Liquid purification or separation – Processes – Preventing – decreasing – or delaying precipitation,...
Reexamination Certificate
2000-02-29
2002-05-21
Hruskoci, Peter A. (Department: 1724)
Liquid purification or separation
Processes
Preventing, decreasing, or delaying precipitation,...
C210S699000, C210S701000, C252S180000
Reexamination Certificate
active
06391207
ABSTRACT:
TREATMENT OF SCALE
The present invention relates to a novel method of treatment for the prevention, reduction of removal of scale in municipal and industrial waste water treatment facilities. The invention employs a new anti-scaling composition which is especially useful for the treatment of waste treatment plants prone to struvite scaling.
The precipitation of struvite is a recurring problem, especially in anaerobic sludge digestion systems. Soluble magnesium combines with ammonia and phosphate, released during the aerobic digestion to form magnesium ammonium phosphate (MgNH
4
PO
4
.6H
2
O) which is frequently referred to as struvite. Struvite is a hard, tenacious solid that is extremely difficult to remove from equipment surfaces and pipelines. It is generally encountered in sludge lines and downstream processes following biological phosphate removal and subsequent anaerobic digestion, although it can form wherever the solubility limit is exceeded. This can also be related to pH changes caused by sludge degassing. Deposits tend to form in pipes where waste waters stand between intermittent flows, but also in areas where there is turbulent flow, pressure change or where cavitation occurs. Thus struvite deposition occurs most frequently in pipes, pipe elbows, valves, centrifuge weir overflows, pumps etc.
Struvite deposition is becoming an increasingly widespread problem for a great many waste water treatment plants, since it reduces throughputs, which could adversely affect subsequent sludge processing. In extreme cases the struvite can reduce the throughput so significantly that the plant operation has to be stopped and then the blocked pipework and other equipment must be cleaned. Generally struvite deposits can only be removed completely by treatment with acid. This can be expensive and involve potentially dangerous handling and disposal of cleaning products (acidic solutions) which can also cause excessive wear to the equipment. In addition the plant closure time during the cleaning operation can also be costly.
Various proposals have been made in order to deal with this problem, many with some degree of success, but none of them have successfully overcome this problem completely.
For instance it is known to attempt to control the release of phosphate biologically. In an article by Nic Booker CSIRO, Molecular Science, entitled, “Struvite formation in wastewater treatments plants: an accident waiting to happen?”, the issue of nutrient removal from wastewater discharges is discussed. The study claims that Biological phosphorus removal plants can reduce phosphorus concentrations to less than 1 mg/L. However there is a tendency for some plants with insufficient ready assimilable carbon in the feedwater to have difficulty in consistently achieving this level. In addition the biological phosphorus plants involve a number of different stages of treatment, including anaerobic, aerobic and anoxic zones and require long liquid residence times of around 24 hours.
There have also been suggestions to prevent struvite formation by removing ammonia and phosphate chemically by the addition of magnesium to raise the pH. However, in practice this was found to be unsuccessful on the whole.
Another proposal has been to add 20 mg/L ferric salts and 0.25 mg/L anionic polymer. However, the addition of such large amounts of ferric salt could have a detrimental effect on the plant equipment.
In JP-A-08-099091 it is proposed to prevent the generation of struvite scale in piping by chemical reaction/crystallisation employing special separation equipment to recover the crystals of struvite. However, installing such struvite precipitating equipment adds an additional burden to the operation of the waste water treatment plant.
There has also been a proposal to combine a sequesterant with a polymeric dispersant. However, to our knowledge we are not aware of the commercial success of such a system.
Thus there exists a need to simply and conveniently prevent or significantly reduce the formation of struvite in waste water treatment facilities. Furthermore there is also a need to find a simple and convenient treatment of aqueous systems containing struvite deposits.
Thus according the present invention we provide a method for reducing or preventing the formation of scale in a municipal or industrial waste water treatment facility comprising introducing an antiscalent composition which comprises:
A) A water soluble polymer of molecular weight below 20,000 and containing anionic groups, and
B) A sequesterant compound containing at least one phosphonate group and salts thereof.
We have found that the special combination of low molecular weight anionic water soluble polymer and phosphate sequesterant give surprisingly effective results in reducing or preventing scale, especially struvite in waste water treatment systems. Thus it is essential to the present invention that the sequesterant comprised in the antiscalent composition of the present invention is a compound containing at least one phosphonate group.
The antiscalent composition may be used for the treatment of any scale deposits in a water treatment facility. The method is of particular relevance when the scale deposits comprise crystalline precipitate formed by the combination of phosphate, ammonia and magnesium, which may exist as magnesium ammonium phosphate (MgNH
4
PO
4
.6H
2
O) which is generally referred to as struvite.
The method may be applied to any waste water treatment facility in which scale deposit are a particular problem, especially where the scale deposit is struvite. We have found the method is especially valuable in removing, reducing or preventing scale deposition in sewage treatment plants. Hence in a particularly preferred form of the invention we provide a method in which struvite deposition is removed, reduced or prevented in a sewage treatment plant employing as a treatment aid the aforementioned antiscalent composition.
A further aspect of the invention relates to the treatment of any aqueous system containing struvite deposit. Thus we provide the use of a composition comprising
A) A water soluble polymer of molecular weight below 20,000 and containing anionic groups, and
B) A sequesterant compound containing at least one phosphonate group and salts thereof,
for the treatment of aqueous systems containing struvite.
The antiscalent composition employed in the present invention preferably comprises 25 to 75% by weight of the water soluble polymer (component A) and 25 to 75% by weight of the sequesterant (component B). Generally we have found that for many treatments it is preferred that the antiscaling composition comprises 30 to 70% by weight, most preferably 40 to 60%, of the water soluble polymer and between 30 and 70% by weight, most preferably 40 to 60%, of the sequesterant.
The water soluble polymer component of the antiscalent composition comprises anionic groups. The polymer is desirably formed from ethylenically unsaturated anionic monomer or anionic monomer blend in which the monomer blend comprises at least one of the anionic monomer. Typically the anionic monomer or anionic monomer blend can be formed from at least one ethylenically unsaturated carboxylic acid, sulphonic acid or phosphonic acid monomer. The monomers include acrylic acid, methacrylic acid, maleic acid, maleic anhydride, fumaric acid, itaconic acid, crotonic acid, vinyl phosphonic acid, vinyl sulphonic acid, allyl sulphonic acid, 2-acrylamido-2-methylpropanesulphonic acid. When more than one monomer is employed in a blend, the monomer blend may comprise any of the aforementioned anionic monomer or alternatively a suitable ethylenically unsaturated nonionic monomer, for instance acrylamide or methacrylamide. The polymers may exist in the free acid form or more preferably they are partially or fully neutralised. A particularly preferred water soluble polymer is the homopolymer of acrylic acid, homopolymer of maleic acid, hompolymer maleic anhydride or a copolymer of acrylic acid with either maleic acid or maleic anhydride.
Typically the water solub
Ciba Specialty Chemicals Water Treatments Ltd.
Crichton David R.
Hruskoci Peter A.
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