How Aerobic Treatment Works
Aerobic systems treat wastewater using natural processes that require oxygen. Bacteria that thrive in oxygen-rich environments work to break down and digest the wastewater inside the aerobic treatment unit. Like most onsite systems, aerobic systems treat the wastewater in stages. Sometimes the wastewater receives pretreatment before it enters the aerobic unit, and the treated wastewater leaving the unit requires additional treatment or disinfection before being returned to the environment. Such a variety of designs exist for home aerobic units and systems that it is impossible to describe a typical system. Instead, it is more practical to discuss how some common design features of aerobic systems work and the different stages of aerobic treatment.
Some aerobic systems include a pretreatment step to reduce the amount of solids in the wastewater going into the aerobic unit. Solids include greases, oils, toilet paper, and other materials that are put down the drain or flushed into the system. Too much solid material can clog the unit and prevent effective treatment. Some pretreatment methods include a septic tank, a primary settling compartment in the treatment unit, or a trash trap. Pretreatment is optional but can greatly improve a unit's performance.
Aerobic Treatment Units
The main function of the aerobic unit is to collect and treat household wastewater, which includes all water from toilets, bathtubs, showers, sinks, and laundry. Aerobic units themselves come in many sizes and shapes-rectangular, conical, and some shapes that defy classification. Figure 1 on this page and figure 2 on page 4 show two aerobic unit design possibilities.
Suspended Growth Unit
The process most aerobic units use to treat wastewater is referred to as suspended growth. These units include a main compartment called an aeration chamber in which air is mixed with the wastewater.
Since most home aerobic units are buried underground like septic tanks, the air must be forced into the aeration chamber by an air blower or a compressor. The forced air mixes with wastewater in the aeration chamber, and the oxygen supports the growth of aerobic bacteria that digest the solids in the wastewater. This mixture of wastewater and oxygen is called the mixed liquor.
The treatment occurring in the mixed liquor is referred to as suspended growth because the bacteria grow as they are suspended in the liquid unattached to any surface.
Unfortunately, the bacteria cannot digest all of the solids in the mixed liquor, and these solids eventually settle out as sludge. Many aerobic units include a secondary chamber called a settling chamber or clarifier (see figure 1) where excess solids can settle. Other designs allow the sludge to accumulate at the bottom of the tank (see figure 2 on page 4).
In aerobic units designed with a separate settling compartment, the sludge returns to the aeration chamber (either by gravity or by a pumping device). The sludge contains bacteria that also aid in the treatment process. Although, in theory, the aerobic treatment process should eventually be able to consume the sludge completely, in practice, the sludge does build up in most units and will need to be pumped out at least once a year so that solids don't clog the unit.
Attached Growth Unit
An alternative design for aerobic treatment is the attached growth system. These units treat wastewater by taking a surface made of material that the bacteria can attach to, and then exposing that surface alternately to wastewater and air. This is done either by rotating the surface in and out of the wastewater or by dosing the wastewater onto the surface. Pretreatment is required. The air needed for the process is either naturally present or is supplied mechanically. Attached growth systems, such as trickling filters and rotating disks, are less common than suspended growth systems, but have certain advantages. For example, there is no need for mixing, and solids are less likely to be washed out of the system during periods of heavy household water use.
Final Treatment and Disposal
Although properly operated and maintained aerobic units are very effective, the wastewater leaving the units is not ready to be returned to the environment and must receive final treatment or disinfection. Methods for final treatment include discharge to a soil absorption field, a sand filter, or an evapotranspiration bed. Sometimes, the wastewater receives disinfection before being discharged to the soil or directly to a body of water. Your health department is familiar with local regulations and the treatment options that are best in your area and for your property.
The amount of dissolved oxygen contained in wastewater from an aerobic unit can help the growth of microorganisms that treat the wastewater in the soil, and can help prevent the pores in the soil from clogging. However, when aerobic units malfunction, they can release solids that can clog the drainfield, which may cancel out any potential benefits.
Soil absorption fields (or drainfields) are the most common method of final treatment used for septic systems. If an aerobic system is being used in place of a septic system or to replace a failing septic system, a drainfield may not be an option. However, an aerobic unit can sometimes help to prolong the life of a drainfield.
Evapotranspiration beds are a less common method of final treatment and use vegetation and evaporation to naturally treat the wastewater. Drip irrigation is another less commonly used method to treat and dispose of wastewater. Sand filters are sometimes used to treat the wastewater from aerobic units. The wastewater is pumped evenly over several layers of sand and gravel, which are located either above or below ground. As with soil treatment systems, the purification process is aided by bacteria that occur naturally in the sand.
Disinfection is another method of treatment commonly used with aerobic units. Some units have the disinfection process incorporated into the unit design. In some cases, disinfection may be the only treatment required of the wastewater from an aerobic unit before the water is released into the environment. One disadvantage of this method is the added cost of the disinfectants, such as clorine.
reprinted from Pipeline, Winter 1996; Vol. 7, No. 1.
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