Wastewater Treatment Plant Design and Operation Modifications to Improve Management of Biosolids: Regrowth, Odors, and Sudden Increase in Indicator Organisms

Abstract

The overall goal of this project was to investigate approaches to manage regrowth, odors, and sudden increase (ROSI) of indicator and pathogenic bacteria in biosolids cake exposed to high-shear dewatering such as centrifuges. The project entailed two phases. The first phase was focused on filling critical research gaps that provided the fundamental insights for developing solutions to these issues and included method development. The second phase focused on evaluating field solutions. The results from this work supported the hypothesis that sudden increase was a result of incomplete inactivation of indicators bacteria. Results suggested that U.S. EPA Method 1680 likely underestimated the viable density of indicators in some samples after thermal treatment, and showed that, to achieve complete inactivation of indicator bacteria, a slightly higher time-temperature regime is likely needed. The project team proposed a new best-practices time-temperature curve. In addition, processes were identified that had a relatively high risk of sudden increase (SI) and regrowth. Gas chromatography-olfactometry was used to identify the persistent/long-term odorants in centrifuged biosolids. The main persistent odorants contributing to cake odors were indole, skatole, p-cresol, and butyric acid. All of these compounds are breakdown products of organics mainly protein as well as carbohydrates and fats, suggesting that processes that remove the readily bioavailable form of these precursors will help reduce odors in biosolids. The results showed that the level of odorants measured after longer-term storage was correlated to the shorter-term total volatile organic compound (TVOSC) concentrations. Several amendments added to centrifuge cake were able to reduce odors and/or regrowth significantly. Incubation of anaerobically digested biosolids with protein-degrading enzymes resulted in additional gas production as well as in a reduction in odorants after dewatering. Also, better digestion was associated with lower odorants, and primary solids produced greater odorant concentrations compared with secondary solids. Longer term storage of biosolids, greater than 20 days at ambient temperatures (15–30°C), was shown in a field trial to produce a product with low odors that met the fecal coliform requirements for Class B biosolids. No benefits between outdoor and indoor storage were measured. This title belongs to WERF Research Report Series ISBN: 9781780404578 (eBook)