May 2013, Vol. 5, No.25

Small communities

Hard data on effluent sewer O&M costs

Grant Denn

Search the Internet for “U.S. sewer bills,” and the first page of results will show examples of communities where monthly household sewer rates are nearing or exceeding three figures. For example, in Islamadora, Fla., a headline dated May 30, 2012, reads, “Sewer bill could run $140/month.”  

More generally, the 2010 report Trends in Local Government Expenditures on Public Water and Wastewater Services and Infrastructure: Past Present and Future, released by the U.S. Conference of Mayors (Washington, D.C.), projects that water and sewer rates will double or quadruple during the next 20 years — with $0.60 of every dollar going to operations and maintenance (O&M). On the flip side, residents of Bethel Heights, Ark., are paying $35/mo; residential customers in the South Alabama Utility District pay $35/mo; and in Yelm, Wash., and Victoria on Prince Edward Island, monthly bills are about $43. 

All these small communities with affordable sewer rates have something in common: All are served by decentralized effluent sewer systems that use a septic-tank effluent pump (STEP) configuration. These systems range from 5 to 19 years old and were designed, equipped, and installed to tight specifications with ongoing O&M support. 


Long-standing technology  

STEP sewer systems are not new. They were first installed in the early 1970s, and the technology has been optimized during the past 4 decades. Consequently, many members of the engineering community now acknowledge that decentralized sewer systems typically are less expensive to purchase and install than conventional gravity sewer systems — but many still are unaware that decentralized sewers also are less expensive to operate and maintain. 

Engineering proposals often overestimate STEP sewer O&M costs by a factor of 2 or more while greatly underestimating O&M costs for gravity sewers. According to the 2004 report Replacing and Securing Water Utility Infrastructure by the National Regulatory Research Institute (Silver Spring, Md.), gravity sewer O&M costs can approach $328/m ($100/ft) or more. The bottom line is that many engineers and communities lack accurate life-cycle costswith which to make decisions. 


Gathering information  

Some of this is completely understandable. STEP O&M data can be difficult to obtain. Because STEP sewers need so little O&M in the first decade, a system has to be in the ground for at least 10 years to start accumulating useful O&M data. Also, STEP sewers need so little O&M overall that often there is little consistency in recordkeeping from town to town and subdivision to subdivision.  

However, based on the documented performance data of thousands of STEP sewer connections collected by one manufacturer, the operational cost of STEP sewers that have been designed, equipped, and installed to the manufacturer’s rigorous specifications, at a 4% interest rate, is about $7.05/mo per residence (see Table 1, p. 66). 


O&M costs and intervals  

As Table 1 shows, O&M and repair and replacement (R&R) costs for STEP sewers fall into four main categories. 

Table 1. Effluent sewer operations and maintenance and R&R costs 

Component  Cost per month per EDU 
Preventive maintenance     $1.60
Reactive maintenance $0.60
Equipment R&R $2.81
Tank pumping $2.04
Total $7.05

EDU = equivalent dwelling unit.
R&R = repair and replacement.

About 30% of monthly O&M costs are for preventive maintenance (PM) and reactive maintenance (RM).For PM, protocols vary widely among systems. Some are given too little attention, some too much. Both approaches create problems. 

In the March 2009 WE&T article “O&M Considerations for STEP Systems,” the author, Mike Saunders, concludes that “[t]he most cost-efficient STEP management approaches balance PM and RM to achieve the lowest overall costs of O&M.” For nearly 10 years, Saunders served as the utility engineer for Charlotte County (Fla.) Utilities — one of the largest STEP systems in the U.S. 

A proper PM program requires only a few tasks every 3 to 5 years, consisting of measuring the solids and scum in the on-lot tank, cleaning the pump and its filter, and verifying the operation of floats and the control panel. Conservatively estimating 1.5 hours per visit at a rate of $40/h, PM costs about $60 per visit in 36 months, or about $1.60/mo per equivalent dwelling unit (EDU). 

As for RM, data from 11 systems in four states (totaling nearly 3100 connections) show that these systems average 1.4 h/mo per 100 EDUs. Conservatively estimating 1.5 h/mo per 100 EDUs yields a cost of $60 per 100 EDUs (using a business-hour labor rate of $40/h, because on-lot tanks have enough reserve capacity to enable operators to handle after-hour emergencies during the next business day). This comes to a monthly RM cost of $0.60 per EDU.  

The greatest-cost category for STEP systems — about 40% of the monthly O&M costs — is for equipment R&R. R&R costs are related directly to equipment quality, especially with regard to pumps. A high-quality pump for STEP systems provides, on average, more than 20 years of service. The average R&R costs shown in Table 2 represent actual costs from several STEP systems. 

Table 2. Effluent sewer R&R schedule and costs  

Component  Frequency  Cost per event (materials + labor)  Cost per month per EDU* 
Pump replacement 20 years $600 $1.62
Float replacement 10 years $100 $0.68
Miscellaneous component R&R 10 years $75 $0.51
Total     $2.81

*Amortized at 4% interest.
R&R = repair and replacement.
EDU = equivalent dwelling unit.

Tank pumping is the fourth and final cost category shown in Table 1. It accounts for nearly 30% of monthly O&M costs and is directly related to pumping frequency (pump-out intervals). Table 3 shows reliable pump-out intervals for households with various sizes of tanks and numbers of occupants derived from an 8-year audit of watertight tanks in Glide, Ore., and a 5-year audit in Montesano, Wash. 

Table 3. Pump-out intervals for 3785-L (1000-gal) tank at 95% confidence level  

Number of occupants 2 3 4 5
Pump-out interval, years 22 11 7 4

Assuming a 3785-L (1000-gal) tank and three occupants in the household, this chart shows a pump-out interval of about 11 years at a 95% level of confidence. Estimating a pump-out fee of $300 at an annual interest rate of 4% and a 10-year frequency, the cost comes to $2.04/mo per connection. 



Pacific Northwest example  

The community of Yelm, Wash., offers a good case study of STEP O&M costs. In the early 1990s, Yelm was growing fast and wished to replace its outdated septic systems with a sewer system that would not burden residents with high rates or the cost of future growth. A phased system with low maintenance costs was crucial. So, in 1994, the city installed a STEP system that now serves nearly 6000 people (1700 STEP connections). 

Like other small communities, Yelm was concerned about two O&M cost categories: periodically pumping and hauling solids from septic tanks, and replacing in-tank pumps. Yelm’s experience during nearly 2 decades shows that these situations rarely occur, meaning costs are very low. 

With regard to pumping, most residential units have two-compartment, 3785-L (1000-gal) tanks (some of which are shared with another home) that are pumped out about every 6 years at a cost of $0.06/L ($0.24/gal). Amortized at 4% interest, this equals about $2.96/mo per EDU. As for pump replacement, between 1994 and 2009, only 28 pumps were replaced out of 1700 connections. With a pump replacement cost of $600 in labor and materials, the monthly cost of each household’s pump is only $2.50. 

Yelm has been able to keep its base sewer rate to $43/mo. 


Southern success  

Another example comes from the growing community of Bethel Heights in Benton County, Ark. Bethel Heights installed its STEP system and a packed-bed filter treatment system in 2003 to accommodate strong interest in new development. The community now has a population of about 2800 and 470 STEP connections. 

To minimize costs for the first several years until connection fees from new subdivisions started coming in, Bethel Heights’ mayor, Fred Jack, obtained a Class I operator’s license and operated the STEP and treatment systems himself. By 2010, the system was regularly maintained by a staff of 1.5 full-time equivalents. 

In a 2010 interview, Jack said, “We focus on preventive maintenance and repair rather than replacement. ... I can repair a pump for about $40 instead of buying a new pump. ... We really work to operate in the black.” Bethel Heights is on course to pay off its 30-year wastewater bond in only 7 years. Its base sewer rate is $35/mo. 


Clients deserve hard data  

After 40 years of technology development, ample data exist to show decentralized sewers save ratepayers in upfront installation costs and long-term life-cycle costs. We no longer have a failure of data; we have a failure to communicate — but the fiduciary duty to clients has not changed. 

Misinformation about STEP sewer O&M costs has been an impediment to wider adoption of affordable, environmentally sustainable effluent sewer systems, causing financial hardship to residents of small communities and consuming funding dollars at double the rate necessary. The data are in. It’s time to do a better job of communicating the message. 


Grant Denn is an engineered systems manager at Orenco (Sutherlin, Ore.).