Hu, Bing
Babcock, Roger W.
Civil Engineering
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The effects of low to medium temperature (44-121℃) thermal hydrolysis (THP) treatment of primary (PS) and secondary (WAS) municipal wastewater sludge on formation of carbohydrates, proteins, Melanoidins and methane generation potential were evaluated. Results show that between 1.5 and 12% of volatile solids (VS) was hydrolyzed into filtered dissolved solids (FDS), with 55-100% represented as carbohydrates and proteins depending on sludge type and THP temperature. Proteins are produced at 160 to 350% of the values for carbohydrates in terms of g gVS-1, and WAS values are 4 to 5 times as large as PS values. Much more low molecular weight (LMW) proteins are formed than high MW (HMW) proteins at all temperatures for both PS and WAS. The same is true for carbohydrates from PS, but the quantities of LWM and HWM produced are similar for WAS. Low-medium temperature THP increased biomethane potential (BMP) of WAS from 145 ml CH4 gVS-1 in untreated WAS to up to 230 ml CH4 gVS-1 (44 to 57% increase depending on temperature), and only nominally increased BMP of PS (by 0 to 7.5%). THP caused the formation of much more Melanoidins in WAS than PS and showed little dependence on temperature in the range evaluated herein. There was a nearly 20-fold increase in supernatant color for both PS and WAS that was well correlated with increases in THP temperature. Overall, the effects of low-medium temperature THP are significant for WAS and limited for PS. Product speciation of soluble components in secondary sewage sludge during THP is investigated in this research. The dissolved components concentrations increased with increasing THP temperatures (between 80 and 220℃) and reaction time (between 0 and 60 min), including dissolved solids, COD, TOC, carbohydrates, VFA, TN, protein, amino acids, and ammonium. At low temperature THP as 80℃, the carbohydrate and protein portion to sCOD is higher than the high temperature THP (110-200℃) due to THP releases nutrients from intercellular to free water but temperature not high enough to break down these structures. Proteins are highly denatured at 200℃ that ammonium concentration goes much higher than other conditions. The liquid brown color which is mainly from melanoidins generated during THP goes darker with temperature up and reaction duration. Melanoidins concentrations are also calculated from high molecular weight material, protein and carbohydrate, whereas the results fail to follow with the color results because of inaccurate conducted in measurements. The effects of THP pre-treatment of PS and WAS on particle size distribution (PSD), apparent viscosity and dewaterability prior to and following anaerobic digestion (AD) were studied. Results showed that when THP there was a shift in particle size distribution during THP of WAS, with particles in the range of 10 to 40 μm decreasing and 2-3 μm particles increasing. The apparent viscosity of WAS at a shear rate of 5 s-1 and 15-min reaction time increased relative to a control (4071 cP) at lower temperatures (80-110℃) due to gelation (up to 5880 cP) and then decreased dramatically in proportion to temperature increases to 140, 170 and 200℃ (2261, 1131 and 678 cP, respectively). Anaerobic digestion of the THP-treated mixed PS+WAS sludge further decreased the viscosity. The dewaterability of digestate from lab-scale ADs fed THP-treated mixed sludge improved compared to a control. The total solids (TS) of centrifuged sludge cake increased from 19.8 to 30.4% for pre-treated at 170℃ THP treated sludge compared to non-THP-treated. This result suggests a lower energy usage in the downstream sludge drying process. THP has been used prior to AD in sewage sludge treatment to increase digestion reaction rate, methane generation, and digested sludge dewaterability. All of these benefits point toward energy savings and the potential for net zero energy. Energy simulations were performed for a medium-sized municipal wastewater treatment plant (WWTP) that is in the process of adding THP, co-digestion of high energy wastes, thermal sludge drying, and combined heat and power (CHP). Laboratory experiments were conducted to determine site-specific efficiencies for THP, AD and BMP as inputs to the model. Historic and future projected facility energy use was also incorporated. The electricity energy input for sludge treatment decreased when applying THP to secondary sludge at 140°C (421 kW input when flow rate is 509 kg hr-1) and 170°C (258 kW input) compared to no THP treatment (463 kW). THP has little effect on the energy balance for PS because the methane yield improvement is minimal compared to WAS. Energy input for PS without THP is 537 kW when the flow rate is 927 kg hr-1, which decreased to 327 kW with THP at 170°C. Net zero energy can be achieved only by conducting co-digestion with a high-energy substrate such as 20% of VS loading added as fats oils and grease (FOG).
Civil engineering, Environmental engineering, biochemical methane potential, compound speciation, dewaterability, energy simulation, sewage sludge, thermal hydrolysis pretreatment
76 pages
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