Rural water pipeline flow and water quality study
Abstract
Many farms and agricultural operations in rural Saskatchewan obtain water from rural water pipelines. These pipelines serve a small number of users spread over a large area, which results in large hydraulic residence times (HRT). The magnitude of the HRTs are a concern because of the degradation of water quality over time and the potential for biofilms to form in the pipeline. Biofilms have been reported to form even in the presence of a substantial chlorine residual, given time and substrate. The danger for users of these systems is that mature biofilms often have a diverse population of bacteria, some of which are reportedly opportunistic pathogens.
Despite the widespread use of rural water pipelines, little is known about the hydraulic patterns and water quality variations in these systems. This study investigated both hydraulic and water quality variations and sampled sections of the two pipelines studied for the presence of biofilms. Of the two pipelines studied, one conveyed raw water, while the other was supplied treated water from a regional water treatment plant.
The data collected over a period of approximately 16 months showed that average demand was highest in the winter months when many farmsteads were wintering cattle. However, peak hourly flow rates occurred in the late summer, typically August. The peak factors determined from the data showed a peak day factor of 1.97 and a peak hour factor of 4.53. Pressures in the extremities of the two networks studied were highly variable, sometimes reaching unacceptably low values during periods of high demand as a result of activity elsewhere in the network.
Peak concentrations of organic carbon and epifluorescent bacteria and high chlorine decay typically coincided with the peak temperatures, occurring in August and September of each year. The recommended threshold values for biofilm control (BDOC < 0.2mg/L, free chlorine> 0.1 mg/L, and temperature < 15°C) were breached for only a portion of this period. The ground temperature, which varies throughout the year, is believed to greatly mitigate biofilm formation as water temperature in these branch networks rarely exceeded the threshold value and few viable organisms were recovered from the pipe samples during biofilm analysis.
Hydraulic modelling was conducted to determine the HRT of the treated system and determine the chlorine decay coefficients (k). HRT values varied from 43 hours near the head of the branch network up to 241 hours at the Farpoint monitoring site. The model also showed another user (not part of the study) at an endpoint of the system may see HRTs as high as 585 hours.
Average values of chlorine decay coefficients determined in the study for the sites monitored varied from 0.3/d to 0.12/d for residence times of 2.2 days and 7.8 days, respectively. The highest individual decay coefficient value coincided with peak temperatures and DOC concentrations. This value was calculated to be 0.42/d for an HRT of 2.1 days. A mathematical relationship between DOC, temperature and chlorine decay coefficient could not be determined within the scope of this project.