Septic Systems - Sizing and Maintenance
This is our second article on septic systems. Our last article talked about Septic System Basics including the components of the system and how they work. In this article we discuss Sizing and Maintenance of domestic septic systems. Please work with your community public officials or with experts when designing and installing your septic system.
Note: Do not confuse a simple holding tank with a septic system. Holding tanks require weekly or monthly pumping out because they have no planned leakage path.
Sizing
Generally the old adage, 'the bigger, the better' applies to septic systems.
The Tank:
The tank should be large enough to retain the liquid layer for not less than 1 day, and preferably 2 to 3 days or more; but, it’s not that simple. As sludge and scum accumulate in the tank, the usable volume is reduced, resulting in less retention time for the next batch of sewage. A properly designed and used septic system should have the capacity to store solids for five years or more ... a typical household septic tank will have a capacity between 1000 and 3000 gallons.
The Absorption Bed:
Sizing an absorption bed is complex and the method varies. Basically, you have to determine the percolation (perc) rate or absorption rate of saturated soil where the bed will be located. It typically ends up somewhere between 10 and 60 minutes per inch (mpi) of water in a prepared test hole. Let’s say for this example the perc rate is 50mpi. You then look up the perc-rate in your local charts to find the square footage of absorption bed per bedroom - say 300 square feet. You then multiply the number of bedrooms by the square footage from the chart to arrive at the total square footage, say 900 square feet for a 3-bedroom home. In this example, if you are using 2 foot wide trenches for your absorption bed, you would need 450 lineal feet of trench. Assuming a trench cannot exceed 100 feet in length, for this example you will need five 90 foot trenches or some equivalent configuration.
Maintenance
When you first start using your new septic system, some experts recommend a starter culture be used to speed up the initial propagation of bacteria. This is unnecessary, but harmless. You should NEVER add chemicals to help the bacteria, even in a small septic system.
The general cleaning interval for a properly designed septic system is 4 to 5 years. I have heard of people cleaning them every 2 years but this is extreme. I have also heard of going 10 years between cleanings; but there is risk involved in waiting this long, since problems are not obvious until you open the cover. When you have your system cleaned, ask the cleaning contractor whether it 'needed' cleaning and adjust the interval accordingly. Also be sure that the contractor checks for damage and correct any problems promptly.
When your septic system is used properly it will require no day-to-day maintenance and will serve you and your family for many years; but always remember, the septic system is a giant bacteria culture and some care is required.
Only allow easily biodegradable substances to flow into your septic system. ABSOLUTELY NONE of the following should be put into your system:
Cigarette butts or cigar butts as these contain toxic nicotine, which can seriously impact the bacteria in your septic system.
Non-biodegradable materials such as condoms, or feminine hygiene applicators or other related products.
Plastic-lined disposable diapers.
Pills (return these to your pharmacy for disposal).
Paints, solvents of any sort, either latex or oil-based.
Insecticides or any strong chemicals.
The following are suggestions that will help your septic system do its job better:
Use non-perfumed, non-tinted toilet paper, as it is easiest to breakdown (and better for you too).
Avoid glass, ceramic, sand, porcelain, and metals going down the drain, as they may clog traps and displace the bacteria in your system.
Avoid over-use of detergents (i.e. only use what you need) especially cold-water detergents which may remain solid
Avoid over-use of strong chlorine and household cleansers although regular quantities diluted in warm laundry water are usually harmless to a mature system.
Trash compactors will work, but vegetable food waste makes great compost so please consider using an outdoor compost recycling system for vegetable waste.
Avoid pouring large quantities of grease, such as fowl or pork grease, into your drains or toilets; they are biodegradable but can clog your sewage pipes and put an abnormal load on the system. These substances may be sealed in a container and disposed of in your regular garbage collection. NEVER put them or any meat products in compost recyclers.
Keep the absorption bed well away from existing trees and do not plant shrubs or trees over the absorption bed or you will have very nice trees and a septic system bed that has to be replaced at great expense. Remember that tree roots of most trees extend far from the trunk.
Avoid driving over your absorption bed with cars or heavy vehicles. Lawn tractors or mowers are fine.
If you' follow these simple guidelines, your septic system will serve you and your family well for a lifetime.
