Bioenergy Potential of Slaughterhouse Waste and Enhancement of Anaerobic Digestion Parameters through Substrate Co-digestion

Abstract

Drastic change in food consumption pattern in South Korea since 1980 has led to increasing amount of livestock slaughtered causing increased waste generation in slaughterhouse. The total amount of slaughterhouse waste generated from South Korea in 2017 was around 260,018 and 127,539 tons for pig and beef cattle, respectively. Proper treatment of slaughterhouse waste in South Korea became an issue because previous disposal method (ocean dumping) was prohibited in 2012. One alternative solution was to treat slaughterhouse waste via anaerobic digestion. However, this process often varied in CH4 production due to variation in waste characteristics. Thus, determination of waste characteristics was essential.

Around 13 million tons of swine slurry were produced in South Korea every year. Swine slurry accumulation might lead to an environmental pollution that needs a proper treatment. Anaerobic digestion is an alternative technology to treat swine slurry. However, it was characterized with low CH4 production that makes anaerobic digestion plant to be uneconomical. Substrate co-digestion might become solution regarding this problem.

Since there was high production of beef cattle slaughterhouse waste (BCSW) and pig slaughterhouse waste (PSW), this study was not aimed to compare the characteristics of BCSW and PSW but rather to determine the characteristics and BCSW or PSW co-digestion effect with livestock waste. As such, this study was divided into two main parts. In the first part, characterization of PSW and swine slurry (SS) was performed by examination of chemical, proximate, ultimate, and energy content. The energy content includes heating value determination by bomb calorimeter and CH4 potential production by batch anaerobic digestion. In addition, effect of PSW co-digestion with SS to anaerobic digestion parameters was also determined. The PSW content in the co-digested mixture was 67%, 50%, or 33% weight per weight volatile solid basis (w/w VS basis). SS was added as remaining substrate to obtain 100% volatile solid in substrate mixture.

PSW heating value was 37 to 93.5% higher than heating value of various energy crops. However, high moisture content (81.8% of fresh matter) of PSW indicated that energy valorization by thermal conversion is uneconomical. In addition, PSW had high volatile solid content (94.5% of dry matter) that might complement the characteristics of SS that had low volatile solid content (67.6% of dry matter) during anaerobic co-digestion. Low volatile solid content in SS indicated that it had high minerals content that might be useful for anaerobic digestion.

Co-digestion experiment showed that high volatile solid content within PSW resulted in 22 to 84% increase of CH4 production potential compare to SS sole digestion. However, low CH4 production potential of SS, 48 to 62% of PSW CH4 production potential, resulted in 7 to 32% decrease of CH4 production potential compare to PSW sole digestion. Still, co-digestion shortened the lag phase period (3.3 to 8.5 days shorter) and effective digestion time (6.5 to 9.1 days faster) compare to PSW sole digestion. Short lag phase and faster digestion time during anaerobic co-digestion might be due to higher cobalt (Co) and nickel (Ni) concentration in SS than PSW. Co and Ni are important cofactor in CH4 production (methanogenesis) stage during anaerobic digestion. It was concluded that, anaerobic digestion was a suitable option to treat PSW. In addition, co-digestion results in improved anaerobic digestion efficiency as seen from increased CH4 production potential with no significant effect on effective digestion time compare to sole SS digestion. From PSW perspective, co-digestion reduced CH4 production potential but improved digestion efficiency as seen from reduced lag phase period and effective digestion time.

In the second part, characterization of BCSW was performed with similar parameters with the first part study. In addition, effect of BCSW co-digestion with SS or cattle dung (CD) to anaerobic digestion parameter was also determined. The BCSW content in the co-digested mixture was 66%, 50%, or 33% w/w OM basis. SS or CD was added as remaining substrate to obtain 100% volatile solid in substrate mixture.

BCSW heating value was 41 to 99% higher than heating value of various energy crops. However, high moisture content (83.3% of fresh matter) of BCSW indicated that energy valorization by thermal conversion is uneconomical. In addition, BCSW had high volatile solid content (93% of dry matter) that might complement the characteristics of SS during anaerobic co-digestion. CD also had high volatile solid content (91% of dry matter).

Co-digestion experiment showed that high volatile solid content within BCSW resulted in 30.7 to 75.8% increase of CH4 production potential compare to SS sole digestion. However, low CH4 production potential of SS, 50 to 64% of BCSW CH4 production potential, resulted in 3.64 to 25.6% decrease of CH4 production potential compare to BCSW sole digestion. Still, co-digestion with SS shortened the lag phase period (2.8 to 7.8 days shorter) and effective digestion time (6.5 to 8.3 days faster) compare to BCSW sole digestion. BCSW anaerobic co-digestion with CD resulted in 3 to 27% decrease of CH4 production potential compare to BCSW sole digestion. In addition, there was no significant effect to lag phase period and effective digestion time indicating CD was not suitable to be co-digested with BCSW. High Co and Ni concentration in the SS was the cause of improved anaerobic digestion efficiency (short lag phase and faster effective digestion time) during anaerobic co-digestion of BCSW and SS. Low Co and Ni concentration in CD was the cause of no improvement in anaerobic digestion efficiency. It was concluded that, anaerobic digestion was a suitable option to treat BCSW. In addition, co-digestion with SS results in improved anaerobic digestion efficiency as seen from increased CH4 production potential with no significant effect on effective digestion time compare to sole SS digestion. From the BCSW perspective, co-digestion with swine slurry reduced CH4 production potential but improved digestion efficiency as seen from reduced lag phase period and effective digestion time.

Based on the results from the first and second experiment, it was concluded that anaerobic digestion was an appropriate technology to treat PSW and BCSW. In addition, co-digestion provides solution to low CH4 production associated with large scale anaerobic digestion of SS. Slaughterhouse waste co-digestion with SS increased CH4 production potential without any adverse effect to the duration of lag phase period and effective digestion time indicating improved digestion efficiency. High concentration of Co and Ni in SS combined with high volatile solid content of slaughterhouse waste were the causes of these observations. This indicated that co-digestion of slaughterhouse waste and SS resulting in complementary effect in term of volatile solid and micro-nutrient availability for anaerobic digestion.