QUESTION: What is an ideal septic system?

ANSWER: Recently someone asked what components I thought would make up the ideal septic system. Of course my initial reaction was that there is no such thing; that the system needs to be matched to the type of residence or establishment and the particular site and soil characteristics.

Giving the question a little more thought, I decided that based on what has been learned about systems over the last 30 years, it is possible to come up with a general profile of an ideal septic system. But we must take into account that adjustments would have to be made based on site characteristics.

TANK TO DISPERSAL

So here is my attempt to describe an ideal system along with some of my reasons for selecting those components:

It starts with a properly sized and maintained septic tank or tanks in series to deliver domestic sewage effluent with no more than 160 mg/L BOD, 60 mg/L TSS and 20 mg/L FOG. This would be followed by some additional pretreatment by an aerobic treatment unit (ATU) or media filter. This pretreatment will lower the levels of BOD, TSS and FOG as well as reduce the fecal coliform levels in the effluent as an indicator for presence of pathogens. This will reduce the amount of treatment necessary in the soil treatment component of the system.

A pump tank would be next, with a pump to deliver effluent by pressure distribution to the soil dispersal system. Effluent delivery would be controlled by a timer rather than allowing on-demand dosing. This will allow for capture of large surge flows and equalize the flow over the entire day rather than having the flow concentrated at specific times, typically in the morning and late afternoon for a residence. Timed dosing also reduces the stress on the soil treatment area during high-use events, such as doing the laundry.

With pressure distribution in the soil treatment area and the pretreated effluent, there is little to no development of a biomat to restrict flow. Controlling the flow and using all of the system reduces the potential movement of pathogens through the soil along preferential flow paths and increases the amount of time the effluent is in contact with the soil, allowing better removal.

In terms of the distribution laterals, having more orifices (holes) is better. I often see designs for laterals that have a 5-foot spacing between them; there is better overall distribution if that number is reduced to 2- to 3-foot spacing, depending on the configuration. This may require a somewhat larger pump, but that’s a good investment from a treatment efficiency standpoint.

BEST TRENCHES

Narrow, shallow trenches installed on the contour would make up the final treatment and dispersal area. A narrow trench would be 1 to 2 feet wide and no more than 1 foot deep in the natural, undisturbed soil. Even better would be an at-grade system with no excavation into the natural soil.

Narrow trenches reduce the linear loading rate along the contour and provide increased soil contact with the effluent. One downside may be that the site does not allow long runs along a given contour, but dividing the area into multiple separate areas allows the opportunity to periodically rest parts of the system to help maintain soil permeability.

Shallow to at-grade placement takes advantage of the best parts of the natural soil for treatment. Natural soil bacteria and other microorganisms play an important role in the treatment processes. About 98 percent of these organisms are found within the upper 16 inches in a typical soil profile. If we are counting on these organisms to help in the treatment process, excavating more than a foot into the soil takes the system out of this biologically active zone. At 30 inches, there is only about 0.5 percent of the total organisms present.

Keeping the trenches shallow also makes use of the most permeable portion of the soil profile and the area where there is the most root growth by vegetation. To the extent plant uptake can help with treatment, the system should be located in this zone. About two-thirds of root growth occurs in the upper 2 feet of the soil profile, with 40 percent in the upper foot. This is also a reflection of where the soil is most permeable.

EASY EVALUATION

The ideal system will also include instruments to evaluate flow and flow characteristics. This means adding time-elapsed meters or cycle counters combined with pump delivery information and pump cycle counters. Inspection ports should be added in the trenches to enable evaluation of potential ponding issues.

Finally, all components of the ideal system will be installed so they are accessible to allow for routine operation and maintenance, which includes tank and pump maintenance as well as the opportunity to periodically clean the pressure distribution laterals.

There you have my outline for an ideal system. Is this what systems look like in your area? If not, is it time to consider some changes? I think the answer should be “yes.” Why not take full advantage of the lessons we’ve learned?