Tertiary treatment of secondary effluents by Coagulation - wseas.us

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Tertiary treatment of secondary effluents by Coagulation- ultrafiltration Comparative Study between two coagulants used: The Ferric Chloride and the Alum MAZARI Lilia, ABDESSEMED Djamal Department of the Environment Engineering University of the Sciences and Technology Houari Boumediene B.P., 32 El Alia 16111, Bab Ezzouar, Algiers Algeria E-mail: [email protected]

Abstract: In this study, a comparison was made between two conventional metal-ion coagulants, alum and ferric chloride to evaluate their impact on the fouling of the ultrafiltration (UF) membrane processes to the treatment of secondary effluent from the sewage treatment plant. Coagulation pre-treatment which is effective in removing a broad range of impurities from water, including colloidal particles and dissolved organic substances can improve the performance of the membrane filtration, by reducing fouling effects by the decrease of the cake resistance, limiting pore blocking and the choice of coagulant is a major variable. The influence of the dose of the coagulant and the water pH on the process performance has been investigated by analysis of the COD and UV254 nm. The ultrafiltration tests were performed in tangential cross-flow mode using Carbosep tubular inorganic membranes. The ultrafiltration of secondary effluent developed significant fouling, with a 54% drop in flux during 2h of the filtration. The flux drop for a pretreated effluent by alum was equal to 43%. The optimal flux with FeCl3 pretreatment gave a significant reduction in organic content with minimal membrane fouling (28%).

Key-Words: Coagulation, ultrafiltration, secondary effluent, reuse, permeate flux, fouling.

fouling of membranes is generally related to the formation of a gel/cake layer by colloidal/particulate organic matter on the membrane surface, and/or adsorption of dissolved organics within the membrane pore structure [9]. The fouling process can be influenced by various factors including the characteristics of the organics in feed water, membrane properties, solution environment and operating conditions [10] [11]. Moreover, a variety of pretreatment technologies have been proposed and investigated to reduce fouling during ultrafiltration treatment, including coagulation, adsorption, pre-oxidation and prefiltration, etc. [12]. Coagulation is the most common process to remove turbidity and natural organic matter (NOM) in drinking or waste water treatment, Applying a coagulation step before membrane filtration can remarkably improve the permeate quality in terms of

1 Introduction In recent years, membrane technology has been used increasingly in water and wastewater treatment [1] and although their performance meets water quality objectives and are costeffective [2]. It can effectively remove microparticles and macromolecules, which generally included inorganic particles, organic colloids (i.e., micro-organism) and dissolved organic matter (DOM) [3]. However, membrane fouling remains a major barrier limiting its application in treating surface water [4], and the major cause of fouling is the accumulation of a complex mixture of humic and fulvic acids, proteins, and carbohydrates[5] [6] [7]. The fouling can result in a marked reduction in product water flux and a significant increase in transmembrane pressure, leading to increased operating and management costs [8]. Organic

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characteristic parameters of the secondary effluent are shown in Table 1.

organic matter content in surface treatment [1317]. Iron/alum salts are widely used as coagulants for pretreatment, which is advantageous on improving, permeate flux [18][19] [20]. They are effective in removing a broad range of impurities from water, including colloidal particles and dissolved organic substances [20][21]. The extent of improvement on the performance of membrane filtration is greatly dependent on the properties of coagulants, dosages, pH [22] [23]. The type of coagulant has been identified as a significant factor influencing the fouling in ultrafiltration [20]. Barbot et al. [23] presented that alum species played an important role on the characteristics of coagulant flocs, which could influence the behaviour of the downstream membrane filtration. Yu et al. [24] found that a much lower transmembrane pressure (TMP) was caused by alum salt compared to that induced by iron (III) salt in the ultrafiltration system, and Konieczny et al. [25] demonstrated that membrane fouling was much more reduced by Al2(SO4)3 than that by Fe2(SO4)3 or FeCl3 in the “in-line” coagulation-ultrafiltration hybrid process. According to Qiao et al. [26], FeCl3 was better for membrane fouling in a large-scale UF process in comparison with polyaluminum chloride (PACl). The aim of this study was to investigate the effect of coagulation as a pretreatment for UF membrane. A comparison has been made between two conventional, metal-ion coagulants, alum and FeCl3 to evaluate their impact on the fouling of the ultrafiltration membrane processes for reuse of secondary effluent from the sewage treatment plant.

Table 1 Average characteristics of secondary effluent Average values

SS (mg/L)

25

19-29

Turbidity (NTU)

9,5

8,75-10,25

BOD5 (mg/L)

8

7,3-8,7

COD (mg/L)

55

43-67

TOC (mg/L)

14,5

11,5-17,5

UV 254 (cm-1)

0,177

0,155-0,200

pH

7,5

7,2-7,8

Pt (mg/L)

1,52

1,40-1,64

Conductivity (µS/cm)

1384

1175-1593

24

20,1-28,5

NT (mg/L)

Range

2.2 Jar-test Alum (Al2 (SO4)3 ,18H2O ) and FeCl3 were used as coagulants in this study. Stock alum and FeCl3 solutions were both prepared at a concentration of 1 g/L in distilled water. Standard jar tests were conducted using a programmable jar-tester. The water sample (1000 mL) was mixed rapidly for 2 min at 260 rpm. After adding coagulant followed by slow mixing at 60 rpm for a duration of 15 min which was then followed by 20 min of quiescent settling. Water samples were collected from 2 cm below water surface for measurements. The water samples were prefiltered using 0.45 µm fiber membrane syringe filter before testing absorbance at 254 nm (UV254); while turbidity and floc zeta potential were directly measured without filtration. Coagulation–flocculation experiments under different solution pH values were conducted after the optimal coagulant dosages were determined. The target coagulation pH values were achieved by adding appropriate quantities of HCl and NaOH solutions with the concentration of 0.1mol/L

2 Materials and methods

2.1 Characteristics of the secondary effluent The secondary effluent with a mean physicochemical characteristics were collected from a conventional wastewater treatment plant (WWTP) located in Reghaïa (Algeria. Thirty liters of water sample was stored at 4 °C, and was warmed to room temperature (20±1 °C) prior to all tests. The main

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Parameters

2.3 Ultrafiltration membrane process

141


The experiments have been developed in a laboratory membrane unit equipped with Carbosep tubular inorganic membranes with a molecular weight cut-off of 40 kDa and the total filtration area is 0.01 m2. A schematic diagram of the experimental setup for the cross-flow UF is shown in Fig.1. The general UF experiment with the wastewater was conducted in tangential cross-flow mode, at temperatures varies among 25°C and 30°C. Transmembrane pressure (TMP) and cross-flow velocity (U) are maintained at 1 bar and 4 m.s−1. Each experiment consisted of three operation periods. The first was an ultrapure water flush, the second using only the feed water and the final using coagulant dosing in the combination process, after the coagulation/flocculation tests, the coagulated water, without prior sedimentation, was transferred to the feed tank of the ultrafiltration module. After each experiment, the membrane was cleaned using two protocols: (1) nitric acid at pH =2 and (2) NaOH at pH= 11. (6)

3.1. Coagulation performance as a function of coagulant dose

(6) (7)

(5)

(5) (8)

(4)

Coagulant (2)

(9)

(5) (3)

(1)

1: Feed tank; 2 : cooling coil; 3: centrifugal pump; 4 : flow meter, 5 : valves; 6: pressure gauge; 7 : tubular UF module; 8 : permeate ; 9: by-pass. Fig. 1 Experimental set-up

2.4 Analytical methods COD has been measured with AFNOR methods. UV254 was measured at 254nm through a 1cm quartz cell using (JASCO UV/VIS 530) spectrophotometer. The pH has been measured with the help of a pH meter calibrated, Hanna instruments pH 211 type. Measurements of zeta potential were performed with a Malvern Zetasizer Nano Serie Zs.

3. Results and discussions

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Variation of coagulation performance vs. coagulant dose with Al2 (SO4)3 and FeCl3 was investigated in terms of residual turbidity, UV254 removal and floc zeta potential, with the results being shown in Fig. 2. From Fig. 2(a), a sharp decrease in residual turbidity was observed with the increasing coagulant dose for both Al2(SO4)3 and FeCl3, followed by a plateau for Al2(SO4)3 or showed slight decrease at high coagulant doses, alum recorded the lowest residual turbidity of 1,24 NTU for the optimum dosage of 70 mg/l. , while a gradual increase was observed for FeCl3 at its dose larger than 40 mg/L it can be seen that the low value of turbidity is 1,3 NTU. As shown in figure 2(b); UV254 removal for all two coagulants increased significantly first with the increasing coagulant dose, and then approached a plateau where showed slight increase at high coagulant doses. Considering both the coagulation efficiency and cost, the optimum dose for Al2(SO4)3 and FeCl3 was fixed at 70 mg/L and 40 mg/L, respectively, where FeCl3 and Al2(SO4)3 gives UV254 removal of around 31,63% and 35,17 respectively. There are many mechanisms that can explain the coagulation process. Action of inorganic salts is based on the charge neutralization mechanism. The repulsive forces between negatively charged colloid particles disappear after the adsorption of the highly charged cations, such as Al3+or Fe3+, and neutralization of surface charge.Variation of floc zeta potential with coagulant dose for the Al2 (SO4)3 and FeCl3 is shown in Fig. 2(c). The floc zeta potential increased with the increasing coagulant concentration [27] [28] , The increase in zeta potential was quite sharp for both coagulants. Coagulation with Al2 (SO4)3 and FeCl3 has been suggested to be achieved by charge-neutralization, sweep flocculation and the bridge-formation mechanism which caused precipitation of amorphous metal hydroxide. [20] [28][29][30]. In this study, the high UV254 removal efficiency by Al2(SO4)3 and FeCl3 was achieved at the optimal dosage of 70 mg/L and 40 mg/L, which probably due to bridge-aggregation, since zeta potential was –5,8 mV and -5,12 mV under these concentration.


3.2. The effect of solution pH on coagulation efficiency

(a)

Change of residual turbidity and UV254 removal, as a function of initial solution pH was investigated under optimum coagulant dose conditions for Al2(SO4)3 and FeCl3 (Fig. 3). Change of residual turbidity and UV254 removal as function of pH were similar for the two coagulant, showing parabolic shape with the inflection points at pH 6,5 and 6 for Al2(SO4)3 and FeCl3, respectively. The Al2 (SO4)3 coagulant was inferior to FeCl3 for turbidity removal in acidic conditions. Change of UV254 removal vs. pH was similar for both coagulants. The UV254 removal coagulant was affected by the pH of solution, and it increased steadily with increasing pH, and then showed decline trend beyond the inflection point. Duan et al. [20] explained that at around neutral pH both Al(III) and Fe(III) have limited solubility, because of the precipitation of an amorphous hydroxide, which can play a very important role in practical coagulation and flocculation processes. The pH values of 6,5 and 6 at the inflection points were therefore selected as the optimum ones for FeCl3 and Al2(SO4)3, respectively, under which condition, UV254 removal varied in order of FeCl3 (44,72%)> Al2(SO4)3 (36,46%) [27]. (a)

Al2(SO4)3 FeCl3

12

Residual turbidity (NTU)

10

8

6

4

2

0 0

20

40

60

80

100

120

Coagulant dose (mg/L)

(b) Al2(SO4)3 FeCl3

40 35

UV254 Removal (%)

30 25 20 15 10 5 0

Al2(SO4)3 FeCl3

4,5

0

20

40

60

80

100

120 4,0

Residual turbidity (NTU)


(c) Al2(SO4)3 FeCl3

-2

3,5 3,0 2,5 2,0 1,5 1,0 0,5

-4

5,0

5,5

6,0

6,5

7,0

7,5

8,0

Zeta potential (mv)

pH

-6

(b)

-8

Al2(SO4)3 FeCl3

50

-10

45

-12

40 35

UV254 removal (%)

-14 -16 0

20

40

60

80

100

120


30 25 20 15 10

Fig. 2 Variation of coagulation performances versus coagulant dose with Al2 (SO4)3 and FeCl3 measured in terms of: (a) residual turbidity, (b) UV254 removal and (c) Zeta potential.

5 0 5,0

5,5

6,0

6,5

7,0

7,5

8,0

pH

Fig. 3 The effect of solution pH on coagulation performance of Al2 (SO4)3 and FeCl3 under optimum dose conditions: (a) residual turbidity, (b) UV254 removal

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3.3. Effect of coagulant pretreatment on membrane fouling 1,0

Normalised permeat (J/J0)

Secondary effluents contain a wide variety of suspended and colloidal particles that cause changes in color and turbidity [31]. Therefore, we tried to find out the effect of organic floc and colloidal matter on the membrane fouling by examining the effect of the coagulation on the changes in permeates flux. The effluent obtained after coagulation treatment using Al2(SO4)3 and FeCl3 as coagulants under optimum dose and the pH of solution were used as the feed water for ultrafiltration to evaluate the membrane fouling potentials of the treated water. Fig. 4 presents the normalized fluxes associated with filtration time. For optimum coagulation conditions, the, samples were fed into the UF membrane after rapid mixing followed by slow mixing. As shown in Figs. 4, the secondary effluent led to the most severe reduction of permeability during UF [32 - 34]. The fluxes dropped to 54% of initial permeate flux. Goren et al. [32], reported that for membranes with MWCO> 20 KDa the retention is due to internal pore adsorption. This phenomenon due to the irreversible character of hydrophobic interactions between the organic matter in the effluent and the membrane surface, leading to pore blocking and internal pore adsorption. However, precoagulation + UF process exerted a lower flux decline. Less flux decline caused by coagulation is considered to be due to the transformation of dissolved organics into a particle that is easily removed by the size exclusion mechanism of UF. That is, during the coagulation process, substantial changes in dissolved organics must occur due to the simultaneous formation of microflocs and NOM precipitates [33]. Therefore, aggregation of small colloids and dissolved organic matter by coagulation may lead to a larger effective particle size, which may result in less specific resistance. Regardless of pretreatment conditions used. When comparing FeCl3 with alum, FeCl3 coagulant caused lower rate of flux decline for membrane ultrafiltration due to enhanced formation of flocs. This result implies that the cake consists of flocs formed on the membrane surface during the filtration of coagulated water, which plays a significant role in preventing fouling. it can be explained that the resistance of the cake layer is mainly depended on the structure of the flocs [35].

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UF alone Al2(SO4)3(70mg/L at pH=6,5)+UF FeCl3 (40 mg/L at pH=6)+UF

0,9 0,8 0,7 0,6 0,5 0,4 0

10

20

30

40

50

60

70

80

90 100 110 120 130

Temps (min)

Fig. 4 Comparison of specific flux decline of ultrafiltration membrane under various pretreatment conditions (J0 = 130 L/h.m2 at 1bar; U= 4 m/s

4 Conclusion Coagulation pre-treatment can enhance the performance of membrane filtration by reducing fouling effects, thereby increasing the membrane working time before cleaning is required. The choice of coagulant is one of the key variables that influences the nature of the flocs and colloid material reaching the membrane, and which in turn affects the development and nature of the cake layer on the membrane surface. In this study a comparison has been made between two conventional, metal-ion coagulants, alum and FeCl3, with respect to their impact on ultrafiltration membrane fouling. The specific findings of this study are as follows: • FeCl3 was an effective coagulant for secondary effluent treatment, with the UV254 removal varied in the following order of FeCl3 (44, 72%)> Al2 (SO4)3 (36, 46%). • For UF filtration without coagulation pretreatment, the flux dropped to 54% of initial permeate flux due to internal pore adsorption. • Applying coagulation process before membrane filtration was found to be very effective in fouling reduction as well as critical flux increase due to the increase in particle size. • The investigation of membrane fouling demonstrated that FeCl3 pre-coagulation resulted in least membrane fouling potential compared to Al2(SO4)3.

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