Maximizing Nutrient Removal in an Existing SBR With a Full-Scale BioMag Demonstration

Brian L. Lubenow 1*, Steven Woodard 2, David W. Stewart 1, Rachel A. Kirkham 1

1 CDM 2 Cambridge Water Technology * Email: lubenowb@cdm.com

ABSTRACT

The 1.1-million-gallon-per-day (mgd) city of Taneytown (Maryland) wastewater treatment plant (WWTP) consists of two sequencing batch reactors (SBRs). The city intends to upgrade the existing plant by incorporating BioMag, by Cambridge Water Technology, to provide enhanced nutrient removal (ENR). A full-scale trial of the BioMag system was conducted in 2010, representing its first application to an SBR. BioMag is an enhanced biological wastewater treatment process that uses magnetite, an inert iron ore (Fe3O4), to increase the specific gravity of biological floc, to increase settling rates. This application demonstrated stable, high-level treatment throughout a wide range of influent concentrations. Effluent total nitrogen and total phosphorus concentrations averaged 1.2 milligrams per liter (mg/L) and 0.11 mg/L, respectively. Key components of this application are the ability to increase mixed liquor suspended solids concentrations and appreciably reduce the duration of the settle phase, increasing aerobic and/or anoxic react phase durations. KEYWORDS: Sequencing batch reactor, BioMag, enhanced nutrient removal, activated sludge, retrofit, mixing, polymer INTRODUCTION Background The city of Taneytown’s wastewater treatment plant (WWTP) is located in Carroll County, Maryland, and was constructed in 2000. The 1.1-million-gallon-per-day (mgd) WWTP receives primarily municipal wastewater and consists of two sequencing batch reactors (SBRs), which were designed to achieve biological nutrient removal (BNR) with effluent concentrations of 6 milligrams per liter (mg/L) of total nitrogen (TN) and 2 mg/L of total phosphorus (TP). In the near future, the plant’s annual National Pollutant Discharge Elimination System (NPDES) permit limits for TN and TP will be lowered to 3 mg/L and 0.3 mg/L, respectively. Table 1 lists the proposed design criteria. The city intends to upgrade the existing plant by incorporating the BioMag system into the existing SBRs to provide enhanced nutrient removal (ENR). A full-scale demonstration of the BioMag system in one of the SBRs was successfully completed in fall 2010.

Table 1. Taneytown WWTP design criteria. Parameter Influent Effluent Units Average day flow rate 1.1 1.1 mgd Maximum month flow rate 1.65 1.65 mgd Maximum day flow rate 3.5 3.5 mgd Peak hour flow rate 5.0 5.0 mgd Average daily BOD 111 mg/L Average daily TSS 178 mg/L Average daily TKN 29 mg/L Required final effluent TN < 3.0 mg/L Assumed average daily TP 4 mg/L Required final effluent TP < 0.3 mg/L

Each SBR provides the specific phases of biological treatment within a single tank. One tank fills and reacts, while the other undergoes react (polishing), settle (clarification), and decant phases. Alternating periods of aeration and non-aeration provide the conditions necessary for biological nitrification and denitrification, respectively. Table 2 lists the dimensions and capacity of each SBR.

Table 2.Parameter

Existing SBR sizing. Number/Size

Number of SBR basins 2 Volume, low water level 590,000 gallons Volume, high water level 944,000 gallons Length 75 feet Width 75 feet Depth, low water level 14 feet Depth, high water level 22.4 feet

BioMag process The BioMag ballasted activated sludge process is an emerging technology by Cambridge Water Technology (CWT) that can achieve low effluent suspended solids, biological oxygen demand (BOD), nitrogen, and phosphorus concentrations in a compact footprint. BioMag has the ability to substantially reduce biological treatment reactor volume requirements and dramatically decrease the footprint needed for BOD, TN, and TP removal. The process can achieve compliance with stringent nutrient standards without the need for tertiary filters or membranes. The key to BioMag’s effectiveness is its marked ability to improve secondary sludge and SBR settling rates and provide high-quality secondary effluent with low suspended solids.

BioMag is an enhanced biological wastewater treatment process that uses magnetite, an inert iron ore (Fe3O4), to increase the specific gravity of biological floc. With a specific gravity of 5.2 and a strong affinity for biological solids, the ballast substantially increases the settling rate of the biomass. Increasing the specific gravity and settling rate of the biological floc provides the

opportunity to increase the mixed liquor concentration, while still maintaining adequate settling and thickening in the SBR.

BioMag improves the treatment capabilities of the existing SBRs through the following mechanisms:

• Ability to operate the activated sludge system at increased mixed liquor suspended solids (MLSS) concentrations, increasing treatment capacity.

• Increased secondary settling and thickening rates, resulting in increased treatment reliability and transfer of time from settle phase to react phase.

• Increasing MLSS concentration results in longer sludge ages, increasing nitrification efficiency.

• Enhanced, reliable removal of suspended solids, nitrogen, and phosphorus.

Figure 1 below shows a typical process flow diagram for BioMag applied to an SBR. The process flow diagram shows that virgin magnetite ballast and recovered magnetite (ballast) are blended with secondary sludge in the ballast mix tank. The ballasted secondary sludge then flows back to the SBR. A small amount of polymer is typically added to the mixed liquor just before it enters the settle phase to enhance sludge settling and thickening. The magnetite recovery process is based on sending the biosolids through a shear mill to separate the ballast from the floc. This stream then flows to a rotating drum lined with fixed magnets, which enable a very efficient separation and recovery of magnetite from the biological floc. The recovered ballast is re-blended with the mixed liquor in the ballast mix tank. The excess biological solids (minus the magnetite) are wasted to the sludge discharge sump.

Figure 1. This trial represents the first application of the BioMag process to an SBR. Objectives The primary objective of this trial was to demonstrate that incorporating BioMag into the existing SBR process will enable the plant to effectively and reliably achieve compliance with the proposed permit limits, including the TN and TP limits of 3 mg/L and 0.3 mg/L, respectively. This trial represents the first application of the BioMag process to a sequencing batch reactor (SBR). Secondary objectives were as follows:

• Demonstrate the peak hydraulic and mass load handling capabilities of the system • Quantify the increased settling rate and investigate polymer feed and mixing parameters • Determine the mixing requirements

METHODOLOGY

CWT entered into a contract with the city to conduct a full-scale BioMag trial at the WWTP. Oversight of the trial was provided by CDM. The five phases of the trial are identified in Table 3, below.

Table 3. Taneytown WWTP BioMag trial phases. Phase Time Operations

1. Mobilization & baseline testing 30 days

Mobilize and install all process equipment. Collect daily samples to establish baseline operational performance.

2. Charge system with magnetite 14 days

Begin running one SBR with BioMag process and fully charge mixed liquor with magnetite.

3. Steady state operation 30 days

Demonstrate consistent compliance with proposed permit limits, including TN and TP. Optimize the process based on dissolved oxygen monitoring. Conduct mixing tests & settling column tests. Optimize BioMag concentration and polymer dosage.

4. Modify SBR cycle structure, increase MLSS concentration & increase mass loading

30 days

Modify SBR cycle structure to decrease settling time and increase react/react fill cycle lengths. Supplement the influent with carbon and nitrogen to simulate high mass loading.

5. Single basin operation 7 days Direct full plant flow to the BioMag SBR.

The performance parameters were set forth in a contract between the City and CWT and are presented in Table 4.

Table 4. Pilot plant performance parameters.

Success Parameters Influent Characteristics

Required Performance Measure

Average daily flow 0.55 MGD1 0.55 MGD 1 Maximum month average daily flow 0.825 MGD1 0.825 MGD1 TKN 25 mg/L2 3.0 mg/L3 30 day average TP 4 mg/L 0.3 mg/L4 30 day average TSS 175 mg/L2 20 mg/L 30 day average BOD 115 mg/L2 20 mg/L 30 day average

1. These flows represent half of the WWTP design flow, since the pilot will only be conducted in one of the two SBRs.

2. These figures represent the typical influent wastewater concentrations at the WWTP over the last two years. Total suspended solids (TSS) is the only parameter that differs significantly from the original design value (140 mg/L).

3. The effluent TKN is measured as total nitrogen and includes refractory nitrogen up to 1.0 mg/L.

4. The effluent TP includes any non-reactive phosphorus up to 0.1 mg/L in the wastewater stream.

Magnetite addition and recovery A full-scale BioMag enhanced nutrient removal trial was conducted in one of the SBRs at the city’s WWTP. SBR No. 1 became the BioMag SBR, while SBR No. 2 served as the control.

The core components of the magnetite feed and recovery system were housed in one of CWT’s mobile BioMag trailers. Mixed liquor was pumped from the SBR to a 2-millimeter (mm) perforated screen using the existing submersible SBR secondary sludge transfer pump. The screen was used to remove rags, hair, and other stringy material that can build up in the shear mill and adversely impact shearing efficiency. The screened secondary sludge then flowed by gravity to the ballast mix tank sump containing two submersible pumps. One of the pumps was used to provide secondary sludge to the well-mixed ballast (magnetite) feed tank. The other pump was used to feed the shear mill (Figure 2), followed by the recovery drum (Figures 3 and 4). An automated feed system (Figure 5) metered dry magnetite into the magnetite mix tank, where the magnetite became enmeshed in the biological floc. Venturi eductors and plant water were used to transport both the powdered and recovered magnetite to the ballast feed tank. The ballasted mixed liquor flowed by gravity into the ballasted mixed liquor sump, where it was pumped back to the head of the SBR. The waste activated sludge (WAS), minus the recovered magnetite, was pumped from the bottom of the recovery drum to the aerobic digester. A polymer makedown/feed system was used to batch feed polymer to the BioMag SBR to aid solids settling and thickening.

Phosphorus removal Ferric chloride was batch fed directly to the BioMag SBR to precipitate phosphorus and demonstrate that 0.3 mg/L TP will be readily achievable using chemical precipitation.

Figure 2. The shear mill separates the magnetite Figure 3. The magnetic recovery drum ballast from the floc. recovers 96 to 99% of the magnetite ballast.

Figure 4. The biomass is separated and transferred to the aerobic digester.

Figure 5. The dry feeder pneumatically adds virgin magnetite to the system.

SBR cycle modifications The BioMag SBR continued to operate with five cycles per day. However, the cycle structure of the SBR was modified to account for the appreciable reduction in settling time. The project team worked with Aqua Aerobic Systems, Inc. (AASI) to modify the SBR programming. The settle phase was reduced from 60 minutes to as low as 20 minutes. The remaining 40 minutes per cycle were used to increase the treatment capacity of the SBR by lengthening the react and react fill phases. This and increased MLSS concentrations resulted in additional treatment capacity for both nitrification and denitrification. The cycle structures were changed regularly to optimize

nitrification and denitrification. The aeration provided was automatically varied based on a temporary dissolved oxygen (DO) control system.

Mixing tests The existing 30-hp AquaDDM SBR mixer was installed as part of the plant construction in 2000. The mixers were designed to achieve complete mixing of the SBR at design TSS of 3,600 mg/L. Mixing tests were conducted using both the existing 30-hp and a new 40-hp AquaDDM mixer.

The testing procedure consisted of pulling samples using a low-flow submersible sample pump at varied locations and depths throughout the tank, and analyzing TSS and magnetite ratio. Tests were conducted during un-aerated mixed SBR phases with TSS values ranging from 6,000 to 13,000 mg/L, including magnetite. Due to the symmetry of the SBR tank, sampling locations were selected within just one (northeastern) quadrant of the tank, as depicted in Figure 6. Sample depths were 1, 5, 10, and approximately 15 feet below the surface. The bottom sampling depth was altered as necessary to be approximately 1 foot above the bottom of the tank.

Figure 6. Due to the symmetry of the SBR tank, mixing test sampling locations were selected exclusively in the northeastern quadrant.

Settling tests and polymer feed and mixing optimization Full-scale SBR settling tests were conducted throughout the demonstration. A series of sampling events were conducted to measure the sludge blanket interface depth and settling rates of the BioMag SBR during the settle phase.

Single basin operation Based on the success of the BioMag SBR throughout the trial, the trial was extended an additional week to operate the WWTP in single-SBR mode, with nearly continuous flow through the BioMag SBR. This single SBR phase was initiated on November 15 at 9:00 a.m. and ran

until 9:00 p.m., and then restarted November 16 at 7:00 a.m. and was monitored until November 18 at 9:00 p.m.

Analytical testing System operation included routine inspections and adjustments of the BioMag test equipment, daily sampling rounds, chemical feed adjustments, and laboratory analyses. BioMag system monitoring is detailed in Table 5.

Table 5. BioMag system monitoring parameters Parameter Location Frequency SBR Cycle Analysis

Dissolved oxygen SBR

Continuous

Mixed Fill/React Fill/React

Auto

Temperature SBR Daily (Mon-Fri) React City pH SBR Daily (Mon-Fri) React City Settled sludge blanket level SBR Daily (Mon-Fri) Decant City Water level in SBR SBR Daily (Mon-Fri) Settle Auto WAS flow NA Daily (Mon-Fri) NA Auto Magnetite feed rate NA Daily (Mon-Fri) NA Auto Polymer feed rate NA Daily (Mon-Fri) NA Auto Aqueous ammonia feed rate NA Daily (Mon-Fri) NA Auto Ferric chloride feed rate NA Daily (Mon-Fri) NA Auto MLSS SBR Daily (Mon-Fri) React City Magnetite in SBR SBR Daily (Mon-Fri) React City MLVSS SBR Daily (Mon-Fri) React City

Magnetite in WAS Sludge holding tank Daily (Mon-Fri) NA City

Residual magnetite in WAS after recovery

Recovery drum Weekly NA City

Influent alkalinity Headworks Daily (Mon-Fri) Mixed Fill/React Fill Hach

Effluent TSS SBR Daily (Mon-Fri) Decant City Effluent VSS SBR Daily (Mon-Fri) Decant City

Influent TP Headworks Daily (Mon-Fri) Mixed Fill/React Fill Hach

Effluent TP SBR Daily (Mon-Fri) Decant Hach Effluent soluble P SBR Daily (Mon-Fri) Decant Hach

Influent BOD Headworks Daily (Mon-Fri) Mixed Fill/React Fill Lab

Effluent BOD SBR Daily (Mon-Fri) Decant Lab

Influent TKN Headworks Daily (Mon-Fri) Mixed Fill/ReactFill Hach

Effluent ammonia SBR Daily (Mon-Fri) Decant Hach Effluent nitrite SBR Daily (Mon-Fri) Decant Hach Effluent nitrate SBR Daily (Mon-Fri) Decant Hach Effluent organic nitrogen SBR Daily (Mon-Fri) Decant Hach

RESULTS

This report section presents and discusses the results of the full-scale BioMag trial. The demonstration began in August 2010 and concluded in November 2010.

Nutrient removal Figure 7 is a time series plot of the BioMag daily (24-hour) flow totals. The BioMag SBR flow averaged 0.22 mgd for the first six weeks of the trial. This represented a very dry period in which wastewater flows were below normal and influent BOD, TSS, TN, and TP concentrations were above normal. A large storm occurred on September 29 and 30 and the 4.3 inches of rain resulted in appreciable infiltration and inflow. The total plant flow on September 30 was 4.5 mgd, 29 percent higher than the design maximum day flow of 3.5 mgd. The system operated in storm mode, which involves eliminating the react polish phase and filling the reactor while it settles. The system performed well, maintaining excellent treatment during these stressed conditions. Despite the fact that the SBR was often filling and decanting at the same time, the sludge blanket remained well under control, full nitrification was maintained, and effluent quality was excellent. After the storm rain event subsided, the BioMag SBR influent flows once again stabilized, varying between 0.2 and 0.7 mgd.

Figure 8 shows the BioMag MLSS concentrations recorded during the trial, not including the weight of magnetite. The MLSS biosolids concentration ranged between 3,000 and 5,000 mg/L, with a general increasing trend. This was roughly twice that of the control SBR, whose MLSS concentrations ranged from 1,500 to 2,400 mg/L.

Figure 7. Influent flow to BioMag SBR Figure 8. BioMag SBR MLSS concentration

Figure 9 shows the influent ammonia concentration throughout the trial. The BioMag SBR treated a surprisingly wide range of influent ammonia concentrations, ranging from 10 to 73 mg/L. Figure 10 shows both the influent and effluent TN concentrations. The influent TN was also highly variable, ranging from 10 to 95 mg/L, while the BioMag effluent TN was low and stable, averaging 1.2 mg/L and peaking at 4.8 mg/L.

Control SBR data was taken less frequently, so there are not as many data points. Regardless, it is clear that the BioMag SBR demonstrated superior TN removal when compared to the control (7.36 mg/L). There are three basic reasons for the enhanced treatment: (1) the increased MLSS concentration results in higher reaction rates and improved nitrogen removal, (2) the enhanced settling rate provides for reduced settling time and increased reaction time, and (3) more flexibility in cycle structure to optimize and stabilize TN removal.

The BioMag effluent TN concentration is shown on a smaller scale on Figure 11. All daily data points except one (the max day flow event on September 30) were less than 3 mg/L.

Figure 12 shows the BioMag effluent nitrogen series, including ammonia, nitrite, and nitrate. The ammonia and nitrite remained low and steady through the trial, staying dependably below 1.0 mg/L. The nitrate was a little more variable, but remained less than 2.0 mg/L, except during startup and one day of storm flow. MicroCTM was added during the final anoxic phase until October 20 to enhance nitrate removal, but was discontinued at that time, as it was discovered that TN removal to the permit limit could be achieved without supplemental carbon addition. In fact, the lowest effluent nitrate concentrations were achieved after discontinuing MicroCTM feed during the final anoxic stage.

Figure 9. Influent ammonia concentration Figure 10. Influent and effluent total nitrogen

Figure 11. Effluent nitrogen Figure 12. Effluent nitrogen series

Figure 13 is a time series plot of the BioMag SBR influent and effluent TP concentrations. Again, the influent was highly variable, ranging from 1.8 to 20 mg/L. The effluent TP was low and stable, averaging 0.11 mg/L after the system stabilized, with only single-point addition of ferric chloride directly to the SBR. The project team demonstrated that the BioMag SBR could average less than 0.1 mg/L by increasing the ferric feed rate during the last 30 days of the trial. Although the experiment was successful, the ferric dose (~18 mg/L as Fe) was relatively high, demonstrating that single-point coagulant addition may not be the most cost effective method of achieving compliance with ultra low TP limits, i.e. 0.1 mg/L or less.

Control SBR data was taken less frequently, so there are not as many data points. Nonetheless, it is clear that the BioMag SBR demonstrated superior TP removal when compared to the control (1.92 mg/L). The increased ferric addition rate to the BioMag SBR certainly played an important role in achieving low effluent TP. Other important factors included BioMag’s consistently low effluent TSS (4.6 mg/L trial average) and overall treatment stability.

Figure 13. BioMag and control SBR Figure 14. BioMag SBR effluent phosphorus effluent phosphorus

Settling tests and polymer feed/mixing trials Settling tests were performed throughout the trial to minimize the required settling time. The SBR’s decanter mechanism pulls effluent from approximately 8 inches below the water surface. For practical purposes, it is typically necessary to have multiple feet of clear water below the water surface to prevent the effluent flow’s velocity from scouring and entraining solids from the top of the blanket. Given the rapid, consistent settling and the distinct, reliable blanket interface of BioMag’s ballasted MLSS, it was typically considered acceptable to go into decant with a 2.5- to 3-foot clear water depth in the BioMag SBR.

The control SBR utilized a settle time of 60 to 70 minutes during the pilot operation. This is longer than the design settle period of 45 minutes, and was required to accommodate the slow settling sludge that is common to SBRs. If a shorter settle time was used, the effluent typically suffered from high TSS, caused by slow settling floc and a poorly defined blanket interface.

For the BioMag SBR, the settling time was reduced to as low as 20 minutes per cycle. This settle time was used in early October, although the BioMag settle time was set at 25 minutes for the majority of the trial. These settle times were utilized both with and without polymer. The comparatively short interval was impressive, considering that the energy from the DDM mixer continues to roll the tank contents for 6 to 10 minutes into the start of the settle phase. The actual settling doesn’t begin until this momentum is dissipated.

Polymer addition was also investigated during the trial. A cationic emulsion polymer was selected from Ashland Chemical and prepared to a 0.4 to 0.5 percent solution using a polymer makedown skid. This polymer makedown skid was used to fill a day tank. Immediately prior to the start of settling, polymer was fed using a peristaltic hose pump at 6 gallons per minute into the SBR tank. Through November 8, the polymer was directed approximately 8 feet from the center of the DDM mixer. It was assumed that this mixer would thoroughly mix the polymer into the tank volume prior to the settle step, developing larger, faster settling floc. The average settling rate over the settle phase during this period ranged from 0.09 feet per second (fps) to 0.25 fps and averaged 0.17 fps.

It was observed throughout the trial that the polymer had a significantly reduced impact in the actual basin compared a jar test at the same dosage. Different full-scale polymer addition timing sequences were investigated, and carrier water was added to try to improve polymer dispersion in the SBR basins. Overall, polymer use did not provide significant benefit in the SBR. TSS reduction and settling speed, two of the major drivers for polymer use, did not significantly improve when polymer was added. This was good news from a chemical use standpoint, but did not provide the rapid settling data that was routinely observed the jar tests. As such, the last two weeks of the trial (November 9 through November 18) were dedicated to optimizing polymer injection and mixing, and demonstrating accelerated settling, so these issues could be adequately addressed in the detailed design of a BioMag SBR system.

It became apparent from detailed testing that the major issue with introducing polymer adjacent to the DDM mixer was excessive shear. A jar test imparts ideal mixing conditions. The polymer is injected quickly and then evenly distributed throughout the sample in a few seconds with a gentle stirring motion. Lab tests with a small laboratory aerator and diffuser stone demonstrated clearly that any vigorous aeration or high-speed propeller mixing quickly began to shear the floc. A negative impact on settling rate was observed in the beaker when the mixing was extended longer than 5 seconds. Because of the limited pilot equipment, dosing the SBR with the proper volume of diluted polymer took 15 to 30 minutes. During this period, the DDM mixer and/or aeration were on, shearing the floc within seconds, and then continuing to destroy the floc over the following minutes.

Full-scale testing was then set up to identify and demonstrate a solution to this challenge. The details of this testing are listed below.

• 400 feet of 1.25” ID poly hose, with holes drilled for distribution, were installed in the SBR around the perimeter at a depth of approximately 9 ft above the floor.

• Larger capacity diluted polymer pumps were installed to batch feed the polymer into the SBR through the poly hose distribution network. The required dose could be fed in approximately 6 minutes, instead of 20 to 30 minutes.

• Additional carrier water was added to accommodate this higher polymer flow and to further improve polymer distribution.

• Several variables were manipulated in an attempt to optimize settling: o The duration of injection o The depth of injection o The period of injection (i.e., continuing to feed polymer during the first few

minutes of the settle step with all mixers off) o Adding a final air “fluff” by turning on a single blower for a few seconds after all

polymer had been injected to provide a final tank “roll”. Visually intense mixing could be observed, with the heavier, faster settling flocs lifted to the top of the tank and introduced to the slower flocs with less ballast

o The polymer dose ranged from 1 to 3 part per million by volume (ppmv) (0.5 to 1.5 mg/L active). In a jar test, rapid settling occurred at any concentration more than 1 ppmv.

The new distribution network improved settling speeds dramatically. The average settling rate over the settle phase during this period ranged from 0.14 to 0.35 fps and averaged 0.25 fps. It

was very clear that proper polymer addition, distribution and mixing, even at lower doses, results in a faster settling, more distinct blanket that keeps pace with decanting.

Mixing tests The existing mixer is a 30-hp AquaDDM and was installed as part of the plant construction in 2000. The AquaDDM mixers were designed to achieve complete mixing of the SBR at design TSS of 3,600 mg/L. Mixing tests were conducted using both the existing 30-hp and a new 40-hp AquaDDM mixer.

The coefficient of variation (standard deviation divided by the mean) was computed for each test and values up to 10 percent were considered to be well mixed. Using the 30-hp mixer, all but two of the 14 tests at locations other than the corner indicated well-mixed conditions. Using the 40-hp mixer, all four tests at locations other than the corner indicated well mixed conditions. The additional inertia associated with the 40-hp mixer extended the period of visible residual mixing (after the mixer had shut off) by several minutes and the settle phase had to be extended accordingly.

Complete mixing in the corner of the square tank was inconsistent. Of the seven tests conducted in the corner using the 30-hp mixer, only three indicated well-mixed conditions. In each of the poorly mixed tests, the sample taken 1 foot off the bottom of the tank greatly exceeded the average TSS, at times by more than a factor of two. Similar results were evident using the 40-hp mixer, as only one of the two tests in the corner indicated well mixed conditions.

Two tests were conducted in the corner using the 30-hp mixer during an aerated cycle to determine the effect of air on mixing. Both tests indicated well mixed conditions, with coefficients of variability of only 2 percent. It appears that while mixing is inconsistent at the bottom of the tank in the corners during anoxic cycles, the additional energy from the operation of the air diffusers during aerated cycles completely mixes the entire tank and resuspends any settled solids.

The results suggest that the 30-hp AquaDDM mixer adequately mixes the SBR, even at TSS levels up to 13,000 mg/L.

Single basin operation Based on the success of the BioMag SBR throughout the trial, the trial was extended an additional week to operate the WWTP in single-SBR mode, with nearly continuous flow through the BioMag SBR. This single SBR phase was initiated on November 15 at 9:00 a.m. and ran until 9:00 p.m., and then restarted November 16 at 7:00 a.m. and was monitored until November 18 at 9:00 p.m.

Flow was directed to the control SBR at selected times during this testing period for several reasons:

• To provide a small amount of food to maintain healthy biomass in SBR #2 • To allow for hydraulic surge conditions during the rain event on November 16. Because

of the fixed decant flowrate, the BioMag SBR level was increasing throughout each

cycle. Flow was diverted to the control SBR for only 2 hours to prevent the SBR from moving into storm flow programming mode

• To allow for monitoring of settling performance during accelerated settle testing led by CDM

It is estimated that the BioMag SBR accepted the full plant flow for approximately 62 of 74 hours of this mini-trial.

Overall, the data were impressive, given that influent was still entering the BioMag SBR at the same time the reactor was undergoing polish, settle, and decant phases. The BioMag effluent TSS concentration averaged 4.5 mg/L, with a maximum value of 10.2 mg/L. The TN remained less than 3.5 mg/L and the TP stayed below 0.3 mg/L. Long-term operation of the system in the single basin mode is not recommended. However, it is comforting to know that adequate effluent quality can still be achieved on occasions when only one SBR is available for treatment.

DISCUSSION AND CONCLUSIONS

The full-scale BioMag demonstration has met all of the project objectives, including the following:

• The BioMag SBR demonstrated stable, high-level treatment throughout a wide range of influent organic and nitrogen concentrations, even when elevated loadings were simulated by the addition of aqueous ammonia and MicroCTM to the influent wastewater. The ability to appreciably reduce the duration of the settle phase, thereby increasing aerobic and/or anoxic react phase durations, are key components of this enhanced treatment efficiency.

• Supplemental carbon feed is not required during the final anoxic phase to achieve consistent compliance with a 3.0 mg/L TN limit.

• Constructing a BioMag system to enhance both SBRs will allow the city to effectively and reliably achieve compliance with the state’s proposed ENR permit limits, including the TN and TP limits of 3 and 0.3 mg/L, respectively, without the need for a tertiary treatment step. Effluent TN concentrations averaged 1.2 mg/L, and effluent TP concentrations averaged 0.11 mg/L.

• BioMag can effectively operate at elevated MLSS concentrations (i.e. 4,000 mg/L compared to 2,000 mg/L) over a wide range of flows and loadings, eliminating the need for additional tankage or tertiary filters to treat present and future loadings.

• BioMag demonstrated excellent TSS removal efficiency, averaging less than 5 mg/L over a wide range of flows and loadings.

• The BioMag system performed very well under maximum day flow conditions, effectively removing TN and TP and providing stable control over the sludge blanket.

• The shearing effect of the magnetite recovery process enhanced biosolids dewaterability, resulting in a 3- to 4-percentage point increase in cake dryness.

• The existing 30-hp mixer is sufficient to maintain adequate mixing of the magnetite ballasted mixed liquor.

• Special consideration should be given to the polymer feed and mixing systems to provide proper polymer distribution and floc development, as well as reduce floc shear in the permanent BioMag installation.

• Although long-term operation in the single basin mode is not recommended, the BioMag system can provide adequate effluent quality on occasions when only one SBR is available for treatment.

ACKNOWLEDGEMENTS

The authors gratefully acknowledge the assistance of the Maryland Department of the Environment, Cambridge Water Technology (CWT), and the city of Taneytown, especially Mr. Andy Bishop of CWT and Mr. Kevin Smeak of the city, who each dedicated themselves to the success of this trial.