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. HNO2) and hydrogen peroxide (H2O2) 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), H2O2 alone (10–80 mg/g TS), and their combinations followed by biochemical methane potential tests. Combined FNA and H2O2 pre-treatment substantially enhanced methane potential of WAS by 59–83%, compared to 13–23% and 56% with H2O2 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.


Figure S1
Confidence regions (95%) of the estimated parameters from the sludge with and without pre-treatment: k slow vs. k rapid .

Figure S2
Confidence regions (95%) of the estimated parameters from the sludge with and without pre-treatment: k slow vs. B 0,slow .

Figure S3
Confidence regions (95%) of the estimated parameters from the sludge with and without pre-treatment: k slow vs. B 0,rapid .

Figure S4
Confidence regions (95%) of the estimated parameters from the sludge with and without pre-treatment: k rapid vs. B 0,slow .

Figure S5
Confidence regions (95%) of the estimated parameters from the sludge with and without pre-treatment: k rapid vs. B 0,rapid .

Figure S6
Confidence regions (95%) of the estimated parameters from the sludge with and without pre-treatment: B 0,slow vs. B 0,rapid .

Figure S7
Biomass specific increase of (A) SCOD, (B) NH 4 + -N and SKN after 24 h pre-treatment of WAS. Error bars show standard errors resulting from triplicate tests. See Table 1 for the pre-treatment conditions shown in abscissa. The SCOD of the WAS in the control with only stirring but without any chemical pre-treatment increased by around 0.05 mg COD/mg VS. In contrast, the SCOD of WAS pre-treated with FNA alone and the combinations increased by 0.10-0.14 mg COD/mg VS, with the highest solubilisation achieved in the combined FNA and H 2 O 2 pre-treatment (1.54 mg HNO 2 -N/L and 30 mg/g TS). Further increasing the H 2 O 2 level did not lead to concomitant SCOD increases. In the case of H 2 O 2 pre-treatment alone, SCOD increased slightly by 0.07 mg COD/mg VS at the dosage of 30 mg H 2 O 2 /g TS, but then substantially decreased to 0.01 mg COD/mg VS at the dosage of 80 mg H 2 O 2 /g TS. The SKN results are consistent with those of SCOD. The decreases in SCOD and SKN at increased H 2 O 2 doses are likely due to oxidation of released organics from cell lysis and EPS matrix disruption 1,2 . However, the NH 4 + results showed a different trend in comparison with those of SCOD and SKN. The NH 4 + concentration in the WAS treated with H 2 O 2 alone increased substantially from 0.0010 mg NH 4 + /mg VS in the control to 0.0047 mg NH 4 + /mg VS at the dosage of 30 mg H 2 O 2 /g TS and thereafter dropped to that of control. This implies that H 2 O 2 was involved in the breakdown of extracellular organics into inorganic fractions at low levels, which accounts for the increase of NH 4 + 3 . The higher levels of H 2 O 2 likely inhibited hydrolytic enzymes, resulting in decreased biological release of NH 4 + 4,5 . No significant increase was observed with FNA pre-treatment alone and with combined pre-treatment, which is potentially attributed to the inhibitory/toxic effects of FNA and/or the by-products of the reactions between FNA and H 2 O 2 on sludge hydrolytic enzymes e.g. protease and/or enzymes responsible for acidogenesis [4][5][6] . Ring vibrations associated with υC-C and υC-OH from aromatic amino acids and nucleotides

Economic analyses of combined FNA and H 2 O 2 pre-treatment for enhancing methane production
A desktop scaling-up study on a full-scale wastewater treatment plant (WWTP) with a population equivalent (PE) of 400,000 and with an anaerobic digester at a hydraulic retention time (HRT) of 20 d was conducted to evaluate the potential economic and environmental benefits of the combined FNA and heat pre-treatment strategy. A system with an annual methane production of approximately 286,000 kg CH 4 was used as a control. The systems with H 2 O 2 pre-treatment alone, FNA pre-treatment alone and combined FNA and H 2 O 2 pre-treatment were designed to obtain an 25%, 60% and 90% increase in methane production (i.e. 357,500, 457,600 and 543,400 kg CH 4 per annum). The methane produced was considered to be combusted in a cogeneration plant in order to produce both power and heat 8 . The costs/benefits associated with H 2 O 2 , FNA, and combined FNA and H 2 O 2 pre-treatment were estimated and compared, as summarized in Table S2. Annual extra power production from methane conversion (compared to the control system) (kwh/y)

367,000
Annual cost associated with WAS pre-treatment ($/y) 154,000 Annual reduced WAS transport and disposal cost (compared to the control system) ($/y) 54,000 Annual extra obtained benefit (compared to the control system) due to the extra heat and power generation ($/y) 124,000

Annual saving (compared to the control system) ($/y) -24,000
System with FNA pre-treatment Methane production (kgCH 4 /y) 345,000 WAS fed to the anaerobic digester (kg VS/y) 2,420,300 WAS removal in the anaerobic digester (on a dry VS basis) 41% Biodegradable COD (bCOD) concentration in the anaerobic digestion liquor (mg/L)  Annualised mixing cost in the pre-treatment reactor ($/y) 1,200 Annualised power consumption for the addition (to the pre-treatment reactor) of the pre-treated WAS (kwh/y)

4,800
Annualised power cost for the addition (to the pre-treatment reactor) of the pre-treated WAS ($/y) 580 Annual extra heat production from methane conversion (compared to the control system) (kwh/y) 1,520,000 Annual extra power production from methane conversion (compared to the control system) (kwh/y) 1,210,000 Annual cost associated with WAS pre-treatment ($/y) 193,000 Annual reduced WAS transport and disposal cost (compared to the control system) ($/y) 186,000 Annual extra obtained benefit (compared to the control system) due to the extra heat and power generation($/y)

410,000
Annual saving (compared to the control system) ($/y) 403,000 Annual saving (compared to the FNA pre-treatment system) ($/y)

46,000
Annual saving (compared to H 2 O 2 pre-treatment system) ($/y) 427,000 a Refer to Metcalf and Eddy. 9 b http://www.alibaba.com/ c Refer to Carballa, et al. 8 . d Refer to Law, et al. 10 . e The capital cost of the bioreactor was estimated using the following equation. 11 493601×(V/1000) 0.7202 , where V=volume of the reactor.