Application of hybrid coagulation – ultra ﬁ ltration for decentralized drinking water treatment: impact on ﬂ ux, water quality and costs

Decentralized membrane-based water treatment represents an attractive and viable approach to safe water supply in low-income areas, but its widespread adoption requires cost-effective antifouling strategies. Although the antifouling mechanisms of Al ‐ based coagulants have been widely investigated, there is little data about their impact on costs and treatment ef ﬁ ciency for decentralized membrane-based systems. In this study, a comparative assessment of two decentralized ultra ﬁ ltration (UF) units with and without polyaluminum chloride (PACl) coagulation was undertaken to evaluate the in ﬂ uence of coagulation on the fouling, water quality, and costs nexus. The results showed that PACl suppressed both total fouling and hydraulically irreversible fouling. A matched-pair analysis also revealed that PACl improved the permeate quality by enhancing the removal of particulates and dissolved organics. Compared with the conventional UF system, the hybrid coagulation – UF system contributed to a 21% increase in the ﬂ ux rate, allowing for a 27% reduction in membrane area and thus, providing cost bene ﬁ ts in terms of both capital and operating costs. These results suggest that PACl coagulation is potentially a cost-effective antifouling method for decentralized membrane-based water systems.


INTRODUCTION
Inadequate access to safe drinking water imperils life, subdues opportunity and subverts human dignity (Watkins ). Yet 2.1 billion people -29% of the global population lack safely managed drinking water services (WHO/ UNICEF ). Low-income countries (LIC) are disproportionately affected by the pernicious and persistent water crisis of today, underpinned by a shortage of fresh water resources due to climate change and escalating levels of water pollution. In many LIC, drinking water treatment is mainly focused on conventional technologies such as media filtration and chlorine disinfection. However, conventional technologies are designed for use in large centralized systems. Therefore, they are highly unlikely to be installed in rural communities due to their high investment costs.
Nowadays, decentralized membrane-based water treatment has emerged as a sustainable approach to safe water supply in LIC. However, membrane fouling remains an overriding obstacle. To alleviate fouling, raw water pretreatment using techniques such as coagulation, biofiltration, adsorption, and oxidation has been widely proposed. Among these techniques, coagulation remains the most successful method for controlling membrane fouling in full-scale water treatment (Arhin et al. ).
Aluminum sulfate is the most commonly used coagulant and can suppress the hydraulically irreversible fouling rate of hollow fiber membranes by 75-100% (Hatt et al. ). However, currently, there is an upsurge in the use of pre-hydrolyzed coagulants, including polyaluminum chloride (PACl). PACl contains substantial proportions of highly charged tridecamer cationic species (Al 13 ), which are effective in neutralizing negatively charged colloids (Arhin et al. ). Much work has been aimed at expounding the antifouling mechanisms of coagulants and at identifying coagulation conditions most effective for fouling abatement (Howe & Clark ). Promising results seem to be contingent on the coagulant type, dosage, mixing conditions, flow configuration, membrane properties, and feed water characteristics. Although a plethora of coagulationultrafiltration (UF) studies have reported favorable results with regards to contaminant elimination and flux amelioration, studies on the impact of coagulation on treatment costs are relatively rare. Yet, information in this regard is essential for the widespread adoption of such systems in LIC.
The aim of this study was, therefore, to evaluate the influence of coagulation on the performance of a pilotscale UF unit for decentralized water treatment in terms of flux, water quality, and costs. Although the hybrid coagulation-UF system is rarely applied in LIC, we presume it could represent a potentially cost-effective approach to alleviating the drinking water crisis in LIC.
This hypothesis is premised on the diminishing cost of UF membranes, the relatively low cost of coagulants and the need to deal with freshwater quality problems in LIC, including eutrophication, cyanobacteria, and disinfection by-product precursors, which make conventional treatments rather costly.

Feed water and study area
The feed water used in this study was collected from Lake Victoriaat the inlet to Ggaba II Water Treatment Plant, Kampala, Uganda. The characteristics of the feed water (Table A1) and further information on the study area are presented in the Appendix (available with the online version of this paper).

Pilot-scale ultrafiltration experiments
Two sets of UF experiments were used to assess the influence of PACl on flux, permeate quality and costs. In the first, commercial liquid PACl solution (Zetafloc 553 L; Abby Laboratories, South Africa) was used to pretreat the feed water prior to UF (herein denoted as system A). Coagulation pretreatment was performed using the optimum conditions observed during bench-scale tests as described previously: PACl dose of 20 mg·L À1 , corresponding to 1.21 mg-Al·L À1 and a hydraulic retention time of 14 min (Arhin et al. ). The pH of the raw water was not adjusted during coagulation in order to mimic full-scale conditions at the local water treatment plant. The setup for system A, therefore, was comprised of coagulation/flocculation followed by UF without sedimentation. In the second set of experiments (system B), the feed water was treated by UF without PACl coagulation (0 mg·L À1 PACl). The setup for the two systems is schematically shown in Figure A2, Appendix (available online).
The treatment efficiency of the two systems was assessed based on the removal of turbidity, color, dissolved organic carbon (DOC), ultraviolet absorbance at 254 nm (UVA 254 ) and specific UVA 254 absorbance (SUVA). Therefore, periodic samples of feed water and permeate were taken and the removal efficiency of each parameter (solute rejection, R) was quantified as: where C p and C f are the permeate and feed concentrations, respectively.
The UF membranes used in this study were polysulfone in NaOCl (100 mg/L) for 3 h, followed by NaOH (0.02 N) for 3 h, then rinsed thoroughly with permeate.

Fouling rate quantification
The universal membrane fouling index (UMFI) was used to quantify the fouling rate in each system. The UMFI is defined as: where J 0 s is the inverse of the normalized specific flux, J s /J os (dimensionless), with J s being the specific flux (J/P), where J is the flux (m/s) and P the transmembrane pressure (Pa), and where J os is the flux at time 0, and V s is the unit permeate throughput (Lm À2 ) (Huang et al. ). The UMFI for total fouling (UMFI T ) was determined by linear fitting of the experimental values of J 0 s against V s for each system ( Figure A3, Appendix, available online). In comparison, the UMFI for hydraulically irreversible fouling (UMFI IR ) and the UMFI for chemically irreversible fouling (UMFI C ) were determined by inserting J 0 s and V s values obtained at the onset of hydraulic backwash and chemical cleaning, respectively, into Equation (2).
The microbiological quality of the permeate was assessed by using the Colilert-18 test kit based on the Most Probable Number method (American Public Health Association ). The log reduction value (LRV) was calculated as: where C f and C p are the concentration of coliform bacteria in the feed water and permeate, respectively. When no coliform bacteria were detected in the permeate (C p < 1 CFU/ 100 mL), LRV was reported as >log C f .

RESULTS AND DISCUSSION
Effect of polyaluminum chloride on permeate flux The variations in the average normalized flux and UVA 254 removal rates for systems A and B are shown in Figure 1(a) and 1(b), respectively. As depicted in Figure 1(a), a two-stage flux decline mode consisting of a rapid flux decline during the initial filtration periods followed by a slow drop was observed in both systems, however, to varying extents. For system B the flux dropped rapidly to 58% on the 9th day.
This rapid flux decline was followed by a gradual decrease over the next duration of the experiment. In contrast, the flux for system A declined rapidly to 82% on the 5th day and this was followed by a relatively stabilized flux in the subsequent days.
This two-stage flux decline mode exhibited by the normalized flux curves suggests potential changes in the fouling mechanisms, as per the pore blockage-cake filtration model described in previous studies (Ho & Zydney ). The rapid flux decline observed during the initial filtration periods suggests that pore blocking was the most dominant fouling mechanism at that stage, attributable to the adsorption of UVA 254 on/in the membrane via hydrophobic interaction.
However, this was followed by cake layer formation in the subsequent days. After 25 days of operation, the flux of system A was 21% higher than system B, suggesting that PACl coagulation could ameliorate permeate flux.
Figure 1(c) presents the nature and extent of fouling in systems A and B. A higher UMFI T value was observed in system B than in system A, suggesting that PACl was effective in reducing the total fouling rate. Furthermore, the value for UMFI IR that indicates the residual fouling after hydraulic backwash was higher in system B than in system A, suggesting that hydraulically irreversible fouling was also suppressed by PACl coagulation. However, our results showed no distinction in the chemically irreversible fouling rate for the two systems.
As shown by the matched-pair analysis in Figure 1(d), the line corresponding to y ¼ 0 that shows whether the UMFI values for systems A and B are equal is within the mean ± 95% confidence interval (CI) range for UMFI C , indicating that there was no statistical difference. This is probably due to the low levels of the UMFI C values in both systems (<0.00005 m 2 L À1 ), which were difficult to quantify as previously observed (Huang et al.

).
Water quality The quality of permeate produced from systems A and B in comparison with the local drinking water standards is presented in Table A3 (Appendix, available with the online version of this paper). As expected, UF was found to be very effective in removing pathogenic indicators (E. coli treated water was determined from the SUVA calculations. As shown in Figure 2(a), SUVA removal increased with PACl coagulation. The average percent removals in systems A and B were 57.1% and 21.9%, respectively. A matchedpair analysis was used to statistically evaluate the impact of PACl on the water quality parameters monitored. As shown in Figure 2(b), the line corresponding to y ¼ 0 is above the mean ± 95% CI range for all the quality parameters assessed, indicating that the differences between system A and B values were statistically different from zero.

Cost analysis and implications for drinking water treatment
In order to compare the costs of the hybrid system with the conventional UF system, a cost analysis was conducted on operating the UF unit with a PACl dose of 1.21 mg-Al·L À1 CAPEX associated with these two elements are given as: where C P is the plant capacity (m 3 /d); C C , C L , C D , C S , and C CS are the cost (USD) of a coagulation unit, land required, dosing pump, chemical storage unit, and chemical sludge treatment unit, respectively; t M , t C , t D t S , and t CS are the lifespan (d) of the membrane, coagulation unit, dosing pump, chemical storage unit and chemical sludge treatment unit, respectively and log (C C ) ¼ 0:222 × (log (C P )) 1:516 þ 3:071 As depicted in Figure 3 costs per kg to be C p , then the OPEX associated with these two components are given as: where t is the membrane life (y), J 1 and J 2 are the fluxes at time t with and without PACl, respectively and d is the PACl dose (kg·m À3 ). As shown in Figure 3(e), PACl dosing at 1.21 mg-Al·L À1 provides OPEX benefits in the first 5 y of membrane life based on the current PACl and membrane replacement cost estimates (Table A4, Appendix, available online). Even with a conservative assumption that PACl costs will increase at 8% per annum (p.a.), but the membrane replacement costs will diminish at 8% p.a. due to advances in membrane technology, PACl dosing provides cost benefits in the initial 3.5 y of membrane life provided the 21% increase in flux rate is sustained (Figure 3(f)). However, considering that the realistic estimate of membrane life is 7 years, PACl dosing will incur a cost penalty of 1 cent/m 3 of water pro-

CONCLUSIONS
PACl coagulation ameliorated the permeate flux in the pilotscale UF unit by significantly suppressing both the total fouling and hydraulically irreversible fouling. Additionally, PACl improved the permeate quality by significantly enhancing turbidity, color, DOC, UVA 254 , and SUVA removal. In comparison with the conventional UF system, the CAPEX of the hybrid coagulation-UF system shows a strong correlation with production capacity. Hence, the hybrid system is more favorable than the conventional UF system at larger capacities. OPEX, on the other hand, appears to be highly correlated to factors such as coagulant and membrane replacement costs, which are also dependent on local variables, including inflation. However, even in the worst-case scenario, OPEX penalties resulting from coagulation could be offset by savings in chlorine dosing. Overall, the results strongly suggest that the hybrid coagulation-UF system is potentially a cost-effective model for decentralized water supply.