Treatment of municipal wastewater reverse osmosis concentrate using biological activated carbon based processes

摘要

Reclamation of municipal wastewater has become an increasingly attractive alternative to supplement the limited fresh water supply all over the world. For this purpose, reverse osmosis (RO) based wastewater reclamation processes are being increasingly used to produce high quality recycled water that is suitable for a wide range of beneficial uses. However, the RO treatment processes generate the waste streams known as RO concentrate (ROC) which contain almost all contaminants present in the RO influent (usually the biologically treated secondary effluent) at elevated concentrations (4-6 times). As these contaminants include many harmful micropollutants and nutrient in addition to the organics recalcitrant to biological treatment, the ROC can pose significant risks to environment and human health if discharged to receiving water environments without proper treatment. The organics present in the ROC are refractory to further biodegradation because these organics are originated from the secondary effluent that has been subjected to extensive secondary treatment. Biological treatment such as biological activated carbon (BAC) is considered as a potentially cost-effective and environmentally benign option for removing organic matter and nutrients from the ROC via adsorption and biodegradation. Some preliminary studies have investigated the potential of BAC treatment in removing organic matter only from the relatively low salinity ROC (TDS &5 g/L). However, there is generally a lack of study on removing both organic matter and nutrients from different types of ROC (e.g., with high salinity, containing industrial process wastewater etc.), and the microbial communities contributing to the ROC remediation. Therefore, effectiveness of BAC was investigated for organic matter and nutrient removal from different types of ROC using different pre-treatment options in this study. The different types of ROC used in this study vary greatly in salinity levels, ionic concentrations, initial organic and nutrient concentrations. The UV/H2O2 was used as pre-treatment of ROC as it improves its biodegradability by degrading recalcitrant organic compounds via oxidation by hydroxyl radicals and making the ROC more amenable for biodegradation by microorganisms in the BAC system. The combined UV/H2O2-BAC treatment of a ROC, which had extremely high salinity (TDS ~ 16 g/L and initial DOC ~36 mg/L) led to effective reductions in organic matter (57% DOC removal) and nitrogen species at the empty bed contact time of 60 minutes. This was attributed mainly to the generation of simpler organic molecules during the oxidative treatment, which were readily removed by the microorganisms embedded in the BAC column. High total nitrogen removal (60%) was achieved with complete nitrification and partial denitrification taking place in the BAC system without supplementing additional carbon source or aeration. However, total phosphorus removal was very low (15%) due to the high salinity of the ROC (&5 g/L), at which plasmolysis of phosphorus removing bacteria would occur. The treated ROC had similar characteristics to the secondary effluent, which was used as the influent for the reclamation process, in terms of DOC, COD and TN. Moreover, the treated ROC was markedly lower in colour and UVA254 compared with the RO influent, confirming that the BAC process could be acclimated to treat the very high salinity municipal wastewater ROC. The BAC treatment system was found to be robust as the organic matter removal was not greatly affected by varied ROC salinity (TDS 7- 16 g/L). However, total nitrogen removal was higher for the ROC at high salinity (TDS 16 g/L) compared with low (7 g/L) and medium (10 g/L) salinity ROC as a result of the considerably higher denitrification at high salinity (39% cf. 23% and 27% at low and medium salinity, respectively). This was attributed to prevalence of diversified halotolerant bacteria which were mostly responsible for denitrification in the BAC treatment system. The major bacterial communities identified in the BAC treatment system were Bacillus sp., Pseudomonas sp. and Rhodococcus sp., as revealed by PCR-DGGE and sequencing, which were able to remove organic matter and the nitrogen species. The combined UV/H2O2-BAC treatment of another type of ROC (TDS 4.5 g/L and initial DOC ~52 mg/L) which was derived from a municipal wastewater containing a significant proportion of petrochemical wastes led to overall 58% DOC removal. The combined treatment of this ROC led to higher phosphorus removal (60%) and low (15%) total nitrogen removal, implying that nutrient removal could be greatly dependent on salinity level of ROC and the groups of bacteria present in the BAC system. The presence of Micrococcus sp. Ralstonia sp., Agrobacterium sp., Sphingopyxis sp. and Pseudomonas sp, which were closely related to phosphorus accumulating organisms (PAOs) in BAC treatment system, were considered to be responsible for phosphorus removal. Furthermore, the BAC treatment system effectively removed the total petroleum hydrocarbon (TPH) from the ROC, thus indicating its good potential for removing petrochemical compounds of interest. For the convenience of comparison, aforementioned two types of ROC were denoted as ROC A (TDS 16 g/L and initial DOC 36 mg/L) and ROC B (TDS 4.5 g/L and initial DOC 52 mg/L). These two ROC types were different in terms of inorganics (as indicated by the TDS concentration) and organics as indicated by DOC concentration and Liquid Chromatography-Organic Carbon Detection (LC-OCD) analysis. The UV/H2O2-BAC treatment of two types of ROC led to comparable DOC reduction (58%) due to considerable reduction of high molecular weight compounds (HA-like) and generating low molecular weight compounds during oxidation, which were more amenable to biodegradation in the BAC treatment. The COD removal was higher (59%) for the ROC B compared with ROC A. It was found that nitrification was consistently higher as more than 90% ammonium nitrogen removal was achieved for both ROC regardless of different inorganic and organic compositions of both ROC. Total nitrogen and phosphorus removals were mainly dependent on the existence of different bacterial communities in the two BAC systems treating different ROC streams. The impact of other pre-treatments including coagulation and sequential coagulation-UV/H2O2 were also evaluated for their capabilities in organic matter and nutrient removal from the aforementioned two types of ROC (ROC A and ROC B). Coagulation pre-treatment achieved & 90% phosphorus removal regardless of the type of the ROC. For the coagulation-BAC treatment, organic matter removal was greater for the ROC A compared with ROC B. This was attributed to the significant removal of higher molecular weight organic compounds due to the formation of more rigid flocs in the higher salinity water environment, which led to better settleability of organic matter. The sequential coagulation-UV/H2O2-BAC treatment on the two types of ROC markedly improved the overall organic matter removal, with a comparable reduction in DOC (62-67%) due to reduced organic load by individual pre-treatment and better oxidation of the remaining organics in coagulated ROC, and consequently more effective biodegradation occurred in the BAC treatment. Ammonium nitrogen removal (90%) was consistently higher for the two types of ROC when using coagulation-BAC and sequential coagulation-UV/H2O2-BAC combinations. This study demonstrated that the BAC based processes are effective and resilient in removing organic matter and nutrients from the municipal wastewater ROC of significantly different natures and water quality characteristics. Since the BAC treatment could lead to significant reductions in chemical consumption (such as H2O2 and coagulant) and energy cost (such as UV light), it is potentially a feasible option for reducing the environmental and health risks associated with the ROC on disposal or reuse. However, the technological feasibility of the processes should be assessed further with larger scale trials, and more detailed cost analyses should be conducted to justify their full-scale applications.