Combined free nitrous acid and hydrogen peroxide pre-treatment of waste activated sludge enhances methane production via organic molecule breakdown

This study presents a novel pre-treatment strategy using combined free nitrous acid (FNA i.e. HNO(2)) and hydrogen peroxide (H(2)O(2)) to enhance methane production from WAS, with the mechanisms investigated bio-molecularly. WAS from a full-scale plant was treated with FNA alone (1.54 mg N/L), H(2)O(2) alone (10–80 mg/g TS), and their combinations followed by biochemical methane potential tests. Combined FNA and H(2)O(2) pre-treatment substantially enhanced methane potential of WAS by 59–83%, compared to 13–23% and 56% with H(2)O(2) pre-treatment alone and FNA pre-treatment alone respectively. Model-based analysis indicated the increased methane potential was mainly associated with up to 163% increase in rapidly biodegradable fraction with combined pre-treatment. The molecular weight distribution and chemical structure analyses revealed the breakdown of soluble macromolecules with the combined pre-treatment caused by the deamination and oxidation of the typical functional groups in proteins, polysaccharides and phosphodiesters. These changes likely improved the biodegradability of WAS.