Maintaining Water Quality in Distribution Systems

Environmental and Science Engineering, January 2004
Ken Collicott, P.Eng.

Water can be considered a perishable product and has a shelf life (detention time), a preservative (chlorine or chloramines), and packaging (pipes and reservoirs). I Water quality may undergo changes after it leaves a treatment plant (or well) due to internal reactions within the water itself, or from interactions between the water and the pipe wall. As the travel time increases through a distribution system, the potential for complex chemical and biological changes also increases, allowing formation of contaminants that may be of concern.

There are several factors that can cause deterioration of water quality in distribution systems, including: the quality of water leaving the treatment plant or well; pipe material and condition; hydraulic detention time; water temperature; operation and maintenance practices; pressure and flow control; and disinfectant residual.

Common water quality problems that can occur in distribution systems can be categorized as biological, chemical/physical and aesthetic.

Microbiological problems include bacterial regrowth, nitrification, and waterborne diseases originating from microbial pathogens such as E.coli and protozoan parasites like Giardia and Cryptosporidium.

Chemical/physical problems include disinfection by-product formation, leaching of lead and copper, pH stability, corrosion and scale formation, by-products of coatings and linings, disinfectant residual, and sediment.

Aesthetic problems include taste and odour as well as colour and general appearance. Aesthetic problems are often linked to chemical/physical problems.

Best Practice
There are some practices that should be used to maintain water quality in distribution systems, but the water treatment processes should be optimized in the first place to produce a high quality water that is biologically and chemically safe, does not precipitate chemical constituents, does not corrode the conveyance and storage systems, and does not cause excessive encrustations.

The free chlorine residual should be greater than 0.2 mg/L throughout the distribution system in order to maintain an adequate disinfectant residual. In large distribution systems, chlorine booster stations may be used within the distribution system (i.e. at pumping stations and reservoirs) to maintain adequate chlorine residual.

A minimum residual water pressure of at least 140 kPa is required to provide a factor of safety against backflow and intrusion of groundwater into the distribution system. There are several measures that can be used to maintain positive water pressures, such as: elevated storage; install surge control equipment; provide standby power for pumps; and open and close valves and hydrants slowly.

Recognizing that water quality may change as it travels through a distribution system, municipalities should implement a comprehensive monitoring program to anticipate, detect and solve water quality problems.

Municipalities should implement cross-connection control and backflow prevention programs in order to mitigate the potential for contamination of the distribution system.

They should implement thorough flushing and swabbing programs to remove sediment, corrosion by-products and biofilm from the distribution system. They should also have an inspection and maintenance program to maintain their valves and hydrants in good condition. The operation of valves and hydrants should be carefully controlled to maintain positive pressures and to mitigate pressure transients that could result in backflow and/or contaminant intrusion.

A biofilm control program should be implemented. Biofilms are microbiological materials that coat the interior of pipes and reservoirs. These can provide nutrients for microorganisms and shield them from disinfection. Biofilms can also release organic contaminants that can react with chlorine to provide undesirable by-products.

The best practices to control biofilm include: control the source of carbon and other nutrients such as phosphorus and nitrogen through biological treatment; provide adequate disinfection and flushing of water main break repairs and new construction; use cleaning practices such as flushing and swabbing; eliminate dead ended mains and low flow areas; maintain positive pressures to prevent contaminant intrusion; avoid sudden changes in flow to prevent the biofilm from sloughing off the pipe wall; implement cross-connection control and backflow prevention programs; maintain adequate disinfectant residuals; provide internal corrosion control; and properly maintain distribution systems and storage facilities.

In general, blending of water sources is not recommended. However, if it is required, blending should always be done in a controlled manner. A detailed blending analysis should be conducted to determine the chemical compatibility of the two sources, predict the blended water quality characteristics, and assess the impacts of blending ratios on pipe materials.

In the design and operation of storage facilities, the best practices to mitigate water quality problems include: storage facilities should not be oversized; do not use standpipes if the total

storage volume is significantly greater than the useful storage volume; all open finished reservoirs should be taken out-of-service until they are covered; linings and coatings should comply with NSF / ANSI Standard 61; the site and roof drainage should be designed so that stormwater runoff drains away from the storage facility; access hatches should be designed to prevent contaminated water and debris from falling into the facility when the hatches are opened.

Also, secondary containment should be provided for fuel tanks and chemicals that are located at nearby pumping stations; design storage facilities to prevent dead zones; a distribution system should be operated to maximize the turnover rate in its storage facilities (automatic controls should be provided to facilitate the draining and filling of the storage facility); storage facilities should be equipped with security systems such as perimeter fencing with lockable gates, lockable access hatches, and alarms that are monitored continuously; continuous monitoring and sampling ports should be installed in a storage facility in order to identify mixing or contamination problems; and in some cases, it may be prudent to install a chlorine booster station at the inlet or outlet of a storage facility.

The best practices for inspection of storage facilities are as follows:

• Routine inspections should be conducted on a daily, weekly or monthly basis to monitor the exterior of the storage facility and grounds for evidence of intrusion, vandalism, coating failures, security and operational readiness.
• Periodic inspections should also be conducted on a monthly or quarterly frequency to check hatches, bug screens, cathodic protection equipment and coatings.
• Comprehensive inspections should be conducted every three to five years. The conventional method for cleaning storage facilities requires that they be taken out-of-service and drained. Storage facilities can also be cleaned using commercial divers and remote operated vehicles.

Water mains should be constructed using approved drinking water materials (NSF/ANSI Standard 61). Wherever possible, water mains should be looped to allow the water to circulate. Most distribution systems have some unavoidable dead ended mains. In these cases, hydrants, blow-offs or automatic flushing devices should be installed at dead ends to allow them to be flushed.

In cases where there are frequent water quality complaints and elevated iron, turbidity and colour are chronic problems, it may be necessary to rehabilitate or replace the main. Iron water mains can be rehabilitated using either structural or non-structural linings. AWWA Manual M28 provides guidance on the rehabilitation of water mains.

Although corrosion control treatment can be used to mitigate leaching of lead, municipalities should adopt a plan for eventual replacement of all lead services in their distribution system. In the interim, public education is an important lead control strategy. All materials in contact with drinking water should comply with standards for health effects (i.e. NSF/ANSI Standards 60 and 61).

AWWA Standard C65l-99 describes a procedure for disinfection of new mains, connection to existing mains, and repair of mains. Municipalities should conduct vulnerability assessments of their water distribution systems and identify the need for physical protection systems, operation systems and consequence mitigation.

If practical/economical, develop a calibrated computer model of the distribution system for the following: constituent analysis; source trace analysis; water age analysis; tank mixing; vulnerability assessment; and emergency response planning.

Formal training programs should be developed that emphasize the importance of maintaining water quality in the distribution system. In addition, more emphasis should be placed on operator certification requirements and ultimately, accreditation of the water utility.

Municipalities should regularly host meetings with all stakeholders, especially the local Medical Officer of Health, to discuss water quality issues, operational protocols, capital planning and emergency response plans.

They should adopt a standard procedure for responding to water quality complaints. Similar to water quality data, this information should be properly documented using electronic databases or GIS. Water quality complaints provide a practical means to monitor public perception and acceptance respecting water provided.

Municipalities should also review each of these practices and develop an action plan to maintain/improve water quality in their distribution system. This action plan should prioritize the needs and include cost estimates as well as a time frame for implementation. Actions that are required to protect public health should be given a high priority.

I American Water Works Association Research Foundation; 1999. Maintaining Water Quality in Finished Water Storage Facilities.

This article provides a summary of a Best Practice report entitled "Water Quality in Distribution Systems." A copy of this report can be downloaded from www.infraguide.gc.ca