PHOSPHATE REMOVAL A SUMMARY OF THE 5 YEARS OPERATION OF THE COMAG TM PHOSPHATE REMOVAL PLANT AT BILLERICA, MASSACHUSETTS, USA

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1 PHOSPHATE REMOVAL A SUMMARY OF THE 5 YEARS OPERATION OF THE COMAG TM PHOSPHATE REMOVAL PLANT AT BILLERICA, MASSACHUSETTS, USA Kalmes, J. and Garabedian, D., Billerica Wastewater Treatment Plant, Billerica, MA, USA Corresponding Author . jkalmes@town.billerica.ma.us Abstract The Billerica CoMag plant in the USA was installed 5 years ago to meet the more stringent consents for Phosphate discharge into the local water course, the River Concord. The innovative CoMag technology enhances conventional coagulation by utilising Magnetite as a ballast, which increases the weight of the flocs and this results in significantly improved effluent quality, in particular TSS and Phosphate. This paper will discuss why P removal technology was required at this location, the operational performance of the CoMag process and will continue by discussing the challenges of introducing then, an untried technology to an established Waste Water Treatment Works and how such challenges were solved. Keywords Billerica, Ballasted Clarification, CoMag, Evoqua, Phosphate, Wastewater Introduction The wastewater treatment plant (WWTP) in the town of Billerica, which is located 20 miles northwest of the city of Boston, was originally built in It has been upgraded several times, once in 1975 and then again in It was the 1990 upgrade that brought the plant to its current capacity of ML/d (5.5 MGD). The treated wastewater is discharged into the Concord River. The discharge is regulated by National Pollutant Discharge Elimination System (NPDES) Permit. This Permit is generally updated every 5 years. In 2005 the Permit was revised to include lower limits for Phosphorus and Aluminium. Following notification of these revised permit limits, the Billerica waste water treatment plant had four years to develop a plan to reduce the phosphorus limits. The WWTP had an original summertime limit (April 1 to October 31) of 0.75 mg/l that was being reduced to less than 0.2 mg/l. The new wintertime (November 1 to March 31) limit was changing from a status of report to a reduced limit of 1.0 mg/l. In addition to this, the Town had been told to expect even more stringent limits by the Massachusetts Department of Environmental Protection (MADEP). Treatment Options Considered The WWTP started using a two-point chemical addition of aluminium salts to meet the new NPDES Permit while exploring the different technologies available. Sodium Aluminate was dosed in the primary effluent which then fed into the aeration tanks. The mixed liquor from the aeration tanks was then treated with PACl (polyaluminium chloride) as it entered the final clarifiers. The combination of these chemicals allowed the WWTP to meet the existing 0.75 mg/l permit for most of the summer season and occasionally approach the new 0.2 mg/l limit. However it was clear that this approach of using two-point chemical addition was not going to be sufficient to achieve the new limits on a consistent basis, so other technologies needed to be considered.

2 As investment in a new process would be required to achieve these new Phosphorus limits. The engineering consultants for Billerica, Woodard and Curran, were commissioned to undertake an evaluation of the most suitable available technologies required to achieve these new onerous Phosphorus limits. The technologies considered were Enhanced Biological Phosphorus Removal, Effluent Cloth Filters, and a process that had just been installed at a similar waste plant at Concord WWTP called CoMag TM. Compared to the other Phosphorus removal technologies the CoMag process appeared to be the most reliable process to achieve the new more stringent Total P levels that would be required. Added to this, a warning had already been given that the WWTP would be expected to achieve a summer TP of 0.1mg/l in the near future. As well as the reliability to meet these stringent limits the other attractive factor was the cost of the CoMag TM solution, which was significantly lower than the other solutions. For the reliability and cost it was decided to go with the CoMag process provided the reliability was proven by a pilot plant trial. A pilot plant (Pic 1) was brought to the Billerica WWTP and operated under various regimes for several months to verify the performance expected. The treatment plant staff collected and tested samples alongside the Cambridge Water Technology engineers. In addition to this, a third sample was collected and sent out to a private laboratory. This was done as a final check on all the work being conducted. After several months of sampling and testing, the results confirmed that CoMag was the treatment process to proceed with. The bid was awarded in December 2008 and construction was complete in October Figure 1: Cambridge Water Technology s (Now Evoqua) Trailer Based Pilot System

3 Figure 2: Pilot System Process Flow Diagram. Graphic courtesy of Evoqua / Cambridge Water Technology. Ferric Chloride Dose Response Curve Alum Dose Response Curve Effluent TP (mg/l) Ferric Chloride (mg/l as Fe) Effluent TP (mg/l) Alum Dose (mg/l as Al) Sodium Aluminate Dose Response PAC (EPIC 58) Dose Response Effluent TP Effluent TP (mg/l) Sodium Aluminate (mg/l as Al) PAC Dose (mg/l as Al) Figure 3: Dose-Response Curves for piloted coagulants. Aluminum Sulfate (Bottom Left) was selected as the most cost effective and desirable coagulant. The CoMag TM Process The origins of the CoMag TM process had been originally conceived at the Massachusetts Institute of Technology (MIT) in Boston where they found that adding Magnetite to a chemical flocculation process had a significant effect on the weight of the flocs and allowed significantly improved settling in the clarification process. This had been developed further by a small local water treatment company

4 (Cambridge Water Technologies (CWT)) that patented the recovery of magnetite from the flocs, thus making the process commercially viable. The CoMag process uses ballasted flocculation, solids contact, and magnetic separation to remove phosphorus and other contaminants from water and wastewater. A typical CoMag TM process flow diagram is illustrated in Figure 1. The process flow ph is adjusted if required and a metal salt (e.g. aluminium sulphate, ferric chloride, ferric sulphate, or poly-aluminum chloride) is added. The chemically produced floc particles are then ballasted in the second reaction tank. Magnetite, an inert iron ore, is added to increase the specific gravity of the chemical floc. This increases and stabilizes the solid s settling rate and produces a thicker (higher DS%) solids blanket in the clarifier. Prior to entering the clarifier, polymer is added in the final reaction tank to enhance the flocculation. Figure 4: General schematic of the CoMag process The ballasted flocs settle rapidly, which, for a given flow rate can result in a small clarifier. A considerable proportion of the settled solids are continuously returned to the reaction tanks with the remaining fraction removed as waste solids. The waste solids pass through a high shear mixer to break apart the chemical floc from the magnetite. The mixture passes through a magnetic drum separator, which captures the magnetite and returns it to the reaction tanks. The waste solids, which contains the contaminants removed from the water, flow under the drum and over an outlet weir where they are collected in a sump. From here the waste solids are sent for further processing and ultimate disposal. Process Theory and Application to CoMag TM Chemical Phosphorus Precipitation Chemical phosphorus removal relies on converting reactive soluble orthophosphate into a precipitate and then separating the newly formed precipitate (and other solid forms of phosphorus) from the water. The process can use different metal salts, such as aluminum sulphate, ferric chloride, ferric sulphate, and poly-aluminium chloride (PAC), to precipitate the phosphate.

5 Soluble P in 9 th European Waste Water Management Conference Equation 1 illustrates a general reaction of a metal salt (Me 3+ ) with phosphate (PO4-3 ). The precipitated solid is in equilibrium with the metal salt and phosphate ions. Me 3+ + PO 4 3 MePO 4 (s) Equation 1 Within limits, excess metal salt can drive the reaction to the right, helping to precipitate more phosphate. The metal salt dose needed to produce low effluent phosphorus levels required by many permits can greatly exceed the stoichiometric relationship of one mole of metal salt to one mole of phosphate. This is due partly to side reactions of the metal salt with other compounds in the wastewater. One such reaction with water forms metal hydroxide sludge. Some metal hydroxide is necessary to coagulate the fine phosphate precipitate into particles large enough to remove by sedimentation; however, too much metal salt wastes coagulant and produces excess hydroxide sludge. It also gives iron in the final effluent, which is a consent parameter in the UK. Figure 5 Coagulant dose response curve Metal Salt : Phosphorus A typical metal salt (coagulant) dose response curve is illustrated in Figure 2. At low metal salt doses, the removal generally follows a linear relation with one mole of metal ion reacting with one mole of phosphate. To achieve very low phosphorus levels (e.g., 0.1 mg/l), the metal salt dose must be much greater than the theoretical molar ratio. Tests at many plants have found the optimum coagulant dose can vary daily, and is typically unrelated to the phosphorus concentration in the tertiary influent. Experience will determine the optimum dose, which will balance the costs of process monitoring and adjustment with the costs of coagulant use and sludge disposal. Non-Reactive Soluble Phosphorus The water to be treated may contain non-reactive, dissolved, acid-hydrolysable phosphorus (nonreactive soluble phosphorus). Non-reactive phosphorus consists of dissolved polyphosphate

6 compounds that do not react in the orthophosphate analysis without first undergoing acid digestion. These compounds can show up sporadically, depending on their source (e.g., industrial and food processor wastewaters). Chemical treatment processes cannot easily remove non-reactive phosphorus because this form of phosphorus does not precipitate using metal salts. The CoMag system can be operated to maximize the removal of reactive phosphorus. This will leave primarily non-reactive phosphorus, which typically is present in low levels, in the effluent and should help the plant produce an average phosphorus concentration for the compliance period that is below the permit limit. Installation of the CoMag System The CoMag phosphorus removal system would occupy the area that two 50 foot DAFT were operating in. This area would house the magnetic recovery drums, magnetite 1 ton supersac for ballast, two-1750 gallon caustic tanks, three-2500 gallon alum tanks, and a space to place gallon plastic totes of polymer, along with the necessary pumps to supply the system. The next big challenge was to convert three-70,000 gallon SHT into 2-24 foot diameter clarifiers as well as a pump room to house the recirculation pumps. Construction of the reaction tank, which consisted of 4-11x11x11 basins, where coagulation and flocculation would occur. In the basement below the DAFTs Five- 11,500 gal/min tertiary pumps would be inserted, these were necessary to pump the secondary effluent up to the CoMag reaction tanks. Figure 6: Gravity Thickeners under construction (left) and complete (right) including the distribution chamber where the three sludge streams are combined, mixed and distributed evenly between the two tanks

7 Figure 7: CoMag TM clarifiers and recycle / waste pump station under construction (left) and complete (right) with the reaction tanks in the background. The three structures were retrofitted within the old sludge holding tanks giving them the distinct round peg in a square hole look Figure 8: Five intermediate pumps were used (left) due to the wide flow range. The magnetic drum separator (right) dumps into the slurry tank where makeup magnetite is added as needed. Another major factor involved in the upgrade of the WWTP included solids handling. This led to two 30-foot gravity thickeners being installed. This would allow the staff to continually waste primary sludge, waste activated sludge as well as the new CoMag(tertiary) sludge. This type of tank was chosen because of the ability to store sludge and have sludge thickening capabilities. The hydraulic loading of this type of tank was also considered because it would allow for the continuous stream of different sludges that would be feeding it. New sludge pumps would be installed to directly feed the Fournier Press (rotary press). Another advantage of the CoMag system was that it fit into the existing footprint of the WWTP and allowed the sludge handling modifications described above to take place. The CoMag had the process capability to withstand some upsets in the secondary system while still achieving results within the 0.2 mg/l T-P permit requirement. However, one of the challenges of installing a new process that had to be reconciled, was the lack of full-scale installation data and a full understanding of the requirements of operating such a new process. Unfortunately, it had also been agreed that the WWTP would install a Supervisory Control and Data Acquisition (SCADA) at the same time as the new CoMag process thus providing additional complication.

8 Following construction and successful commissioning of the CoMag plant by Cambridge Water Technologies it was handed over to the operators at the Billerica wastewater treatment plant in October It was a very busy time as the new SCADA system also became operational that month. The engineer from CWT involved with the design, installation, and operations of the system had made himself available to train all of the wastewater operators in the theory and mechanical processes of CoMag. It was understood that the CoMag system would be monitored and run by a SCADA program. The WWTP operatives had no previous experience with SCADA or computerization of our process in the past, so this was going to be a new experience and challenge for all. We had anticipated a learning curve with the senior operators as they appeared to be intimidated by the new technology. It is one thing to run a biological treatment plant without the use of computers, compared to now running a high-tech CoMag system using SCADA as well. Over time and through constant exposure, the men began to understand the system and interpret the trends on the computer to what it means in the process to the point that it is now an invaluable tool. When the WWTP operatives took over the operations of CoMag, the original set points were maintained, which included chemical dosages and operational parameters where CWT had left them. As a result of a lack of confidence in the new systems we were facing we were reluctant to make changes to a system that we were just starting to understand. However, we soon discovered that we just couldn t seem to get the same results that CWT had achieved on a consistent basis. We had a ph probe at the beginning of the reaction tank that was monitoring the CoMag influent ph, which was targeting a ph set point of 6.3 to 6.8 to ensure that the full reaction with phosphorus was achieved. When we found that the ph probe had never really fluctuated (SCADA trend chart) over time, staying at 6.3, we became suspicious of the operation of the probe. After calibrating the probe we found it to be faulty, so we replaced it with a new one. The ph of the water was actually 7.5, which explained why we were not achieving the Phosphorus performance that had been seen during commissioning. Following this incident, we started experimenting with changes such as changing of alarm set points in some monitoring probes, adding more magnetite, adjusting polymer dosage, alum dosages, wasting frequency and duration (and we thought SCADA could run itself). After adjusting the processes, we then ran into other issues that were confusing to us. The Dynablend polymer delivery system would constantly clog and fail to form floc, which resulted in poor settling causing the clarity of the finished water to suffer as washouts occurred. When that happened the turbidity probe on the final effluent would then send alarms, but by the time it called out we were already in trouble. The problem was that the polymer would turn to a glue-like texture and plug the main lines, which were roughly 80 feet long and underground for part of the way. This would prevent formation of floc. We have 2 redundant side-by-side polymer systems. It was a real problem as this happened on a regular basis. It would involve using 2 men most of the day to flush the polymer from the line so we could have it available for backup. Due to this type of problem we had several polymer companies come out and bench test new polymer products, and we were able to find a more compatible product. We also did not appear to have enough free magnetite in the system and wondered where it went as we were adding 50 lbs per day. We discovered that some of the magnetite was being lost to the gravity thickeners and some was being lost to the piping going back out to the reaction tank. Some of the initial installation of the CoMag process had to be retrofitted to help us achieve a better result. An example is the installation of a flush water system to inject water into the return line that carries the magnetite from the slurry tank back to the reaction tank; it had been collecting magnetite that had settled in the pipe.

9 TP (mg/l) 9 th European Waste Water Management Conference We also experienced ph problems that were the result of caustic pump(s) that would clog. Although these issues were not catastrophic, they were frustrating as we couldn t figure out the causes and effects of what chemical or process was giving us the problem. We learned through time that the ph and alum dosage were critical in the success as was the regular monitoring and review of process control through SCADA trends as well as the accurate interpretation of trend charts. We have come to realize how valuable and helpful the SCADA system is to have for reviewing historical trends. We have had issues, but they are all valuable in that we learn from them and understand how to overcome those issues. As confidence and experience has been gained in the new process and our understanding of the critical parameters has improved, we now have a CoMag system that is operating beautifully Billerica Functional Performance Test Results - Total Phosphorus Effl TP - CWT Effl TP - WWTP NPDES Permit Avg Feed TP - CWT Feed TP - WWTP Feed TP - Lab CoMag Alum Dose: 45 PPMV: Start to 10/1/10 60 PPMV: 10/1/10 to Feed Pumps Surging Instantly from 60 Day Running Average Permit Limit Figure 9: Functional performance test results. The CoMag TM system produced effluent below the permit limit at all times with the exception of periods where the intermediate pumps malfunctioned and pumped significantly more than design flow with a single clarifier online. Conclusion We have now developed a daily routine of doing a checklist twice a day to visually walk and inspect the CoMag process. This has helped the operators to understand what normal operations should look like so when they see something out of the ordinary they can bring it to someone s attention. The operators who at first seemed to reject and avoid the SCADA computer program now appear to be more comfortable navigating throughout the different operational process pages and making adjustments as needed and logging their actions.

10 Acknowledgements Cambridge Water Technology Evoqua Town of Billerica Woodard and Curran