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Low-Pressure Dosing Systems

The Low Pressure Dosing System (LPDS) originated in North Carolina and Wisconsin. They were developed as an alternative to conventional soil absorption systems to eliminate problems such as clogging, mechanical sealing of the soil trench during construction, anaerobic conditions due to continuous saturation and high weather table. The LPDS is a shallow pressure-dosed soil absorption system with a network of small diameter perforated pipes placed 10 to 18 inches deep in narrow trenches 12 inches wide

Why Use a Low Pressure Dosing System

Process Description

Design Criteria

Operation & Maintenance

Advantages & Disadvantages

        Why Use a Low Pressure Dosing System

In a conventional system effluent discharges from the septic tank in direct response to wastewater entering the tank and flows by gravity through the perforated pipes with in the leach field. Effluent is unevenly distributed within the leach field. It trickles out of the holes nearest the leach field inlet or at points of lowest elevation. Eventually a biological mat forms beneath these areas, and the reduced infiltrative capacity of the biomat forces incoming effluent to seep further down through the trench gravel to an area of fresh soil. Over time, the biomat progresses until the entire leach field is covered and a stabilized slow infiltration rate is attained. Even as this mature state little or no effluent reaches the far end of the perforated pipe. It has been distributed down the trench through the trench's rock or gravel.

Uneven distribution of effluent with the leach field may result in localized overloading of the soil. This may cause insufficient treatment as effluent infiltrates rapidly into soil around the localized distribution points, or an accelerated biomat may cause clogging of the leach field pipes.

Low Pressure Dosing Systems were developed as an alternative to conventional soil absorption systems to eliminate problems mentioned above. The LPDS has the following features that overcome these problems: (1) shallow placement, (2) narrow trenches, (3) continuous trenching, (4) uniform distribution, (5) resting and re-aeration between doses.

        Process Description

There are three main components to the LPDS system: (1) a treatment unit such as a septic tank or secondary treatment device, (2) a pump chamber or dosing system, and (3) disposal system consisting of small diameter distribution laterals with small perforations.

         The Septic Tank

As mentioned in the conventional systems section the septic tank is a large, buried container, typically constructed of concrete, fiberglass or polyethylene. Wastewater from the building being served flows into the tank. Heavy solids, or sludge, settle to the bottom where anaerobic bacterial partially decomposes them. Lighter solids, scum, such as fats, oils and greases rise to the top. The septic tank is design in the same manner as described in previous sections. The clarified effluent from the septic tank flows, by gravity, through an effluent filter and into the pump chamber.

         The Pump Chamber

The pump chamber is typically a modified septic tank with a pump, pump controls and alarm system. The pump is submerged in the effluent in the pump chamber. When enough water is present to dose the disposal system, a float turns on a programmable timer that turns the pump on until the initial timed dose is complete. The timer not only provides a specific design dose to the distribution laterals it also provides a specific design “resting” period for the disposal field. For example the timer may allow the pump to operate for ten minutes in every four hours, provided there is sufficient effluent in the pump chamber. The “resting” period assures the disposal field has the proper amount of time to allow the previous dose of water to work its way through the subsoil before another dose of water is applied. The pump provides adequate pressure to pass the water through the supply line and to the distribution laterals and orifices in the disposal field.

As mentioned above the pump chamber is also equipped with an alarm. The alarm is activated by a float to warn of pump or system problems. The alarm float is set so that an alarm goes off if the effluent rises above a predetermined level.

In the event of extreme water usage, a timer override float is installed in the tank to force the pump to dose the mound in the event the pump tank is about to overflow. This should happen very rarely, and in the event this occurs the homeowner should immediately curtail water usage.

Pump Chamber

         Disposal System

The pump moves the effluent through the supply line and manifold to the distribution laterals in the trenches under a low pressure (typically five feet of head). These laterals are a network of PVC pipes that have small, drilled perforated holes usually from 1/8 to 1/4 inches in diameter and spaced at three to five feet intervals (exact dimensions are determined for each system).

The laterals are placed in narrow gravel-filled trenches ten to 18 inches deep and spaced five or more feet apart. The narrow (12 inch) trenches allow enough storage volume so that the depth of the effluent does not exceed two or three inches of the total trench depth during each dosing cycle.

        Design Criteria

Two critical factors that affect the performance of an LPDS are dosing and distribution of the effluent. The dosing and resting periods help maintain aerobic conditions in the soil and around the distribution trench. Uniform distribution cannot be overemphasized in the performance of the system. The effluent must be distributed evenly over the soil absorption field without hydraulically overloading it.

         Soil Requirements

A LPDS should be located in soils that have suitable or provisionally suitable texture, structure, consistence, depth and permeability. A minimum of 48 inches of usable soil is required between the bottom of the absorption trenches and any underlying restrictive horizons such as bedrock and a minimum of 60 inches is required between the bottom of the absorption trench and the seasonal water table.

         Space Requirements

The size of the distribution network will depend on the size of the residence it serves and the soil conditions. An area of equal size must also be available for possible future replacement of the system.

         Topography

There are special design considerations for LPDS located on slopes. If the pumping system is lower than the distribution field the dose must exceed the pipe volume. If the field is lower than the pumping system, the system must be designed to ensure that effluent will not leave the pump chamber with the pump turned off. Care must be taken to assure there is no more than a ten percent difference in head between any of the lateral orifices. In colder climates, the system should be designed so effluent does not remain in the supply line.

        Operation & Maintenance

With a few simple maintenance procedures, a properly designed and installed LPDS will last for several years. This system does have some mechanical equipment that will need care and replacement.

The system is designed to provide treatment and disposal for normal domestic sewage. No non-biodegradable material should be introduced into the wastewater treatment and disposal system. Plastic and paper (except toilet paper) are examples of non-biodegradable materials that should not be placed down the drain. Normal amounts of dirt and small non-biodegradable debris (buttons, dental floss, etc.) from washing will inevitably get into the system. These solids will be retained in the septic tank until it is pumped during its normal maintenance. Oils and grease should not be placed down the drain in excess quantities. Normal washing of greasy dishes is not considered excessive. Routinely draining fat from a frying pan, deep fryer, or roasting pan down the drain would be considered excessive. A garbage disposal may be used on the system but its use should be restricted. A garbage disposal should not be used for the bulk disposal of food preparation waste.

The septic tank as described above retains heavy and light materials and provides anaerobic digestion. Eventually the materials retained in the septic tank will have to be removed. There is a filter on the tank outlet that may require cleaning more often. This is accomplished by simply removing the filter and hosing it off back into the septic tank and re-installing the filter.

The pump chamber should be checked for sludge and scum buildup and pumped as well as the septic tank. The pump and floats should be checked annually and replaced or repaired as necessary. All electrical parts and conduits should be checked for proper operation and corrosion. Follow all of the manufacturer’s operation and maintenance instructions. All equipment must be tested and calibrated according to the manufacturers specifications. It is critical that both the septic tank and pump chamber be watertight.

The maintenance required for the disposal field is minimal. To reduce the possibility of failure to the disposal field all surface water and roof drainage should be diverted around the area. As a minimum, the laterals should be flushed annually. Also, check the field for the following conditions:

        Spongy areas developing or ponding in the absorption area

        Clogging of the distribution system.

If the effluent level in the pump chamber reaches the alarm float, the alarm light and buzzer will activate. The following items should be checked if the alarm activates:

        Check to see if a circuit breaker has been tripped or a fuse is blown. The pump should have a separate circuit with its own breaker or fuse. If the pump is on a circuit with other equipment, that equipment can trip the breaker.

        If the electrical connections are the plug-in type check to see if a pump or float switch has come unplugged, make sure the switch, and pump plugs are making good contact in their outlets.

        The floats or other parts in the pump chamber such as the electrical power cord or lifting rope can become tangled. Make sure the floats can operate freely in the pump chamber.

        Be sure the floats or support cables are clean of debris that could limit the mobility of the float switches.

Make sure to turn off the power supply at the circuit breaker and unplug all power cords before handling the pump or floats. Do not enter the pump chamber, gases inside are poisonous and that coupled with a lack of oxygen, causes conditions that can be fatal. Most alarms are equipped with a silence button located on the control panel. By using water conservatively, the reserve storage in the pump chamber should allow enough time to correct the problem.

        Advantages & Disadvantages

Some Advantages and Disadvantages are listed below.

Advantages:

        Shallow placement of trenches in LPDS installations promotes evapotranspiration and enhances growth of aerobic bacteria.

        Absorption fields can be located on sloping ground or on uneven terrain.

        Improved distribution through pressurized laterals disperses the effluent uniformly throughout the entire leach field area.

        Periodic dosing and resting cycles enhance and encourage aerobic conditions in the soil.

        Shallow, narrow trenches reduce site disturbances and thereby minimize soil compaction and loss of permeability.

        LPDS systems allow placement of the leach field area upslope of the building being served.

        LPDS systems overcome the problem of peak flows associated with gravity-fed systems.

Disadvantages:

        Small orifices have an increased potential for clogging by solids.

        Need for electricity and mechanical equipment (in some circumstances siphons can be used in place of pumps).

        Increased monitoring and maintenance is required.

        Orifice sizing and spacing is critical especially in areas with steep topography.