Effect of a nanofiltration combined process on the treatment of high-hardness and micropolluted water

The quality of raw water and the current high level of pollution presents a phenomenon of high hardness and micropollution. An experimental study was conducted of the nanofiltration (NF) pilot-scale process combined with biological contact oxidation precipitation and ultrafiltration (UF) as the pretreatment process to treat this water. The study investigated the removal efficiency and membrane fouling of the NF process under the continuous and stable operating conditions of the combination process and studied the influence of high-hardness water on the membrane pollution of the combination process. The results showed that the combined process had a positive removal effect on conventional pollutants and characteristic pollutants, and the removal rates of conventional pollutants, such as turbidity, UV254 and CODMn, were 95%, 90% and 85%, respectively. The removal efficiency of total hardness, total alkalinity and soluble total solids reached 98%, 86% and 91%, respectively, and that of total desalination was above 95%. The removal rates of fluorescent organic substances, such as tryptophan, tyrosine, soluble microbial products (SMPs), fulvic acid and humus-like substances, as well as the precursors of disinfection byproducts reached over 88% and 50%, respectively. The pollutant removal efficiency of the combined process was mainly concentrated in the NF unit. The pretreatment process had certain removal effects on turbidity and macromolecular organic substances in the raw water, which provided a perfect operating environment for the NF process. Under long-term operation, the main elements of scaling on the surface of the NF membrane included C, O, Na, Mg, Al, Si, S, Cl, Ca, Ti and Fe, which were mainly concentrated at the outlet of the membrane and mainly came from monomers or compounds composed of inorganic salts in the raw water and some organic compounds. High-hardness water accelerated the change in membrane process parameters, and the surface of the membrane had abundant inorganic scaling. The inorganic scale on the surface of the NF membrane increased noticeably when filtering water with high hardness. Regular cleaning of the UF and NF membranes could effectively restore the parameters of the process and prolong the service life of the membrane process.