What's Up Down Under? Australian Experience with Recuperative Thickening

The capacity of the mesophilic anaerobic digestion process in a municipal water resource recovery facility (WRRF) is typically limited by the need to maintain a minimum solids retention time (SRT). In conventional applications, the hydraulic retention time (HRT) is essentially equal to the SRT. Recuperative thickening allows the SRT and HRT to be decoupled, thus enabling the digestion process to be intensified. Recuperative thickening has been implemented at a number of large wastewater treatment plants in Australia, for the purpose of increasing process capacity and improving final biosolids quality. For new digesters, recuperative thickening may be used to significantly reduce the digester volume, resulting in substantial capital savings. In existing digester systems, recuperative thickening may be used to either increase capacity, or to increase the operating SRT. A number of Australian facilities have used recuperative thickening to operate at higher SRT, often as high as 30 days. Benefits of operating at high SRT include improved volatile solids destruction, increased biogas production and reduced cake odor potential. Successful implementation of recuperative thickening requires careful consideration of the impact of thickening the digester solids on sludge rheology, which in turn determines how various equipment associated with digestion, such as heat exchangers, mixers and pumps, are affected by recuperative thickening. These challenges are addressed by determining site-specific rheological properties of the digester sludge. Once these properties are known, the designer can determine their impact on the equipment, and provide upgrades as required.