If you have been looking at garbage disposals, you may have noticed the phrase, safe for properly sized septic tanks. If you live in a city, you may never have heard of septic tanks. Septic systems are used in houses that are not connected to any sewer system. The septic tank is a key component in a septic system.
A septic system is a small sewage system with no connections to other sewage systems. In Europe septic systems are not very common but in North America about 25% of the households use septic systems. Most of them are located in rural areas.
A septic system has two main components, the septic tank and the (septic) drain field. Typically, a septic system is very simple, no machines are involved. A properly maintained septic system can last for decades. The only maintenance needed is removing the solids out of the tank. The sludge layer at the bottom needs to be pumped out of the tank. How often this must be done depends on the size of the tank and how much solid waste is disposed by the household. Properly sized tanks should not need to be pumped more than every three years or so. But septic tanks can also be used ten years or longer without pumping.
This is a simple septic tank. Modern septic tanks typically have two compartments. This makes it easier to prevent the sludge layer from entering the drain field. New tanks often have risers with lids at the surface. This makes it easier to inspect and pump the tank.
The septic tank is large, thousand gallons is a normal size for small septic tanks. It is generally made of concrete, glassfiber or polyethylene. The septic tank receives the waste water from the household and disposes it into a drain field. The septic tank is typically buried in ground near the house. The water that flows out to the drain field is relatively clean. Note that some states require septic tank owners to have a reserve drain field on the property.
Oil and grease is stored at the top of the tank, the so called scum layer. Solid waste is stored at the bottom of the tank, creating the sludge layer. Some, but not all, of the solids are broken down through natural anaerobic processes. It is essential that neither the scum nor the sludge layer is growing out of control and flowing into the drain field. Generally, the septic tank and the drain field is located so that gravity will distribute the liquid waste from the tank into the field. Local regulations typically restrict the design of the drain field. This is to protect the groundwater in the area.
A garbage disposal increases the sludge layer of the septic tank. If you dispose fat, the scum layer will also grow quicker. If your septic tank is not properly sized, a garbage disposer may push it past its limit. Unfortunately, there is no easy way of estimating how large septic tank you need. The main factors determining how often the tank needs to be pumped are the size of the tank, the amount of waste water generated and the volume of solids in the waste water.
A septic tank should be inspected annually by a professional. This way you know when it is time to empty the tank. As a rule of thumb, once the scum layer is within 12 inches of the bottom of the outlet or the sludge layer is within 6 inches of the bottom of the outlet, the tank need to be pumped. It can be very expensive to replace an unusable septic system and very often the reason for the failure is due to neglected maintenance. Note that failed septic system will also lower the value of your property and may incur fines.