Changes in dewatering properties between the thermophilic and mesophilic stages in temperature-phased anaerobic digestion systems

Temperature-phased anaerobic digestion (TPAD) has become increasingly appealing in recent years because of the pathogen destruction capabilities of the system. However, there has also been concern about the dewatering properties of the solids created by these systems. A laboratory study was conducted at Virginia Polytechnic Institute and State University, Blacksburg, to determine the effect of thermophilic solids retention time (SRT) on dewatering properties of wastewater solids to characterize system parameters associated with dewatering and to understand the mechanisms causing changes in dewatering properties between the thermophilic and mesophilic phases. The study showed that, while anaerobic digestion caused dewatering properties to deteriorate, the solids varied little with thermophilic SRT. Acidogenesis was essentially complete after 1.5 days. Subsequent mesophilic digestion resulted in little change to dewatering properties and modest reductions in conditioning doses, but substantial reductions in biopolymer (protein and polysaccharides) occurred. It seems that thermophilic anaerobic digestion creates or releases colloidal materials that cause dewatering to be poor and subsequent mesophilic digestion for 15 days does little to improve the dewatering properties of solids in TPAD systems.