Improvement of biogas productivity from distillery wastewater by partial potassium reduction pretreatment using two-step microfiltration and nanofiltration

The main purpose of the current study was to employ filtration pretreatment to lower the K concentration in distillery wastewater (DW) from a very high level (8800 mg/L) close to severe inhibition (>12000 mg/L) to 4350 mg/L, which was in the moderate toxic range (2500-4500 mg/L) for methanogens. The filtration pretreatment system consisted of the two steps of microfiltration (MF) to remove large solid particles and nanofiltration (NF) to reduce K concentration in the retained DW. Both steps of MF and NF were operated in batch mode with continuous recirculation. The permeate of the MF step was fed to the NF unit in conjunction with different dilution ratios (dilution water volume-to-feed volume) to lower the K content in the retentate. The higher the cumulative dilution ratio, the lower the K concentration in the retentate of the NF step. However, it has to be traded off against the increasing total volume of permeate with the higher cumulative dilution ratio. Thus, at the optimum cumulative dilution ratio of 0.5:1, the DW from filtration pretreatment with a high COD value of 111500 mg/L and a low K content of 4350 mg/L was found to have significantly higher methanogenic productivities in terms of average production rate and yields of both biogas and methane with a higher optimum COD loading rate, as compared to those of the untreated DW. Moreover, the use of the two-step filtration in this investigation could significantly lower the dilution ratio as compared to the sole dilution method (0.5:1 against 2:1).