Aerobic vs. Anaerobic Wastewater Treatment

Aerobic processes use bacteria that require oxygen, so air is circulated throughout the treatment tank.These aerobic bacteria then break down the waste within the wastewater.

Some systems utilize a pretreatment stage prior to the main treatment processes, as well as more treatment and sanitizing prior to release into the environment.

Electricity is required for system operation.

Pretreatment Process: Some systems reduce the solids that the aerobic bacteria could not easily break down (such as toilet paper) prior to the main treatment to reduce the chance of clogging the system.

Main Treatment: Treats household wastewater, and includes both domestic greywater and blackwater.

Forced air from an air blower or compressor is mixed with the wastewater, where the aerobic bacteria feed on the waste in the water and reproduce.

Solid wastes that the bacteria are unable to process settle out as sludge. Some aerobic treatment systems include a secondary settling tank to facilitate this settling process.

The sludge is frequently pumped out of the system to reduce opportunities for clogging.

Final Treatment and Disposal:

The final treatment processes prepare the water for return to the environment. These processes may include disinfection using chlorination or UV light, discharging the water to a soil absorption field, filtering through sand filters, drip irrigation, or evapotranspiration.

Anaerobic bacteria (bacteria that live in environments that contain no oxygen) transform organic matter in the wastewater into biogas that contains large amounts of methane gas and carbon dioxide.
Energy-efficient process.

Often used to treat industrial wastewater that contains high levels of organic matter in warm temperatures.

Can be used as a pretreatment prior to aerobic municipal wastewater treatment.

Used when site or soil conditions are not amenable for septic (anaerobic) systems.

Used when the water table is too high for effective septic system use.

Provides a high-quality alternative to septic systems when properly cared for.

Often used to replace failing septic systems.

Sludge is safe to use as a soil enrichment.

Uses less energy and fewer chemicals than aerobic treatment.

The biogas produced can be used for a renewable energy source as a replacement for fossil fuels such as oil and natural gas.

Small-scale systems can be used to treat individual households or several households in a shared facility. In developing countries where centralized wastewater treatment systems are cost-prohibitive to build and implement, small scale anaerobic treatment systems are a viable option, and may include on-site, community on-site or off-site systems.

More expensive to maintain than septic tanks.

In most cases, systems require professional inspection and maintenance.

Solids must be pumped out of units frequently.

Bacterial activity decreases at cooler temperatures.

Due to these temperature constraints, anaerobic systems that are utilized in temperate climates require the use of insulation and heating systems. Generally, only a small volume of wastewater being treated in cold climate countries is treated using anaerobic process, with the majority of water volume treated using aerobic processes that utilize aerators in open and closed ponds.

The methane-rich biogas produced through anaerobic treatment must be collected, treated and used to avoid release into the atmosphere, as methane is a potent greenhouse gas and has a bad odor associated with it.

The biogas can be used in cogeneration units to produce electricity and heat.

Minimally, the methane should be flared to reduce its impact on the environment.