Chemolithoautotrophic elimination of H2S from biogas in a pilot-scale biotrickling filter for agricultural applications

Abstract

Hydrogen sulfide (H2S) removal is the single most essential treatment step for most biogas applications. This study compiles an exploratory analysis of a pilot-scale biotrickling filter (BTF) for H2S removal under anoxic conditions to evaluate its performance. The system utilized real biogas and diluted digestate from an agricultural biogas plant as the trickling liquid. The process relies on the biological degradation of H2S by chemolithoautotrophic bacteria naturally present in the digestate. The pilot plant was operated continuously for 69 days showing stability under H2S inlet load (IL) variations (12.9 and 69.5 gS-H2Sm−3h−1). The influence of pH on process performance was evaluated by operating the BTF without pH regulation (6.8 ± 0.24) and with pH control (7.7 ± 0.25). An H2S removal efficiency (RE) above 99 % (maximum elimination capacity = 66.3 gS-H2Sm−3h−1) was achieved with and without pH control, although pH control led to higher process stability. Two different nitrate salts (KNO3 and Ca(NO3)2) were used as electron acceptors, and their effects on the process efficiency were assessed. No difference in process performance was observed between both salts. The conversion of S-H2S to S-SO42– was on average 81.6 ± 15.1 %, and the mean denitrification rate was 1.6 ± 0.6 mgN-NO3–L−1h−1. The chemical composition of the trickling liquid, analyzed after 31 days of operation, was compared to fresh diluted digestate to assess its potential as a crop fertilizer. Analysis indicated enrichment of the trickling liquid with sulfate, elemental sulfur, and other components such as ammonium. These findings suggest that the trickling liquid remains viable for continued use as a fertilizer.