Abstract
Although the membrane bioreactor technology is gaining increasing interest because of high efficiency of wastewater treatment and reuse, data on the anaerobic transformations of retentate are scarce and divergent. The effects of transmembrane pressure (TMP) in microfiltration (MF) and ultrafiltration (UF) on the pollutant rejection, susceptibility of ceramic membrane to fouling, hydraulic parameters of membrane module, and biogas productivity of retentate were determined. Irrespective of the membrane cut-off and TMP (0.2–0.4 MPa), 97.4 ± 0.7% of COD (chemical oxygen demand), 89.0 ± 4.1% of total nitrogen, and 61.4 ± 0.5% of total phosphorus were removed from municipal wastewater and the permeates can be reused for irrigation. Despite smaller pore diameter, UF membrane was more hydraulically efficient. MF membrane had 1.4–4.6 times higher filtration resistances than UF membrane. In MF and UF, an increase in TMP resulted in an increase in permeate flux. Despite complete retention of suspended solids, strong shearing forces in the membrane installation changed the kinetics of biogas production from retentate in comparison to the kinetics obtained when excess sludge from a secondary clarifier was anaerobically processed. MF retentates had 1.15 to 1.28 times lower cumulative biogas production than the excess sludge. Processing of MF and UF retentates resulted in about 60% elongation of period in which 90% of the cumulative biogas production was achieved.
Highlights
To achieve high-quality effluents in the municipal wastewater treatment plants (WWTPs), membrane bioreactors (MBRs) are increasingly employed [1,2]
The replacing secondary clarifier with a membrane unit would be an advantageous solution because in the MBR technology the sludge settling ability does not influence the final quality of the effluent
The membrane fouling decreased the effective diameters of the pores, which allowed for the retention of molecules that were smaller than would be expected based on the nominal pore sizes [29]
Summary
To achieve high-quality effluents in the municipal wastewater treatment plants (WWTPs), membrane bioreactors (MBRs) are increasingly employed [1,2]. Conventional activated sludge systems are still cheaper to use than MBRs, the costs of membranes and MBR operation are decreasing and MBRs offer greater opportunities for water reuse as the most rapidly developing solution in wastewater treatment [3]. Microfiltration (MF) and ultrafiltration (UF) membranes that replace secondary clarifiers in the MBRs are complete barriers for the flocs, which allows maintaining a high biomass concentration in the bioreactor, up to 30 g/L. Effective biomass retention leads to a very long sludge age and low sludge production [3]. Long solids retention time (SRT), and low food-to-microorganisms ratio (F/M), bacteria primarily utilize the energy supply for their maintenance
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