Surface water treatment using nanofiltration — pilot testing results and design considerations

Integrated membrane systems (IMS) were pilot tested for treatment of a high TOC river water. Pretreatment methods included microfiltration, in-line coagulation–microfiltration, and coagulation–sedimentation–filtration. Fouling was minimized at lower flux, lower recovery and with the addition of a biocide. Such experiments incurred no fouling. A cellulose acetate nanofilter was susceptible to biological degradation therefore a biocide was necessary to ensure membrane integrity. A polyamide membrane was sensitive to oxidation by monochloramine and rejection characteristics were compromised. The rate of fouling between the three nanofilters tested increased with increasingly negative surface charge and increasing surface roughness. Organic adsorption therefore did not follow the charge–repulsion theory but may have been negatively influenced by the greater surface area associated with a rougher surface. Modeling of productivity decline by a form of the resistance model using permeate volume and TOC concentration provided a better fit than that of the linear model of productivity with time. Rejection of organic and inorganic parameters increased with decreasing nanofilter molecular weight cut-off (MWCO). Rejection was influenced by diffusion and size exclusion mechanisms. Log removals of Bacillus subtilis spores by IMS ranged from 5.4 to 10.7 log with the highest removals achieved by microfiltration pretreatment followed by a low (200 dalton) MWCO polyamide nanofilter.

6.1. Productivity The order of membrane fouling, from most to least fouling, was ESNA>LFC1>CALP. The CALP system requires addition of an oxidant such as monochloramines to control biological degradation of the membrane film. The CALP system required no chemical cleanings over a 6-month period. The 10% decline in Kw observed was independent of pretreatment process and operating conditions. Projected CALP nanofilter cleaning frequency is once every 9 months. The ESNA was irreversibly fouled and was not a viable membrane for this source. Bacterial growth contributed to LFC1 fouling; cleaning cycles were improved with the addition of monochloramine. The LFC1 suffered damage due to oxidation from the monochloramine and would need a non-oxidative biocide for use at this site. The order of membrane surface charge, from most to least negative, as well as surface roughness, from most to least rough, was ESNA>LFC1>CALP, which followed the order of fouling. A more negatively charged membrane surface correlated with increased fouling, indicating electrostatic repulsion, was not effective for fouling control. Increased surface area due to greater surface roughness for the more negatively charged membranes may have contributed to organic adsorption fouling. Fouling was reduced with addition of monochloramine, lower flux, and lower recovery. Experiments operated under such conditions did not foul and affirmed the viability of IMSs for treatment of this source. Statistical regression of productivity decline data showed that a resistance model using permeate volume and TOC concentration better fit Kw decline data than the linear model and other less complex resistance models. Further development of the resistance model is recommended. 6.2. Water quality MF can remove significant amounts of particles, turbidity, and pathogens and cannot reject color, DBPFP or DOC unless augmented by another process. The nanofiltration systems provide a higher quality water compared to conventional coagulation treatment (CSF) or MF treatment. The LFC1 nanofilter provided a higher quality permeate than the CALP nanofilter. This can be attributed to the composition and lower MWCO of the LFC1 compared to the CALP. The CALP nanofilter can meet current and Stage 1 D/DBPR regulations. Stage 2 regulations may require a coagulant-based pretreatment or an alternate disinfectant. The LFC1 nanofilter can meet Stage 2 regulations with any of the pretreatment processes evaluated. Log removals of the sporulated-form of the pathogen surrogate Bacillus subtilis by IMSs was found to be a function of pretreatment unit as well as nanofilter. Spore removal of the least capable IMS averaged 5.4 log compared to 10.7 log for the most capable system. The MF systems outperformed CSF and the LFC1 nanofilter outperformed the CALP nanofilter. Organic and inorganic rejection were influenced by both diffusion- and sizeexclusion mechanisms. Higher molecular weight compounds were controlled more by size exclusion.