Composting of solids separated from anaerobically digested animal manure: Effect of different bulking agents and mixing ratios on emissions of greenhouse gases and ammonia

We investigated the effects of bulking agents (BA) and mixing ratios on greenhouse gas (GHG) and NH3 emissions from composting digested solids (DS), separated from anaerobically digested manure and other bio-wastes, in small-scale laboratory composters. BA evaluated were plastic tube pieces (PT), woodchips (WC), bio-char (BC), barley straw (BS) and lupin residues (LR) and were included at a DS:BA of 3:1 or 6:1, resulting in nine treatments: CTDs (control, DS only), PT3:1, PT6:1, WC3:1, WC6:1, BC3:1, BC6:1, ES3:1 and LR3:1. Depending on treatment, C losses via CO2 and CH4 emissions accounted for 41.2 -65.3 g C kg(-1) initial total solids (TS) and 4.4-191.7 mg C kg(-1) TS (8.4-16.1% and 0.001 -0.05% of initial total-carbon), respectively, while N losses as N2O and NH3 emissions comprised 2.1-13.6 mg N kg(-1) TS and 2.7-4.8 g N kg(-1) TS (0.01-0.04% and 9.1-13.0% of initial total-nitrogen), respectively. Most of the CH4 emissions occurred during the thermophilic temperature phase, which had little or no effect on N2O emissions. BS addition to DS resulted in the lowest cumulative NH3-N and N2O-N losses. BC was as effective as BS in reducing cumulative NH3-N losses, but had non-significant effect on CH4-C emissions. Decreasing the mixing ratio from 6:1 to 3:1 reduced losses of CH4-C and N2O-N (except for BC) without any increase in NH3-N losses. BC and BS proved most effective in reducing emissions of total GHG (as CO2-equivalents). Composting of DS with C-rich BA can thus be an effective means of conserving N in DS, while also reducing GHG emissions. (c) 2014 IAgrE. Published by Elsevier Ltd. All rights reserved.