Biofilm control for plate heat exchangers using surface seawater from the open ocean for the OTEC power plant

At the proposed site for the Ocean Thermal Energy Conversion power plant on the east coast of India, biofilm formation and its control using intermittent chlorination in plate heat exchangers was studied prior to commissioning of the plant. Settlement of fouling larvae was not observed on the plates, which may be attributed to the low density of larvae of fouling organisms in the open ocean (65 km from the coast). Significant reduction in biofilm thickness was observed after chlorination at a dosage of 1.2 ppm residual. Microalgae were absent on untreated and chlorinated heat exchanger plates. Significant differences were also observed with respect to bacterial density and diversity between chlorinated and control surfaces. Total counts of viable bacteria in untreated controls increased with time. Relative to controls, a significant reduction in bacterial density in biofilms was observed upon chlorination. Sulphate reducing bacteria were absent in chlorinated biofilms, whereas their numbers increased with time in controls. Counts of heterotrophic bacteria in biofilms also showed an increase with time in controls and were significantly lower on chlorinated surfaces. Counts of slime formers, e.g. Pseudomonas sp. and Aeromonas sp., were low compared to total viable bacterial counts.