System for removing phosphorus from waste water

Liquid purification or separation – Processes – Treatment by living organism

Reexamination Certificate

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C210S631000, C210S714000, C210S721000, C210S724000, C210S747300, C210S151000, C210S170050, C210S199000, C210S203000, C210S206000, C210S532200, C210S903000, C210S906000

Reexamination Certificate

active

06623642

ABSTRACT:

Discharged domestic wastewater often contains phosphorus, in the form of dissolved phosphate, arising from household detergents, etc. Dissolved phosphate is a problem because, in a body of open water (e.g a lake), it can lead to blooms of algae, to the detriment of other life-forms.
In some cases, the limitation on the number of dwellings that can be permitted around a lake is defined by the phosphorus in the effluents from the dwellings. It is becoming common for authorities to impose levels for P-content, in discharged water, typically at 1 mg/liter, or less. Indeed, maximum permitted levels of 0.3 mg/liter are becoming standard.
BACKGROUND TO THE INVENTION
As is well-known, one way by which dissolved phosphate can be taken out of solution in wastewater is by adsorption of the phosphate onto a suitable sorbing medium. The present invention is not concerned with adsorption, but with taking the phosphorus out of the water by mineral precipitation reactions. In the invention, the aim is to convert the dissolved phosphate, by chemical reaction, into an insoluble solid, which precipitates.
It has been conventional, in some municipal sewage treatment plants, to address the problem of an excessive P content by dumping bags of alum into the water. The alum serves as a source of aluminum sulphate. Alum is very soluble. When the alum enters the water, Al
3+
and SO
4
2−
ions quickly enter solution. The dissolved Al
3+
ions combine with any phosphate PO
4
3−
ions that might be present, to form aluminum phosphate. Under the conditions of (approximately) neutral pH likely to be encountered in a sewage treatment system, the aluminum phosphate is insoluble, and precipitates. The precipitant may comprise the mineral variscite, AlPO
4
.2H
2
O.
The use of alum can be effective to drive down the phosphate-P content to 1 mg/liter, or less. However, one problem with the use of alum is that dissolved Al
3+
ions do not remain in solution for long. The Al
3+
ions have an affinity for phosphate, and so the phosphate PO
4
3−
ions that happen to lie close to the point at which the alum enters the water react with the dissolved aluminum Al
3+
ions, as desired, to form aluminum phosphate, which precipitates; however, the dissolved Al
3+
ions from the sulphate which do not immediately pick up phosphate PO
4
3−
ions, soon tend to react with the water, and to form aluminum hydroxide. Aluminum hydroxide, Al(OH)
3
, like the aluminum phosphate, is also insoluble at normally-encountered pH levels, so the hydroxide, too, precipitates, generally in the form of the mineral gibbsite.
The problem with putting alum into the water, to take out the phosphate, is that alum is very soluble, and the alum dissolves too quickly; but only a few of the many Al
3+
ions that enter the water actually reach, and react with, the phosphate ions to precipitate as aluminum phosphate; the remainder of the large quantity of aluminum ions that enter solution precipitate out as aluminum hydroxide (gibbsite). Because so many of the Al
3+
ions from the alum precipitate as the hydroxide before they can react with a phosphate ion, a large excess of alum is generally needed, in order to draw a given concentration of phosphate out of solution, by precipitating aluminum phosphate.
Thus, the technique of treating water contaminated with phosphorus by adding alum to the water results in the unwanted precipitation of large quantities of gibbsite (aluminum hydroxide). Besides, adding alum is labour-intensive; because alum dissolves quickly, the bags have to be added one at a time at regular intervals, rather than just once as a large volume that would last for a long period.
In a municipal sewage plant, at least the inconvenience associated with the use of alum can be managed, and it is conventional, in municipal plants, to treat phosphorus by adding alum to the water. But when it comes to domestic water treatment systems, e.g single-dwelling septic-tank systems, of course it is out of the question, as a practical system for treating excess phosphorus, to expect householders, every few days, to add a bag of alum into the water emerging from the septic-tank.
Gibbsite is (almost) insoluble, and, in the municipal systems, the large volumes of excess gibbsite precipitate as a flocculant in the sewage treatment. This adds to the sludge produced by the plant, which is a nuisance. It is recognised that, because alum is so soluble, though suitable as a treatment material for use in municipal systems, is only marginally suitable for use in domestic systems.
On the other hand, the ion-association reaction, in which dissolved phosphate PO
4
3−
combines with dissolved aluminum Al
3+
to form aluminum phosphate, is indeed an effective reaction for getting rid of the phosphate. The aluminum reaction is advantageous because aluminum phosphate is highly insoluble, and precipitates out of the water rapidly. The concentration of dissolved phosphate in the water can easily be reduced to 1 mg/liter, or less, when this reaction is able to take place properly and fully.
The invention is aimed at focussing the aluminum ion-precipitation reaction more efficiently onto the phosphate, whereby the phosphate can be precipitated, without the unwanted precipitation of large excess quantities of other unwanted minerals.
It is recognised that the ion-exchange reaction can be made to work with metals other than aluminum, but the aluminum reaction is the most economically practicable, and the invention is described herein as it relates to the aluminum reaction.
The invention is aimed at treating phosphorus by providing conditions for the ion-exchange reaction to take place, economically, and in a manner that requires little by way of on-going attention or maintenance. In particular, it is an aim of the invention to provide a phosphorus treatment system which requires no more attention than a conventional septic tank system. To be acceptable in a domestic water treatment context, a phosphorus treatment system must not require the householder to add a bag of treatment material into the water every few days; nor must it require the householder to clean out accumulated deposits of precipitated minerals. For the system to be acceptable, the service and maintenance demands should be compatible with those of a domestic septic tank water treatment system, i.e no more than once every year or two.
This is not to say that the invention is restricted only to domestic wastewater systems. It is recognised that the invention is suitable for use generally, in cases where the wastewater containing the dissolved phosphorus also contains dissolved ammonium.
THE GENERAL FEATURES OF THE INVENTION
It is known that the solubility of aluminum hydroxide (gibbsite) increases as the pH of the water decreases. For example, a typical septic-tank wastewater effluent, when at a neutral pH, has a solubility of aluminum hydroxide of only about 0.01 milligrams per liter; whereas, once the pH drops below about 5.5, the solubility of aluminum hydroxide in otherwise the same water increases sharply. The solubility of aluminum hydroxide reaches 1 mg/l at a pH of about 4.8, and 10 mg/l at a pH of about 4.5.
The invention involves creating the conditions whereby the pH of the phosphorus-contaminated water is lower than about 5.5, and preferably lower than about 5. It is recognised, in the invention, that when the pH has dropped to this low value, now the source of the Al
3+
ions needed for the formation and precipitation of aluminum phosphate can be aluminum hydroxide (gibbsite). Thus, instead of gibbsite being the insoluble substance that, unfortunately, precipitates in large quantities following the introduction of excessive quantities of aluminum sulphate (alum) into solution, now, at the low pH, gibbsite is soluble enough, itself, to serve as the source of the Al
3+
ions. As the water becomes acidic, more gibbsite dissolves, and more Al
3+
ions go into solution; if phosphate is present in the water, the Al
3&pl

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