Chemical phosphorus removal

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Chemical phosphorus removal is a wastewater treatment method, where phosphorus is removed using salts of aluminum (e.g. alum or polyaluminum chloride), iron (e.g. ferric chloride), or calcium (e.g. lime). Phosphate forms precipitates with the metal ions and is removed together with the sludge in the separation unit (sedimentation tank, flotation tank, etc.). [1]

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Aluminum sulfate treatment to reduce phosphorus content of lakes

One method of eutrophication remediation is the application of aluminum sulfate, a salt commonly used in the coagulation process of drinking water treatment. Aluminum sulfate, or “alum” as it is commonly referred, has been found to be an effective lake management tool by reducing the phosphorus load. [2]

Sediment core sampled from a Minnesota lake. Aluminum Sulfate flocs are depicted as white clumps near the sediment surface. Aluminum Sulfate Floc in Lake Sediment.png
Sediment core sampled from a Minnesota lake. Aluminum Sulfate flocs are depicted as white clumps near the sediment surface.

Alum was first applied in 1968 to a lake in Sweden. [2] Its first application to an American lake followed in 1970. [3] Today, alum has been utilized with improved effectiveness and understanding. In a large scale study, 114 lakes were monitored for the effectiveness of alum at phosphorus reduction. Across all lakes, alum effectively reduced the phosphorus for 11 years. While there was variety in the longevity, (21 years in deep lakes and 5.7 years in shallow lakes), the results express the effectiveness of alum at controlling phosphorus within lakes. [4]

Mechanism

Alum treatment begins with the addition of aluminum sulfate salt to a water body. Once added, the salt dissolves and dissociates, introducing Al(III) ions to the water. The aluminum ions participate in a series of hydrolysis reactions, forming different aluminum species across pH ranges. As more aluminum sulfate is added, water pH decreases. At higher pH, the soluble species Al(OH)4− is present. In neutral pH ranges (6-8), the insoluble aluminum hydroxide (Al(OH)3) occurs. As pH decreases further, the Al(III) ion remains present. [2]

Maintaining optimal pH is important for the removal of phosphorus from water. Phosphorus is most effectively removed at the neutral pH range, when the insoluble aluminum hydroxide is present. This hydroxide functions as a Lewis acid, [5] creating a flocculation environment similar to conventional wastewater treatment. The insoluble Al(OH)3 floc adsorbs phosphorus, as well as other species, and removes them from the water column. As floc adsorption continues, the floc becomes larger, eventually settling to the bottom of the water column in the sediment. The resulting aluminum hydroxide layer covering the lake bottom additionally blocks the diffusion of phosphorus from sediment into the water column, further regulating internally loaded phosphorus. [4]

Implementation

For most alum treatments, aluminum sulfate salt is applied to substrate at the lake's bottom, within the hypolimnion. [6] The alum then reduces phosphorus levels by inactivating the phosphorus released from these lake sediments, thereby controlling phosphorus in the entire water column. This phosphorus supplied from within the lake sediments is known as "internally loaded" phosphorus, as opposed to "externally loaded" phosphorus supplied by sources outside the lake, such as runoff. [7]

Although alum is typically applied to the hypolimnion, reducing phosphorus universally within the lake, it may also be applied to the epilimnion or locally to point sources. [8] This style of alum treatment is similar to the use of alum in conventional water treatment, and is more effective at reducing externally loaded phosphorus than universal application of alum to the hypolimnion. When applied to the epilimnion, boats powered by an outboard motor are deployed onto a lake carrying aluminum sulfate. After determining the necessary dosage and location of the application, the aluminum sulfate is added to the surface of the lake near the wake of the outboard motor. This provides sufficient mixing of the aluminum sulfate within the epilimnion. [9]

The necessary dosage of alum is determined by a variety of parameters. Changes in pH, dissolved oxygen levels, metal content of lake sediment, and lake size are all important for consideration. [10] Alum dosage is calculated by scientist and engineers to increase the effectiveness.

Limitations

Alum treatment is less effective in deep lakes, as well as lakes with substantial external phosphorus loading. [7] In deep lakes, the inactivation of phosphorus is not spread throughout the entire water column, as it is in shallower lakes due to the localization of aluminum hydroxide to the hypolimnion. Furthermore, externally loaded phosphorus often diffuses slowly downward from the lake surface, limiting its interaction with aluminum hydroxide within the hypolimnion and allowing phosphorus accumulation higher in the water column. [8] Therefore, alum treatment is most effectively applied to shallow lakes with primarily internally loaded phosphorus. One exception is point sources of externally loaded phosphorus, which can be effectively regulated by direct application of aluminum sulfate to the source. [8]

Another physical property to be considered is the ability of a lake to withstand mixing in the water column. Lakes with a higher Osgood Index, a parameter used to determine the amount of mixing a that occurs in a lake due to wind, have been found to result in more effective alum treatment. Another parameter is the ratio of the watershed area to the lake surface area. Lakes with lower watershed to lake area ratios experienced greater longevity following treatment. These lakes tend to be correlated with longer residence times and tend to be influenced by internally loaded phosphorus which aids in successful treatment. [4] Regardless of application strategy, repeated alum treatment is often necessary for most lakes every 5 to 15 years. [8] The necessity of repeated treatment requires continuous management and phosphorus monitoring to ensure optimal effectiveness.

Biological implications are another important consideration of alum treatment. Treatments increase water clarity, which has been correlated with increased plant growth at greater depths within the lake. [8] Increased plant growth within lakes changes the character of the substrate, which is sometimes a factor in biodiversity. Lakes with benthic feeding fish such as carp tend to have lower success at removing phosphorus. These species forage in lake sediments which disturbs the aluminum hydroxide flocs binding phosphorus to the lake bottom. [4] An additional concern is that aluminum salts can acidify lakes, making them potentially toxic to aquatic organisms. [7] However, the aluminum sulfate dosage used for lake treatment is not often high enough to pose significant toxicity to fish, although declines in algae and invertebrates have been observed in treated lakes. The alum dosage is also insufficient to cause toxicity in humans, and is often similar to alum doses used in conventional drinking water treatment. [8] To reduce negative biological effects, the accepted limit for dissolved aluminum concentrations in a water body is 50 μg Al/L and pH should be restricted to a range of 5.5-9. [2]

Related Research Articles

<span class="mw-page-title-main">Eutrophication</span> Excessive plant growth in response to excess nutrient availability

Eutrophication is the process by which an entire body of water, or parts of it, becomes progressively enriched with minerals and nutrients, particularly nitrogen and phosphorus. It has also been defined as "nutrient-induced increase in phytoplankton productivity". Water bodies with very low nutrient levels are termed oligotrophic and those with moderate nutrient levels are termed mesotrophic. Advanced eutrophication may also be referred to as dystrophic and hypertrophic conditions. Eutrophication can affect freshwater or salt water systems. In freshwater ecosystems it is almost always caused by excess phosphorus. In coastal waters on the other hand, the main contributing nutrient is more likely to be nitrogen, or nitrogen and phosphorus together. This depends on the location and other factors.

<span class="mw-page-title-main">Iron(III)</span> The element iron in its +3 oxidation state

In chemistry, iron(III) refers to the element iron in its +3 oxidation state. In ionic compounds (salts), such an atom may occur as a separate cation (positive ion) denoted by Fe3+.

<span class="mw-page-title-main">Aluminium hydroxide</span> Chemical compound

Aluminium hydroxide, Al(OH)3, is found in nature as the mineral gibbsite and its three much rarer polymorphs: bayerite, doyleite, and nordstrandite. Aluminium hydroxide is amphoteric, i.e., it has both basic and acidic properties. Closely related are aluminium oxide hydroxide, AlO(OH), and aluminium oxide or alumina, the latter of which is also amphoteric. These compounds together are the major components of the aluminium ore bauxite. Aluminium hydroxide also forms a gelatinous precipitate in water.

<span class="mw-page-title-main">Water purification</span> Process of removing impurities from water

Water purification is the process of removing undesirable chemicals, biological contaminants, suspended solids, and gases from water. The goal is to produce water that is fit for specific purposes. Most water is purified and disinfected for human consumption, but water purification may also be carried out for a variety of other purposes, including medical, pharmacological, chemical, and industrial applications. The history of water purification includes a wide variety of methods. The methods used include physical processes such as filtration, sedimentation, and distillation; biological processes such as slow sand filters or biologically active carbon; chemical processes such as flocculation and chlorination; and the use of electromagnetic radiation such as ultraviolet light.

<span class="mw-page-title-main">Water treatment</span> Process that improves the quality of water

Water treatment is any process that improves the quality of water to make it appropriate for a specific end-use. The end use may be drinking, industrial water supply, irrigation, river flow maintenance, water recreation or many other uses, including being safely returned to the environment. Water treatment removes contaminants and undesirable components, or reduces their concentration so that the water becomes fit for its desired end-use. This treatment is crucial to human health and allows humans to benefit from both drinking and irrigation use.

<span class="mw-page-title-main">Epilimnion</span> Top layer of water in a thermally-stratified lake

The epilimnion or surface layer is the top-most layer in a thermally stratified lake.

<span class="mw-page-title-main">Potassium alum</span> Chemical compound

Potassium alum, potash alum, or potassium aluminium sulfate is a chemical compound: the double sulfate of potassium and aluminium, with chemical formula KAl(SO4)2. It is commonly encountered as the dodecahydrate, KAl(SO4)2·12H2O. It crystallizes in an octahedral structure in neutral solution and cubic structure in an alkali solution with space group P a −3 and lattice parameter of 12.18 Å. The compound is the most important member of the generic class of compounds called alums, and is often called simply alum.

<span class="mw-page-title-main">Flocculation</span> Process by which colloidal particles come out of suspension to precipitate as floc or flake

In colloidal chemistry, flocculation is a process by which colloidal particles come out of suspension to sediment in the form of floc or flake, either spontaneously or due to the addition of a clarifying agent. The action differs from precipitation in that, prior to flocculation, colloids are merely suspended, under the form of a stable dispersion and are not truly dissolved in solution.

<span class="mw-page-title-main">Ion exchange</span> Exchange of ions between an electrolyte solution and a solid

Ion exchange is a reversible interchange of one kind of ion present in an insoluble solid with another of like charge present in a solution surrounding the solid with the reaction being used especially for softening or making water demineralised, the purification of chemicals and separation of substances.

<span class="mw-page-title-main">Aluminium sulfate</span> Chemical compound

Aluminium sulfate is a salt with the formula Al2(SO4)3. It is soluble in water and is mainly used as a coagulating agent (promoting particle collision by neutralizing charge) in the purification of drinking water and wastewater treatment plants, and also in paper manufacturing.

Electrocoagulation (EC) is a technique used for wastewater treatment, wash water treatment, industrially processed water, and medical treatment. Electrocoagulation has become a rapidly growing area of wastewater treatment due to its ability to remove contaminants that are generally more difficult to remove by filtration or chemical treatment systems, such as emulsified oil, total petroleum hydrocarbons, refractory organics, suspended solids, and heavy metals. There are many brands of electrocoagulation devices available and they can range in complexity from a simple anode and cathode to much more complex devices with control over electrode potentials, passivation, anode consumption, cell REDOX potentials as well as the introduction of ultrasonic sound, ultraviolet light and a range of gases and reactants to achieve so-called Advanced Oxidation Processes for refractory or recalcitrant organic substances.

<span class="mw-page-title-main">Lake ecosystem</span> Type of ecosystem

A lake ecosystem or lacustrine ecosystem includes biotic (living) plants, animals and micro-organisms, as well as abiotic (non-living) physical and chemical interactions. Lake ecosystems are a prime example of lentic ecosystems, which include ponds, lakes and wetlands, and much of this article applies to lentic ecosystems in general. Lentic ecosystems can be compared with lotic ecosystems, which involve flowing terrestrial waters such as rivers and streams. Together, these two ecosystems are examples of freshwater ecosystems.

<span class="mw-page-title-main">Phosphorus cycle</span> Biogeochemical movement

The phosphorus cycle is the biogeochemical cycle that describes the movement of phosphorus through the lithosphere, hydrosphere, and biosphere. Unlike many other biogeochemical cycles, the atmosphere does not play a significant role in the movement of phosphorus, because phosphorus and phosphorus-based compounds are usually solids at the typical ranges of temperature and pressure found on Earth. The production of phosphine gas occurs in only specialized, local conditions. Therefore, the phosphorus cycle should be viewed from whole Earth system and then specifically focused on the cycle in terrestrial and aquatic systems.

Monomictic lakes are holomictic lakes that mix from top to bottom during one mixing period each year. Monomictic lakes may be subdivided into cold and warm types.

<span class="mw-page-title-main">Sodium alum</span> Inorganic compound

Sodium aluminium sulfate is the inorganic compound with the chemical formula NaAl(SO4)2·12H2O (sometimes written Na2SO4·Al2(SO4)3·24H2O). Also known as soda alum, sodium alum, or SAS, this white solid is used in the manufacture of baking powder and as a food additive. Its official mineral name is alum-Na (IMA symbol: Aum-Na).

<span class="mw-page-title-main">Phototrophic biofilm</span> Microbial communities including microorganisms which use light as their energy source

Phototrophic biofilms are microbial communities generally comprising both phototrophic microorganisms, which use light as their energy source, and chemoheterotrophs. Thick laminated multilayered phototrophic biofilms are usually referred to as microbial mats or phototrophic mats. These organisms, which can be prokaryotic or eukaryotic organisms like bacteria, cyanobacteria, fungi, and microalgae, make up diverse microbial communities that are affixed in a mucous matrix, or film. These biofilms occur on contact surfaces in a range of terrestrial and aquatic environments. The formation of biofilms is a complex process and is dependent upon the availability of light as well as the relationships between the microorganisms. Biofilms serve a variety of roles in aquatic, terrestrial, and extreme environments; these roles include functions which are both beneficial and detrimental to the environment. In addition to these natural roles, phototrophic biofilms have also been adapted for applications such as crop production and protection, bioremediation, and wastewater treatment.

Clarifying agents are used to remove suspended solids from liquids by inducing flocculation, causing the solids to form larger aggregates that can be easily removed after they either float to the surface or sink to the bottom of the containment vessel.

Phoslock is the commercial name for a bentonite clay in which the sodium and/or calcium ions are exchanged for lanthanum. The lanthanum contained within Phoslock reacts with phosphate to form an inert mineral known as rhabdophane. Phoslock is used in lake restoration projects to remove excess phosphorus from aquatic systems, thereby improving water quality and inducing biological recovery in impaired freshwater systems.

Deep-water aeration, also known as hypolimnetic aeration, describes the provision of oxygen from the atmosphere to meet oxygen demand in deep water without disrupting the natural stratification of the water above. This process promotes the development of aerobic conditions in deep water, leading to a significant reduction in phosphate dissolution and an improvement in sediment mineralization. Scientific studies support the effectiveness of implementing technical ventilation measures to maintain year-round aerobic conditions in the deep water, thereby restoring the natural balance of lakes.

Oilfield scale inhibition is the process of preventing the formation of scale from blocking or hindering fluid flow through pipelines, valves, and pumps used in oil production and processing. Scale inhibitors (SIs) are a class of specialty chemicals that are used to slow or prevent scaling in water systems. Oilfield scaling is the precipitation and accumulation of insoluble crystals (salts) from a mixture of incompatible aqueous phases in oil processing systems. Scale is a common term in the oil industry used to describe solid deposits that grow over time, blocking and hindering fluid flow through pipelines, valves, pumps etc. with significant reduction in production rates and equipment damages. Scaling represents a major challenge for flow assurance in the oil and gas industry. Examples of oilfield scales are calcium carbonate (limescale), iron sulfides, barium sulfate and strontium sulfate. Scale inhibition encompasses the processes or techniques employed to treat scaling problems.

References

  1. ↑ Wastewater engineering : treatment and reuse (4 ed.). McGraw-Hill. 2003. p. 501. ISBN   0070418780.
  2. 1 2 3 4 Kennedy, Robert H.; Cook, G. Dennis (June 1982). "Control of Lake Phosphorus with Aluminum Sulfate: Dose Determination and Application Techniques". Journal of the American Water Resources Association. 18 (3): 389–395. doi:10.1111/j.1752-1688.1982.tb00005.x. ISSN   1093-474X.
  3. ↑ Garrison, Paul J.; Knauer, Douglas R. (January 1984). "Long-Term Evaluation of Three Alum Treated Lakes". Lake and Reservoir Management. 1 (1): 513–517. doi: 10.1080/07438148409354568 . ISSN   1040-2381.
  4. 1 2 3 4 Martyn, Huser, Brian J. Egemose, Sara Harper, Harvey Hupfer, Michael Jensen, Henning Pilgrim, Keith M. Reitzel, Kasper Rydin, Emil Futter (2016). Longevity and effectiveness of aluminum addition to reduce sediment phosphorus release and restore lake water quality. Uppsala universitet, Limnologi. OCLC   1233676585.{{cite book}}: CS1 maint: multiple names: authors list (link)
  5. ↑ "Aluminium Coagulant - an overview | ScienceDirect Topics". www.sciencedirect.com. Retrieved 2022-03-15.
  6. ↑ Cooke, G. D., Heath, R. T., Kennedy, R. H., & McComos, M. R. (1982). Change in lake trophic state and internal phosphorus release after aluminum sulfate application 1. JAWRA Journal of the American Water Resources Association, 18(4), pp. 699-705.
  7. 1 2 3 Cooke, G. D., Welch, E. B., Martin, A. B., Fulmer, D. G., Hyde, J. B., & Schrieve, G. D. (1993). Effectiveness of Al, Ca, and Fe salts for control of internal phosphorus loading in shallow and deep lakes. Hydrobiologia, 253(1), 323-335.
  8. 1 2 3 4 5 6 "North American Lake Management Society (NALMS)". North American Lake Management Society (NALMS). Retrieved 2022-04-12.
  9. ↑ Ree, William R. (March 1963). "Emergency Alum Treatment of Open Reservoirs". Journal - American Water Works Association. 55 (3): 275–281. doi:10.1002/j.1551-8833.1963.tb01028.x. ISSN   0003-150X.
  10. ↑ Rydin, Emil; Welch, Eugene B (October 1998). "Aluminum dose required to inactivate phosphate in lake sediments". Water Research. 32 (10): 2969–2976. doi:10.1016/s0043-1354(98)00055-4. ISSN   0043-1354.
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