THyPAD – From Pilot to Full Scale Application at Hamar WWTP
During 2001-2003 the city and county of San Francisco completed a two-year pilot study of pretreatment 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:
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).
Thermal Hydrolysis combined with Thermophilic/Mesophilic Digestion – called here THyPAD, gave the best combination of results in terms of both digester loading and VS% destruction. Digester loading was 5 kgVS/m3/day (0.312lbs/cuft) and VS reduction (VSR) peaked and stabilized at 70%. The combined system was extremely stable in operation with quality biogas.
Hamar WWTP (HIAS), Norway 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 1300m3 (340,000 gals) and the VS loadings had been peaking at 7kgsVS/m3/day (0.44lb/cuft) 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 %VSR.
The plant was extended in the fall of 2005 with an equal size digester but set up to operate the THyPAD system. The thermophilc digester was commissioned during January to May 2006. The addition of the second digester has increased gas yield by 22%. The biogas production has continued to increase throughout the commissioning period. 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 end or May 2006. The full scale plant bears out the findings of the original pilot work.