Quality of anaerobic compost from paper mill and municipal solid wastes for soil amendment

A previous work has shown the technical and economic feasibility of the solid substrate anaerobic digestion (DASS) of municipal and non-hazardous industrial solid wastes. However, the anaerobic compost (AnC) quality for direct application as a soil amender or other alternative uses are issues that have received scarce attention. This research aimed at reviewing the advances in two areas: anaerobic compost quality determination, and its post-treatment by aerobic composting. A factorial experiment 4x2x2 was run. The factors were feedstock type (4 mixtures of food waste, FW, and lignocellulosic fraction, LG: 100%FW, 67%FW-33%LG, 33%FW-67%LG and 100%LG or FS1, FS2, FS3 and FS4 respectively), temperature (35 and 55°C), and mass retention time (MRT, 16 and 23 days). The LG fraction consisted of paper mill sludge cake. Anaerobic compost from DASS reactors operated at 23 day/35°C was fed to lab scale semi-continuous aerobic composters (5 day and 10 day MRT, 35 and 55°C). The AnC coming from feedstocks with increasing proportion of LG fraction (FS3 and FS4) gave the lowest chemical oxygen demand (COD), biochemical oxygen demand (BOD), volatile organic acids (VOA) and total ammonia nitrogen (TAN) concentrations in the extract and the highest germination indices (GI). The AnC from thermo-digesters showed a lower quality than those from mesophilic reactors (presumably associated to higher contents of VOA and TAN in the extracts), while AnC from digesters at 23 day-MRT had a better quality than those from reactors operated at 16 day-MRT. Overall, AnC generated in reactors fed with FS3 and FS4, at 35°C and 23 day-MRT showed the highest quality. Heavy metal concentrations in all the AnC were lower than the maximum levels indicated in USA and European compost quality standards. However, high total oxygen uptake (UAD), moderate-to-high concentrations of VOA and GIs under 60% indicated that the AnC was not suitable for direct use as a soil improver. Regarding the aerobic post-composting, operation at longer MRT (10 day) and 55°C gave aerobic composts of better quality than those coming from 5 day-MRT composters. Aerobic post-composting caused considerable reductions of TAN, VOA, UAD, immediate oxygen demand rate, and increased compost GI up to approximately 100%.