An ever-rising number of communities and new developments are choosing wastewater management solutions that use alternative wastewater collection system technologies, such as the septic tank effluent pump (STEP). STEP systems collect wastewater from each property in an onsite 3800- to 5700-L (1000- to 1500-gal) interceptor tank and then pump screened primary effluent to a treatment facility or neighboring sewer. Most organic solids are retained in the tank to degrade anaerobically while the screened effluent is conveyed offsite through small-diameter, shallowly buried pressure mains.
Applied properly, STEP wastewater collection systems can offer several benefits for existing communities and new developments, including reduced up-front capital costs, lower life-cycle costs, and lower treatment costs than conventional gravity sewer collection systems.
Pinpointing O&M Costs
STEP systems often are selected because the construction costs for a gravity sewer collection system are prohibitive or because existing site conditions make installing a gravity sewer system logistically impractical. In most instances, STEP systems are selected with anticipation of operations and maintenance (O&M) costs being higher than those of a comparable gravity sewer system.
STEP system manufacturers often will state that “properly constructed and maintained” STEP systems can provide comparable or possibly lower long-term O&M costs than a gravity sewer system. Conversely, engineers and utility operators will point to an existing STEP wastewater collection system as a testament to higher O&M costs. When we consider these two opposing perspectives, the obvious question is, What is a properly constructed and maintained STEP system, and how can we identify one?
First, we must consider the quality of the STEP system, as well as the system’s major O&M protocols. The quality of the system components must provide the best value, rather than the lowest initial capital cost. Protocols must be designed to ensure that the system provides optimal service and is the most cost-effective option for the community. If a STEP system does not satisfy these objectives, it is not properly constructed and maintained.
The O&M costs for a STEP system can be broken into two major components. The first is the onsite system, which includes the tank, effluent filter, pumps and controls, building sewer, and service lateral. The second is the conveyance component, which includes the collection mains, air-release valves, odor-control filters, and shutoff valves.
In the context of the overall wastewater collection system cost, the O&M of STEP collection mains is typically insignificant. Such an O&M program generally includes only valve maintenance in accordance with the manufacturer’s recommendations and replacement of media in any existing odor-control filters. Although most STEP systems are constructed with cleanouts that facilitate pigging or flushing the mains, this is rarely, if ever, necessary. Breaks in or leaks from collection mains are rare and inexpensive to repair.
O&M costs for STEP systems are almost entirely attributable to the onsite components. The largest proportion of these costs is attributable to tank pump-outs, filter cleaning, and pump replacement. O&M costs for major components are directly influenced by component quality, installation quality, maintenance protocols, and ease of access to the site for maintenance. Accordingly, the decisions managers make about a limited number of onsite O&M factors can have a profound impact on the overall cost of the STEP system.
O&M responsibilities for conventional gravity sewer systems generally are limited to infrastructure located within the public right-of-way. If this O&M policy is applied to a STEP system, it would be necessary to transfer O&M responsibility for onsite components to the property owner, or, alternatively, the onsite infrastructure would have to be located within the public right-of-way. Successful STEP systems generally provide centralized O&M for onsite infrastructure that is located on private property. The costs for O&M are included in the fees charged to property owners.
O&M typically is facilitated by a combination of easements, service tariffs, and service agreements that are intended to grant O&M personnel access while preserving the rights of both the management entity and the customer. In practice, homeowners are almost always willing to grant access to a STEP system on their property for O&M, rather than maintain wastewater infrastructure themselves. Functionally, STEP infrastructure located on private property has provided an accessible, safe, and secure location for the components and O&M without exposing the system owner to increased liability.
Some STEP systems have been in service more than 30 years. In its simplest form, the STEP system can be achieved by using a standard septic tank and placing a pump on a concrete block. Conversely, a system can be a pre-engineered package that integrates the tank, pump, filter, and controls into a single package. In reality, many of the older systems fall somewhere in between. As the technology has evolved, several manufacturers have developed and marketed system components or, in some cases, complete STEP packages. All components or packages should be evaluated carefully to establish the best value for the utility.
Each material should be evaluated relative to longevity, cost, and reliability. For example, STEP pumps can range from a $150 cast-iron effluent pump that will provide 5 years of service to a $500 multistage, high-head effluent pump that should last more than 20 years. Effluent filters can range from less than 0.05 m2 (0.5 ft2) of flow-through area of 6.4-mm (0.25-in.) mesh up to filters with more than 1.9 m2 (20 ft2) of flow-through area of 3.18-mm (0.125-in.) mesh. Smaller flow-through area can necessitate more frequent site visits for cleaning clogged filters. Tanks can range from structurally unsound tanks that crack or collapse during installation and leak immediately to watertight tanks with watertight connections that can provide up to 50 years of anticipated life with proper installation. When substandard materials are used, either in new construction of all components (including building sewers) or for replacement parts, an increase in O&M costs is a certainty.
Reactive and Preventive Maintenance
O&M of onsite STEP systems typically is divided into two activities: reactive maintenance (RM) and preventive maintenance (PM). The protocol for onsite system maintenance varies greatly, even when comparing similar systems.
Most systems have been operated with varying degrees of emphasis placed on PM activities, the frequency of which is dictated by the most sensitive components. Extremely aggressive PM programs have been able to reduce RM service calls to a point that they become relatively insignificant in the overall O&M costs. Unfortunately, an overly aggressive PM program also can result in higher overall O&M costs when PM activities unnecessarily target components that have a significant level of reliability with less PM. A proper balance is the key to an optimal maintenance program.
A less well-known RM protocol is a run-to-fail approach, whereby maintenance is only performed when equipment fails. Essentially, the operator waits for an alarm. While this approach is fairly uncommon, it has some important implications. While daily O&M costs (at least in the early years) may be extremely low, increasing frequency of major repairs and replacement activities will escalate as the system suffers from neglect. Additionally, public perception of a run-to-fail approach is generally poor, because this approach may place the customer in a more responsible position of identifying system alarms and reporting them to the management entity. Most studies reveal that customers want as little responsibility as possible related to their systems.
In practicality, the most cost-efficient STEP management approaches balance PM and RM to achieve the lowest overall cost for O&M. The STEP systems that typically achieve the best overall O&M cost tend to extend PM cycles to the maximum time increment possible until RM maintenance starts to become cost-prohibitive. PM intervals can be affected significantly by material quality, filter flow-through area, and tank quality and sizing. Selecting a PM protocol and cycle time should be derived based on historical information garnered from established systems with similar characteristics. In the absence of historical data, a PM program initially should be executed annually, with a gradual increase in the time between PM intervals once the integrity of the system is established.
One of most important components of STEP O&M cost is tank pump-out intervals. Extended tank pump intervals can be achieved through the use of larger tanks and through the inspection and measurement of solids and scum accumulation. While lower capital costs can be achieved by using smaller tanks, the cost of larger tanks easily can be paid back through extended pump-out intervals. Some studies have found that managed STEP tank pump-out programs can achieve pump-out intervals up to and sometimes in excess of 10 years.
Many STEP systems have excessive O&M costs that are attributable to excessive investments in service equipment or labor costs. While STEP systems can be maintained with basic tools, a small pickup truck, and sufficient replacement-part inventory, it is common to see heavy-duty utility vehicles in use. Additionally, while most tasks can be performed by a single service technician during normal business hours, many high-cost system management entities employ two or more service personnel, often working during overtime hours. It is more typical for a single technician with a pickup truck to maintain about 2000 STEP connections with high-quality STEP systems.
Public education is important. New customers should be given guidance that explains their wastewater collection system, along with the dos and don’ts that they must adhere to when using their wastewater facilities. Annual mailers to customers should be used to provide ongoing reminders of the initial guidance. When system alarms are attributable to misuse or a lack of knowledge regarding the system, service employees should be prepared to provide additional training to customers through direct contact, door hangers, or follow-up calls.
In time, all components of a wastewater collection system will require repair and replacement (R&R). R&R is an important element to consider when determining the life-cycle cost or the best value of a wastewater collection system. The largest R&R cost for wastewater collection systems that use onsite pumps typically is related to the pump and controls. A high-quality STEP multistage effluent pump will include warranties of up to 5 years and can provide up to 20 years of service. These pumps typically feature a run-dry capability, will be UL-listed, will be rated for continuous operation, and will be manufactured from corrosion-resistant materials. Additionally, pumps should be rebuildable, either by replacement of individual components or replacement of either the liquid end or the motor end. By comparison, a low-quality pump may require complete replacement at average intervals of less than 10 years.
A “properly constructed and maintained” STEP system can provide a sustainable and cost-effective solution for wastewater collection. If you are tasked with a comparative analysis between STEP and other wastewater collection alternatives, data from existing STEP systems should be scrutinized and validated carefully prior to use. There are STEP systems throughout the United States that vary in age, quality, and O&M protocols. Calling STEP system operators and asking for their annual costs rarely will yield a reliable number consistent with the technology currently available.
If you’re an existing STEP user or are tasked with improving an existing system, significant O&M cost savings can be achieved by focusing on a few major cost elements within your program.
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