Facultative Aerated Lagoon

The Definition
 

The basic function of the lagoon system is to adequately treat wastewater for ultimate return to the environment. The primary indicator of the amount of pollution in wastewater is the biochemical oxygen demand or B.O.D. Generally speaking the B.O.D. of untreated average municipal sewage is 200 to 250 mg/l (milligrams per liter). Derry is currently permitted to discharge treated wastewater with B.O.D.'s  up to 30 mg/l. Derry's actual quality of treated wastewater varies between 9 mg/l and 13 mg/l.  
 

Lagoon processes are biological treatment plants where living bacteria, plants and animals consume B.O.D. The B.O.D. is incorporated into these living things. The rate that the B.O.D. is removed or the length of time it takes for these bacteria, plants and animals to break down the waste is dependent on many things, but most importantly on temperature. Generally speaking, the warmer the water the faster the "bugs" work.  
 

In an "Aerated Facultative Lagoon", "bugs" breakdown the B.O.D. and the solids in the wastewater settle on the lagoon bottom where other bugs digest what they can. All of the liquid in the lagoon is aerobic (has oxygen). The bacteria and animals require oxygen, as do humans, to live. All of the oxygen is supplied to the lagoon from the air by the blowers and through the BIOLAC system. Below the liquid is the sludge which does not have oxygen available. Other bacteria which live without oxygen (anaerobic) digest about 80% of the sludge. The other 20% will build up on the lagoon bottom. Sludge levels are usually measured annually and have not been significant enough to require any removal at this time. Any removal required has been due to liner damage and not sludge levels.