Method for treating sludge from wastewater treatment

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 aluminum, the pH of the sludge being adjusted to below 4, preferably below 2, for dissolving the content of the 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 FePO.sub.4 at a pH of 2-3; and precipitated FePO.sub.4 is separated. The method is characterized in that the solution which remains after separation of FePO.sub.4 and which contains said metal from the precipitation chemicals, is recycled to the wastewater purification. The phosphorus content of the solution is precipitated as FePO.sub.4 by adding an at least equivalent amount of Fe.sup.3-.

The present invention relates to a method for treating sludge from
 wastewater treatment. More specifically, the invention relates to such a
 process, in which the precipitation chemicals are recovered from the
 sludge and recycled to wastewater treatment.
 In the treatment of wastewater, as a rule first mechanical separation of
 solid impurities is carried out, for instance with the aid of screens and
 grit chambers and by allowing the solid impurities to settle in a
 preliminary settling device. Moreover, the wastewater is treated by
 chemical purification and preferably also by biological purification. The
 chemical purification is done such that precipitation chemicals, such as
 iron salts or aluminum salts, are added to the water and, by flocculation,
 precipitate and collect impurities in the wastewater such as phosphates
 and particles. In biological purification, which can take place, for
 instance, by an activated sludge process or by means of a trickling
 filter, the wastewater is purified by means of microorganisms. In
 wastewater treatment, large quantities of sludge are obtained, which must
 be taken care of. This can be carried out by digesting the sludge, in
 which case organic substances are converted into inorganics, assisted by
 anaerobic microorganisms. The-sludge obtained after digestion, i.e.
 digested sludge, can be used for landfilling or as fertilizer. If the
 digested sludge is to be used as fertilizer, its content of heavy metals,
 i.e. metals from the group consisting of chromium, nickel, copper, zinc,
 cadmium, lead and mercury, should first be removed. Besides, the sludge
 contains the added precipitation chemicals, and from the economic point of
 view these should, if possible, be recovered and reused. In the present
 invention, precipitation chemicals relate to iron and/or
 aluminium-containing compounds, such as ferric chloride, ferrous sulphate,
 ferric sulphate, aluminium sulphate and polyaluminium chloride.
 Different methods for treating sludge from waste-water treatment are known,
 and as an example of prior-art technique, reference is made to W096/20894,
 which was published on Jul. 11, 1996. According to this reference,
 wastewater sludge is treated by acidifying the sludge to dissolve metals
 and phosphorus from the sludge. After separation of the remaining sludge,
 the precipitation chemicals iron and aluminum are recovered as phosphates
 by adjusting the pH to about 2-4. After separation of precipitated
 phosphates, a further precipitation is carried out, this time of dissolved
 heavy metals, which are precipitated by increasing the pH to about 7-9 and
 adding precipitants, such as sulphides. After separation, the heavy metal
 sulphides are deposited, while the filtered water can be recycled to the
 wastewater treatment procedure. The resulting phosphate deposit, which
 contains iron phosphate and possibly also aluminum phosphates, can be
 treated for recovery of the precipitation chemicals iron and aluminum by
 adding an alkali hydroxide, such as sodium hydroxide, thereby forming
 insoluble iron hydroxide and a solution containing soluble alkali
 phosphates and aluminum hydroxide. The iron hydroxide can be dissolved in
 an acid, such as hydrochloric acid, sulphuric acid or nitric acid, to give
 a solution of the corresponding iron salt which is usable as precipitation
 chemical.
 According to the above-mentioned WO96/20894, the iron content of the sludge
 is present in trivalent form, or the iron is oxidised to trivalent form by
 adding an oxidant such as hydrogen peroxide. No external addition of
 trivalent iron takes place. However, the external addition of phosphorus
 in the form of phosphoric acid or phosphate can take place to adjust the
 molar ratio of phosphoric acid to phosphorus to about 1:1.
 As is apparent from above, WO 96/20894 accomplishes a treatment of
 wastewater sludge, the sludge being relieved of undesired metals such as
 heavy metals and phosphorus. The content of metals originating from
 precipitation chemicals, such as iron and aluminum, in the sludge, is
 recovered as phosphates and cannot be recycled directly to the wastewater
 purification process to be used as precipitation chemicals, but must first
 be converted by additional dissolving and precipitating procedures. Since
 each dissolving and precipitating procedure means a risk of decreased
 yield of the chemical at issue, it would be advantageous if a process
 could be provided in which the metals used in the precipitation chemicals,
 after being dissolved from the wastewater sludge, can be recycled directly
 to the wastewater purification process, without any intermediate
 precipitating and dissolving steps.
 According to the present invention, the above-mentioned drawbacks are
 obviated or reduced, and a method is provided for treating sludge from
 wastewater purification, in which iron and/or aluminium from the
 precipitation chemicals is dissolved from sludge, and the resulting
 solution is recycled to the wastewater treatment.
 More specifically, the invention provides a method for treating sludge from
 wastewater purification, said sludge containing phosphorus and at least
 one metal which originates from precipitation chemicals and which is
 selected among Fe.sup.2+ and Al.sup.3+,
 the pH of the sludge being adjusted to below 4 for dissolving the content
 of phosphorus and said metal in the sludge;
 separating the remaining sludge;
 treating the solution which is relieved of sludge and which contains
 phosphorus and said metal, for precipitation of the content of phosphorus
 in the solution as FePO.sub.4 at a pH of 2-3; and separating the
 precipitated FePO.sub.4. The invention is characterized in that the
 remaining solution, which contains said metal from the precipitation
 chemicals, is recycled to the wastewater treatment.
 Further advantages and distinctive features of the invention will be
 evident 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 a presently preferred
 embodiment of the invention.
 Sludge from a wastewater purification plant (not shown) containing, inter
 alia, phosphorus in the form phosphate and metals originating from the
 precipitation chemicals that are used in the wastewater purification, is
 supplied to a first step I for dissolving the content of phosphorus and
 metals in the sludge from the precipitation chemicals. According to the
 invention, the metal or metals originating from the precipitation
 chemicals are iron and/or aluminum, on the condition that iron is present
 in divalent form (Fe.sup.2+). Originally, the iron is present in trivalent
 form (Fe.sup.3+) in the precipitation chemical, but when the precipitation
 chemical in the chemical purification step of the wastewater treatment has
 been added, flocculated and passed to the sludge phase, the iron is
 reduced to divalent form, for instance when digesting the sludge.
 In the first step I, the content of phosphorus, iron and/or aluminium in
 the sludge is dissolved by acidifying the sludge. This is effected by
 subjected the sludge to an acid hydrolysis at a pH below 4, preferably
 below 2, with an acid, e.g. sulphuric acid. The hydrolysis is effected
 under conditions that result in the desired dissolution. Neither
 temperature nor pressure is critical in the hydrolysis, and ambient
 temperature and pressure car be used. If desired, an increased temperature
 and/or pressure, however, can be used in order to, for instance,
 accelerate the hydrolysis. Normally the temperature can be in the range of
 about 0-200.degree. C., and preferably the temperature is increased such
 as about 100-140.degree. C., to accelerate the hydrolysis.
 Correspondingly, the pressure may vary from ambient pressure (atmospheric
 pressure) up to about 1 MPa depending on the hydrolysis temperature. It is
 in many cases sufficient for the pH to be just below 4 in the hydrolysis,
 but preferably the pH in the hydrolysis is below 2 for complete
 dissolution of the content of phosphorus, iron and/or aluminium in the
 sludge.
 After completion of the hydrolysis, the remaining sludge and hydrolytic
 fluid are supplied to a second step II for separating the remaining
 sludge, for instance by filtration or centrifugation.
 After separating the sludge, the solution relieved of sludge which contains
 dissolved phosphorus and metal from the sludge in the form of phosphate
 and dissolved metal salts, is supplied to a third step III for separating
 heavy metals, if any. By heavy metals are meant, 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 can be
 neglected, this step can be omitted.
 In the heavy metal separation step III, heavy metals are separated by
 adding a substance which forms an insoluble compound with heavy metals.
 Preferably, this substance is a sulphide ion source, such as sodium
 sulphide, such that the heavy metals are precipitated as heavy metal
 sulphides (HMS).
 Alternatively, the content of phosphorus in the solution can first be
 precipitated as FePO.sub.4 according to steps IV and V described below
 before the heavy metals are precipitated by adding a sulphide ion source.
 If heavy metal sulphides can be accepted in the sludge which is separated
 after the acid hydrolysis, a further alternative implies that a sulphide
 ion source is added even before or in connection with the acid hydrolysis
 for binding any heavy metals present as sulphides. In this case, the
 subsequent, specific sulphide precipitation step III can be omitted.
 When the solution has been relieved of heavy metals, it is supplied to a
 fourth step IV, which is a step in which the pH of the solution is
 adjusted to 2-3, preferably 2-2.8. The pH is adjusted by adding a suitable
 base, such as sodium hydroxide or magnesium oxide. The adjustment of the
 pH is effected as a preliminary step before the subsequent precipitation
 of the content of phosphorus in the solution as FePO.sub.4, which is
 insoluble in the stated pH range.
 If the pH of the solution has already been adjusted, in the first step
 above, in which the pH is adjusted to below 4, to be in the range of 2-3,
 no subsequent adjustment of the pH is necessary after separating the
 sludge.
 The solution from step IV, which contains phosphate (PO.sub.4.sup.3-),
 divalent iron (Fe.sup.2+) and/or aluminum (Al.sup.3+), is then supplied to
 a fifth step V for precipitating the phosphorus content of solution as
 iron phosphate (FePO.sub.4). This takes place by adding to the solution a
 source of trivalent iron (Fe.sup.3+), e.g. ferric chloride. With a view to
 achieving complete precipitation of the phosphorus content of the
 solution, the trivalent iron is preferably added in an at least equimolar
 amount, i.e. 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 1-1.5:1. As mentioned above, trivalent iron phosphate is insoluble
 in the pH range of 2-3, preferably 2-2.8, and, in this range, is
 precipitated in very pure form. To achieve as complete precipitation as
 possible of the phosphorus content of the solution, further a certain
 sojourn time should pass between the adding of the source of trivalent
 iron and the separation of the formed iron phosphate. Suitably, the
 sojourn time is from about 5 min to about 6 h, preferably from about 30
 min to about 1 h. The precipitated iron phosphate is then removed from the
 solution in a per se known manner, for instance by filtration or
 centrifugation.
 Even if the method has been described above such that the step IV with pH
 adjustment of the solution is carried out before step V with the addition
 of a source of trivalent iron (Fe.sup.3+), it should be understood that
 the relative order of steps IV and V is optional in the present invention.
 Thus it is quite possible, and in many cases also preferred first to add
 the source of trivalent iron and only then to adjust the pH to 2-3. In the
 latter case, the above-mentioned sojourn time is placed in connection with
 the pH adjustment.
 After separation of the precipitated iron phosphate, the remaining solution
 contains the content of metal originating from the precipitation chemicals
 and being Fe.sup.2+ and/or Al.sup.3+, in the original sludge. This
 solution, which is relieved of sludge, heavy metals and phosphate, is
 recycled to the wastewater purification for renewed use of the content of
 iron and aluminum in the solution as precipitation chemicals. To make the
 iron content of the solution function actively as a precipitation
 chemical, it must be transformed from divalent to trivalent form. This is
 preferably carried out by the wastewater purification comprising an
 aerobic biological purification step, and the solution being added to this
 purification step, the divalent iron being oxidized to trivalent iron in
 the aerobic biological purification step. If the recycled solution
 contains divalent iron, it will thus for this reason be added to the
 wastewater purification prior to or in the aerobic biological purification
 step. If the solution contains aluminum only, it can in principle be added
 at an optional point in the wastewater purification. It goes without
 saying that it is also possible to oxidize the divalent iron content of
 the solution to trivalent iron in some other manner, for instance by
 adding hydrogen peroxide. In this case, the solution can be added to the
 wastewater purification at an optional point.
 The ferric phosphate (FePO.sub.4) resulting from the above described
 precipitation can be used as fertilizer in agriculture. It is also
 possible to recover the trivalent iron from the ferric phosphate for
 renewed use as precipitation reagent by treating the precipitation of
 ferric phosphate with an alkali, such as sodium hydroxide, to form ferric
 hydroxide which is separated and treated with an acid, such as
 hydrochloric acid or sulphuric acid, to form the corresponding ferric
 salt, which can then be used as precipitation reagent.
 It will be appreciated that the invention offers a simple and smooth method
 for recovering, from sludge, metal originating from precipitation
 chemicals and recirculation of this metal to the wastewater purification
 for renewed use. By the metal, in the method, being constantly kept in
 solution, and there being no separation of the metal in one or more
 precipitation steps, the loss of the metal is minimized in the inventive
 method. The possibility of recovering the trivalent iron which constitutes
 precipitation reagent for the phosphorus content of the sludge also makes
 the inventive method being highly economic.
 In addition to the recycling of iron and aluminum to the wastewater
 purification for renewed use as precipitation chemicals, a sludge is
 achieved by the present invention, which is relieved of undesired
 impurities and which can be used as, for instance fertilizer. Any heavy
 metals included are recovered in the invention preferably as a separate
 precipitate which can be deposited or be further processed for recovering
 the heavy metals. Finally, also the phosphorus content of the original
 sludge is recovered, according to the invention, separately in the form of
 ferric phosphate, which as stated above can be treated for recovering the
 trivalent iron. In this recovery of the trivalent iron, the phosphate is
 obtained as sodium phosphate (Na.sub.3 PO.sub.4) which, for instance, can
 be used as raw material for the production of fertilizer in agriculture or
 as raw material in the detergent industry.
 As is obvious from that stated above, the emission of noxious or undesired
 substances is eliminated or reduced to a minimum level according to the
 invention, and consequently the invention provides an extremely
 environmentally friendly method for treating sludge from wastewater
 purification.
 In order to further illustrate the invention, some embodiments will be
 described below, which, however, are not intended to limit the scope of
 the invention.

EXAMPLE 1
 Sludge from wastewater purification in a pilot plant was subjected to acid
 hydrolysis at a pH of 1.6 and a temperature of about 140.degree. C. for
 about 1 h. The precipitation chemicals comprised both ferric chloride and
 ferric sulphate and polyaluminum chloride, and the sludge therefore
 contained both Fe.sup.2+ and Al.sup.3+. After the hydrolysis, the
 remaining sludge was separated by centrifugation, and the solution
 relieved of sludge (clear phase) was used in the test which was carried
 out at a temperature of about 20.degree. C. A trivalent iron salt, which
 is stated in more detail in Table 1, was added to the solution under
 agitation to avoid settling in such an amount that the molar ratio
 Fe.sup.3+ : PO.sub.4.sup.3- was 1:1. Then NaOH was added to the solution
 under continued agitation in order to adjust the pH of the solution to
 2.6. In the pH adjustment, iron phosphate (FePO.sub.4) was precipitated
 from the solution and after agitation and precipitation for 1 h, the
 resulting iron phosphate precipitation was separated from the solution by
 filtration through a GF/A filter. The remaining solution was then
 analyized in respect of the content of Fe.sup.2- and Al.sup.3+. The
 results are stated in Table 1. In Table 1, Fe.sup.2+ in and Al.sup.3+ in
 designate the content of Fe.sup.2+ and Al.sup.3+, respectively, of the
 original sludge. Fe.sup.2+ out and Al.sup.3+ out designate the Fe.sup.2+
 and Al.sup.3+ content of the final, recycled solution. P recycled,
 Fe.sup.2+ recycled and Al.sup.3+ recycled designate the percentage amount
 of P, Fe.sup.2+ and Al.sup.3+, respectively, which is recycled to the
 waste-water purification.
 TABLE 1
 Fe.sup.3+ /P in P recycled Fe.sup.2+ in Fe.sup.2+ out Fe.sup.2+
 recycled Al.sup.3+ in Al.sup.3+ out Al.sup.3+ recycled
 Fe.sup.3+ -source (mole/mole) (%) (mg/l) (mg/l) (%)
 (mg/l) (mg/l) (%)
 JKL.sup.1) 1:1 3.6 726 631 87 27 20
 74
 PIX-111.sup.2) 1:1 3.0 726 654 90 27
 19 70
 PIX-115.sup.3) 1:1 8.4 726 670 92 27
 20 74
 .sup.1) JKL = iron chloride sulphate with 11.6% by weight Fe.sup.3+ and
 max. 20% by weight Cl.sup.- and 20% SO.sub.4.sup.2-. JKL can be obtained
 from Kemira Kemwater, Helsingborg, Sweden.
 .sup.2) PIX-111 = iron chloride with 13.7% by weight Fe.sup.3+ and 26-28%
 by weight Cl.sup.-. PIX-111 can be obtained from Kemira Kemwater,
 Helsingborg, Sweden.
 .sup.3) PIX-115 = iron sulphate with 11.5% by weight Fe.sup.3+ and 32% by
 weight SO.sub.4.sup.2-. PIX-115 can be obtained from Kemira Kemwater,
 Helsingborg, Sweden.
 As appears from Table 1, the invention allows recycling of about 90% of
 Fe.sup.2+ and about 75% of Al.sup.3+ from the precipitation chemicals in
 the sludge.
 EXAMPLE 2
 Wastewater sludge from a commercial wastewater purification plant which
 used ferric chloride and iron sulphate as precipitation chemicals, was
 subjected, in two different tests (Tests 1 and 2) to acid hydrolysis at a
 pH of 1.8 and a temperature of about 140.degree. C. for about 1 h. After
 the hydrolysis, the remaining sludge was separated by means of a
 centrifugal decanter, and the solution relieved of sludge (clear phase)
 was used in the test which was carried out at a temperature of about
 50-60.degree. C. A trivalent iron salt was added to the solution in a
 mixing tank such that the molar ratio of Fe.sup.3+ to PO.sub.4.sup.3- was
 about 1:1. The added trivalent iron was an iron chloride product
 containing 13.7% by weight Fe.sup.3+ and 26-28% by weight Cl.sup.-. This
 product can be obtained from Kemira, Kemwater, Sweden, under the
 designation PIX-111. The sojourn time in the mixing tank was 30 min. In a
 subsequent mixing tank, NaOH was added for adjusting the pH to 2.1-2.8.
 The sojourn time in this mixing tank was 30 min. In the pH adjustment,
 iron phosphate (FePO.sub.4) was precipitated, which was separated by the
 solution from the mixing tank being pumped to a centrifugal decanter. A
 cationic polymer Zetag 89 supplied by Allied Colloid, Great Britain, was
 added to ensure good separation in the decanter. An analysis was made of
 the Fe.sup.2+ content (Fe.sup.2+ in) of the original sludge and of the
 Fe.sup.2+ (Fe.sup.2+ out) content of the final solution. The results from
 Tests 1 and 2 are stated in Table 2, and the values concern average values
 in the tests, which were carried out during 4 h (Test 1) and 6 h (Test 2).
 Table 2 shows that at least 80% of the Fe.sup.2+ content of the sludge can
 be recycled (Fe.sup.2+ recycled) to the water purification procedure.
 TABLE 2
 FE.sup.3+ /P Fe.sup.2+ in Fe.sup.2+ out Fe.sup.2+
 recycled
 Fe.sup.3+ -source (mole/mole) (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