Onsite Wastewater Systems: How They Work
If your home or business is not connected to a central sewer system, you will need an onsite wastewater system. All your household wastewater will be collected and treated on the spot (on site). The treated effluent will be returned to the soil, and, after soil filtration, to the groundwater. Some components are common to all onsite systems. Others depend on your site conditions.
Primary Treatment in the Septic Tank
All onsite systems start with an underground septic tank that receives the wastewater from the house. There, the raw wastes naturally separate into settled solids, floating material, and liquid effluent -- a process called primary treatment.
Primary treatment is a passive and very reliable process. It provides 70% of the necessary wastewater treatment with no energy input. As long as the tank is watertight, the solids remain in the tank for years, while microorganisms slowly break them down to a fraction of their volume.
Every 8-12 years, the remaining solids are pumped out of the tank by a service provider, who manages them as directed by local regulations. In many areas, they are applied to land as a soil amendment.
In contrast, the clarified liquid effluent remains in the tank for only a couple of days. Then it is pumped (or flows by gravity, if possible) from the tank to the next treatment step.
Dispersal to the Drainfield
Sometimes the clarified effluent can be returned directly to the soil by means of a drainfield. This is an array of perforated pipes placed in sand, gravel, or plastic chambers. Effluent flows (either by pump or gravity) to the drainfield and trickles into the sand. There, microorganisms remove the nutrients in the effluent, purifying it. Plant roots take up some of the remaining water, and the rest makes its way down to the groundwater.
A drainfield requires soil with the proper permeability -- not too much, and not too little. When your house was built, an engineer or designer evaluated the soil and determined whether or not a drainfield was possible.
Three Main Types of Onsite Systems
Standard Gravity Systems: Depending on the terrain of your lot, you may be able to install an onsite system that discharges effluent to the drainfield via a gravity system. As wastewater flows into the tank at the inlet, it pushes out clear effluent at the outlet. A Biotube Effluent Filter on the outlet of your septic tank will protect the drainfield from particles that could clog and damage it.
Standard Pumping Systems: Other onsite systems discharge effluent to the drainfield using an effluent pumping system. This is necessary when the drainfield is not downhill of the septic tank. One advantage of an Orenco pumping system is that it delivers wastewater to the drainfield in small, uniform doses throughout the day. This keeps the drainfield in a uniformly moist condition favorable to the growth of treatment organisms.
In Orenco's effluent pumping systems, the pump is protected by a Biotube Effluent Filter and operated by a control panel. The panel may activate the pump based on demand, when the effluent in the tank reaches a certain level. Or it may activate it on a timed schedule.
Advanced Treatment Systems: If your site is unsuitable for a drainfield of conventional size, or if your site has poor soils or is in an environmentally sensitive area, you will need a secondary or advanced treatment system. These systems treat household waste to very high, advanced (secondary) treatment levels: 10 mg/L BOD and TSS*, or better. Learn more about BOD and TSS
Many of the treatment organisms need oxygen to do their work. In a municipal wastewater treatment plant, and in many, less efficient advanced treatment systems, machinery or blowers continuously aerate the liquid effluent to supply the necessary oxygen.
However, a more stable and energy-efficient practice is to trickle the effluent over a bed of porous material, called a media filter. The filter media develops a thin coating of microorganisms. Nutrients continuously wash over them, and oxygen from the air passively permeates the thin layer of moisture. Periodically trickling effluent with a small pump uses much less electricity than continuously aerating liquid with a blower. Treatment efficiency is also considerably greater, with biosolids and management significantly reduced.
The original media filters, developed more than a hundred years ago, were made of sand and gravel. Intermittent Sand Filters are still used at rural homes with spacious yards. Watch a video about them.)
However, engineered filtering media provides more surface area than sand to facilitate the growth of microorganisms, and provides more open pore space for the movement of oxygen. For example, in Orenco's AdvanTex Treatment Systems, effluent circulates over hanging curtains of "engineered textile." The large internal surface area of this material allows a three-by-eight-foot AdvanTex pod to treat an average household’s wastewater, which otherwise would require a sand filter of hundreds of square feet.
Reuse for Landscaping
The effluent that comes out of a sand filter or the AdvanTex Textile Treatment System is so clean and odorless that, in many cases, it can be used for drip irrigation of landscaping. Many jurisdictions around the United States allow the use of drip irrigation for the dispersal of effluent. Orenco's Shallow Pressurized Dispersal Systems (SPDSs) offer a simpler, less expensive way to to disperse of the highly treated effluent.
BOD = Biochemical Oxygen Demand; a measure of the amount of organic contamination in wastewater
TSS = total suspended solids, another measure of contamination