Facultative lagoon
Encyclopedia
Facultative lagoons are a type of stabilization pond
used for biological treatment of industrial and domestic wastewater
. Sewage or organic waste from food or fiber processing may be catabolized in a system of constructed ponds where adequate space is available to provide an average waste retention time exceeding a month. A series of ponds prevents mixing of untreated waste with treated wastewater and allows better control of waste residence time for uniform treatment efficiency.
system. Heavy solids will settle to the bottom of the lagoon, and lighter solids will float. This facultative lagoon lacks the sludge
removal capability of a primary clarifier, so a population of anaerobic organism
s will colonize accumulated sludge on the bottom of the lagoon. The surface area of the lagoon should be large enough to provide an atmospheric oxygen
transfer rate adequate to prevent anaerobic conditions on the lagoon surface. Intermediate depths of the lagoon support facultative micro-organisms capable of oxidizing both the dissolved and suspended organics from the original wastewater and the products of anaerobic catabolism
on the bottom of the lagoon.
from the final polishing pond may be suitable for discharge to natural receiving waters.
can provide surface oxygen during daylight hours, but algal respiration can require additional oxygen during darkness. Ice or scum mats can reduce the oxygen transfer surface. Some facultative lagoons use mechanical surface aerators to increase atmospheric oxygen transfer, but aerator mixing depth should not re-suspend anaerobic sludge from the bottom of the lagoon. Aerator operation may be limited to periods of heavy waste loads, high temperatures, darkness, low wind velocity, or other conditions threatening to cause anaerobic conditions on the lagoon surface.
Precipitation
falling on the surface of the lagoons and polishing ponds will increase the volume of wastewater requiring disposal. Conversely, the volume of wastewater may be reduced by evaporation from the water surface in arid climates.
Wastewater nutrients may cause continuing growth of algae in the polishing ponds after the original wastes have been catabolized. Algae may cause measurable contribution to biochemical oxygen demand
(BOD) and total suspended solids
(TSS) concentrations where discharge regulations include limitations on those concentrations. The TSS contribution of algae tends to peak in the summer months, but the long-term BOD of decomposing algae may not be evident within the typical 5-day test. United States Environmental Protection Agency
regulations describe facultative lagoons as providing "treatment equivalent to secondary treatment" when 65 percent of influent BOD and TSS are removed and effluent BOD and TSS concentrations do not exceed a 7-day average of 65 mg/L and a 30-day average of 45 mg/L. Individual States may establish alternative effluent limitations.
as the first pond of the series. Aerated lagoons have mechanical aerators which minimize anaerobic zones by completely mixing the lagoon to achieve catabolism through a process called extended aeration
.
Stabilization pond
Stabilization pond technology - sometimes also called facultative pond technology - is a natural method for wastewater treatment.-Technology:Stabilization ponds consist of shallow man-made basins comprising a single or several series of anaerobic, facultative or maturation ponds...
used for biological treatment of industrial and domestic wastewater
Wastewater
Wastewater is any water that has been adversely affected in quality by anthropogenic influence. It comprises liquid waste discharged by domestic residences, commercial properties, industry, and/or agriculture and can encompass a wide range of potential contaminants and concentrations...
. Sewage or organic waste from food or fiber processing may be catabolized in a system of constructed ponds where adequate space is available to provide an average waste retention time exceeding a month. A series of ponds prevents mixing of untreated waste with treated wastewater and allows better control of waste residence time for uniform treatment efficiency.
The first pond is a facultative lagoon
The facultative lagoon in the pond sequence functions like the primary clarifier of a conventional sewage treatmentSewage treatment
Sewage treatment, or domestic wastewater treatment, is the process of removing contaminants from wastewater and household sewage, both runoff and domestic. It includes physical, chemical, and biological processes to remove physical, chemical and biological contaminants...
system. Heavy solids will settle to the bottom of the lagoon, and lighter solids will float. This facultative lagoon lacks the sludge
Sludge
Sludge refers to the residual, semi-solid material left from industrial wastewater, or sewage treatment processes. It can also refer to the settled suspension obtained from conventional drinking water treatment, and numerous other industrial processes...
removal capability of a primary clarifier, so a population of anaerobic organism
Anaerobic organism
An anaerobic organism or anaerobe is any organism that does not require oxygen for growth. It could possibly react negatively and may even die if oxygen is present...
s will colonize accumulated sludge on the bottom of the lagoon. The surface area of the lagoon should be large enough to provide an atmospheric oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
transfer rate adequate to prevent anaerobic conditions on the lagoon surface. Intermediate depths of the lagoon support facultative micro-organisms capable of oxidizing both the dissolved and suspended organics from the original wastewater and the products of anaerobic catabolism
Catabolism
Catabolism is the set of metabolic pathways that break down molecules into smaller units and release energy. In catabolism, large molecules such as polysaccharides, lipids, nucleic acids and proteins are broken down into smaller units such as monosaccharides, fatty acids, nucleotides, and amino...
on the bottom of the lagoon.
Subsequent polishing ponds
Overflow from the facultative lagoon may be routed through one or more polishing ponds supporting lower populations of anaerobic micro-organisms and a higher proportion of aerobic organisms adapted to survival in lower concentrations of organic material. EffluentEffluent
Effluent is an outflowing of water or gas from a natural body of water, or from a human-made structure.Effluent is defined by the United States Environmental Protection Agency as “wastewater - treated or untreated - that flows out of a treatment plant, sewer, or industrial outfall. Generally refers...
from the final polishing pond may be suitable for discharge to natural receiving waters.
Oxygen transfer
Objectionable odors are likely when the rate of oxygen transfer from the lagoon surface is less than the rate of oxygen consumption in the lower levels of the lagoon. A 1 acres (4,046.9 m²) facultative lagoon might provide 50 pounds of oxygen per day (5 grams of oxygen per square meter per day) for biochemical catabolism. Biological activity within a facultative lagoon varies directly with temperature. Warm weather will require large oxygen transfer rates, and waste accumulation during cold weather can cause short-term warm weather oxygen requirements to exceed long-term waste loading rates. AlgaeAlgae
Algae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms, such as the giant kelps that grow to 65 meters in length. They are photosynthetic like plants, and "simple" because their tissues are not organized into the many...
can provide surface oxygen during daylight hours, but algal respiration can require additional oxygen during darkness. Ice or scum mats can reduce the oxygen transfer surface. Some facultative lagoons use mechanical surface aerators to increase atmospheric oxygen transfer, but aerator mixing depth should not re-suspend anaerobic sludge from the bottom of the lagoon. Aerator operation may be limited to periods of heavy waste loads, high temperatures, darkness, low wind velocity, or other conditions threatening to cause anaerobic conditions on the lagoon surface.
Minimum temperature
Vertical stratification including an aerobic surface layer, an anaerobic bottom layer, and a facultative intermediate layer is essential to proper functioning of a facultative lagoon ecosystem. Stratification is maintained by a thermal gradient of cool, dense water at the bottom of the lagoon overlain by warmer, less dense water on the surface. This thermal gradient becomes unstable when water reaches its maximum density at 4 degrees Celsius (39 degrees Fahrenheit). Facultative lagoons are impractical in cold climates, because the lagoons become non-functional when cooler air temperatures depress water temperatures below this critical value.Other considerations
Inert solids in wastewater will accumulate on the bottom of the lagoon and gradually reduce depth until there is inadequate room for the facultative zone. Lagoon depths between 2 and 5 feet (60 to 150 cm) are preferred for effective treatment. Parallel facultative lagoons with common polishing ponds allow wastewater treatment to continue while one lagoon is out of service for sludge removal.Precipitation
Precipitation (meteorology)
In meteorology, precipitation In meteorology, precipitation In meteorology, precipitation (also known as one of the classes of hydrometeors, which are atmospheric water phenomena is any product of the condensation of atmospheric water vapor that falls under gravity. The main forms of precipitation...
falling on the surface of the lagoons and polishing ponds will increase the volume of wastewater requiring disposal. Conversely, the volume of wastewater may be reduced by evaporation from the water surface in arid climates.
Wastewater nutrients may cause continuing growth of algae in the polishing ponds after the original wastes have been catabolized. Algae may cause measurable contribution to biochemical oxygen demand
Biochemical oxygen demand
Biochemical oxygen demand or B.O.D. is the amount of dissolved oxygen needed by aerobic biological organisms in a body of water to break down organic material present in a given water sample at certain temperature over a specific time period. The term also refers to a chemical procedure for...
(BOD) and total suspended solids
Total suspended solids
Total suspended solids is a water quality measurement usually abbreviated TSS. It is listed as a conventional pollutant in the U.S. Clean Water Act. This parameter was at one time called non-filterable residue , a term that refers to the identical measurement: the dry-weight of particles trapped...
(TSS) concentrations where discharge regulations include limitations on those concentrations. The TSS contribution of algae tends to peak in the summer months, but the long-term BOD of decomposing algae may not be evident within the typical 5-day test. United States Environmental Protection Agency
United States Environmental Protection Agency
The U.S. Environmental Protection Agency is an agency of the federal government of the United States charged with protecting human health and the environment, by writing and enforcing regulations based on laws passed by Congress...
regulations describe facultative lagoons as providing "treatment equivalent to secondary treatment" when 65 percent of influent BOD and TSS are removed and effluent BOD and TSS concentrations do not exceed a 7-day average of 65 mg/L and a 30-day average of 45 mg/L. Individual States may establish alternative effluent limitations.
Similar facilities
The facultative lagoon may be replaced by an aerated lagoonAerated lagoon
An aerated lagoon or aerated basin is a holding and/or treatment pond provided with artificial aeration to promote the biological oxidation of wastewaters. There are many other biological processes for treatment of wastewaters, for example activated sludge, trickling filters, rotating biological...
as the first pond of the series. Aerated lagoons have mechanical aerators which minimize anaerobic zones by completely mixing the lagoon to achieve catabolism through a process called extended aeration
Extended aeration
Extended aeration is a method of sewage treatment using modified activated sludge procedures. It is preferred for relatively small waste loads, where lower operating efficiency is offset by mechanical simplicity.-Conventional sewage treatment:...
.