Filtrate rDON and Ortho-P Control Through Coagulant Addition During Dewatering of Thermal Hydrolysis Pretreatment-Enhanced Anaerobic Digester Sludge

In 2024, the Washington Suburban Sanitary Commission (WSSC Water), the largest water/wastewater utility in Maryland, will commission a centralized biosolids processing facility (Bioenergy Facility) utilizing a thermal hydrolysis pretreatment (THP)-enhanced anaerobic digester (AD) process. The Bioenergy Facility will receive 100 MGD worth of biosolids from six Water Resource Recovery Facilities (WRRFs). The filtrate from the Bioenergy Facility's dewatering process will be treated using the ANITATM MOX process and then discharged to the headworks of the 30 MGD Piscataway WRRF. Prior evaluations estimated that the total nitrogen (TN) in Piscataway's effluent could increase by as much as 1 mg/L due to the production of recalcitrant dissolved organic nitrogen (rDON) via the Maillard reaction resulting from the THP process as detailed in our previous study (Zhang, et al., 2021). The anticipated increase in rDON loading will place the Piscataway WRRF at risk of exceeding the 3.25 mg/L TN effluent limit. In addition, due to operational changes in coagulant dosing and phosphorous removal strategies at the six raw biosolids source plants, higher ortho-phosphorus (ortho-P) presence is expected in the Bioenergy Facility's AD effluent and dewatering filtrate, potentially causing struvite formation in downstream equipment. To address these issues, a temporary, easy-to-implement approach was tested to control rDON and ortho-P that can be applied at this and other WRRFs operating the THP-AD process. Therefore, this study aims to assess the impact of dosing Al3+ versus Fe3+ in AD effluent prior to dewatering on rDON and ortho-P levels in filtrate. Samples of dewatered mixed sludge were collected from WSSC Water's four largest WRRFs (Piscataway, Western Branch, Seneca, and Parkway) with a blending ratio (wet mass) of 4.3 : 3.3 : 2.4 : 1 (Figure 1). The blended cake samples were diluted to 16% TS followed by processing through a pilot-scale Cambi THP unit (Cambi, Asker, Norway) operated at 165 SMƒ for 30 minutes. The pretreated sludge was then mixed with AD inoculum collected from DC Water Blue Plains WWTP and anaerobically incubated at 36.5 SMƒ until the cumulative biogas production reached a plateau. Three variables were investigated in this study: coagulant type and dose, and the dewatering polymer dose. Briefly, various doses of polyaluminum chloride (PACl) or ferric chloride (FeCl3) solutions were added to the digested sludge followed by dosing polymer at either optimal polymer dose (OPD) or half OPD as described in Table 1. The lab-scale dewatering protocol includes shearing of the sludge-polymer-coagulant mixture and piston compression of the solids. Samples of dewatering filtrate were collected for characterization and testing for recalcitrant species. The recalcitrant species test consisted of ammonia (TAN) stripping of the dewatering filtrate to minimize background noise when calculating DON = sTN - NOx-N - TAN and subsequent aerobic incubation to eliminate biodegradable DON, with the remainder treated as rDON. Effect on DON and rDON removal Preliminary results from this study revealed that adding medium (5.5 % Al3+ or Fe3+) and high (8.2%) doses of PACl or FeCl3 along with the optimal dose of polymer during dewatering could reduce > 60% DON (Figure 2a). When the polymer dose was reduced by half but the coagulant doses were maintained at medium (5.5%), the effect of DON removal was only slightly diminished, increasing the DON level in the filtrate to approximately 45% remaining (Figure 2a). This slight decrease in DON removal efficiency at half OPD indicates that it was the coagulants rather than the polymer playing a dominant role in DON removal. rDON test is in progress. Effect on ortho-P removal Without any coagulant dosing, only 5 and 11% of ortho-P can be removed from the liquid phase at half and full OPD of polymer during dewatering, respectively (Figure 2b). However, it is interesting to observe that even adding low doses (2.8%) of PACl or FeCl3 was able to remove nearly 100% of ortho-P in the filtrate, eliminating the risk of struvite formation. Effect on filtrate pH The results showed that increasing the PACl and FeCl3 doses prior to dewatering significantly dropped the pH almost linearly to as low as 6.1 (Figure 2c). This can be explained by the hydrogen ion release from the formation of metal-hydroxide complexes after metallic coagulant dosing (Persson, 2018). Therefore, to maintain the dewatering filtrate within the suitable pH range of 6.5 to 7.5 for ANITATM MOX's ammonia-oxidizing bacteria and anammox bacteria activities (Hu, et al., 2023) without pre-pH adjustment, adding low or medium doses of coagulants would be preferred. Effect on cake dryness and cation overdosing Figure 2d showed that adding a low dose (2.8%) of coagulants during dewatering with OPD substantially aggravated cake dryness from 27.2% to less than 25% due to cation overdosing. With the coagulant doses increasing to medium (5.5%) and high (8.2%) doses, the cake dryness at OPD further deteriorated to as low as 23% and 22.4% TS, respectively, which corroborated the belief that overdosing has occurred as a result of coagulant addition. However, reducing 50% polymer dose while maintaining a low (2.8%) PACl and FeCl3 doses improved cake dryness to 27.5% and 27.2% TS. However, adding more coagulants beyond 2.8% with ½ OPD deteriorated cake dryness again due to cation overdosing.