Oil Bio Degradation

8/3/2019 Oil Bio Degradation

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Oil biodegradationPetroleum oil is toxic, andpollution of the environment by oil causes major ecologicalconcern. Oil spills of coastal regions and the open sea are poorly containable and mitigationis difficult; much of the oil can, however, be eliminated by the hydrocarbon-degrading

activities of microbial communities, in particular the hydrocarbonoclastic bacteria (HCB).These organisms can help remedy the ecological damage caused by oil pollution of marinehabitats. HCB also have potential biotechnological applications in the areas ofbioplastics and

biocatalysis.[1]

Biomagnification, also known as bioamplification orbiological magnification, is theincrease in concentration of a substance that occurs in a food chain as a consequence of:

Persistence (can't be broken down by environmental processes)

Food chain energetics

Low (or nonexistent) rate of internal degradation/excretion of the substance (often due towater-insolubility)

The following is an example showing how biomagnification takes place in nature: Ananchovy eats zooplankton that have tiny amounts of mercury that the zooplankton has pickedup from the water throughout the anchovie's lifespan. A tuna eats many of these anchovies

over its life, accumulating the mercury in each of those anchovies into its body. If themercury stunts the growth of the anchovies, that tuna is required to eat more little fish to stayalive. Because there are more little fish being eaten, the mercury content is magnified.

Biological magnification often refers to the process whereby certain substances such aspesticides or heavy metals move up the food chain, work their way into rivers or lakes, andare eaten by aquatic organisms such as fish, which in turn are eaten by large birds, animals orhumans. The substances become concentrated in tissues or internal organs as they move upthe chain. Bioaccumulants are substances that increase in concentration in living organismsas they take in contaminated air, water, or food because the substances are very slowlymetabolized or excreted.

Although sometimes used interchangeably with 'bioaccumulation,' an important distinction isdrawn between the two, and with bioconcentration, it is also important to distinct betweensustainable development andoverexploitationin biomagnification.

Bioaccumulation occurs within atrophic level, and is the increase in concentration of asubstance in certain tissues of organisms' bodies due to absorption from food and theenvironment.

Bioconcentration is defined as occurring when uptake from the water is greater thanexcretion (Landrum and Fisher, 1999)

Thus bioconcentration and bioaccumulation occur within an organism, and biomagnificationoccurs across trophic (food chain) levels.
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Biodilution is also a process that occurs to all trophic levels in an aquatic environment; it isthe opposite of biomagnification, thus a pollutant gets smaller in concentration as it

progresses up a food web.

Lipid, (lipophilic) or fat soluble substances cannot be diluted, broken down, or excreted inurine, a water-based medium, and so accumulate in fatty tissuesof an organism if the

organism lacks enzymesto degrade them. When eaten by another organism, fats are absorbedin the gut, carrying the substance, which then accumulates in the fats of the predator. Since ateach level of the food chain there is a lot of energy loss, a predator must consume many prey,including all of their lipophilic substances.

For example, though mercury is only present in small amounts in seawater, it is absorbed byalgae (generally as methylmercury). It is efficiently absorbed, but only very slowly excreted

by organisms (Croteau et al., 2005). Bioaccumulation and bioconcentration result in buildupin the adipose tissue of successive trophic levels: zooplankton, small nekton, larger fish etc.Anything which eats these fish also consumes the higher level of mercury the fish haveaccumulated. This process explains why predatory fish such asswordfish and sharks or birdslike osprey andeagles have higher concentrations of mercury in their tissue than could beaccounted for by direct exposure alone. For example, herring contains mercury atapproximately 0.01 ppm and shark contains mercury at greater than 1 ppm (EPA 1997).

Contents[hide]

1 Current status

2 Substances that biomagnify

2.1 Novel organic

substances

2.2 Inorganic substances

3 See also

4 References

[edit] Current statusIn a review of a large number of studies, Suedel et al. (1994) concluded that although

biomagnification is probably more limited in occurrence than previously thought, there isgood evidence that DDT,DDE, PCBs,toxaphene, and the organic forms ofmercury andarsenic do biomagnify in nature. For other contaminants, bioconcentration and

bioaccumulation account for their high concentrations in organism tissues. More recently,Gray (2002) reached a similar substances remaining in the organisms and not being diluted tonon-threatening concentrations. The success of top predatory-bird recovery (bald eagles,

peregrine falcons) in North America following the ban on DDT use in agriculture is testamentto the importance of biomagnification.

[edit] Substances that biomagnifyThere are two main groups of substances that biomagnify. Both are lipophilic and not easilydegraded. Novel organic substances are not easily degraded because organisms lack previousexposure and have thus not evolved specific detoxification and excretion mechanisms, asthere has been no selection pressure from them. These substances are consequently known as

'persistent organic pollutants' or POPs.
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Metals are not degradable because they are elements. Organisms, particularly those subject tonaturally high levels of exposure to metals, have mechanisms to sequester and excrete metals.Problems arise when organisms are exposed to higher concentrations than usual, which theycannot excrete rapidly enough to prevent damage. These metals are transferred in an organicform.

Treatment of industrial wastewaterThe different types of contamination of wastewater require a variety of strategies to removethe contamination.[3][4]

[edit] Solids removal

Most solids can be removed using simple sedimentation techniques with the solids recoveredas slurry or sludge. Very fine solids and solids with densities close to the density of water

pose special problems. In such case filtration orultrafiltration may be required. Although,flocculationmay be used, using alumsalts or the addition ofpolyelectrolytes.

[edit] Oils and grease removalMain article:API oil-water separator
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A typical API oil-water separator used in many industries

Many oils can be recovered from open water surfaces by skimming devices. Considered adependable and cheap way to remove oil, grease and other hydrocarbons from water, oilskimmers can sometimes achieve the desired level of water purity. At other times, skimmingis also a cost-efficient method to remove most of the oil before using membrane filters andchemical processes. Skimmers will prevent filters from blinding prematurely and keepchemical costs down because there is less oil to process.

Because grease skimming involves higher viscosity hydrocarbons, skimmers must beequipped with heaters powerful enough to keep grease fluid for discharge. If floating greaseforms into solid clumps or mats, a spray bar, aerator or mechanical apparatus can be used tofacilitate removal.[5]

However, hydraulic oils and the majority of oils that have degraded to any extent will alsohave a soluble or emulsified component that will require further treatment to eliminate.Dissolving or emulsifying oil using surfactants orsolventsusually exacerbates the problemrather than solving it, producing wastewater that is more difficult to treat.

The wastewaters from large-scale industries such as oil refineries,petrochemical plants,chemical plants, and natural gas processing plants commonly contain gross amounts of oiland suspended solids. Those industries use a device known as an API oil-water separatorwhich is designed to separate the oil and suspended solids from their wastewatereffluents.The name is derived from the fact that such separators are designed according to standards

published by the American Petroleum Institute(API).[4][6]

The API separator is a gravity separation device designed by usingStokes Law to define therise velocity of oil droplets based on theirdensityand size. The design is based on the
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specific gravitydifference between the oil and the wastewater because that difference ismuch smaller than the specific gravity difference between the suspended solids and water.The suspended solids settles to the bottom of the separator as a sediment layer, the oil rises totop of the separator and the cleansed wastewater is the middle layer between the oil layer andthe solids.[4]

Typically, the oil layer is skimmed off and subsequently re-processed or disposed of, and thebottom sediment layer is removed by a chain and flight scraper (or similar device) and asludge pump. The water layer is sent to further treatment consisting usually of aElectroflotationmodule for additional removal of any residual oil and then to some type of

biological treatment unit for removal of undesirable dissolved chemical compounds.

A typical parallel plate separator[7]

Parallel plate separators[7] are similar to API separators but they include tilted parallel plateassemblies (also known as parallel packs). The parallel plates provide more surface forsuspended oil droplets to coalesce into larger globules. Such separators still depend upon thespecific gravity between the suspended oil and the water. However, the parallel platesenhance the degree of oil-water separation. The result is that a parallel plate separatorrequires significantly less space than a conventional API separator to achieve the same degreeof separation.

[edit] Removal of biodegradable organics

Biodegradable organic material of plant or animal origin is usually possible to treat usingextended conventional wastewater treatment processes such asactivated sludgeortricklingfilter.[3][4] Problems can arise if the wastewater is excessively diluted with washing water or ishighly concentrated such as neat blood or milk. The presence of cleaning agents,disinfectants, pesticides, or antibiotics can have detrimental impacts on treatment processes.

[edit] Activated sludge process

Main article:Activated sludge
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A generalized, schematic diagram of an activated sludge process.

Activated sludge is abiochemical process for treating sewage and industrial wastewater thatuses air (oroxygen) and microorganismsto biologically oxidize organic pollutants, producinga waste sludge (orfloc) containing the oxidized material. In general, an activated sludge

process includes:

An aeration tank where air (or oxygen) is injected and thoroughly mixed intothe wastewater.

A settling tank (usually referred to as a "clarifier" or "settler") to allow thewaste sludge to settle. Part of the waste sludge is recycled to the aeration

tank and the remaining waste sludge is removed for further treatment andultimate disposal.

As a general process for most of the Industrial waste water the followingTechnologies are used.

1. ASP : Activated Sludge process 2. SAFF system of Submerged aerobic fixed film system3. MBBR : Moving bed bio reactor ( Anox invented this now is considered generictechnology) 4. MBR : Membrane Bioreactor 5. DAF clarifiers 6. TBR : Turbo bioreactorTechnology ( A patented technology of Wockoliver) 7. Filtration technologies Moreinformation about above can be found on various commercial manufactures likeWOIL

[edit] Trickling filter process

Main article: Trickling filter
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Image 1: A schematic cross-section of the contact face of the bed media in a

trickling filter

A typical complete trickling filter system

A trickling filter consists of a bed ofrocks,gravel,slag,peat moss, or plastic media overwhich wastewater flows downward and contacts a layer (or film) ofmicrobial slime coveringthe bed media. Aerobic conditions are maintained by forced air flowing through the bed or bynatural convection of air. The process involves adsorption oforganic compounds in thewastewater by the microbial slime layer, diffusion of air into the slime layer to provide theoxygen required for the biochemical oxidation of the organic compounds. The end productsinclude carbon dioxide gas, water and other products of the oxidation. As the slime layerthickens, it becomes difficult for the air to penetrate the layer and an inner anaerobic layer isformed.

The components of a complete trickling filter system are: fundamental components:
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A bed of filter medium upon which a layer of microbial slime is promoted anddeveloped.

An enclosure or a container which houses the bed of filter medium.

A system for distributing the flow of wastewater over the filter medium.

A system for removing and disposing of any sludge from the treated effluent.The treatment of sewage or other wastewater with trickling filters is among the oldest andmost well characterized treatment technologies.

A trickling filter is also often called a trickle filter, trickling biofilter, biofilter, biologicalfilterorbiological trickling filter.

[edit] Treatment of other organics

Synthetic organic materials including solvents, paints, pharmaceuticals, pesticides, cokingproducts and so forth can be very difficult to treat. Treatment methods are often specific tothe material being treated. Methods includeAdvanced Oxidation Processing,distillation,adsorption, vitrification,incineration, chemical immobilisation or landfill disposal. Somematerials such as some detergents may be capable of biological degradation and in suchcases, a modified form of wastewater treatment can be used.

[edit] Treatment of acids and alkalis

Acids and alkalis can usually be neutralised under controlled conditions. Neutralisationfrequently produces aprecipitate that will require treatment as a solid residue that may also

be toxic. In some cases, gasses may be evolved requiring treatment for the gas stream. Someother forms of treatment are usually required following neutralisation.

Waste streams rich inhardness ions as from de-ionisation processes can readily lose thehardness ions in a buildup of precipitated calcium and magnesium salts. This precipitation

process can cause severefurringof pipes and can, in extreme cases, cause the blockage ofdisposal pipes. A 1 metre diameter industrial marine discharge pipe serving a majorchemicals complex was blocked by such salts in the 1970s. Treatment is by concentration ofde-ionisation waste waters and disposal to landfill or by careful pH management of thereleased wastewater.

[edit] Treatment of toxic materials

Toxic materials including many organic materials, metals (such as zinc, silver, cadmium,thallium, etc.) acids, alkalis, non-metallic elements (such as arsenic orselenium) aregenerally resistant to biological processes unless very dilute. Metals can often be precipitatedout by changing the pH or by treatment with other chemicals. Many, however, are resistant to

treatment or mitigation and may require concentration followed by landfilling or recycling.Dissolved organics can be incineratedwithin the wastewater byAdvanced OxidationProcess.

[edit] See also

Pre-treatment

Pre-treatment removes materials that can be easily collected from the raw waste water before

they damage or clog the pumps and skimmers of primary treatment clarifiers (trash, treelimbs, leaves, etc.).
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[edit] Screening

The influent sewage water is screened to remove all large objects like cans, rags, sticks,plastic packets etc. carried in the sewage stream.[5] This is most commonly done with anautomated mechanically raked bar screen in modern plants serving large populations, whilstin smaller or less modern plants a manually cleaned screen may be used. The raking action of

a mechanical bar screen is typically paced according to the accumulation on the bar screensand/or flow rate. The solids are collected and later disposed in a landfill or incinerated. Barscreens or mesh screens of varying sizes may be used to optimize solids removal. If grosssolids are not removed they become entrained in pipes and moving parts of the treatment

plant and can cause substantial damage and inefficiency in the process. [6]:9

[edit] Grit removal

Pre-treatment may include a sand or grit channel or chamber where the velocity of theincoming wastewater is adjusted to allow the settlement of sand, grit, stones, and brokenglass. These particles are removed because they may damage pumps and other equipment.For small sanitary sewer systems, the grit chambers may not be necessary, but grit removal is

desirable at larger plants.[6]:10

An empty sedimentation tank at the treatment plant in Merchtem, Belgium.

[edit] Flow equalization

Clarifiers and mechanized secondary treatment are more efficient under uniform flowconditions. Equalization basins may be used for temporary storage of diurnal or wet-weatherflow peaks. Basins provide a place to temporarily hold incoming sewage during plantmaintenance and a means of diluting and distributing batch discharges of toxic or high-strength waste which might otherwise inhibit biological secondary treatment (including

portable toilet waste, vehicle holding tanks, and septic tank pumpers). Flow equalizationbasins require variable discharge control, typically include provisions for bypass andcleaning, and may also include aerators. Cleaning may be easier if the basin is downstream ofscreening and grit removal.[7]

[edit] Fat and grease removal

In some larger plants, fat and grease is removed by passing the sewage through a small tankwhere skimmers collect the fat floating on the surface. Air blowers in the base of the tank
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may also be used to help recover the fat as a froth. Many plants, however, use primaryclarifiers with mechanical surface skimmers for fat and grease removal.

[edit] Primary treatment

In the primary sedimentation stage, sewage flows through large tanks, commonly called

"primary clarifiers" or "primary sedimentation tanks." The tanks are used to settle sludgewhile grease and oils rise to the surface and are skimmed off. Primary settling tanks areusually equipped with mechanically driven scrapers that continually drive the collectedsludge towards a hopper in the base of the tank where it is pumped to sludge treatmentfacilities.[6]:9-11 Grease and oil from the floating material can sometimes be recovered forsaponification.

The dimensions of the tank should be designed to effect removal of a high percentage of thefloatables and sludge. A typical sedimentation tank may remove from 50 to 70 percent ofsuspended solids, and from 30 to 35 percent ofbiochemical oxygen demand(BOD) from thesewage.

[edit] Secondary treatment

Secondary treatment is designed to substantially degrade the biological content of thesewage which are derived from human waste, food waste, soaps and detergent. The majorityof municipal plants treat the settled sewage liquor using aerobic biological processes. To beeffective, thebiota require both oxygenand food to live. Thebacteria andprotozoa consume

biodegradable soluble organic contaminants (e.g. sugars, fats, organic short-chain carbonmolecules, etc.) and bind much of the less soluble fractions into floc. Secondary treatmentsystems are classified asfixed-film orsuspended-growth systems.

Fixed-film or attached growth systems include trickling filters, biotowers,and rotating biological contactors, where the biomass grows on media andthe sewage passes over its surface.[6]:11-13 The fixed-film principal has furtherdeveloped into Moving Bed Biofilm Reactors (MBBR), and Integrated Fixed-Film Activated Sludge (IFAS) processes. An MBBR system typically requiressmaller footprint than suspended-growth systems. (Black & Veatch)

Suspended-growth systems include activated sludge, where the biomass ismixed with the sewage and can be operated in a smaller space than tricklingfilters that treat the same amount of water. However, fixed-film systems aremore able to cope with drastic changes in the amount of biological materialand can provide higher removal rates for organic material and suspendedsolids than suspended growth systems.[6]:11-13

Roughing filters are intended to treat particularly strong or variable organic loads, typically

industrial, to allow them to then be treated by conventional secondary treatment processes.Characteristics include filters filled with media to which wastewater is applied. They aredesigned to allow high hydraulic loading and a high level of aeration. On larger installations,air is forced through the media using blowers. The resultant wastewater is usually within thenormal range for conventional treatment processes.
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A generalized, schematic diagram of an activated sludge process.

A filter removes a small percentage of the suspended organic matter, while the majority ofthe organic matter undergoes a change of character, only due to the biological oxidation andnitrification taking place in the filter. With this aerobic oxidation and nitrification, the organicsolids are converted into coagulated suspended mass, which is heavier and bulkier, and cansettle to the bottom of a tank. The effluent of the filter is therefore passed through asedimentation tank, called a secondary clarifier, secondary settling tank or humus tank.

[edit] Activated sludge

Main article:Activated sludge

In general, activated sludge plants encompass a variety of mechanisms and processes that usedissolvedoxygen to promote the growth of biological floc that substantially removes organicmaterial.[6]:12-13

The process traps particulate material and can, under ideal conditions, convertammonia tonitrite and nitrate ultimately to nitrogen gas. (See also denitrification).

A Typical Surface-Aerated Basin (using motor-driven floating aerators)
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[edit] Surface-aerated basins (Lagoons)

Many small municipal sewage systems in the United States (1 million gal./day or less) useaerated lagoons.[8]

Most biological oxidation processes for treating industrial wastewaters have in common the

use of oxygen (or air) and microbial action. Surface-aerated basins achieve 80 to 90 percentremoval of BOD with retention times of 1 to 10 days.[9] The basins may range in depth from1.5 to 5.0 metres and use motor-driven aerators floating on the surface of the wastewater. [9]

In an aerated basin system, the aerators provide two functions: they transfer air into the basinsrequired by the biological oxidation reactions, and they provide the mixing required fordispersing the air and for contacting the reactants (that is, oxygen, wastewater and microbes).Typically, the floating surface aerators are rated to deliver the amount of air equivalent to 1.8to 2.7 kgO2/kWh. However, they do not provide as good mixing as is normally achieved inactivated sludge systems and therefore aerated basins do not achieve the same performancelevel as activated sludge units.[9]

Biological oxidation processes are sensitive to temperature and, between 0 C and 40 C, the

rate of biological reactions increase with temperature. Most surface aerated vessels operate atbetween 4 C and 32 C.[9]

[edit] Constructed wetlands

Constructed wetlands (can either be surface flow or subsurface flow, horizontal or verticalflow), include engineered reedbeds and belong to the family of phytorestoration andecotechnologies; they provide a high degree of biological improvement and depending ondesign, act as a primary, secondary and sometimes tertiary treatment, also see

phytoremediation. One example is a small reedbed used to clean the drainage from theelephants' enclosure at Chester Zoo inEngland; numerous CWs are used to recycle the waterof the city of Honfleur in France and numerous other towns in Europe, the US, Asia and

Australia. They are known to be highly productive systems as they copy natural wetlands,called the "Kidneys of the earth" for their fundamental recycling capacity of the hydrologicalcycle in the biosphere. Robust and reliable, their treatment capacities improve as time go by,at the opposite of conventional treatment plants whose machinery age with time. They are

being increasingly used, although adequate and experienced design are more fundamentalthan for other systems and space limitation may impede their use.

[edit] Filter beds (oxidizing beds)

Main article: Trickling filter

In older plants and those receiving variable loadings, trickling filterbeds are used where the

settled sewage liquor is spread onto the surface of a bed made up ofcoke (carbonized coal),limestone chips or specially fabricated plastic media. Such media must have large surfaceareas to support the biofilms that form. The liquor is typically distributed through perforatedspray arms. The distributed liquor trickles through the bed and is collected in drains at the

base. These drains also provide a source of air which percolates up through the bed, keepingit aerobic. Biological films of bacteria, protozoa and fungi form on the medias surfaces andeat or otherwise reduce the organic content.[6]:12 Thisbiofilmis often grazed by insect larvae,snails, and worms which help maintain an optimal thickness. Overloading of beds increasesthe thickness of the film leading to clogging of the filter media and ponding on the surface.Recent advances in media and process micro-biology design overcome many issues withTrickling filter designs.

[edit] Soil bio-technology

Main article: Soil bio-technology
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A new process called soil bio-technology (SBT) developed at IIT Bombayhas showntremendous improvements in process efficiency enabling total water reuse, due to extremelylow operating power requirements of less than 50 joules per kg of treated water. [10] TypicallySBT systems can achieve chemical oxygen demand (COD) levels less than 10 mg/L fromsewage input of COD 400 mg/L.[11] SBT plants exhibit high reductions in COD values and

bacterial counts as a result of the very high microbial densities available in the media. Unlikeconventional treatment plants, SBT plants produce insignificant amounts of sludge,

precluding the need for sludge disposal areas that are required by other technologies.[12]

In the Indian context, conventional sewage treatment plants fall into systemic disrepair due to1) high operating costs, 2) equipment corrosion due to methanogenesis and hydrogensulphide, 3) non-reusability of treated water due to high COD (>30 mg/L) and highfecalcoliform (>3000 NFU) counts, 4) lack of skilled operating personnel and 5) equipmentreplacement issues. Examples of such systemic failures has been documented by SankatMochan Foundation at the Gangesbasin after a massive cleanup effort by the Indiangovernment in 1986 by setting up sewage treatment plants under the Ganga Action Planfailed to improve river water quality.

[edit] Biological aerated filters

Biological Aerated (or Anoxic) Filter (BAF) or Biofilters combine filtration with biologicalcarbon reduction, nitrification or denitrification. BAF usually includes a reactor filled with afiltermedia. The media is either in suspension or supported by a gravel layer at the foot of thefilter. The dual purpose of this media is to support highly active biomass that is attached to itand to filter suspended solids. Carbon reduction and ammonia conversion occurs in aerobicmode and sometime achieved in a single reactor while nitrate conversion occurs inanoxicmode. BAF is operated either in upflow or downflow configuration depending on designspecified by manufacturer.

Schematic diagram of a typical rotating biological contactor (RBC). The treated

effluent clarifier/settler is not included in the diagram.

[edit] Rotating biological contactors

Main article: Rotating biological contactor

Rotating biological contactors (RBCs) are mechanical secondary treatment systems, which

are robust and capable of withstanding surges in organic load. RBCs were first installed inGermany in 1960 and have since been developed and refined into a reliable operating unit.The rotating disks support the growth of bacteria and micro-organisms present in the sewage,
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which break down and stabilise organic pollutants. To be successful, micro-organisms needboth oxygen to live and food to grow. Oxygen is obtained from the atmosphere as the disksrotate. As the micro-organisms grow, they build up on the media until they are sloughed offdue to shear forces provided by the rotating discs in the sewage. Effluent from the RBC isthen passed through final clarifiers where the micro-organisms in suspension settle as a

sludge. The sludge is withdrawn from the clarifier for further treatment.A functionally similar biological filtering system has become popular as part of homeaquariumfiltration and purification. The aquarium water is drawn up out of the tank and thencascaded over a freely spinning corrugated fiber-mesh wheel before passing through a mediafilter and back into the aquarium. The spinning mesh wheel develops abiofilmcoating ofmicroorganisms that feed on the suspended wastes in the aquarium water and are alsoexposed to the atmosphere as the wheel rotates. This is especially good at removing wasteurea and ammonia urinated into the aquarium water by the fish and other animals.

[edit] Membrane bioreactors

Membrane bioreactors(MBR) combine activated sludge treatment with a membrane liquid-

solid separation process. The membrane component uses low pressure microfiltration orultrafiltration membranes and eliminates the need for clarification and tertiary filtration. Themembranes are typically immersed in the aeration tank; however, some applications utilize aseparate membrane tank. One of the key benefits of an MBR system is that it effectivelyovercomes the limitations associated with poor settling of sludge in conventionalactivatedsludge (CAS) processes. The technology permits bioreactor operation with considerablyhigher mixed liquor suspended solids (MLSS) concentration than CAS systems, which arelimited by sludge settling. The process is typically operated at MLSS in the range of 8,00012,000 mg/L, while CAS are operated in the range of 2,0003,000 mg/L. The elevated

biomass concentration in the MBR process allows for very effective removal of both solubleand particulate biodegradable materials at higher loading rates. Thus increased sludge

retention times, usually exceeding 15 days, ensure complete nitrification even in extremelycold weather.

The cost of building and operating an MBR is usually higher than conventional wastewatertreatment. Membrane filters can be blinded with grease or abraded by suspended grit and lacka clarifier's flexibility to pass peak flows. The technology has become increasingly popularfor reliably pretreated waste streams and has gained wider acceptance where infiltration andinflow have been controlled, however, and the life-cycle costs have been steadily decreasing.The small footprint of MBR systems, and the high quality effluent produced, make them

particularly useful for water reuse applications.[13]

[edit] Secondary sedimentation

Secondary Sedimentation tank at a rural treatment plant.
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The final step in the secondary treatment stage is to settle out the biological floc or filtermaterial through a secondary clarifier and to produce sewage water containing low levels oforganic material and suspended matter.

[edit] Tertiary treatment

The purpose of tertiary treatment is to provide a final treatment stage to raise the effluentquality before it is discharged to the receiving environment (sea, river, lake, ground, etc.).More than one tertiary treatment process may be used at any treatment plant. If disinfection is

practiced, it is always the final process. It is also called "effluent polishing."

[edit] Filtration

Sand filtration removes much of the residual suspended matter.[6]:22-23 Filtration overactivatedcarbon, also called carbon adsorption, removes residual toxins.[6]:19

[edit] Lagooning

A sewage treatment plant and lagoon in Everett, Washington, United States.

Lagooning provides settlement and further biological improvement through storage in largeman-made ponds or lagoons. These lagoons are highly aerobic and colonization by nativemacrophytes, especially reeds, is often encouraged. Small filter feeding invertebrates such as

Daphnia and species ofRotiferagreatly assist in treatment by removing fine particulates.

[edit] Nutrient removal

Wastewater may contain high levels of the nutrientsnitrogen andphosphorus. Excessiverelease to the environment can lead to a build up of nutrients, calledeutrophication, which

can in turn encourage the overgrowth of weeds,algae, and cyanobacteria (blue-green algae).This may cause an algal bloom, a rapid growth in the population of algae. The algae numbersare unsustainable and eventually most of them die. The decomposition of the algae by

bacteria uses up so much of oxygen in the water that most or all of the animals die, whichcreates more organic matter for the bacteria to decompose. In addition to causingdeoxygenation, some algal species produce toxins that contaminatedrinking watersupplies.Different treatment processes are required to remove nitrogen and phosphorus.

[edit] Nitrogen removal

The removal of nitrogen is effected through the biologicaloxidation of nitrogen fromammonia to nitrate (nitrification), followed by denitrification, the reduction of nitrate to

nitrogen gas. Nitrogen gas is released to the atmosphere and thus removed from the water.
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Nitrification itself is a two-step aerobic process, each step facilitated by a different type ofbacteria. The oxidation of ammonia (NH3) to nitrite (NO2

) is most often facilitated byNitrosomonas spp. (nitroso referring to the formation of a nitroso functional group). Nitriteoxidation to nitrate (NO3

), though traditionally believed to be facilitated byNitrobacterspp.(nitro referring the formation of a nitro functional group), is now known to be facilitated in

the environment almost exclusively byNitrospira spp.Denitrification requires anoxic conditions to encourage the appropriate biologicalcommunities to form. It is facilitated by a wide diversity of bacteria. Sand filters, lagooningand reed beds can all be used to reduce nitrogen, but the activated sludge process (if designedwell) can do the job the most easily.[6]:17-18 Since denitrification is the reduction of nitrate todinitrogen gas, an electron donoris needed. This can be, depending on the wastewater,organic matter (from faeces), sulfide, or an added donor likemethanol. The sludge in theanoxic tanks (denitrification tanks) must be mixed well (mixture of recirculated mixed liquor,return activated sludge [RAS], and raw influent) e.g. by usingsubmersible mixersin order toachieve the desired denitrification.

Sometimes the conversion of toxic ammonia to nitrate alone is referred to as tertiarytreatment.

Many sewage treatment plants use axial flow pumps to transfer the nitrified mixed liquorfrom the aeration zone to the anoxic zone for denitrification. These pumps are often referredto asInternal Mixed Liquor Recycle (IMLR) pumps.

[edit] Phosphorus removal

Each person excretes between 200 and 1000 grams of phosphorus annually. Studies of UnitedStates sewage in the late 1960s estimated mean per capita contributions of 500 grams in urineand feces, 1000 grams in synthetic detergents, and lesser variable amounts used as corrosionand scale control chemicals in water supplies.[14]Source control via alternative detergent

formulations has subsequently reduced the largest contribution, but the content of urine andfeces will remain unchanged. Phosphorus removal is important as it is a limiting nutrient foralgae growth in many fresh water systems. (For a description of the negative effects of algae,

seeNutrient removal). It is also particularly important for water reuse systems where highphosphorus concentrations may lead to fouling of downstream equipment such as reverseosmosis.

Phosphorus can be removed biologically in a process calledenhanced biological phosphorusremoval. In this process, specific bacteria, called polyphosphate accumulating organisms(PAOs), are selectively enriched and accumulate large quantities of phosphorus within theircells (up to 20 percent of their mass). When the biomass enriched in these bacteria isseparated from the treated water, these biosolids have a high fertilizervalue.

Phosphorus removal can also be achieved by chemicalprecipitation, usually with salts ofiron(e.g. ferric chloride), aluminum(e.g. alum), or lime.[6]:18 This may lead to excessive sludge

production as hydroxides precipitates and the added chemicals can be expensive. Chemicalphosphorus removal requires significantly smaller equipment footprint than biologicalremoval, is easier to operate and is often more reliable than biological phosphorus removal .Another method for phosphorus removal is to use granularlaterite.

Once removed, phosphorus, in the form of a phosphate-rich sludge, may be stored in a landfill or resold for use in fertilizer.

[edit] Disinfection

The purpose ofdisinfectionin the treatment of waste water is to substantially reduce thenumber ofmicroorganisms in the water to be discharged back into the environment for the
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later use of drinking, bathing, irrigation, etc. The effectiveness of disinfection depends on thequality of the water being treated (e.g., cloudiness, pH, etc.), the type of disinfection beingused, the disinfectant dosage (concentration and time), and other environmental variables.Cloudy water will be treated less successfully, since solid matter can shield organisms,especially from ultraviolet light or if contact times are low. Generally, short contact times,

low doses and high flows all militate against effective disinfection. Common methods ofdisinfection include ozone,chlorine, ultraviolet light, or sodium hypochlorite.[6]:16

Chloramine, which is used for drinking water, is not used in waste water treatment because ofits persistence. After multiple steps of disinfection, the treated water is ready to be released

back into the water cycle by means of the nearest body of water or agriculture. Afterwards,the water can be transferred to reserves for everyday human uses.

Chlorination remains the most common form of waste water disinfection inNorth Americadue to its low cost and long-term history of effectiveness. One disadvantage is thatchlorination of residual organic material can generate chlorinated-organic compounds thatmay be carcinogenicor harmful to the environment. Residual chlorine or chloramines mayalso be capable of chlorinating organic material in the natural aquatic environment. Further,

because residual chlorine is toxic to aquatic species, the treated effluent must also bechemically dechlorinated, adding to the complexity and cost of treatment.

Ultraviolet (UV) light can be used instead of chlorine, iodine, or other chemicals. Because nochemicals are used, the treated water has no adverse effect on organisms that later consumeit, as may be the case with other methods. UV radiation causes damage to the geneticstructure of bacteria, viruses, and otherpathogens, making them incapable of reproduction.The key disadvantages of UV disinfection are the need for frequent lamp maintenance andreplacement and the need for a highly treated effluent to ensure that the targetmicroorganisms are not shielded from the UV radiation (i.e., any solids present in the treatedeffluent may protect microorganisms from the UV light). In the United Kingdom, UV light is

becoming the most common means of disinfection because of the concerns about the impacts

of chlorine in chlorinating residual organics in the wastewater and in chlorinating organics inthe receiving water. Some sewage treatment systems in Canada and the US also use UV lightfor their effluent water disinfection.[15][16]

Ozone(O3) is generated by passing oxygen (O2) through a highvoltage potential resulting ina third oxygen atombecoming attached and formingO3. Ozone is very unstable and reactiveand oxidizes most organic material it comes in contact with, thereby destroying many

pathogenic microorganisms. Ozone is considered to be safer than chlorine because, unlikechlorine which has to be stored on site (highly poisonous in the event of an accidentalrelease), ozone is generated onsite as needed. Ozonation also produces fewer disinfection by-

products than chlorination. A disadvantage of ozone disinfection is the high cost of the ozonegeneration equipment and the requirements for special operators.

[edit] Odour control

Odours emitted by sewage treatment are typically an indication of an anaerobic or "septic"condition.[17] Early stages of processing will tend to produce foul smelling gases, withhydrogen sulfide being most common in generating complaints. Large process plants in urbanareas will often treat the odours with carbon reactors, a contact media with bio-slimes, smalldoses ofchlorine, or circulating fluids to biologically capture and metabolize the obnoxiousgases.[18] Other methods of odour control exist, including addition of iron salts,hydrogen

peroxide, calcium nitrate, etc. to manage hydrogen sulfidelevels. High-density solids pumpsare suitable to reduce odours by conveying sludge through hermetic closed pipework.

[edit] Package plants and batch reactors
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To use less space, treat difficult waste and intermittent flows, a number of designs of hybridtreatment plants have been produced. Such plants often combine at least two stages of thethree main treatment stages into one combined stage. In the UK, where a large number ofwastewater treatment plants serve small populations, package plants are a viable alternativeto building a large structure for each process stage. In the US, package plants are typically

used in rural areas, highway rest stops and trailer parks.[19]

One type of system that combines secondary treatment and settlement is the sequencing batchreactor(SBR). Typically, activated sludge is mixed with raw incoming sewage, and thenmixed and aerated. The settled sludge is run off and re-aerated before a proportion is returnedto the headworks.[20] SBR plants are now being deployed in many parts of the world.

The disadvantage of the SBR process is that it requires a precise control of timing, mixingand aeration. This precision is typically achieved with computer controls linked to sensors.Such a complex, fragile system is unsuited to places where controls may be unreliable, poorlymaintained, or where the power supply may be intermittent. Extended aerationpackage plantsuse separate basins for aeration and settling, and are somewhat larger than SBR plants withreduced timing sensitivity.[21]

Package plants may be referred to as high chargedorlow charged. This refers to the way thebiological load is processed. In high charged systems, the biological stage is presented with ahigh organic load and the combined floc and organic material is then oxygenated for a fewhours before being charged again with a new load. In the low charged system the biologicalstage contains a low organic load and is combined withflocculate for longer times.

[edit] Sludge treatment and disposalMain article: Sewage sludge treatment

The sludges accumulated in a wastewater treatment process must be treated and disposed of

in a safe and effective manner. The purpose of digestion is to reduce the amount oforganicmatterand the number of disease-causingmicroorganisms present in the solids. The mostcommon treatment options include anaerobic digestion,aerobic digestion, and composting.Incineration is also used albeit to a much lesser degree.[6]:19-21

Sludge treatment depends on the amount of solids generated and other site-specificconditions. Composting is most often applied to small-scale plants with aerobic digestion formid sized operations, and anaerobic digestion for the larger-scale operations.

[edit] Anaerobic digestion

Main article:Anaerobic digestion

Anaerobic digestion is a bacterial process that is carried out in the absence of oxygen. Theprocess can either be thermophilic digestion, in which sludge isfermentedin tanks at atemperature of 55C, ormesophilic, at a temperature of around 36C. Though allowingshorter retention time (and thus smaller tanks), thermophilic digestion is more expensive interms of energy consumption for heating the sludge.

Anaerobic digestion is the most common (mesophilic) treatment of domestic sewage in septictanks, which normally retain the sewage from one day to two days, reducing the BOD byabout 35 to 40 percent. This reduction can be increased with a combination of anaerobic andaerobic treatment by installingAerobic Treatment Units (ATUs) in the septic tank.

One major feature of anaerobic digestion is the production ofbiogas (with the most usefulcomponent being methane), which can be used in generators for electricity production and/or

in boilers for heating purposes.
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[edit] Aerobic digestion

Aerobic digestion is a bacterial process occurring in the presence of oxygen. Under aerobicconditions, bacteria rapidly consume organic matter and convert it into carbon dioxide. Theoperating costs used to be characteristically much greater for aerobic digestion because of theenergy used by the blowers, pumps and motors needed to add oxygen to the process.

Aerobic digestion can also be achieved by usingdiffuser systems orjet aerators to oxidize thesludge. Fine bubble diffusers are typically the more cost-efficient diffusion method, however,

plugging is typically a problem due to sediment settling into the smaller air holes. Coarsebubble diffusers are more commonly used in activated sludge tanks (generally a side processin waste water management) or in the flocculation stages. A key component for selectingdiffuser type is to ensure it will produce the required oxygen transfer rate.

[edit] Composting

Composting is also an aerobic process that involves mixing the sludge with sources of carbonsuch as sawdust, straw or wood chips. In the presence of oxygen, bacteria digest both the

wastewater solids and the added carbon source and, in doing so, produce a large amount ofheat.[6]:20

[edit] Incineration

Incineration of sludge is less common because of air emissions concerns and thesupplemental fuel (typically natural gases or fuel oil) required to burn the low calorific valuesludge and vaporize residual water. Stepped multiple hearth incinerators with highresidencetime and fluidized bed incinerators are the most common systems used to combustwastewater sludge. Co-firing in municipal waste-to-energy plants is occasionally done, thisoption being less expensive assuming the facilities already exist for solid waste and there isno need for auxiliary fuel.[6]:20-21

[edit] Sludge disposal

When a liquid sludge is produced, further treatment may be required to make it suitable forfinal disposal. Typically, sludges are thickened (dewatered) to reduce the volumestransported off-site for disposal. There is no process which completely eliminates the need todispose of biosolids. There is, however, an additional step some cities are taking to superheatsludge and convert it into small pelletized granules that are high in nitrogen and other organicmaterials. InNew York City, for example, several sewage treatment plants have dewateringfacilities that use large centrifuges along with the addition of chemicals such as polymer tofurther remove liquid from the sludge. The removed fluid, called centrate, is typicallyreintroduced into the wastewater process. The product which is left is called "cake" and that

is picked up by companies which turn it into fertilizer pellets. This product is then sold tolocal farmers and turf farms as a soil amendment or fertilizer, reducing the amount of spacerequired to dispose of sludge in landfills. Much sludge originating from commercial orindustrial areas is contaminated with toxic materials that are released into the sewers from theindustrial processes.[22] Elevated concentrations of such materials may make the sludgeunsuitable for agricultural use and it may then have to be incinerated or disposed of tolandfill.

[edit] Treatment in the receiving environment
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The outlet of the Karlsruhe sewage treatment plant flows into the Alb.

Many processes in a wastewater treatment plant are designed to mimic the natural treatment

processes that occur in the environment, whether that environment is a natural water body orthe ground. If not overloaded, bacteria in the environment will consume organiccontaminants, although this will reduce the levels of oxygen in the water and maysignificantly change the overall ecology of the receiving water. Native bacterial populationsfeed on the organic contaminants, and the numbers of disease-causing microorganisms arereduced by natural environmental conditions such as predation or exposure toultravioletradiation. Consequently, in cases where the receiving environment provides a high level ofdilution, a high degree of wastewater treatment may not be required. However, recentevidence has demonstrated that very low levels of specific contaminants in wastewater,includinghormones (from animal husbandry and residue from human hormonalcontraceptionmethods) and synthetic materials such asphthalatesthat mimic hormones intheir action, can have an unpredictable adverse impact on the natural biota and potentially onhumans if the water is re-used for drinking water.[23]In the US andEU, uncontrolleddischarges of wastewater to the environment are not permitted under law, and strict waterquality requirements are to be met, as clean drinking water is essential. (For requirements inthe US,see Clean Water Act.) A significant threat in the coming decades will be theincreasing uncontrolled discharges of wastewater within rapidly developing countries.

[edit] Effects on Biology

Sewage treatment plants can have multiple effects on nutrient levels in the water that thetreated sewage flows into. These effects on nutrients can have large effects on the biologicallife in the water in contact with the effluent. Stabilization ponds(or treatment ponds) can

include any of the following: Oxidation ponds, which are aerobic bodies of water usually 12 meters in

depth that receive effluent from sedimentation tanks or other forms ofprimary treatment.

Dominated by algae

Polishing ponds are similar to oxidation ponds but receive effluent from anoxidation pond or from a plant with an extended mechanical treatment.

Dominated by zooplankton

Facultative lagoons, raw sewage lagoons, or sewage lagoons areponds where sewage is added with no primary treatment other than

coarse screening. These ponds provide effective treatment when the
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