Using Biogas Plant Slurry to Optimize pH Dynamics and Control Nitrification in Hydroponic Systems

Abstract

The biogas digestate liquid slurry used in hydroponic farming effectively brings in a large amount of organic matter along with several complementary nutrients, improving nutrient quality and enhancing microbial activities within the hydroponic medium to boost plant growth and productivity. Liquid slurry from biogas digestate, applied after liquid-solid separation, supports sustainable farming by recycling organic matter and nutrients derived purely from production wastes. Hydroponics: This process allows continuous plant growth without being affected by seasonal or environmental conditions, making it particularly beneficial in areas where agricultural land is too limited, such as small urban spaces. Growing soilless vegetables and recycling liquid slurry from biogas plants as fertilizers is highly advantageous for food production and sustainable organic waste management. We tested this by growing lettuce (Lactuca sativa capitata) with biogas digestate in a hydroponic nutrient film technology system, using cabbage (Brassica oleracea var.) as the test crop. We removed large amounts of ammonium from the digestate through nitrification, either outside the system or within an integrated hydroponic culture system using moving bed biofilm reactors. We looked at differences in pH, crop growth, shoot water content, and shoot mineral content between treatments with different amounts of nitrification and digestate input diameters. The results indicated that increasing the growing time by about one week (equivalent to 20%) resulted in yields in biogas-slurry-based hydroponics comparable to those obtained with synthetic fertilizers in conventional hydroponics. The addition of digestate with automatically adjusted pH showed no significant difference from the mineral fertilizer reference system, as evidenced by the shoot dry weight. However, when considering how well they fit into a circular system, the benefits become clearer. When designing a hydroponic system that uses liquid slurry from biogas plants as fertilizer, it is critical to consider the amount of ammonium and the pH at which the slurry works best. Additionally, it is worth thinking about the moment when nitrogen changes form.