Why are we so concerned about Phosphorus in the environment ?
Phosphorus is a mineral nutrient essential for life. Cells require it to grow and proliferate. Elemental Phosphorus (P) does not occur as such. Being highly reactive, it is usually present as phosphorous (P-bearing) compounds, most common being PHOSPHATE (PO4-3). P is found in many ionic forms. Some of the common ionic forms of phosphate are the oxidized species of phosphoric acid (H3PO4), phosphorous acid (H2PO3), and hypophosphorous acid (H3PO2), along with their salts.
Often, in nature, there are 2 elements that may become limiting, controling the growth - Nitrogen and Phosphorus. One or the other impose constraints on growth, deciding just what will grow and what will not. It has been the limited availability of P in the special environment of the Florida Everglades that formed the whole ecology and appearance of the region.
As expanding and intensified agriculture and urban development contribute excess of P into the environment, undesirable changes occur. Toxic blue-green algae that can "fix" nitrogen from the air particularly proliferate when P becomes available. As the natural cycle of phosphorus has become disrupted, P has become a primary concern and challenge for the region.
Phosphorus is present in water primarily in 2 forms - organic phosphate and orthophosphate. Other species can be present, usually in smaller quantities. Total Phosphorus (TP) is typically the sum of organic and orthophosphate.
How do we control it and how do we remediate and remove that phosphorus ? Below are some P-removal techniques.
However, the fact remains - we currently do NOT have cost-effective techniques to control and remove P from the environment to low parts per billion (ppb, micrograms/liter) - on a huge scale as particularly desirable for the Everglades.
Proliferation of harmful algae in P-rich waters
Because it is a limiting nutrient, PHOSPHORUS is the most important element in the Everglades.
At the same time, it is crucial to the agriculture where it substantially improves crop yields. Unfortunately, it washes off the fields and most of it ends up where it creates ecological havoc in the environment. One way of minimizing this draw-back effect is to improve on the availability of P to the plants and make thus for its more efficient uptake by the crop plants.
Watch a short VIDEO showing how AVAIL Fertilizer Advancer does it :
Technology / Process Description
Illustration / Comment
PRECIPITATION and Solid / Liqud Separation
Usual best practical performance:
solubility of P,
By-Product: chemical sludge
Chemical phosphorus removal is brought about by addition of salts of multivalent metal ions to form precipitates of sparingly soluble phosphate. The commonly used chemicals are aluminum [Al(III)], ferric ion [Fe(III)] and calcium [Ca(II)]. The chemistry of P precipitation with Fe or Al is quite complex due to the formation of various metal-P complexes and metal hydroxyl complexes, as well as adsorption of P onto the precipitates. Depending on the dose ratio, either metal phosphate precipitation alone occurs, or both metal hydroxide and metal phosphate precipitation occurs. The lowest soluble P level that can be obtained is determined by the solubility of P, which depends on the dose ratio and pH. At pH around 7, the lowest residual soluble P concentrationof observed is 10 to 20 ppb (micro-g/L) for aluminum and 40-50 ppb for iron(III). Residual soluble P less the solubility limit has been observed as a result of P adsorption onto the precipitates or onto adsorbents added. With calcium and at high pH, residual P less than 6 ppb can be obtained. One tertiary treatment facility with a combination of biological or primary chemical P removal followed by tertiary polishing step achieved 170 ppb, another reached 70 ppb TP with tertiary sedimentation/filtration process involved.
♦ Soda ash
♦ Magnesium hydroxide
♦ Caustic soda
Pelletizing P-removal reactor:
Usual practical performance:
of P, min.
By-Product: biological sludge
Biological P removal relies on the function of a specific group of microorganisms that are capable of taking up P as intracellular storage and then the P is physically removed by "wasting" (separating) the biomass. Full-scale performance data indicate that effluent TP less than 1000 ppb is reliably achieved with bio-systems. The lowest TP observed at biological treatment facilities ranges from 100-300 ppb. Orthophosphate concentrations as low as 20 ppb have been achieved, below which the P concentration becomes limiting for biological reaction. This and other "process disturbances" are the reasons for frequently combining bio and chemical treatments. Coagulants could be added at different process points (1-3 examples below for an activated sludge wastewater treatment plant):
There are 2 limitations of the process:
♦ P-concentration limitation for the bios;
♦ solid-liquid separation of fine particles
This imposes a definite ceiling on the process performance and P concetrations achievable in the outflow.
♦ CONSTRUCTED WETLANDS
~ 20 ppbP
"Engineered" wetlands are artificially constructed with planted plants optimized and complementing each other in taking up phosporus. These systems that could be using also additional adsorbent on the floor, with selected micro-plants and microbes and with controlled flowrates and possible recycle flows, have been shown to produce effluent soluble P < 20 ppb. Variations of constructed wetlands are currently being used already on a large scale to reduce phosphorus in waters that proceed further to feed the Everglades (Stormwater Treatment Areas - STAs). Their performance has not been conclusively established as yet, mainly because they are relatively new and have been often operated under inflow P-overload conditions. The costs of land, construction and operation will together determine the feasibility of this P-removal alternative currently experimentally practiced for the Everglades.
For its optimum performance, the sorption process is most often configured as a packed-bed flow-through column - or multiple colums. The packing material (sorbent) has a special property of retaining the compound of interest (sorbate) from the fluid stream. The packed bed process configuration could be taken advantage of in having it function in varied configurations also as a deep-bed filter for retaining particulate matter. This dual function is particularly important in P removal since most often we deal with both forms of P - particulates and dissolved. Among inorganic sorbent materials, aluminum, iron and titanium based sorbents and waste materials containing these compounds (iron ore residues, slag, aluminum 'red mud', fly ash, etc.) are effective. SorbtiveMEDIA utilizes a high surface area and oxide coating, which sorbs dissolved phosphorus and physically fi lters particulate bound phosphorus by capturing fine sediment (Total Suspended Solids). Ion exchange is also a sorption process. Commercially produced (Dowex, Rohm & Haas) polymeric anionic exchange resins are effective in picking up phosphate ions. Their efficiency increases when suitably combined with hydrated ferric oxide nano-particles (HFO).
An important feature of the sorption process is the possibility of multiple regeneration and reuse of the sorbent material and the potential of recovering the collected compound.
Sorption columns scale up by multiplication of optimum-size column units. They could also function as deep-bed filters for fine particles.
By-Product: concentrated brine
Certain types of waste bio-organic materials can serve as potent biosorbents of ionic species. The main advantage of biosorbents, compared to other sorbent materials is their cost effectiveness which may be particularly significant for large-scale applications. Crab shells, containing chitin, are an example of an anionic type of a waste-based biosorbent suitable for removing anionic phosphate species from solutions.
Total phosphorus in water consists predominantly of particulate P and soluble P. To achieve effluent TP concentrations close to the soluble limit, particulate P must be completely removed. Solid-liquid separation processes that provide near complete removal of particulates (suspended solids and colloids) are therefore required such as deep-bed dual stage filtration, microfiltration (MF) or membrane biological reactor (MBR). For treatment goals of effluent TP lower than the solubility limits, adsorption using adsorbent media or reverse osmosis will be required.
MICROFILTRATION and REVERSE OSMOSIS
< 5 ppbP
By-Product: concentrated brine
Both are membrane-based prcesses. The combination of the two is currently the most advance, and the most expensive, treatment that can produce TP below detection limits (~5 ppb). While MF membranes (pore size 0.04-0.2 micro-m) remove all particulate P, high-pressure RO membranes further remove the soluble phosphate by ~(95-97)% to consistently achieve very low effluent P levels.
Reverse osmosis is similar to the membrane filtration treatment process. However, there are key differences between reverse osmosis and filtration. The predominant removal mechanism in membrane filtration is straining, or size exclusion, so the process can theoretically achieve perfect exclusion of particles regardless of operational parameters such as influent pressure and concentration. RO (Reverse Osmosis), however involves a diffusive mechanism so that separation efficiency is dependent on influent solute concentration, pressure and water flux rate . It works by using pressure to force a solution through a membrane, retaining the solute on one side and allowing the pure solvent to pass to the other side. This is the reverse of the normal osmosis process, which is the natural movement of solvent from an area of low solute concentration, through a membrane, to an area of high solute concentration when no external pressure is applied.
The membranes used for reverse osmosis have a dense barrier layer in the polymer matrix where most separation occurs. In most cases the membrane is designed to allow only water to pass through this dense layer while preventing the passage of solutes (such as salt ions). This process requires that a high pressure be exerted on the high concentration side of the membrane, usually 2–17 bar (30–250 psi) for fresh and brackish water, and 40–70 bar (600–1000 psi) for seawater, which has around 24 bar (350 psi) natural osmotic pressure that must be overcome.
This relatively "hi-tech" process uses high pressure and (sensitive) membranes. While widely used in desalination for fresh water, it is expensive to build and to operate.
Capable of below detection
By-Product: concentrated brine
Electric current pulls the ions through membranes that are permeable to only the positive or negative ions. Alternating positive or negative membranes, which number in the hundreds, are bound by a frame and form narrow compartments to trap the ions. When a DC electric current is applied, the positively charged ions tend to migrate through the membranes permeable to positive ions and the negatively charged ions tend to migrate through the membranes permeable to negative ions. By this process, ions move between the compartments and become more concentrated in one section while purified water remains in the other. Careful pre-treatment of the water to remove undesirable materials is usually necessary to protect the membranes.
The electrical power consumption required to separate the ions from the water, proportional to the feed ion concentration,would be cheaper than the resin and chemicals used in ion exchange. However, the scale of the process is somewhat limited.
Newer Technologies to achieve Extremely Low Phosphorus (ELP)
Depending on process combinations:
By-Product: chemical sludges, biological sludge,
The treatment technologies that have been successful in achieving very low effluent TP at pilot or full-scale applications that are commercially available include:
♦ Dual stage Parkson Dynasand filtration
It consists of continuous self-cleaning filters in series for ultimate filtration performance. The first contains large sand grain and the second polishing one fine sand. The third component of the system is the Lamella gravity settler which treats rejects from both filters. Chemicals are usually dosed before the first filter. Overall, the process claims to produce effluent with (10-50) ppb TP.
♦ Blue Water Vandal-ION technology
It includes a pre-reactor for chemical mixing and a moving-bed reactive filter filled with iron-oxide coated sand (IOCS), a good adsorbent for many types of pollutants. The filter provides both filtration and adsorption effects. Pilot studies produced (50-80) ppbTP.
♦ Microfiltration (MF) and Membrane Biological Reactor (MBR) processes
MF processes and MBRs both use membranes (0.04-0.2 micro-m pores) for solid-liquid separation.
The process can eliminate nearly all particulate matter including chemical precipitants and produce effluent TP close to the solubility limit of (20-50) ppbTP. Using MF as tertiary treatment with low TP feeds (~80 ppb), effluent TP of less than 10 ppb was achieved.
?? Experimental, below detection
~ 5 ppbP
Scaling up unknown
By-Product: (unknown sludges)
A number of developing technologies are capable of producing low effluent TP as evidenced by laboratory and pilot-scale studies. The developing technologies include:
♦ Packed columns with sorbent materials such as iron oxide particles, blast furnace slag, zeolite, iron/calcium oxides, crushed limestone, aluminum oxide, etc.
♦ Packed columns with biosorbent materials capable of removing dissolved P. These columns could be designed and operated to also effectively remove particulate P to very low TP values.
♦ Immobilized bacteria or algae on hollow cellulose fibers/beads at lab-scale produced effluent soluble P <20 ppb.
♦ Iron nano-particles, treated in different ways, have been examined with encouraging results. Nanotechnology may hold quite a promise that has not been exploited yet.