Effects of Residual Hypochlorite Ion on Methane Production during the Initial Anaerobic Digestion Stage of Pig Slurry

Carbon fixation via volatile fatty acids recovery from sewage sludge through electrochemical-pretreatment-based anaerobic digestion

Capturing the carbon in volatile fatty acids (VFA) produced from the anaerobic digestion (AD) of sewage sludge has the potential to not only provide economic benefits but also reduce greenhouse gas production. This study demonstrates a chemical-free method to collect VFA from an AD instead of methane that involves electrochemical pretreatment (EPT) of sludge. Experimental results show that applying 15 V EPT for 45 min enhances acidogenesis and selectively inhibits methanogenesis, leading to a substantial VFA accumulation (2563.1 ± 307.9 mg COD/L) and achieving 2.5 times more carbon fixation than via methane production. Interfacial thermodynamic analysis shows that EPT induces a decrease in both the repulsive electrostatic energy (from 152.9 kT to 12.2 kT) and the energy barrier (from 57.0 kT to 2.6 kT) in the sludge, leading to increased sludge aggregation and entrapment of microorganisms. Molecular docking sheds lights on how the methanogens interacts with the organic matter released from EPT (e.g., alanine-tRNA ligase), showing that these interactions potentially interfere with the proteins that are associated with the activities of the methanogens and the electron transfer pathways, thereby impeding methanogenesis. Integrating EPT into AD therefore facilitates the recovery of valuable VFA and the capture of carbon from freshwater sludge, providing notable economic and environmental benefits in sewage sludge treatment.

Sewage sludge digestion beyond biogas: Electrochemical pretreatment for biochemicals

Low economic gains from biogas drive research on shifting to volatile fatty acid (VFA) production during anaerobic sludge digestion. pH control and methanogenesis inhibition are widely used strategies for VFA production via anaerobic digestion of sludge. However, these strategies require perpetual dosing of chemicals, increasing cost and operation complexity. Here, we applied electrochemical pretreatment (EPT) (12 V/30 min) for VFA production during anaerobic sludge digestion. The underlying mechanisms of the VFA production induced by EPT were explored systematically through analyses of the changes in the EPT operation parameters, the sludge characteristics, and the microbial community structure and functional enzymes involving in the subsequent sludge digestion. EPT with carbon-based electrodes selectively inhibited methanogenesis by down-regulating heterodisulfide reductase without affecting enzymatic acidogenesis and hydrolysis, resulting in accumulation of VFAs (up to 389±12 mg acetic acid equivalent/L). Propionate and acetate were, respectively enriched to 89 and 75% of the total VFAs after carbon- and graphite- EPT. Titanium-EPT produced lower levels of VFA; instead, biogas yield increased by ∼20%. We anticipate that EPT will advance VFA recovery from diverse organic wastes to meet the global challenge of resource supply and waste management.

Treatment of petroleum oil spill sludge using the combined ultrasound and Fenton oxidation process

In this paper, advanced oxidation process (AOP) combining ultrasound (US) and Fenton's process was proposed for the treatment of total petroleum hydrocarbons present in oil spill sludge. The effect of several parameters like pH, ultrasonic power, weight ratio of hydrogen peroxide to iron [H₂O₂/Fe²⁺], Fenton reagent dosage, addition of salts and contact time were analyzed for the reduction of Petroleum Hydrocarbons (PHCs) in terms of hydrocarbon fractions (nC7-C10, nC11-C20). Chemical characterization of oil spill sludge was analysed by gas chromatography- mass spectrum (GC-MS) Elemental analyser, Fourier Transform Infra Red (FT-IR) Analyser and particle size analyser. Experiments were conducted for identifying the wide range of hydrocarbons fractions (nC7-C10, nC11-C20 and nC21-C30). Results shown that maximum solubilisation and PHC removal rate of up to 84.25% could be achieved at a pH of 3.0, sludge/water ratio of 1:100, ultrasonic power of 100 W with 40-50% ultrasonic amplitude, a H₂O₂/Fe²⁺ weight ratio of 10:1, and an ultrasonic treatment time of 10 min. The lower and medium fractions (nC7-C10, nC11-C20) were amenable to degradation due to ultrasound treatment compared to the heavier carbon fraction (nC21-C30). The study concludes that the combined sono-Fenton (SF) process significantly enhanced the degradation of oil spill sludge as compared to ultrasound treatment and Fenton oxidation alone. The enhanced solubilisation achieved by US alone is highly beneficial when we couple this with biodegradation which will be greatly facilitated by the enhanced solubility.

The effects of cleaning/disinfecting agents on biogas production in the anaerobic digestion of cow dung slurry

Cleaning/disinfecting agents (CDA) are compounds known to affect the growth of microbes and could have impacts on biogas production in a digester. In this study, three commonly used CDAs (harpic, hypo and izal) in livestock pens and slaughter houses were applied at different digestion stages and concentrations to cow dung (CD) slurry, with the aim of determining their effects on biogas production. Three application stages (early addition, late addition and graduated addition) were carried out for each CDA, and a CD treatment with no CDA applied was set up as the control. The results showed that application stage had different effects on the parameters of CD monitored during the digestion. Significant (p ≤ .05) effect was recorded on: temperature of izal + CD treatment, pH of harpic + CD and hypo + CD treatments, total bacterial count of hypo + CD and izal + CD treatments and biogas of harpic + CD, hypo + CD and izal + CD treatments. Early addition and increased concentration of hypo appeared toxic to anaerobic bacteria, whereas early addition of izal seemed not to affect their development. While early addition and increased concentration of harpic and hypo inhibited biogas production, the contrary was observed with izal. At the end of digestion, biogas yield (% of control) in the CDA + CD treatments ranged between 8% and 69.4% (harpic + CD), 5.4-23.5% (hypo + CD) and 16.1-52.5% (izal + CD). Among the CDAs used, hypo had the highest inhibitory effect on biogas production from CD slurry. The introduction of harpic at fourth week of digestion showed the least inhibitory effect.

Effects of Benzalkonium Chloride, Proxel LV, P3 Hypochloran, Triton X-100 and DOWFAX 63N10 on anaerobic digestion processes

In this study, the individual and synergistic toxicity of the following xenobiotics: Benzalkonium Chloride (BKC), Proxel LV (PRX), P3 Hypochloran (HPC), Triton X-100 (TRX), and DOWFAX 63N10 (DWF), on anaerobic digestion (AD) process, was assessed. The experiments were performed in batch and continuous (up-flow anaerobic sludge blanket, UASB) reactors with biochemical-industrial wastewater, as substrate. In batch experiments, half-maximal inhibitory concentrations (IC50) for the tested xenobiotics were found to be 13.1, 1003, 311.5 and 24.3 mg L(-1) for BKC, PRX, DWF and TRX, respectively while HPC did not affect the AD process. Furthermore, the xenobiotics mixture tested did not present any synergistic inhibitory effect on the AD process. In continuous experiments, BKC and xenobiotics' mixture induced even stronger (more than 85%) of inhibition, expressed as IC50, compared to the levels observed from the batch reactors. Oppositely, TRX showed no inhibition in continuous mode, while inhibition was detected at batch mode.