Improving Clarifier Performance and Capacity through Full-scale Implementation of High-rate Contact Stabilization

When combined with carbon efficient nutrient removal systems (short-cut nitrogen removal), high-rate activated sludge (HRAS) systems provide a potential pathway towards energy neutrality at Water Resource Recovery Facilities (WRRFs) (Rahman et al., 2020). In well operated HRAS systems, 50-60% of influent wastewater COD can be captured and sent to anaerobic digestion through carbon redirection and enhanced settling in clarifiers. Carbon redirection depends on short solids retention time (SRT) to minimize aerobic oxidation while solids capture in clarifiers is a function of bioflocculation (Jimenez et al., 2015; Rahman et al., 2016). However, the operational conditions of HRAS system can lead to a very dynamic process resulting in inconsistent clarifier performance and poor effluent quality, which in turn, reduces the net COD captured and energy recovery through anaerobic digestion (Miller et al., 2016). The secondary treatment or HRAS system at Blue Plains Advanced WWTP in Washington, DC operates at 1to 3 days SRT. The secondary clarifier performance is positively impacted when the HRAS system is bioaugmented with waste sludge from the downstream, enhanced nitrogen removal (ENR) system, and negatively impacted by recycle from the solids thickening and dewatering processes. Bioaugmentation with ENR sludge allows for better settling characteristics through improved bioflocculation (Mancell-Egala et al., 2017b; Van Winckel et al., 2019) and reduced energy demand through increased oxygen transfer efficiency (Garrido-Baserba et al., 2020). On the other hand, the solids recycle flow (solids processing building return flow, or SPB) has high levels of colloidal material that cannot settle without being adsorbed onto flocs and thus increases fines in the secondary effluent (Mancell-Egala et al., 2017b; Van Winckel et al., 2019). Previous research at Blue Plains showed that biofloculation in HRAS systems is the limiting factor in achieving good effluent quality, especially with high colloidal loads coming from the solids recycle flow (Mancell-Egala et al., 2017b; Van Winckel et al., 2019). The same studies showed that extraccellular polymeric substances (EPS) production was the critical factor impacting flocculation properties and settling characteristics. High-rate Contact Stabilization (CS) or RAS aeration has shown to enhance EPS production and bioflocculation by imposing a feast-famine regime (Rahman et al., 2016). It was therefore suggested that a high-rate CS system rather than a high-rate step-feed system (current configuration) should allow for enhanced bioflocculation within the Blue Plains secondary treatment system. Full-scale CS implementation was easy to implement to the current existing infrastructure at Blue Plains as it only required to bypass feed flow to the later stages to allow for a RAS aeration zone or stabilization zone to be created (Figure 1). Overall, this study focused on evaluating the impact and potential benefits of full-scale high-rate CS implementation, including the complexities of bioaugmentation and SPB flows on settling behavior and clarifier performance.