Dewatering Benefits of SLG Technology at the McDowell Creek Water Resource Recovery Facility

Dewatering Benefits of SLG Technology at the McDowell Creek Wastewater Treatment Plant Anish Luthra, PE, PMP, CDM Smith Kathryn Quaid, CDM Smith Muriel Steele, PhD, PE, Charlotte Water Charlotte Water's McDowell Creek Wastewater Treatment Plant (WWTP) has historically faced biosolids dewatering challenges related to achieving typical industry cake dryness for belt filter press dewatering of anaerobically digested biosolids. This presentation will discuss the dewatering benefits experienced by the plant by piloting a solids-liquid-gas (SLG) pretreatment process ahead of belt filter press dewatering. The plant's processes include screening, grit removal, primary clarification, 5-stage BNR for nitrogen and phosphorus removal, secondary clarification, deep-bed sand filters, UV disinfection, cascade aeration, gravity belt WAS thickening, mesophilic anaerobic digesters, and dewatering belt filter presses. Anaerobically digested solids from four primary digesters overflow into a single, common secondary digester. From the secondary digester, digested solids are pumped to the dewatering facility for final dewatering via Ashbrook belt filter presses. Dewatered cake is conveyed by a belt conveyor to a covered storage area before being loaded onto hauling trucks typically for land application, or alternatively landfill disposal. In recent years, the plant has struggled to produce dewatered cake total solids concentrations above 12.5% on average. In early 2023, Charlotte Water elected to pilot the solids-liquid-gas (SLG) pretreatment process offered by Orege to optimize the dewatering operations at the McDowell Creek WWTP. The SLG technology is composed of three parts - the SLG skid, an air compressor, and a deaerator. The SLG technology uses compressed air to modify the rheology of the digested sludge upstream of belt filter press. Low pressure compressed air is injected into the digested sludge, creating a floating emulsion sludge. Polymer is then added to the emulsion to complete the flocculation process ahead of dewatering. The emulsion's larger solids surface area improves polymer adherence, resulting in better water release and higher cake dryness. Before the pilot was commissioned, Charlotte Water developed pilot evaluation criteria containing a variety of measurable and subjective goals to determine pilot success. Pilot logistics were then established through a collaboration between Charlotte Water, CDM Smith, and Orege. It was determined that the plant's existing digested sludge pumps that normally feed the belt filter presses would not have enough head to overcome the pressure of the SLG unit. As such, a temporary 1000-gallon sludge buffer tank and additional sludge pump were provided. Temporary piping was installed to feed the buffer tank, sludge pump, SLG skid, deaerator, and belt filter press. A variety of polymer selections were also evaluated as part of the pilot to determine compatibility with the SLG emulsion. Once the pilot was commissioned, the baseline verification period began. The project team observed Charlotte Water's operation of the belt filter press under typical operating conditions and collected and analyzed baseline samples to compare against historical data. After a week of baseline verification, the pilot moved into the optimization period. During this period the belt filter press was started under typical operating conditions by Charlotte Water and baseline samples were collected before the SLG unit was started up for the day. During the three months of optimization, the project team made process adjustments to the SLG unit and belt filter press to determine the optimal operating parameters for pilot operation. Once the optimal pilot operating parameters were determined, the field staff performed a two-week field test. During the field test, the belt filter press was started under normal operating conditions and baseline samples were collected and analyzed. Then, the SLG unit was started up and optimal pilot operating parameters were set. Multiple samples were taken throughout the SLG run time before the unit was shut down and the belt filter press was adjusted back to normal parameters for a second round of baseline sampling. This process was repeated for 10 days, during which baseline parameters remained consistent and SLG parameters remained consistent. The pilot results suggested the SLG pretreatment process improved cake dryness by an average of approximately 2% total solids concentration. This is a significant improvement from the historical performance of McDowell Creek WWTP's dewatering operations. Polymer treatment rate and filtrate quality remained similar to baseline conditions. Table 1 contains average field test results and historical data. Figures 1-3 present daily average cake, filtrate, and polymer treatment rate results from the field test period. The project team is currently performing a lifecycle cost analysis to evaluate the capital cost investment of the SLG pretreatment process and the annual cost savings associated with the operational improvements of the SLG process, which will be ready for presentation by the time of the 2024 Water Environment Federation (WEF) Residuals and Biosolids Conference.