Holistic Biofiltration Optimization Can Lower Overall Treatment Costs Without Compromising Treatment Goals

Historically, biofiltration optimization has focused on characterizing and optimizing biofilter performance, considering biofilter as an isolated unit process. In contrast, holistic biofiltration optimization and integration advocates for biofiltration optimization in tandem with upstream unit processes. Through holistic optimization approach, utilities may realize significant cost savings through reduction in chemical and power use, and minimization of sludge production and management. In collaboration with the City of Columbus, the Water Research Foundation (WRF) tailored collaboration (TC) project 4731 intended to combine much of the knowledge gathered from previous biofiltration studies and demonstrate holistic biofiltration optimization and integration at full-scale facilities. Bench- and full-scale tests were conducted at the City of Columbus' Dublin Road Water Plant (DRWP) and Hap Cremean Water Plant (HCWP). Biofiltration optimization tests included phosphorus supplementation, pre-chlorination, and biofilter maintenance strategies during a filter shutdown. Upstream process optimization included coagulant dose and O3:TOC ratio optimization.
Phosphorus (50 to 450 µg/L) was fed to With-P filter influent. Tests were conducted at filter loading rates of 1.3 and 2.5 gpm/ft2. The effects of phosphorus supplementation were evaluated by comparing hydraulic, water quality, and microbial parameters in No-P and With-P filters. Both No-P and With-P filters performed similarly (Figure 1), suggesting that the phosphorus supplementation did not improve system performance. The impact of pre-chlorination was evaluated targeting approximately 0.25 mg/L free Cl2 in filter influent of the Test filter (i.e., With-Cl2 filter). Pre-chlorination significantly lowered headloss in Test filter compared to the Control filter (i.e., No-Cl2 filter) (Figure 2). TOC removal was not significantly different in the Test and Control filters and the biofilters removed a majority of DBPs formed during pre-chlorination.
Maintenance strategies for filter shutdown focused on assessing the importance of backwashing a filter before shutting down, comparing the effects of chlorinated versus non-chlorinated backwash before filter shutdown or immediately before resuming filter operation, and dry versus wet storage during filter shutdown. Filters stored wet were assessed for the effects of weekly water replenishment, daily 10-min aeration without water replacement, or daily low-rate backwashing. Furthermore, the shutdown testing also included evaluating the effects of short-term (i.e., 72-hour shutdown) and long-term shutdown (i.e., two-week shutdown). While the shutdown did not affect hydraulic performance in the filters, TOC removal was lower after resuming filter operation compared to before shutdown, which was likely due to lower temperature. Turbidity remained similar in all filters before and after shutdown. The results also indicated that it may not be necessary to backwash a filter before shutting down. Two coagulation approaches were evaluated at the bench-scale: (1) normal approach (i.e., using 85 mg/L alum) practiced at DRWP and (2) alternate approach (i.e., using 50 mg/L alum with 5 mg/L cationic polymer). The test results indicated that 0.3 mg/L TOC could be transferred to biofilters without affecting the filtered water TOC concentration. At the full-scale, alum dose was sequentially lowered from 90 mg/L to 55 mg/L without affecting filter effluent TOC (Figure 3) or turbidity (Figure 4).
Ozone dose was sequentially lowered at the full-scale system from 3 mg/L to 2 mg/L, which resulted in O3:TOC ratio ranging from 0.82 to 1.2 in the biofilter influent. Lowering the O3:TOC ratio did not significantly affect TOC removal across the biofilters. With the holistic optimization approach, it was determined that DRWP would save more than $600,000 per year (Table 1).
This presentation will discuss results of biofilter performance strategy testing and holistic biofiltration optimization and associated potential benefits. Utilities that are operating or considering biofilters for drinking water treatment are anticipated to benefit from the discussion.