Treatment of waste thermal waters by ozonation and nanofiltration

Szép A.; Kiss Zs. L.; Kertész Szabolcs; Hodúr Cecília; László Zsuzsanna: Treatment of waste thermal waters by ozonation and nanofiltration. [Tanítási erőforrások, tanulási objektumok]

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Angol cím

Treatment of waste thermal waters by ozonation and nanofiltration

Angol nyelvű abstract, leírás

In the southern part of Hungary thermal waters are widely used for heating e.g. greenhouses or housing estates. The residual thermal (waste) water may cause environmental problems due to their high ion content, sometimes high organic content related to high chemical oxygen demand or toxic compounds. The methods most frequently used to remove salinity are membrane separation processes. The main limitation of membrane processes is the flux decline caused by membrane fouling, which lowers the economical efficiency of membrane processes by reducing the rate of production treated water and shortening the membrane life. The aim of our work was to purify residual thermal water with high ion and phenolic contents and chemical oxygen demand (COD). This study was also designed to investigate the effects of ozone on the membrane flux, and of preozonation on the filtration efficiency. During the experiments model thermal water was used. Ozone was produced from oxygen. The ozone-containing gas was bubbled continuously through a batch reactor during the treatment. The ozone concentrations of the bubbling gas before and after the reactor were followed by an UV spectrophotometer. The phenol index was measured with the help of photometric determination based on the 4-aminoantipyrine colour reaction. Determination of the COD was based on the standard method involving potassium-dichromate oxidation; TOC was measured by a TOC analyzer, Na and K content was measured by ion chromatograph; the conductivity, salinity and pH with an electrochemical multimeter. The membrane filtration measurements were carried out on laboratory membrane filter (UWATECH 3DTA) using nanofiltration membranes. In the first series of experiments, the effect of ozone treatment on the concentration of organic (oxidizable) compounds was investigated. It was found that phenolic compounds can be eliminated from the water, while the total COD and TOC decreases slowly (81.8% and 35.7% could be eliminated respectively). The conductivity increased with ozonation time, which can be explained by arising small organic acids originated from phenol. This phenomenon is in accordance with the changes of pH: after short ozonation time the pH is decreased due to presence of organic acids, then the pH set back to the original value. The results showed that the nanofiltration alone greatly decrease the turbidity (nearly 100%), the COD (80%), but the phenol content has not decreased below the limit (3 mg/L) and the TOC decreased with only 20.95%. In the next series of experiments ozone treated thermal water was nanofiltered. It was found that the phenol retention decreases by ozone treatment due to decomposition of phenol molecules to small molecules, than after longer ozonation times it increases. The TOC removal increases with ozonation time; the retention of COD decreases after ozonation. The ion chromatographic measurements showed that preozonation changes the Na+ -ion and Cl- -ion retention during nanofiltration. Summarizing the effect of ozone treatment, of nanofiltration and of nanofiltration after preozonation the elimination efficiency of COD, TOC, phenolic index and ion content were compared. The results show that the chemical oxygen demand and TOC can be effectively decreased by combination of ozone retreatment and membrane filtration. The ozone treatment is more effective for phenol elimination than nanofiltration alone – by combination of two processes 100% elimination efficiency can be achieved. The elimination efficiency of ion content by combined processes was found to be similar to the efficiency of nanofiltration. It can be concluded that the combination of ozone treatment and nanofiltration is applicable to eliminate the organic pollutants from the water, but the ion content – although decreases by the treatment is more than 50% - remains high.

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