Enhancing Post-Treatment Efficiency of Anaerobic Digestate: A Comparative Evaluation of Chemical Applications for Volume Reduction and Resource Recovery

<p dir="ltr">Anaerobic digestate has a high-water content, making it volumous and difficult to handle. Digestate produces foul odours and contains excess nutrients and pathogens that risk human health and the environment. The conventional polymer approach only reduces volume without addressing excess phosphorus (P), pathogen levels, or odour concerns. Due to strict regulations regarding sludge treatment and disposal high cost of polymer, these challenges must be addressed. By post-treating anaerobic digestate, volume and odour reduction challenges can be effectively resolved, phosphorous removal can be improved, and biosolids that comply with regulatory guidelines can be produced. Digestate treatment can be improved using organic polymers, trivalent metal coagulants and other oxidants. The current study focused on a comprehensive post-treatment technique that included chemical conditioning and dewatering of conditioned digestate, followed by analyzing the process' efficacy using volume reduction, nutrient recovery, odour reduction potential, and biosolids quality assessment regarding pathogen content. Analyzing and evaluating various dewatering parameters was done using a polymer alone, a polymer and ferric chloride (FeCl3) dual addition, and a polymer, FeCl3, and hydrogen peroxide (H2O2) combined application at an alkaline pH. Single-stage mesophilic digestate (MD), thermophilic digestate (TD), and two-stage temperature-phased anaerobic digestate (TPAD) are conditioned by chemical mixtures. Analysis of dewatering indices determined the best chemical combination and digestate type to enhance digestate post-treatment. According to the study, MD displays the least volume reduction, P removal, and odour reduction, resulting in biosolids classified as Class B. Compared to MD and TPAD, TD consumes 20 to 50% more chemicals. TPAD demonstrated the best post-treatment efficiency when 2.5 kg/t dry solids (DS) polymer was combined with 2.1 kg/t DS FeCl3 and 600 mg/l H2O2 at pH 8.0. A significant reduction of capillary suction time (CST) by 94%, turbidity and specific resistance to filtration (SRF) by 99%, centrate P and fecal coliforms by 100%, total sulphide (TS2-) concentration by 91%, protein to polysaccharide (PN/PS) ratio of 86% and an increase of cake solid content by 11% compared to raw TPAD were observed following the conditioning of TPAD using combined chemicals. As TPAD was chemically conditioned, 40% cake solid content and 100% centrate P elimination were achieved, resulting in significant volume reduction and recovery of resources. The combined chemicals at the best dose also showed significant odour reduction potential while producing class A biosolids with only 57 most probable number (MPN)/g DS fecal coliform. </p><p dir="ltr">In contrast, dual chemical conditioning yielded the highest post-treatment efficiency for TD at pH 8.0 with 3.1 kg/t DS of polymer and 2.5 kg/t DS of FeCl3 as it resulted in an increase in cake solids by 10%, a decline in CST by 98%, centrate P removed by 100%, and PN/PS ratio reduced by 89% from raw TD. In addition to TPAD, TD also produced Class A biosolids due to H2O2 addition, but even after chemical conditioning, MD and TD showed the possibility for pathogen regeneration. Heat treatment at 100˚C destroyed E. coli in all three types of digestate. </p><p dir="ltr">Keywords: Anaerobic digestate, post-treatment, combined chemical conditioning, centrifuge dewatering, resource recovery, P removal, odour reduction, volume reduction, biosolids quality improvement, class A biosolids, pathogen regrowth.</p>