Solid anaerobic digestion batch of bio-waste as pre-treatment for improving amendment quality: The effect of inoculum recirculation

•Integrated SADB with post composting of organic waste was compared with composting.•Not inoculated and inoculated SADB pre-treatment were investigated.•Higher TOC, HA, FA and DH were detected for the amendment from not inoculated SADB with post composting.•Biogas generation from not inoculated SADB was 267 NL/kgVS instead of 300 NL/kgVS from inoculated SADB.

The effect of solid anaerobic digestion batch (SADB) on bio-waste performed with and without inoculum on the quality of the final amendment was investigated by means of determining the content of organic carbon, humic and fulvic acids and the degree of humification. Two different processes were compared: composting and SADB with post-composting. Six parallel tests were performed. In three of these tests the SADB was inoculated mixing the bio-waste with the digestate from the previous run in a 1:1 ratio by weight. The amendment obtained by the SADB with post-composting treatment, in which the SADB was not inoculated, had an organic carbon content ranging from 15.5% TS to 30.3% TS resulting from 1% up to 14% higher than that of the corresponding composting processes. Similar results were achieved for the degree of humification. On the other hand SADB in which the inoculum was used generated about 300 NL/kgVS of biogas instead of about 267 NL/kgVS for non-inoculated runs.