Treatability Evaluation of Domestic Wastewater for a Rational Selection of Treatment Processes

1. Treatability Evaluation of Domestic Wastewater for a Rational Selection of Treatment Processes for Water Reuse Introduction Rational selection of wastewater treatment processes is important to meet the requirement on treated water quality for various reuse purposes. In this paper, the authors conducted a fundamental study to classify the main impurities in the raw domestic wastewater in Xi’an City, China according to their physical and chemical properties. On this basis, treatability evaluation was conducted regarding the impurities of each category through three kinds of typical wastewater treatment processes, i.e. enhanced primary treatment, secondary treatment and advanced treatment. Results & Discussion Treatability Evaluation of Impurities in Domestic Wastewater Classification of Impurities in Domestic Wastewater Figure 3 shows the treatability of dissolved and suspended matters of each category by three kinds of treatment processes, where enhanced primary treatment was done by jar test using poly-aluminium chloride (PAC) as coagulant, secondary treatment was done at an wastewater treatment plant using an oxidation ditch process and advanced treatment was done by granular activated carbon adsorption with pre-ozonation after the secondary treatment. Regarding the suspended components, all the processes are effective in the removal of almost all the components except for suspended nitrogen where enhanced primary treatment shows much lower ability of nitrogen removal than the other two processes. However, it is noticeable that enhanced primary treatment is equally effective in SS, COD(s), BOD(s), P(s) and NH3-N(s) removal as the advanced treatment – both of them can almost completely remove these suspended impurities. Regarding the dissolved matter, enhanced primary treatment is less effective than the secondary and advanced treatment for most of the components. However, complete removal of the dissolved phosphorous is only achieved by the enhanced primary treatment. By GC-MS analysis, dissolved organic matter were fractionated into three groups according to their functional bonds. As is shown in Figure 3, the enhanced primary treatment seems to be effective to remove aliphatic acids but less effective to remove chain hydrocarbons and cyclic hydrocarbons. As shown in Figure 1, suspended solids (SS), chemical oxygen demand (COD), biological oxygen demand (BOD), total nitrogen (TN) and total phosphorous (TP) were chosen as parameters representing the main impurities encountered in the raw domestic wastewater. Regarding each kind of impurity, a 0.45mm filtration method was applied to classify it into dissolved and suspended matters and a thermal ignition method was applied to classify it into organic and inorganic matters. A matrix was thus formed to show the composition of each kind of impurity. It is noticeable that most of the BOD and COD are of suspended state in the raw wastewater, as well as half of TP and 20% of TN. Regarding SS, COD, TP and TN, the inorganic parts take about 65%, 30%, 60% and 70% respectively. Figure 1 Classification of Impurities in Domestic Wastewater HPLC Chromatograms for Molecular Weight Evaluation Figure 2 is an evaluation of the molecular distribution of dissolved organic matter before and after the treatment by different processes using high performance liquid chromatography (HPLC) with UV detection at 254nm. The secondary treatment, i.e. biological degradation, cannot reduce the peaks of large organic molecules very much. In contrary to this, by enhanced primary treatment and advanced treatment most of the organic matter with molecular weight larger than 1000 can be effectively removed. Several new peaks appear in the smaller molecular range after secondary treatment and advanced treatment. This is believed to be resulted from biological degradation and chemical oxidation by ozone where larger molecules may be broken into smaller ones. Figure 2 HPLC Chromatograms for Molecular Weight Evaluation Figure 3 Treatability of Various Suspended and Dissolved Matters by Different Treatment Processes Conclusions Impurities in the domestic wastewater are classified according to their size and chemical composition. More than 60% of the organic substances and an amount of TP and TN in the domestic wastewater are with a size larger than 0.45 mm showing the property of suspended matter. Enhanced primary treatment can achieve almost 100% removal of the suspended impurities, part of the soluble COD/BOD, and a complete removal of the soluble phosphorous. Secondary treatment is effective in removing most of the soluble impurities. However, if very high quality effluent is required, additional advanced treatment should be applied. Acknowledgement: This study is supported by the National Natural Science Foundation of China (Grant No. 50138020)