Enhancing Post-Treatment Efficiency of Temperature-Phased Anaerobic Digestate Through Combined Chemical Conditioning for "Class A" Biosolids Production

A post-treatment process is required to reduce the volume temperature-phased anaerobic digestate (TPAD), so that it can be more easily transported and disposed of. The digestate liquid fraction contains a lot of nutrients as well and biosolids might contain pathogen. Traditional dewatering process that uses expensive polymers, separates solid and liquid parts to produce condensed biosolids. Polymer conditioning focuses only on volume reduction. TPAD has a high solid content which makes dewatering difficult. Other challenges associated to digestate dewatering such as nutrient and resource recovery, odour control pathogen contamination in biosolids is not addressed in traditional polymer conditioning. It is hypothesised that metal salts as coagulant and oxidants are added along with polymer to enhance the post-treatment of TPAD that would enhance dewatering, nutrient and resource recovery and pathogen elimination in dewatered biosolids. Through the combination of chemicals, the current research seeks to increase TPAD post-treatment efficiency by improving the removal of P in the liquid fraction while producing a Phosphorus (P) rich and pathogen free class A biosolids. To compare the effects of polymer conditioning alone, conditioning with polymer with ferric chloride (FeCl3), polymer, FeCl3 along with hydrogen peroxide (H2O2) before centrifugal dewatering, cationic polymer alone, combined polymer with FeCl3, and cationic polymer with FeCl3, H2O2 in a pH-controlled environment. The best combination of chemicals was 2.5 kg/t DS polymer, 2.1 kg/t dry solids (DS) FeCl3 and 600 mg/l H2O2 at pH 8.0 with the lowest turbidity 11 NTU, specific resistance to filtration (SRF) 0.08 Tm/kg, capillary suction time (CST) 11.5s. The combined chemical dose shows a 94 to 99% improvement in dewatering indices such as CST, turbidity and SRF than raw TPAD. Around 88 to 90% decrease in centrate, TS2- and PN/PS ratio was also observed that contributed to odour causing potential and increased dewatering efficiency. 100 % P removal was achieved with no P in the centrate and P rich biosolids was produced with 40% cake solid content after combined chemical conditioning. However, the fecal coliform level of raw TPAD cake from the full-scale plant exceeds the regulatory limit. The dewatered cake turned into class A biosolids with just 57 MPN/g DS fecal coliform content after combined chemical conditioning in the lab scale study. In comparison with untreated TPAD cake, pathogens were reduced 100%. Combined chemical application reduces polymer dose by 40% as well as associated cost. The novel combined chemical treatment with cationic polymer, FeCl3, and H2O2 at adjusted pH are used to enhance digestate efficiency as well as reduce costs, storage requirements, and address nutrient imbalance, pathogens, and odors that offered more sustainable management. Wastewater treatment plants are thereby recommended to adopt the novel combined chemical conditioning the synergistic chemical combination of polymer, FeCl3 and H2O2, adjusting pH at 8.0 with Ca(OH)2 for TPAD post-treatment while assessing the full scale adaptability through pilot studies.