Effect Of Combined Chemicals On Anaerobic Digestate Dewaterability

Anaerobic digestate from municipal wastewater treatment plants (MWWTPs) contains more than 90% water. The high volume of the digestate imposes handling and disposal challenges. Often anaerobic digestate undergoes dewatering, which helps decant the water from the digestate, improves cake solid content, and reduces storage, transportation, and disposal costs. Chemical conditioning before the dewatering process facilitates solid-liquid separation that enhances the efficacy of the dewatering process. Organic polymers have wide application in MWWTPs for the enhancement of dewatering. The experiment aims to evaluate the effect of different chemicals individually and in combination on the municipal anaerobic digestate dewaterability and phosphorus release. Anaerobic digestate is conditioned using cationic polyacrylamide (CPAM) alone, with the dual application of ferric chloride (FeCl3) and CPAM and a combination of FeCl3, CPAM, and hydrogen peroxide (H2O2). CPAM suppresses and neutralizes surface charges of digestate particles and bridge the gap between the particles to hold them in a floc structure. Overdose of the polymer may cause disaggregation, redispersion of the particles, and increase the solution's viscosity. Therefore, polymer overdose increases chemical costs, which creates the necessity to identify the optimum polymer dose. Full-scale anaerobic digestate conditioned with 0.35% and 12 to 14 kg/ton polymer doses were replicated and compared in lab-scale experiments with various doses of 0.5% CPAM. Lab-scale data achieved a 17% increase in the cake solid content and a 94% reduction in capillary suction time (CST) with an optimum and lower polymer dose of 2.2 kg/ton DS. Remarkable improvement of dewatering was observed compared to municipal anaerobic digestate when FeCl3 was added along with CPAM. A combined dose of 2.2 kg/ton DS CPAM and 1.0 kg/ton DS FeCl3 shows a minimum CST 11.8s and maximum cake solid content 30% with 94% reduction in CST and 13% increase in cake solid percent compared to those of digestate. However, a combined dose of 2.2 kg/ton DS CPAM and 1.5 kg/ton DS FeCl3 exhibits the lowest turbidity 32 NTU, 84% decrease in turbidity than digestate. A combined dose of 2.2 kg/ton DS CPAM and 2.1 kg/ton DS FeCl3 show a minimum SRF 0.24 Tmin/kg, a 99% decrease in SRF than raw digestate. Trivalent cation Fe(III) forms colloidal ferric hydroxide (Fe(OH)3 ) with very low solubility, which acts as bridges to strengthen the aggregation between particles and reinforces polymer flocculation and polymer bridge formation. Along with 0.5% CPAM and 0.5% FeCl3, 3% Hydrogen peroxide (H2O2) was also added to observe the effect of advanced oxidation on dewatering and phosphate removal efficiency. Combined dose of 2.2 kg/ton DS CPAM, 1.5 kg/ton DS FeCl3 and 400 mg/l H2O2 shows the lowest CST 5.4s and turbidity 25 NTU with a reduction of both the indices by 97% and 87.5%, respectively. Biosolids' quality improved in cake solid content by 37% with the same dose of combined chemicals. However, a combined dose of 2.9 kg/ton DS CPAM, 1.0 kg/ton DS FeCl3 and 400 mg/l H2O2 showed the lowest SRF by 0.07 Tmin/kg with a 99% reduction in SRF compared to that of digestate. Combined dosing of CPAM, FeCl3, and H2O2 converted the fixed iron (Fe) in anaerobic digestate to Fe ++ then Fe +++. Converted Fe +++ subsequently reacted and formed phosphate (PO43-) precipitates, dropping the sedentary total phosphorous (TP) content of anaerobic digestate by 77% in the sludge conditioned by the combined dose of 2.2 kg/ton DS CPAM, 1.5 kg/ton DS FeCl3 and 400 mg/l H2O2. The experiment also demonstrated a reduction of 70% of soluble reactive orthoPO43- while digestate conditioned by a combined dose of 2.2 kg/ton DS CPAM, 1.5 kg/ton DS FeCl3and H2O2 500 mg/l. The experiment results confirm the effect of combined chemical addition on enhancing dewaterability by improved CST, turbidity, SRF, and phosphorus release for municipal anaerobic digestate and improving biosolids quality by increasing the cake solid content.