Abstract
To evaluate the feasibility of swine manure treatment by a proposed Dry Thermophilic Anaerobic Digestion (DT-AD) system, we evaluated the methane yield of swine manure treated using a DT-AD method with rice straw under different C/N ratios and solid retention time (SRT) and calculated the mass and energy balances when the DT-AD system is used for swine manure treatment from a model farm with 1000 pigs and the digested residue is used for forage rice production. A traditional swine manure treatment Oxidation Ditch system was used as the study control. The results suggest that methane yield using the proposed DT-AD system increased with a higher C/N ratio and shorter SRT. Correspondently, for the DT-AD system running with SRT of 80 days, the net energy yields for all treatments were negative, due to low biogas production and high heat loss of digestion tank. However, the biogas yield increased when the SRT was shortened to 40 days, and the generated energy was greater than consumed energy when C/N ratio was 20 : 1 and 30 : 1. The results suggest that with the correct optimization of C/N ratio and SRT, the proposed DT-AD system, followed by using digestate for forage rice production, can attain energy self-sufficiency.
Highlights
The number of scaled pig farms in Asia has greatly increased in recent years, while the disposal methods of swine manure are relatively underdeveloped
Different levels of biogas were produced at different C/N ratios after 80 days following the start of the experiment, with an solid retention time (SRT) of 80 days (Figure 2(a))
Treatments with C/N ratios greater than 20 produced relatively high biogas yields, mostly greater than 400 mL/g VSadded. When these higher levels are compared with CN10, the benefit of adding more rice straw to assist in anaerobic digestion of swine manure is clear, with increases up to approximately 2 and 3 times the average biogas production of CN20 (386 ± 139 mL/g VSadded) and CN30 (474 ± 99 mL/g VSadded)
Summary
The number of scaled pig farms in Asia has greatly increased in recent years, while the disposal methods of swine manure are relatively underdeveloped. China accounts for approximately 45–50% of the global pig production during the past decade [1, 2]. This has resulted in a significant increase in pig farm wastewater discharge, which has become an important source of water body pollution [3, 4]. During the process of industrial treatment of swine manure, the solids and liquid are first separated [5], followed by further treatment using both liquid and solid processes. Some of the nutrients contained in swine manure cannot be recovered and are discharged with the wastewater treatment plant effluent. Organic matter contained in the swine manure cannot be efficiently recovered as an energy source
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