Establishment of HPC(R2A) for regrowth control in non-chlorinated distribution systems Wolfgang Uhl a, * , Gabriela Schaule b a Water Technology Department, Gerhard-Mercator-University Duisburg, Bismarckstr. 90, D-47048 Duisburg, Germany b Microbiology Department, IWW Rheinisch-Westfa ¨lisches Institut fu ¨r Wasser Beratungs-und Entwicklungsgesellschaft mbH, Moritzstr. 26, D-45476 Mu ¨lheim an der Ruhr, Germany Abstract Drinking water distributed without disinfection and without regrowth problems for many years may show bacterial regrowth when the residence time and/or temperature in the distribution system increases or when substrate and/or bacterial concentration in the treated water increases. An example of a regrowth event in a major German city is discussed. Regrowth of HPC bacteria occurred unexpectedly at the end of a very hot summer. No pathogenic or potentially pathogenic bacteria were identified. Increased residence times in the distribution system and temperatures up to 25 jC were identified as most probable causes and the regrowth event was successfully overcome by changing flow regimes and decreasing residence times. Standard plate counts of HPC bacteria using the spread plate technique on nutrient rich agar according to German Drinking Water Regulations (GDWR) had proven to be a very good indicator of hygienically safe drinking water and to demonstrate the effectiveness of water treatment. However, the method proved insensitive for early regrowth detection. Regrowth experiments in the lab and sampling of the distribution system during two summers showed that spread plate counts on nutrient-poor R2A agar after 7-day incubation yielded 100 to 200 times higher counts. Counts on R2A after 3-day incubation were three times less than after 7 days. As the precision of plate count methods is very poor for counts less than 10 cfu/plate, a method yielding higher counts is better suited to detect upcoming regrowth than a method yielding low counts. It is shown that for the identification of regrowth events HPC(R2A) gives a further margin of about 2 weeks for reaction before HPC(GDWR). D 2003 Elsevier B.V. All rights reserved. Keywords: Drinking water; Distribution; Regrowth; Residence time; Temperature; Plate counts; HPC bacteria; R2A; AOC; Internal alert limit 1. Introduction According to the German Drinking Water Regula- tion (GDWR; Anonymous (2001)), which is the national version of the European directive (Anony- mous (1998), final disinfection of drinking water is required only if the water does not meet the hygienic standards, i.e., if the water is not free from pathogens (no E. coli, coliforms or enterococci may be detected in 100 ml) and/or if pour plate counts for heterotro- phic bacteria on a defined nutrient-rich agar (to be determined at 20 and 36 jC incubation for 48 h) exceed the guidance value of 100 cfu/ml. In many cases, in Germany, drinking water is distributed without disinfection as the water already meets the hygienic standards and careful management of the distribution system by the utilities and pur- 0168-1605/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.ijfoodmicro.2003.08.010 * Corresponding author. Fax: +49-89-2443-36969. E-mail address: w.uhl@uni-duisburg.de (W. Uhl). www.elsevier.com/locate/ijfoodmicro International Journal of Food Microbiology 92 (2004) 317 – 325