Mitigation of greenhouse gas emission through anaerobic digestion of livestock waste

INTRODUCTION

Pork is the largest economic meat source for the Taiwan population, with a per capita pork consumption of about 36.5 kg/ycar. As such, pig farmers produced biogas production due to the large presence of pig farms in Taiwan and availability of fresh manure for biogas collection. As a result, the collected biogas from pig farms is used for power generation or direct combustion in Taiwan (Su & Chen 2015).

Greenhouse gas (GHG) emissions from the agricultural sector in Taiwan was 2,712 kilotons of C02-cq in 2016, accounting only for 0.93% of the country’s total GHG emissions (293,125 kilotons of C02-cq). From 1990 to 2016, GHG emissions from the agricultural sector show the agriculture sector decreased by 30.66% with an average annual growth rate of -1.40% (TEPA 2019). In 2016, N20 emission from agricultural soil, livestock manure management, and agricultural waste burning accounted for 28.0, 1.62, and 0.02% of total agricultural GHG emission (4,701 kilotons of C02-cq), respectively (TEPA 2019).

The livestock greenhouse gas emission was mainly from anaerobic digestion of manure and wastewater. Traditional anaerobic digestion (AD) is liquid anaerobic digestion using high water content feedstocks such as wastewater and slurry. However, anaerobic digestion can be divided into different types depending on the total solid content (TS). temperature, and ways of feeding the digester (Kothari et al. 2014). Based on the TS content, AD can be classified into liquid anaerobic digestion, which contains less than 15% TS and is applied for the treatment of wastewater (de Laelos et al. 1997), and solid-state anaerobic digestion (SSAD), which contains more than 15% TS (Gc et al. 2016). Liquid AD is a technology that had been used for a long time, while SSAD for the treatment of municipal solid waste was initially installed in Europe and has gradually increased since the 1990s (Baere & Mattheeuws 2010). Compared to SSAD. liquid AD generates a large amount of wastewater as well as sludge production (Pezzolla et al. 2017). In contrast, SSAD generates a lower amount of wastewater and requires less energy for mixing as well as heating (Kothari et al. 2014, Xu et al. 2014).

INVESTIGATION OF BIOGAS PRODUCTION FROM ANAEROBIC DIGESTION OF PIGGERY WASTEWATER

Waste-water treatment systems in Taiwan in selected pig farms

The most widespread piggery waste-water treatment system in Taiwan is the three-step piggery waste-water treatment system (TPWT), which includes the stages of solid/liquid separation, anaerobic digestion and activated sludge treatment (Figure 1). The anaerobic digestion basin is a plug-flow, top-opened, horizontal and underground waste-water basin covered with a plastic lid and constant pressure device: biogas can be collected from the top of the plastic cover. Among this literature, the TPWT system is the only typical wastewater treatment system for manure management applied in Asia (Su et al. 1997).

Three integrated pig farms were selected from Miaoli (9,000-pig farm), Changhua (15,000— 1,8000-pig farm) and Tainan (10,000-pig farm) Counties located in northern, central and

Diagram of the three-step piggery wastewater treatment (TPWT) system (Su and Chen, 2018)

Figure 1. Diagram of the three-step piggery wastewater treatment (TPWT) system (Su and Chen, 2018).

southern Taiwan. The daily wastewater volume was 294. 300, and 400 nrVd, respectively. Thus, the hydraulic retention time (HRT) of anaerobic digesters was 6.5, 20.4. and 3.0 d, respectively. Because the Tropic of Cancer cuts across central Taiwan, dividing Taiwan between the tropical and subtropical zone, with unique landscapes and rich natural resources of different climates. Hence, the three pig farms were chosen in northern, central and southern Taiwan in order to represent different climatic regions.

Production of biogas after anaerobic digestion of piggery waste water

The emission factors of GHG were calculated from the data of analyzed on-site samples, taken from the gas outlets of the selected anaerobic piggery wastewater treatment facilities prior to pressure stabilizers in northern, central and southern pig farms (Table 1). Daily average biogas production per farm across the temperature zones ranged from 625 to 958 (P < 0.05), 1,851 to 2,129 (P > 0.05) and 628 to 696 nvVday (P > 0.05) in the northern, central and southern pig farms, respectively. Results implied that the biogas production rates might reach their maximum when hydraulic retention time (HRT) of anaerobic digesters is >20 days. In contrast, there might be inadequate retention time for biogas production when HRT of anaerobic digesters was only 3 days.

The analytical results demonstrated the average GHG compositions in the biogas for CH4, CO2 and NiO were 0.65 ± 0.035, 0.30 ± 0.011 and 0.0004 ± 0.00021, respectively. Additionally, the average emission levels of CH4. CO2 and N2O were 10.8-19.0, 12.3-25.3 and 0.03-0.09 kg/head/year, respectively (Table 1). Statistical results implied that both average biogas production and GHG contents in biogas were significantly different among three pig farms, except for N2O. These results might be due to different climates and manure management techniques (slatted vs. unsalted) among the three pig farms in the current study.

Table 1. Average biogas production in the northern, central, and southern pig farms (Su and Chen, 2018).

Biogas production

Farm locations

Average

P-value

Northern

Central

Southern

Daily average per farm (nrVd)

8651162

19261168

664185

1.1511653

<0.001

Average per head (nv'/head/d)

0.08810.016

0.12810.011

0.06610.008

0.09410.031

<0.05

CH4 (kg/head/yr)

13.2910.27

19.0210.09

10.8210.05

14.3814.21

<0.001

C02 (kg/head/yr)

17.7310.34

25.2710.16

12.3410.16

18.4516.49

<0.001

N26 (kg/head/yr)

0.02910.016

0.05010.011

0.08610.002

0.05510.029

NS

Data presented as mean ±S.D. NS, not significant.

The results suggested that the average emission factor of CH4 from anaerobic waste-water treatment of pig farms (14.4 kg/head/ycar) is lower than that (1-23 kg/head/year in temperate and warm regions) estimated by IPCC (2006) (Su & Chen 2018). This difference may result from differences in the organic concentrations in waste water and liquid manure (or slurry).

However, emission of CH4 for pig operation in the three climate regions, cool (<15 °C), temperate (15-25 °C) and warm (>25 °C), estimated by IPCC in 1996 was 1, 4 and 7 kg/head/ year, respectively (IPCC 1996).

 
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