for complete removal of viruses for several reasons (Asano et al., 2007). First, whereas the terms micro-and ultrafiltration nominally refer to pore sizes that have cutoff characteristics as shown in Figure 4-3, the actual pore sizes in today’s commercial membranes often vary over a wide range. Second, experience has shown that today’s membrane systems sometimes experience problems with integrity during use for a variety of reasons. Although membrane integrity tests have been developed and these tests are widely used, they are not suitable for detecting imperfections small enough to allow viruses to pass.
Nevertheless, it is generally believed that the new generation of filtration systems has significantly improved performance for microbial removal. For example, Orange County Water District (OCWD) compared the MF filtration result of their current groundwater replenishment system (GWRS) operation initiated in 2008 (see Table 2-3) with data collected during Interim Water Factor 21 (IWF21), the precursor to the GWRS project, started in 2004. Although the influent water quality was similar for both projects, IWF21 MF filtrate showed breakthrough of total coliform in 58 percent of the samples and Giardia cysts in 23 percent of samples, whereas both were absent in the GWRS MF filtrates (OCWD, 2009). However, MF did not eliminate viruses. Coliphages were present in GWRS after MF treatment. The geometric mean of male-specific coliphage was 134 plaque-forming units (pfu)/100 mL in MF-treated water (OCWD, 2009). Combining MF with chlorination is likely to improve the rate of virus removal. The OCWD reports significant reduction of coliphage in the MF feed in the presence the chloramine residual. Male-specific coliphage dropped from a geometric mean of 1,800 pfu/100 mL in the previous year to 28 pfu/100 mL in the MF feed and they were absent in the MF filtrate (OCWD, 2010).
MF and UF membranes sometimes in combination with coagulation can also physically retain large dissolved organic molecules and colloidal particles. Effluent organic matter and hydrophobic trace organic chemicals can also adsorb to virgin MF and UF membranes, but this initial adsorption capacity is quickly exhausted. Thus, adsorption of trace organic chemicals is not an effective mechanism in steady-state operation of low-pressure membrane filters.
Nanofiltration or Reverse Osmosis
For reuse projects that require removal of dissolved solids and trace organic chemicals and where a consistent water quality is desired, the use of integrated membrane systems incorporating MF or UF followed by NF or RO may be required. RO and NF are pressure-driven membrane processes that separate dissolved constituents from a feedstream into a concentrate and permeate stream (Figure 4-4). Treating reclaimed water with RO and NF membranes usually results in product water recoveries of 70 to 85 percent. Thus, the use of NF or RO results in a net loss of water resources through disposal of the brine concentrate. RO applications in water reuse have been favored in coastal settings where the RO concentrate can be conveniently discharged to the ocean, but inland applications using RO are restricted because of limited options for brine disposal (see NRC [2008b] for an in-depth discussion of alternatives for concentrate disposal and associated issues). Thus, existing inland water reuse installations employing RO membranes are limited in capacity and commonly discharge brine to the sewer or a receiving stream provided that there is enough dilution capacity.
Most commonly used RO and NF membranes provide apparent molecular weight cutoffs of less than 150 and 300 Daltons, respectively, and are therefore highly efficient in the removal of organic matter and selective for trace organic chemicals. Some of the organic constituents that are only partially removed by NF and RO membranes while still achieving total organic carbon (TOC) concentrations of less than 0.5 mg/L are low-molecular-weight organic acids and neutrals (e.g., N-nitrosodimethylamine [NDMA], 1,4-dioxane) as well as certain disinfection byproducts (e.g., chloroform) (Bellona et al., 2008). Recent advances in membrane development have resulted in low-pressure RO membranes and NF membranes that can be operated at significantly lower feed pressure while providing approximately the same product water quality. However, certain monovalent ions (e.g., Cl–, Na+, NO3–) are only partially rejected by NF, and NF membranes result in product water with higher TDS than RO (Bellona et al., 2008).
Today, most integrated membrane systems applied in reuse employ RO rather than NF. However, certain low-pressure NF membranes offer opportunities for