Question

Aerobic and anaerobic process in wastewater treatment

Answer 1

There are two types of biologically-based wastewater treatment processes: aerobic and anaerobic wastewater treatment.

The following chart explains the main differences between these two treatment processes:

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 treatments on wastewater are normally implemented when treating more concentrated wastewater. The anaerobic sludge contains various groups of micro organisms that work together to eventually convert organic material to biogas via hydrolysis and acidification.  Biogas typically consists of 70% methane (CH4) and 30% carbon dioxide (CO2) with residual fractions of other gases (e.g. H2 and H2S). The methane can be used as an energy source. Anaerobic reactors can be implemented in a variety of ways. The figure shows a contact reactor and an upflow reactor.

The contact reactor is comparable with a conventional active sludge system, but under anaerobic conditions. The sludge is mixed with wastewater in the reactor and is then separated in the sedimentation tank and returned to the reactor.

In the anaerobic upflow reactor, the influent is introduced at the bottom of the vertical reactor. The sludge in the reactor is primarily grain shaped and forms a blanket in the reactor, with the most compact sludge grains at the bottom and the lighter grains and heavier sludge floccules above it. Very light sludge floccules will be released by the upward flow, but can potentially be collected in a sedimentation tank. The biogas is collected and disposed of at the top of the reactor, separately from the partly purified water and the sludge.