilled into the inner tank and filtration occurred. The filter effluent piping was observed for first discharge to determine the retention time. This time was also noted as the beginning of filtration time. Flow rate to the filter was continuously observed for consistency with the specified test flow rate and the valve/pump throttle was adjusted as necessary. If adjusted, flow rates before and after adjustment and actual time of the adjustment were recorded. Automatic samplers were manually enabled. Time of sample collection, filter influent flow and filter head were recorded at each sampling interval, typically every 5 minutes. Sample collection and notations continued until the water level in the filter reached the top of the filter tank. Actual time the water level reached the top of the filter tank was recorded and the samplers were then

6733

WEFTEC®.06


disabled. This was considered the end of the filter run and flow to the filter was stopped by turning off the pump and closing the influent valve to the filter. Backwash was then initiated by opening the backwash valve to allow the filter contents to drain to the aeration tank influent channel (Aeration Basin No. 6 first pass). The filter effluent valve was then closed and the blower was turned on to circulate and scrub the filter media inside the filter pod. Approximately 100 gpm (~5 gpm/sf) backwash makeup water was added to raise water level inside the filter pod. The filter was typically backwashed for 40 minutes. The backwash makeup water valve was then closed and the blower turned off. The filter effluent drain valve was then opened to allow the filter pod and underdrain contents to be discharged into the aeration basin influent channel. Once the filter completely drained, the filter was ready to be put back into service. Primary effluent, primary influent, and secondary effluent wastewaters were tested in this pilot study. The characteristics of each during the four-week study are summarized in Table 2. Two rain events occurred during testing of primary effluent on Monday, September 26, 2005 and Wednesday, September 28, 2005. This resulted in peak flows of 252 MGD and 215 MGD, respectively. Average flow through the plant during testing was approximately 117 MGD for both rain events. All wastewater received primary treatment and flow in excess of 110 MGD was bypassed around secondary treatment. Although the setup to the pilot unit was not changed, the pilot unit was treating actual Secondary Bypass quality wastewater for that trial. Table 2 – CMF Pilot Study Influent Wastewater Characteristics Wastewater Type

Max TSS (mg/l)

Min TSS (mg/l)

Average TSS (mg/l)

Max TP (mg/l)

Min TP (mg/l)

Average TP (mg/l)

Primary Influent 256 93 167 4.04 2.48 3.18 Primary Effluent 92 31 56 7.15 1.12 2.97 Secondary Effluent 5.0 2.8 4.4 1.17 0.47 0.74

The testing protocols for this Study were developed with input from the City and WWETCO to evaluate the effectiveness of WWETCO’s CMF for treating wet weather flow and to determine design recommendations for such treatment. A coordinated effort was made to keep trials similar to those performed during the Krüger ACTIFLO pilot and the IDI DensaDeg pilot so the three processes could be compared. A prioritization of trials was provided by the City prior to testing. Primary effluent testing was most important followed by primary influent and then secondary effluent. Several trials for each of the following protocol were completed:

• Primary Effluent at 200, 300 and 400 gpm flow rates (approximately 10, 15 and 20 gpm/sf).

• Primary Influent at 100, 200, 300 and 400 gpm flow rates (approximately 5, 10, 15 and 20 gpm/sf).

• Secondary Effluent at 200, 300 and 400 gpm flow rates (approximately 10, 15 and 20 gpm/sf).

6734

WEFTEC®.06


RESULTS The following results provide an overview of the detailed results of each pilot unit evaluation. The detailed results are too voluminous to present in this publication. Total Suspended Solids and Total Phosphorus The primary test parameters during the pilot unit operations were total suspended solids (TSS) and total phosphorus (TP). They are two of the key test parameters relating to compliance with NPDES Permit effluent limitations. And, sampling and testing can be performed at the WPCS laboratory with results available in a short time frame. A series of protocol were performed for each pilot unit to initially optimize operating conditions and chemical addition, where applicable for the ACTIFLO and DensaDeg processes. Alum was selected as the preferred coagulant as a result of the initial trials. A summary of the TSS and TP removal performance for primary effluent (Secondary Bypass) using alum is presented in Figures 7 and 8 for the ACTIFLO and DensaDeg pilot units. A summary of TSS and TP removal performance for the CMF pilot unit is presented in Figure 9.

Figure 7 - ACTIFLO - TSS/Total Phosphorus Removal

0

10

20

30

40

50

60

70

0 50 100 150 200

Alum Dose - mg/l

Tota

l Sus

pend

ed S

olid

s (T

SS) -

m

g/l

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

Tota

l Pho

spho

rus

(P) -

mg/

l

Influent TSS Effluent TSS NPDES Limit Influent P Effluent P

6735

WEFTEC®.06


Figure 8 - DensaDeg - TSS/Total Phosphorus Removal

0

10

20

30

40

50

60

70

80

0 20 40 60 80 100 120 140Alum Dose - mg/l

Tota

l Sus

pend

ed S

olid

s (T

SS)

-m

g/l

0.0

1.0

2.0

3.0

4.0

5.0

Tota

l Pho

spho

rus

(P) -

mg/

l


Figure 9 - CMF - TSS/Total Phosphorus Removal

0

10

20

30

40

50

60

70

80

0 100 200 300 400 500 600

Hydraulic Rate (gpm)

Tota

l Sus

pend

ed S

olid

s (T

SS)

- m

g/l

0.0

1.0

2.0

3.0

4.0

5.0

Tota

l Pho

spho

rus

(P) -

mg/

l


6736

WEFTEC®.06


Carbonaceous Biochemical Oxygen Demand (CBOD) The CBOD removal capability of the high-rate treatment units is linked to the percent of soluble CBOD in the wastewater. A limited number of samples were tested to assess CBOD removal. Soluble CBOD (SCBOD), fecal coliform and E. coli were evaluated for the ACTIFLO pilot tests using an outside laboratory. The tests were not administered properly by the laboratory and resulted in inconclusive data. ACTIFLO tests on primary effluent to determine CBOD removal efficiencies revealed a reduction from an average influent CBOD concentration of 48 mg/l to an average of 18 mg/l with alum while ferric reduced the CBOD to an average of 19 mg/l. Therefore, both coagulants performed essentially equally for CBOD removal. The target CBOD concentration (10 mg/l) was not met. Although, because ACTIFLO is a physical, not biological process, it was not expected to directly reduce CBOD. DensaDeg tests on primary effluent revealed a reduction from an average influent CBOD of 53 mg/l to an average of 20 mg/l with alum while ferric reduced the CBOD to an average of 15 mg/l. While ferric performed better than alum for these tests, it should be noted that alum’s dose was not sufficient to reach targeted TSS and Total P treatment for half of its trials and influent CBOD concentrations were higher during tests with alum than with the ferric tests. The target CBOD concentration was not met. Although, DensaDeg is a physical process similar to ACTIFLO and, hence, was not expected to directly reduce CBOD. Tests were performed on primary effluent to determine CBOD removal efficiencies for the CMF pilot study. Average CMF influent CBOD was 53.1 mg/l. Filtration reduced the CBOD to an average of 45.6 mg/l. The CMF performance was not expected to meet the NPDES permit limit of 10 mg/l because it is a physical, not biological, process. Considering the mechanism of treatment, an average 52% CBOD removal efficiency while treating primary influent (70.0 mg/l to 32.8 mg/l average) is notable. However, primary effluent removals only averaged 13%. The CBOD results are summarized in Table 3. Trials summarized in Table 3 were performed while treating primary influent and effluent at various flow rates. Table 3 – CMF CBOD Results

CBOD (mg/l)

Wastewater Type

Hydraulic Loading Rate (gpm/sf) Influent Effluent

Percent Removal

5.1 52.5 27.6 47.4 Primary Influent 15.3 87.5 38.0 56.6

15.3 47.2 45.2 4.2 15.3 63.5 50.2 20.9

Primary Effluent

20.4 48.8 41.5 15.0

6737

WEFTEC®.06


pH During the ACTIFLO pilot test, pH was monitored to ensure that NPDES permit limits were not exceeded. The average influent pH was 7.31 for primary effluent and 7.36 for primary influent. When using ferric to treat primary effluent wastewater, the ACTIFLO effluent pH ranged from 6.22 to 7.12 and averaged 6.80. The only pH values that exceeded the NPDES permit limit of 6.5 were when ferric was dosed at 135 mg/l and 170 mg/l to establish a dose curve. When feeding the most efficient, effective Ferric dose of 40 mg/l the resulting effluent pH averaged 6.94. Therefore, while Ferric lowers the pH, at the low dose necessary for effective treatment of this wastewater, it does not pose a problem with the permit. When treating primary influent, Ferric resulted in an ACTIFLO effluent pH of 6.21 to 7.04, with an average of 6.67. The pH of 6.21 corresponded to a Ferric dose of 110 mg/l. Doses of 90 mg/l and 70 mg/l also resulted in pH values lower than permit limits. The coagulant doses that exceeded permit limits are higher than the anticipated full-scale doses. When using alum to treat primary effluent, the ACTIFLO effluent pH ranged from 6.42 to 7.12 and averaged 6.89. The only pH values that exceeded the NPDES permit limit of 6.5 were when alum was dosed at 205 mg/l and 185 mg/l. These doses are significantly higher than Alum’s most efficient, effective dose of 50 mg/l, which resulted in an average effluent pH of 7.01. Therefore, at the low dose necessary for effective treatment of this wastewater, alum does not pose a problem with the permit. A limited number of trials were performed using alum when treating primary influent. Therefore, conclusions cannot be made regarding the different strength wastewaters. However, alum has less effect overall on pH than ferric. During the DensaDeg pilot test, pH was monitored to ensure that NPDES permit limits were not exceeded. The average influent pH was 7.5 for primary effluent and 7.6 for primary influent. When using ferric to treat primary effluent wastewater, the DensaDeg effluent pH ranged from 6.7 to 7.4 and averaged 7.1. No pH values violated the NPDES permit limit of 6.5. When feeding the most efficient, effective ferric dose of 50 mg/l the resulting effluent pH averaged 7.1. Therefore, while ferric lowers the pH, it does not pose a problem with the permit at the dose necessary for effective treatment of this wastewater. When treating primary influent, ferric resulted in a DensaDeg effluent pH of 6.7 to 7.5, with an average of 7.0. (While NPDES pH limits were not exceeded for any trial, caution should still be exercised if using ferric, especially at CSO locations where higher coagulant doses may be applied to higher strength wastewater. These higher doses could cause pH permit violations and toxicity issues.) When using alum to treat primary effluent, the DensaDeg effluent pH ranged from 7.0 to 7.4 and averaged 7.1 No pH values violated the NPDES permit limit of 6.5. Higher doses of alum are required for treatment; however, alum does not pose a problem with the permit. When treating primary influent, alum resulted in a DensaDeg effluent pH of 7.0 to 7.5, with an average of 7.2. Overall, alum has less effect on pH than ferric.

6738

WEFTEC®.06


The CMF uses no chemical additives to reduce wastewater pollutants. And, the hydraulic retention time in the filter cells is short. There were no measurable changes in pH between CMF influent and effluent samples. Fecal Coliform Samples were taken as part of each pilot test to measure changes in fecal coliform counts before and after treatment in the high rate process technology. Similar to CBOD, there was no specific expectation of fecal coliform reduction except that which occurs coincidental to removal of biological solids in the process wastewater. Samples taken during the ACTIFLO pilot test were not administered properly by the contract laboratory. The resultant test data is considered inconclusive and is not presented herein. Samples were taken during the DensaDeg pilot test and evaluated by a contract laboratory. When treating primary effluent the average DensaDeg influent fecal coliform was 2,325,000/100 ml and effluent fecal coliform was 91,000/100 ml (96% removal). DensaDeg was not expected to meet the fecal coliform NPDES permit limit of 1,000/100 ml because it does not include disinfection. All trials were performed while treating primary effluent at a rise rate of 40 gpm/s.f. and a dry anionic polymer dose of 2.0 mg/l. Samples were taken during the CMF pilot test and evaluated by a contract laboratory. When treating primary effluent the average WWETCO CMF influent fecal coliform was 1,230,000/100 ml and effluent fecal coliform was 662,000/100 ml (46% removal). The CMF was not expected to meet the fecal coliform NPDES permit limit of 1,000/100 ml because it does not include disinfection. An important consideration in the pilot testing was whether each high rate process would impact the disinfection ‘treatability’ of the treated effluent. Sodium hypochlorite is currently used to disinfect WPCS effluent and Sodium bisulfite is used to dechlorinate. The same chlorination and dechlorination process is also expected to be implemented at CSO locations initially with storage basins located upstream of the high rate process. However, the City wants to maintain the option of using UV for disinfection at the WPCS and/or at remote CSO locations where maintaining a chemical inventory would pose challenges. ACTIFLO effluent samples using both ferric and alum were collected and analyzed to determine treatability using UV. One test to determine the effectiveness of using UV is UV transmittance. UV transmittance (UVT) is a measurement of the quantity of UV light that can pass through a sample of wastewater (UVT=100% for pure water). Therefore, higher UVT values indicate more feasible and economical disinfection using UV. As expected, higher coagulant doses removed more TSS, thereby allowing the UVT value to be better for the higher doses. However, the results are still good for the lower doses, which also achieve TSS and Total P treatment goals. Samples with UVT values above 35% to 40% are considered treatable using medium pressure UV lamps. However, as the UVT decreases from 65% to 50% the energy required for disinfection approximately doubles, thereby making UV disinfection more costly. While typical

6739

WEFTEC®.06


settled CSO UVT values are about 40% to 50%, the UVT for ACTIFLO effluent is much higher. Therefore, ACTIFLO provides enough benefit with respect to the UVT that disinfection using UV is feasible. A collimated beam test considers particle size and density in evaluating wastewaters to determine their dose responses. Two samples were sent to Trojan Technologies, Inc. to perform this test. One sample was treated using 50 mg/l alum and another sample using 40 mg/l ferric. Trojans results indicated that the wastewater treated using ferric could not meet Akron’s 30-day fecal coliform NPDES permit limit of 1000/100ml even at the highest UV dose tested (80 mWs/cm2). This is because iron coats particles, thereby making them larger in size and more difficult for UV to penetrate them. The results indicated that the wastewater treated with ferric had a higher number of larger particles than the wastewater treated with alum. Conversely, the wastewater treated using alum could meet the NPDES permit fecal coliform limit with doses greater than approximately 26 mWs/cm2. At the maximum dose tested on the alum-treated wastewater (86 mWs/cm2), the resulting fecal coliform was 136/100 ml. Therefore, if UV disinfection is to be used, alum is the recommended coagulant. DensaDeg effluent samples using both ferric and alum were collected and analyzed to determine treatability using UV light. DensaDeg also provides enough treatment benefit, with respect to the UVT, that disinfection using UV is feasible. Two samples were sent to Trojan Technologies, Inc. to perform the collimated bean test for the DensaDeg pilot. One sample was treated using 50 mg/l ferric and another sample using 75 mg/l Alum. Trojans results indicate that the wastewater treated using ferric could not meet Akron’s 30-day fecal coliform NPDES permit limit of 1000/100ml even at the highest UV dose tested (60 mWs/cm2), presumably for the same reasons stated above for ACTIFLO. The wastewater treated using alum could meet the NPDES permit fecal coliform limit with doses greater than approximately 23 mWs/cm2. At the maximum dose tested on the alum-treated wastewater (60 mWs/cm2), the resulting fecal coliform was 173/100 ml. Therefore, if UV disinfection is to be used, alum is the recommended coagulant. One CMF effluent sample, treating primary effluent, was collected and analyzed to determine treatability using UV light. Because the CMF does not utilize chemical additives, there should be no interferences similar to those encountered with iron in the ACTIFLO and DensaDeg pilots. However, to be consistent with previous pilot studies, the collimated beam test was performed on one sample of WWETCO’s CMF effluent, even though chemicals were not added for filtration. Trojan’s results indicate that the CMF effluent could meet the NPDES permit fecal coliform limit (1000/100 ml) with doses greater than approximately 49 mWs/cm2. At the maximum dose tested on the filtered wastewater (80 mWs/cm2), the resulting fecal coliform was 290/100 ml. DISCUSSION The test results indicate that all three high rate treatment processes, ACTIFLO, DensaDeg and the CMF, are capable of treatment primary effluent strength wastewater and providing a significant reduction in TSS. The consequential reduction in TP and CBOD is also notable but is

6740

WEFTEC®.06


limited by the percent insoluble solids in the process wastewater. And, therefore, TP and CBOD are not infinitely predictable for all treatment scenarios that may be encountered. The pH can be maintained within permit conditions depending on the coagulant dose applied and has higher likelihood of meeting the current NPDES permit limit of 6.5 if alum is the selected coagulant for both ACTIFLO and DensaDeg. All three treatment technologies provide an effluent that can be normally disinfected using chlorinated chemicals, or UV if alum is the chosen coagulant. Additional test protocols were completed that provided further insight in to the optimum operation of each process technology if applied at the Akron WPCS. The following is a summary of those test results for each technology. Optimum Operating Conditions ACTIFLO - Anionic polymer is needed for successful operation and either a dry or emulsion type is acceptable. Anionic polymer doses from 0.7 mg/l to 0.9 mg/l were optimal. Ferric and alum were effective coagulants when dosing at least 36 mg/l and 50 mg/l, respectively, for the treatment of primary effluent. Doses may need to be higher to treat higher strength wastewater. PACL (Poly-aluminum Chloride) can also effectively treat primary effluent when dosing approximately 60 mg/l or more. Rise rates from 29 gpm/s.f. to 80 gpm/s.f. were effective in treating primary effluent. Lower rise rates, from 40 gpm/s.f. to 60 gpm/s.f., were more effective in the treatment of primary influent. Several start-up modes may be used for a full scale ACTIFLO system. If a wet weather event is expected within 7 days of a previous wet weather event, the units should be shut down, but not put on stand-by. Wastewater would remain in the tanks and a wet start-up, which was successful in this pilot study, would ensue at the time of the next wet weather event. In summer months when freezing is not possible, the intermittent flush stand-by mode could be used; and when freezing is possible, the continuous flush stand-by mode should be used. These stand-by modes would result in a wet method, dry start-up, which was successful in this pilot study. DensaDeg - Anionic polymer is needed to treat this wastewater and either a dry or emulsion type is acceptable. The optimal dose was found to be 1.5 mg/l for primary effluent and 2.0 mg/l for primary influent. Ferric and alum were effective coagulants when dosing at least 50 mg/l and 100 to 120 mg/l, respectively, for the treatment of primary effluent. When treating primary influent, optimum ferric and alum doses were found to be 90 mg/l for both coagulants. Generally, a higher coagulant dose is required when influent solids are higher and a higher polymer dose is required when the rise rate is higher. Rise rates from 30 gpm/s.f. to 55 gpm/s.f. were effective in treating primary effluent. Lower rise rates, from 30 gpm/s.f. to 40 gpm/s.f. were more effective in the treatment of primary influent. Several start-up modes may be used for a full-scale DensaDeg. If a wet weather event is expected within 6 hours of a previous wet weather event, the units should be shut down, but not drained. After 6 hours, the units may be drained except for three feet of depth in the clarifier. Both of these scenarios, which would include keeping the sludge collector running while the system is idle, would maintain a sludge inventory and a wet start-up would ensue at the time of the next wet weather event. After 12 hours the tanks should be completely drained to prepare for

6741

WEFTEC®.06


a dry start-up. This study showed that sludge age could hinder performance, which establishes that the above guidelines should be followed. CMF - Multiple factors can effect operating conditions for a CMF process. They include: filter cell size, influent flow rate, influent TSS concentrations and particle size, differential head across the filter (i.e. compression of the filter membrane), the number of active and inactive (out-of-service for backwash) filters, hydraulic loading rate, backwash rate and filter run time. The CMF proved effective at removing solids at various ranges of flow for each wastewater process tested (primary influent, primary effluent and secondary effluent). However, unique run time results were observed for each hydraulic loading rate. Typically, as hydraulic and TSS loading rates increased, filter run times decreased. Therefore, a balance of run time and loading rates is desired for optimum performance. Sludge Production Sludge production rates and sludge characteristics need to be understood in order to plan the application and management of any of the piloted treatment technologies. The following paragraphs present data and observations associated with the sludge and backwash generated by the three process technologies. ACTIFLO - The sludge volume produced is about 4.8% of the ACTIFLO design flow because the sludge recirculation pump rate is approximately 6% of the design flow and the hydrocyclone splits that flow such that 80% is sludge discharged out of the process. This rate cannot be changed, even if actual flows through ACTFLO vary from the design flow because the hydrocyclone relies on the pressure drop produced at the design rate for proper operation. Sampled sludge characteristics are described below. ACTIFLO sludge samples were taken for various trials throughout the Study to determine how the treatment variables, especially the doses of different coagulants and polymer, affected the sludge. Table 4 lists the ACTIFLO sludge percent solids analyzed with respect to these process variables.

6742

WEFTEC®.06


Table 4 – ACTIFLO Sludge Analysis

Wastewater Type

Rise Rate (gpm/s.f.)

Coagulant Type

Coagulant Dose (mg/l) Polymer Type

Polymer Dose (mg/l)

Average Sludge Solids (%)

Primary Effluent 60 Ferric 40 Dry Anionic 0.77 0.21 Primary Effluent 60 Ferric 85 Dry Anionic 0.77 0.26 Primary Effluent 60 Alum 50 Dry Anionic 0.78 0.19 Primary Effluent 60 Alum 100 Dry Anionic 0.78 0.22 Primary Influent 30 Ferric 70 Dry Anionic 0.67 0.15 Primary Influent 45 Alum 110 Dry Anionic 1.30 0.13 Primary Influent 45 Alum 140 Dry Anionic 1.50 0.14 If ACTIFLO is used at the WPCS, the sludge could be discharged to the plant’s headworks or could be mixed with the other plant sludges and sent to the Komline-Sanderson gravity belt thickeners. Samples of sludge were sent to Komline-Sanderson for a treatment evaluation. The samples evaluated were approximately a 50/50 mixture of ACTIFLO sludge with plant sludge since this is the highest amount of ACTIFLO sludge that would likely be present in the plant sludge mixture. Sample E-123 included ACTIFLO sludge that was treated using 40 mg/l ferric and sample E-126 included ACTIFLO sludge that was treated using 50 mg/l alum. Both samples were a result of ACTIFLO trials held at a rise rate of 60 gpm/s.f. and a dry anionic polymer dose of 0.78 mg/l. Generally, Komline-Sanderson concluded that either sludge mixture could be effectively treated using the same procedures that the plant currently uses to process sludge. Specifically, the performance requirements were met, including a 6% minimum thickened sludge, 95% minimum solids capture and 7.5 lbs./ton dry solids (DS) maximum active polymer dose. The plant polymer was found to be the best polymer tested during the evaluation and was dosed at 6.9 to 7.1 lbs./ton dry solids. Neither sludge mixture was limited hydraulically since the plant’s typical hydraulic loading rate is 200 gpm/meter and both samples could be fed up to 280 gpm/meter. The expected thickened solids for the ferric sludge mixture was 6.9% to 7.8% and for the alum sludge mixture was 6.9% to 7.4%. DensaDeg - The sludge volume produced by DensaDeg varies because the sludge waste frequency changes based on the influent stream’s solids content. When the DensaDeg process wastes sludge it is wasted at a rate of 5% of the influent flow rate for a two minute duration. During the pilot study, the optimum sludge waste frequency was 120 minutes when treating primary effluent wastewater and 15 minutes when treating primary influent wastewater. Sampled sludge characteristics are described below. DensaDeg sludge samples were taken for various trials throughout the Study to determine how the treatment variables, especially the doses of different coagulants, affected the sludge. Table 5

6743

WEFTEC®.06


lists the DensaDeg sludge percent solids analyzed with respect to these process variables. When treating primary effluent wastewater, a high dose of ferric produced the highest percent solids, although not significantly more than when using alum as a coagulant. Sludge percent solids were overall higher when treating primary influent wastewater, since more solids were being removed from the wastewater. The low dose of alum used to treat primary effluent did not produce acceptable sludge since not enough coagulant was being fed. Generally, all sludge sampled could easily be transported without concern of plugged piping. Table 5 – Densadeg Sludge Analysis

Wastewater Type Rise Rate (gpm/s.f.)

Coagulant Type

Coagulant Dose (mg/l)

Polymer Type

Polymer Dose (mg/l)


Primary Effluent 40 Ferric 50 Dry Anionic 2.0 0.38 Primary Effluent 40 Ferric 100 Dry Anionic 2.0 0.69 Primary Effluent 40 Alum 75 Dry Anionic 2.0 0.09 Primary Effluent 40 Alum 150 Dry Anionic 2.0 0.63 Primary Influent 40 Alum 90 Dry Anionic 2.0 0.85 Primary Influent 40 Ferric 90 Dry Anionic 2.0 0.83 IDI claims that full-scale sludge solids concentration should be higher than the pilot data shown because full-scale units are deeper, allowing a thicker sludge blanket to develop. Sludge samples were sent to Komline-Sanderson for a treatment evaluation following the same approach as with the ACTIFLO sludge samples. Their evaluation concluded that the Densadeg sludge will meet the performance specifications. The plant polymer was found to be the best polymer tested during the evaluation and was dosed at 5.3 to 6.1 lbs./ton dry solids. Neither sludge mixture was limited hydraulically since the plant’s typical hydraulic loading rate is 200 gpm/meter and both samples could be fed up to 265 gpm/meter. The expected thickened solids for the ferric sludge mixture was 6.2% to 7.0% and for the alum sludge mixture was 6.1% to 6.8%. CMF - If a WWETCO CMF facility is constructed at the WPCS, WWETCO recommends mixing the backwash with the other plant flow in the aeration basin influent. During operation at peak flow, this would reduce the 110 MGD rated secondary treatment capacity, subsequently increasing the amount of additional wet weather treatment units needed.

6744

WEFTEC®.06


Figure 10 – CMF Process Flow Diagram for Primary Effluent Filtration. (Source: WWETCO, LLC)

CMF backwash samples were taken for various trials throughout the Study to determine how the treatment variables affected the sludge. Table 6 lists the CMF backwash percent solids analyzed with respect to flow rate. Backwash percent solids were overall higher when treating primary influent wastewater, since more solids were being removed from the wastewater. Generally, all backwash sampled contained a low percent solids which could easily be transported using conventional solids handling pumps. Table 6 – CMF Backwash Solids Analysis

Wastewater Type

Flow Rate (gpm/sf)


20.4 0.09 Primary Effluent 15.3 0.12

15.3 0.13 Primary Influent 10.2 0.14

CONCLUSIONS All three high rate treatment processes piloted at the Akron WPCS are viable alternatives for treating peak wet weather flows that would otherwise bypass secondary treatment. All three processes would require a footprint much smaller than if conventional secondary treatment processes were constructed, with the ACTIFLO process using the least area at about 1/10th that of an aeration basin and settling tanks. However, each process has its limitations for pollutant reduction and is not a complete replacement of a conventional biological process for compliance with all NPDES effluent parameters. They are physical-chemical processes (ACTIFLO and DensaDeg), or just physical (CMF), and they do not directly reduce CBOD or phosphorus. Reduction of either parameter is related to the TSS reduction that occurs and the associated percent of insoluble CBOD and TP.

WWETCO Filters

Biological Treatment

Backwash Water12 MGD

182 MGD at 100 mg/l

98 MGDCl2

Cl2

110 MGD

182 MGD

170 MGD

12 MGD Backwash Waste

64% Removal at 12 to 15 gpm/sf

40% to 50% Removal in Primary Clarifiers 75% to 85%

Total TSS Removal

Primary Treatment/ Primary Bypass/ SRT Overflow Influent

280 MGD

6745

WEFTEC®.06


The ACTIFLO-treated primary effluent wastewater averaged 51 mg/l TSS, 2.3 mg/l Total P, and 48 mg/l CBOD. Alum is the recommended coagulant so that fecal coliform goals could be met with the use of UV disinfection. The optimum chemical doses were found to be 50 mg/l alum and 0.8 mg/l anionic polymer, which resulted in an average pilot unit effluent of 10 mg/l TSS, 0.75 mg/l Total P, and 22.7 mg/l CBOD. The effluent pH using this alum dose did not exceed NPDES permit limits. While 60 gpm/s.f. is the standard design hydraulic rise rate, rates up to 80 gpm/s.f. resulted in effective treatment and with process retention times as low as seven minutes. The waste sludge averaged 0.2% solids and was continuously produced at a rate of approximately 5% of the design flow. The DensaDeg-treated primary effluent wastewater averaged 49 mg/l TSS, 2.4 mg/l Total P, and 53 mg/l CBOD. Alum was the recommended coagulant so that fecal coliform goals could be met with the use of UV disinfection. The optimum chemical doses were found to be 90 mg/l alum and 2.0 mg/l anionic polymer, which resulted in an average pilot unit effluent of 11 mg/l TSS, 0.30 mg/l Total P, and 20.3 mg/l CBOD. The effluent pH using this alum dose did not exceed NPDES permit limits. While 40 gpm/s.f. is the standard design hydraulic rise rate, rates up to 65 gpm/s.f. resulted in effective treatment and with process retention times as low as 11 minutes. The waste sludge averaged 0.4% solids and was wasted for two minutes every two hours at a rate of 5% of the design flow. Both the ACTIFLO and DensaDeg process rely heavily on coagulants and/or coagulant aids, in conjunction with ballast, to accomplish TSS reduction. For ACTIFLO, if any one of the coagulant, polymer or sand ballast is lost, the TSS reduction efficiency is notably reduced. The DensaDeg process reacts similarly if either chemical feed is lost or the sludge recycle is discontinued. With both processes the loss of chemical feed results in the more significant reduction of treatment. The Compressed Media Filter (CMF) process requires no chemical addition. As its name implies, it operates as a filter to accomplish pollutant reduction from the wastewater. There are concerns with loss of treatment due to “chemical feed failure” scenarios. However, the CMF pilot unit did not provide a level of treatment comparable to the ACTIFLO and Densadeg pilot units. The CMF-treated primary effluent wastewater averaged 52 mg/l TSS, 2.2 mg/l Total P, and 60 mg/l CBOD. The average primary effluent suspended solids concentration was reduced from 52 to 25 mg/l using this system, which fails to meet the treatment goal of 15 mg/l. Fecal coliform goals could be met with the use of UV disinfection. The optimum hydraulic loading rate for Secondary Bypass (primary effluent) was found to be 13 gpm/sf. Because no chemicals are used with the CMF, there is no affect on the wastewater pH or concern for disinfection using ultraviolet light. A side benefit of the CMF technology, if used at a wastewater treatment facility, is that it can be used to filter secondary effluent during dry weather. While 13 gpm/s.f. is the optimum design hydraulic rise rate for Secondary Bypass, rates up to 20 gpm/s.f. were successfully tested when filtering secondary effluent. The CMF backwash averaged 0.1% solids and totaled 6.8% of the design flow. The average runtime for the CMF processing Secondary Bypass quality wastewater was 3.3 hours between backwashes. The backwash is recycled to the

6746

WEFTEC®.06


secondary treatment process, and thus, would reduce the effective capacity of the secondary process. Additional CMF capacity would be provided to account for this. Advantages of all three technologies include a small footprint, quick start-up time and high removal efficiencies. Limitations for the two ballasted flocculation processes include their reliance on chemical coagulants and ability to remove only particulate pollutants. Being physical/chemical processes they do not remove soluble pollutants. The compressed media filter also is unable to remove soluble pollutants. Its footprint is also several times larger than either ballasted flocculation process. In final comparison of the three technologies, ACTIFLO can operate at the highest rise rate – 60 to 80 gpm/sf. This allows for a smaller footprint and shorter retention time, enabling a faster start-up and recovery from failure scenarios as compared to DensaDeg. ACTIFLO can also achieve similar treatment to DensaDeg while using lower doses of chemicals. DensaDeg does not have screening requirements as fine as ACTIFLO and the CMF. DensaDeg and the CMF may be placed in a completely dry stand-by mode, whereas ACTIFLO requires water in the tanks even when not in operation. DensaDeg also produces a smaller volume of sludge with a higher percent solids concentration than ACTIFLO and the CMF. Both ballasted flocculation technologies require a chemical storage and feed facility; ACTIFLO also requires a sand storage and handling facility. The CMF process requires no chemical facilities. The results of these pilots show promise for use of these technologies for wet weather flow treatment. Further, during dry weather periods, all three technologies could be used for tertiary treatment. However, chemical costs and chemical/sand pump maintenance for the ballasted flocculation processes must be considered for ACTIFLO and DensaDeg. Both processes have shown effectiveness removing phosphorus, or for primary treatment in lieu of conventional primary settling tanks. This treatment benefit is not achievable with the CMF process as designed. All three processes may be automated for use at remote sites in the collection system. Before any high rate process technology may be further considered for the Akron WPCS, it will first be necessary to establish clear NPDES permit conditions. REFERENCES “City of Akron, Ohio Water Pollution Control Station Secondary Bypass Treatability Study”. Prepared by ARCADIS FPS, Inc., March, 2004. “City of Akron, Ohio Water Pollution Control Station Secondary Bypass Treatability Study, Phase II”. Prepared by ARCADIS FPS, Inc., December, 2004. “City of Akron, Ohio Water Pollution Control Station Secondary Bypass Treatability Study, Phase III”. Prepared by ARCADIS, December 2005.

6747

WEFTEC®.06


Side By Side By Side - The Evaluation of Three High Rate

Documents

Transcript of Side By Side By Side - The Evaluation of Three High Rate