TRANSIENT EFFECTS OF RAPID TEMPERATURE INCREASE IN THERMOPHILIC ANAEROBIC DIGESTION: BIOCHEMICAL STABILITY AND PRODUCTION OF VOLATILE SULFUR COMPOUNDS

The City of Los Angeles, Bureau of Sanitation, has converted its digestion processes at Hyperion and Terminal Island Treatment Plants (HTP and TITP) to thermophilic operation. A two-stage continuousbatch process was established at HTP, while a single-stage sequencing batch process was established at TITP. This was to evaluate compliance with the Class A pathogen reduction requirements of U.S. EPA 40 CFR Part 503. The first part of this contribution includes full-scale data on the production of volatile sulfur compounds (VSCs), biochemical stability of the processes, and digester performance during shortterm episodes of rapid changes in the digester temperature in the range of about 55 to 65 °C. The objective to increase the temperature at HTP was to comply with Alternative 1 of 40 CFR 503.32, which required a relatively high temperature because of the limited batch digester capacity (minimum temperature of 56.3 °C at 16 hours holding). TITP operations were already complying with the Alternative 1 time-temperature relation (minimum temperature of 550C at 24 hours), but the digester temperature was increased to evaluate whether this could help preventing fecal coliform recurrence in post-digestion biosolids. Rapid increases in the digester temperature at TITP (up to 65.5 °C) and HTP (up to 58 °C) caused biochemical instability, declining digester performance, and increased production of methyl mercaptan and, to a lesser extent, hydrogen sulfide. It is likely that these effects observed at fullscale were transient responses to rapid changes in temperature. The second part of this contribution includes a steady-state temperature-stress study conducted as part of the Hyperion Advanced Digestion Pilot Program. These pilot-scale studies indicated stable, steady-state operation at temperatures as high as 56 — 58 °C with minimum impact on digestion performance. A full-scale study is currently being conducted at HTP to further evaluate the effect of high temperature on the biochemical stability and production of VSCs, but under steady-state conditions with relatively small increases of the thermophilic digester temperature.