What maintenance costs can be expected with each type of system?  The table below shows what the typical maintenance costs will be for an Aerobic System and a Low Pressure Dosing (LPD) system.

Assumptions:

• This cost comparison is over a 10 year period
• Average usage - 240-300 Gallons Per Day usage
• Reasonable care taken of system, to avoid damage.  Neglect will increase these costs

Aerobic Systems

Aerobic systems are often the primary option chosen for sites with unfavorable conditions, either because of shallow ground water, poor surface drainage, or poor soils, particularly clay soils.  The principle of aerobic systems is that it treats the wastewater sufficiently, so that it is allowed to be disposed of on the surface, most often via sprinklers.  This provides the guarantee that no mater the weather conditions, or how much rain there has been, the system will get rid of the wastewater. This is the primary advantage, but there are several major disadvantages to be aware of.  There are also a number of myths about aerobic systems.

Advantages

• Can be installed on sites that are not acceptable for conventional systems
• Typically lowest cost to install on sites with poor soils

Disadvantages

• Highest maintenance cost of any system
• Greater potential for disease transmission, particularly if the system is not operating properly
• Largest area required
• Most sensitive to usage

Maintenance Costs

In Texas, aerobic systems are required to have a maintenance contract, which typically runs between $175-225 per year, depending on location and the specifics of the system.  They also require disinfection, which can be as low as $25-40 a year for liquid chlorinators, up to $150-300 per year for tablets.  Then there is the eventual replacement of both motors, the Aerator and the Water Pump.  Depending on your maintenance provider each can cost between $350-800 to replace. (This is primarily driven by how much they mark up the parts).  Finally, there is the electrical usage, which starts with the aerator which uses 100watts or more and runs continuously, and the water pump which is 1800 watts, but typically only runs for 30-45 minutes a day.

Disease Transmission

Aerobic systems are designed to treat and remove 95-99% of contaminants in the effluent under optimal conditions.  Thus, even theoretically, the water is NOT SAFE TO DRINK.  It is also not allowed to be sprayed on food crops, including vegetable gardens, fruit orchards, and pecan trees.  In the real world, the quality of the effluent can vary much more.  The chlorine or other disinfection method is supposed to eradicate the remainder of the contaminants.  However, if the quality of the water decreases, so does the effectiveness of the disinfection.  Therefore, if the system is not treating properly, the risk for disease goes up tremendously, especially if the sprayed effluent comes in contact with other surface water, such as during heavy rains.

Area Required

This varies with the soil on the property and your location in the state, but generally an aerobic system with spray irrigation will require twice the area of a conventional system.  Further, Spray irrigation has larger setbacks than most other systems.  Consider a couple of examples:

The setback means that for a conventional system there is only a 5' strip along the property line that cannot be used for the drain fields, whereas for the aerobic there is a 20' strip of land around the edge of the property that cannot be used to achieve the required area.  The rules do allow for a reduction to 10' if the system is set to spray only at night, but this can add from $250 to nearly $1,000 to implement.

Both of these examples assume the worst possible soil for the conventional systems.  Better soil (More sand / Loam) will reduce the area required.  For the aerobic systems, the area is based on the general evaporation rate of the area, and thus will not vary with the soil.

Sensitive to Usage

Aerobic systems work best with the same usage day in and day out.  Large fluctuations in usage, or even short times of very high usage will cause the system to not properly treat the wastewater.  The most common cause is laundry, if one day a week is laundry day.  Additionally, antibiotic consumption or overuse of strong chemical cleaners will kill the bacteria in the system, leading to odor problems.

Myths

• Aerobic Systems never need to be pumped - All systems need to be pumped.
• The water is safe to drink - NEVER EVER EVER
• It is your only option - NEVER - there are always alternatives.  They may cost more, but there are always options.  If someone tells you otherwise, find another contractor to work with!
• The State/County REQUIRES an aerobic system.  State law does not allow this.  If you are told this, either someone is lying, or the government agency involved has exceeded their authority from the legislature.

Low Pressure Dosing (LPD) Systems:

Low Pressure Dosing systems are basically a modified conventional system. The treatment method and quality is the same, traditional septic tanks, which typically remove about 50% of the biological content of the effluent(wastewater) The remainder of the treatment takes place in the soil. The disposal portion of the system is what sets it apart from conventional systems. Instead of gravity flowing into the drain field, the effluent is pumped to the drain field, where each trench is separated from the others, and each has a valve to reduce the pressure of the water in that trench. Typically the pressure is reduced to 1 psi or less, the the Low Pressure. This change provides several significant benefits over conventional systems:

1. The drain field no longer needs to be located down grade from the tanks and house.
2. The effluent is applied in the top 18-24" of soil, where absorption, transpiration and evaporation are maximized.
3. The effluent is applied in periodic doses instead of trickling out into the field when the water is used.

The Dosing component of LPD's is very powerful. Traditionally, when water was used in the home, the water slowly trickled out into the drain field over the next 30-45 minutes, and because of this tended to migrate to the same spot in the drain field. This in turn lead to that spot staying saturated and growing a bacterial slime known as Bio-Mat. Over the course of years, this Bio-Mat slowly spread through the entire drain field and ultimately lead to the drain field failing. By dosing the effluent, the entire drain field gets an even dose, and is given several hours to absorb that dose, and dry some. This dosing cycle has been scientifically proven to tremendously improve the long term reliability of the drain field. The main concern to keep in mind is that the system still depends on the soil to absorb the effluent, so weather and site conditions still impact the long term viability of the system, as well as usage. Any system will fail if the designed usage is exceeded.

Pros & Cons

Pros:

• Low maintenance cost
• No surface application, thus no chance of coming in contact with the effluent
• Highest reliability of the subsurface systems

Cons:

• Higher Installation Cost
• No long term guarantee of functioning, if usage or rainfall exceed design parameters