Computer Simulation Reduces Cost of Compliance

A recent application of computer simulation helped one utility reduce by 60 percent the cost of complying with new drinking water regulations.

The Environmental Protection Agency (EPA) recently reduced the allowable content of trihalomethanes from 100 to 80 parts per billion (ppb) because they are a suspected carcinogen. Reducing trihalomethanes concentration while maintaining drinking water safety requires increasing disinfection efficiency so less chlorine can be used.

Simulation with computational fluid dynamics (CFD) helped develop an optimized baffle design that increased the efficiency of a reservoir at an Arizona water treatment plant from 10 percent to 35 percent. This design complies with the new regulations, at a cost of about $1 million compared to an estimated $2.5 million that would have been required to achieve compliance using conventional methods.

Trihalomethane Formation

The Safe Water Drinking Act established a 100 ppb annual average maximum residual for total trihalomethanes in 1979. EPA?s Stage 1 Disinfectant and Disinfection Byproduct Rule reduced the maximum to 80 ppb. Large surface water systems are required to comply with the Stage 1 rule by December 2001. Ground water systems and small surface water systems must comply by December 2003.

Trihalomethane Formation

For the majority of water treatment plants, the least expensive method of compliance is increasing the efficiency of their chlorination process in order to reduce the amount of chlorine required to kill bacteria. The problem at some water treatment plants is that water flows through disinfection reservoirs with a relatively short exposure to the chlorine. As a result, chlorine concentration needs to be maintained at high values in order to thoroughly disinfect the fraction of the water with the lowest contact time.

Trihalomethane Formation

A reservoir?s efficiency is calculated by injecting an arbitrary amount of tracer and waiting for 10 percent of that tracer to exit the reservoir. The time needed for the tracer to exit the reservoir divided by the theoretical residence time yields the efficiency. The theoretical residence time is simply the volume of the reservoir divided by the flow rate.

Trihalomethane Formation

The efficiency of most reservoirs is low, in the 10 percent to 15 percent range.

Scale Models

The efficiency of a reservoir can usually be improved by adding baffles that increase the length of the path that water must travel from the inlet to the outlet. The value added by the baffles must be weighed carefully against their cost and installation time.

Scale Models

Another problem is the difficulty in determining the number of baffles required to raise the efficiency of the reservoir by a certain amount and their ideal configuration and location.

Scale Models

In order to overcome this problem, Carollo engineers have in recent years begun using computational fluid dynamics (CFD) software to simulate the flow of water through disinfection reservoirs.

Scale Models

A CFD analysis provides fluid velocity, pressure and solute concentration values throughout the solution domain for problems with complex geometries and boundary conditions. As part of the analysis, a researcher may change the geometry of the system or the boundary conditions such as inlet velocity, flow rate, etc. and view the effect on fluid flow patterns or concentration distributions. CFD also can provide detailed parametric studies that can reduce the amount of experimental work necessary to develop prototype equipment.

Finite Element Approach

Carollo selected FIDAP CFD software from Fluent Inc., Lebanon, N.H. Disinfection reservoirs are very challenging from a modeling standpoint because of the difficulty in capturing the complicated shear effects near the walls of the reservoir. The finite element approach used by FIDAP provides high accuracy on problems of this type. Carollo engineers have found that FIDAP provides ?5 percent accuracy in predicting the efficiency of disinfection reservoirs.

Finite Element Approach

In one recent application, the city of Phoenix asked Carollo to design modifications to an existing 40 million gallon reservoir at the Val Vista Water Treatment Plant that would increase its efficiency from the current 10 percent to the 35 percent level required to meet the new EPA regulations while minimizing construction costs and downtime. The Val Vista plant processes 220 mgd for the communities of Phoenix and Mesa. Without the benefit of simulation, Carollo engineers would have had to play it safe by designing a five baffle modification that would have certainly improved efficiency to the required level but would have cost approximately $2.5 million. Instead, engineers constructed a two-baffle configuration that increased reservoir efficiency to 35 percent.

Finite Element Approach

Engineers wrote FIDAP subroutines to create special boundary conditions at the inlet and around the walls of the reservoir. An RNG turbulence module was used to model the separation, recirculation and swirl regions in the flow. Engineers then used the software to simulate the addition of a tracer chemical to the reservoir and measured the elapsed time before tracer appeared at the outlet. The results matched experiments on the existing reservoir within ?5 percent.

Finite Element Approach

The next step was modifying the model to evaluate a range of different baffle configurations. In only one month, the engineers examined six different design iterations, the best of which achieved the required efficiency level at a cost of $1 million.

Finite Element Approach

About the Author: Sanjay Reddy, P.E., is Project Manager for Carollo Engineers, PC, in Phoenix, Arizona.