مقایسه چند سیستم تصفیه فاضلاب روستایی از نظر برخی ویژگی‌های شیمیایی و عناصر سنگین

Introduction: Water scarcity is an important challenge worldwide, especially in arid and semi-arid regions. In these areas, the excessive exploitation of groundwater for irrigation, inefficient irrigation methods, irrigation with low-quality water and uncontrolled utilization of fertilizers in agricultural lands in addition to contamination of water resources by domestic and industrial wastewater in urban as well as rural regions, have led to water pollution problems. Furthermore, pollution and transportation of pollutants through wastewater have been considered as an environmental issue. Wastewater is a term that is used to describe waste materials that includes liquid waste and sewage waste. Wastewaters from single houses in the countryside that are not connected to sewers are generally treated on-site by septic tank systems or individual domestic wastewater treatment systems. Study on wastewater quality derived from refining systems in rural areas has been rarely taken into account. This study investigates the efficiency of some refining systems in the reduction of wastewater pollution indices and heavy metal concentrations.
Materials and Methods: This study was done in four rural areas including Dehmilan, Hotkan, Sarbagh and Sekukan which are located around the city of Zarand in the Kerman province. Recently, some refining systems have been established in these areas in order to mitigate the environmental issues. An experiment was done to assess the efficiency of these refining systems and to determine the pollution indices for such small communities. Wastewater sampling was done in 10 replicates each at one week interval from four refineries. Different variables including BOD, COD, TOC, EC, TSS, TDS, DO, TKN, TP, pH, temperature, turbidity (Turb), alkanity (Alk) and also the concentrations of Cd, Zn, Pb, Ni and Mo were measured using standard methods. To quantify the performance of each system, the amount of each variable at the outlet was compared to the value of the same variable at the inlet. Also, the percentage change of wastewater properties at the outlet ratio to the inlet values in the refining systems was calculated. Meanwhile, the efficiency was evaluated using permissible values reported by the Environmental Protection Organization of Iran.
Results and Discussion: The results showed that after purification, the amounts of DO, Cd and Mo were not significantly different among the studied systems, while, the other parameters were found to be similar. In almost all the cases, the amounts of pollutants decreased at the outlets, nevertheless considering the permissible standards, just in few cases the pollution was reduced. Moreover, in comparison to the standard values, the amount of TP increased, while Turbidity decreased. Additionally, the amount of DO was higher than the threshold values. As a result of purification in all the studied systems, the concentrations of Cd and Pb were reduced, whereas the concentration of Ni increased. Also, the concentrations of heavy metals, except Mo were less than the standard values.
Conclusion: It was concluded that the selected refining systems had limited performance in the purification of wastewater in the studied rural areas. However, the amounts of pollutants showed some reductions at the outlets, based on the permissible standards reported by the Environmental Protection Organization of Iran. In just a few cases the pollution indices were reduced. In fact, the septic tank systems could not remove the chemical pollutants from wastewaters, although the best performance was observed for TSS and Turbidity, which were reduced with respect to permissible levels. The amounts of BOD and COD were higher than the standard values, indicating low efficiency of the refining systems in removal of chemical and biological agents. Also, the concentration of TP was found to be higher than the permissible level. The entrance of phosphorous into the surface runoff and water bodies may lead to eutrophication. The results of assessment of heavy metals indicated that the refining system could reduce the concentrations of Cd and Pb, whereas, the concentration of Ni increased. Anyway, the concentrations of heavy metals, except Mo were less than the standard values. The source of Mo seems to be within the wastewater generated by the rural communities, which can lead to serious environmental problems. The main concern arises from the high concentration of Mo, which was 4000 percent greater than the permissible level. Therefore, more studies are needed on the possible source of Mo in the rural study region. Also, a modification in the current systems particularly in removal of chemical agents is necessary.