Enhancing Sludge Dewaterability and Phosphate Removal Through a Novel Chemical Dosing Strategy Using Ferric Chloride and Hydrogen Peroxide

Enhancing Sludge Dewaterability and Phosphate Removal Through a Novel Chemical Dosing Strategy using Ferric Chloride and Hydrogen Peroxide Ian Watson c, Vahid Ghodsi a,b, Siva R. Sarathy a,b, John R. Walton c, Elsayed Elbeshbishy d, Domenico Santoro b,c a Trojan Technologies, London, Ontario Canada b Department of Chemical and Biochemical Engineering, University of Western Ontario, London, Ontario, Canada c USP Technologies, Atlanta, Georgia United States d Department of Civil Engineering, Ryerson University, Toronto, Ontario Canada Introduction Sludge processing and disposal constitutes 50-60% of the total treatment cost in WRRFs, and dewatering operations represent a large fraction of those costs. Those dewatering costs may be direct (as in the trade-off between polymer dose and cake solids content, and how that impacts disposal costs) or indirect (as in the removal of sulfide and phosphorus from the return streams, and how that impacts aeration and solids processing costs). A particular inefficiency confronting many WWRFs is the recycling of phosphorus into the water line via the dewatering return stream, which can comprise 25-50% of the total P entering the aeration unit. In this study, we aimed to develop a sludge conditioning process for Anaerobically Digested Sludge (ADS) with the goal of breaking this phosphorus recycle loop, and doing so within the constraints of current operations (i.e., with minimal capital investment and quick implementation within existing budgets). The Higgins Modified Centrifugation Test (MCT) was used as it allows a solids cake to form atop a screen platform while capturing the centrate below. The initial work was to calibrate the MCT parameters (screen mesh size, centrifugation speed and time, and polymer dose) to plant operational performance. Then, different dosages of ferric chloride (FeCl3), hydrogen peroxide (H2O2) and/or cationic polymer were applied and performance measures were collected for cake solids content and centrate TSS, sulfide, and o-phosphate. Results Conditioning ADS with H2O2 only (without polymer added). At a 200 mg/L H2O2 dose (Figure 1a), centrate o-phosphate levels dropped by 30% after 10 minutes, and by 50% after 20 minutes (no significant phosphate reduction was observed at higher H2O2 doses). Such incomplete phosphate elimination indicates insufficient and/or unavailable endogenous iron, and suggests a need to supplement by externally dosing FeCl3. Conditioning ADS with FeCl3 only (without polymer added). FeCl3 was added to ADS and centrate o-phosphate levels were measured (Figure 2). Dosing 90 mg/L Fe resulted in approx. 40% decrease in o-phosphate levels, increasing to 80% removal when dosing 220 mg/L Fe. In the case of FeCl3 dosing, five minutes was enough for Fe to react with o-phosphate (Figure 2). Conditioning ADS with FeCl3 and H2O2 (without polymer added). Figure 3 shows a 40% drop in o-phosphate levels after dosing 90 mg/L FeCl3, which increased to nearly 60% after dosing 300 mg/L H2O2. Raising the FeCl3 dose to 155 mg/L and 220 mg/L, with the same 300 mg/L H2O2 dose, increased o-phosphate removals to about 80% and 85%, respectively. Similarly, a 90 mg/L Fe dose reduced o-phosphate levels by 40%, increasing to nearly 60% when combined with a 400 mg/L H2O2 dose. Raising the Fe dose to 155 mg/L and 220 mg/L (with the same concentration of H2O2) increased P-removal to about. 80% and 85%, respectively. Conditioning ADS with FeCl3 and H2O2 (with polymer added). As expected, higher concentrations of polymer yielded a drier cake and lower centrate TSS levels (Figure 4a). Centrate TSS levels dropped by nearly 70% when the polymer dosage increased from 0.8% to 1.6%. Also, the cake solids content of the ADS treated with the combination of FeCl3 and H2O2 was about two percentage points higher (compared to unconditioned ADS with no FeCl3 and no H2O2). This indicates potentially significant cost savings on polymer consumption and/or solids disposal. Compared to ADS that was not conditioned, the centrate from ADS conditioned with FeCl3 and H2O2 had far lower o-phosphate levels (Figure 4b). This confirms that Fe (III) is capable of playing several simultaneously roles in wastewater treatment, such as a phosphate precipitant and a TSS flocculant. Economic assessment The results in Figures 3 and 4 can be combined to estimate the additional costs of different H2O2 / FeCl3 combinations to reach increasing levels of treatment (P-removal) — see Table 1. The first conclusion for this particular study is that the benefit of adding FeCl3 exceeds that of adding H2O2. This conclusion could change, however, were: 1) the endogenous Fe content in the feed sludge to increase (due to e.g., upstream Fe addition in the collection system or WRRF); or 2) the sulfide levels in the feed solids significantly increase (consuming FeCl3) or if durational odor control (at the sludge disposal facility) was needed. The second finding is that the higher cake solids content achieved by H2O2 and FeCl3 dosing can offset the costs for these chemicals by 25-30% in this particular case. This amount will be more for situations where disposal costs are higher than $50 per WT or where the degree of dewaterability improvement (2% points higher cake solids) is greater. Conclusions The results of this work confirmed that iron is key to binding phosphorus into the dewatered cake, though it is often not present in anaerobic digested solids in the amounts or forms needed to bind both sulfide and phosphorus. In those cases, the endogenous Fe bound to sulfide can be regenerated by adding an oxidizing agent (H2O2) or, if the Fe content is still insufficient, the regenerated Fe can be supplemented with FeCl3. In this study, the latter was shown to be the case: combined H2O2 and FeCl3 dosing was shown to greatly reduce centrate sulfide and phosphorus levels while producing a dewatered cake with two percentage points higher solids content (at the same polymer dose).