Alternate: Combined Chemical Conditioning Municipal Anaerobic Digestate for Dewatering Enhancement and Improvement of Biosolids Quality

Anaerobic digestate from municipal wastewater treatment plants (MWWTP) 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 reduce the polymer dose while introducing other chemicals to improve the dewatering and quality of biosolids. Several dewatering indices such as capillary suction time (CST), Cake Solid Content, turbidity and viscosity, soluble protein, soluble polysaccharide, phosphates are measured. Cake solid content, centrate total suspended solids (TSS), and fecal coliform of the cake after centrifuge dewatering indicate biosolids quality improvement. The objective of the experiment is to optimize the cationic polyacrylamide (PAM) dose, a double chemical dose of Ferric Chloride (FeCl3), and combined chemical dose of PAM, FeCl3, and Hydrogen Peroxide (H2O2) doses and study its impact on biosolids quality improvement. Anaerobic digestate is conditioned using cationic polyacrylamide (PAM) alone, with a combination of Ferric Chloride (FeCl3) and cationic PAM and a combination of (FeCl3), PAM, and hydrogen peroxide (H2O2) to find the optimum combination of chemical doses. Cationic PAM 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 chemical dose. Full-scale anaerobic digestate conditioned with 0.4% and 14 kg/ton polymer doses were replicated and compared in lab-scale with 0.5% polymer. Lab-scale data achieved a 4% increase in the cake solid content and a 93% reduction in CST with optimum and lower polymer dose of 2.2 kg/ton DS. Remarkable improvement of dewatering was observed as compared to municipal anaerobic digestate when FeCl3 was added along with cationic PAM. A combination of 0.5% P with 0.5% FeCl3 exhibited a minimum CST of 5.1s and a higher cake solid content of 30% with a polymer dose of 2.2 Kg/T DS in combination with 1.0 Kg/T DS of FeCl3. Trivalent cation Fe(III) forms colloidal Ferric Hydroxide (Fe(OH)3), which acts as bridges to strengthen the aggregation between particles and reinforces polymer flocculation and polymer bridge formation. Approximately 28% reduction of phosphate concentration was achieved in anaerobic digestate at optimum polymer dose. While dual chemical conditioning 2.2 kg/ton polymer with 2.1 kg/ton DS FeCl3 improved the phosphate reduction by 50%. Along with cationic PAM and FeCl3, 3% Hydrogen peroxide (H2O2) was also added to observe the effect of oxidation on dewatering efficiency. Combined Polymer Dose 2.2 kg/ton DS, FeCl3 2.1 kg/ton DS and 400 mg/l H2O2 shows minimum CST 5.63s and maximum cake solid content 41%. Biosolids' quality improved in terms of solid cake content by 15% on combined chemical addition. Combined dosing of polymer, FeCl3, and H2O2 converted the fixed iron in anaerobic digestate to Fe ++ and Fe +++, which subsequently reacted and formed phosphate precipitates, further dropping the sedentary phosphorous content of anaerobic digestate by more than 50%. The experiment results confirm the effect of combined chemical addition on enhancing dewaterability for municipal anaerobic digestate. Centrate suspended solids, turbidity, and viscosity will be measured to find the optimum dual and combined dose of chemicals. Soluble protein and polysaccharides will also be measured as a measure of dewatering efficiency over optimum chemical doses. Cake fecal coliform content will also be measured to study the disinfection efficiency of H2O2 over optimum chemical amounts. Fecal coliform testing is an essential measure of biosolid's quality improvement. The Study results will confirm the enhancement of biosolids quality over combined addition of H2O2 along with cationic PAM and FeCl3, thereby improving the performance of MWWTP dewaterability.