If the site conditions are so poor that neither a conventional septic system or an Advanced Wasteater Treatment System (AWWTS) can be installed, then a holding tank is the only viable option. A holding tank is a steel, water tight, tank, without an outlet. The typical size, for a private residence, varies from 2000 to 4000 gallons. All of the wastewater generated from the home goes into this tank, and once it is full, it must be pumped out. The system is equipped with a high level alarm (located within the house) to notify the residents when the tank is full.
The cost of installing a holding tank varies depending on the site conditions, but one can expect to pay from $10,000 to $15,000, including engineering, permit fees, and installation costs. Although a holding tank is significantly less expensive to install than an AWWTS, it costs more to operate. Depending on water useage, the holding tank may have to be pumped several times per month.
Municipal design standards conservatively dictate that a septic system be sized to handle 150 gallons per day per bedroom (which equates to 2 people per bedroom using 75 gallons per day each). Through the use of flow restrictors on the sinks/showers, low volume flush toilets, and the optimum utilization of the dishwater and washing machine (washing only full loads), the flow rates can be significantly reduced, perhaps as low as 35 gallons per day per person. Since approximately 40% of the wastewater generated on a daily basis comes from flushing toilets, anything which can be done to reduce toilet flushing volumes will have a significant impact. The installation of composting toilets, or urinals would be an extreme example of the steps one could take to reduce wastewater volume.
Because the homeowner must pay for the removal and disposal of every gallon of wastewater, it is imperative that water conservation be practiced. The cost to pump a tank will vary depending upon the size of the tank, the location (accessiblity), and the season of the year. For a 4 person family, served by a 4000 gallon holding tank, assuming 40 gallons per person per day, the tank will have to be pumped every 25 days. This corresponds to 15 pumpings per year (assume $250/each) for a total annual cost of $3,750.
Something to Consider
Although the cost of operating a holding tank may be more than a conventional septic system or an AWWTS, it is not necessarily more expensive to own when one considers the capital and operating costs combined. In short, the owners of such systems will have periodic repairs (new pumps, air compressors, electrical components, etc.) as well as the possibility of having to periodically upgrade portions of the system, such as the drainfield.
Holding tanks, unlike septic systems, have no components to fail, other then the alarm, and their useful life expectancy is about 20 years (the tank eventually rusts out and starts to leak). The only costs associated with owning a holding tank is the original capital cost, and the monthly pumping costs. When these costs are weighed against some of the higher priced AWWTS systems, it becomes clear that holding tanks are perhaps unjustly stigmatized. Another redeeming quality regarding holding tanks is that they, unlike septic systems, are 100% reliable.
If you find yourself in the position of marketing or buying a home with a holding tank, perhaps the most important thing you can do is educate yourself regarding the economics of operating a holding tank and weigh those costs against the capital costs associated with installing a conventional septic systems or an AWWTS system. In some cases, you will find that a holding tank is the most economical solution.