in the following description , for purposes of explanation and not limitation , specific details are set forth , such as particular applications , processes and plants , etc . in order to provide a thorough understanding of the present invention . however , it will be apparent to the man skilled in the art that the present invention may be practised in other embodiments that depart from these specific details . with reference to fig1 a plant for treatment of waste according to an embodiment of the present invention will be described . the oxidation stage is mainly a system or supercritical water oxidation ( scwo ) and includes a supply conduit 11 for wastewater , a high - pressure pump 12 , a heat exchanger 13 , a heater 14 , and a supply conduit 15 for oxygen , a scwo - reactor 16 , and output conduit 17 , a cooler 18 and a separator 19 . the scwo - reactor 16 includes two inlets 16 a and 16 b for the supply conduits 11 and 15 , and an outlet 16 c , to which the output conduit 17 is connected . the separator 19 comprises an inlet 19 a for the output conduit 17 , and three outlets 19 b , 19 c and 19 d for separation of gases , for instance co 2 , o 2 and n 2 , cleaned water and residue products . the recovery stage comprises a supply conduit 20 and a recovery tank 21 provided with * two inlets 21 a , 21 b and two outlets 21 c , 21 d . the supply conduit 20 is connected between the outlet 19 d of the separator and the inlet 21 a of the recovery tank . in operation , an alkali or an acid is supplied to the recovery tank via the inlet 21 b , wherein the alkali or the acid dissolves the product that is be recovered and this is output from the tank 21 through the outlet 21 c , whereas unsolved products or so called residue waster is exhausted from the tank 21 through the outlet 21 d . the recovery stage may be provided with further devices for separating different species dissolved in the alkali or the acid . more about this will be closer discussed below with reference to fig2 and 3 . the plant of fig1 is in its simplest version intended to treat waste containing organic material and a precipitation chemical , for instance an iron or aluminum salt , and a brief . description of the operation of such a plant follows . the waste may typically be sludge from a cleaning or a water plant . if the waste does not already contain water in suitable amount in order to be a pumpable sludge and in order to be oxidizable via supercritical water oxidation the waste is put into such consistence by means of thickening or diluting . a suitable dry content ought to be 10 - 15 %, and in order to obtain an effective plant the highest possible dry content is utilized . the sludge is pumped through the supply conduit 11 by means of a high - pressure pump , which increases the pressure to a supercritical pressure for water ( 22 , 13 mpa ). the sludge is further pumped through a heat exchanger and is preheated by hot , already reacted sludge , which is pumped from the scwo - reactor 16 through the output conduit 17 . the preheated sludge is further pumped towards the - heater 14 , which heats the sludge to a supercritical temperature ( 374 ° c .) for water , or close to such a temperature . the sludge is further pumped into the scwo - reactor 16 through the inlet 16 a . simultaneously herewith , oxygen or other oxidant is supplied to the scwo - reactor 16 through the inlet 16 b . as a result hereof the organic material contained in the waste is substantially completely oxidized through supercritical water oxidation and the precipitation chemical is converted into an oxide . if the sludge before the reaction has not really reached a supercritical temperature for water the sludge will anyhow react and the released reaction heat will increase the temperature to a supercritical temperature for water and the reaction velocity is increased . the reacted sludge is transported through the outlet 16 c and is guided through the heat exchanger 13 in order to give off heat to the still non - reacted sludge in the supply conduit 11 . then the reacted sludge is cooled in the cooler 18 or in other apparatus in order to take care of the heat , whereafter the sludge is introduced into the separator 19 through one inlet 19 a . the separator separates the products mainly by means of the phase , such that gases are guided through the outlet 19 b at the top , solid material , a so called residue product , is settled such that the main portion of the liquid phase , i . e . water may be exhausted through the outlet 19 c , and finally the solid phase is exhausted through the bottom outlet 19 d . the process in the scwo - reactor 16 converts rapidly and effectively organic material , which substantially comprises carbon and hydrogen , to carbon dioxide and water at a temperature and pressure above the critical point for water ( 374 ° c . and 22 , 13 mpa ) while releasing energy . the process is completely closed and the destruction efficiency is often higher than 99 . 9 %. if the non - reacted sludge only contains water , organic waste and the precipitation chemical , clean carbon dioxide is emitted through the outlet 19 b , optionally together with oxygen if a surplus of oxygen has been used in the reaction , while the residue product , which is exhausted at 19 d , comprises an oxide of the precipitation chemical . heavy metals present in the process are converted to their oxides , whereas the phosphorous is oxidized and is exhausted as a residue product at 19 d . smaller amounts of nitrogen compounds , amines and ammonia , which exist in the waste , are converted to molecular nitrogen and not to nox , which is an acidifying and fertilizing residue product , which is therefore not wanted in the gaseous phase . the nitrogen is outlet at 19 b together with the other gases . the residue products exhausted through the outlet 19 d exist , due to the nature of the scwo - reaction , in a finally , reactive form and is well suited for treatment to recover , inter alia , the precipitation chemical . thus , the residue products are transported through supply line 20 into the recovery tank 21 and alkali ( if the residue products comprise phosphorous ) or hydrochloric acid , particularly using shortage of acid ( if the oxidized residue products comprise aluminum ) is added through inlet 21 b , wherein the phosphorous is dissolved or the aluminum is dissolved to directly form polyaluminum chloride . insoluble material is settled and is exhausted through the outlet 21 d as a residue waste , while the recovered phosphorous or polyaluminum chloride is exhausted through outlet 21 c , possibly after filtration . with reference next to fig2 which illustrates a recovery system 21 usable in the plant of fig1 a first particular embodiment of the invention will be described . the residue product from the scwo - reactor is in this embodiment a slurry with a fine , very reactive powder comprising iron oxide , phosphorous oxide ( p 2 o 5 ), possibly oxides of heavy metals , and silicates and other insoluble residue products , which may originate for instance from sewage sludge from a cleaning plant . this residue product is collected in a thickener / storage tank 22 and is then filtered through a filter 23 in order to increase the dry content . the thickened and filtered residue product is collected in a reactor 24 , to which caustic solution ( naoh ) is added . it has surprisingly been seen that the phosphorous very effectively is dissolved in the caustic solution and forms sodium phosphate , which is believed to depend on the very finely divided and reactive nature of the residue product from the supercritical water oxidation , where the phosphorous and the iron are separated . the remaining matter iron oxide , possibly oxides of heavy metals , and silicates and other insoluble residue products remain in solid phase and can easily be filtered away in a filter 25 . as an alternative to caustic solution , ammonia may be used for formation of ammonium phosphate in solution . the solution is then transported to a second reactor 26 , to which lime ( cao or ca ( oh ) 2 ) is added . hereby calcium phosphate is precipitated , which can thus be separated from the solution in a third filter 27 . the caustic solution is then returned to the reactor 24 . a smaller amount of caustic solution has to be added due to losses in the system . calcium phosphate with a dry content of about 70 % is obtained at the third filter 27 , which is an excellent raw material for commercial fertilizers . obviously , the filter cake from the second filter 25 , which contains iron oxide , possibly oxides of heavy metals , and silicates and other insoluble residue products may be further treated in order to recover the iron salt and separate the heavy metals . this may be performed for instance by leaching in an acid and reduction followed by a respective filtering step or by means of other processes known within the technical field . with reference now to fig3 which illustrates a recovery system 21 , which may be used in the plant of fig1 a second particular embodiment of the invention will be described . the residue product from the scwo - reactor is in this embodiment a slurry with a fine , very reactive powder comprising an aluminum salt , possibly oxides of heavy metals , and silicate and other insoluble residue products , which may originate for instance from a water cleaning plant . this residue product is collected in a thickener / storage tank 30 and is then filtered through a filter 31 or other kind of dewatering device in order to increase the dry content . the thickened and filtered residue product is collected in a reactor 32 to which hydrochloric acid is added — preferably in stoichiometric shortage , particularly in 50 % shortage . the reactor 32 operates preferably at atmospheric pressure and at a temperature below the boiling point of water . it has surprisingly been seen that the aluminum very effectively and directly forms polyaluminum chloride , which is believed to depend on the finely divided and very reactive nature of the residue product from the supercritical water oxidation . thus , it is directly obtained with 50 % basicity without having to utilize pressure boiling or to produce the aluminum chloride by means of the stoichiometric reaction and thereafter to produce highly basic polyaluminum chloride by means of adding an alkali , which may be performed according to conventional processes . by means of the process for forming polyaluminum chloride according to the invention less amounts of hydrochloric acids are wasted and no alkali is needed . oxides of heavy metals will also be dissolved , but the heavy metals will be directly precipitated in a heavy metal reduction stage , in which pure aluminum is added in order to obtain the reduction here exemplified with copper . the heavy metals may then be filtered away in a filter 34 and may be taken care of / deposited in a suitable manner . the solution is then transported to a storage tank for storage of polyaluminum chloride , which may be reused as a flocculant / precipitation chemical . the particular embodiments of the invention illustrated in fig2 and 3 may certainly be combined in case the residue product contains both phosphorous and aluminum . it shall be realised by the man skilled in the art that the invention is applicable for waste other than those containing precipitation chemicals , for instance industrial and food stuff industry waste comprising organic material and phosphorous or heavy metals . the phosphorous may be recovered and the heavy metals can be separated in the above - described manner . it is obvious that the invention can be varied in a plurality of manners . all such variations , which are obvious for a man skilled in the art , are intended to fall within the scope of the appended patent claims .