Scaling control of RO membranes and direct treatment of surface water

Since 1992 Amsterdam Water Supply has been conducting pilot-plant (2 x 9 m( 3)/h) research on the use of reverse osmosis (RO) for direct surface water treatment from the Rhine River. The investigations were made as a feasibili ty study for an extension with 13 Mm(3)/y of the existing treatment plant ( 70 Mm(3)/y). Research was carried out with ultralow-pressure membranes and an organophosphonate antiscalant (Permatreat 191). With this antiscalant no biofouling was introduced upon its use. Because of its low environmental i mpact, sulphuric acid was preferred above hydrochloric acid, but the barium sulfate scaling risk was increased. Experiments were conducted with both t ypes of acid in increasing recovery steps from 80% up to 90%. While startin g experimental difficulties with dosing the antiscalant occurred and BaSO4- scale formation was a fact, especially in the installation with sulphuric a cid, a severe MTC decrease was seen. Successive cleaning was possible with an alkali cleaner containing a complex builder. From the barium mass balanc e over this 3-month period, the precipitated barium amount was calculated. Barium analysis of the cleaning solution showed an almost complete recovery of the barium from the membrane elements. Moreover, the MTC was restored c ompletely. From this it was concluded that the precipitated barium sulphate was washed out completely. Recoveries up to 87% are possible without BaSO4 -scale formation and run times up to 1 year. The working inhibition time of the antiscalant in concentrate from increasing recoveries was determined e xperimentally.