Towards sustainable agricultural waste management using dry anaerobic digestion
26 July 2016
Treatment of wastewater containing high concentrations of nitrogen from pig farming is one of the ongoing environmental problems in Japan. After treatment of wastewater from large-scale pig farms, nitrogen concentration is reduced below the standard limit for industrial wastewater; however small-scale farmers have not been able to treat wastewater to meet the standard. One reason is that the financial and labour conditions of small-scale farmers are not sufficient to manage wastewater treatment facilities.
Therefore, we focused on the dry anaerobic digestion technology for treatment of pig manure including piggery wastewater. In anaerobic digestion, organic matter is converted into methane (CH4) and carbon dioxide (CO2) by several micro-organisms in the absence of oxygen. The produced CH4 can be used as renewable energy. Anaerobic digestion is classified into two types by water content in the digestion sludge: wet anaerobic digestion and dry anaerobic digestion (Figure 1).
Wet digestion is anaerobic digestion with less than 15% total solid content, while dry digestion involves more than 15% total solid content. These water contents result in distinct differences in digestate (residue of digestion) property. Wet digestion produces liquid digestates, which should be treated in wastewater facilities. On the contrary, the residue of dry digestion is ‘claylike’. Therefore, dry digestion does not emit any wastewater during digestion.
Using dry anaerobic digestion for treating pig manure, we propose a novel pig farm management system. Figure 2 shows conventional and the proposed pig farming systems. In the conventional system, most of the pig feed is imported from foreign countries. The solid phase of the pig manure is composted and the liquid phase is treated in a wastewater treatment facility. In the proposed one, pig manure can be treated through dry anaerobic digestion with rice straw.
The digested residue is applied to rice fields as fertiliser. In the rice cultivation process, we plan to cultivate forage rice for producing feed in Japan itself. Grain of forage rice is supplied as pig feed and rice straw is mixed with pig manure and then digested. In this manner, pig manure can be recycled into energy and fertilisers without wastewater treatment. In order to verify the feasibility of the proposed system, we carried out lab-scale and field studies.
Anaerobic digestion and agricultural waste
Through anaerobic digestion, agricultural waste can be converted into energy and fertilisers. It has been widely used for treatment of organic wastes. In the application of anaerobic digestion for treatment of pig wastewater, inhibition of the digestion by the high concentration of ammonium is a shortcoming. We also confirmed that no CH4 was produced when dry digestion was carried out by mixing only pig manure and inoculum. To overcome this problem, we mixed rice straw with manure and digesting them.
Through this mixing, ammonium concentration was decreased in the feed material and inhibition of dry anaerobic digestion was prevented. Furthermore, since rice straw is rich in organic carbon, which is substrate for anaerobic digestion, stable and high CH4 production was achieved. Thus, dry anaerobic digestion of pig manure is feasible by mixing rice straw.
How can we treat digestion residue? Digestion residue contains ammonium from pig manure. Therefore, it can be utilised as a fertiliser. Our experimental trial suggested that rice can be grown with the application of the digestion residue, replacing chemical fertilisers. This is significant advantage for farmers towards reducing costs for fertilisers. Moreover, fossil fuel consumption for production of chemical fertilisers can be also reduced.
However, in the rice cultivation, soil is flooded with water, creating favourable conditions for CH4 production in the soil. CH4 is known to be 25 times stronger as a greenhouse gas than CO2. We found that CH4 emission from rice fields applied with the digestion residue was higher than those with chemical fertilisers (Figure 3).
As mentioned above, CH4 production becomes active in an oxygen-free environment. Therefore, we temporarily drained water to supply oxygen into the soil. This water management practice is called intermittent irrigation, and is traditionally used in Japanese rice cultivation; this can mitigate CH4 emission significantly in rice fields with the residue.
As shown in Figure 2, dry anaerobic digestion influences material flow in the pig farming system. Nitrogen flow into the environment is the most important part of this system, because the motivation of this study was wastewater management in pig farming. In the proposed system, pig manure (mixture of dung and urine) is treated by dry anaerobic digestion. Therefore, wastewater containing nitrogen is not produced during the manure treatment process.
However, the digested residue contains nitrogen, and therefore, the risk of nitrogen leaching in the forage rice paddy field should be considered. In the field study, nitrate (NO3−) and nitrite (NO2−) concentrations of the soil water in a rice field with the digestate were much below the Japanese environmental standard of the ground water (10 mg N/L).
One of the reasons for such a low concentration of the NO3− and NO2− is the absence of oxygen in the flooded soil, and ammonium in the digestate could not convert into NO3− and NO2−. NO3− and NO2− would be converted into dinitrogen (N2) gas, because this reaction is active in the absence of oxygen. The low levels of NO3− and NO2− is a characteristic of rice paddy wetlands.
Because rice cultivation is widely practiced in Japan, this novel system is suitable for the Japanese situation. Our study also showed that application of the digestion residue increased NO3− concentration in the soil in an upland condition. Therefore, special attention should be paid when using the digestate in upland fields for vegetable or wheat production.
Our proposed system is for small-scale pig farmers. Therefore, the dry anaerobic digestion system must be as simple and cheap as possible. Major commercial dry anaerobic digesters, such as Hitz Kompogas system and the DRANCO (DRy ANaerobic Composting) system, are widely used for organic waste management. However, these digesters require expensive instruments. Now, we are seeking more simple and cheap means to digest pig manure and rice straw towards sustainable agricultural waste management.
The 2nd International Conference on Pollution Control and Resource Recovery for the Livestock Sector takes place in NUI Galway next month, providing a forum to present, discuss and develop innovative technologies and practices for managing livestock waste and recovering resources. Dr Shohei Riva will present on ‘System stability and pathogen inactivation during dry co-digestion of food waste and pig manure.’ Prof Masaaki Hosomi will host a session on anaerobic digestion.
The research work was supported by the Environment Research and Technology Development Fund (1B-1103 and 1-1404) from the Ministry of the Environment, Japan.
Shohei Riya (1), Sheng Zhou (2), Koki Toyota (3) and Masaaki Hosomi (1)
(1) Department of Chemical Engineering, Faculty of Engineering, Tokyo University of Agriculture and Technology, Tokyo, Japan
(2) Eco-environmental Protection Research Institute, Shanghai Academy of Agricultural Sciences, Shanghai, China
(3) Institute of Symbiotic Science and Technology, Tokyo University of Agriculture and Technology, Tokyo, Japan
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Riya S, Katayama M, Takahashi E, Zhou S, Terada A, Hosomi M. ‘Mitigation of Greenhouse Gas Emissions by Water Management in a Forage Rice Paddy Field Supplemented with Dry-Thermophilic Anaerobic Digestion Residue.’ Water Air and Soil Pollution, 2014; 225. 2118, DOI: 10.1007/s11270-014-2118-3
Zhou S, Zhang J, Zou G, Riya S, Hosomi M. ‘Mass and energy balances of Dry Thermophilic Anaerobic Digestion treating swine manure mixed with rice straw.’ Biotechnology Research International, 2015; 2015:895015-Article ID 895015, doi:10.1155/2015/895015