Application of ultrafiltration membranes for removal of humic acid from drinking water

Humic acids are primarily a result of the microbiological degradation of surrounding vegetation and animal decay and enter surface waters through rain water run-off from the surrounding land. This often gives rise to large seasonal variations, high concentrations in the wet season and lower concentrations in the dry season. Alone humic acid is just a colour problem but when present in conventional treatment processes like chlorination, carcinogenic by-products like trihalomethane and haloacetic acid are formed. This, in addition to the demand for clean potable drinking water, has sparked extensive research into alternative processes for the production of drinking water from various natural/industrial sources. One of the major areas of focus in these studies is the use of membranes in microfiltration, ultrafiltration and nanofiltration. In this report the humic acid removal efficiency of ultrafiltration membranes with 3 kDa, 5 kDa and 10 kDa MWCO is examined. The membranes were made of regenerated cellulose and were in the form of cassette providing a 0.1 m2 surface area. At first distilled and deionised water, known as milliQ water, was used as the background feed solution to which humic acid powder was added. It was found that all three membranes removed humic acid with an efficiency of approx. 90% and were capable of reducing initial concentrations of 15mg/L to below the New Zealand regulatory limit of 1.17 mg/L. The permeate flux at a transmembrane pressure of 2.1 bar was approx. 20 l/m2/h (LMH) and 40 LMH, respectively through the membranes with MWCO 3 kDa and 5 kDa. These membranes experienced significant surface fouling resulting in retentate flow rates as low as 11 litres per hour after just four runs compared to the recommended 60–90 l/h. Cleaning with 0.1 M NaOH slightly improved the retentate flow rate, but well below those obtained with fresh membranes. The 10 kDa membrane provided high retentate flow rates which evidently minimised fouling by providing a good

• The ultrafiltration membranes, of MWCO 10 kDa, 5 kDa and 3 kDa, are capable of removing humic acid from the model feed solutions to levels below those required under New Zealand regulation. • The 3 kDa and 5 kDa membranes experienced severe fouling possibly due to the adsorption of humic acid molecules on the surface and inside the membrane pores. • The 10 kDa membrane provided highest permeate flux with minimum fouling and is capable of reducing an initial humic acid concentration of 15 mg/L to below the maximum allowable level of 1.17 mg/L. This was concluded to be the optimal membrane size as it provides best humic acid removal and operability. • Transmembrane pressure was found to have little influence over the humic acid removal efficiency of the 10 kDa membrane but for faster processing it is recommended that the TMP be in the range 2–3 bar. • Increasing the initial humic acid feed concentration from 10 mg/L to 50 mg/L resulted in an increase in the humic acid removal efficiency, which is suspected to be a result of pore blockage and eventually restricting the molecules to pass through. • Fouling of the membrane surface was found to be the common form of fouling for all three membranes and was observed by a decrease in the retentate flow rate over time. Only the 10 kDa membrane was found to experience pore fouling and this was observed as a decrease in the permeate flux through the membrane. • Both UV absorption at a wavelength of 254 nm and total organic carbon gave accurate indications of humic acid removal. TOC measurements generally gave lower removal efficiencies than UV254 since it was more sensitive to the smaller aliphatic compounds which could more easily penetrate through the pores.