In the last O&M Matters article, we started a series discussing the concept of the treatment train as a way to view and evaluate an onsite wastewater treatment process and to provide insights for professionals who want to perform operations and maintenance service as a line of business.
Having discussed the first component of the treatment train (the source) in the last article, we now move downstream to the pretreatment tank. Most onsite wastewater treatment systems, whether standard gravity or advanced systems, employ some type of pretreatment tank. It may be a trash tank, a septic tank or a pump or process tank.
As standard gravity systems are still the preferred option, we will focus here on the septic tank. The septic tank plays two important roles in the treatment train. First, it separates and stores unwanted solids that could compromise downstream processes, most notably the soil treatment unit (drainfield). Second, it provides some biological treatment by anaerobic degradation.
A septic tank is a simple system, but not as simple as it may seem. It includes distinct parts that serve essential functions and must be in good condition if the tank is to perform properly. A professional performing O&M or conducting an inspection needs to know what to look for in assessing a tank’s condition and diagnosing trouble.
Dual compartments
While single-compartment septic tanks are still in use, most septic tanks installed today are designed with two compartments — a larger compartment on the inlet side, and a smaller outlet compartment.
In either case, the first point of reference in a service or inspection is to verify that the tank water level is at the invert of the outlet pipe. This indicates that water is flowing to the drainfield without obstruction.
If the liquid level is below the invert of the outlet pipe, the tank may have a leak. If the liquid is higher than the invert of the outlet pipe, there may be obstructions in the distribution line, in a distribution box or in the drainfield itself. This condition might also be the result of the tank settling due to inadequate bedding, causing the distribution line to lose it’s desired slope and become higher than the tank outlet.
After verifying the tank is at its normal operating level, we can move to the tank inlet side, where wastewater from the source (the house or business) flows into the tank through an inlet baffle or sanitary tee, which diverts the flow into the mid-depth level of the tank, referred to as the clear zone.
Here, in a quiescent environment, separation of solids takes place. Lighter materials such as fats, oils, greases and synthetic materials float to the top and form a scum layer. At the same time, heavier materials such as dirt, fillers from detergents, and decomposing bacteria, settle to the bottom and form a sludge layer.
All the time, the decomposing bacteria are working to break down the organic materials entering the tank. This separation and breakdown process depends on detention time — how long the wastewater is held in the tank.
Excess accumulation of solids or excess wastewater flow will reduce detention time and will affect both separation and the biological treatment process. In the case of single-compartment septic tanks and trash tanks, managing solids is even more important, as there is no internal baffle to serve as a buffer if solids levels become excessive and migrate through the tank.
In the more commonly used two-compartment septic tank, the internal baffle separates the inlet and outlet compartments and serves as a sort of dam that holds solids in the larger inlet compartment for processing. Some solids, mostly de-composing bacteria, move through the clear zone passage between the two compartments and settle to the bottom of the outlet compartment.
The next important component of the septic tank is the outlet fitting. This may be the same fitting used at the inlet — a baffle or sanitary tee. However, the best option is an effluent screen (filter) that protects the next downstream process from unwanted solids. And last but not least are watertight access risers. Risers not only protect the system from inflow and infiltration but allow easy access for routine inspection and maintenance.
Taking a look
So what do we look for in a routine maintenance inspection of a septic tank? Here is a basic checklist.
1. Are the tank risers and covers watertight so as to keep surface water or groundwater from entering the tank? Excess inflow will cause hydraulic overloading and reduce detention time.
2. Is the tank at a normal operating water level, with the liquid at the invert of the outlet pipe?
3. Is the inlet fitting intact to prevent turbulence and mixing of separated solids in the clear zone?
4. Is there excessive solids build-up thus reducing detention time?
5. Are there unwanted inorganic solids in the tank that will not decompose?
6. If a dual-compartment tank, is the internal baffle intact, providing a positive barrier for solids separation?
7. If a dual-compartment tank, are there excessive solids in the outlet compartment?
8. Is the outlet fitting or effluent filter in place and clean?
9. Is the tank structurally sound, and are the covers properly secured so as not to pose a health or physical hazard?
10. Is there evidence of a physical or biological problem in the tank, such as unusual odors, scum on both sides of the tank, or no scum layer at all?
Where to learn more
To learn more about why these various conditions are a problem and how to clearly evaluate and resolve them, check with your local onsite association or with the National Association of Wastewater Transporters (NAWT) to find out what training programs are available in your area.
You also might consider attending the two-day NAWT O&M training program in Louisville Feb. 28-March 1, just before the 2011 Pumper & Cleaner Environmental Expo. In our next O&M Matters article, we will look at the gravity drainfield, the next component of a standard gravity onsite treatment system.
Kit Rosefield is an adjunct instructor at Columbia Community College and a trainer for NAWT and the California Onsite Wastewater Association. His company, Onsite Wastewater Management in Mi Wuk Village, Calif., has a consumer education service at www.septicguy.com. Reach him at 209/ 770-6760 or kit@septicguy.com.
