Sulfur from biogas desulfurization: Fate of S during storage in manure and after application to plants

Summary

Residues from hydrogen sulfide (H2S) removal in biogas filters contain sulfur (S) in various forms (sulfate, sulfide,
elemental S) that, if properly stored, is potentially valuable as crop fertilizer. We investigated 1) the turnover
of the S compounds from filter materials during storage in untreated and digested cattle manure (CM), and 2) the
S fertilizer replacement value (SFRV) of the filter materials applied in pure form or mixed manure with and without
storage. The S filter materials from four H2S removal processes (biological and physical-chemical) containing
mostly sulfate and/or elemental Swere added to untreated CMor digested CMand stored at 10 °C for sixmonths.
Afterwards, a pot experiment was established to assess the S availability in an oil-seed rape (Brassica napus) crop.
Microbial reduction of sulfate into sulfide took place rapidly after 69 days storage of untreated CM. A lower reduction
rate was observed in digested CM mixtures. After six months, 68% and 32% of the initial sulfate content were
still present in mixtures containing the S filter materials from biological desulfurization with digested CM and untreated
CM, respectively. Sulfate reduction was inhibited for 120 days when digested CM was mixed with S saturated
solution from an ash filter, probably due to high pH (≥8.2) and redox potential (>−100 mV) levels.
Oppositely, elemental S was immediately and simultaneously both reduced and oxidized. Relatively low losses
of total S were observed during the present storage conditions. Despite S turnover, the SFRV of CM and digested
CM significantly increased from 15–19% (of total S applied) to 56–90% when S filter materials were added. The
storage of S filter materials in digested manure reduced the risk of sulfide production and potential S volatilization.
The S filter materials were a valuable source of plant-available S.