Claims about gravity septic systems misleading

I read with interest the guest opinion of Marye Flowers in the March 24 edition. Marye’s motivation to maintain Flathead’s water quality is commendable, but I found some of her science references and conclusions to be very flawed and misleading.

At issue is the use of gravity vs pressurized drainfield systems for the treatment of septic tank effluent. She rightly points out that pressurized systems can extend drainfield life by distributing effluent more uniformly over the drainfield trenches, but that does not equate to better treatment. Marye calls the uniform pressure system a “more protective” system for the groundwater. This is not true nor is it science based. 

Marye states that a gravity system creates “hot spots” of saturation that allows nitrogen and phosphorus to bypass treatment and head straight for our groundwater and lakes. Quite the opposite is true, the soil becomes clogged with anaerobic slime which actually prevents effluent from seeping through to the ground water. In a sense, the drainfield becomes too efficient from a treatment standpoint and effluent no longer seeps downward but instead comes to the surface. A dosing system does help in maintaining aerobic conditions which can extend the life of a drainfield but it does not improve the treatment. 

The problem of a “failed” septic system, whether it is gravity or pressurized, is that the effluent comes to the ground surface and therein causes a public health problem and a potential runoff problem, but it is not a direct groundwater pollution threat.

Septic drainfields are extremely efficient treatment systems. The upper soil mantle is biologically very active and is efficient at filtering out and basically composting organic matter. It also filters out bacteria and attaches viruses within a very short distance of the drainfield. 

Drainfield soils filter out and are chemically attached to nutrients like phosphorous and to a lesser extent to nitrogen (when in the form of ammonia) where they can be utilized by plant roots. Drainfields need to be properly sited in the proper soils and they should be installed as shallow as possible to be within reach of the roots of plants which will remove some of available nutrients, especially phosphorus. All of this should be considered by the health department when granting a permit.

Marye calls on our elected officials to be fully informed by the science already at their fingertips. The science shows that when septic tank drainfields are properly sited and constructed that they are a very reliable and a very good waste treatment system. 

Dosing can be accomplished on gravity systems with the use of automatic syphons or pumping. Pumping should be required only when it is needed to make available a more suitable drainfield site than what is available by gravity. This is especially true around lakes or bodies of water which need special protections. Options must be left open because pressurizing will not improve treatment.

I commend our elected officials for taking a stand not to increase the cost on all drainfield systems. Drainfields are necessary for much of the county and spending more for pumping when nothing is gained from a treatment standpoint is simply ill advised. The end result will increase housing costs, which are already out of reach for far too many.

Terry Druyvestein is a retired civil engineer with a master's in environmental engineering. His thesis was written on the travel of sewage pollutants in the soil and groundwater systems. He lives in Rollins.