Polishing Pond
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Facultative lagoons are a type of
waste stabilization pond Waste stabilization ponds (WSPs or stabilization ponds or waste stabilization lagoons) are ponds designed and built for wastewater treatment to reduce the organic content and remove pathogens from wastewater. They are man-made depressions confine ...
used for biological treatment of industrial and domestic
wastewater Wastewater (or waste water) is water generated after the use of freshwater, raw water, drinking water or saline water in a variety of deliberate applications or processes. Another definition of wastewater is "Used water from any combination of do ...
.
Sewage Sewage (or domestic sewage, domestic wastewater, municipal wastewater) is a type of wastewater that is produced by a community of people. It is typically transported through a sewerage, sewer system. Sewage consists of wastewater discharged fro ...
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.


Fundamentals


First pond

The facultative lagoon in the pond sequence functions like the primary clarifier of a conventional
sewage treatment Sewage treatment is a type of wastewater treatment which aims to remove contaminants from sewage to produce an effluent that is suitable to discharge to the surrounding environment or an intended reuse application, thereby preventing water p ...
system. Heavy solids will settle to the bottom of the lagoon, and lighter solids will float. This facultative lagoon lacks the
sludge Sludge (possibly , or some dialect related to slush) is a semi-solid slurry that can be produced from a range of industrial processes, from water treatment, wastewater treatment or on-site sanitation systems. It can be produced as a settled sus ...
removal capability of a primary clarifier, so a population of
anaerobic organism An anaerobic organism or anaerobe is any organism that does not require oxygen, molecular oxygen for growth. It may react negatively or even die if free oxygen is present. In contrast, an aerobic organism (aerobe) is an organism that requires an o ...
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 is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...
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 () is the set of metabolic pathways that breaks down molecules into smaller units that are either oxidized to release energy or used in other anabolic reactions. Catabolism breaks down large molecules (such as polysaccharides, lipid ...
on the bottom of the lagoon.Metcalf & Eddy (1972) pp.552-554 Areas with a consistently cool, but frost-free, climate may sustain facultative conditions in the first stabilization pond when treating lightly polluted water at low temperatures favorable to high concentrations of
dissolved oxygen Oxygen saturation (symbol SO2) is a relative measure of the concentration of oxygen that is dissolved or carried in a given medium as a proportion of the maximal concentration that can be dissolved in that medium at the given temperature. It can ...
with low
metabolic rate Metabolism (, from ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run cellular processes; the co ...
s. Facultative pond stratification becomes unstable during cold weather increasing release of malodorous gas when water temperatures drop below 4 degrees Celsius (39 degrees Fahrenheit); and formation of ice on the pond surface will effectively prevent transfer of atmospheric oxygen to the pond biome. Stabilization ponds in climates with significant seasonal temperature variation may release malodorous gas during the season of rising temperatures as the pond biome consumes wastes accumulated during cold weather with increasing metabolic rates exceeding the atmospheric oxygen transfer rate at the pond surface.


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.
Effluent Effluent is wastewater from sewers or industrial outfalls that flows directly into surface waters, either untreated or after being treated at a facility. The term has slightly different meanings in certain contexts, and may contain various pol ...
from the final polishing pond may be suitable for discharge to natural receiving waters.Hammer (1975) pp.399-402


Oxygen transfer and algae

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 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.
Algae Algae ( , ; : alga ) is an informal term for any organisms of a large and diverse group of photosynthesis, photosynthetic organisms that are not plants, and includes species from multiple distinct clades. Such organisms range from unicellular ...
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. Facultative stabilization ponds stratify with an aerobic surface layer and an anaerobic layer below the surface. The aerobic surface layer limits release of malodorous gas from the anaerobic benthic zone. Algae and
cyanobacteria Cyanobacteria ( ) are a group of autotrophic gram-negative bacteria that can obtain biological energy via oxygenic photosynthesis. The name "cyanobacteria" () refers to their bluish green (cyan) color, which forms the basis of cyanobacteri ...
typically grow in the aerobic zone and provide bacteria in the pond with plenty of oxygen during the daytime. However, algal
photorespiration Photorespiration (also known as the oxidative photosynthetic carbon cycle or C2 cycle) refers to a process in plant physiology, plant metabolism where the enzyme RuBisCO oxygenates RuBP, wasting some of the energy produced by photosynthesis. Th ...
may consume oxygen during night time when it is dark. Waste stabilization ponds with large algal populations may show significant diurnal fluctuation in oxygen concentrations with a peak in the late afternoon, and a minimum at dawn. Kinds of algae growing in treatment ponds include green, red and brown algae.Cincinnati, O. (2010, June 29).
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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.


Design 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 In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls from clouds due to gravitational pull. The main forms of precipitation include drizzle, rain, rain and snow mixed ("sleet" in Commonwe ...
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 (also known as BOD or biological oxygen demand) is an analytical parameter representing the amount of dissolved oxygen (DO) consumed by aerobic bacteria growing on the organic material present in a water sample at a s ...
(BOD) and
total suspended solids Total suspended solids (TSS) is the dry-weight of suspended particles, that are not dissolved, in a sample of water that can be trapped by a filter that is analyzed using a filtration apparatus known as sintered glass crucible. TSS is a water qu ...
(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 The Environmental Protection Agency (EPA) is an independent agency of the United States government tasked with environmental protection matters. President Richard Nixon proposed the establishment of EPA on July 9, 1970; it began operation on De ...
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.U.S. Environmental Protection Agency (EPA), Washington, D.C. "Treatment equivalent to secondary treatment." ''Code of Federal Regulations,'' . Accessed 2013-05-04.


Similar processes

The facultative lagoon may be replaced by an
aerated lagoon An aerated lagoon (or aerated pond) is a simple wastewater treatment system consisting of a pond with artificial aeration to promote the biological oxidation of wastewaters. There are many other aerobic biological processes for treatment of waste ...
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 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.Steel & McGhee (1979) p.501 Convent ...
.


See also

*
List of waste-water treatment technologies This page consists of a list of wastewater treatment technologies: See also *Agricultural wastewater treatment *Industrial wastewater treatment *List of solid waste treatment technologies * Waste treatment technologies *Water purification ...


Notes


References

* * * * * * EPA (2002)
"Facultative Lagoons."
Wastewater Technology Fact Sheet. Document no. EPA 832-F-02-014. {{pond Environmental engineering Pollution control technologies Sanitation Sewerage Sewerage infrastructure Waste treatment technology Water treatment