The impact of anionic polyacrylamide (APAM) on ultrafiltration efficiency in flocculation-ultrafiltration process

With the cost reduction and technique maturation of the ultrafiltration (UF) membrane, UF technology has been increasingly applied in the renovation and new construction of water supply plants. In order to intensify the contaminant removal ability and relieve membrane fouling, various pre-treatment methods, such as coagulation, adsorption and pre-oxidation, have been coupled with the UF process (Huang et al. 2009). Depending on its effectiveness, low cost and easy operation, coagulation with inorganic coagulants has become the most widely used membrane pre-treatment method (Huang et al. 2009; Lai et al. 2015), for which the technological process mainly includes flocculation-UF, and coagulation- sedimentation-UF, coagulation-sedimentation-sand filtration-UF. Compared with other technological processes, the flocculation-UF process has been proved to have some advantages, such as a shorter flow process, smaller floor area, and lower investment (Vickers et al. 1995; Liang et al. 2008; Yu et al. 2014).

In traditional drinking water treatment processes, polyacrylamide (PAM), as a kind of organic polymer coagulant-aid, is always used in the treatment of low-temperature and low-turbidity water, as well as high-turbidity water. According to its charged situation, PAM can be divided into three kinds: cationic polyacrylamide (CPAM), non-ionic polyacrylamide (NPAM) and anionic polyacrylamide (APAM). The coagulation efficiency of PAM is much better than inorganic coagulants. A small dosage can effectively increase the floc size and density, thus improving the subsidence effect (Jarvis et al. 2008).

Based on the above fact, we naturally considered what impact would be generated if PAM was used in the flocculation-UF process. A small number of research results involving this question can be found in the literature. The research of Huang et al. suggests that trace amounts of CPAM in addition to PACl coagulation can effectively increase the permeate flux when microfiltration is coupled with pre-treatment by coagulation (Huang et al. 2004). Conversely, Wang et al. found that CPAM would aggravate membrane fouling because of the electrostatic attraction between the positively charged polymer and the negatively charged membrane surface, but the fouling layer on membrane surface could improve dissolved organic matter rejection (Wang et al. 2011, 2013). In order to avoid the electrostatic attraction between CPAM and the membrane, Yu et al. took NPAM as a coagulant aid for alum before a UF membrane and examined its potential benefit and impact on membrane fouling. They found that the addition of NPAM cannot improve the removal of natural organic matter, but the membrane fouling can be obviously mitigated at a NPAM dosage of 0.2 mg/L (Yu et al. 2013). However, a higher NPAM dosage of 1 mg/L would aggravate membrane fouling. Yao et al. studied the influence on UF membrane fouling when adding NPAM in the floc breakage process. According to their research, broken flocs re-formed with additional NPAM have a larger size and lower effective density, which is available to form a loose and porous cake layer on the membrane surface (Yao et al. 2015). In this way, reversible fouling and irreversible fouling can be simultaneously decreased. However, their NPAM dosage is higher, reaching 5 mg/L.

In general, the addition of PAM in the flocculation-UF process will firstly lead to a change in the floc structure and floc physicochemical properties. Then, the changed flocs will form a fouling layer with a specific structure and function on the UF membrane surface. This formed fouling layer will not only affect the membrane fouling, but also influence the membrane cleaning and contaminant removal. Up to now, the impact of CPAM and NPAM on membrane fouling in the flocculation-UF process has been relatively systematically studied. However, APAM has not been studied. As a kind of hydrophilic linear polymer, there are many carboxyl groups in the APAM molecule. In aqueous solution, the carboxyl groups make APAM negatively charged and it is easy to form a hydrogen bond with other polar groups. Therefore, compared with CPAM and NPAM, APAM is expected to mitigate membrane fouling due to the electrostatic repulsion between the negatively charged polymer and the negatively charged membrane surface. Moreover, other hydrophilic contaminants in water are expected to integrate with APAM based on hydrogen interaction, and this action is beneficial to improving the hydrophilic contaminants removal.

In this study, the impact of APAM on UF efficiency in the flocculation-UF process was systematically summarized. APAM was used as a coagulant-aid with PACl coagulation. The optimal dosage of APAM was obtained according to the flux decline curve. Scanning electron microscopy (SEM) and a contact angle goniometer were employed to support the conclusions. Membrane cleaning with ultrapure water or 0.5% NaOH was conducted to verify whether APAM would affect the membrane cleaning feasibility. The occurrence of pharmaceutical and personal care products (PPCPs) in natural water is an emerging environmental problem. Antipyrine (ANT), a commonly used analgesic, is one of the mostly commonly identified PPCPs. The ecotoxicity of ANT is still largely unknown, but it has high environmental persistence (Tan et al. 2013). Here, the hydrophilic ANT was further used to study the impact of APAM on UF effluent quality. The present work can be a theoretical guidance for the application of APAM as a coagulant-aid in the flocculation-UF process.

Poly-aluminium chloride (PACl, 28% quality calculated as Al₂O₃) was used as the coagulant in this study. The dosage of PACl (2.0 mg/L as Al³⁺) was fixed in all conditions. APAM (Henan Jucheng Chemical Reagent Co. Ltd, China) with a molecular weight of 10 million g/mol was used as a coagulant-aid in this study. Before use, the APAM powder was firstly prepared to 100 mg/L stock solution and left overnight.

In the coagulation procedure, PACl (2.0 mg/L as Al³⁺) coupled with a certain amount of APAM coagulant-aid were added into the prepared synthetic raw water. Simultaneously, the water sample was mixed rapidly at 200 rpm (G ≈ 184 s⁻¹) for 2 min, and then reduced to 50 rpm (G ≈ 23 s⁻¹) for 20 min.

The UF procedure includes five steps: (1) the new membranes were soaked in ultrapure water for 48 h prior to use; (2) a piece of fresh membrane was pre-pressured with ultrapure water at 0.1 MPa until the water flux became constant; (3) 300 ml suspension liquid after flocculation was gently decanted from the flocculator to the UF beaker without sedimentation; (4) the filtration process was started (0.1 MPa, 20 ± 2 °C) until 50 ml suspension liquid remained in the UF beaker; (5) step 3 and step 4 were repeated until the accumulated filtration time meet the experimental requirements.

The concentration of ANT was determined by ultra-high performance liquid chromatography (Acquity, Waters, USA). The chromatographic column was Poroshell 120EC-C18 (50 mm*4.6 mm, 2.7 nm, Agilent). Column temperature: 30 °C. Sample volume: 10 μL. Mobile phase: V (methanol): V (ultrapure water) = 45:55. Flow rate: 0.1 ml/min. Detection wavelength: 242 nm. Retention time: 5 min. The external standard method was adopted to determine the ANT concentration based on the peak area. Water turbidity was determined by a turbidimeter (2100AN, HACH, USA). The zeta potentials of 10 mg/L APAM solution at different pH were measured with a zeta potential analyser (Nano-Z, Malvern, UK). The floc size was measured with a laser diffraction instrument (Mastersizer 2000, Malvern, UK). A SurPASS streaming potential analyzer (EKA, Anton Paar GmbH, Austria) was adopted to measure the membrane surface Zeta potential using 1 mM KCl aqueous solution, with the pH value being adjusted using 0.1 M of HCl or NaOH. The new and fouled membrane samples were characterized by SEM (QUANTA 200, FEI, The Netherlands) and a contact angle goniometer (SL200B3, Solon, China). All the membranes used for characterization should be dried in a vacuum drying oven at 25 °C for 48 h prior to analysis.

The flux decline and change of membrane surface property are respectively the macro-behaviour and micro-behaviour of membrane fouling. In this section, filtration experiments, SEM and the contact angle goniometer are adopted to understand the impact of APAM on membrane fouling.

The foulants in the raw water evenly dispersed in the form of colloids, with a particle size smaller than the UF membrane pore size. They can easily enter the membrane pores and result in membrane pore blockage. This action could lead to serious and continuous flux decline. Flocculation pre-treatment can cause the collision and aggregation of colloids, thus forming flocs with a particle size larger than the UF membrane pore size. The flocs will be transported to the membrane surface by drag force and form a cake layer with a pore structure, thus hindering membrane pore blockage. Therefore, membrane fouling was mitigated when PACl was added to the flocculation-UF process. According to the research of Li and Yao, adding NPAM in the flocculation process can form larger floc clusters and a cake layer with more porosity/permeability on the membrane surface (Yu et al. 2013; Yao et al. 2015). As a coagulant-aid, APAM can play a similar role at a low APAM dosage of 0.1 mg/L and 0.2 mg/L. Besides, the negative APAM in the fouling layer can repel the negative humic acid because of electrostatic repulsion. This repulsion action can protect the membrane from being fouled by humic acid. However, when the dosage increased to 0.4 mg/L and 0.8 mg/L, excess APAM formed a residue in solution or resulted in re-stabilization of flocs. All changes caused higher residual APAM on the surface of the cake layer and membrane, which was adverse to membrane fouling control.

As shown in Figure 4, there are clearly visible pores distributed on the new membrane surface. Different flocculation conditions led to cake layers with different structures being deposited on the membrane surface. The fouling layer formed by the raw water without coagulation consisted of denser and smaller particles, and this structure would have higher hydraulic resistance. By comparing Figure 4(c) and 4(d), we can find that addition of 0.1 mg/L APAM formed a flake-like fouling layer with more porosity. This distinction explains why a low dosage of APAM can mitigate membrane fouling and slow down flux decline. However, when the dosage of APAM increased to 0.8 mg/L, excess APAM acted as a cross linking agent and led to a much denser fouling layer in Figure 4(d). As a result, membrane flux decreased at a higher rate.

Figure 5 indicates that new membrane was relatively hydrophobic, with a contact angle of 96.81°. The contact angle of the membrane fouled by the coagulated suspension with PACl decreased to 80.23°. This change is attributed to the formation of hydrolyzed precipitate layers, rich in hydroxyl, formed by the PACl. The addition of APAM, a kind of hydrophilic polymer, further decreased the contact angle to 63.97°. Although the hydrophilic fouling layer itself is a resistance for mass transfer, it is generally accepted that improving hydrophilicity is beneficial to flux increase and fouling control (Gohari et al. 2014; Rajabzadeh et al. 2015). In the research of Chu et al., PAM was used to alter and control the interfacial properties of the membrane surface, so that improved the membrane anti- fouling ability (Chu & Sidorenko 2013). Thereby, a low dosage of APAM can mitigate membrane fouling and slow down flux decline. However, as the dosage of APAM increased to 0.8 mg/L, the negative impact occupied the main position in spite of a lower contact angle (63.97°).

In general, a low dosage of APAM as coagulant-aid is beneficial to fouling control. Particularly, when the dosage of APAM is 0.1 mg/L, coupled with PACl as the coagulant, the flux decline rate is at its lowest, the membrane fouling layer is porous, and the membrane contact angle is low. Therefore, 0.1 mg/L APAM can be regarded as the optimal coagulant-aid dosage.

Although pre-treatment with flocculation can mitigate membrane fouling, fouling cannot be completely avoided. Membrane cleaning is required when there is a serious drop in permeate flux. Therefore, it is important to clarify whether the addition of the optimal dosage of APAM (0.1 mg/L) would increase the difficulty of membrane cleaning.

As shown in Figure 6, cleaning with ultrapure water was ineffective for both kinds of fouled membranes, as the flux recoveries were only 19.17% and 19.69%. This is because the water molecule itself cannot break up the adsorption interaction between the fouling layer and the membrane surface. Nevertheless, the similar flux recoveries indicate that the addition of 0.1 mg/L APAM did not increase the difficulty of physical membrane cleaning. As for the chemical cleaning with 0.5% NaOH, the different flux recoveries of the two fouled membranes suggested that the addition of 0.1 mg/L APAM improved the chemical cleaning efficiency, with the FR up to 60.72%, which was much higher than 40.55%.

Decreasing the membrane fouling rate and membrane cleaning difficulty, and increasing the effluent quality, are three different aspects of an ideal membrane process. An effective membrane process should be able to reject a pollutant as much as possible. A low dosage of APAM (0.1 mg/L) in the flocculation process has been demonstrated to be beneficial for fouling control and membrane cleaning in the above sections. As a kind of PPCPs, the molecular weight of ANT is 188.23 g/mol, much lower than the UF membrane cut-off molecular weight (100 kDa). ANT is hydrophilic and easy to dissolve in water. According to the size exclusion model, ANT cannot be rejected by the UF membrane. Therefore, if the addition of APAM (0.1 mg/L) in the flocculation process can improve the rejection of ANT, it can be inferred that the impact of APAM on effluent quality is favourable.

As shown in Table 1, the turbidity removal rate of flocculation with PACl was 86.3%, while that of flocculation with PACl + APAM increased to 91.5%. Meanwhile, the floc sizes for flocculation with PACl and PACl + 0.1 mg/L APAM were 136 μm and 248 μm, respectively. Flocs with a larger size usually have better settleability. Therefore, flocculation with PACl + 0.1 mg/L APAM has higher turbidity removal efficiency. As shown in Figure 8(a), the ANT removal of the single flocculation process with PACl was only 5.57%. The addition of 0.1 mg/L APAM increased the removal to 10.54%, which indicates the positive impact of APAM on ANT removal in the flocculation process. This is because APAM can increase the floc size and activity. The larger flocs containing APAM had more active adsorption sites, which can adsorb more ANT and form a ‘coalition’, as mentioned above. This ‘coalition’ can be removed from the system by sedimentation. Figure 8(b) indicates that new UF membrane without fouling had a weak removal ability for ANT (7.88%) as well, although the molecular weight of ANT was much lower than the UF membrane cut-off molecular weight. This phenomenon can be attributed to the adsorption interaction between the membrane material and ANT. When the fouling layer formed by the coagulated suspension with PACl appeared on the membrane surface, ANT removal increased to 20.61%. In the research of Ma et al., they found the precipitate layer of aluminum sulfate hydrate on the UF membrane surface could adsorb humic acid of low molecular weight, thus improving the removal rate (Ma et al. 2013). Similarly, the fouling layer formed by the coagulated suspension with PACl can absorb the ANT and increase the ANT removal rate. When the fouling layer formed by the coagulated suspension with PACl and 0.1 mg/L APAM appeared on the membrane surface, ANT removal further increased to 33.34%. As mentioned earlier, the addition of 0.1 mg/L APAM makes the fouling layer more porous and hydrophilic. Figure 7 has indicated that the zeta potential of the APAM solution at pH = 7.5 was between −21.53 mV to −22.23 mV. Therefore, this fouling layer would have more active adsorption sites due to the negative charge and rich polar groups of APAM. As illustrated in the chemical structural of the ANT molecule in Figure 1, there are two nitrogen atoms in one ANT molecule. According to the molecular conformational analysis, the lone pair of electrons of the nitrogen atoms would attract the H⁺ intensely. Meanwhile, the asymmetric structure of the ANT molecule endows it with strong polarity. Thus more ANT molecules can be adsorbed in the fouling layer because of electrostatic attraction and hydrogen interaction.

In this study, APAM was used as a coagulant-aid with PACl coagulation. The impact of APAM on UF efficiency in the flocculation-UF process has been investigated with regard to membrane fouling, membrane cleaning and effluent quality. The obtained results are concluded as follows. (1) The optimal dosage of APAM was 0.1 mg/L coupled with 2 mg/L PACl (as Al³⁺). Under this optimal condition, the membrane fouling can be mitigated, the membrane fouling layer is porous, and the membrane contact angle is low. Higher dosage of APAM will aggravate membrane fouling. (2) APAM did not increase the difficulty of physical membrane cleaning. Specifically, the addition of APAM can improve the cleaning efficiency of chemical cleaning with 0.5% NaOH due to the disintegration of the fouling layer when APAM was dissolved under strong alkaline conditions. (3) When 0.1 mg/LAPAM was used as the coagulant-aid with PACl, more active adsorption sites were formed both in the flocs and membrane fouling layer, thus more ANT molecules in the raw water were able to be adsorbed and removed in the flocculation-UF process.

The authors gratefully acknowledge the financial support provided by the State Key Laboratory of Urban Water Resource and Environment (HIT, Grant no. 2016DX11), and National Natural Science Foundation (51508383).