Low strength graywater characterization and treatment by direct membrane filtration

The potential of direct membrane filtration on either ultrafiltration (30, 200 and 400 kDa MWCO) or nanofiltration (200 Da MWCO) membranes was studied as a method of treatment of low load graywater for on site reuse. The graywater collected from a sports center’s public showers and characterized for its chemical composition and physical properties fitted well to a low strength type (29.8 mg/l TSS and 170.3 mg/l as COD). Particle distribution analysis showed that colloidal size particles are the dominant fraction in terms of number distribution, while the much fewer, larger particles make up most of the particle volume (mean particle diameter ≈ 0.1 µm). When treated by ultrafiltration, COD and turbidity concentration of permeates complied with established restrictions (45–70% and 92–97% reduction range, respectively), however BOD values were still above the requirements in all cases. Permeate produced by nanofiltration was of high quality with high rejection of soluble organic matter (>90%) and ionic species (50%). It can be concluded that direct dense-membrane filtration is a favorable candidate for efficient treatment of graywater for unrestricted reuse. MWCO optimization still needs to be done in order to achieve better economics at an acceptable quality of permeate produced.

The graywater characterized in this study are of low strength type, with major pollutants being suspended solids and organic matter. Treated by UF, permeate quality obtained increased with decreasing MWCO of the membrane used. Despite compliance with COD and TSS concentration restrictions, it is assumed that BOD requirements are not met, for the permeate obtained by the UF membranes, even for the denser one (MWCO 30 kDa). Permeate produced by nanofiltration was of high quality and is suitable for unrestricted reuse with high rejection of soluble organic matter and partial rejection of ionic species. Organic matter rejection data, expressed as COD, obtained by the various pore size/MWCO rated membranes, enabled a rough estimation of MW distribution of the organic matter present in graywater, as presented in Table 7. Based on this, it can be seen that approx. 60% of the organic matter is below 1 mm particle diameter. Particle Table 7 Estimated molecular weight/particle size distribution of organic matter in low load graywater % of total CODa a >1µm 400 kDa (0.002µm)–1 µm 200 kDa–400 kDa 30 kDa–200 kDa 200 Da–30 kDa <200 Da