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FLOW-3D Helps Prevent Nitrification in Water Storage Reservoir

Simulations with FLOW-3D recently played a key role in helping to reduce the potential for nitrification occurring in a municipal water system that is converting to chloramines disinfectant.

figure 1 image
Figure 1: This wall inlet generates a bottom jet that
spreads and rises upon intersecting the opposing tank
wall. This circulation pattern leaves a region in the
center part of the tank that receives less mixing
than at the bottom and surface of the reservoir.

figure 2 image
Figure 2: This plan view shows the distribution of
fresh (disinfected) water after 500 seconds at the
manifold and diffuser level in a water tank
retrofitted with a new mixing system. Better
mixing is achieved with this type of system
compared to a single inlet system.

figure 3 image
Figure 3: The distribution of disinfectant concentration is
shown in two curves – one for the original and the
other for the new mixing condition. The initial ambient
water concentration is assumed to be 0.20 milligrams
per liter. The influent concentration is 0.24 milligrams
per liter. The water tank capacity is 500,000 gallons

To comply with the mandates of the federal Safe Water Drinking Act, many municipal water utilities are converting from the use of chlorine and chlorine compounds (which form disinfection byproducts linked to certain cancers and reproductive and developmental defects) to alternative disinfectants such as chloramines. Chloramines, however, break down through a nitrification process that can provide nutrients for bacteria if they are allowed to remain in dead spots. For this reason, utilities moving to chloramine disinfectants face the challenge of insuring adequate circulation within water storage reservoirs.

One such municipality making this conversion, the City of Brisbane, CA, adopted advanced technology to address these issues. The first task was to evaluate the circulation within Brisbane’s tanks, which range from 33 to 92 feet in diameter and hold from 200,000 to over 1,000,000 gallons. The original tank designs used a single pipe that penetrated the floor or shell of the tank. Positive head was applied to this pipe to fill the tank and negative head was used to drain it.

In the past, the only method engineers had to evaluate reservoir circulation was to build a scale model and perform experiments or conduct tracer studies. These approaches are expensive and time-consuming and, more problematic, may yield inaccurate results because of scaling factors and the limited number of measurement points. For these reasons, INCA Engineers (Bellevue, WA) was hired to perform CFD analysis on the existing tank configuration as well as alternative designs. The alternative designs are based on constructing multiple inlets off of a central manifold. The inlets are fitted with diffuser valves.

INCA Engineers had developed confidence in FLOW-3D's modeling accuracy because the model's results compared well to laboratory studies of diffuser valves and, for this case, found the multi-block model especially valuable because of the need to mesh small areas of the domain in very fine detail while using a coarser grid elsewhere in order to minimize solution times. This capability played an important role in modeling valve nozzles as small as 2 inches in diameter without loss of accuracy.

Text output data, readily available from FLOW-3D results files, was gathered for disinfectant concentrations in each fluid cell at three different time intervals during each simulation condition. A statistical program developed by INCA's Jim Dexter was used to compute a disinfectant concentration cumulative frequency curve for each set of results. The curve was plotted as the disinfectant concentration versus percent of water in the tank that is less than or equal to the indicated value.

The simulation results for the original configuration showed that the single-inlet generated a bottom jet that spreads and rises upon intersecting the opposing tank wall. This circulation pattern left a region in the center of the tank that received less mixing than at the bottom and surface of the reservoir.

A modified manifold system, which introduced water near the center and to the far side of each reservoir, was then studied. The new configuration had a cluster of smaller inlet valves located in the center third of the tank diameter and two larger inlet valves located near the far tank wall. The outlet valves are located near where the pipe penetrates the tank. Special diffuser valves were used to maintain higher jet velocities at lower flow rates. The multiple turbulent jet directions developed by this manifold entrain a larger ambient water region as a result of creating more complex mixing patterns.

The simulation of the new manifold configuration showed dramatic improvements in mixing efficiency relative to the initial configuration. The results indicated that nearly the entire (85%) tank volume was being mixed by the system at the end of a 25-minute simulation interval. The statistical program also showed substantial improvements in disinfectant distribution. As the accompanying figure shows, 50% of the storage volume had reached a concentration of 0.21 compared to only about 2% under the original condition

Spiess Construction installed the new manifolds in Brisbane's four tanks well in advance of its switch to chloramines, so that each system could be tested in advance. Measurements of free chlorine were performed in the tanks with the new manifolds as a marker for chloramines disinfectant. They showed that at all fluid levels, disinfectant was consistently maintained at levels above 0.25 mg/L, indicating excellent mixing and the elimination of dead spots. These improvements will ensure the safety of the city's water supply for years to come. This application clearly demonstrates the value of applying CFD to every-day engineering problems and the power of FLOW-3D in particular for water utilities that are planning to improve drinking water quality.

Appreciation to Jim Dexter of JD Hydraulics, LLC, for his contributions to this note.