MXPA99008418A - A method for treating sludge from wastewater treatment - Google Patents

Abstract

A method for treating sludge from wastewater purification is described. In the method, a sludge is treated, which contains phosphorus and at least one metal which originates from precipitation chemicals and which is selected among divalent iron and aluminium, the pH of the sludge being adjusted to below 4, preferably below 2, for dissolving the content of phosphorus and said metal in the sludge;the remaining sludge is separated;the solution relieved of sludge and containing phosphorus and said metal is treated for precipitation of the phosphorus content of the solution as FePO4 at a pH of 2-3;and precipitated FePO4 is separated. The method is characterised in that the solution which remains after separation of FePO4 and which contains said metal from the precipitation chemicals, is recycled to the wastewater purification. The phosphorus content of the solution is precipitated as FePO4 by adding an at least equivalent amount of Fe3+.

Description

METHOD FOR TREATING SLUDGE FROM WASTEWATER TREATMENT DESCRIPTIVE MEMORY The present invention relates to a method for treating sludge resulting from wastewater treatment according to the preamble of claim 1. Specifically, the invention relates to a process in which precipitation chemical compounds are recovered from the sludge and They are recycled for wastewater treatment. In wastewater treatment, as a rule, first the mechanical separation of solid impurities is carried out, for example with the help of sieves and preliminary sedimentation chambers and allowing solid impurities to settle to the bottom in a mechanical device. preliminary sedimentation. In addition, wastewater is treated by chemical purification and preferably also by biological purification. Chemical purification is carried out with precipitation chemical compounds, such as iron salts or aluminum salts, which are added to water and, by flocculation, precipitate and collect impurities in the wastewater, such as phosphates and particles. In biological purification, which can be carried out, for example by the activated sludge process or by a trickling filter, the waste water is purified by microorganisms. In the treatment of wastewater, a large amount of mud is obtained that must be taken care of. This can be carried out by digestion of the sludge, in which case the organic substances become inorganic, aided by anaerobic microorganisms. The sludge obtained after digestion, ie the digested sludge, can be used for sanitary landfills or as a fertilizer. If the digested sludge will be used as a fertilizer, the content of heavy metals, ie metals from the group consisting of chromium, nickel, copper, zinc, cadmium, lead and mercury, must first be removed. In addition, the sludge contains added chemical precipitation compounds, and from the economic point of view, they must be recovered and, if possible, reused. In the present invention, precipitation chemical compounds refer to iron and / or aluminum containing compounds, such as ferric chloride, ferrous sulfate, ferric sulfate, aluminum sulfate and polyaluminum chloride. Different methods for treating sludge resulting from the wastewater treatment are known, and as an example of the prior art reference is made to WO96 / 20894, which was issued on July 11, 1996. According to this reference, the wastewater sludge it is treated by acidification of the mud to dissolve the metals and the mud matches. After the separation of the remaining sludge, the chemical compounds of precipitation, iron and aluminum, are recovered as phosphates adjusting the pH to approximately 2 to 4. After the separation of the precipitated phosphates, another precipitation is carried out, this time of dissolved heavy metals, which precipitate by increasing the pH to approximately 7 to 9 and adding precipitants, such as sulfides. After separation, the heavy metal sulphides are deposited, while the filtered water can be recycled with the wastewater treatment process. The resulting phosphate deposit, which contains iron phosphate and possibly also aluminum phosphates, can be treated to recover the chemical compounds of iron and aluminum precipitation by adding an alkali metal hydroxide, such as sodium hydroxide, thus forming an insoluble iron hydroxide. and a solution containing soluble alkaline phosphates and aluminum hydroxide. Iron hydroxide can be dissolved in an acid, such as hydrochloric acid, sulfuric acid or nitric acid, to give a solution of the corresponding iron salt that can be used as the precipitation chemical. According to WO 96/20894, mentioned above, the iron content of the sludge is present in trivalent form, or the iron is oxidized to its trivalent form by adding an oxidant, for example hydrogen peroxide. There is no external addition of trivalent iron. However, it is stated that the external addition of phosphorus in the form of phosphate or phosphoric acid can be performed to adjust the molar ratio of phosphoric acid to phosphorus to about 1: 1. As is evident from the above, WO 96/20894 performs a sewage sludge treatment, the sludge is released from unwanted metals, for example heavy metals, and phosphorus. The content of metals that originate from precipitation chemical compounds such as iron and aluminum, in the mud are recovered as phosphates and can not be recycled directly to the wastewater purification process to be used as chemical precipitation compounds.; they must first be converted by additional precipitation and dissolution procedures. Because each dissolution and precipitation procedure means a reduced performance risk of the chemical under discussion, it would be helpful if a procedure could be given in which the metals used in the precipitation chemical compounds, after dissolving in the water mud residual, could be recycled directly to the process of purification of wastewater, without any step of precipitation and intermediate dissolution. According to the present invention, the disadvantages mentioned above are counteracted or reduced, and a method is provided for treating the sludge resulting from the purification of wastewater, where the iron and / or aluminum of the chemical compounds of the sludge is dissolved from the sludge. precipitation, and the resulting solution is recycled to wastewater treatment. More specifically, the invention provides a method for treating the sludge from the purification of wastewater, said sludge containing phosphorus and at least one metal that originates from the precipitation chemical compounds and that is chosen between Fe2 + and Al3 + , the mud pH is adjusted below 4 to dissolve the phosphorus content and said metal in the mud; to separate the remaining mud; to treat the solution that is released from the sludge and that contains phosphorus and the aforementioned metal, for the precipitation of the phosphorus content in the solution as FePO4 with a pH of 2 to 3; and to separate the FePO4 precipitate. The invention is characterized in that the remaining solution, which contains the aforementioned metal of the precipitation chemical compounds, is recycled for the treatment of waste water. Other advantages and distinctive features of the invention will be apparent from the following description and the appended claims. The invention will now be described in more detail with reference to the accompanying drawing, which schematically shows one embodiment of the invention that is currently preferred. The sludge from a wastewater purification plant (which is not shown) contains, among others, phosphorus in the form of phosphate and metals that originate from the chemical precipitation compounds used in the purification of wastewater. to a first step I to dissolve the content of phosphorus and metals in the sludge that were produced from the precipitation chemical compounds. According to the invention, the metal or metals that originate from the precipitation chemical compounds are iron and / or aluminum, with the condition that the iron is presented in divalent form (Fe2 +). Originally, the iron is presented in trivalent form (Fe3 +) in the chemical precipitation compound, but when the precipitation chemical compound in the chemical purification step of the wastewater treatment is added, it is flocculated and passed into the phase of mud, the iron is reduced to its divalent form, for example when the mud is digested. In the first step I, the content of phosphorus, iron and / or aluminum in the sludge is dissolved by the acidification of the sludge. This is done by subjecting the sludge to an acid hydrolysis with a pH of less than 4, preferably less than 2, with an acid, for example, sulfuric acid. The hydrolysis is carried out under conditions which result in the desired dissolution. Neither temperature nor pressure are important in hydrolysis, and can be performed at room temperature and pressure. However, if desired, it can be done at an increased temperature and / or pressure, for example, to accelerate hydrolysis. Normally the temperature can be on an approximate scale between 0 and 200 ° C, and preferably the temperature is increased to approximately between 100 and 140 ° C, to accelerate the hydrolysis. According to the above, the pressure can vary from the ambient pressure (atmospheric pressure) to about 1 MPa depending on the hydrolysis temperature. In many cases it is sufficient that the pH is just below 4 in the hydrolysis, but preferably the pH in the hydrolysis is less than 2 for a complete dissolution of the content of phosphorus, iron and / or aluminum in the mud. After the hydrolysis is completed, the remaining sludge and the hydrolytic fluid pass to a second step II to remove the remaining sludge, for example by filtration or centrifugation. After separating the sludge, the solution released from sludge containing phosphorus and dissolved metal from the sludge in the form of phosphate and dissolved metal salts, goes to a third step III to separate the heavy metals, if any. Heavy metals are, as mentioned above, metals from the group consisting of chromium, nickel, copper, zinc, cadmium, lead and mercury. If there are no heavy metals or if they are insignificant, this step can be omitted. In step III of heavy metal separation, the heavy metals are separated by the addition of a substance that forms an insoluble compound with heavy metals. Preferably, this substance is a source of sulfur ion, such as sodium sulfide, such that the heavy metals are precipitated as heavy metal sulphides (HMS). Alternatively, the phosphorus content in the solution can first be precipitated as FePO4 according to steps IV and V described below, before the heavy metals are precipitated by the addition of a sulfide ion source. If the heavy metal sulphides can be accepted in the sludge that is separated after the acid hydrolysis, another alternative implies that the source of sulfide is added even before or together with the acid hydrolysis to bind any heavy metals present as sulfides. In this case, step II of specific, subsequent sulfur precipitation can be omitted. When the solution has been freed of heavy metals, a step IV is provided, which is the step in which the pH of the solution is adjusted between 2 and 3, preferably between 2 and 2.8. The pH is adjusted by the addition of a suitable base such as sodium hydroxide or magnesium oxide. The pH adjustment is performed as a preliminary step before the subsequent precipitation of the phosphorus content in the solution as FePO4, which is insoluble in the indicated pH scale. If the pH of the solution has already been adjusted in the first step mentioned above, in which the pH is adjusted below 4, so that it is on the scale between 2 and 3, it is not necessary to make any subsequent adjustment of pH after separating the mud. The solution of step IV containing phosphate (PO43"), divalent iron (Fe2 +) and / or aluminum (Al3 +), then a step V is provided to precipitate the phosphorus content of the solution as iron phosphate (FePO4). It is done by adding a trivalent iron source (Fe3 +), such as ferric chloride, to the solution.To achieve a complete precipitation of the phosphorus content of the solution, the trivalent iron is preferably added in at least an equimolar amount, it is say, in such an amount that the molar ratio of the trivalent iron to the phosphorus content of the solution is at least about 1: 1, such as about 1 to 1.5: 1. As mentioned above, the trivalent iron phosphate is insoluble on a pH scale between 2 and 3, preferably between 2 and 2.8, and, on this scale, it is precipitated in a very pure form to achieve as complete a precipitation as possible of the phosphorus content of the solution , there must be a certain additional time of temporary permanence between the addition of the trivalent iron source and the separation of the iron phosphate formed. Suitably the temporary residence time is between approximately 5 minutes and 6 hours, preferably approximately between 30 minutes and approximately 1 hour. Subsequently, the precipitated iron phosphate is removed from the solution in a manner known per se, for example by filtration or centrifugation. Even if the method has been described above in such a way that step IV with pH adjustment of the solution develops before step V with the addition of a trivalent iron source (Fe3 +), it should be understood that the relative order of steps IV and V is optional in the present invention. Therefore, it is possible, and in many cases it is also preferred to first add the trivalent iron source and only then adjust the pH between 2 and 3. In the latter case, the aforementioned temporary dwell time is placed in connection with the pH adjustment. After separation of the precipitated iron phosphate, the remaining solution contains the metal content that originated from the precipitation chemical compounds and is Fe2 + and / or Al3 +, in the original mud. This solution, which is released from sludge, heavy metals and phosphate, is recycled towards the purification of wastewater for a renewed use of the iron and aluminum content in the solution for precipitation chemicals. To make the iron content of the solution work as an active chemical precipitation compound, it needs to be transferred from the divalent form to the trivalent form. This is preferably carried out by the purification of wastewater comprising an aerobic biological purification step, and by adding the solution in this purification step, the divalent iron is oxidized to trivalent iron in the aerobic biological purification step. If the recycled solution contains divalent iron, then it will be added for this reason to the purification of wastewater before or during the aerobic biological purification step. If the solution contains only aluminum, it can in principle be added at an optional point in the purification of wastewater. It is carried out without mentioning that it is also possible to oxidize the divalent iron content of the solution to trivalent iron in another way, for example by the addition of hydrogen peroxide. In this case, the solution can be added to the purification of wastewater at an optional point. The ferric phosphate (FePO4) resulting from the precipitation described above can be used as fertilizer in agriculture. It is possible to recover the trivalent ferric phosphate iron for a renewed use as precipitation reagent by treating ferric phosphate precipitation with an alkali, such as sodium hydroxide to form ferric hydroxide which is separated and treated with an acid, as hydrochloric acid or sulfuric acid, to form the corresponding ferric salt which can then be used as a precipitation reagent. It will be appreciated that the invention offers a simple and leveled method for recovering from the sludge the metals that originate from the chemical precipitation compounds, and the recirculation of this metal towards the purification of wastewater for a renewed use. In the method, by keeping the metal constantly in solution, and without separation of the metal in one or more steps from precipitation, the loss of the metal is minimized in the method of the invention. The possibility to recover the trivalent iron constituting the precipitation reagent for the phosphorus content of the sludge also makes the method of the invention very economical. In addition to recycling the iron and aluminum with the purification of wastewater for a renewed use as precipitation chemical compound, a sludge is achieved by the present invention, which is released from unwanted impurities and which can be used, for example, as a fertilizer . Any included heavy metals are preferably recovered in the invention as a separate precipitate that can be deposited or subjected to another process to recover the heavy metals. Finally, also the phosphorus content of the original mud is recovered, according to the invention, separately in the form of ferric phosphate, which, as indicated above, can be treated to recover the trivalent iron. In this recovery of trivalent iron, the phosphate is obtained as sodium phosphate (Na3PO4) which, for example, can be used as a starting material for the production of a fertilizer for agriculture or as a starting material in the detergent industry. As can be understood from the above, the emission of noxious or unwanted substances is eliminated or reduced to a minimum level according to the invention, and subsequently the invention provides a method that does not affect the environment in any way to treat the sludge from water purification residuals To further illustrate the invention, some embodiments will be described below; however, they are not intended to limit the scope of the invention.

EXAMPLE 1 The sludge from the purification of wastewater in a pilot plant was subjected to an acid hydrolysis with a pH of 1.6 and a temperature of approximately 140 ° C, approximately for 1 hour. The precipitation chemical compounds contained both ferric chloride and ferric sulfate and polyaluminium chloride, and therefore the sludge contained both Fe2 + and AL3 +. After hydrolysis, the remaining sludge was separated by centrifugation, and the solution released from the sludge (clear phase) was used in the test which was carried out at a temperature of approximately 20 ° C. A trivalent iron salt, which is explained in more detail in Table 1, was added to the solution under agitation to avoid sedimentation, in such an amount that the molar ratio of Fe3 +: P? 43"was 1: 1. added NaOH to the solution under continuous stirring to adjust the pH of the solution to 2.6.After adjusting the pH, the iron phosphate (FePO4) was precipitated from the solution and after stirring and precipitation for 1 hour, the precipitation of phosphate The resulting solution was separated from the solution by filtration through a GF / A filter, the remaining solution was then analyzed with respect to the content of Fe2 + and Al3 + The results are indicated in the table In the Ul Ul TABLE 1 Source of Fe37P Fe2 + Fe3 + inside P recycled Fe2 + inside Fe + outside recycled Al3 + inside Al3 + outside At 3 recycled (mol / mol) (%) (mg / l) (mg / l) (%) (mg / l) (mg / l) (%) JKL 1) 1: 1 3.6 726 631 87 27 20 74 PIX-111 2) 1: 1 3.0 726 654 90 27 19 70 PIX-115 3) 1: 1 8.4 726 670 92 27 20 74 1) JKL = iron chloride sulfate with 11.6% by weight of Fe3 + and 20% by weight of CI "and 20% of SO42- .. JKL can be obtained from Kemira Kemwater, Helsingborg, Sweden 2) PIX-111 = chloride of iron with 13.7% by weight of Fe3 + and from 26 to 28% by weight of CIL PIX-111 can be obtained from Kemira, Kenwater, Helsingborg, Sweden 3) PIX-115 = iron sulphate with 11.5% by weight of Fe3 + and 32% by weight of SO42". PIX-115 can be obtained from Kemira, Kenwater, Helsingborg, Sweden.

Table 1, Fe2 + inside and Al3 + designated the content of Fe2 + and Al3 +, respectively, of the original mud. Fe2 + outside and Al3 + outside designated the content of Fe2 + and Al3 + of the final recycled solution. P recycled, Fe2 + recycled and Al3 + recycled designated the amount in percentage of P, Fe2 + and Al3 +, respectively, which is recycled for the purification of wastewater.

As can be seen in Table 1, the invention allows the recycling of about 90% Fe2 + and about 75% Al3 + of the precipitation chemical compounds in the slurry.

EXAMPLE 2 The sewage sludge from the commercial wastewater purification plant that used ferric chloride and iron sulphate as chemical precipitation compounds was subjected, in two different tests (tests 1 and 2) to an acid hydrolysis with a pH of 1.8 and a temperature of approximately 140 ° C, for about 1 hour. After hydrolysis, the remaining sludge was separated by centrifugal settling, and the sludge-released solution (clear phase) was used in the test which was developed at a temperature between 50 and 60 ° C. A trivalent iron salt was added to the solution in a mixing tank, such that the molar ratio of Fe3 + to PO43"was approximately 1: 1. The added trivalent iron was an iron chloride product containing 13.7% by weight of Fe3 + and between 26 and 28% by weight of CIL This product can be obtained from Kemira, Kenwater, Sweden, under the designation PIX-111. The temporary residence time in the mixing tank was 30 minutes. After mixing, NaOH was added to adjust the pH between 2.1 and 2.8.The temporary residence time in this mixing tank was 30 minutes., iron phosphate (FePO4) was precipitated, which was separated by solution from the pumped mixture tank to a centrifugal decanter. A Zetag 89 cationic polymer from Allied Colloid, Great Britain, was added to ensure good separation in the decanter. An analysis was made of the Fe2 + content (Fe2 + inside) of the original mud and the Fe2 + content (Fe2 + outside) of the final solution. The results of tests 1 and 2 are set out in table 2, and the average values on values in the tests, which were developed during 4 hours (test 1) and 6 hours (test 2). Table 2 shows that at least 80% of the Fe2 + content of the sludge can be recycled (Fe2 + recycled) to the water purification process.

Ni O Ul Ul TABLE 2 Source of Fe3 + Fe37P Fe2 + inside Fe2 + outside Fe2 + recycled (mol / mol) (mg / l) (mg / l) (%) Test 1 PIX-111 1.07: 1 56 47 84 Test 2 PIX-111 1.30: 1 39.5 32.6 83

Claims (9)

NOVELTY OF THE INVENTION CLAIMS

1. - A method to treat sludge from the purification of wastewater, said sludge contains phosphorus and at least one metal that originates from the chemical compounds of precipitation and that is chosen between Fe2 + and AI3 \ the pH of the was adjust below 4 to dissolve the phosphorus content and said metal in the mud; the separation of the remaining mud; the treatment of the solution that is released from the mud and containing phosphorus and the aforementioned metal, for the precipitation of the phosphorus content in the solution as FePO4 with a pH between 2 and 3; and the separation of the precipitated FePO; further characterized in that the remaining solution, which contains the aforesaid metal obtained from the precipitation chemical compounds, is recycled for the treatment of wastewater.

2. The method according to claim 1, further characterized in that the metal obtained from the chemical precipitation compounds is Fe2 +.

3. The method according to claim 1 or 2, further characterized in that the pH of the sludge is adjusted below 2, to dissolve the phosphorus content and the metal mentioned in the mud.

4. The method according to any of claims 1 to 3, further characterized in that the phosphorus content in the solution is precipitated as FePO4 by the addition of at least an equivalent amount of Fe3 +.

5. The method according to any of claims 1 to 4, further characterized in that the remaining solution contains Fe2 + and is recycled for the treatment of wastewater before an aerobic biological purification step.

6. The method according to any of claims 1 to 4, further characterized in that the remaining solution contains Fe2 + and is recycled to an aerobic biological purification step in the treatment of wastewater.

7. The method according to any of claims 1 to 6, further characterized in that the heavy metals in the mud, which have dissolved by adjusting the pH of the mud below 4, and those after the separation of the Remnant sludge are present in the solution released from sludge, precipitated as sulfides by the addition of a sulfur ion source to the solution after separation of the remaining sludge.

8. The method according to claim 7, further characterized in that the sulfur ion source is added to the solution before precipitation of the phosphorus content in the solution.

9. The method according to any of claims 1 to 8, further characterized in that the content of phosphorus and the said metal that originates from the chemical precipitation compounds and is present in the sludge. dissolves by acid hydrolysis at a temperature on a scale between 0 and 200 ° C.