Life in the Septic Tank

You’ll be better equipped to help your customers if you understand how the onsite system works and what can throw it out of kilter

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What can affect septic tank biology?



One question often asked by pumpers and other service providers who regularly see the inside of operating septic tanks is: Why do I see such a wide range of conditions? And what can cause septic tanks to operate improperly?

Before addressing factors that can affect the performance of a septic tank, I offer a brief review of what a properly operating septic tank should look like and what is happening in the tank in terms of treatment.



A properly operating septic tank will have three distinct zones. There should be a sludge layer at the bottom, a clear zone and a floating scum layer. Effluent delivered from the clear zone should have values of 150-175 mg/L BOD-5; 40-60 mg/L TSS (total suspended solids) and no more than 20 mg/L FOG (fats, oils and grease). A septic tank will usually have a pH between 6 and 7.5; and for bacteria activity to occur, a temperature above 40 degrees F.

Treatment in the tank consists of providing a quiet zone for settling of the gross solids (sludge) and through use of baffles capturing and storing the floating soap and grease scum at the top of the tank. When the percentage of the combined sludge and scum depth versus the operating depth of the tank exceeds 25 percent, the tank should be pumped. The bacterial digestion process in a septic tank is anaerobic; that means it is occurring without the presence of dissolved oxygen (<1 mg/L).

There are three general types of bacteria: aerobic, facultative and anaerobic. Aerobic bacteria require the presence of free oxygen to survive and flourish. Facultative are organisms that can operate in the presence or absence of oxygen. Anaerobic bacteria can operate in environments where the only oxygen present is bound within other compounds; they can slit these compounds and utilize the oxygen.

Aerobic bacteria are more efficient at breaking down and utilizing the organic waste as their food source. They are larger in size than anaerobic bacteria. They are more sensitive to environmental changes. Aerobic bacteria and treatment processes are used for treatment in aerobic treatment units and media filters. Anaerobic bacteria are smaller and less efficient in breaking down the waste; but they are tougher and can withstand larger changes in the environment.



What can cause a tank to be “upset” and not exhibit the three distinct layers? As in any troubleshooting process, we need to evaluate the water-use habits inside the house and the use of high water-consuming devices that do not allow settling and separation to occur. Examples of this may be as simple as washing multiple consecutive loads of laundry or dishes.

This can both hydraulically overload the tank and – in the case of dishwashers – add a large organic load. Most new dishwashers have built-in garbage disposals that add solids to the tank and are harder to break down. In both cases the hydraulic surges can stir up septic tank contents. On a positive note, if the tank has an effluent screen in place, the solids are being captured by the screen and not delivered to the soil treatment unit, where the soil will become plugged leading to failure.

Another similar situation is found with the use of large-volume whirlpool tubs. Not only can the tubs create higher water usage, but drained water can build several feet of head if delivered from the second or third floor of the house. That gives water entering the septic tank a higher pressure, resulting in turbulence in the tank.

In all of the cases described, the service provider can work with the homeowner to reduce flows and spread out the wash and cleaning events to minimize the hydraulic impact on tank operation.

Once it is determined that flow or water use is not the problem, look at chemical products being used. These products can interfere with biological activity and/or cause chemical reactions resulting in bulking of the sludge. This means gas is emitted around the sludge, making it buoyant so it is floating or suspended. Before effluent screens, these solids were often delivered to the soil treatment area before anyone was aware there was a problem.



When used excessively or regularly, cleaning products can have cumulative effects in the tank. Most cleaners now highlight antibacterial properties. And if they are antibacterial, they don’t discriminate between the bacteria killed on the sink, toilet or countertop and the bacteria present in the septic tank.

The common use of antibacterial liquid hand soaps also can cause a problem, slowing down the already slow anaerobic digestion process and leading to increased sludge and scum buildup and requiring increased maintenance. Use of automatic cleaners should also be discouraged. These are primary related to toilet bowl cleaning. Again, automatic probably indicates more product is used than necessary.

Homeowners should be encouraged to look at the labels of all cleaners ultimately flushed down the drain. If the product is labeled with the word “danger,’’ it will kill bacteria and its use should be minimized. If the label says “warning,” it means limited use of the product should have little impact. And if the label says “caution,” the product should have minimal impact. Toxic drain cleaners that remove clogs and blockages from plumbing usually fall into the danger category.

One item that has become more of a problem over the last decade is the increased use of bath and body oils. This can lead to elevated FOG numbers, which if passed on to the soil treatment area can increase development of biomat, reducing the ability of the soil to accept water. Again, working with the homeowner to limit use of these products is recommended.

Prescription drugs are another area of concern. Some of these drugs kill bacteria or inhibit their ability to break down waste. Solving this problem can require additional tank maintenance.

As we perform more rigorous maintenance on systems in the future, we will identify additional problems and better understand how to keep septic tanks operating properly.


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