Phosphorus elimination

Among phosphorus removal (also phosphorus removal , phosphorus removal , phosphate elimination or P-elimination ) are understood in the waste water treatment , the removal of phosphorus compounds from waste water in sewage treatment plants or when bathing ponds a water purification method. The term “elimination”, which is unusual in chemistry (in chemistry one speaks more of “conversion”), is derived from eliminare ( Latin : “drive out of the house”) and was introduced in wastewater technology in the 1980s. Phosphorus compounds act as fertilizers in water and are the main cause of the eutrophication of water. In particular, detergents containing phosphates, as they were almost exclusively produced until the 1990s, made a significant contribution to the accumulation of phosphate in municipal sewage systems and receiving waters .

In municipal sewage treatment plants, phosphates are nowadays removed from domestic sewage; two methods are suitable for phosphorus elimination:

1. chemical P-precipitation by adding precipitants and
2. the biological P-elimination in biological wastewater treatment plants.

In the case of bathing ponds, the same effect is achieved by letting pumped water flow through the water filter , binding it with sprinkled calcium peroxide and "harvesting" plants and algae that have grown in the water .

Chemical P precipitation

Dissolved phosphates can be converted into undissolved phosphates with the help of suitable precipitating agents and removed as solids from the wastewater (simultaneously with other solids). The separated phosphates are then part of the sewage sludge and either return to the natural cycle as a fertilizer or are enriched in the ash by incinerating sewage sludge and i. d. Usually deposited, but withdrawn from the natural cycle.

The following precipitants are used in sewage treatment plants:

• Iron chlorides (FeCl ₂ or FeCl ₃  )
• Iron chloride sulfate (FeClSO ₄  )
• Iron (II) sulphate / green salt (FeSO ₄  )
• Aluminum sulfate (Al ₂ (SO ₄ ) ₃  )
• Sodium aluminate (NaAl (OH) ₄  )
• Milk of lime (Ca (OH) ₂  )

Iron chloride sulfate and iron sulfate (green salt) are products that are created as secondary products in the extraction of titanium dioxide . With the exception of sodium aluminate and milk of lime, these are acidic Fe or Al salts which, when used, can reduce the pH value of the water and introduce additional anions, making biological wastewater treatment more difficult (depending on the acid buffer capacity of the water). The typical precipitation reaction is:

${\displaystyle \mathrm {[PO_{4}]^{3-}\ +\ FeCl_{3}\longrightarrow \ FePO_{4}\ +\ 3Cl^{-}} }$

${\displaystyle \mathrm {[PO_{4}]^{3-}\ +\ NaAl(OH)_{4}\longrightarrow \ AlPO_{4}\ +\ NaOH\ +\ 3OH^{-}} }$

${\displaystyle \mathrm {2[PO_{4}]^{3-}\ +\ 3Ca(OH)_{2}\longrightarrow \ Ca_{3}(PO_{4})_{2}\ +\ 6OH^{-}} }$

In municipal wastewater, the average phosphorus concentration is between 1 and 5 mg P / l and must be 0.5 to 1, depending on the size of the sewage treatment plant and the type of receiving water (small stream, large river, lake or sea) according to the provisions of the Waste Water Tax Act . 0 mg P / l can be reduced. The required amount of precipitants is calculated according to the stoichiometry:

• For 1 g P, 1.80 g Fe or 0.87 g Al or 1.94 g Ca or their corresponding salts are required.
• From 1 g of P, 4.87 g of FePO ₄ or 3.94 g of AlPO ₄ or 5.00 g of Ca ₃ (PO ₄ ) _{2 are formed} .

Most of the time, the precipitants are added directly to the biological purification stage (simultaneous precipitation), sometimes also into a waste water side stream or for pre-precipitation or post-precipitation. Practically every sewage treatment plant today is equipped in such a way that chemical precipitation can be carried out depending on the measured phosphate concentrations. In order to save precipitants, many sewage treatment plants also practice biological P removal.

Electrolytic precipitation

In swimming ponds , the electrolytic dissolution of iron electrodes is also used to release dissociated iron ions, because the anions of precipitants used in open water can affect the natural balance (especially zooplankton ).

The Fraunhofer Institute for IGB developed an electrochemical process using a magnesium - sacrificial anode , with the phosphates and nitrates from water in the form of struvite (ammonium magnesium phosphate hexahydrate: NH ₄ MgPO ₄ * 6 H ₂ O) can be deposited. This ammonium phosphate, which can also be extracted from wastewater, can be used as a low-uranium phosphate fertilizer (see also drinking water # uranium pollution and nitrate fertilizers ) in the context of the global shortage of phosphate .

Types of precipitation

Simultaneous precipitation

The simultaneous precipitation is a precipitation reaction in which dissolved in the water ions through the addition of a precipitating agent in a water-insoluble compound converted, and then filtered off. If two ions are precipitated at the same time with one precipitant, then there is simultaneous precipitation.

Example: In wastewater treatment, simultaneous precipitation is a process in municipal sewage treatment plants that removes phosphorus as phosphate from the wastewater. The precipitant (mostly iron sulfate , iron chloride or an aluminum salt ) is added before the aeration tank. The precipitation reaction takes place here at the same time (= simultaneous) with the biological cleaning processes in the aeration tank. The resulting poorly soluble metal-phosphorus compounds (iron and aluminum phosphates) remain in the secondary clarifier with the activated sludge and are thus removed from the wastewater. This leads to an increase in excess sludge, as the metal-phosphorus compounds are also produced in addition to the normal activated sludge.

Furthermore, the addition of the precipitants leads to an improved sludge structure and better settling properties. Simultaneous precipitation is therefore often carried out not only for reasons of nutrient removal, but also to improve the operation of the plant, since it improves the separation efficiency of the secondary clarifier.

Pre-precipitation

Pre-precipitation (also “pre-flocculation”) is the name given to the removal of phosphorus from the wastewater by converting it into undissolved metal salts . This precipitation takes place before the biological stage. For example, the primary clarifier can be used for settling.

The main focus in pre-precipitation is not the phosphate precipitation itself, but the relief of the biological stage. Outdated sewage treatment plants in particular are often overloaded. If the renewal is delayed for various reasons, the current discharge values ​​can also be maintained with these systems with the help of a pre-precipitation. Disadvantages of this process are a very high volume of primary sludge and a very high consumption of precipitants in the form of triple and higher valued metal salts, which are also particularly cost-intensive.

Post-precipitation

The phosphorus precipitation takes place after the secondary clarification. Trivalent metal salts are used. The post-precipitation ensures very good phosphorus runoff values, but the consumption of precipitant is very high. In addition, a downstream separation of the resulting flakes is necessary.

Biological phosphorus elimination

In biological phosphorus elimination (Bio-P for short), polyphosphate-accumulating organisms (PAO) are brought into a stressful situation in an anaerobic tank. If these microorganisms do not have oxygen available, they cannot breathe, which means that they should actually die. To prevent this, they release the phosphates stored in their cells, which releases energy that they use to survive. To support this process, the microorganisms must be provided with easily degradable organic substrate . When the microorganisms then get into an aerobic area of life, they take up the previously dissolved phosphate and store more phosphates in their cells, which then reduces the total phosphate content in the water.

Favorable conditions for Bio-P

• Cascade design and front-end loading
• High BOD ₅ in the anaerobic tank
• Little pre-dismantling in the sewer system

Unfavorable conditions for Bio-P

• Oxygen or nitrate water
• Primary treatment with good efficiency
• Thin and cold wastewater (a lot of extraneous water)

Because the Bio-P process is sensitive, chemical and biological phosphorus removal are combined in most sewage treatment plants.

Phosphorus elimination in bathing ponds

Phosphate absorbers used in water filters, such as zeolites , calcium peroxide- calcium carbonate mixtures, granulated iron (III) hydroxide , yellow-rusty iron or ion exchangers , bind phosphates by adsorbing them or reacting chemically to form poorly soluble phosphate compounds .

Alternatively, iron ions can also be released electrolytically (direct current electrolysis ) (see above ). The attempt to use rusting steel instead of stainless steel ladders and stairs can indeed contribute to the phosphate binding, but will not reduce the phosphate content of the bathing water sufficiently and cause problems with the grounding (see there ).

The resulting "poorly" soluble phosphate compounds have a substance-specific (and temperature-dependent) solubility in water. In accordance with this solubility, a phosphate residue always remains in the water in chemical equilibrium , which cannot be removed with the respective agent. This residual content can only be removed by plants and algae and "harvesting" them. To do this, plant remains (cut underwater plants or perennial stalks and leaf mass from marsh plants) must be removed from the pond or algae must be nested or filtered off.

The aim of phosphorus elimination in swimming ponds is to keep the nutrient content as low as possible, so that regularly occurring algal blooms occur less frequently at larger intervals. With every supply of fresh water (in Central Europe more evaporates from membrane ponds than is replenished by rainfall ) (also contained and permitted in drinking water ) phosphate levels are introduced again , which is why a drop in the water level is taken into account in swimming ponds (and taken into account in the bank design and membrane cover) and only refilled in autumn or spring.

With the regular control of the capillary barrier of foil ponds (because root pressure can shift the soil there) it is checked at the same time whether nutrient-rich surface water is flushed into the pond during heavy rain events, in order to then take countermeasures.

Problem

Sewage sludge ash from mono-incineration with a phosphorus content of 6–8%

In view of the finite nature of the earth's phosphate deposits and the vital importance of phosphorus (no plant growth is possible without phosphates), efforts are being made to recover phosphorus from waste water or sewage sludge using suitable methods, e.g. B. through the "Phostrip process", through the digestion of phosphate-rich sewage sludge ash or through MAP precipitation (with struvite = ammonium magnesium phosphate ).

In the Phostrip process, part of the phosphate-containing excess sludge from biology is sedimented in a separate tank (stripper) and the phosphate-rich decantate is treated separately with precipitants, e.g. B. with milk of lime ( calcium phosphate precipitation) or magnesium chloride ( struvite precipitation). Crystalline precipitation products are formed, which can be separated off relatively easily and fed to industrial use. However, because of the additional costs, these methods have not yet become established. In Berlin, however, fertilizer which is obtained through MAP precipitation is already being marketed on a large scale as fertilizer (" Berlin plant ").

literature

• Sewage technology . Springer Verlag, ISBN 3-540-13038-1
• Nitrogen and phosphorus in rivers . Working report of the ATV committee 2.1. In: Korrespondenz Abwasser , 11/87
• Phosphorus elimination through an alkaline clay solution . In: Korrespondenz Abwasser , 1/90

Web links

• Example of phosphate precipitation in a sewage treatment plant

Individual evidence

1. ↑ ePhos®: Electrochemical process for the recovery of phosphorus
2. ^ Daniel Frank: Experimental investigation and modeling of the precipitation of potassium-magnesium-phosphate ; Dissertation; Stuttgart, 2013 PDF file
3. ↑ Wastewater Lexicon: Phosphate Elimination .