Improvement of pulp-mill wastewater for anaerobic digestion
Abstract
Anaerobic digestion is the most cost-effective biological-treatment process available for generating energy. However, the use of certain pulp-mill wastewater streams for anaerobic digestion is not commonly implemented, mainly due to toxic and recalcitrant compounds. By using certain pre-treatment methods, most of the toxic and recalcitrant compounds can be removed, which improves the suitability of these streams for anaerobic digestion. In this work, sulphite evaporator condensate (SEC) was pre-treated and evaluated at the substrate level, after which the pre-treatments were evaluated using bench-scale upflow anaerobic sludge blanket (UASB) reactors. Characterisation of the SEC showed a high chemical oxygen demand (COD) concentration (19 000 mg/L) and was largely composed of volatile fatty acids (VFAs), furfural, lactic acids, polyphenols and lignosulphonate. Additionally, the wastewater consisted of a high concentration of carbonate alkalinity and had a low pH. The high concentration of VFAs, furfural, lactic acids and carbonate alkalinity were favourable for methane production and the stability of anaerobic digestion. Polyphenols and lignosulphonate are inhibitory to anaerobic digestion and high sulphate concentrations may reduce methane production. The pre-treatment methods were therefore focused on removing the polyphenols and lignosulphonate, without affecting the potential substrates for anaerobic digestion. Laccases and coagulants were used during the pre-treatments due to the effectiveness on phenol-containing compounds, with few side-effects. Laccase11 was the best-performing enzyme and increased the molecular weight of lignosulphonate by 60%, and the removal was 34% and 33% for lignosulphonate and polyphenols, respectively. Polydiallyldimethylammonium chloride (PolyDADMAC) was the best-performing coagulant and removed 62% lignosulphonate and 57% polyphenols. However, PolyDADMAC also removed 50% of the VFAs. Three batch pre-treatments were performed on the SEC, which were used as feed to the reactors. The SEC in each batch was adjusted to a pH of 7 and treated with PolyDADMAC, Laccase11 or a control. Characterisation of the batches revealed that Laccase11 removed more than 30% and PolyDADMAC more than 50% of the inhibitory compounds from the SEC. The biological oxygen demand did not change significantly. Additionally, PolyDADMAC removed 34% sulphate. These pre-treatments enabled higher volumetric hydraulic loading (VHL) rates to the respective reactors to achieve the same organic loading rate (OLR) as the control. Treatment of SEC with PolyDADMAC was the most effective, allowing the VHL to increase by 1.13 times to obtain the same OLR as the control. At all OLRs tested, the PolyDADMAC reactor removed the most COD and had the highest specific methane yield. At the highest OLR where all three reactors were stable (13kgCOD/m³d), the PolyDADMAC reactor had a COD removal efficiency of 60%, a specific methane yield twice the value of the control and the methane produced was more than double. At the highest OLR tested (16 kgCOD/m³d), the COD removal efficiency of the reactors using Laccase11- and PolyDADMAC-treated effluent was below 55%, with the PolyDADMAC reactor performing 7% better. At the same OLR, the reactor fed with the control batch went "sour", with less than 28% COD removed. Therefore, PolyDADMAC was the most effective pre-treatment and may be a financially feasible option to enhance anaerobic digestion.
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