Biogas-producing characteristics of anaerobic digestion of protected horticulture residues

Abstract: With the large-scale popularization of protected horticulture, many protected horticulture residues, which include vegetable stalks and fruit scrap, followed in recent years; and it is imperative to adopt a method of properly handling the residues. Protected horticulture residues have characteristics of high water content, high volatile solids content, and high biodegradability. If the residues are treated simply as general living garbage, it will lead to not only high costs but also a waste of resources. There are many studies abroad using anaerobic digestion processes to dealing with protected horticulture residues, while the domestic relevant reports are fewer. To obtain more detailed data about the biogas production characteristics of typical protected horticulture residues, and to explore the possibility of the application of Separated two phase anaerobic digestion (STP), in this study, five typical different protected horticulture residues are chosen as research subjects. The characteristics of five typical different protected horticulture residues-cucumber vine, tomato stem and leaves, pakchoi, watermelon vine, and grass are analyzed in this paper. Under a warm fermentation condition (38℃), the biogas production characteristics of these five raw materials are tested. In the condition of 38℃, watermelon vine, which has the best gas production performance, is chosen to test its adaptability of Separated two phase anaerobic digestion (STP), the biogas production characteristics of watermelon vine under two different reflux modes of spray and immersion is comparatively analyzed, and the main emphasis is on the pH variation of of leachate, biogas production, and methane concentration under two different reflux modes. All of these studies afford useful reference for the resource utilization of protected horticulture residues. The results show that the five protected horticulture residues have the same gas production tendency, reach a peak of gas production in the 3~4 days, and then the gas production declines and persists at a high level. Twenty days later, the gas production is low and stable; after 42 days, almost no gas is generated. These five protected horticulture residues have good gas production of anaerobic fermentation characteristics under warm temperature conditions, and all of them could be a good source for biogas fermentation. The sequence of the biogas potential in the five protected horticulture residues in watermelon vine, pakchoi, cucumber vine, tomato stem and leaves, and grass, and their unit dry mass gas production is 508.85, 458.93, 442.41, 410.17, and375.03 mL/g. Protected horticulture residues have good adaptability to Separated two phase anaerobic digestion (STP), which is shown as higher biogas production and higher methane concentrations. The spray mode of leachate is better suited than the immersion mode, and it has more biogas and higher methane concentrations. The total biogas production of the spray mode is 506.39L and its unit dry mass gas production is 481.21 L/kg, which is 10.81 percent higher than the immersion mode, and its methane concentration is basically stable between 53-56 percent, which was 6-12 percent higher than the immersion mode. In addition, the spray mode is generally more suitable for STP because of its greater stability.