Practitioner's Tool / Types of Sewage Lagoons
Excerpt from the United States Environmental Protection Agency’s (EPA’s) Wastewater Technology Fact Sheet on Anaerobic Lagoons. Anaerobic lagoons are not aerated, heated, or mixed. The typical depth of an aerated lagoon is greater than eight feet, with greater depths preferred. Such depths minimize the effects of oxygen diffusion from the surface, allowing anaerobic conditions to prevail. In this respect, anaerobic lagoons are different from shallower aerobic or facultative lagoons, making the process analogous to that experienced with a single stage unheated anaerobic digester, except that anaerobic lagoons are in an open earthen basin. Moreover, conventional digesters are typically used for sludge stabilization in a treatment process, whereas lagoons typically are used to pretreat raw wastewater. Pretreatment includes separation of settleable solids, digestion of solids, and treatment of the liquid portion.
Purdue University's Extension program provides diagrams of anaerobic lagoons.
Excerpt from the EPA’s Wastewater Technology Fact Sheet on Facultative Lagoons. Facultative waste stabilization ponds, sometimes referred to as lagoons or ponds, are frequently used to treat municipal and industrial wastewater. These earthen lagoons are usually 1.2 to 2.4 m (4 to 8 feet) in depth and are not mechanically mixed or aerated. The layer of water near the surface contains dissolved oxygen due to atmospheric reaeration and algal respiration, a condition that supports aerobic and facultative organisms. The bottom layer of the lagoon includes sludge deposits and supports anaerobic organisms. The intermediate anoxic layer, termed the facultative zone, ranges from aerobic near the top to anaerobic at the bottom.
Excerpt from the EPA’s Wastewater Technology Fact Sheet on Aerated, Partial Mix Lagoons.
In aerated lagoons, oxygen is supplied mainly through mechanical or diffused aeration rather than by algal photosynthesis. Aerated lagoons typically are classified by the amount of mixing provided. A partial mix system provides only enough aeration to satisfy the oxygen requirements of the system and does not provide energy to keep all total suspended solids in suspension.
Aerobic lagoons, otherwise known as polishing ponds, are shallow impoundments, typically no more than 1 meter in depth. Dissolved oxygen concentrations are typically high due to wind and wave action that allow for natural reaeration. Aerobic ponds typically follow facultative or aerated ponds to provide final polishing by reducing biological demand, total suspended solids and pathogens through natural disinfection. Natural disinfection occurs as ultraviolet light penetrates the water column and kills the pathogens.
For more information: Safe Use of Household Greywater
Advanced Integrated Wastewater Pond Systems (AIWPS) have been developed by Professor William J. Oswald and his co-workers at the University of California, Berkeley over the past four decades. AIWPS are potentially feasible for application in the developing world. Although AIWPS may appear to be an adapted traditional pond system, each AIWPS facility designs and incorporates a series of low-cost ponds or earthwork reactors. A typical AIWPS facility consists of a minimum of four ponds in series: advanced facultative ponds, secondary facultative ponds or algal high rate ponds, algae settling ponds and maturation ponds.
AIWPSs do not require sludge management as it takes decades for sludge residues to accumulate to the point that it requires removal and disposal. Carbon is transformed in AIWPSs through two important mechanisms: methane fermentation and biological assimilation by microalgae. The conversion of waste organic solids to methane, nitrogen gas and carbon dioxide via methane fermentation and the assimilation of organic and inorganic carbon to algal biomass via photosynthesis provides the basis for primary, secondary and tertiary treatment in AIWPSs.
For more information: Advanced Integrated Wastewater Pond Systems (AIWPS).