Nitrate-rich wastewater discharged from a bio-trickling filter can be reused as a moisture conditioning agent for organic waste composting

Abstract Biotrickling filter (BF) is an effective ammonia-contain odour purification device for organic waste composting. However, proper disposal of nitrogen-rich trickling liquid (TL) discharged from the BF is consider ‘the last-mile problem’ for the application of this coupled system. Although our previous study demonstrated that reusing a TL as moisture conditioning agent for a composting system could promote compost maturity and nitrogen content, the exploration of the intrinsic promotion mechanism was obscured by the complexity (i.e., multiple forms of nitrogen contained) of the TL. Thus, a simulated TL (STL, pH =7.18) containing exclusively nitrate nitrogen was reused to condition the moisture of composting in this study. Effects of STL addition phase [adding STL in mesophilic (days 0-9), thermophilic (days 4-13), and cooling period (days 13-22)] on the performance of composting were explored. Results revealed that the organic degradation was partly promoted by adding STL in thermophilic or cooling period, but mightily inhibited in mesophilic period. Furthermore, adding STL in thermophilic period stimulates the denitrification of nitrate-N into dinitrogen at high temperature, whereas most of added nitrate-N was retained in compost product when adding STL in cooling period. Eventually, STL addition effectively increased total nitrogen (TN) by 2%–9%, especially the nitrate nitrogen by 10-11 times. Notably, adding STL rarely affected nitrous oxide and greenhouse gas emission. Therefore, STL can be reused to condition the moisture of compost in a proper composting phase (i.e., thermophilic period) as defined in term of the enhancement of TN and nitrate content of compost product.