One of the main global issues facing society today is the rapidly increasing generation of organic wastes. In Korea, the generation of organic wastes including food waste (FW) and sewage sludge about eighteen million tons in 2017 while raising concerns over sustainable management of organic wastes are critical issues since the ocean dumping ban by the London Convention. Currently, over 80% of organic wastes are recycled by compost or fertilizer. However, a large amount of wastewater is produced during processes, and the demand for these products is very low.
In recent years, interests in anaerobic digestion (AD) have increased as an alternative for management of organic wastes owing to its merits; it can simultaneously stabilize various organic wastes and generate green energy. AD is known as a complex multistage process that depends on various microbial groups involved in hydrolysis, acidogenesis, acetogenesis, and methanogenesis, which finally converts organic polymers to methane (CH4).
Sewage sludge is generated in the wastewater treatment plant (WWTP), and AD is one of promising processes for sewage sludge-to-energy conversion. Sewage sludge is classified as primary sludge and waste activated sludge (WAS). Primary sludge is known to be easily biodegradable because its major fraction is readily biodegradable organic compounds while WAS mainly consists of microbial cells which is more difficult to be decomposed, causing resistance to biodegradation. In order to enhance AD of WAS, application of pretreatment is a well-accepted way of disintegrating cell walls and hence improving biogas performance by increasing hydrolysis. Various pretreatment methods, including physical, chemical, hydrothermal, and biological ways, have been applied. To date, the most common approach for pretreatment of WAS has been to use physical, chemical, and hydrothermal ways, which are energy-intensive, and need additional equipment and chemicals. On the other hand, biological approach such as enzymatic pretreatment has been explored in recent years. It is usually considered as more eco-friendly and economical, but more intensive research activities are needed at the fundamental. Many commercial enzymes (e.g. amylase, cellulase, protease, and lipase.) have been reported to play an important role in hydrolysis of particulate organic polymers till now. In light of the above research background, the first purpose of this study was to investigate methane production through AD of WAS in response to application of enzymatic pretreatment using commercial enzymes. A biochemical methane potential (BMP) test was subsequently conducted and the results showed that pretreatment by combined commercial enzyme was found to be superior in CH4 production over single enzymatic pretreatment. The methane yield of pretreated WAS by the mixture of amylase, protease, lipase, and cellulase was 216.0 mL·CH4/g·COD, corresponding to a 2.9 times higher than the control. This implies that more enhanced performance of AD of WAS can be obtained when applying the mixture of commercial enzymes, which makes the entire AD process become non-cost effective.
The enzymes are generally involved in the interaction with the specific target substrates, and each of those needs optimum environmental conditions such as pH and temperature. In addition, as WAS mainly consists of proteins, lipids, and carbohydrates, the mixture of enzymes (protease, lipase, amylase) needs to be employed for efficient hydrolysis. Therefore, there is a need for a strategy that can economically obtain the mixture of enzymes. Thus the second purpose of this study was to investigate the feasibility of AD of pretreated WAS by using crude enzymes (CE) extracted from acid fermentation of FW. The sample pretreated by using CE obtained 211.9 mL·CH4/g·COD of methane yield, equivalent to a 286% higher than that of the control. This result implied that application of enzymatic pretreatment by CE was economically feasible. However, it was necessary to know how much CE had an effect on improving methane production compared to a mixture of commercial enzymes.
Therefore, the last objective of this study was to quantitatively evaluate the performance of CE on AD compared to the methane production from AD of WAS pretreated by commercial enzyme mixtures at different concentrations (0.2-1.6 g/g·COD of WAS). Methane yield of samples with mixture of commercial enzymes was 142.0-349.1 mL·CH4/g COD. Methane yield of WAS at various concentration of commercial enzyme mixtures was fitted with R2 > 0.99. It showed that the methane yield of WAS pretreated with CE was found to correspond to the methane yield of 0.34 g/g·COD of WAS with commercial enzyme mixtures.
In this study, CE extracted from acid fermentation of FW was found to be an alternative to mixture of commercial enzymes, making it possible to exploit the economic potential of organic waste treatment and bioenergy production.