Mix it Up

As regulations tighten, utilities striving for optimum water quality are turning their attention to the distribution system.

It is in the distrubution system that even the highest quality water leaving a treatment facility can significantly deteriorate before reaching customers. Stored water can become stagnant and stratify due to factors such as underutilization, shortcircuiting, pipeline distance and size, solar heating, and inlet/outlet pipe configuration. Under these conditions, especially in the thermally isolated upper layers, water ages and can completely lose its disinfectant residual, the final barrier that prevents bacterial regrowth and protects consumers from waterborne illness. Despite today's advanced treatment and disinfection technologies, water utilities are discovering that the simple process of mixing is a cost-effective way to maintain the highest possible quality water in storage facilities.

Public water distribution systems store larger-than-normal volumes to provide for peak demand, pressure regulation, fire protection and other emergencies, and future growth. Often daily volume levels do not fluctuate more than 5% to 10%, leaving underutilized tanks with poor circulation. Those who designed traditional storage facilities with common inlet/outlets were unaware of the impact of mixing.

Impact of Regulatory Changes

Historically, regulations focused on treatment plant effluent quality with only limited requirements for quality testing in the distribution system. This changed significantly in 1996 when the Safe Drinking Water Act (SDWA) was amended to establish more stringent requirements for pathogen removal, including maximum levels for harmful disinfectant byproducts (DBPs). Recent major rule changes include the Stage 2 Disinfectants and Disinfection Byproducts Proposed Rule and the Long Term 2 Enhanced Surface Water Treatment Rule. Together these changes are compelling water utilities to find more effective ways of controlling microbial contaminants while reducing disinfectant byproducts.

Consequently, many utilities are shifting from chlorine to chloramine disinfection because chloramine residuals are more stable and tend to form fewer DBPs. While more cost-effective than upstream removal of DBP precursors, chloramine disinfection has introduced its own problems because as it breaks down it forms ammonia. Naturally occurring bacteria thrive on that excess ammonia.

Nitrification

Lack of mixing also creates conditions favorable for nitrification in chloraminated systems, i.e., warmer temperatures and loss of disinfectant residual to control bacterial regrowth in the upper layers. If caught early, onsite addition of hypochlorite often can prevent full nitrification. If not remedied at the earliest stages, nitrification can result in:

  • Further reduction or complete loss of disinfectant residual throughout the entire storage volume
  • Elevated bacteria levels requiring a public boil water alert
  • Loss of consumer confidence.

More severe events may require tanks to be taken out of service for cleaning at a high cost to the utility. Even when nitrification is caught on time, pumping or pouring straight hypochlorite solution into a problematic tank can increase DBP levels.

While preventing nitrification is the primary reason for reservoir mixing, water utility operators also understand the importance of preventing stagnation. Any tank showing loss of disinfectant residual poses an imminent threat to water quality and unnecessary risk to public health.

A Simple Solution

Costly new treatment facilities are limited in addressing water quality deterioration when the cause is the reservoir itself. Mixing to eliminate stratification and ensure uniform distribution of disinfectants is a viable and effective long-term strategy to control residual loss and associated nitrification. Further, mixing reduces the formation potential of DBPs and can therefore help utilities stay ahead of the upcoming EPA Stage II regulations. While mixing is not on the regulatory horizon, more and more states are recommending a mixing system for all new and rehabilitated tank projects as a best management practice.

Mixing systems are classified as active and passive. Passive systems use the momentum of influent water through directional nozzles to mix. Active systems are powered and sized to deliver a specified mixing performance.

Active systems provide the operational flexibility to accommodate any chemical addition to the tank if needed to boost residual levels, any time of the day or night, without sacrificing storage volume for fires and other emergencies. Giving operators control over mixing in the storage reservoirs, where today most rely upon normal demand and nighttime pumping operations, is a key driver of active mixing technology.

Mixing is a simple, cost-effective, sustainable way to manage water quality in reservoirs, and reflects a shift in thinking toward optimal use of chemicals already in the system. Lightweight submersible reservoir mixing systems can effectively reduce the amount of chlorine-based disinfectants needed while maintaining the highest quality and consistency of water delivered.

Jason Oppenheimer is vice president of marketing for PAX Water Technologies Inc., San Rafael, Calif.