The synergy of combined free nitrous acid and Fenton technology in enhancing anaerobic digestion of real sewage waste activated sludge

Background Recently, free nitrous acid (FNA) pre-treatment of sewage waste activated sludge has been introduced as an economically attractive and environmentally friendly technique for enhancing methane production from the anaerobic digestion process. Fenton pre-treatment of sewage sludge, as an advanced oxidation process, has also been introduced as a powerful technique for methane improvement in a couple of studies. This study, for the first time, investigates the synergy of combined FNA and Fenton pre-treatment technologies in enhancing the methane production from the anaerobic digestion process and reducing waste sludge to be disposed of. Actual secondary waste activated sludge in laboratory-scale batch reactors was used to assess the synergistic effect of the pre-treatments. The mechanisms behind the methane enhancement were also put into perspective by measuring different microbial enzymes activity and solubilisation of organic matter. Result This study revealed that the combined pre-treatments release organic matter into the soluble phase significantly more than the bioreactors pre-treated with individual FNA and Fenton. For understanding the influence of pre-treatments on solubilisation of organic matter, soluble protein, soluble polysaccharide and soluble chemical oxygen demand (SCOD) were measured before and after the treatments and it was shown that they respectively increased by 973%, 33% and 353% after the treatments. Protease and cellulose activity, as the key constituents of the microbial community presenting in activated sludge, decreased considerably within the combined pre-treatments (42% and 32% respectively) and methane production enhanced by 43-69%. Furthermore, total solids and volatile solids destruction improved by 26% and 24% at the end of anaerobic digestion, which can reduce transport costs of sludge and improve the quality of sludge for application in farms and forests. Conclusions The results obtained from the experiments corroborate the synergic effect of the combined FNA and Fenton pre-treatment technologies in degrading the organic and microbial constituents in waste activated sludge, which improved methane production accordingly. This is of paramount importance because the total costs of wastewater treatment plants operation and greenhouse gas emission from sludge treatment and disposal processes would reduce considerably, which pave the way for the implementation of these technologies.