Biogas from anaerobic processes mainly consists of CH4 and CO2. But H2S in ppm level can also be found along with CH4 and CO2. What causes the gas to be produced?
Anaerobic conditions may promote the activity of sulfate reducing bacteria. If the source solution contains sulfate, it may be used by these micro-organisms as an electron acceptor, thus reducing it to H2S.
As Odded has indicated you, under anaerobic conditions usually sulphate is reduced into H2S. This can reduce the amount of CH4 produced for the methanogenic bacteria since in the production of H2S in addition to sulphate the same compounds used in the production of CH4 are used.
Thank you all for the inputs. I have received more information from my friends working in industry. According to them, the generation of H2S from the palm oil mill effluent (under anaerobic conditions) is mainly due to the presence of sulfur in fruit bunch of palm oil. Therefore, the refining process of crude palm oil will cause the wastewater to contain sulfur component which later will be used by microorganisms to produce H2S.