This paper reports on the effect of pH on the retention of dry matter (DM), total ammonia-nitrogen (TAN), potassium, phosphorus and volatile fatty acids (VFAs) during the filtration of pretreated swine manure by three highly selective reverse osmosis (110) membranes. The manure was pretreated using various combinations of biological and physical technologies, namely anaerobic digestion (AD), vacuum filtration through diatomaceous earth (DE), nanofiltration (NF), and a first stage RO filtration. The objective was to establish the level of acidification required to optimize permeate quality while minimizing chemical addition. Pretreatment by NF or RO increased feed pH to values greater than 8.5, but decreased the volume of acid required to lower pH because of decreased alkalinity. Acidification improved TAN retention by all membranes, down to a pH of about 6.5. The increase in TAN retention mirrored the reduction in the fraction of TAN present as free ammonia (NH3) as feed pH was lowered. With the most selective membrane operated at 55.2 bar and a permeate recovery rate of 80%, TAN retention averaged 94% at pH 7.8 while it remained above 99% at pH < 6.5. The retention of VFAs was reduced at pH < 5, presumably because of the formation of unionized VFAs which permeated more readily through the membranes. Potassium retention tended to increase slightly with feed acidification, but remained higher than 98% at all pH values and with all membranes. Acidification did not appear to have an effect on the level of membrane fouling, but the water flux recovery rate following alkaline cleaning tended to increase with decreasing feed pH, possibly indicating decreased fouling by inorganic precipitates at lower pH. Crown Copyright (C) 2008 Published by Elsevier B.V. All rights reserved.