H2S emission in sludge conditioning with different inorganic salt coagulants and its relationships with sludge properties

Abstract

Inorganic coagulants are always used to pre-condition wastewater sludge prior to high-pressure processing since their hydrolyzed products can function as flocculants and skeleton builders to support floc structure and reduce sludge compressibility. In this study, the sludge conditioning efficiency of four representative inorganic coagulants (FeCl₃, FeSO₄, Al₂(SO₄)₃ and polyaluminum chloride (PAC)) was evaluated based on their specific resistance to filtration (SRF). Hydrogen sulfide (H₂S) release and its relationships with sludge properties, such as solution chemistry conditions and extracellular polymeric substances (EPS) under chemical conditioning, were also studied. It was found that the amounts of H₂S released with four inorganic coagulants were as follows: H₂S_FeCl3 > H₂S_Al2(SO4)3 > H₂S_FeSO4 > H₂S_PAC. The amount of H₂S released increased gradually with dosage of inorganic coagulants added, and the amount of H₂S released was a maximum when the FeCl₃ dosage was 10% (g g⁻¹ dry solid). Inorganic coagulant conditioning resulted in acidification of the sludge system, which was the major mechanism of H₂S emission. Meanwhile, S²⁻ could form precipitates with Fe³⁺, Al³⁺, and Fe²⁺ and inhibit H₂S release. Pearson correlation analysis showed that there was a strong negative correlation between H₂S concentration and sludge pH. In addition, H₂S concentration correlated well with humic acid, fulic acid and protein-like substances in the soluble EPS fraction under inorganic coagulant conditioning.