Combination of membrane technology and limestone filtration to control drinking water quality

• Published in: Desalination Vol. 131; no. 1; pp. 271 - 283
• Main Authors: Kettunen, Riitta, Keskitalo, Pertti
• Format: Journal Article Conference Proceeding
• Language: English
• Published: Amsterdam Elsevier B.V 20.12.2000; Elsevier
• Subjects: Alkalinity; Aluminium; Applied sciences; Drinking water and swimming-pool water. Desalination; Exact sciences and technology; Finland; Fluoride; Limestone filtration; Membrane filtration; Nanofiltration; Pollution; Reverse osmosis; Water treatment and pollution; Europe; Finland; Finland, Laitila; Nanofiltration; Fluoride; Membrane filtration; Reverse osmosis; Aluminium; Alkalinity; Limestone filtration; Drinking water treatment; Energy consumption; Filtration; Combined process; Fluorides; Alkalinization; Calcite; Membrane separation; Granular bed filters; Calcium Carbonates; Drinking water treatment plant; Technicoeconomic study; Water quality; pH; Aluminium Compounds; Total organic carbon; Cost analysis; Performance; Ground water
• ISSN: 0011-9164; 1873-4464
• DOI: 10.1016/S0011-9164(00)90025-0

Certain areas in Finland have a problem of high fluoride and aluminium in groundwater because of soil properties. In 1999, the City of Laitila constructed a membrane filtration plant (16–25m 3/h) to control fluoride and aluminium concentration in drinking water. The plant has two trains, one with reverse osmosis (RO) and the other with nanofiltration (NF). Only a fraction (ca. 1/4) of water distributed is membrane filtered. The rest flows directly to alkalization. The alkalization process includes two limestone filters: one for membrane-filtered water (RO+NF) and the other for untreated groundwater. During the first year of operation, both RO and NF were shown applicable for fluoride removal from soft, high fluoride (ca. 4 mgF/l) ground water. The fluoride removals were on average > 95% in RO at 7.3 bar and 76% in NF at 5.7 bar. More than 78% of aluminium was also removed in membrane filtration. Temperature (2–16°C) affected permeated conductivities in the NF unit but not in the RO unit. Limestone filtration was shown applicable for alkalization of membrane filtered water. The alkalinity of membrane-filtered water rose back to the level it was before membrane filtration. However, alkalinity achieved with membrane-filtered water was lower than with groundwater while pH obtained was higher. This mainly resulted from the difference in carbon dioxide concentration of these waters. Since start-up of the RO/NF plant, fluoride and aluminium concentrations in the water distribution system have been at acceptable levels (≤1.5 mgF/l, ≤0.2 mgAl/l) while limestone filtration has ensured a stable alkalinity and pH. The cost of membrane filtration was ca. Euro 0.2/m 3 permeate.