Effects of the chemical injection points in pre-treatment on reverse osmosis (RO) plant performance

Good characterization of raw water quality and subsequent identification of the most appropriate pre-treatment are not sufficient to ensure successful functioning of a reverse osmosis plant. The Tan Tan demineralization plant was set up in July 2003 for 20 l/s production capacity. The brackish water with total dissolved salt (TDS) of 3 ppm is supplied from Khang Lahmam underground water located 70 km from the plant. Since it started working, the main problem encountered in the Tan Tan demineralization plant was the continuous presence of a membrane biofouling problem which provokes high pressure drop values and requires an increased washing frequency, as often as twice a month. The diagnosis undertaken to remedy this anomaly has concerned all the pre-treatment steps including the raw water, quality of the applied chemical reagent types and concentrations, and the composition of the biofilm. Collected data confirm the good raw water quality specially the absence of heterotrophic bacterium (aerobic-anaerobic analyzed at 37°C+1°C and to 22°C+1°C) and low oxydation values (<3 ppm). Chlorine-raw water contact time, about 17 h, is largely sufficient to insure its efficiency. Analyses of the biofilm have shown the abundance of heterotrophic germs Pseudomonas sp. and ferobacter absence despite the reddish color of the biofilm. The hypothesis retained to interpret the origin of the biofouling is the sodium bisulfate injection point, upstream of the microfilter. The microfilter then plays the role of a bioreactor favoring a bacterial proliferation upstream of the membranes. The simple displacement of the injection point of that of sodium bisulfate will allow remedying definitively the disorder caused by frequent appearance of biofouling. The present article, through the analysis of operational data, and particularly of results and actions undertaken, focuses on the crucial role of reagent injection point in optimization of reverse osmosis plant operation.

Even if the undertaken actions did not permit operators to re-establish the start-up pressure drop values in each level of the plant, it allowed focusing on the role of the SBS injection point choice. Then it is important to retain in particular: • The choice of SBS injection point, meant to neutralize the residual chlorine, is very important for inhibition of the biofouling appearance. In Tan Tan demineralization plant the SBS neutralization upstream the microfilters generated a high biological activity in these filters which played a bioreactor role. The direct consequence was then membranes biofouling. • The chemical cleaning applied did not allow removing the biofouling, the formal is not the appropriate reagent to reach this target. • The use of the residual chlorine, after having changed the SBS injection point, gave immediate and efficient results. The chemical cleaning protocol to be optimized have probably to use the sodium hypochlorite or an equivalent reagent to re-establish the start-up working conditions in Tan Tan reverse osmosis demineralization plant.