Treatment of reverse osmosis concentrate produced from municipal effluent by UV/H2O2 processes

Reverse osmosis (RO) technology is increasingly used for municipal wastewater recycling. However, a major drawback of RO is the production of RO concentrate (ROC), which may contain a high level of organic pollutants. Consequently, ROC poses a threat to the environment and therefore must be treated to reduce or even eliminate the negative impact associated with the organic pollutants. Three ROCs produced from three municipal effluents with different properties were treated by UVC/H2O2 processes. UVC irradiation alone was not sufficient to achieve satisfactory removal of the organics in the three ROCs. Coupling H2O2 with UVC irradiation greatly enhanced the oxidation performance. The reduction in absorbance at 254 nm (A254) and colour of the three ROCs was much faster than in chemical oxygen demand (COD) and dissolved organic carbon (DOC). Complete decolourisation of the three ROCs was usually achievable by UV/H2O2 processes. At a given H2O2 dose, the COD reduction followed a pseudo first-order reaction for the first 30 min and then followed a different reaction mechanism because the H2O2 concentration was insufficient to produce a steady concentration of •OH afterward due to its consumption. With the same irradiation time, UVC/H2O2 treatment increased the biodegradability of the remaining organics 2.6-6.7 times, depending on the type of ROC and H2O2 dose. However, some DOC in the three ROCs (from 10% to 20%) was not converted to BDOC under the conditions used. The highest DOC removal after the sequential UVC/H2O2-BDOC process for the three ROCs was 78-93%, depending on the ROC source, corresponding to a residual DOC of less than 5 mg/L. Molecular weight analysis revealed that enhancement of biodegradability of the organics was due to the decrease in the average molecular weight and the degree of conjugation. The final DOC removal from the ROC derived from Western Treatment Plant after 2 h UVC/H2O2-BDOC (3 mM H2O2) increased from 69% to 90% when the pH was decreased from 10 to 4. However, the salinity was found to only marginally affect the oxidation performance. Although initial DOC concentration of the ROC was increased from 20 mg/L to 30 mg/L, 2 h UVC/3 mM H2O2 followed by BDOC could still reduce the DOC to 5.4 mg/L, corresponding to 82% removal of DOC. The potential health risks associated with the organics in the ROC before and after the UVC/H2O2 or UVC/H2O2-BDOC treatment were assessed by Microtox tests and trihalomethane formation potentials (THMFP). The electrical energy per order value showed that the energy efficiency of the UVC/H2O2-BDOC treatment was decreased after 30 min, although preliminary calculations of energy requirements indicated that the UVC/H2O2-BDOC process was potentially suitable for practical application. Hence, with further research and development, UV/H2O2 followed by biological treatment may be a practical option for treating ROC produced from municipal effluent