DoTT: Phosphorus recovery

Phosphorus is one of the – so called – “elements of life”. Given the fact that – except singular cases – there are no phosphorus sources in Europe, and the ore deposits located mostly near the banks of north-western Africa are extensively exploited, European Commission in 2014 has added phosphate rock to the list of 20 Critical Raw Materials, for which supply security is at risk and economic importance is high. The consumption of phosphate-based fertilizers on a global scale is very high, because of the demand from food / agriculture sector.

All of the above resulting in search for alternative phosphorus sources, the ones that allow the phosphorus recovery. The fact that sewage sludge is a potential source of phosphorus is obvious for years already. Thanks to many projects and huge amount of research work that is still being performed the phosphorus recovery from sewage sludge concept has many well documented variations. Some of them are being close to or already are viable. Examples of well-documented technologies of phosphorus recovery from sewage sludge:

KREPRO method

a Kemira company technology – wet method, which was created in the 90’s. It is a typical hydrolysis process using a strong oxidant (sulfuric acid). Typical sludge concentration for this process is 4-6% dry matter. Such sludge, after being mixed with sulfuric acid at pH of approx. 1-2,5, is subject to pressure heating in chemical reactor to a temperature of about 1400C at a certain overpressure. After hydrolysis, the reaction mass is emptied into a buffer tank and after cooling, the contents are subject to filtration process either on filter presses or centrifuges. Sludge (organic fraction) with a dry matter content of approx. 45% is obtained. It can be further utilized, e.g. by thermal methods. The solution is placed in a mixer, where after adding a little coagulant in the form of a solution of iron sulphate III and neutralization with sodium hydroxide solution (for pH increase) iron orthophosphates precipitate. Such mixture goes thru filtration process, where a cake in the form of a phosphate with a content of about 35% of dry matter and impure water is obtained. Water is recycled at the waste water treatment plant.

Kemira took the Krepro process to the next stage, which is called the Kemicond process. In this process, which is also a hydrolysis, hydrogen peroxide solution of approx. 50% is used. Hydrolysis takes place at a slightly higher pH, i.e. about 3.5 at a temperature of about 200C, and the hydrogen peroxide is used as a strong oxidant to prevent the reduction of iron III ions, which are beneficial for the precipitation of iron orthophosphates. Kemira uses both of these methods especially where the wastewater treatment process is carried out with the help of coagulants in the form of iron sulphate or chloride III, precipitating phosphorus very effectively. The combination of these methods also causes that in addition to orthophosphate precipitation, some of the iron salts are also recovered as coagulants.

Biocon technology

uses ash generated by sludge incineration at a temperature of approx. 900°C. Because ashes – by definition – have diverse granulation, they must be grinded before starting proper process in the reactor for a more efficient reaction with the added chemicals. The ashes are mixed with sulfuric acid, which is a strong oxidant, until a low pH of about 1 is achieved. Under these conditions, orthophosphoric acid contaminated with various metals is created. To get rid of metals, ion exchange is used (anion and cation). Using KCl and HCl for example, metals are converted into solutions of their chloride salts and removed. The resulting pure orthophosphoric acid is neutralized with calcium to become an absorbable character of phosphate in the form of calcium phosphate.

Aqua Reci technology 

It is a wet technology because it uses sewage sludge e.g. after thickening. This method involves the oxidation of the organic fraction of the sediment in an atmosphere of pure oxygen under supercritical water conditions. Sodium hydroxide is added to the inorganic fraction, with the help of which phosphorus is leached, further subject to react with calcium oxide, resulting in acid calcium phosphate. This process occurs at high pH, due to which the heavy metals contained in the sludge remain undissolved.

Sephos technology

uses ash from sludge incineration, containing – among the others – aluminium compounds and heavy metals. The ashes are mixed with sulfuric acid in the reactor, maintaining the pH below 1.5. The post-reaction suspension is transferred into a filtration device, e.g. a filter press or centrifuge, where the so-called filtrate is separated from the filter cake. The solution is fed to another reactor and sodium hydroxide is added to raise the pH to about 3.5. This process produces aluminium phosphate. At the same time, most heavy metal compounds precipitate. The slurry from the reactor is subject to another filtration process, where the solution (filtrate) is separated in the form of waste reaction acid, which is recycled to the beginning of the process and mixed with ashes. Meanwhile the precipitate created after filtration is fed to another reactor, to which sodium hydroxide is added to raise the pH to a value of about 13. As a result of this neutralization process, the aluminium phosphate formed in the previous process is dissolved and the remaining metals are separated and transform into a solid state. Such suspension is subject to filtration, as a result of which a cake with heavy metals and other impurities is separated from the phosphate solution. Phosphate solution, in another reactor is treated with calcium oxide. As a result of this process, easily absorbable calcium phosphates are precipitated. The post-reaction suspension is subject to final filtration process, after which the cake is obtained in the form of easily absorbed phosphates. The effluent is recycled to the sewage treatment plant. The filter cake in the form of easily absorbed phosphates can be mixed with other types of fertilizers or directly used for fertilizing.

LOPROX technology

Based on the oxidation of sewage sludge with pure oxygen. The process takes place in the reactor under increased pressure of about 12-28 bar, at a temperature of about 160-220°C in an acidic environment at a low pH of <1,5. An acidic environment is created by adding sulfuric acid to the reaction. As a result of the reaction, the content of organic compounds is significantly reduced. In addition, compounds such as pharmaceuticals, organic micro-impurities and polycyclic aromatic hydrocarbons are degraded. Phosphates migrate into solution. They are mainly in the form of phosphoric acid. The solution is separated from the solid waste by filtration. Acidic solution after the filtration is neutralized with e.g. calcium oxide, resulting in easily digestible fertilizer phosphates.

Main parameter: Phosphorus recovered

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