| Conditions contributing to the growth of biofilm bacteria, particularly coliforms, in simulated drinking water distribution systems were investigated. Key experimental variables were substrate loading, temperature, hydraulic residence time, presence of chlorine, pipe material, and the initial growth rate of the introduced coliforms. Pilot distribution systems comprised (a) annular reactors with carbon steel pipe as the outer cylinder and (b) a five segment pipe loop of 4" carbon steel. The pilot systems and laboratory annular reactors were operated to simulate water quality and operational conditions typical of drinking water distribution systems.; A mathematical model was developed to calculate growth rates of the suspended and attached heterotroph and coliform populations using experimental data. Inclusion of a potential for planktonic growth to that of the biofilm increased the precision of the model.; The initial growth rate of the coliforms was critical to their ability to persist long-term in mixed population biofilms. The longest chemostat residence time tested (20 hr) produced the most competitive organisms.; The influence of substratum material was tested in laboratory systems. Biofilms grown on mild steel contained more heterotrophs and coliforms than those sampled from polycarbonate. However, growth rates on the polycarbonate were higher.; In lab and pilot reactors, higher substrate loading resulted in increased bacterial numbers, but not in increased growth rate. No correlation was found between substrate loading, organism numbers or growth rate.; Temperature increase from 10{dollar}spcirc{dollar} to 20{dollar}spcirc{dollar} C did not result in elevated coliform numbers or growth rates. Heterotroph growth rate was enhanced.; Chlorination was not effective in controlling coliforms, regardless of the substratum composition. The numbers of culturable heterotrophs on polycarbonate were reduced when chlorine was present; this effect was not seen on mild steel.; Coliforms were more abundant at the shortest (2 hr) hydraulic residence time while heterotrophs were unaffected at increased residence times of 4, 8, and 16 hr.; Comparisons between data from parallel annular reactors and pipe loops showed that the qualitative response of the organisms to operational parameters was similar. The annular reactors are recommended as the preferable tool for experimental and industrial use. |
