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Onsite Wastewater Systems: How Do They Work?

If you’re not familiar with septic systems at all, start with Homeowner Basics. Otherwise, you can skip to Choose a System. That’s where we provide info on our three main types of residential systems: standard gravity septic systems (without a pump), pressurized septic systems (with a pump), and advanced treatment systems (for environmentally sensitive sites).

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. This process is 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 there for years and microorganisms slowly break them down to a fraction of their original volume. Every 8-12 years, the remaining solids are pumped out of the tank by a service provider, who manages them according to local regulations. In many areas, they’re applied to land as a soil amendment. The clarified liquid effluent stays in the tank for only a couple of days. Then, it’s 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 consistently 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 might activate it on a timed schedule.

Advanced Treatment Systems:

If your site’s not suitable for a drainfield of conventional size, or if your site has poor soils or is in an environmentally sensitive area, you’ll 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.

Many wastewater treatment organisms need oxygen to do their work. In a conventional municipal wastewater treatment plant and in some advanced treatment systems, machinery or blowers continuously aerate the liquid effluent to supply the necessary oxygen. This process requires a lot of energy.

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. This process also provides more efficient treatment and reduces the amount of biosolids produced.

Reuse for Landscaping

The effluent that comes out of a sand filter or an AdvanTex Textile Treatment System is so clean that, in many cases, it can used to irrigate landscaping. Many jurisdictions around the United States allow the use of drip irrigation systems for the dispersal of effluent. Orenco’s Shallow Pressurized Dispersal Systems (SPDSs) offer a simple, less-expensive way to to disperse of the highly treated effluent.

Direct Discharge

If a property isn’t suitable for on-lot wastewater disposal, some jurisdictions allow for direct discharge or surface discharge via an NPDES permit. While the treatment limits for NPDES permits are higher than for other regulations, properly configured AdvanTex systems with disinfection capabilities (UV disinfection, for example) can successfully meet those requirements.

Make sure to consult local regulations to determine the treatment and O&M requirements that apply in your area.