Comparison of the Impacts of Thermal Pretreatment on Waste Activated Sludge using Aerobic and Anaerobic Digestion

Abstract

Thermal pretreatment systems are typically employed to improve waste-activated sludge (WAS) dewaterability and to treat sludge prior to anaerobic digestion. It is important to understand how WAS properties are affected during pretreatment to be able to assess the performances of processes utilizing pretreated WAS (PWAS). However, there are no generally accepted means of characterizing and comparing pretreatment processes. A pretreatment model for high temperature thermal hydrolysis was developed previously for one pretreatment condition. The motivation for this project stemmed from the need to extend the range of thermal pretreatment conditions to span the range of conditions commonly employed in practice and to evaluate the impact of these pretreatment conditions on WAS chemical oxygen demand (COD) fractionation. The two main objectives of this study were to fractionate the COD of WAS before and after pretreatment for several high temperature thermal pretreatment conditions and to compare the impact of pretreatment on aerobic and anaerobic biodegradability. The secondary objectives were to investigate how pretreatment affected the rate and extent of aerobic and anaerobic digestion of WAS. The data employed in this study was collected by others following the work of Staples-Burger (2012) and was generated by pretreatment of sludges at 125°C, 150°C, and 175°C for 10, 30 and 50 minutes. Physical and biochemical properties were measured for raw WAS (BR WAS) and PWAS. Offline and online respirometric data were used to evaluate the aerobic biodegradability of BR WAS and PWAS and to fractionate the COD of the BR WAS and PWAS. Biochemical methane potential (BMP) tests were conducted for BR WAS and PWAS to evaluate the anaerobic biodegradability of BR WAS and PWAS. BioWin® was used to aid in determining the WAS COD fractionation before and after pretreatment, and to determine whether pretreatment changed the aerobic and anaerobic biodegradability of the WAS. It was found that the high pressure thermal hydrolysis (HPTH) pretreatment conditions employed substantially solubilized the COD, organic nitrogen and volatile suspended solids (VSS) in the range of 30 – 55%, 23 – 41% and 30 – 89% respectively. Total COD (TCOD) was however not reduced by pretreatment indicating that organics were not mineralized. These findings closely agreed with the conclusions made in the literature. Pretreatment did not increase the overall extent to which WAS could be aerobically biodegraded. The fraction of non-biodegradable COD as represented by endogenous decay products (Ze) in the BR WAS were not converted to biodegradable form by pretreatment. However, pretreatment increased the rate at which WAS could be aerobically biodegraded as indicated by an increase in the fractions of readily biodegradable COD (Sbsc) in the PWAS. Pretreatment increased both the rate and extent of anaerobic biodegradability. The ultimate methane yield and the methane production rate were both increased when compared to the ultimate methane yield and methane production rate observed in BMP tests conducted on BR WAS. The experimental results were combined with BioWin® modeling to determine that the BR WAS consisted of 79% Zbh and 18% endogenous decay products (Ze). The endogenous decay products fraction remained at 18% through pretreatment and the concentration of active biomass (Zbh) in PWAS was deemed to be negligible. HPTH pretreatment at the employed temperatures and durations transformed the biodegradable fraction of BR WAS (Zbh) to 16.5 – 34.6% Sbsc and 45.8 – 63.6% slowly biodegradable COD (Xsp) of the TCOD concentration. The same PWAS COD fractionations were employed in anaerobic biodegradability test modeling and it was concluded that the aerobic and anaerobic biodegradability of PWAS was different. Up to 50% of the endogenous decay products were converted to biodegradable substrate (Xsp) due to HPTH pretreatment. It was determined that both pretreatment temperature and duration were important in solubilizing organic matter in the WAS. Increasing the pretreatment temperature and duration generally increased the organics solubilization. However, the impact of pretreatment temperature and duration on WAS COD fractions were inconclusive. The increase in organics solubilization did not correspond to how much of the biodegradable COD of BR WAS was converted to Sbsc by pretreatment.