THERMOPHILIC ANAEROBIC DIGESTION: 3-YEARS OF CLASS A PERFORMANCE

In 1992, the Greater Vancouver Regional District opted to build North America's first allthermophilic anaerobic digestion process, to produce a Class A biosolids and achieve maximum solids destruction. Built as part of a 475 million Cdn. project to upgrade the Annacis Island WWTP to secondary treatment, the process employs multiple stages in series to ensure adequate time-temperature exposure for Class A pathogen destruction.Three of the four new 12,000 m3 anaerobic digesters were started as a single-stage thermophilic process beginning in late 1996, just months before the first phase of the secondary treatment process started up to treat roughly 40% of the flow. Digested solids production increased from 16 dt/d to 26 dt/d with the addition of the waste secondary sludge. In May 1998, failure of the mesophilic digestion process at another District plant resulted in transfers of up to 80,000 L/d of thickened raw primary sludge to Annacis. Though the digester HRT fell to as low as 10 days, the thermophilic process continued unfazed. Full secondary treatment came on-line in November 1998, which increased the digested biosolids production from 26 to 41 dt/d. The thermophilic process accommodated this loading increase without upset. In April 1999, the “extended” process came on-line with three retrofitted digesters in series following the first stage digesters.Monitoring data shows that the single stage process met the Class A criteria more than 85% of the time. The thermophilic process maintained rock steady digestion performance throughout the start up of the secondary process and variable solids loadings. The paper will present and discuss the operating performance and biosolids dewaterability of this unique thermophilic digestion process over the past three years. Conversion to the multi-stage extended thermophilic process is complete and showing improved pathogen and volatile solids destruction. By the time of the paper presentation in March 2000, we will have almost a year of operating data with the full process to present.