Reclaimed water use in industrial cooling circuits: Compatibility with TP11 biocides

Cooling water circuits represent a high-water saving potential that has not been fully exploited yet due to the risk of Legionella outbreaks. According to legislation, cooling circuits are subjected to specific biocide treatments which must be considered when water reuse is planned. Biocide effectiveness mainly depends on the type of installation, its operating regime, and the interactions with other chemicals. Therefore, water reuse is critical because its physical, chemical and microbiological characteristics, beside deposit and corrosion problems, may reduce the biocidal power and produce a health risk. However, there is a lack of knowledge on the compatibility of reclaimed water with biocides, and on long-term maintenance problems. This paper validates the long term performance of an industrial cooling tower using reclaimed water, obtained after the treatment of bottle washing water. The compatibility study of the reused water with three non-oxidant commercial products from the TP11 group (quaternary ammonium, tetrakis hydroxymethyl phosphonium sulphate and isothiazolone) shows a reduction lower than 20% of the biocide efficiency between periodic additions. Isothiazolone-based biocide was tested with reclaimed water during 1 year at industrial scale, without detecting any problem. Results show that water quality was always within the values allowed by legislation and slightly better when using reclaimed water (lower ecotoxicity of the purge and higher biocidal residual concentration). With the reuse strategy, water savings of 3750 m3/y were obtained. Furthermore, to minimize the Legionella and corrosion risks, an optimal control of the cooling tower has been developed based on correlations.