This invention is concerned with the recovery for recycling of inorganic coagulants used in water or waste water treatment processes from the coagulant-containing sludges produced in such processes. The process described has the dual benefit of reducing the volume of sludge for disposal, and reducing the amount of fresh coagulant needed and hence saving on coagulant costs.

Bibliographic details of the publications referred to in this specification are collected at the end of the description.

In order to satisfy the ever increasing thirst of our industrialised and urbanised societies, water authorities must turn to treatment of more and more polluted water sources using complex technologies. This creates a secondary problem - the disposal of residues/sludges formed as a result of the treatment. Usually such residues contain substantial quantities of inorganic coagulant such as aluminium or ferric hydroxides which create much bulk and make the sludge less than beneficial to the environment into which it is discharged.

Coagulants are used to encourage the agglomeration of colloidal impurities in many water/wastewater purification processes; commonly the agglomerated impurities are then removed from the product water by sedimentation or filtration. The coagulants may be either organic or inorganic in nature. The inorganic coagulants are usually based on iron or aluminium, most commonly used are aluminium sulphate (alum) or ferric sulphate or chloride.

Past attempts to recover and reuse alum in conventional water treatment processes were usually motivated by problems associated with the handling and disposal of the aluminium hydroxide containing sludges. Recover}' of alum reduces the amount of solids and improves the dewatering and filtration characteristics of the residue. Reuse of the recovered alum also reduces the chemical cost.

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Simple solubilisation of aluminium hydroxide with sulfuric acid or sodium hydroxide i technically feasible, but not all recovered aluminium can be reused due to the presenc of a variety of impurities. Solubilisation of ferric hydroxide with acid leads to simila results.

Dissolution of the aluminium hydroxide with sulfuric acid followed by a simpl separation of the solid residue from the solubilised aluminium by sedimentation o filtration has been studied in laboratory, pilot plant and full-scale plant experiments ^(1_8 ). The pH at which maximum aluminium recovery occurred ranged from 1-3 depending on the sludge treated. It appears that pH 2.0 was a reasonable compromise. The proportion recovered ranged from 50-90% and up to 1.8 times the theoretical acid requirement was needed. The performance of the recovered alum in coagulation was generally inferior to fresh alum. Accumulation of impurities, such as metals and organic acids, in the recycled stream was suggested also as a possible problem. The danger that such impurities could transfer into the product water, as well as reduce the effectiveness of the recycled aluminium, is also present.

Recovery of aluminium from a waterworks sludge by alkalisation with NaOH or Ca(OH)

'2 was also demonstrated under laboratory conditions &\ Even though it was possible to recover 80% of the aluminium the authors recommended recycling less than 50% of the total dosing rate because of the impurity problem. Lime was more effective than sodium hydroxide for removing heavy metals from the recovered coagulant.

While aluminium sulfate is the most commonly used coagulant, polymerised aluminium salts (PAC) are becoming increasingly popular world-wide for their excellent coagulation properties and low residual aluminium levels. The possibility of recovering and reusing the polymeric species by acidification was demonstrated in a laboratory study < ¹⁰⁾ which showed that the recovered coagulant could be reused. The results also indicated reduced effectiveness of the recovered species, compared to the fresh material.

Whilst it is possible to recover the hydrolysed coagulant by redissolving it with acid or

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alkali there are a number of problems to overcome. Firstly, the amount of acid or alkali required is affected by both the amount of water present and the age of the sludge. Because the cheap dewatering processes are slow the sludge ages during the process. Secondly, during redissolution, organic acids and heavy metals are liberated if present in the residue. Aged sludge requires lower pH for redissolution of the polymerised aluminium hydroxide which also increases the concentration of other undesirable components in the recovered stream. In the present state of the art only approximately 50-70% of the recovered coagulant can be reused safely.

To improve the aluminium recovery rate and to enhance its separation from impurities, techniques such as liquid ion exchange ⁽¹¹⁾ , electrochemical coagulant regeneration ⁽¹²⁾ , ultrafiltration < ¹³ and selective two-step aluminium recovery employing composite membrane ⁽¹⁴ >, have been evaluated in the laboratory. It remains to be seen whether such techniques are economic in a practical situation.

Thus there is a need for a simple technique which would allow recovery of the inorganic coagulant selectively, leaving the unwanted impurities, such as humic and fulvic acids, behind. We have discovered a relatively simple separation process that meets this need and which may be incorporated into water treatment plants that use inorganic coagulants.

Accordingly this invention provides a method for recovering inorganic coagulants from water/wastewater treatment sludges in a condition which enables effective reuse of the coagulant, which method comprises the following steps:

1. Treatment of the sludge with acidic or alkaline material as appropriate in order to dissolve the coagulant.

2. Precipitation of humic acids by adjustment of liquor pH (usually to 2 - 3 .

3. Addition of polyelectrolyte and optionally a solid adsorbent to the liquor to cause precipitation of impurities.

4. Separation of the liquor containing dissolved coagulant from the residue. 5. Contact of the supernatant from step 4 with a positively charged and/or hydrophobic solid adsorbent to remove soluble organic materials.

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The polyelectrolyte used in step 3 may be any one of those well known in the water treatment art. While cationic and non-ionic polyelectrolytes are useful, anionic polyacrylamides such as Cyanamid Superfloc A130 or A 2125S are preferred.

The solid adsorbents used in step 3 may be any suitable particulate adsorbent. Preferably the adsorbent is a natural or artificial clay mineral or a particulate carbon.

The positively charged and/or hydrophobic adsorbent (hereinafter referred to as the "PC/H Adsorbent") used in step 5 may be selected from a wide range of materials. Suitable materials may be selected from silicas or minerals treated with organosilicon compounds containing a long hydrocarbon chain, eg Sep-Pak treated with C ₁₈ hydrocarbons, "white carbons " such as XAD resin, and anion exchange resins. Anion exchange resins are preferred PC/H Adsorbents.

The contact with the PC/H Adsorbent may be brought about in any suitable known way such as passing the liquor through a fixed or fluidised bed of PC/H Adsorbent or circulating the adsorbent through the liquor.

The adsorbents used have a finite capacity but may be regenerated. The solid adsorbent used in step 3 is preferably cheap enough to be discarded but may be regenerated by alkali. Surprisingly we have discovered that in step 5 of the process of this invention if the PC/H Adsorbent is positively charged, the impurities are removed by forming a coating around the PC/H Adsorbent. Most of such coating may be removed by physical means from the impurity coated PC/H Adsorbent; eg by washing with a stream of water, by agitating a suspension of the loaded PC/H Adsorbent in water, or by backflushing or air scouring a column of PC/H Adsorbent. The partially cleaned PC/H Adsorbent can then be readily separated from the resulting impurity suspension which is discarded. The water used for washing is not critical; it may be conveniently either purified product water or impure feed water. This washing procedure leaves a minor portion of absorbed impurities which accumulate. Preferably the washed PC/H Adsorbent is periodically further regenerated by treatment with a suitable reagent such as an alkaline solution.

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1— —

Accordingly, in another aspect, this invention provides a process for recovery _of inorganic coagulant from water treatment sludges which comprises steps 1 - 5 above, followed by (as and when necessary) subjecting the impurity coated positively-charged PC/H Adsorbent to a physical process thereby to remove the coating of impurities from the PC/H Adsorbent, separating the removed impurities from the PC/H Adsorbent and returning the cleaned PC/H Adsorbent to the process.

The process of the invention is further described in the following non-limiting examples.

EXAMPLE 1

A water treatment sludge was treated at pH 2.5 using dilute sulphuric acid. The liquor was then separated from the sludge and treated with polyelectrolyte to precipitate a second sludge. The supernatant from polyelectrolyte treatment was an aluminium rich solution with an initial colour of 460 PCU and a Total Organic Carbon (TOC) content of 190 mg/L. This solution was passed through columns of Amberlite IRA 910 in either the hydroxyl or sulphate form. The hydroxyl column eluent had an average colour of 100 PCU and TOC 40mg L; the sulphate column eluent had an average colour of 120 PCU and 50mg/L TOC. Each column passed all of the aluminium initially present. When the impurity coated resins were washed with water, the hydroxyl column released impurities equivalent to 55% of the initial colour and 42% of the TOC while the sulphate column released 28% of the colour and 24% of the TOC. Further treatment of each column with 0.5M NaOH released impurities equivalent to 45% of the initial colour and 58% of the initial TOC from the hydroxyl column and 72% of the initial colour and 58% of the initial TOC from the sulphate column.

EXAMPLE 2

A sludge from the SIROFLOC® process was treated with acid at pH 2.5, a cationic polyelectrolyte was added to the supernatant from the acid treatment. A flocculant precipitate formed which was allowed to settle. The supernatant from the polyelectrolyte treatment had pH 2.95, an aluminium content of 317 mg/L, apparent colour = 1080 PCU, and TOC = 164 mg/L.

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A number of adsorbents were tested for their efficacy in removing organics from this supernatant. Anion exchange resins, solid phase extractants, activated carbon and "white carbons" were evaluated. The anion exchange resins were used in the chloride form. The conditions and results are set out in Table 5 below. The experiments were carried out with very small beds as quantities of some materials were very limited. The bed sizes ranged from 1 to 5 mL. Amount of material adsorbed was measured after elution with 0.5 M NaOH

SIROFLOC® is a registered trade mark of CSIRO. The SIROFLOC® process is patented and licensed to Austep Pty Ltd, a subsidiary of Davy John Brown.

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Table 5. Adsorption of humic substances from acidic regeneration effluent by ion exchange resins.

CO c O

c

H rπ

m m

H

Whilst the nature of these experiments was a preliminary screening only, Jhe result tended to fall into three categories.

The functionalised adsorbents (i.e. anion-exchange resins) tended to adsorb organi colour well, allowing all aluminium to pass through. They regenerated quite stoichiometrically.

The non-functionalised adsorbents which depend on pore size for their adsorption capacity tended to adsorb organics well regardless of pore size but regeneration was only partial. The C ₁₈ bonded to silica adsorbents which depend on hydrophobic interaction also adsorbed organics successfully. These materials are presently high cost relative to ion exchange resins.

Activated carbon was also successful in adsorption of organics but on regeneration the TOC appeared to regenerate better than colour. This might indicate that the larger molecules which tend to have a higher density of chromophores remain trapped in the pores of the carbon. Silica gel which is both hydrophilic and non functionalised performed poorly.

Jar tests performed using eluate from the anion-exchange columns revealed that coagulation performance of the Al ³⁺ ions present was restored to more than 90% of its original coagulating ability.

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- 9 -

REFERENCES

1. J.M. Roberts and C.P. Roddy; Recovery and Reuse of Alum Sludge at Tamp JAWWA 52,7, 1960, 857-866.

2. G.P.Fulton; Disposal of Wastewater from Water Filtration Plants, JAWWA, 61, 1969, 322-326.

3- G.P. Fulton; Recover Alum to Reduce Waste-Disposal Costs, JAWWA, 66 1974, 312-318.

4. P. . Pope, B.D. Waters and T. Wardle; Aluminium Sulfate Recovery in Pilo Plant, Water Treatment and Examin., 24,4, 1975, 278-294.

5. R.M. Gruninger; Disposal of Waste Alum Sludge from Water Treatment Plants, JWPCF, 47,3, 1975, 543-552.

6. B.H.H. Chen, P.H. King and C.W. Randall; Alum Recovery from Representative Water-Treatment Plant Sludges, JAWWA, 68,4, 1976, 204-207.

M.M. Bishop, A.T. Rolan, T.L. Bailey and D.A. Cornwell; Testing of Alum Recovery for Solids Reduction and Reuse, JAWWA, 79,6, 1987, 76-83.

J. Masides, J. Soley and J. Mata-Alvarez; A Feasibility Study of Alum Recovery in Wastewater Treatment Plants, Water Research, 22,4, 1988, 399-405.

J. Masschelein, R. Devleminck and J. Genot; The Feasibility of Coagulant Recycling by Alkaline Reaction of Aluminium Hydroxide Sludges, Water Research, 19,11, 1985, 1363-1368.

10. J.E. van Benschoten, G.R. Stuart, J.J. Jensen; Recovery and Reuse of Polyaluminium Chloride Sludges, Proc. Amer.WWA Annual Conference, June

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23-27, 1991, Philadelphia, PA.

11. D.A. Cornwell, G.C. Cline, J.M. Przybyla, D. Tippin; Demonstration Testing of Alum Recovery by Liquid Ion Exchange, JAWWA, 73,6, 1981, 326-332.

12. Ju. V. Epifanov and E.S. Matskevich; Technological Properties of Solutions of Electrochemically Regenerated Coagulants Containing Aluminium, Khimiya i Teknologiya Vody, 12,2, 1990, 91-96.

13. L.E. Ernest and C. Tongkasame; Recovery and Reuse of Alum from Water Filtration Plant Sludge by Ultrafiltration, AIChE Symp. Ser.,. 71,1975 151-157

14. A.K. Sengupta and Bo Shi; Selective Alum Recovery from Clarifier Sludge: A New Two-Step Process Using Composite Membranes, Proc. of the Amer.WWA Annual Conference, June 23-27, 1991, Philadelphia, PA.

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