Talk Synopsis

Treated water storage presents several water quality risks including water ageing, dead-spots, chemical stratification, stagnation and bacterial growth. Standing water can also support growth of biofilm on surfaces and sedimentation on floors of tanks. We could address many of the risks with mixing. What’s more, service reservoirs and storage tanks provide an ideal asset base in which to do this. Optimising storage assets would leverage more value and capability; enhancing services for the same cost and, in most cases, more sustainably. Win, win, win.

Treatment and network assets are often managed separately, with different priorities and objectives. Utilities focus on delivering excellent system water quality treatment plants, and pressure management, calm networks and leakage reduction in distribution. While this make sense, it doesn’t have to be so binary. Storage assets already deliver storage objectives and can be optimised to support water quality through distribution. In effect, becoming low-cost, decentralised, treatment assets operating closer to customers and reducing the burden and capital expenditure requirements on distant treatment works.

Mixing:

Mixing overcomes the risks of standing water by producing a homogenous body of blended water in tanks which improves water quality in several ways:

• Improved disinfectant blend time, making disinfection more efficient and possibly lowering chemical use
• Eliminating chemical stratification
• Removing dead-spots and creating a uniform water age and quality in the tank, resulting in:
  • Calmer and more predictable networks
  • Lower numbers of bacti failures
  • Fewer taste and odour complaints that are a result of fluctuations in disinfectant level (customers notice changes as opposed to specific concentrations)
• Reduced amounts of disinfection by-products (DBPs), such as Trihalomethanes (THMs)
• Reduced build-up of sediments and biofilm increasing time required between cleans and fewer interruptions to supply
• Better management of nitrification in chloraminated systems

While most tanks would benefit generally from mixing, it is a foundational technology that can be enhanced further for problem tanks or DMAs to enable smart residual control or disinfection by-product (DBP) removal:

Smart residual control:

Disinfectants (chlorine or chloramine) are used in potable water networks to maintain water quality from treatment works to customers. Samples taken from mixed tanks are representative of actual disinfectant levels in the tank; disinfectant need only be introduced when the level is too low. Residual levels can be automatically managed in real-time by integrating three existing discrete functions:

1. Mixing to achieve complete disinfectant and chemical homogenisation
2. Active monitoring to respond to dynamic tank conditions in real-time
3. Instructing pumps to dose until the required set-point is achieved.

DBP removal:

DBPs, such as trihalomethanes (THM), occur when organic matter meets chlorine and cannot be avoided in potable water networks. However, many DBPs are volatile. Therefore, with the right conditions we can significantly reduce the level of DBPs in treated water as follows:

1. Mixing to achieve a blended body of water which regularly cycles water at the surface
2. Aerating water if the air-water interface needs to be increased
3. Venting the tank to prevent DBP concentration build-up in the tank headspace.

Conclusion:

• Storage tanks and SRVs are underutilised assets that can be optimised to deliver water quality objectives at the same time as storage and hydraulic objectives
• Mixing is the first step towards optimising storage assets and has several benefits
• Mixing can be enhanced with an automatic sampler and dosing rig to automatically manage tank disinfectant levels in real-time
• Mixing can be enhanced with aeration and ventilation to reduce DBPs 

Further information on this technology can be seen here:

• https://pantonmcleod.co.uk/outside-the-fence
• http://www.paxwater.com/tank-mixer
• http://www.4psi.net/monoclor-overview.php
• http://www.paxwater.com/dbp-removal