Septic Tanks

 

 

 

By B. Bosshard, PE  _  www.wastertreatmentplant.org

As a professional engineer, I generally receive questions on the why’s and how’s of wastewater treatment. First we need to ask why we need treatment. Then discuss the minimum requirements which are generally accepted to minimize polluting our environment. Our next discussion explores the Environmental disadvantages of using septic tanks, and finally what are some of the alternative septic tank solutions in use today.

Why Treatment?

First we need to ask, “Why do we need to treat our wastewater?” In the old days we merely dug a hole and place an “outhouse” on top of the hole. We would then deposit our biological waste directly into the hole.  When the hole became full we would just move the “outhouse” and cover up the waste, dig a new hole and place the “outhouse” over the new hole. In this scenario, the waste does not readily break down, and can remain toxic for 100’s of years. The waste can readily be exposed by digging animals, or leach down to the surrounding groundwater. Both paths promote pathogen pathways directly to us, which can readily spread disease.

To minimize our wastewater effects on the human and other animal populations we can take a few steps to allow the wastewater to naturally break down, reducing health risks. The most common and least expensive method is the use of Septic Tanks. The septic tanks are configured to take all our wastes, biological and wash waters (Black and white waters) combine them into a tank (of adequate size) and then discharge the treated wastewater to the surrounding soil.

 

 

 

 

Minimum Requirements:

Tank Sizing

Generally, minimum septic tank sizing is specified as 2.1 times the maximum expected daily flow. This size promotes the wastewater to slowly flow through the tanks, requiring about 2 plus days to exit the tank. This process is commonly referred to as “residence time.” Furthermore, the tanks are constructed to allow the solid wastes to be removed first and then further treatment of the off-liquid is treated in a separate chamber. The first tank compartment is generally referred to as a solids tank, or “chunk tank.” Here the lighter solids like toilet paper floats to the top and heavier particles and solids settle to the bottom of the tank.

A pass-though line, about 4-inches in diameter, allows solid-free water to flow to the second tank compartment for further treatment. Generally the first tank compartment is 60 to 70 percent of the total tank volume.

Build up of anaerobic bacteria (bugs that live without oxygen) in the two tanks treat the wastewater continuously. I usually put up a fuss when my wife pours chlorine into the toilets to clean them, since the chlorine has a tendency to kill the beneficial bacteria in the septic tank.

The treated wastewater that exits the tank flows to a leach bed. A leach bed is designed with a 30-year life consideration. The size of the leach area is generally driven by the soil conditions. Obviously, a sandy, more permeable soil will require less leach area to serve the system. Generally the leach line, or bed is constructed with a 2 to 4-ft layer of 1 to3 inch rock. A perforated leach line is place on top of the leach rock, covered with leach rock and then a geotextile layer to keep dirt from clogging the leach rock below. Then the leach field is covered with about 12 inches of native soil.

In considering a 30-yr. life span of the leach area we must consider that the leach field will clog up over time. As the wastewater is deposited into the leach field, further biological activity occurs and a matting-like layer forms at the bottom and sides of the leach field. This layer generally expands with time, so at the end of a 30-yr life, the leach field cannot take all the wastewater volume produced by the residence. There are treatments available which will dissolve this layer. I prefer the NORWECO Bio-gem and Bio-perc products to restore leach field health. We will discuss leach field and septic tank treatment and conditioning in subsequent articles.

 

 

Soil Conditions

Soil conditions are crucial to ensure that the wastewater has enough time in the soil to remove the majority of pathogens and other toxic components. Generally, a minimum vertical separation from a limiting area is drawn at 10-ft. A limiting area is where no further treatment of the wastewater will occur, once the wastewater reaches the limiting layer. A limiting layer can be groundwater, or bedrock, etc.

Environmental Disadvantages of Septic Tanks

Nitrogen Loading

One of the largest concerns with using septic tanks is that we load excessive nitrogen into the ground. The nitrogen loading is promoted because we do not treat the nitrogen in the process. Generally, in the United States, regulatory bodies set a minimum size developed lot for a 3-bedroom home at one-acre. This minimum size promotes an acceptable nitrogen load on the groundwater below.

Groundwater Contamination

Other groundwater contaminations are in the form of pathogens. Pathogens are usually tracked as colony forming units (CFUs) per ml. As we increase the number of homes to less than one home per acre, we also start to stress the underlying aquifers with pathogens. The soils can only take so much of a loading rate before the pathogens start increasing in surrounding groundwater wells.

Water Resources

Therefore, a down-side to septic tanks as the primary residential wastewater treatment method is the effect that it has on our water resources. The primary contamination of our groundwater comes from total nitrogen loading and pathogens. As we increase the density (the number of homes per acre) we need to consider other wastewater treatment methods to minimize the impact on our water resources and the resultant health risks to our population.

Alternatives to Septic Tanks

New technologies over the last 100 years, has significantly reduced nitrogen and pathogen loading to our environment. Wastewater treatment methods included extended aeration, De-nitrifying systems, Geotextile filters, and hybrids.

Extended Aeration

In extended aeration we can take a standard septic tank as noted above, and blow air into the tanks which will promote aerobic bacteria to form. The aerobic bacteria (bacterial that breathe oxygen) are much more aggressive than the anaerobic bacteria in standard septic tanks. The aerobic bacteria grow quickly and break down the waste much faster. The resulting wastewater stream has much lower concentrations of pathogens and other wasteproducts. Leach field degradation is greatly reduced, and can be much smaller in size and still maintain that 30-yr. plus life span.

De-nitrifying systems

Once we treat the wastewater in an extended aeration plant, we still have a lower amount of total nitrogen than a septic tank, but the total nitrogen is still high. Generally, in areas that have total nitrogen problems in groundwater, de-nitrifying systems are required to be installed on backend of extended aeration plants to further reduce nitrogen loading of underlying groundwater.  De-nitrifying systems require anaerobic conditions (absence of oxygen). Some hybrid systems like the NORWECO SINGULAIR system, utilizes a Bio-kinetic modules to promote anaerobic conditions and the further breakdown of total nitrogen before the water leaves the unit. With recirculation, the SINGULAIR system yields total nitrogen levels less than drinking water standards in the United States. BnB Alternative Septic Solutions recommends this product as the proven leader in de-nitrifying residential systems.

Geotextile filters

Geotextile filters consists of plates or filters that the wastewater flows through the filters or plates allow bacterial to form on the substrate and promotes breakdown and some dentrification of the wastewater stream. One large downside to filters or plates is that they generally require replacement in 10 years or so, due to clogging. This can be a large expense to the homeowner. We therefore, do not recommend using these products.

Hybrids

Crafty engineers are always coming up with hybrids of extended aeration, flow equalization, and geotextile filters. We are generally skeptic on claims, and look for demonstrated performance over many years, prior to endorsing many of these products. The one product we like today is the NORWECO SINGULAIR which is a hybrid system, which has evolved over the last 100 years to its current configuration. Extended aeration, combined with geo-textile technology is patented and well proven, with low costs and the best proven treatment performance on the market.