February 2009, Vol. 21, No.2

Small Communities

Benefits of STEP Systems

Michael Hines

 

Septic tank effluent pump (STEP) systems have revolutionized cluster system development. Prior to the 1980s, sewer systems in rural or suburban locations were restricted by technology to gravity collection systems designed using conventional municipal sewer system design criteria. This usually resulted in small developments being served by sewers 200 mm (8 in.) and larger, laid with sufficient slope to transport solids, and equipped with numerous manholes, cleanouts, and lift stations. Invariably, these systems leaked and usually hydraulically overloaded the treatment units at the end of the line — generally packaged, extended aeration, activated sludge systems — during and after wet weather events.

Subsequently, the pump industry developed the high-head turbine pump rated for use in the corrosive sewer environment. This pump is installed in the pump vault of a watertight tank and protected by a screen to prevent solids larger than 3 mm (0.125 in.) in diameter from entering the vault. Prior to that, the STEP industry relied on sump pumps, which were less susceptible to influent solids but were unable to develop the higher heads of high-head turbine pumps. This development gave STEP designers more flexibility when using smaller-diameter pipes laid at a uniform depth, thereby eliminating expensive excavation and installation requirements and expensive manholes and cleanouts of conventional sewers that are still the major sources of infiltration.

Design Considerations
A STEP system consists of two major component groups — the STEP unit at a building and the collection system receiving flow from all attached STEP units. To serve the intended purpose of providing a watertight pumping system, the STEP system consists of a watertight, structurally sound concrete or fiberglass tank, a high-head turbine pump, a screened pump vault, and the control panel to operate the pump.

STEP pumps are selected to produce sufficient flow at the system head to transfer all of the received wastewater flow at a rate that will neither allow the tank to surcharge nor cause sufficient turbulence in the tank to resuspend settled solids. Typically, a high-quality 0.37-kW (0.5-hp), 115-V, STEP pump will be able to discharge approximately 23 L/min (6 gal/min) at a total dynamic head (TDH) of 55 to 73 m (180 to 240 ft), depending on the manufacturer.

The pump may be set inside of a screened pump vault or in a vault that is preceded by a protective screen that retains all solids larger than a nominal size in the tank. Some pumps are equipped with flow restrictors to avoid excess tank turbulence when pumping at low TDH.

The single most important criterion related to STEP system success is installing and maintaining a watertight system from the house to the end of the system. The most common sources of conventional septic tank and drain field hydraulic overloads are leaking building sewers and tanks. Watertight STEP tanks usually are fabricated of either concrete or fiberglass and equipped with integral rubber boots for entry of the building sewer or inlet line into the tank. Watertight risers from the tank to the surface are sealed to riser rings that are an integral part of the tank, and a watertight, gasketed, and bolted lid is installed at the top of the riser. All pump and float cords exit the tank through watertight grommets in the riser wall.

STEP pumps discharge to small-diameter collection systems that also must be installed and maintained as watertight systems. Successful STEP collection systems have used thermally fused high-density polyethylene pipe and polyvinyl chloride pipe connected with elastomeric slip joints. These collection systems must be as free as possible from entry of groundwater or surface water. Hence, they do not contain manholes and ideally should contain no cleanouts.

Septic tank discharge collection systems, or effluent sewers, consist of some combination of STEP units and septic tank effluent gravity (STEG) units. STEG units also consist of a watertight, structurally sound concrete or fiberglass tank with an effluent filter to retain solids within the tank. Collection systems can be pure STEG, STEP, or a mixture. Gravity tanks can discharge to the collection system at any place where the elevation of the tank discharge is above the elevation of the collection system hydraulic grade line.

STEP collection lines frequently are installed at a constant depth below whatever is the local frost depth. The shallow depths and sizes of pipes favor trenching the lines. Wide, deep ditches are not necessary. Tracer wire along every section of pipe including service connections will allow for future line repair or for line markings prior to other digging.

Combination air–vacuum relief (CAV) valves may be necessary at the top of high points in the lines. Although STEP pumps usually can overcome the extra head represented by the gas bubble if the pipe run is short, it is good practice to minimize these spikes in headloss through the use of these CAVs and minimization of high points. CAV valves are most necessary on systems in which TDH comprises a significant portion of the available pump operating head. When the collection system includes both STEP and STEG tanks, siting of CAV valves becomes more critical. In residential locations, these valves can be a source of odor complaints if they are constructed without odor-control appliances, such as soil filters.

Operations and Maintenance
STEP systems are generally low-maintenance systems. I currently manage decentralized utility operations with approximately 1000 STEP and STEG customers. In our earlier systems, the electrical splice boxes and telemetry control panels required the greatest maintenance. We eliminated splice boxes and ran each float and pump cord through individual conduits to the control panel. Control panels are simple, two-float “dumb” panels with elapsed-time meters. While maintenance calls have been limited, most have involved line blockages between the house and the tank or high-water alarms resulting from someone turning breakers off or shutting the service connection ball valve at the street.

Collection system maintenance is almost universally related to digging in the vicinity of our line absent the mandatory notice of pending excavation. Invariably, this results in broken lines, requiring emergency repairs.

Potential Impact on Sewer System Choices
Most municipal and other governmental entities have not yet embraced the STEP−STEG concept. Generally, they are unwilling to have “septic tanks” in their system. Some have indicated that STEP tanks will require too much maintenance. This claim is belied by the fact that most utilities require municipal lift stations to be inspected at high frequencies — sometimes daily — and STEP tanks would not have to be inspected more than once annually.

Many municipalities have service areas, portions of which cannot be served cost-effectively by the conventional sewers because of steep terrain or the extreme cost of installing such sewers in rocky ground. As STEP collection systems do not rely on gravity and can go in at a much shallower depth and a much smaller trench, these systems could serve such areas. Some progressive municipalities are beginning to consider STEP systems as possible additions to their distributed wastewater management arsenal.

 

Michael Hines is founding principal of Southeast Environmental Engineering LLC (Concord, Tenn.) and a member of the Water Environment Federation (Alexandria, Va.) Small Communities Committee.