Analysis of foulant layer in all elements of an RO train

Membrane samples were recovered from all RO elements of the first and second stage (six elements each) of a RO unit fed with pre-treated surface water to analyse fouling layer composition. Chemical and microbiological data were averaged based on the results obtained from analysis of samples from five different membrane leaves from each element, but electron microscopy was performed on a single membrane sample per element due to the homogeneity of the fouling layers. Membranes from all elements were covered by a thick, compact, adherent, slimy, lightly brown coloured fouling layer, which contained microbial cells but no salt deposits, except for the last element of the second stage, where a patchy but sometimes thick layer of BaSO4 crystals was detected. Total microbial counts varied between 105 and 106 cells/cm2 in the elements from both stages. Viable counts dropped from 105–106 colony forming units/cm2 in the first two elements of the first stage to 102–104 cells/cm2 in subsequent elements. Dry weight of the fouling layers varied between 0.15 and 0.20 mg/cm2 in all elements except elements 1 and 4 in stage 1 (0.15 and 0.25 mg/cm2, respectively). The proportion of organic matter as a percentage of dry weight increased in the first stage from approximately 30% in the first two elements to 55% in the elements 4–6 and remained constant in all elements of the second stage. The protein content in the organic fraction decreased linearly from around 70% in element 1, stage 1 to about 40% in element 6, stage 1 and remained constant at the latter level in all elements of stage 2. The proportion of total carbohydrate in the organic fraction of about 3% was the same in all elements of both stages. The content of O, S, P, Si, Fe and Al was similar in the inorganic fractions of foulant layers from all elements. Ba and Sr increased linearly from the lead to the rear element in stage 1 and remained constant throughout stage 2. BaSO4 crystals were detected in significant amounts only in fouling layers from the last element of stage 2.