Pathak, Ankit

Ankit is a wastewater process engineer with Hazen and Sawyer since 2018. His areas of expertise include watewater process engineering as well as...

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About Pathak, Ankit

Ankit is a wastewater process engineer with Hazen and Sawyer since 2018. His areas of expertise include watewater process engineering as well as applied data science, including the use of tools such as Power BI for creative data analytics and visualization.

Pathak, Ankit

Titles from this speaker

Description: Assessing Impacts of Thermal Hydrolysis on Mainstream Non-Biodegradable Organic...

Assessing Impacts of Thermal Hydrolysis on Mainstream Non-Biodegradable Organic Carbon and Nutrients at the Broad Run WRF

Abstract

Background The Broad Run Water Reclamation Facility (BRWRF) is owned and operated by Loudoun Water (LW). The BRWRF has a design flow of 11 million gallons per day (mgd) and is an advanced treatment facility that complies with very stringent effluent standards, including monthly average effluent chemical Oxygen Demand (COD), Total Kjeldahl Nitrogen (TKN), Total Nitrogen (TN) and Total Phosphorus (TP) limits of 10 mg/L, 1.0 mg/L, 4.0 mg/L and 0.1 mg/L, respectively at current flows and loads. Currently, liquids treatment at the BRWRF employs primary clarification, 5-stage biological reactor basins and membrane filtration (MBRs) for biological nutrient removal. Granular activated carbon contactors are used as a final polishing step for organic carbon, nitrogen, and phosphorus removal. Solid stream treatment employs anaerobic digestion (AD) for stabilization of primary sludge and thickened waste activated sludge. Stabilized solids are then dewatered via centrifugation. Dewatering and thickening centrate are returned to the liquid stream process without further treatment. Figure 1 shows a schematic diagram of the treatment process at the BRWRF. Motivation As part of planning efforts to increase solids treatment expansion, sludge pre-treatment via thermal hydrolysis (TH) was identified as one of the alternatives that could help Loudoun Water meet solids treatment capacity needs at the BRWRF. TH has the potential to provide the BRWRF with a class A biosolids product while reducing digester volume requirement for solids stabilization. However, evaluating the impact of increased production of potentially non-readily biodegradable organic compounds in the combined THP-MAD process on COD, TKN, TN and TP concentrations in the mainstream as well as the plant discharge was critical, given the stringent discharge requirements. This pilot study was therefore aimed at benchmarking digester and solids process performance with upstream THP, as well as understanding the potential impacts of THP pretreatment on the effluent discharge from the BRWRF. The pilot study was performed over a 15-month period, from February 2021 through May 2022. Set-up and Methods The pilot treatment process included pre-dewatering of blended primary sludge (PS) and thickened waste activated sludge (WAS), followed by TH, AD and post-dewatering to produce the final cake solids. Pre-dewatered solids were diluted to a TS concentration of 10 12% total solids (TS) before batch treatment in a TH pilot unit provided by Cambi ® (shown in Figure 2). Digestion was performed using semi-continuous benchtop anaerobic digesters with a 5-liter working volume (Figure 3), while THP was accomplished using a pilot unit. Pre-dewatering was performed with a pilot-scale screw press, while post-dewatering was performed by a combination of centrifugation and filtration. Digester operation was performed in three phases: -Phase 1 involved digester operation without TH pretreatment, at a 23-day solids retention time (SRT). The digesters were fed with a blend of primary sludge and thickened waste activated sludge (TWAS) at a solids concentration of 4.5%, in line with current, full-scale operations. -Phase 2 involved digester operation with TH pretreatment, at a 23-day SRT. Three TH temperatures were tested in this phase, 135 ºC, 150 ºC, and 165 ºC. The digesters were gradually acclimated to TH over a period of four months. -Phase 3 involved digester operation with TH pretreatment, at a 15-day SRT. This phase mimicked projected full-scale operation with TH at the BRWRF. Similar to Phase 2, three TH temperatures were tested in this phase, 135 ºC, 150 ºC, and 165 ºC. To achieve the target retention time, digester SRT was shortened by half a day on a daily basis, over a one-month period to prevent shock changes to operating conditions. During each phase, digester operation and performance were closely monitored by analyzing pH, alkalinity, volatile fatty acid concentrations (VFA) as well as COD and nutrient species concentrations in samples of the digestate, as well as the post-dewatering centrate. Additionally, biodegradability assays were performed on the centrate to quantify the non-biodegradable fractions of COD, nitrogen, and phosphorus in the centrate recycle. These results helped inform the impacts of the centrate recycle on final effluent COD and nutrient concentrations. Summary of Results Impacts to Anaerobic Digestion Increased solids loading to the digesters (by a factor of 2 or 3 over current loading) was able to achieve same digester SRT with 50 66% lower digester volume in operation. The pilot digesters were able to acclimate to a solids feed treated using TH over a period of 3 4 months through a gradual ramping up of solids and organic loading to ensure that key indicators of digester health (such as pH, VFA, Alkalinity, gas production and solids destruction) continued to remain in optimal ranges. Overall, greater solids destruction and gas production were observed across the combined TH and AD process relative to AD without TH pretreatment. A fundamental shift was observed in digester operational parameters and digestate characteristics. The higher solids and organic loading to the digesters, coupled with increased solubilization of carbon and nutrients in the TH process led to increased concentrations of sCOD and nutrient species in the digesters. The higher solids loading also resulted in higher TS content in the digested solids, increasing from approximately 2.5% to 4.5% TS. TH pretreatment was found to result in the production of increased COD and nutrient species in the centrate, especially soluble COD, TKN and ammonia. Impacts to Centrate Characteristics Concentrations of dissolved organic nitrogen and phosphorus (DON and DOP) in the centrate were also found to be significantly higher with TH pretreatment versus without TH. In addition to an increase in DON, DOP and sCOD, the non-biodegradable fractions of the constituents (i.e., nbDON, nbDOP and nbsCOD) were also higher in the centrate when TH pretreatment was applied. Table 1 shows the difference in post-dewatering centrate characteristics with and without TH pretreatment. €ƒ Impacts on Plant Effluent and Mainstream Data from pilot testing indicated that implementing TH could potentially increase final effluent sCOD concentrations to be in the range of 8 to 12 mg/L. This would trigger a need for additional treatment to meet the monthly average permit limit for COD. Final effluent TKN was estimated to increase to between 0.50 and 0.75 mg/L, which is within 20% of the monthly TKN limit. However, in order to meet TN of 2 mg/L and lower at future flows and loads (i.e., TKN< 0.5 mg/L and TIN < 1.2 mg/L), Loudoun Water may need to implement additional treatment. Final effluent TP was estimated to increase to between 0.02 and 0.05 mg/L. This encroaches upon LW's safety buffer for the monthly TP limit. However, in order to meet effluent TP concentrations of 0.05 mg/L and lower at future flows and loads (i.e., Orthophosphate < 0.02 mg/L and DOP < 0.02 mg/L), Loudoun Water may need to implement additional treatment. Cumulatively, these results suggest that the increases in sCOD, TKN and TP that may result from implementation of TH at BRWRF may present a challenge for meeting current and future permit limits at the BRWRF. Consequently, implementation of TH at BRWRF would likely trigger the implementation of additional treatment for nutrients onsite or diversion of centrate to an offsite facility for treatment or disposal.

This paper was presented at the WEF/IWA Residuals and Biosolids Conference, May 16-19, 2023.

SpeakerPathak, Ankit

Presentation time

9:00:00

9:30:00

Session time

8:30:00

11:15:00

SessionSession 21: Advances and Innovations in Thermal Processes II

Session number21

Session locationCharlotte Convention Center, Charlotte, North Carolina, USA

TopicDigestion & Stabilization

Author(s)

A. Pathak

Author(s)A. Pathak1, W. Khunjar2, K. Sveum3, B. Schmitz4, R. Iboleon, Z. Wang6,

Author affiliation(s)Hazen and Sawyer1; Loudoun Water2; Virginia Tech3

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date May 2023

DOI10.2175/193864718825158794

Volume / Issue

Content sourceResiduals and Biosolids

Word count18

Troubleshooting Nitrification Process Upsets with a Robust Testing Protocol

Abstract

Introduction & Approach Wastewater utilities must often navigate biological process upsets due to the vulnerability of microbial processes to toxicity. Nitrification is particularly susceptible to impact from inhibitory substances (Bye et al., 2012; Xiao et al., 2015). This paper describes three WWTPs where deterioration in nitrification performance was troubleshooted successfully. Information in this paper is intended to support industry efforts to navigate process upset events at WWTPs. For each facility, operating data including flows, NH3, TN, TKN, airflows, DO, MLSS, and SRT were assessed. Additionally, WWTP influent and waste streams were analyzed for typical (ammonia, TKN, COD, sCOD) and potentially toxic (QACs, surfactants, amines, isothiazolones, phenols, peracetic acid, metals) compounds. Aerobic biomass activity was evaluated through specific oxygen uptake rate (sOUR) and substrate uptake rate (SUR) testing (Adani et al., 2001). Results and Conclusions Case Study 1 WWTP1 is a BNR facility with capacity of 7 mgd annual average (AA) flow and effluent TN limit of 8 mg/L AA. The treatment process contains aerobic basins and secondary clarifiers with no primary treatment. WWTP1 experienced loss of nitrification in 2022 and 2023, evidenced by elevated ammonia concentrations in the effluent. In 2022, sampling of the plant influent, as well as in the collection system indicated high QAC and zinc concentrations. Batch testing was performed on plant biomass spiked with the influent and collection system streams, showing inhibition of nitrification in the plant biomass as well as in a control biomass. Figure 1 shows data suggesting the continued presence of toxic substances in the plant. No increase in nitrification activity was observed using decanted biomass, suggesting a continued influx of toxicant to the WWTP. Increasing the aerobic volume in service resulted in partial recovery. The process ultimately recovered as influx of toxic substances in the plant influent abated and viable nitrifier inventory increased (1.5 months). In 2023, analyses of plant data indicated that the likely cause of elevated nitrogen in the effluent was nitrifier washout due to over-wasting. The facility was able to recover nitrification by reducing wasting rate and building up nitrifier inventory. The batch testing approach was found to be robust in determining the cause of the inhibition and informing recovery efforts through data analysis in combination with sOUR and SUR tests. A decision tree was provided to support future recovery efforts, outlining checks and proactive actions for prevention and timely resolution of issues (Fig 2). Case Study 2 WWTP 2 is a BNR facility with treatment capacity of 7.5 mgd AA and effluent TKN and TN limits of 3 mg/L (monthly) and 8 mg/L AA, respectively. The process includes primary clarifiers, BNR basins, secondary clarifiers, and gravity sand filters. In February 2022, the facility experienced near complete loss of nitrification. Influent characterization did not indicate toxicity in the plant influent. However, sOUR and SUR analyses on decanted biomass resulted in a clear and immediate increase in activity, suggesting the presence of inhibitory compounds in the liquid biomass matrix (Figure 3). sOURs did not change when using influent from WWTP2 versus a control facility, indicating that carbon removal was not impacted. Consequently, the facility increased wasting rate to accelerate washout of the embedded toxicant(s) from the process. The basins were then re-seeded with a fully nitrifying biomass from a nearby facility and a decision tree was provided (Figure 4). The chosen strategy of biomass wasting and re-seeding were successful in helping the facility recover nitrification. Case Study 3 WWTP 3 is BNR facility with Treatment capacity of 15 mgd AA and seasonal NH3-N limits (Winter: 18.5 mg/L; Summer: 6.5 mg/L). The process includes primary clarifiers, BNR basins, secondary clarifiers, and disinfection. In October 2020, the facility experienced a process upset characterized by slime growth, loss of nitrification, low DO, high SVIs, and elevated effluent cBOD, TSS and NH3-N. Operational data, microscopy, sampling, and toxicity testing indicated excessive sulfur may have caused the issues. Additionally, a number of industrial dischargers were identified, that had potentially caused previous process upsets. To combat future upsets, the following were recommended: (1) Routine examination of MLSS to identify early signs of deteriorating performance. (2) Addition of a selector zone for the selection of floc forming bacteria. (3) RAS chlorination (4) Installation of SentryTM probes to provide real-time monitoring of influent wastewater. Recommendations A comprehensive and robust protocol can help identify and address the process upset at WWTPs. The protocol proposed in this study includes review of facility performance leading up to and during the performance deterioration, waste stream characterization, and biomass activity testing. periodic sampling of non-routine constituents such as surfactants that are common nitrification toxicants, proactive monitoring of influent characteristics using sensors and sampling to provide early warning of potential detrimental impacts are recommended. Prompt response and robust testing can provide clear insight into causes of process upsets.

This paper was presented at the WEFTEC 2024 conference in New Orleans, LA October 5-9.

SpeakerPathak, Ankit

Presentation time

14:00:00

14:30:00

Session time

13:30:00

15:00:00

SessionInnovative Approaches to Address Traditional Operational Challenges

Session number211

Session locationRoom 344

TopicFacility Operations and Maintenance, Intermediate Level, Municipal Wastewater Treatment Design, Nutrients

Author(s)

Pathak, Ankit, Khunjar, Wendell, Seyedi, Saba, Blate, Micah, Sayles, Molly, Noteboom, Haley

Author(s)A. Pathak1, W. Khunjar2, S. Seyedi1, M.D. Blate3, M. Sayles1, H. Noteboom4

Author affiliation(s)1Hazen and Sawyer, VA, 2Hazen & Sawyer PC, VA, 3Hazen and Sawyer, PA, 4Hazen & Sawyer, NY

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date Oct 2024

DOI10.2175/193864718825159496

Volume / Issue

Content sourceWEFTEC

Word count10

Base text size -

Pathak, Ankit

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