The performance of sequencing batch reactors (SBR) in municipal wastewater treatment and risk assessment of SBRs effluent in agricultural irrigation

Nowadays, with the increasing urban population on the one hand and growing water consumption per capita on the other, the use of treated wastewater has been the subject of much attention, especially in arid and semi-arid areas. The city of Yazd, in central Iran, with its hot and dry climate, is the driest major city in Iran, with annual precipitation of 50–60 mm. Since there is no surface water, the city has relied on its groundwater system. In past decades, underground aqueducts, called Qanats (a series of well-like vertical shafts, connected by gently sloping tunnels) were used to irrigate farmland in this area. Over the past few decades, the qanats have experienced decline and deletion due to low rainfall and the excessive use of groundwater resources. Considering that the qanats are failing, water shortage is a critical challenge in this area. Therefore, treated wastewater is a good alternative, especially in agricultural applications. In the Yazd municipal wastewater treatment plant (YMWTP), the stabilization ponds had been used for sewage treatment until 2013, but after that, the advanced sequencing batch reactors (SBRs) became operational, and treated wastewater and sludge are used in agricultural applications. The most important aspect of wastewater application is a concern for public health.  Therefore, the main objectives of the present study are: 1) risk assessment of the SBRs effluent in agricultural irrigation based on the comprehensive pollution index (CPI) and 2) evaluation of SBRs performance and comparison with the efficiency of stabilization ponds and artificial wetlands.

The YMWTP is located in northern Yazd, close to the main road of Yazd airport. In the YMWTP, wastewater passes two initial treatment units including a screening and grit chamber and is then discharged into the SBRs. In the YMWTP, six SBRs are employed. The dimension of each reactor in meters is as follow length 40, width 23.7, and depth 6.6. In the SBR process, five stages including filling, reaction (mixing and aeration), settling, effluent, and idle are conducted in each reactor. The treated WW is decanted from SBR units to the disinfection section and then transferred to irrigate the green space. It should be noted that extra sludge is also discharged from the system in other treatment processes including digestion, dewatering, and drying to use in farmlands. All of these phases plus the total retention time is 4.9 h. For sample collection, composite sampling was carried out daily (every four hours) from October (2018) to September (2019). In this regard, special polyethylene bottles (1 L) for wastewater sampling were utilized to collect samples from the influent and effluent of the SBR system in the YMWTP. The American Public Health Association (APHA) method was applied To the measurement of turbidity, biological oxygen demand (BOD), total suspended solids (TSS), chemical oxygen demand (COD), ammonia (NH3), total phosphorus (TP), electrical conductivity (EC), total dissolved solids (TDS), and power of hydrogen (pH). The Statistical Package for the Social Sciences (SPSS) was also used for data analysis. This study adopted a simplified approach to risk assessment named CPI. The CPI was evaluated by using the measured concentration of parameters concerning their permissible limit in irrigation wastewater quality prescribed by the department of environment of Iran (DOE) and the United States environmental protection agency (USEPA). 

Based on our results, the annual average of the studied parameters in the influent, effluent, and also the percent of removal efficiency were; turbidity 173.5, 7.59, 96%, BOD 325.6, 14.9, 95%, TSS 293.7. 17.4, 94%, COD 650.2, 43.5, 93%, NH3 44.6, 5, 88%, TP 5.6, 2.6, 52%, EC 1881.1, 1463.1, 22%, TDS 981.5, 825.5, 16%, and pH 7.65, 7.27 and 5%. The results also indicated that the efficiency of SBR is higher than the stabilization ponds and artificial wetlands. Considering the DOE limits, the results of the calculation of CPI showed that the effluent from the SBR units of the YMWTP is suitable for agricultural purposes and irrigation of green spaces because the CPI rate was less than 0.5 in all months; and according to the annual average CPI (0.17), the effluent quality is in the clean category. But considering the limits set by the USEPA and the obtained average annual CPI (1.63), the quality of effluent from the YMWTP SBR units is placed in the category of medium pollution. It should be mentioned that in some months, such as Bahman and Farvardin, the monthly average of the CPI index exceeded the number of 2, and the quality of effluent was placed in the category of severe pollution. It was also found that the most effective parameters in increasing CPI are turbidity and TDS parameters. Our results revealed that the annual average of EC in the YMWTP SBRS was 1601 ± 196 µS/cm, which according to the Wilcox classification, the quality of the effluent is placed in the high salinity category; therefore, it is at the medium level for agricultural uses. According to the correlation analysis, a positive significant relationship was found between the EC and TDS and also turbidity and TSS in the influent. There was a negative significant correlation between the TSS, turbidity, and NH3 in the influent, while it was positive in the effluent. Our data showed a negative significant correlation between the TP, TDS, and NH3 in the effluent.

According to our results, the highest efficiency of the SBR units is in removing turbidity, BOD, TSS, and COD, respectively. Although according to the USEPA limits, the quality of the effluent from the SBR units of the YMWTP is in the medium pollution category and it is risky to use YMWTP effluent for agricultural purposes, according to the DOE limits, the quality of the effluent is in the clean category and it is suitable for agricultural use. Considering the characteristics of the treated wastewater of this city and considering the presence of many industries in the Yazd-Ardakan plain and the lack of water in this area, it is suggested that in future research, the feasibility of using treated wastewater for industrial purposes such as cooling towers, steam boilers, product production process, fire extinguishing, dust control, construction industry and artificial feeding of underground water should also be investigated.