Thermal hydrolysis pre-treatment for high solids anaerobic digestion

The City and County of San Francisco completed a two-year pilot study of pre-treatment processes to mesophilic anaerobic digestion for improved biosolids stabilization and production of biosolids suitable for unrestricted reuse (class A biosolids). The results of this effort will help in the design of the new Solids Handling Facility (SHF) at the 11 m3/s Southeast Water Pollution Control Plant (SEP), which ten conventional anaerobic digesters are near the end of their useful life. The pre-treatment options investigated included:
  1. A thermophilic (55 °C; TP) process with 5 days hydraulic retention time (HRT).
  2. A thermal hydrolysis (170°C; TH) process with 25 minutes contact time.
  3. A combination of options 1 and 2
  4. No pre-treatment (i.e., control digester).

The different pre-treated sludges were fed to mesophilic (35-37 °C) digesters with 10-15 days HRT. The 15-day maximum loading used in the SHF design translates into a digester HRT of 15 days. The TP (temperature phase) and TH processes were selected on the basis of the performance of full-scale facilities in North America and Europe (Schafer, et al., 2002; Kepp, et al., 2000). The objectives of the study were to compare the potential for biosolids stabilization and gas production of the four process trains selected; to analyze process performance during steady state stability; to evaluate the stability of the best performing process under reduced and to assess the ability of the three process trains to produce Class A biosolids.

Methodology. A semi-automated pilot plant that includes four egg-shaped digesters with capacities between 113 and 227 liters was built for this project. One 113-liter digester was used as a control digester. It is fed Combined Primary and Activated Sludge (CPAS). One 227-liter digester was fed thickened, thermally hydrolyzed CPAS. The other two digesters were operated in series, the first one in the series at the thermophilic temperature range, and the second in the mesophilic range. The digesters in series were fed CPAS or thickened, thermally hydrolyzed CPAS. Thermal hydrolysis was achieved by injecting live steam to thickened primary and waste activated sludge in a 2-liter reactor to maintain a temperature of 170 ºC for 25 minutes, after which pressure was released instantaneously by transferring sludge into an non-pressurized flash tank and holding it until cooling to less than 100 ºC. Table 1 summarizes pre-treatment, digester hydraulic retention times and feed percent solids for the different tests carried out during the project. The increase in feed solids, which results in increased volatile solids and COD loadings to the digester, is one of the advantages of thermal hydrolysis pretreatment. Routine parameters monitored in the different streams included total and volatile solids, COD, ammonia, total volatile fatty acids, pH, alkalinity, gas production and methane content and fecal coliforms. All analyses were performed according to Standard Methods (APHA, 1998).

Results. Process stability was assessed by tracking the total volatile acids (TVAs) to alkalinity ratio. The mesophilic bacterial consortia used as seed for the pilot digesters was well acclimated to the SEP CPAS and therefore the CPAS-fed digesters operated at a ratio of 0.1 since start-up. High organic loading and higher soluble organic fraction in HCPAS resulted in transient unbalanced digestion after TH pretreatment, but measured ratios during steady-state operation paralleled those of the CPAS-fed digesters. However, TVAs, alkalinity and ammonia concentrations were approximately twice as much in the TH digesters than in the CPAS-fed digesters due to the increased organic loading. Short unbalances were observed in the TH digester after each HRT reduction.

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