Effect of poly aluminum chloride on dark fermentative hydrogen accumulation from waste activated sludge

Poly aluminum chloride (PAC), an inorganic coagulant being accumulated in waste activated sludge (WAS) at substantial levels, are generally thought to inhibit WAS anaerobic fermentation. However, its effect on dark fermentative hydrogen accumulation has not been documented. This work therefore aimed to explore its effect on hydrogen accumulation and to elucidate the mechanism of how PAC affects hydrogen accumulation. Experimental results showed that with an increase of PAC addition from 0 to 20 mg Al per gram of total suspended solids (TSS), the maximal hydrogen yield from alkaline fermentation (pH 9.5) increased from 20.9 mL to 27.4 mL per gram volatile suspended solids (VSS) under the standard condition. Further increase of PAC to 30 mg Al/g TSS didn't cause a significant increase of hydrogen yield (p > 0.05). The mechanism explorations revealed that although PAC reduced the total short-chain fatty acid (SCFA) production, this reduction was mainly enforced to propionic acid fermentation type, which did not contribute hydrogen production. PAC suppressed all the microbial processes relevant to anaerobic fermentation to some extents, but its inhibition to hydrogen consumption was much severer than that to hydrogen production. Illumina Miseq sequencing analysis revealed that PAC did not affect the populations of SCFA and hydrogen producers, but the two hydrogen consumers, Acetoanaerobium and Desulfobulbus, were almost washed out by PAC. Among the three types of Al species present in the anaerobic fermentation systems, Ala (monomeric species) significantly affected the maximal hydrogen production potential while Alb (medium polymer species) and Alc (species of sol or gel) posed impacts on hydrogen production rate and the lag time.