TECHNICAL REPRINTS

TN Community's Grinder Pumps Provide Positive O&M Statistics

 

Pressure sewers are especially advantageous for lakefront homes where lift stations can be eliminated and house elevations can be set without regard for the elevation of the sewer main.

by Donald D. Gray, Ph.D., P.E., West Virginia University, Department of Civil Engineering

Small communities considering the installation of grinder pump pressure sewers are often concerned about whether they will be able to afford the continuing maintenance these systems require. While there is no universal answer to this question, the experience of another community may be the most reliable indication.

Fairfield Glade is a privately developed resort and retirement community nine miles from Crossville, Tennessee. Located on the Cumberland Plateau, the irregularly shaped site contains 11 lakes and has elevations ranging from 1665 to 2200 feet above sea level. Prior to its development in 1970, the area was forested with rock outcrops.

Today, Fairfield Glade has a population of 4,500 and has four golf courses. Approximately 16,000 single-family lots, ranging in size from 1/4 to 3/8 acre, have been plotted and 14,500 lots have been sold. As is common with developments of this type, the actual buildout has been slow, with fewer than 1,400 single-family houses in place. In addition to single family houses, the development includes town houses and condominiums.

Fairfield Glade uses a mix of technologies for sewage collection. Approximately 15 percent of the single family houses use onsite septic systems, while the remaining houses use conventional gravity sewers or grinder pump pressure systems.

"Each system has its good applications," says Tom Swafford, vice president of engineering for Fairfield Glade, Inc., the company designing the system. "Gravity sewers have expensive mains and low-cost taps, so high density and rapid buildout throw the advantage to gravity sewers. Pressure sewer costs are reversed, so it has the advantage where densities are low and buildout is slow. When costs seem close, I make the choice based on a detailed economic analysis of the alternatives. I expect to continue to use both systems."

Swafford finds that pressure sewers are especially advantageous for lakefront homes where lift stations can be eliminated and house elevations can be set without regard for the elevation of the sewer main.

With the combination of undulating terrain, rocky soil, and low population densities over most of the site, Swafford knew early in the project that grinder pumps would be a key part of the collection system. By 1978, 20 grinder pumps were in operation. An average of 60 grinder pumps per year have been added with 821 pumps currently in operation. According to Swafford, there are presently 31 miles of 2- to 6-inch PVC pressure mains.

Environment One grinder pumps have been used exclusively in the project. A key reason for this choice was the manufacturer's estimate of an annual operation and maintenance (O&M) cost of $40 to $50 per year per pump.

Recognizing the importance of O&M costs for this type of system, Swafford directed Fairfield Glade's maintenance foreman Wade Davenport to keep detailed records on all grinder pump service calls. Davenport oversees three employees who maintain the gravity and pressure sewers as well as the conventional biological treatment plant.

According to Davenport, approximately 25 to 40 percent of their efforts are devoted to pressure sewer O&M, including both preventive maintenance and service calls. In many small communities there is a general maintenance department that services several types of utilities; therefore, it is difficult to isolate the costs related specifically to sewers. Because his staff deals exclusively with sewage systems, Davenport's records are unusually reliable.

The Fairfield Glade data tell a reassuring story for communities considering grinder pump pressure sewers. Figure 1 shows the amount spent (in 1990 dollars to remove the effects of inflation) for O&M per pump in each year since 1978. Except for 1979 and 1982, the costs have been less than originally estimated. The average for the entire period is $36.07.

The downward trend of O&M costs is even more encouraging when the aging of the system is taken into account. Figure 2 shows the actual amount spent for O&M (in 1990 dollars) per pump as a function of the average age of the pumps. Because new pumps have been added each year, the average age increases less than one year per calendar year. Figure 2 shows that the average age of the pumps operating on January 1, 1990, was 5.03 years.

Because the number of pumps increases each year, it may be misleading to compare the average costs from different years. One way to remove this bias is to calculate a cumulative annual O&M cost by dividing the total O&M cost since 1978 by the total pump-years of service. Figure 3 presents this cumulative annual O&M cost (in 1990 dollars) as a function of the average age of the pumps. Once again the trend is downward, with the latest figure being $28.80 per pump per year.

Swafford offers two reasons for the surprising decline in O&M costs. "Our people have become better at maintenance and Environment One has made improvements which reduce maintenance requirements." Another reason may be that the relative importance of "infant mortality" has declined as the system has grown and aged. Whatever the reasons, the news is good for communities considering grinder pump pressure sewers.

Small Flows, Volume 5, Number 4, October 1991