Septic System Design
Septic system design is a mix of art and science. Review this site and you will learn a lot about the waste management system that is many rural properties. Discuss septic systems in urban communities and they look at you like you are a Beverly Hillbilly. But the fact is that septic systems are incredibly efficient and simple. And contrary to what is commonly mentioned regarding ground water quality it is rare for properly designed systems to have any more effect on ground water than traditional sewer plants.
The science of the septic system is that there are simple calculations to be done regarding the burden you wil be placing on the system and the amount of waste water your soil can absorb and process. For example hard soil absorbs less wastewater than does sandy soil. The traditional way that the absorption rate of your soil is determined is through a test known as a perc or percolation test.
This is a test that simply measures the rate at which water is absorbed in your soil. The percolation test is normally conducted by a soils engineer. The perc test will also include an evaluation of the type of soil, geology of the area, and location and depth of groundwater. Different cities and counties have varied guidelines on how to design systems based on the results of the perc test.
The septic system design uses the perc test results and soils evaluation and combines this into a system that will disburse the “gray water” that is produced from the system. This is probably a good point to discuss how a traditional system operates. Basically sewage from your sinks, toilets, and washers flow out sewer pipes from your home into a septic tank. This tank is specially built so that the solid waste falls to the bottom of the tank and the liquid waste floats to the top. As sewage enters the tank the water flows out the top of the system out into a disbursement system into the surrounding soil.
The types of disbursement systems into the surrounding soil is the foundation for the rest of the septic system design. There are several types of disbursement systems.
The first type is the most common and it is a gravity based system. These types of systems have the wastewater or gray water that flows out of the tank into a square box called the distribution or d-box. The d-box allows the gray water to flow evenly out into other long 4 inch pipes that are at a slight downward angle from the d-box. These pipes have holes evenly spaced down the length. The pipes are buried. The gray water flows down the pipes and spills out the holes down the length of the pipes. This water distribution then seeps into the ground. This is the basic operation of a gravity septic system design.
A pressure septic system operates similarly to the gravity system except as the name implies there is a pump to distribute the gray water across the top of the soil. Where the gravity system uses buried perforated pipes to drip the gray water underground the pressure pump system sprays it to the top of the soil and then it seeps in. Either way the gray water is allowed to absorb into the soil. Usually the gravity system is used in areas with deep groundwater. The pump system is for areas with shallow groundwater. The spray on top of the soil allows for further separation from the underground natural ground water.
A sand septic system is a hybrid of a pump system and natural soil system. Without getting too technical this system basically has two separate septic tanks. The first tank forces the gray water through compacted sand in the tank.
This first filtering through sand is sort of a pretreatment before the gray water is then sent to a second tank where it effectively enters into a pressure pump system. Think of this system like using spray and wash before washing clothes. The sand septic tank is the pre-wash before the gray water hits the main treatment tank. Because the water is pre treated the gray water on eventual pump spraying is can be pumped onto a much smaller area. Therefore this system is more common when the home is on a small lot. The mound septic system design is mainly for areas where ground water is an issue or where sandy soils require a separated soil area to treat the gray water.
Basically a mound system is a gravity fed system into a mound traditionally above ground. For example if the gravity system is to be in a 800 sq ft area underground the mound would be 800 sq ft above ground and the system built into the mound. Because the mound is above ground then the gray water will be absorbing into soil and be further separated from the underlying ground water. When mound systems are built underground it is because the soil is too sandy and therefore the gray water filters straight down to the ground water. In this scenario the mound is an underground area of soil that has a higher absorption rate than the surrounding soils. Either way these systems are designed to counteract ground water concerns.
This is the conclusion of this simple septic system design overview. There are many more designs that we will be covering as this site is developed further. For now we wanted to cover the basics. If you have questions or need septic advice and are in Southern California feel free to drop us an email. For northern California it depends on the area you are in. But drop us an email either way – we are always interested in hearing from readers. If you need a referral to a septic installer soils engineer or perc test company – just let us know. Our email is support@septicsystemdesign.net.
Thanks for reading.