During 2001-2003 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 pre-treatment options investigated included:
- A thermophilic (55 °C; TP) process with 5 days hydraulic retention time (known in the US as a TPAD - temperature phased anaerobic digestion)
- A thermal hydrolysis (170°C; TH) process with 25 minutes contact time.
- A combination of options 1 and 2
- No pre-treatment (i.e., control digester).
Thermal Hydrolysis combined with Thermophilic/Mesophilic Digestion gave the best combination of results in terms of both digester loading and VS% destruction. Digester loading was 5 kgVS/m3/day and VS reduction (VSR) peaked and stabilized at 70%. The combined system was extremely stable in operation with quality biogas. Due to these positive results Hamar WWTP (HIAS), Norway, decided to implement this solution on a full scale.
HIAS has been operating a Cambi thermal hydrolysis plant since 1995. The success of the project has led to an increase in imports of sludges from other local plants. The Cambi plant has a capacity of about 3,850 dry tonnes per year. The original mesophilic digester is only 1,300m3 and the VS loadings had been peaking at 7 kg VS/m3/day and adversely affecting digestion rate. HIAS had recently installed cogeneration and wished to maximize biogas production using the Cambi system by adding another digester, which would also add redundancy to the existing 10 year old digester. It had already been demonstrated that just extending the HRT of the mesophilic digester did not have a major impact on %VS reduction.
The HIAS plant was therefore extended in the autumn of 2005 with an equal size thermophilic digester. The digester was commissioned during January to May 2006 and has increased the plant's gas yield by 25% when corrected to methane. VSR has risen from 55% during 2005 to 65% during the first part of 2006. The thermophilic digester appears to operate in an acid/gas mode and this, combined with the scavenging of the mesophilic digester, gives a high rate of digestion. The plant data is shown for the period before the retrofit and up to the current period. The full-scale plant is yet to completely confirm the findings of the original pilot work.