Liquid purification or separation – Processes – Treatment by living organism
Reexamination Certificate
1999-09-14
2001-04-17
Simmons, David A. (Department: 1724)
Liquid purification or separation
Processes
Treatment by living organism
C210S712000, C210S722000, C210S724000, C210S737000, C210S906000, C423S140000, C423S311000
Reexamination Certificate
active
06217768
ABSTRACT:
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
2+
and Al
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
4
at a pH of 2-3; and separating the precipitated FePO
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
2+
). Originally, the iron is present in trivalent form (Fe
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° C., and preferably the temperature is increased such as about 100-140° 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 hy
Hansen Bengt
Jokinen Simo
Birch & Stewart Kolasch & Birch, LLP
Kemira Kemi AB
Prince Fred
Simmons David A.
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