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ExtremeSTP Cold Climate Sewage Treatment Plants

Selecting the Proper Model

Treatment Process Description

ExtremeSTPs utilize aerobic treatment inserts manufactured by Bio-Microbics®, Inc. The FAST® process begins with pretreatment to remove floating and settling solids by trapping them in an anaerobic settling zone. This step is followed by aerobic biological fixed activated sludge (FAST®) treatment in a compartment with a submerged growth media that is colonized by the microbes that are naturally present in sewage.

A blower forces air into an airlift pump inside the FAST® unit. The airlift pump vigorously mixes the air with the sewage and circulates the aerated sewage through channels in the growth media. This brings oxygen and nutrients into contact with the microbes that are attached to the growth media so that they can decompose the impurities. Nitrification occurs in the aerobic zone, and as the airlift pump circulates the water, a small percentage of the water continuously returns to the anaerobic zone where denitrification occurs.

FAST® units can be purchased as modules that can be fitted into fiberglass, steel, or concrete tanks to meet a wide variety of treatment applications. Individual FAST® units are available in sizes ranging from 150 gallons per day (gpd) to 9,000 gpd. Several units can be manifolded together in series or parallel configurations to treat greater flows. The Bio-Microbics®, Inc. link on this website has further information about FAST® systems.

Lifewater Engineering Company's ExtremeSTPs begin with the two steps described above, namely pretreatment and aerobic FAST® treatment. However, in order for the systems to work in cold climates two additional steps must be taken.

Before treated effluent is discharged onto the ground surface or into a water body, it must be disinfected. In ExtremeSTPs this is generally done using either UV or chlorination. Most of the smaller system use UV and the larger systems use chlorination. A dechlorination step can be added if needed.

Effluent flow control prevents effluent from trickling out the effluent line and freezing in the line itself or on the ground close to the point of discharge. Whenever site conditions will allow, flow control is done using an automatic dosing siphon. This device has no moving parts except water and air, yet it retains treated effluent in the ExtremeSTP until a preset amount is accumulated and the siphon is triggered. Then effluent is then discharged rapidly (30 gpm on the smaller systems) until the liquid in the effluent compartment drops to the level where siphoning stops. In this way, relatively large doses of warm effluent are discharged onto a splash plate (to prevent erosion) and the effluent soaks into the natural vegetative mat and flows away from the treatment plant before it freezes.

In cases where there is insufficient head to allow use of a dosing siphon, an effluent pump is used to achieve about the same dosing rate and volume while lifting the effluent to a suitable discharge location. Discharge can also go to a leach field either with or without a dosing siphon or effluent pump. Systems can be customized to suit particular site conditions.

Hydraulic Loading

All wastewater treatment systems must be sized to handle both the hydraulic and organic loadings to which they will be subjected. Since ExtremeSTPs use aerobic treatment inserts manufactured by Bio-Microbics®, Inc., the hydraulic capacities of ExtremeSTPs are determined by the hydraulic capacities of the Bio-Microbics treatment inserts.

Bio-Microbics' model numbers designate hydraulic capacities in thousands of gallons per day. For example their Model MCF0.5 treatment insert is rated at 0.5 thousand gallons (or 500 gallons) per day and their Model MCF9.0 is rated at 9,000 gallons per day. ExtremeSTP model numbers designate hydraulic capacity in gallons per day. For example, a Model XSTP500 system is rated at 500 gallons per day.

Fixed activated sludge treatment (FAST®) systems can process surge flows very well. In FAST® systems, the microbes that decompose the organic matter are fixed on a growth medium rather than suspended in the liquid. Sludge cannot wash out of the clarifier because there is no clarifier. There is a large reserve of treated wastewater inside the FAST® insert and when more sewage enters the treatment plant, an equal amount of treated wastewater is displaced out.

If actual flows are not available, conventional engineering techniques should be used to estimate or predict flows as closely as possible. However, excessive efforts need not be made to refine a flow estimate because in most cases it is not hydraulic capacity that determines which system should be used.

Organic Loading

The organic capacities of ExtremeSTPs are determined by the organic capacities of the Bio Microbics treatment inserts that are used, and in most cases, especially with modern water-saving fixtures, it is organic loading that determines which size system must be used. Bio-Microbics® rates their residential systems in terms of a range of people they will handle. However, when applying their numbers you must keep in mind that the high end of the range represents a short-term peak rather than a long-term average number of people. If necessary, these numbers can be approximately converted to lbs of BOD5 per day by multiplying the average of the range of persons per module times the typical factor of 0.2 lbs BOD5 per person per day.

Discharge

If possible, the first choice for discharge of treated effluent from any on-site sewage treatment system is a subsurface soil absorption system. Soil absorption systems should be designed in accordance with generally accepted practices for the local area in which the treatment system will be located. However, since ExtremeSTPs produce an oxygen-rich effluent with low suspended solids and low BOD5, design of the absorption system can be based solely on the hydraulic capacity of the soil. This means that reductions in absorption system size from what is typically required for a septic system (perhaps up to 50 percent) can sometimes be achieved.

However, since ExtremeSTPs were originally designed for permafrost areas where discharge into the ground is not practical, surface discharge is commonly done from ExtremeSTPs. In Alaska, each discharge location must be approved by the regulatory body (typically the Alaska Department of Environmental Conservation) on a case-by-case basis as part of the plan approval process.

We have found that surface discharge works best where there is a thick, natural, well-drained vegetative mat. In the Interior of Alaska, black spruce forests provide an ideal environment. If permafrost is an issue, the discharge location should not be close to a foundation or other structure that could be damaged by thermokarsting (ground subsidence due to thawing of unstable permafrost). If thermokarsting is a concern, we recommend use of a thermo siphon under the ExtremeSTP and possibly underneath the point of surface discharge.

An automatic dosing siphon is used to prevent effluent from freezing in the discharge pipe or at the point of discharge on top of the ground. The dosing siphon has no moving parts other than the air and water within it, yet it retains effluent in the effluent compartment until it rises to a predetermined level. At that point, a rapid discharge begins and continues until the level drops to the point where the siphoning action is broken and the discharge from the effluent compartment abruptly stops. After the water in the effluent pipe drains, essentially no more is discharged until the next dose occurs. On sloping ground where permafrost is not a problem but soil conditions are poor and a soil absorption system will not work, consider using a below ground system with an above ground discharge.

Elevation Requirements

USTP500, USTP750, and USTP900 models without an effluent dosing siphon have a 3 to 5-inch drop from the 4-inch inlet pipe to the 4-inch outlet pipe.

USTP500, USTP750, and USTP900 models with an effluent dosing siphon have an additional 16-inch drop, for a total drop of 20 to 22 inches from the center of the 4-inch inlet pipe to the center of the 2-inch outlet pipe.

XSTP150 and XSTP500 models come standard with an effluent dosing siphon. These models have a 20 to 22-inch drop from the center of the 4-inch inlet pipe to the center of the 2-inch outlet pipe.

If an effluent pump option is chosen, elevation is typically of minimal concern.

A drawing and table showing tank dimensions and inlet and outlet elevations appear at the end of this page.

Inlet & Outlet Piping

Lifewater Engineering Company recommends that the inlet and outlet pipes be insulated with 2 to 3 inches of polyurethane foam with a k-factor of about 0.17 BTU/hr/ft2/ºF. The foam should be protected from damage by UV rays, animals, etc., especially where it is exposed above ground. We also recommend that the effluent line be heat traced.

Movement of buildings, pipe supports, and the sewage treatment system can be expected, especially in above ground installations on permafrost. Projected movement should be taken into consideration when designing pipe slopes. For example, when an above ground system is set on top of a thawed active layer, one would expect the treatment system to rise somewhat when the ground freezes, especially if a thermosyphon is used to enhance freezing. In this case, the slope of the sewer line should be set slightly greater than ¼-inch per foot to accommodate the expected movement.

Inlet and outlet piping connections to the ExtremeSTP should be made with rubber couplers with stainless steel bands. These will accommodate some movement between the treatment system and the pipes and will provide "weak points" at the inlet and outlet so that neither the pipes nor the treatment plant will be damaged if movement is severe.

Venting Requirements

Because air is pumped into the ExtremeSTP air must be vented out. ExtremeSTPs are constructed so that there is a common air space above all of the internal compartments. For residential and small commercial systems, this air space is commonly vented through the sewer line and out the main stack vent on the roof of the house or building being served. We have found that homeowners like this venting system because it helps keep the roof vent from freezing shut during the winter. Larger systems, whether residential or commercial will require separate venting to the atmosphere. Vent diameters must be in accordance with Bio-Microbics'® specifications for each particular model (see links page).

Electrical Requirements

ExtremeSTPs are typically constructed so that there is one point of power connection for the whole unit. For the XSTP150, XSTP500, XSTP750, and XSTP900 gpd models, one 20 Amp, ground-fault protected circuit that is dedicated to the ExtremeSTP is normally all that is required, unless the owner wants a separate ground-fault protected circuit for alarm purposes, such as for a high water alarm.

Permafrost Considerations

For sites with permafrost, the engineer should consider use of a thermosyphon to enhance freezing in the ground. In every case where a thermosyphon has been used, the treatment system has stayed in place and level. In one case, the yard underwent extensive thermokarsting and the house needed frequent leveling, yet the treatment plant remained in position because a thermosyphon kept the ground frozen.

Typically, above ground systems can be installed at any time of year because minimal excavation is required. When installing on permafrost, the best time of year may be in the fall or early winter. At this time of year, only the surface of the active layer has refrozen, so the ground is easy to excavate to prepare a pad for the system, yet it freezes at night, thereby facilitating equipment operation.

Foundation thoughts for permafrost areas:

The best rule for permafrost areas is this: Do not rely only on these general thoughts to design your tank foundation, rather consult your "local" foundation expert even if that person is 1000 miles away!

o Place on mineral soil - ExtremeSTPs should be placed either on an insulated gravel pad (such as is often constructed for a building on permafrost) or on mineral soil. In permafrost areas, it is best to place the unit in a very small clearing shaded by trees (such as black spruce). The natural vegetative mat under the tank foundation should be removed so that it does not compress later causing the tank to shift. If the vegetative mat is removed, it should only be removed under the tank foundation and should be left in place undisturbed everywhere else to preserve ground insulation and minimize permafrost thaw.
o Leave an air space - When installing an XSTP on permafrost consider leaving an air space underneath the tank. This will allow cold air to flow under the tank in winter and reduce ground temperatures so that it is less likely to thaw during the warmer months. One way to do this is with a pressure treated timber foundation.
o Pressure-treated timber foundation - This type of foundation eliminates direct contact between the tank and the permafrost soil, thus reducing heat flow into the ground. We suggest using 3"x12" or 4"x12" timbers that are specially treated and rated for ground contact. Since soil moves seasonally, especially in permafrost areas, consider a timber arrangement similar to that shown below that will allow shims to be paced between the timbers as needed to keep the tank level. See conceptual sketch below.
o Extra 2 inches of rigid foam - When the tank must be in direct contact with the ground, consider placing 2 inches of rigid foam between the tank and the ground to slow heat transfer into the ground.

(Note: the above sketch is intended only to convey the concept of a timber foundation. It is not a design drawing!)

Thermosyphon - If your property is already experiencing thermokarsting due to permafrost thaw, or if you prefer to take the conservative approach (which we recommend) you should install a thermosyphon underneath the tank to help refreeze the ground and/or keep it frozen. Consult a local foundation engineer or Arctic Foundations in Anchorage, Alaska (1-907-562-2741, www.arcticfoundations.com) for further information and advice.

Flat loop evaporator for thermosyphon - When you are uncertain of the need for a thermosyphon but want to maintain the option of having one if necessary, we recommend you install a flat loop evaporator beneath the tank or timber foundation at the time of installation. This can be done at a small extra cost and will allow a condenser and gas/liquid charge (the expensive parts of a thermosyphon) to be added later if necessary. Keep in mind that if you know you'll need a thermosyphon, it is generally less expensive to install a standard thermosyphon initially than to install just the flat loop evaporator first and the condenser and gas/liquid charge later. Some suggested generic installation instructions appear below, but we recommend that you contact a local foundation engineer or Arctic Foundations in Anchorage, Alaska (1-907-562-2741, www.arcticfoundations.com) for further information and site specific recommendations.

Flat loop evaporator installation instructions:

For model XSTP150, XSTP500, XSTP750, and XSTP900 systems, use 60 feet of ½-inch diameter Type K copper tubing for the evaporator. Solder a cap onto each end of the tubing before taking it out of a clean environment. It is important that the inside of the tubing remain completely clean and dry.

Before setting the tank or its foundation, bed the tubing in 6 inches of sand, with 3 inches of sand below and 3 inches above the tubing. Set the tubing so that both ends are together and extend up to near the top edge of the tank. The rest of the tubing should loop back and forth under the tank so as to get the best coverage (see sketch below). Compact the sand carefully so as not to damage the tubing.

Set the tank or its foundation on top of the compacted sand and secure the ends of the tubing near the top edge of the tank so that a condenser can later be connected to it. Leave the soldered caps in place on the ends of the tubing until they are removed by the technician who installs the condenser.




Installation & Field Assembly

Installation is normally done by a certified on-site sewage system installer and is not included in Lifewater's pricing. Installation includes setting the unit in place, running the sewer pipe from the house to the unit, running the outlet pipe from the unit to the point of discharge, and providing electrical power to the unit. A homeowner who is familiar with applicable building codes and normally does his/her own wiring and plumbing can in most cases install a system (if this is allowed in your jurisdiction).

ExtremeSTPs are shop assembled as much as possible, but depending on how and where they will be shipped some field assembly may be required. For below ground units, some field assembly is always required. When the units are being installed at a location close to a distributor, such as near Fairbanks, Anchorage, or Kenai, Alaska, field assembly of ExtremeSTP components is typically included in the sales price. Please ask about this when ordering a system.

Handling ExtremeSTPs

XSTP models (above ground fiberglass tanks) should only be picked up using either a forklift with extended forks, a boom and two or more fabric straps, or a fabric sling designed for the purpose. Cables, chains, or other methods that could damage the fiberglass should never be used. Steel USTP models (underground steel tanks) typically have a lifting ring or rings welded onto the top. These steel tanks should be picked from above by means of the lifting rings.

Notes

• All inlet diameters (except larger models) are 4 inches
• D means automatic dosing siphon
• P means effluent pump
• … means with or without disinfection
• No suffix means no disinfection, no dosing siphon, no pump
• * means dimension can be specified at time of ordering
• XSTP (above ground) models come standard with an automatic dosing siphon but can be ordered with an effluent pump
• All dimensions subject to change without notice