Digestate application in landfill bioreactors: Nitrogen removal and digestate stabilization

Anaerobic digestion (AD) of organic fraction of municipal solid waste (OFMSW) or food waste has aroused attention increasingly as it can achieve both environment and economic benefits. The by-product from AD, the digestate, still rich in putrescible components and nutrients, is mainly considered for reutilization as a bio-fertilizer or a compost after its treatment. However, the limitations exist for digestate agricultural application because of nutrients surplus, fertilization seasons, high cost of long-distance transportation and low market acceptance. Besides, digestate from OFMSW and food waste needs to be disposed as “waste” when it does not comply with the regulation requirements. The chapter 1 of the work is an overview on the digestate characterizations, regulations of agricultural use and application techniques. The concept of Back to Earths Alternatives (BEAs) introduced in the literature review involves that the residues, after appropriate treatment, "should be returned to their non-mobile state as they were before they were extracted from the ground to be used as raw materials ". The specific aim of BEA is to bring elements and materials back into the environment either as soil amendments or functional components for landfills. The novel concept can deal with the two dilemmas of digestate management and achieve the close of material loop. Aim of the Chapter 2 was to develop an alternative option to use digestate to enhance nitrified leachate treatment through a digestate layer in landfill bioreactors. Two identical landfill columns (Ra and Rd) filled with the same solid digestate were set and nitrified leachate was used as influent. Nitrate nitrogen removal efficiency of 94.7% and 72.5% were achieved after 75 days and 132 days, respectively. These results suggest that those solid digestates could be used in landfill bioreactors to remove the nitrogen from old landfill leachate. The study of Chapter 3 aims to evaluate the application of digestate in simulated landfill columns to enhance nitrogen conversion via short-cut nitrogen removal. Two identical simulated landfill reactors filled with solid digestate were setup and partial nitrified leachate was fed through the columns. After 109 days of operation, the average nitrite nitrogen and nitrate nitrogen removal efficiencies were 92.6% and 85.8%, respectively. The high concentrations of nitrite (1004 mg-N/L) did not significantly inhibit methane production during denitrification in the digestate-added landfill columns. Aim of the Chapter 4 was to use a digestate layer in aged refuse bioreactors to enhance the denitrification capacity of aged refuse, stabilize digestate and mitigate the ammonia emission from digestate leaching with leachate recirculation. Six identical landfill columns filled with 0% (R0), 5% (R5), and 15% (R15) of solid digestate above aged refuse, were set and nitrified leachate was periodically fed and recirculated. Nitrate removal rate in R5 and R15 were 3.4 and 10.0 times higher than that of control (no digestate added). The results showed that instead of land use, solid fraction of digestate could be exploited as an inexpensive functional layer embedded in old landfill site to enhance denitrification capacity and achieve digestate stabilization with little ammonia leaching from digestate. In conclusion, solid digestate was effective in reducing the oxidized nitrogen (nitrate or nitrite) content of nitrified leachate or partial nitrified leachate. Meanwhile, the organic matter content of the solid digestate was degraded under anaerobic conditions with low methane emission. Solid digestate with or without further treatment can be utilized as an engineered landfill biocover. Besides, solid digestate could be applied in external reactors to ex-situ treat nitrified leachate. At last, solid digestate can be used as functional layer embedded landfill to removal nitrogen in those old landfills with low denitrification capacities.