Pilot Scale Evaluation of a Two-Stage, Low-Cost Low-Tech Class A Biosolids Treatment Process

Many water resource recovery facilities (WRRF) currently producing Class B biosolids are interested in upgrading to Class A biosolids production. However, the conventional treatment technologies approved for Class A biosolids are often beyond the means of many small WRRFs. Low-cost, low-tech (LCLT) options for producing Class A biosolids are available. However, widespread adoption of LCLT processes has been hindered by a lack of data regarding the conditions necessary for achieving the required pathogen and indicator organism (PIO) inactivation. Thus, the overall goal of this multi-phase, multi-year project is to improve our understanding of the key factors controlling PIO inactivation rates, and ultimately develop a rational and universal approach for the design of LCLT processes for Class A biosolids production. In the phase of the project reported on in this paper, a pilot scale investigation of a combined long-term storage and air drying LCLT treatment process that was performed at the Portage Lake Water and Sewer Authority (PLWSA). In the first stage, anaerobically digested and dewatered biosolids were placed in triplicate inside and outside test beds. After a year in storage, the biosolids were removed and placed in air drying piles that were turned twice a week. PIO monitored during treatment were fecal coliforms, seeded viable Ascaris ova, and poliovirus. Total solids, volatile solids, pH, ammonia, volatile fatty acids and alkalinity were monitored to investigate potential PIO inactivation mechanisms. Ambient weather conditions and biosolids temperature were also monitored to demonstrate the differences between the inside and outside treatments. Sufficient fecal coliform reduction was observed during long-term storage to meet the Class A standards, however bacterial regrowth occurred after sufficient rainfall. Viable Ascaris ova levels also decreased but did not achieve the log removal required for Class A equivalency due to insufficient heating and drying during the air-drying stage. Poliovirus data are not available yet but are expected to meet Class A requirements. Potential PIO inactivation mechanisms include temperature and drying effects. Chemical inactivation by ammonia is unlikely at the pH values observed because the ammonia is in the nontoxic ionized form. However, volatile fatty acids are toxic at these low pH values and may contribute to inactivation.