CITY OF WILSON WRF ACHIEVEMENTS IN TOTAL NITROGEN AND PHOSHPORUS REDUCTION BELOW ENR LEVELS

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1 CITY OF WILSON WRF ACHIEVEMENTS IN TOTAL NITROGEN AND PHOSHPORUS REDUCTION BELOW ENR LEVELS Samuel A. Ledwell*, Environmental Operating Solutions, Inc. 160 MacArthur Blvd, Bourne, MA Jimmy Pridgen, City of Wilson Water Reclamation Facility Nicholas Eatmon, City of Wilson Water Reclamation Facility Rodney Harris, City of Wilson Water Reclamation Facility ABSTRACT The City of Wilson Water Reclamation Facility is a 14 MGD facility discharging to Contentnea Creek, one of the largest sub basins in the Neuse River Basin. The process consists of primary and secondary clarifiers, seven 4-stage Bardenpho trains with the option of running a 5 th stage for EBPR, downflow tertiary denitrification filters, newly renovated solids dewatering and processing facilities and anaerobic digesters. The facility currently has a Total Nitrogen (TN) discharge target of 3 mg/l and Total Phosphorus (TP) discharge target of 1 mg/l. The City has made the conscious good faith effort to reduce TN and TP discharges as part of the alliance with the Neuse River Compliance Association (NRCA) and Lower Neuse Basin Association (LNBA). With an end of pipe discharge concentration of 2.13, the City of Wilson ranked 8 th among all 23 NRCA members and 3 rd among municipal NRCA members when comparing end of pipe average TN concentration achieved in The focus of this paper is to summarize recent activities at the facility resulting in even further reduction of Total Nitrogen and Total Phosphorus. In the fall of 2012, the facility transitioned from a pure methanol based carbon source to a mixed alcohols solution called MicroC MicroC 3000 is a mixture of methanol, ethanol, propanols, higher alcohols and water and has a COD concentration of 1.18 kg COD/L, similar to that of methanol. The carbon source is injected into the second anoxic zone of the 4-stage biological process. Upon transitioning to the new carbon source and feed rate optimization, the effluent TN and TP concentrations averaged 1.91 mg/l and 0.16 mg/l respectively. This compared to the prior year TN performance of 2.15 mg/l and TP performance of 0.83 mg/l. Perhaps the most interesting observation has been the reduction in TP despite the recent elimination of the sodium aluminate feed prior to the secondary clarifiers. As nitrate in the second anoxic zone is driven to very low levels, the zone essentially becomes anaerobic. The hypothesis is that the higher weight alcohols, propanols and butanols, are being converted to volatile fatty acids (VFA s) that can then be used for polyhydroxyalkanoate storage resulting in orthophosphate release and subsequent uptake in the final aeration stage. The paper will explore the benefits of using a mixture of different alcohols as opposed to pure methanol with respect to TN removal and TP removal via Enhanced Biological Phosphorus Removal (EBPR). KEYWORDS BNR, ENR, DENITRIFICATION, PHOSPHORUS REMOVAL, MICROC, METHANOL

2 INTRODUCTION The scope of this study was to evaluate the conversion from methanol to a mixture of low molecular weight alcohols called MicroC There were several drivers for the evaluation including economics, interest in enhanced denitrification performance and the possibility of enhanced phosphorus removal performance. MicroC 3000 is less expensive than methanol on a $/lb N removed basis. A mixture of alcohols could provide an advantage compared to methanol as a mixture supports a broader ecology compared to methanol which supports solely methylotrophic organisms. Lastly, the higher molecular weight alcohols in MicroC 3000 could potentially support EBPR resulting in lower TP effluent discharges. The City of Wilson WRF is designed for 14 MGD with an average daily flow of 8.5 MGD. The biological process consists of primary and secondary clarifiers bracketing a conventional 4 Stage Bardenpho process preceded by a common 5 th stage called the BPR tank. There are seven 4 Stage Bardenpho trains and a single BPR tank that is broken into five zones. Effluent from the primary clarifiers, the BPR tank and a portion of the RAS from the secondary clarifiers feeds into the front end of the Bardenpho trains. Tertiary downflow filters follow the secondary clarifiers. These filters are designed for denitrification however the operators found it more efficient to feed methanol to the second anoxic zone of the Bardenpho trains. Historical methanol usage averaged 200 gpd. Solids processing typically occurs Monday to Friday and does noticeably impact nitrogen and phosphorus loading to the biological process. The gravity belt thickener operates Monday to Friday and the belt filter press operates Tuesday to Friday. Figure 1 shows the process flow and Figure 2 shows a satellite view of the facility. Figure 1: Process Flow Diagram:

3 Figure 2: Satellite View Biological Phosphorus Removal In the Biological Phosphorus Removal process or EBPR process, phosphorus accumulating organisms (PAOs) assimilate volatile fatty acids (VFA s) and produce intracellular polyhydroxyalkanoate storage products in anaerobic conditions. This storage process results in the breakdown of polyphosphate molecules resulting in a release of orthophosphate to the wastewater. In aerobic conditions, PAOs utilize the stored PHA energy resulting in the uptake of orthophosphate in the wastewater in an amount greater than that released in the anaerobic zone. PAOs can contain as much as 20-30% phosphorus by dry weight compared to 2-3% for a normal heterotroph. The City of Wilson has an anaerobic zone, called the BPR Tank, up front of the 4 Stage Bardenpho process (Figure 1). The anaerobic zone is currently fed with RAS from the secondary clarifiers however it is set up to take flow from the primary clarifiers. The BPR tank provides the proper environment for the enrichment of PAOs. Due to the aggressive Total Nitrogen reduction goals at the facility, the nitrate concentration in the second anoxic zone in the 4 Stage Bardenpho can at times be depleted resulting in anaerobic conditions and creating the potential for phosphorus release in the second anoxic zone and subsequent uptake in re-aeration. Furthermore, some VFA s can be stored as PHA even in the presence of nitrate, namely acetic acid, propionic acid and formic acid (Sedlak, 1991).

4 Methanol does not directly support EBPR. MicroC 3000 contains approximately 9% propanol and 3 % butanol. Propanol and butanol could be metabolized a number of different ways and could be used directly for denitrification, fermented to VFA s that can then be used by PAOs, or internalized by PAOs and converted to PHA. One plausible pathway is a relatively simple conversion of propanol to propionic acid and butanol to butyric acid which are both VFA s. METHODOLOGY Carbon Sources Prior to the transition the facility used virgin methanol supplied in bulk tanker. Carbon is fed with LMI diaphragm pumps through a common manifold that splits the carbon source to the 7 second anoxic zone trains. Feed rate adjustments are made manually based upon daily sample results from clarifier effluent and final effluent nitrate and nitrite concentrations. MicroC 3000 is a carbon source supplied by Environmental Operating Solutions, Inc. (EOSi). The carbon source is an environmentally sustainable mixture of alcohols derived from the conversion of renewable agricultural inputs. The alcohol mixture is a co-product of this process and is considered green methanol. The properties of MicroC 3000 and methanol are in Table 1. MicroC 3000 Methanol Specific Gravity ph 11 ~ 7 Flammability Flammable Flammable COD Methanol 66% 100% Ethanol 10% 0% Propanols 9% 0% Butanols 3% 0% Water 14% 0% Price/gallon $1.18- $1.25 $1.06- $1.49 The advantages of a mixture of alcohols compared to methanol are the enhanced kinetics of the other alcohols, resulting in higher denitrification rates and specific growth rates minimizing the risk of denitrifier washout, improving nitrogen removal performance and better response to fluctuations in nitrogen loading. There is a potential for the higher weight alcohols to provide a benefit for EBPR presuming they can be fermented and subsequently converted intracellularly to PHA. The transition from methanol to MicroC 3000 was fairly straightforward. The methanol inventory in the storage tank was depleted to fairly low levels and a 6500 gallon delivery of MicroC 3000 was added to roughly several hundred gallons of virgin methanol. The chemistry of methanol and MicroC 3000 are similar and the two carbon sources are easily miscible. Therefore there were no modifications required to pumps, feed lines, rotameters, etc. Data Collection The City of Wilson WRF has advanced laboratory capabilities and a robust sampling and analysis schedule. This data provided the baseline information necessary to compare historical performance with methanol to the performance during the MicroC 3000 period.

5 In addition to the regular plant sampling, intrazone sampling across individual Bardenpho trains were also performed to identify performance differences between the individual trains. Also two Hach Nitratax UV analyzers capable of measuring Nitrate+Nitrite were installed in the second anoxic zone of Bardenpho train #3 to provide real time data. Data transmissions were made daily between the City of Wilson and EOSi. RESULTS The results include data beginning on January 1, 2012 and ending on June 30, The transition from methanol to MicroC 3000 occurred on October 12 th, The monthly average data is derived from 24 hour composite samples acquired every weekday, or the average of roughly 20 composite samples per month. Nitrogen Removal Performance Figure 3 shows the monthly average Total Nitrogen (TN) discharge for the methanol and MicroC 3000 periods and the Total Nitrogen % removal that compares the reduction in TN from the influent and effluent composite samples. The monthly average TN discharges ranged from 1.85 to 2.5 mg/l and averaged 2.15 mg/l over this period. TN % removal averaged 93.1% over the same period of time. During the MicroC 3000 feed period the monthly average TN discharges ranged from 1.39 mg/l to 2.71 mg/l and averaged 1.91 mg/l. TN % removal averaged 93.6% over the same period of time. The methanol feed rate averaged 200 gpd and the MicroC 3000 feed rate averaged 214 gpd during the respective periods. The higher than normal TN discharge in May of 2013 was not definitively identified however one possible explanation may have been a result of increased load from solids processing activities. Another possibility is that due to higher flows a second primary clarifier was online that reduced the organic loading to the BNR process. For both periods, the TN discharge performance is exceptional. As expected, the transition to MicroC 3000 was seamless and did not result in a temporary process upset. There was a slight reduction in TN discharge from the methanol period to the MicroC 3000 period from 2.17 mg/l to 1.91 mg/l however TN % removal was approximately the same, 93.0% and 93.6% respectively.

6 Figure 3: Total Nitrogen Removal Total Nitrogen Removal Performance mg/l TN % 95.0% 90.0% 85.0% 80.0% 75.0% 70.0% Methanol MicroC 3000 TN% Removed Hach Nitratax Analyzers were installed in the second anoxic zone of the 3rd train for a period of the trial. Although the analyzers have provided incredible value in other projects they did not contribute much meaningful information in this particular application (Figure 4). The probe data was compared with filtered samples analyzed by the plant laboratory and were consistently higher compared to the laboratory values. One useful takeaway was the diurnal nitrogen loading that impacts the second anoxic zone between 3PM and 6PM. These peaks tend to be highest when solids processing activities are underway. Final effluent TN discharges are consistently lower on Sunday and Monday and gradually increase through the rest of the week. The operators are well aware of the impact of solids processing on the nutrient loading to the system and manage those activities for the benefit of effluent quality. The facility has modified the dewatering schedule over the years to match side stream loading with plant influent flows and organic loading. The facility does have a single Hach Nitratax and ammonia sensor in train #5 that serves as a guide for nitrification and denitrification performance.

7 Figure 4: Nitrate and Nitrite Analyzer Data Nitrate and Nitrite Analyzer Data Influent Effluent Methanol Feed MicroC 3000 Feed 0 9/23/2012 9/28/ /3/ /8/ /13/ /18/2012 Basin profiling was also performed by the plant staff to investigate if specific trains were performing better than others. Basin profiling requires sampling of each transition point between the zones within the 4 Stage Bardenpho trains. Sampling all seven trains (35 samples) is a time consuming exercise and as a result the operators sampled one train at a time. Differing performance between trains is typically driven by uneven flow distribution which is often times unpreventable. Other factors driving differences in performance may be aeration efficiency, mixing efficiency, short circuiting, back mixing of flow, etc. Basin profiling did reveal some potential differences between the trains however the amount of resources required to monitor small process changes was not justified considering small room for improvement in TN discharges. Other improvements in recent years (prior to this study) include the addition of VFD s to the influent pump station that resulted in a TN reduction of 5-10%, modification of the methanol distribution lines for better feed splitting between the seven trains and communication with the city water facility to minimize ferric sludge discharges to the sewer. Phosphorus Removal Performance Figure 5 shows the monthly average Total Phosphorus (TP) discharge for the methanol and MicroC 3000 periods and the Total Phosphorus % removal that compares the reduction in TP from the influent and effluent composite samples. The monthly average TP discharges ranged from 0.35 to 1.63 mg/l and averaged 0.83 mg/l over this period. TP % removal averaged 85.4% over the same period of time. During the MicroC 3000 feed period the monthly average TP discharges ranged from <0.1 mg/l to 0.44 mg/l and averaged 0.16 mg/l. TP % removal averaged 96.4% over the same period of time. During the methanol feed period, approximately 1000 lbs/day of 38% sodium aluminate was added to the secondary clarifier to trim effluent phosphorus. The sodium aluminate feed was discontinued in January of 2013 as a result of the EBPR activity in the second anoxic zone resulting in a savings of approximately $175/day. This occurred despite an increase in primary clarifier effluent orthophosphate load from 319 lbs/day during the methanol period to 375 lbs/day during the MicroC 3000 period. The only reasonable

8 explanation for the reduction in effluent TP is the increased EBPR activity as a result of the higher alcohol components in MicroC 3000 Figure 5: Total Phosphorus Removal Total Phosphorus Removal Performance mg/l TP % 100% 95% 90% 85% 80% 75% 70% Methanol MicroC 3000 TP % Removed DISCUSSION The transition from methanol to MicroC 3000 occurred without issue resulting in a 12% reduction in TN discharge to an average of 1.91 mg/l. Most interestingly was the noticeable improvement in TP effluent discharge from 0.90 mg/l to 0.16 mg/l. In addition to the reduction in effluent TP, the sodium aluminate feed to the secondary clarifiers was discontinued in January Furthermore the phosphorus loading from the primary clarifiers was higher during the MicroC 3000 period. Table 2 summarizes the results. Units Methanol MicroC 3000 Final Effluent TN mg/l TN % Removed 93.1% 93.6% Final Effluent TP mg/l TP % Removed 85.4% 96.4% Feed Rate gpd % Sodium Aluminate Feed lbs/day 1000 Discontinued The ability of MicroC 3000 to support EBPR will be evaluated in the bench scale using a method similar to that proposed by Wachtmeister. Sludge from Wilson WRF will be divided to two anaerobic reactors, one will dosed with methanol and the second will be dosed with MicroC The phosphorus release will be measured over a 3 hour period. The reactors will then be supplied with air and phosphorus uptake will be measured over a 2.5 hour period. Very little phosphorus release and uptake is

9 expected in the methanol dosed reactor compared to the MicroC 3000 reactor. These results will be presented in the podium presentation at the 93 rd NC AWWA-WEA Annual Conference in Concord, NC. CONCLUSION This study found that historical effluent Total Nitrogen performance with methanol as the carbon source was maintained with a mixture of low molecular weight alcohols. The study found that the mixture of alcohols also provided a benefit to effluent Total Phosphorus performance via EBPR. Future goals include: Automate carbon source feed Install Turbo Blowers with VFD s to better control airflow to basins. Equalize dewatering filtrate to constant feed over 24 hours, or based on incoming flow rate. Feed a carbon source to the BPR to enhance and/or stabilize the EBPR process ACKNOWLEDGEMENT The authors would like to acknowledge the outstanding work and operational excellence of the City of Wilson Water Reclamation Facility and its 31employees. REFERENCES Sedlak, Richard, (1991). Phosphorus and Nitrogen Removal from Municipal Wastewater. 2 nd Edition. CRC Press. ISBN Wachtmeister A., et al. (1997) Development of a Sludge Characterization Assay for Aerobic and Denitrifying Phosphorus Removing Sludge Wat. Res 31(3)