Laboratory studies were conducted to evaluate the performance and operational stability of a Temperature‐Phased Anaerobic Digestion (TPAD) system modified to operate in the sequential‐batch mode. The system fed with a 40:60 mixture (dry weight) of primary sludge (PS) and waste activated sludge (WAS) at 5.5% solids showed stable performance with minimum variation in operational parameters such as biogas production, VFA to alkalinity ratio, pH, and foam accumulation at system retention times as short as 12 days. The maximum volatile solids removal (VSR) of 52.5% was achieved at a system retention time of 16 days. The system did not show any effects of “shock loading” at the retention times studied and outperformed a “conventional” mesophilic system operated at a longer retention time. The system was effective in reducing the densities of pathogenic indicator organisms in the biosolids to levels lower than those specified by U.S. EPA for Class A designation.
The stability and performance of a sequential batch Temperature-Phased Anaerobic Digestion (TPAD) scheme aimed at satisfying the 40 CFR Part 503 Class A time-temperature requirements (24 hours at 55 o C) were evaluated. The system, fed with a 40:60 mixture (dry weight basis) of primary sludge and waste activated sludge at 5.5% total solids, showed stable performance at system retention times as short as 12 days. The maximum volatile solids removal (VSR) of 49.6% was achieved at a system retention time of 16 days. The system did not show any effects of shock loading at the retention times studied and the system performance was comparable to a semi-continuously fed TPAD system. The sequential-batch TPAD system outperformed a "conventional" single stage mesophilic system operated at a longer retention time with respect to VSR. The methane recovery efficiency of the system ranged from 0.62 -0.65 liter methane per gram of volatile solids destroyed, which was comparable to the single-stage mesophilic system.
With rising energy costs and concerns over global warming, sustainable treatment with a reduced carbon footprint is fast becoming the goal of major wastewater treatment utilities. Sustainability goals are driving the industry to take on the challenge of transforming wastewater treatment from an energy-consuming and waste-producing activity to one with positive net energy production and minimal residuals. Raw wastewater contains ten times the energy needed to treat it (Shizas and Bagley, 2004). However, most of the soluble organic compounds that contribute to the measured energy are mineralized to carbon dioxide or synthesized into new cell matter during treatment. The remaining energy is recoverable from biosolids either as steam or waste heat following thermal processes or as a methane-rich gas following anaerobic digestion. To be able to recover energy from biosolids for meeting energy needs onsite or for electricity generation and to displace the fossil-based energy used is very central to the theme of sustainability. Methods of energy recovery from biosolids are at more advanced stages of development than technologies for energy recovery from the liquid stream. This paper provides an overview of some of the newer thermal conversion technologies and their potential for energy recovery in comparison with the conventional anaerobic digestion pathway.
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