Upgrading secondary wastewater plant ef ﬂ uent by modi ﬁ ed coagulation and ﬂ occulation, for water reuse in irrigation

In this study, the feasibility of using coagulation, ﬂ occulation, and sedimentation (CF-S) for advanced treatment of secondary ef ﬂ uent released from the Yazd Intermittent Cycle Extended Aeration System was investigated. Four coagulants including ferric chloride (FeCl 3 ), polyaluminum chloride (PAC), ferrous sulfate(FeSo 4 ),andpotassiumferrate(K 2 FeSo 4 )alongwithG ﬂ ogC-150as ﬂ occulantpolymerwereused. In this study, returned chemical sludge was considered as a modi ﬁ cation. Preliminary CF-S processes showed that FeSO 4 and K 2 FeO 4 had low removal ef ﬁ ciencies. Thus, these two coagulants were abandoned and CF-S processes were continued only with PAC and FeCl 3 coagulants which had higher ef ﬁ ciencies in the removal of biological oxygen demand (BOD 5 ), chemical oxygen demand (COD), total suspendedsolids(TSS),andturbidity.Removalef ﬁ ciencywashigherwhenhalfofthechemicalproducing sludge was returned as compared with using both coagulants simultaneously along with 2 mg L (cid:1) 1 of C-150 as ﬂ occulant. In the optimum dosage, when half of PAC and FeCl 3 sludge were returned, the volume of produced sludge was reduced by 40% and 28%, respectively, as compared without returned sludge. For the PAC coagulant in the optimum dosage with half of the sludge returned, all 2012 EPA standards of irrigation were met for both ‘ processed and non-processed type ’ agricultural crops. This study was conducted to determine the feasibility of coagulation, ﬂ occulation, and sedimentation (CF-S) processes for upgrading secondary ef ﬂ uent discharged from the Yazd Intermittent Cycle Extended Aeration System (Yazd-ICEAS). The purpose of this study was to achieve the best coagulants, most effective technique, and optimum concentrations of coagulants used to reach EPA standards and the conditions speci ﬁ ed by the regional employer (Yazd Water and Wastewater Company).


INTRODUCTION
The city of Yazd, with a mean annual precipitation of 60 mm, is known as one of the most arid desert regions in Iran, where water shortage is considered to be a critical challenge. Climate change, water resources shortage, and increasing rate of conversion of water into wastewater due to population growth have led to serious consideration of water reuse. Reclaimed water can be used in different sectors, such as industry, recreation, and irrigation. The reuse of water has created great motivation to use innovation technology for upgrading wastewater effluent regarding agricultural irrigation (Löwenberg et al. ). In some parts of the world, especially in developing countries including Iran, where advanced technologies of wastewater treatment are not available, simple available technologies should be applied for advanced treatment of wastewater to be used in irrigation in agricultural sectors (Üstün et al. ; Väänänen ). Sustainable water resource development is an This study was conducted to determine the feasibility of coagulation, flocculation, and sedimentation (CF-S) processes for upgrading secondary effluent discharged from the Yazd Intermittent Cycle Extended Aeration System (Yazd-ICEAS). The purpose of this study was to achieve the best coagulants, most effective technique, and optimum concentrations of coagulants used to reach EPA standards and the conditions specified by the regional employer (Yazd Water and Wastewater Company).

MATERIALS AND METHODS
Analytical procedure and materials used Accessibility and ease of use were the two main factors considered in selection of the chemical materials. The four coagulants used in this study were ferric chloride (FeCl 3 , purity: 98%, Merck), polyaluminum chloride (PAC, 99% ±2, Merck), ferrous sulfate (FeSo 4 , 99% ±1, Merck), and potassium ferrate (K 2 FeSo 4 , 97%, Sigma-Aldrich). Gflog C-150 was the polymer flocculant used in this study, with a purity of 97% (Aqua Tech, Portugal), which is referred to as C-150 hereafter.
A comparative study by Yazd Water and Wastewater Company had previously confirmed the high efficiency of C-150 polymer flocculant in the Yazd-ICEAS treatment plant. Accordingly, we did not conduct any further investigation on other flocculants and C-150 was used as the sole flocculant in the present study.
In this study, both before and after each stage of CF-S process, conductivity and pH were measured by a conductivity/ TDS meter and a pH-meter using a Hach HQ40d multi meter (Hach Co., Loveland, CO, USA). Turbidity was measured using a Hach 2100P turbidity meter. Hach DR5000 UV-Vis spectrophotometer was used to determine color, chemical oxygen demand (COD), and suspended solids (SS). Instruction standard method of 5210-B was used to determine biological oxygen demand (BOD 5 ). Fecal coliform (FC) before and after CF-S operation, as well as disinfection with chlorine, were performed by multiple tubular technique (MPN).

Sampling and CF-S process
Sampling, testing, and CF-S process were performed in winter (the coldest season in Yazd), representing the worst possible efficiency situation for Yazd-ICEAS treatment plant effluent to ensure generalization of the results to any other conditions of effluent discharged from Yazd-ICEAS.
Jar test experiments and sedimentation were used as a batch reactor in order to perform CF-S processes. The samples were transferred to the laboratory and different CF-S processes using varying coagulants and C-150 flocculant were performed. A total of 80 samples from Yazd-ICEAS effluent treatment plant were taken for laboratory analysis. About half of the samples were used in the process of a pry test to achieve optimum conditions for determining the duration and velocity of slow and fast mixing, as well as optimum conditions for the sedimentation term.
The survey was conducted on the actual wastewater discharged from Yazd-ICEAS treatment plant. In other words, pH, concentration, and dilution were not artificially defined.
According to previous studies, changing wastewater pH requires a lot of chemicals due to abundant organic matter and thus it is not economical (Ismail et  The CF-S process was conducted in several stages for each of the coagulants to achieve optimum condition. Eventually, after several pre-test operations, coagulant concentrations between 5 and 55 mg L À1 , rapidly mixed (150 rpm) for 1 min, with a paddle velocity of 30 rpm for 10 min and settling for 15 min were selected as the optimum settings. These settings were selected as the base for all CF-S processes. In a study on the treatment of raw wastewater, PAC and ferrous sulfate were used with flash mixing started at 350 rpm and continued for 1 min along with addition of coagulants with dosages of 30, 60, 80, 100, and 120 mg L À1 (Ismail et al. ). In another study on the treatment of raw wastewater, a series of jar test experiments were performed at 100 rpm for 1 min and 30 rpm for 20 min followed by 30 min for settling. In the mentioned study, concentrations between 150 and 450 mg L À1 of alum were applied at pH ranging from 4 to 10 and room temperature (Guida et al. ). Given the difference between used wastewaters (secondary treated versus raw), the coagulant concentrations used in the mentioned studies were different (clearly higher concentrations compared to those used in the current study). In a study on tertiary wastewater

Selection of the most effective coagulants
In the present study, four coagulants of FeCl 3 , FeSO 4 , K 2 FeO 4 , and PAC were used to perform CF-S processes.
The BOD 5 , COD, turbidity, and total suspended solids (TSS) removal efficiencies were used as performance criteria to choose the best coagulants. Figure 1 illustrates TSS removal efficiency by each of the four coagulants. The percentage of TSS removal by K 2 FeO 4 , especially in lower dosages, was much lower than the other three coagulants. However, in general, TSS removal efficiencies by FeCl 3 and PAC (especially in high concentrations) were higher compared to those obtained using K 2 FeO 4 and FeSO 4 . Turbidity removal efficiencies in different dosages of the four coagulants are presented in Figure 2. The differences in turbidity removal efficiencies by various coagulants were slightly higher compared to TSS. The ordered turbidity removal efficiencies, respectively, belonged to PAC (60-90%), FeCl 3 (30-70%), K 2 FeO 4 (0-45%), and FeSO 4 (5-40%). Figure 3 shows BOD 5 removal efficiencies by four different coagu-     The results of TSS removal efficiency using FeCl 3 and PAC coagulants are presented in Figure 6. In both coagulants, the better TSS removal efficiency, especially at low coagulant concentrations, is obtained when half of the sludge is returned. In general, among the six states illustrated in Figure 6, the TSS removal efficiency is the highest when PAC is used along with half of the produced sludge. Using a coagulant with high removal efficiency of turbidity and TSS has also shown to be an effective way for the removal of many other contaminants that can be adsorbed by colloids, such as metals, toxics, organic matter, viruses, and radionuclides (Stechemesser & Dobiáš ).    Table 1 shows the volume of sludge deposited by CF-S processes in six cases of PAC and FeCl 3 applications in the states of optimum concentrations. When half of the PAC sludge was returned and when 2 mg L À1 of C-150 polymer was used, the sludge volume reduced, respectively, by 40% and 33% compared with using PAC coagulant alone. For FeCl 3 coagulant, in the corresponding cases, sludge reduction was recorded as 28% and 34%, respectively.

Disinfection process
The CF-S operation effluents for optimum dosages of PAC and FeCl 3 (45 mg L À1 PAC and 55 mg L À1 FeCl 3 ) were disinfected by 1 mg L À1 chlorine in all cases, including each coagulant along with half of the returned sludge, each coagulant in addition to 2 mg L À1 of C-150 polymer, and each coagulant alone. The amount of residual chlorine and FC were measured after 20 min. The results of disinfection processes are presented in the last two rows of Table 2.
The residual chlorine ranged from 0.1 to 0.3 mg L À1 .
Optimum dosages for irrigation reuse Table 2 Table 2) were met for both 'processed and non-processed type' agricultural crops.
The final quality of treated effluent using CF-S process in optimum dosages of PAC and FeCl 3 were indicated in Table 3 for some important parameters in agricultural use.
• Using PAC and FeCl 3 each with half of the produced sludge as a modification or each with 2 mg L À1 of C-150 polymer, resulted in higher removal efficiencies of BOD 5 , COD, TSS, and turbidity compared with using each coagulant alone. Moreover, both coagulants, especially PAC, showed the highest removal efficiency when half of the produced sludge was returned compared with using 2 mg L À1 of C-150 as flocculant along with the coagulants.
• At the optimum dosage, when half of PAC sludge was returned or 2 mg L À1 of C-150 flocculant was used, respectively, 40% and 30% of the sludge volume was reduced as compared with using PAC alone, while in the corresponding cases, these amounts for FeCl 3 were calculated as 28% and 34%, respectively.
• At the optimum dosages, except for using FeCl 3 alone , in the other five cases the 2012 EPA standards related to irrigation of 'processed type' crops were met after disinfection using 1 mg L À1 chlorine in 20 min retention time. Also, when half of the sludge was returned to the PAC coagulant CF-S process and after disinfection using 1 mg L À1 chlorine in 20 min retention time, all recommendations set by Yazd Water and Wastewater Company and 2012 EPA standards of irrigation were met for both 'processed and non-processed type' agricultural crops.
Investigations on the processes of combination of coagulation/flocculation and filtration would be of interest to evaluate operational experience. Also, it is recommended to examine the process of a combination of coagulation/ flocculation and multi-oxidants to study the development of tertiary treatment. Some natural coagulants, such as chitosan, are more favorable in wastewater treatment due to their environmental friendly characteristics, thus it is recommended that they be investigated.