Reap the Rewards of Effective Storage Tank Mixing with
Tideflex® Mixing System (TMS)
Thousands of miles from the coast, lake communities serve as the center of Midwestern fun. However, as millions of families flock to their favorite lakeside spots, municipalities close to the shore will see water usage rates more than double the yearly average. Hot weather, increased people in the area – it all adds up to higher water use. To help us better understand the ways water engineering solutions help cities cope with these shifting needs, here is an example of a community in Iowa that required larger capacity and a creative mixing system for their community water system.
Obvious as it may be, increased water usage calls for greater city water storage capacity. However, not every municipality can afford to build a new, larger water tower, nor may they see the benefit with such large discrepancies between high and low usage months. Take the City of Wahpeton, Iowa, for example. Located near popular West Lake Okoboji, the City struggled to provide consistent water service to its community during the summer months, due largely in part to the municipal water tower’s limited holding capacity of 50,000 gallons. To make up for the seasonal gap in supply and demand, the City supplemented water use from the nearby City of Milford. This is not an entirely uncommon solution for lake communities, but one that may seem odd for Wahpeton, which overlooks the State of Iowa’s second largest fresh water source. Recognizing they were missing out on an opportunity to capitalize on such a great, nearby resource, the City decided to pursue the next water solution of discussion: increased water storage capacity.
The City of Wahpeton worked with their City Engineer, ISG (a Midwestern consulting firm), to review their potential options. A larger water tower (200,000 gallons) was proposed to not only account for the high-water use but also to serve as a beacon for the community. Typical water usage during the winter months was approximately 0.050 MGD but water demand increased during the summer months to peaks of 0.250 MGD. This additional storage would aid in water demand during the summer months but had risk of water stagnation and freezing during the winter months. For cities that decide to move forward with building larger water towers, a different set of considerations come into play, especially as they relate to bacterial growth and by-products stemming from water treatment processes. While having greater storage capacity helps maintain supply during peak months, it also creates new challenges throughout days of relatively lower demand (winter), including the tendency for water to sit stagnant in large towers for longer periods of time before use. According to the American Water Works Association (AWWA), water should not be allowed to age for more than seven days in any system. If water does sit idle for longer than the suggested window, it’s susceptible to disinfection by-products (commonly referred to as DBPs) and chlorine residual loss, which can cause increased levels of bacterial regrowth, contamination, nitrification, stratification, and other negative effects.
The City of Wahpeton was faced with this challenge of water stagnation when designing a new 200,000-gallon water tower replacement for their previous 50,000-gallon tank. To keep water moving within the tower, ISG integrated a custom-designed Tideflex® Mixing System (TMS). TMS – which can also be installed in existing distribution storage tanks during routine system inspection/rehabilitation periods – are powered entirely by energy facilitated from already-occurring fill and draw cycles. In addition to not needing any additional energy source, TMS designs also require virtually no maintenance, making them a great investment for municipalities. These items combined would assist in proper mixing of the water tower, alleviate water stagnation and freezing, improve chlorine residual or remove the potential for loss of residual, and reduce detects of DBP/TTHMs. The City’s opinion of the advantage of the TMS included the following:
- Any mechanical repairs required could be accomplished at the base of the tower with the supplemental mixing pump (added for additional mixing if desired) rather than climbing the tower to make any repairs
- Costs of the system was in par or below alternatives
- Timely responses of the manufacturer and manufacturer representative, and
- CFD modeling was also provided by the manufacturer to show the city proper mixing analysis for the system was completed.
Previous winters, even with the 50,000-gallon water tower, provided water stratification in the tower due to extreme external temperatures. Additionally, this led to loss of chlorine residual and increase in disinfection by-products (DBPs). With the new Tideflex® Mixing System in the new, larger capacity water tower; chlorine and temperature were monitored and maintained consistent through the entire depth of the tower. Additionally, DBPs were drastically reduced throughout the system thanks to the TMS.