Comparison Between Mesophilic and Thermophilic Anaerobic Digestions of Thermal Hydrolysis Pretreated Organic Wastes

Abstract

With the rapid growth of urban population and expansion of industries, the excessive production of organic wastes (e.g., food waste, swine manure, and waste activated sludge, etc.) has become a major environmental issue since last few decades. Anaerobic digestion (AD), by which microorganisms decompose organic compounds and convert them into renewable energy – methane, is widely accepted as a low-cost and sustainable alternative for organic wastes management.

The conventional AD is not profitable due to the low biodegradability of raw feedstocks. The rigid lignocellulosic biopolymers in swine manure, and to some extent, in food waste, the large fraction of cell wall/membrane and flocs found in waste activated sludge, encapsulate biodegradable organics and resist to be degraded by enzymes as well as microorganisms. Those natural characteristics greatly hinder the hydrolysis step in AD and limit biogas generation potential of aforementioned organic wastes.

In order to solve this problem, various physico-chemical methods prior to anaerobic digestion have been proposed for generating easily-degradable products, such as mechanical grinding, ultrasonic disintegration, chemical methods, thermal pretreatment, enzymatic and microbial application, etc. Among them, thermal hydrolysis pretreatment (THP), which utilizes high temperature and pressure to rupture rigid structures as well as disintegrate flocs, has been widely applied to sludge stabilization. However, the effects of THP on organic wastes are closely related to their intrinsic variabilities (e.g., nature, composition, and structure, etc.). The detailed comparison of THPs impacts on the biodegradability characteristics of food waste, swine manure, and waste activated sludge has not been revealed yet.

Generally, the AD of organic wastes is performed in a wide range of experimental conditions, from batch to continuous mode, and from mesophilic (30–45℃) to thermophilic (45–60℃) temperature regimes. In early researches, the influences of THP on anaerobic biodegradability of organic wastes are mostly evaluated at mesophilic temperature in lab-scale batch tests. The optimal temperature during subsequent AD after THP process is still unknown.

Therefore, the main objective of this study is to compare the impacts of THP and temperature (i.e., mesophilic and thermophilic) on various types of organic wastes (i.e., food waste, swine manure, and waste activated sludge). The biodegradability properties and overall mass balance of the three organic wastes with/without THP implement were investigated in BMP test. The AD feasibility and performance of raw/pretreated swine manure under both mesophilic and thermophilic temperatures were evaluated over a relatively long-term continuous stirred-tank reactors (CSTRs) operation.

The characterization results statistically revealed that THP enhanced volatile suspended solid (VSS) hydrolysis degree and solubilization of chemical oxygen demand (COD) for food waste, swine manure, and waste activated sludge. THP led to a maximum 145.0 and 118.2% methane yield increase of swine manure and waste activated sludge, respectively except for that of food waste. Compared with mesophilic condition, thermophilic temperature did not show distinctive advantages as for enhancing methane generation potential of the three organic wastes in BMP test (p > 0.05). The mass balance analysis showed that the differences in overall methane production between raw and pretreated swine manure under the two temperature settings were not significant (p > 0.05) due to the loss of organic matters during pretreatment process.

The results of CSTRs operation demonstrated that the AD of pretreated swine manure obtained higher specific methane yield, greater organic solids reduction efficiency and higher COD solubilization level than that of raw substrates. Thermophilic temperature did not improve methane production of THP-treated swine manure, while leading to a lower methane generation from raw substrate due to a short-term free ammonia inhibition. The concentration of acetic acid exceeded the inhibiting threshold in thermophilic reactors, and the accumulation of propionic acid caused disturbance in the higher temperature situation.

In conclusion, THP was effective to improve the anaerobic biodegradability of swine manure and waste activated sludge. The methane reduction caused by organic substances loss should be taken into account. Thermophilic temperature did not significantly increase the methane production, and suffered higher risks of process unbalance and instability than that of mesophilic digestion.