Use of a biofilm reactor with sand filter to remove nutrients was studied at an operating heat exchanger plant. An exptl. reverse osmosis test cell was run upstream and downstream of the sand filter. Permeate flow declined within 10 days of operation by 20% upstream. After the filter, no flux decrease was obsd. Biofilm thickness was 30 mm upstream and 3 mm downstream of the filter. Biodegradable dissolved org. C (BDOC) decreased from 0.35 to 0.12 mg/L. To maintain biofilm growth below a defined interference threshold, it is sufficient to reach a BDOC concn. of 0.1 mg/L.