Metals, heavy metals and microorganism removal from spent ﬁ lter backwash water by hybrid coagulation-UF processes

Spent ﬁ lter backwash water (SFBW) reuse has attracted particular attention, especially in countries that experience water scarcity. It can act as a permanent water source until the water treatment plant is working. In this study, the concentrations of Fe, Al, Pb, As, and Cd with total and fecal coliform (TC/FC) were investigated in raw and treated SFBW by hybrid coagulation-UF processes. The pilot plant consisted of pre-sedimentation, coagulation, ﬂ occulation, clari ﬁ cation, and ultra ﬁ ltration (UF) units. Poly-aluminum ferric chloride (PAFCL) and ferric chloride (FeCl 3 ) were used as pretreatment. The results showed that, at the optimum dose of PAFCl, the average removal of TC and FC was 88 and 79% and with PAFCl-UF process, it reached 100 and 100%, respectively. For FeCl 3 , removal ef ﬁ ciency of TC and FC were 81 and 72% and by applying FeCl 3 -UF process, it reached 100 and 100%, respectively. In comparison with FeCl 3 , PAFCl showed better removal ef ﬁ ciency for Fe, Pb, As, and Cd, except residual Al concentration. Coagulation-UF process could treat SFBW ef ﬁ ciently and treated SFBW could meet the US-EPA drinking water standard. Health risk index values of Fe, AL, Pb, AS, and Cd in treated SFBW indicate no risk of exposure to the use of this water.


INTRODUCTION
Recently, water reuse of spent filter backwash water (SFBW) has attracted particular attention in most countries because of water scarcity. The main point is that the SFBW acts as a permanent source until the water treatment plant is working. During most water treatment processes, SFBW is generated from 2 to 10% of the total plant production (Raj et al. ). Filter backwashing is conducted to remove all captured material through the filter bed during filtration. Thus, there are some concerns regarding its reuse because of high concentration of metals, heavy metals, natural organic matters, microorganisms, and colloidal materials (Walsh et al. ).
Heavy metals' exposure routes include absorption, inhalation, and ingestion. Ingestion through drinking water is the major source of heavy metals' exposure in some areas.
Heavy metal contamination in drinking water causes health problems such as shortness of breath, neurotoxic, mutagenic and teratogenic effects with various types of cancers depending on the heavy metal type (Chowdhury et al. ). It is obvious that direct recycling of SFBW can jeopardize the quality of the treated water because of the high concentration of contaminants. SFBW treatment can be conducted by different methods. Previous studies have shown that membrane separation is an effective way. Membrane filtration such as microfiltration and ultrafiltration (UF) remove particulates, colloids, and pathogens effectively and has been believed to be the water treatment technique of the 21st century. They need a smaller footprint, have low energy consumption and produce clean water (Ang et al. ); however, fouling is a big problem in membrane processes, especially for SFBW treatment.
To overcome this problem, a combination of membrane system with chemical or physical processes, such as coagulation and sedimentation, has been introduced (Wang The purpose of this study was to understand the level of contaminants and health risk related to metals (Fe and Al), heavy metals (Pb, As, and Cd), and microorganisms (total and fecal coliforms (TC/FC)) in the SFBW of Isfahan Water Treatment Plant before and after treatment with pilot-scale hybrid coagulation-UF processes, and comparison between the two coagulants: poly-aluminum ferric chloride (PAFCl) and FeCl 3 , as pretreatment.

Raw SFBW
Samples of SFBW were collected from Babashaikhali water treatment plant in Isfahan, Iran, during the winter season.
This plant treats 12 m 3 /s of water using coagulation, flocculation, clarification, and sand filtration processes. In this plant, PACl is used as a coagulant and 48 filters purify the water.
Each filter backwash needs 500 m 3 of water. Considering 48 filters with a 24 h cleaning interval, it accounts for over 2.25% of the raw water entering the plant. Hence, during the water treatment process, approximately 24,000 m 3 /d of SFBW is generated. The characteristics of raw SFBW are listed in Table 1. In this study, about 3,000 L of raw SFBW was collected from the water treatment plant. It was transferred to the laboratory and used for examination.

Experiment protocol
In this study, continuous processes including primary sedimentation, coagulation, flocculation, clarification, and UF were used for SFBW treatment. For all the sections of the pilot unit, except the UF membrane, the flow rate was 10 L/h. Hydraulic retention time (HRT) for this section, except the UF membrane, was 60, 6, 48, and 192 min. (40 mg/L) was continuously added into the rapid mixing unit (speed was 80 rpm and HRT was 6 min). The Fe (mg/L) 4 ( ± 0.14) 0.13 ( ± 0.03) 0.3 Al (mg/L) 0.4 ( ± 0.028) 0.068 ( ± 0.008) 0.05-0.2 Pb (μg/L) 217 ( ± 9.9) 4 ( ± 1.41) 10 As (μg/L) 2.36 ( ± 0.5) 1 ( ± 0.28) 10 Cd (μg/L) 4 ( ± 0.7) 0.5 ( ± 0.14) 5 TC (MPN/ 100 mL) 7,500 ( ± 707) 3,300 ( ± 210) 0 FC (MPN/ 100 mL) 2,200 ( ± 550) 900 ( ± 280) 0 coagulated water then passed though the two flocculation tanks, with a 40 rpm mixing intensity. After this, water was introduced for clarification, and then the treated water was fed to the UF membrane module. Investigation for the optimum dose selection was carried out for both coagulants in a continual manner for 4 days separately ( Figure 1, section 4). The optimum dose was selected to produce the best water quality based on turbidity and color results. These parameters were analyzed after two HRT of the second clarification (inflow was 10 L/h). After that about 1,000 L of raw SFBW was treated with PAFCl and FeCl 3 separately. Then, 800 L of treated water with PAFCl and 800 L of treated water with FeCl 3 entered the UF membrane process separately (inflow was 8 L m À2 h À1 ). Some parameters, such as turbidity, color, pH, TC, and FC were detected about ten times during the pilot operation, but metals and heavy metals were detected three times in the optimum dose and quality of treated water.
The UF membrane was made of hollow-fiber polypropylene, with a nominal pore size of 0.01-0.2 μm. The total membrane area of UF was 0.1 m 2 /module. The UF module was operated in dead-end mode with constant filtration of about 8 L m À2 h À1 at a trans-membrane pressure of 300 Pa.
It was operated in a cycle of 60 min filtration and 1 min backwashing with permeate in the reverse direction.

Water characteristics
From Table 1 it can be seen that the SFBW sample had high turbidity, color, Fe, Al, and heavy metals' concentration in comparison with raw water. Of course, the quality of raw water was very high because of very low turbidity, metals and heavy metals' concentration. From Table 1 it can be concluded that the concentrations of metals and heavy metals in raw water were lower than the EPA guideline.
On the other hand, raw SFBW has a very high concentration of turbidity, color, Fe, Al, Pb, TC, FC and, to some extent, Cd. Results showed that during the water treatment process, water contaminants were removed or accumulated on filter beds. Subsequently, filter backwash removed this material from the filters. Low concentrations of TC and FC in SFBW may be related to pre-ozonation and pre-chlorination in the water treatment plant. Metals and heavy metal concentrations in SFBW samples were found to be in the order: Fe > Al > Pb > Cd > As.

Microbial quality of treated SFBW
The effect of various doses (5 to 60 mg/L) of PAFCl and FeCl 3 is presented in Figure 2, to determine the optimum For the membrane process, the quality of input water is very important because of fouling problems. In this study, by applying optimum doses of PAFCl and FeCl 3 , treated water turbidity reached 2.4 and 3.9 NTU, respectively. After this, treated SFBW was used in the UF membrane process and microbial quality of treated water was investigated.
With regards to coagulation and flocculation, most bacteria and protozoa are considered as particles, and most viruses as colloidal organic particles. Thus, removal of turbidity has an indirect relationship with microbial reduction. In this result, the two coagulants showed a different influence on microbial reduction. From Figure 3 it can be seen that PAFCl showed good removal efficiency in com-

Metals and heavy metals quality of treated SFBW
The quality of SFBW treated with coagulation and the UF membrane process is presented in Table 2. Coagulation  was conducted with PAFCl and FeCl 3 at optimum doses. It can be seen that the two coagulants showed a different influence on SFBW quality. From Table 2, it can be concluded that PAFCl showed good removal efficiency for all the experimental parameters, except residual Al concentration.
As indicated in Table 2, residual turbidity and true color after coagulation with PAFCl and FeCl 3 reached 2.4 ± 0.14, 3.9 ± 0.14 NTU and 2 ± 0, 5 ±  has, to some extent, more Al concentration than SFBW treated with FeCl 3 , and SFBW treated with FeCl 3 has, to some extent, more Fe concentration than SFBW treated with PAFCl. Maybe it is related to composition property of the coagulant that repels some Fe or Al in treated water. However, the concentrations of Fe and Al in treated SFBW were low and met the drinking water standards.
The concentration of Pb, As, and Cd in raw SFBW was 217 ± 9.9, 2.36 ± 0.5 and 4 ± 0.7 μg/L, respectively. After coagulation with PAFCl (at optimum doses), it reached 0, 0, and 0.19 ± 0.012 μg/L, respectively. Also, for FeCl 3 , it reached 0, 0. and 0.35 ± 0.08 μg/L, respectively. The concentration of Pb, As, and Cd in SFBW treated with PAFCl-UF process was 0, 0, and 0.15 ± 0.01 μg/L, respectively, and for SFBW treated with FeCl 3 -UF process, it was 0, 0, and 0.3 ± 0.07 μg/L, respectively. It can be seen that both coagulants had good efficiency for heavy metals' removal. However, PAFCl removed heavy metals better than FeCl 3 . Metals and heavy metals' concentrations in SFBW treated with PAFCl were found to be in the order: Al > Fe > Cd > As and Pb. For coagulation with FeCl 3 , this order was Fe > Al > Cd > As and Pb. After the PAFCl-UF process, the order of metals and heavy metals was Al > Cd > Fe, As and Pb. Eventually, this order for the FeCl 3 -UF process was Fe > Al > Cd > , As and Pb.
It can be seen that coagulation reduced most of the metals and heavy metals in treated SFBW, therefore, it can be concluded that the highest amount of metals and heavy metals are related to constituents that are attached to particles, organic matter, clay or silt during the water treatment process.
In this study, coagulation with FeCl 3 that occurred under Thus, it is predicted that the dominant mechanism for metals and heavy metals' removal by PAFCl is adsorption.

Health risk assessment
Health risk indicators such as chronic daily intakes (CDIs) and health risk indexes (HRIs) of metals were calculated for SFBW treated with coagulation and UF processes. The chronic daily intake of metals (CDI) (μg/(kg·day)) and heavy metals through water ingestion was calculated using Equation (1) where C m (μg/L) is the concentration of metals or heavy metal in water, I w (L/day) is the average daily intake of water (2 L/day for adults and 1 L/day for children), and W b (kg) is the average body weight (72 kg for adults and 32.7 kg for children), respectively.
Health risk indexes (HRIs) were calculated using Equation (2) (Shah et al. ): where RfD (μg/(kg·day)) is the oral toxicity reference dose. It represents the daily dosage that the exposed individual can sustain at this level of exposure over a long period of time without experiencing any harmful effects. The oral reference doses (RfD oral) for the respective toxicants were used. RfD values for Fe, Al, Pb, Ar, and Cd are illustrated in Table 3 (EPA ; Muhammad et al. ).
The CDI and HRI values of the selected metals and heavy metals, before and after treatment, are summarized in Tables 4 and 5. HRI value less than one is considered to be safe for consumers (Khan et al. ).
The results showed that CDI values of Fe, Al, Pb, As,    lower than that of the FeCl 3 -UF process, and Pb and As concentration were zero for both processes. All the concentrations of metals and heavy metals in the SFBW treated with both processes met the drinking water standard according to EPA guidelines. Based on the quality of SFBW treated with the PAFCl-UF process, the HRIs of selected metals and heavy metals were found to be in the order: Cd > Al > Fe ¼ Pb ¼ As (HRIs values for Fe, Pb, and As were zero). For the FeCl 3 -UF process, this order was Cd > Fe > Al > Pb ¼ As (HRIs values for Pb and As were zero).

CONCLUSIONS
• SFBW treated with the coagulation-UF membrane process was colorless and had a turbidity of 0.1 NTU, and TC/FC were undetectable.
• All concentrations of metals and heavy metals in SFBW treated with both processes met the drinking water standard according to EPA guidelines.
• PAFCl showed good removal efficiency in comparison with FeCl 3 .
• Based on the quality of SFBW treated with the PAFCl-UF process, the HRIs of selected metals and heavy metals were found to be in the order: Cd > Al > Fe ¼ Pb ¼ As (HRIs values for Fe, Pb, and As were zero). For the FeCl 3 -UF process, this order was Cd > Fe >Al > Pb ¼ As (HRIs values for Pb and As were zero).
• HRIs indices' values of Fe, Al, Pb, As, and Cd in SFBW treated with PAFCl-UF and FeCl 3 -UF processes were less than 1, which indicates no risk of exposure to the use of this water.