The effect of coagulation with MF/UF membrane filtration for the removal of virus in drinking water

doi:10.1016/j.memsci.2005.12.023

Journal of Membrane Science

Volume 279, Issues 1–2, 1 August 2006, Pages 364-371

https://doi.org/10.1016/j.memsci.2005.12.023 Get rights and content

Abstract

Removal of phage MS2 in drinking water by pre-coagulation and filtration through ultrafiltration (UF) and microfiltration (MF) membranes was investigated in a laboratory test unit. Two commercial aluminium-based coagulants (ALG and PAX) were used. The low (≤1–30 pfu/mL) numbers of infective virus detected in permeates show that pre-coagulation/flocculation in combination with both loose UF and MF membrane filtration was an effective hygienic barrier against MS2 virus. Without pre-coagulation/flocculation, no (MF) or only a minor (UF) virus removal was observed. The two coagulants provided high virus removal efficiencies (>7.4 logs) after UF and MF membrane filtration using doses of 5 mg Al/L. Reducing the dose of coagulants to 3 mg Al/L had only a small effect on the removal efficiency, which remained high after UF membrane filtration, and ALG/MF membrane filtration (>7.1 logs). The virus removal obtained with PAX (3 mg Al/L) and MF membrane filtration was slightly less (6.7 logs) than for the other conditions. Combined removal of virus and colour by pre-coagulation/flocculation and MF membrane filtration was also investigated. The low concentrations of both virus and colour in the permeate indicate that low-pressure MF filtration in combination with pre-coagulation/flocculation is a potential efficient technology for removal of virus and colour in drinking water.

Introduction

Low pressure membrane filtration (microfiltration (MF) and ultrafiltration (UF)), are today used worldwide for reduction of particle concentration and natural organic material (NOM) in drinking water [1]. Central to this application is also the ability of the various membrane filtration processes to remove pathogenic microorganisms such as protozoa, bacteria and viruses. UF technology has been shown to meet current water regulation for turbidity and Giardia. However, the retention of virus is dependent on the type of membrane and membrane characteristics, module design and operating conditions. The efficacy of low-pressure MF/UF as a hygienic barrier is of concern. Some national regulations only recognize nanofiltration (NF) membranes as hygienic barriers, based on their low nominal pore size, usually less than 10 nm.

In general, a correlation may be expected between the retention of particles of a certain size, and the nominal pore size or the molecular cut-off (MWCO) of the membrane. On the basis of nominal cut-off size of membranes, viruses can be expected to be completely retained by UF membranes in the lower (10–100 kDa) MWCO range. However, varying levels of virus removal by MF/UF membranes have been reported in the literature. Complete removal of poliovirus [2] and more than 6 logs of MS2 virus removal [3] was obtained using 30 and 100 kDa MWCO membranes, respectively, whereas incomplete Qβvirus retention (2.5 logs) has been observed using UF membranes in the 30 kDa range [4]. Virus removal has also been reported with MF membranes; 0.5 logs of MS2 was achieved with a 0.4 μm nominal pore size membrane [5], and 0.2–1.0 log of MS2 [3] and <2 logs poliovirus [2] with membranes of 0.2 μm nominal pore size. Retention of viruses cannot be predicted solely based on nominal pore size or the MWCO characteristics specified for a membrane. Small viruses can penetrate membranes defined as NF and UF membranes, and retention values varying from 99 to 99.9999% (translates into 2–6 log removal) have been demonstrated [4]. Membranes are known to have varying pore size distributions with respect to a measured nominal pore size as a function of membrane material and manufacturing method, and observed virus leakage on NF/UF membranes may be due to the presences of abnormally large pores. More efficient virus retention can potentially be expected in a combined process where pre-coagulation/flocculation is used with UF or MF membrane filtration when viruses are adsorbed to or included in larger flocks that are retained by the membrane. More than 7 log removal of the Qβvirus removal [6] and more than 6 log removal of MS2 and Qβvirus [7] was obtained after coagulation pre-treatment and filtration through ceramic MF membranes.

Different viruses may respond quite differently to coagulation conditions. After pre-coagulation with alum and sedimentation the numbers of MS2 and human enteric poliovirus in drinking water are reduced more efficiently than phage PRD-1 and enteric echovirus, whereas the coagulant polyaluminium chloride provide higher reduction of MS2 than of PRD-1, poliovirus and echovirus [8]. MS2 is reduced more efficiently than phage φX174 with alum as coagulant, which was therefore regarded as a more conservative and reliable surrogate for human enteric virus removal by conventional chemical treatment than MS2 [9]. However, mechanisms of particle separation are different for sedimentation and UF/MF membrane filtration processes. A conservative virus surrogate obtained from studies of pre-coagulation/sedimentation processes will therefore not necessarily be the best virus surrogate for pre-coagulation/membrane filtration processes. In this study MS2 has been chosen.

In many regions of the world surface water is used as a drinking water source. In temperate regions the water is commonly characterized by high concentrations of NOM. For example in Norway, about 90% of the water supplies are from surface water sources, generally from lakes. The raw water is characterized by high colour, low pH, low alkalinity and very low turbidity. The removal of NOM is therefore a major treatment requirement where raw water concentrations are typically in the 30–80 mg Pt/L true colour range and TOC 3–8 mg C/L. These waters also have typical DOC values in the 2–5 mg DOC/L range giving SUVA values around 4 and higher. The nature of the NOM based on these values can therefore be characterized as being mostly aquatic humics with high hydrophobicity and high MW compounds. Turbidity is commonly between 0.2 and 1.0 NTU and alkalinity varying between 0.1 and 0.5 meq/L. UF and MF membrane filtration in combination with pre-coagulation/flocculation are potential alternatives to coagulation and granular filtration for colour (NOM) removal [10], [11]. One of the expected benefits of the membrane process is a reduction of coagulant demand.

The use of membrane filtration in drinking water treatment has increased in recent years. Polymer membranes are dominating and increasingly used in membrane filtration processes for drinking water production. The objective of the present study was to investigate the efficacy of pre-coagulation/flocculation combined with low pressure membrane filtration through polymer UF and MF membranes for virus and NOM removal in drinking water production.

Section snippets

Membranes

Two membranes have been investigated in this study, a polypropylene (PP) hollow fibre UF membrane with a molecular weight cut-off of 30,000 and a polyethersulfone (PES) hollow fibre MF membrane with a nominal pore size of 0.2 μm. The membranes are characterized according to manufacturer specifications (Table 1). The hydrophobic PP membrane was wetted prior to experiments.

Virus

In order to measure the virus retention, the bacteriophage MS2 (ATCC 15597-B1) was seeded to autoclaved tap water.

Results and discussion

The use of UF and MF membranes as hygienic barriers in drinking water production depends on their ability to retain different types of pathogens, and in particular viruses. The efficiency of the virus removal process can be expressed by a log reduction value (LRV), which is defined as LRV = log C_r/C_p, where C_r and C_p represent the virus concentrations in the reservoir and permeate, respectively. When no viruses are detected in the permeate, i.e. C_p < 1 pfu/mL, LRV can be reported as >log C_r. In this

Conclusions

The low numbers of infective virus detected in permeates of all experiments with pre-coagulation/flocculation show that pre-coagulation/flocculation in combination with both UF and MF membrane filtration was an effective hygienic barrier against MS2 virus. The MF membrane retained the virus to a similar extent as the UF membrane, i.e. the average LRVs were ≥6.7 for all experimental conditions. Without pre-coagulation/flocculation, no virus removal was observed for the MF module and only a minor

Acknowledgements

This work was supported by a grant from the Norwegian Research Council NFR. The authors thank research assistants Turid Wist and Rolf Magnus Åhl for technical assistance during the experiments.

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The effect of coagulation with MF/UF membrane filtration for the removal of virus in drinking water

Abstract

Introduction

Section snippets

Membranes

Virus

Results and discussion

Conclusions

Acknowledgements

Desalination

J. Membr. Sci.

J. Membr. Sci.

Water Res.

J. Membr. Sci.

Water Sci. Technol.

Mechanism of Cryptosporidium, Giardia, and MS2 virus removal by MF and UF

AWWA

Virus removal by ceramic membrane microfiltration with coagulation pretreatment

Water Sci. Technol. Water Supply