Water Quality Deterioration in Distribution Systems

About the author: Thomas L. O’Connor, BSEE, MBA, is the project director for H2O’C Limited, Columbia, Missouri.

John T. O’Connor, EngD, P.E., is the prinicipal for H2O’C, Limited, Columbia, Missouri. Phone: 877-22-WATER; e-mail: [email protected].

Thomas L. O'Connor and John T. O'Connor

Seasonal Temperature Effects

While temperature is acknowledged to be an important factor in water treatment, remarkably little study has been made of the adverse influence of low temperatures on physical treatment process effectiveness. An early study concluded that "there is no preventative or retarding effect on alum floc formation with low raw water temperatures."1

This conclusion prompted Camp et al. (1940)2 to further evaluate the effect of temperature on the rate of floc formation. Camp utilized the direct measurement of iron or aluminum in lieu of turbidity. He concluded that temperature did not have a measurable effect on the time of floc formation.

However, pilot plant studies of aluminum sulfate-coagulated river water demonstrated the overall adverse effect of low temperatures on sedimentation and filtration.3 These researchers advised, "where raw water temperature is low, the jar tests must be run on samples held at the same temperature if results are to be used in plant control."

It was not until 1984 that a systematic evaluation of the adverse effects of low temperature on water treatment plant performance was undertaken.4 The investigators reported significant temperature effects on coagulation that accounted for observed decreases in turbidity removal efficiency, particularly when aluminum sulfate was used. Using decreases in alkalinity and measurements of the metal coagulant in solution, the researchers determined that the reduction in turbidity removal efficiency was not due to reduced metal hydroxide precipitation rate but to retardation of the floc growth. Under equivalent conditions, iron salts produced larger flocs than aluminum salts and resulted in lower residual turbidity values. The implications of these results became more significant in light of observed major temperature effects on organism removals.5

While low water temperatures have been shown to severely impair microorganism removals by physical water treatment processes,5, 6, 7, 8 microbiological violations due to coliform or heterotrophic plate count organisms are most commonly reported by U.S. water utilities during warmer (summer) months.9 Alternately, for 14 of the 21 large water distribution systems studied, finished water turbidities were lowest when water temperatures were highest. As a result, percent positive total coliform samples and finished water turbidity were either unrelated or inversely related.

Figure 1 illustrates the repetitive seasonal variation in finished water turbidity for Kansas City, Mo. While significant annual reductions in finished water turbidities from 0.8 to 0.2 ntu were achieved by the Kansas City Water Department, the recovery of positive total coliform samples from the distribution system, initially low, actually increased slightly during this five-year period.

Figure 2 compares the percent recovery of positive total coliform samples in summer and winter for several distribution systems. The marked seasonal differences indicate that the value of total coliform as an indicator of microbial contamination is severely compromised when water temperatures are low.

Those water utilities that used chloramine as a residual in their distribution systems recovered 8.6 times fewer positive total coliform samples than those using chlorine (Figure 3). Table 1 shows that percent positive coliform recoveries differed by a factor of more than 1,000 (7 percent to 0.006 percent) among the 21 distribution systems surveyed.

While the divergent seasonal variations in turbidity and total coliform are seemingly inconsistent, the data would suggest that the penetration of increased numbers of organisms into the distribution system during cold weather is not evidenced by distribution sampling for total coliform. This is because their influx is not accompanied by the subsequent regrowth during distribution that is experienced in the summer. These utility data illustrate the fundamental weaknesses in current operational microbiological standards. They confirm the need for alternative methods to directly visualize and quantify source water microorganisms entering and migrating through the distribution system.10

Case History: Aftergrowth

A study of the Philadelphia (Pa.) Suburban Water Company (PSWC) by Donlan and Pipes (1988)11 provided an example of a distribution system experiencing aftergrowth. HPC populations (periphytic) that developed on cast iron test specimens suspended in PSWC water mains were measured and related to the distribution system (planktonic) HPC values. No statistically significant relationships were found between the population of HPC attached to the cast iron specimens and measured water quality parameters, including total organic carbon, ammonium ion, phosphate and pH.

However, temperature was found to have a major influence. Both the attached and distribution system (planktonic) HPC were most abundant at higher temperatures. The density of HPC organisms attached to the cast iron test specimens ranged from 100 cfu/cm2 at 5° C to 37,000,000 cfu/cm2 at 23° C. High attached HPC also corresponded to high planktonic HPC in the distribution system.

Direct microscopic total bacterial cell counts made of the bacteria scraped from some of the cast iron specimens were 3 to 134 times greater than the HPC enumerated. These results were consistent with studies that showed microscopic cell counts to be orders of magnitude greater than the number of HPC colonies obtained on culturing distributed drinking waters.12

Within the PSWC distribution system, HPC increased markedly as water temperatures rose. Increased water temperatures (24° C) led to the more rapid depletion of chloramine. HPC was highest (8,318 cfu/ml) where the chloramine residual was almost totally depleted. Alternately, where chloramine was maintained at 1.3 g Cl/m3, only 10 colony forming units per millilitre were observed.

Download: Here