The wastewater effluent from primary treatment flows into secondary treatment which uses natural biological treatment processes to reduce the organic matter by 90%. Microorganisms found in the wastewater use the organic matter as a food supply in the presence of oxygen. This process can be accelerated by adding oxygen to the wastewater beyond that which is already present. Typically, two different processes are most commonly used. They are called the attached growth process and the suspended growth process.
The attached growth or a fixed film process takes place on the surface of stones or plastic. The wastewater is spread over the media along with air and trickles down slowly allowing the biomass containing microorganisms to consume the organic matter and produces carbon dioxide. These systems though they may vary in type are very successful and they include trickling filters, rotating biological contactors, and bio towers. Problems occur when there are sharp temperature differentials which may slow the process down, the storage space between the different types of media become filled, there is a reduction in the air present within the system, groups of organisms, their waste products, and byproducts crack off the stones or other parts of the media, and the underdrainage system to capture the effluent becomes clogged or deteriorates.
In the suspended growth process, the biodegradable organic material and organic nitrogen- containing material through the use of pumped-in oxygen or air and agitation of the fluid allows an oxygen transfer to convert ammonia nitrogen to nitrates, which then may be processed further. This creates large numbers of microorganisms and speeds up the process. As the activated sludge accumulates, part of it is removed and part of it is returned to the aeration tank to be mixed with the incoming wastewater and this activated sludge must be kept in suspension to make the system work. Most activated sludge processes are more expensive to operate than trickling filters because of the cost of high-energy use to run the aeration system. Toxic chemicals from industry also impact the process.
Membrane bioreactors may be used for secondary treatment. This process combines the activated sludge treatment with the use of a membrane that separates the solids from the liquid, leading to effective removal of the soluble and biodegradable materials at higher loading rates. The problems are that they cost more than the usual secondary treatment and the membranes can become less effective over time.
Wastewater lagoons, which are scientifically constructed 3-5 feet deep ponds that allow sunlight, algae, bacteria, and oxygen to interact, especially in smaller communities where land size is not a problem, serve as a secondary treatment method for about 25% of the municipal secondary treatment facilities. It typically works as well as secondary treatment of other types in removing biodegradable organic material and some of the nitrogen if the lagoon is constructed properly and operated properly. Cold temperatures have a significant effect on the operation of the lagoon. Of great concern is the breaching of the lagoon, especially those containing animal waste during storms, which will have a major impact on the environment and health of people.