Maintaining optimum water quality—from the source all the way to the tap—is a top priority of our nation’s water utilities. Recent regulatory changes have water professionals increasingly focused on water quality deterioration within distribution systems. When finished water of even the highest quality leaves a treatment facility and then resides for extended periods of time in storage tanks and reservoirs, even the best water can significantly deteriorate before reaching the first customer.
Until recently, storage facility design neglected to consider the water quality implications of poor mixing and inadequate turnover/circulation. Storage tanks tend to be over-sized, and the majority of existing tanks and reservoirs constructed do not have separated inlet and outlet pipes. Prior to recent (and increasingly stringent) regulation of disinfectant byproducts, heavier disinfectant use masked this design deficiency. However, to meet new EPA requirements, innovative utilities are implementing new practices to minimize DBP formation, and are driving the conversation from chemical to mechanical solutions. Tighter regulations are accelerating a shift in thinking away from additional onsite dosing and toward optimal use of the chemicals already present in the distribution system.
Many water utilities face one or more of these interrelated challenges, all three of which can be addressed by a single solution: effective, efficient mixing.
1: Stagnation and Stratification
Sized to meet future peak demands and comply with requirements for fire protection or other emergencies, storage volumes can be ten or more times the daily demand. This mismatch between tank size and water demand leads directly to low turnover and stagnation.
As cooler water from the distribution system enters the inlet pipe at the bottom of a tank, it slides underneath the warmer water already present in the tank, and very little natural mixing occurs. A thermocline forms in the tank—even a few degrees Fahrenheit is enough to limit natural mixing. If a tank is exposed to the sun, stratification is magnified. Testing of the upper layers of water reveals elevated temperatures and excessive water age, and this warmer water begins to rapidly lose disinfectant residual—leaving the tank vulnerable to bacterial re-growth.
Any tank exhibiting the loss of disinfectant residual poses a threat to water quality and unnecessary risk to public health.
The water quality challenges present in a thermally stratified tank
2: Regulatory Compliance
EPA regulations regarding microbial contaminants such as Cryptosporidium and Giardia, and disinfectant byproducts (DBPs) have recently become more stringent. The most 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 at the same time reducing disinfectant byproducts.
As a result, many water agencies have converted to chloramine disinfection because it is more stable, persists longer in the distribution system, and tends to form lower levels of DBPs. However, use of chloramines can create conditions favorable to nitrification.
3: Nitrification
Deficient mixing creates conditions favorable for nitrification in chloraminated systems; it often leads to warmer temperatures and a resulting loss of disinfectant residual, which controls bacterial re-growth in the upper layers. Warmer temperatures also lead to faster rates of growth for nuisance bacteria. If not remedied at the earliest stages, nitrification can result in further reduction (or a complete loss) of disinfectant residual and elevated bacteria levels that would trigger the need for a public boil water notice. More severe events require tanks to be taken out-of-service for cleaning at a high cost to the utility.
Even when nitrification events are caught early, the most common response—breakpoint chlorination—introduces its own concerns. This practice of adding chlorine solution directly into a problematic tank can produce high levels of DBPs from the rapid oxidation of organic precursors in tank biofilms.
All three of these challenges can be addressed by a single solution: effective, efficient mixing.