What critical factor determines the suitability of leachate for recirculation within a landfill?

The critical factor determining the suitability of leachate for recirculation within a landfill is its chemical composition, specifically the concentrations of inhibitory substances. Leachate recirculation involves reintroducing collected leachate back into the landfill mass to accelerate waste decomposition and biogas production. However, high concentrations of certain compounds can inhibit the microbial activity responsible for breaking down the waste. These inhibitory substances can include heavy metals, ammonia, and certain organic compounds. If these substances are present in excessive concentrations, they can negatively impact the landfill's biological processes, leading to reduced gas production, increased leachate strength, and potential liner damage. Therefore, a thorough analysis of the leachate's chemical composition is crucial to determine its suitability for recirculation, ensuring that it will enhance rather than hinder the waste stabilization process. Specifically, ammonia levels above a certain threshold can be toxic to methanogens, the microorganisms responsible for methane production.