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For installation in conduits or tunnels.

Movement of People and businesses to the Sunbelt areas of the U.S. has put pressure on many water utilities in those regions to expand their treatment plants. Jointly owned by the cities of Phoenix and Mesa , Arizona , the Val Vista Water Treatment Plant was one of those that underwent a modification program to increase its capacity. One of the principal components installed during the project was an in-line motionless mixer believed to be the largest ever supplied to a water plant which has produced some attractive benefits in the operation of the plant.

When it was built in 1975, this still new plant had a rated capacity of 80 mgd. This has now been increased to 140 mgd. Water from six lakes, four on the Salt River Basin and two on the Verde River Basin , is channeled through canals and controlled by the Salt River Project to provide Val Vista with its supply.

Water quality is normally good. For instance, during the past year, turbidity has averaged 10 Nephelometric Turbidity Units (NTU) or less. In the past, on some occasions when rain run-off has been high, turbidity has gone up to 8,000 NTUs. Periods of high turbidity usually last around 24 hours, but has lasted as long as two months.

The plant’s treatment train is typical. Water from the canal passes through bar screens, flows by gravity into a grit basin with a 15-min. retention time. Heavy grit and mud particles are removed from this chamber by a rake into a sump, from which they are pumped into a centrifuge for liquids/solids separation.

The next step is pre-sedimentation with a 1.5-hr retention time. Smaller particles settle out before the water reaches the flocculation basins. These particles are drawn off by clarifier type rakes to the center of the basin, where a valve is opened and they are slushed back into a canal system.

Aluminum sulfate and chlorine are continuously added to the raw water feed and in the dosing channel which connects the pre-sedimentation and flocculation basins. It was at this point that the mixing system was given careful consideration in the design phase of the project to expand the plant.

In the original configuration, 7-5-hp mechanical mixers were used to disperse the chemicals into the stream through the 8-sq-ft flume joining the two basins. Two 4-bladed units were installed in parallel. While this method proved adequate for treatment of water at 80 mgd, it was concluded that a new system would have to be installed to handle the up-rated capacity.

The firm selected to design the plant expansion, James M. Montgomery, Consulting Engineers, Inc. of Pasadena , California , studied several alternatives to the mechanical misers then in service at Val Vista. Their work concluded that the static mixer manufactured by Komax Systems, Inc. of Long Beach would be the preferred unit and would provide cost-effective, high-efficiency mixing. The supplier was asked to custom design a mixer which would fit into the eight-foot square channel.

Alternatives considered were determined to be either too expensive or inadequate. Mechanical mixers would have cost significantly more than the $30,000 price of the mixer chosen. Other baffle-type motionless mixing designs would have created too much head loss, and they were not available in a square configuration at that time.

Installed in less than on week, the Komax mixer consists of a series of interlocking left and right handed mixing elements which, under turbulent flow, produce elliptical vortices rotating in opposite directions of wither side of each element. The axis of each vortex is at right angles to the axis of the main water flow, producing a back-mixing effect that disperses the chemicals evenly. Chemicals are added through several spargers connected at the head of the mixer.

The 20-ft-long unit, which has two mixing elements, is fabricated of carbon steel and bolted to the concrete channel. Treated with placite to resist corrosion, the mixer has a maximum head loss of 2 ft (under 1 psi) and a total effective mixing time of about 5 seconds at the maximum design flow rate of 140 mgd.

After the water passes through the motionless mixer, it flows into a flocculation basins, where water and chemicals remain in contact for 35 minutes. From there the water goes into secondary settling basins, where the floc settles out. The water then passes through dual media bed filters (anthracite coal and sand) and is either held in a reservoir or channeled to the distribution systems of the two cities.

According to a preliminary cost analysis based on several months of operational data, the new mixer has reduced costs in a significant way. The unit, of course, requires no external energy to operate. Using guidelines established by the American Water Works Association and the American Society of Civil Engineers, Mechanical flash mixers should have between 0.25 and 1 hp for each mgd of plant flow. At Val Vista’s new flow rate of 140 mgd. The power range would fall between 35 hp and 140 hp. Taking 75 hp as an average, annual energy costs of running such a mixer (at 7¢/kwh, 24/hr day, 365 days/yr) would be $34,300.00. At the 140-hp maximum value, the annual energy costs would obviously be a lot $64,000.00.

At the same time, since there are no moving parts in this type of mixer, maintenance problems have been eliminated. It was estimated that mechanical mixers could require about $500 per year, after their first year of operation, to maintain. Finally, the new mixer generates no noise, which produces a definite improvement in the work environment.