Effect of a combined chlorine-monochloramine process on the inhibition of biofilm regrowth in potable water systems

Abstract

To investigate the effect of a combined chlorine-monochloramine disinfection process on the inhibition of biofilm regrowth, surface water and groundwater were used as the test water sources while stainless steel and galvanized mild steel were used for the study of biofilm regrowth in a laboratory-scale unit. Disinfection was carried out using 2.5 mg.L-1 chlorine followed by 1.5 mg.mg.L-1 monochloramine. The bactericidal effectiveness of the process relied on heterotrophic plate count (HPC) and total bacteria. The concentrations of organic and inorganic compounds such as dissolved organic carbon, calcium, magnesium, chemical oxygen demand, total nitrogen, phosphate and sulfate were also taken into account. The formation of biofilm occurred 24 h after the exposure of test pipe materials to both chlorinated surface water and groundwater (with the average counts of 2 log.cfu.cm-2 for viable bacteria in both systems, 5 log.cells.cm-2 and 6 log.cells.cm-2 in surface water and groundwater systems respectively). A concentration of 0.45 ± 0.5 mg.L-1 free chlorine could not inhibit the formation of biofilm on the surface of pipe materials. Whereas the yield for attached bacteria was initially similar under chlorination treatment (24 h after chlorination), a significant difference in bacterial biofilm counts occurred in chlorinated water systems with the addition of monochloramine. Less than one attached viable and total bacteria colonized slides exposed to both combined chlorine-monochloramine surface water and groundwater systems. This same pattern was detected up to 7 d and 21 d on the surface of test pipe materials exposed to surface water and groundwater respectively. The present study demonstrates the inability of bacteria to colonize surfaces of stainless steel and galvanized mild steel in the combined chlorine-monochloramine system. In such a treatment the susceptibility of attached viable bacteria was observed only when the concentration of monochloramine persisted up to 0.35 mg.L-1 and 0.2 mg.L-1 in surface water and groundwater respectively. A lower monochloramine residual of 0.15 mg.L-1 led to the phenomenon of biofilm regrowth. Because the residual monochloramine depleted earlier in the groundwater system than in the surface water system, biofilm inhibition lasted longer in the latter system than in the former. A LSD test showed no significant difference in attached bacterial counts (at p < 0.05) when chlorine was used as primary disinfectant and monochloramine as second disinfectant for the disinfection of surface water and groundwater. Although the high concentrations (average) of organic (Surface water: DOC = 5.55 mg.L-1, groundwater: DOC = 12.6 mg.L-1 and inorganic compounds (surface water: Ca2+ = 12.17 mg.L-1, Mg2+ = 9.20 mg.L-1, COD = 11.35 mg.L-1, N = 2.70, SO43- = 21.40 mg.L-1, PO43- = 0.4 mg.L-1 = 0.54 groundwater: Ca2+ = 57.13 mg.L-1, Mg2+ = 77 mg.L-1, COD = 7.14 mg.L-1, N = 9.02, SO43- = 69.25 mg.L-1, PO43- = 0.4 mg.L-1) could influence bacterial biofilm regrowth in treated water systems, statistical data supported the findings that a combined chlorine-monochloramine process and the persistence of monochloramine in the combined systems constitutes one of the most important factors linked to the inhibition of bacterial biofilm in a drinking water system.