Semiconductor device manufacturing: process – Making field effect device having pair of active regions... – Having insulated gate
Reexamination Certificate
2000-03-13
2001-02-27
Chaudhari, Chandra (Department: 2813)
Semiconductor device manufacturing: process
Making field effect device having pair of active regions...
Having insulated gate
C438S130000
Reexamination Certificate
active
06194275
ABSTRACT:
BACKGROUND TO THE INVENTION
In many ponds, lakes, and other bodies of water, the nutrient balance is such that phosphorus is the nutrient that limits the growth of algae. An increase in the amount of phosphorus in the water can cause an excess of algae. This can deplete the oxygen and the other nutrients in the water, to the detriment of fish and other life forms. It is therefore important to prevent excess phosphorus from entering the body of water.
Often, excess phosphorus comes from sewage. Sometimes, the excess phosphorus is in the effluent from small-scale domestic septic tank systems, and in some cases the factor that limits how many dwellings can be located in an area near a lake is the fact that any further dwellings will cause an unacceptable increase in the amount of phosphorus entering the lake.
The traditional way of limiting phosphorus has been to limit the number of dwellings. Conventional sewage treatment, especially of the kind carried out in a traditional domestic septic tank, has not been aimed at removing, and does not remove, the phosphorus to any substantive degree.
The invention is aimed at treating sewage water, to attenuate the phosphorus content thereof. Another aim is to provide a phosphorus treatment system which operates on the water as the water is passing through the septic-tank or other sewage treatment system, whereby the costs attributable solely to the phosphorus treatment system are minimised. Another aim is to provide a treatment system which requires nothing, or very little, by way of on-going attention and maintenance, over long periods of use.
GENERAL FEATURES OF THE INVENTION
The invention lies in placing a body of RID material in the water to be treated, and keeping the water, and the RID material, under anaerobic conditions. Preferably, the anaerobic conditions are so strict that the Eh voltage of the water becomes very, low, or negative.
For the purposes of this specification, RID material means Reductive Iron Dissolution material. That is to say, RID material is material that contains ferric iron, and the ferric iron therein is capable of undergoing a change in oxidation state, i.e is capable of being reduced, to ferrous iron, under conditions of anaerobicity, i.e conditions of low or negative Eh voltage.
As to pH, generally sewage water tends to of neutral, or near-neutral, pH, under which conditions the newly-created ferrous ions are much more soluble than the ferric ions, and readily pass into solution in the water.
The phosphorus-attenuation systems as described herein make use of the fact that iron in the ferric oxidation state Fe
3+
is very insoluble at neutral pH values, but in its ferrous oxidation state Fe
2+
is much more soluble. The invention recognises that in anaerobic (low Eh) waters, ferric iron is unstable and can be solubilized by reduction to Fe
2+
. The dissolved Fe content in anaerobic waters can thus rise to tens of milligrams Fe per liter of water (and considerably more than that in some cases).
The presence of large amounts of dissolved iron in the water leads to phosphate precipitation by two mechanisms. First, in the anaerobic environments, when phosphate is present, increased iron levels lead to the formation of ferrous-phosphate minerals, such as vivianite Fe
3
(PO
4
)
2
.8H
2
O which is insoluble enough to precipitate. Second, when the effluent is subsequently oxidised, any still-remaining dissolved ferrous ions tend to start to oxidise back to ferric ions, but because of the still-dissolved phosphate, the ferric ions now tend to form ferric-phosphate compounds, such as strengite FePO
4
.2H
2
O which again is insoluble enough to precipitate.
Thus, the dissolved ferrous iron provides two mechanisms for drawing the phosphate ions out of solution, and causing them to precipitate
In fact, a third mechanism can be present also. Oxidation of the ferrous ions back to ferric will result in precipitation of some ferric hydroxide or oxyhyroxide, besides the strengite and other ferric-phosphate minerals. Ferric oxyhydroxide serves as an excellent medium for adsorbing phosphate ions out of solution.
When all three mechanisms are present, the phosphate is removed (a) by precipitation in the anaerobic area as vivianite, (b) by precipitation in the aerobic area as strengite, and (c) by adsorption. All three mechanisms, however, require first that insoluble ferric iron be converted to soluble ferrous iron, which requires a low Eb; and it is recognised that raw sewage, held under anaerobic conditions, provides an excellent medium for promoting the reductive dissolution of ferric iron.
The RID material should be rich in ferric iron. Ferric hydroxide Fe(OH)
3
or oxy-hydroxide FeOOH compounds are suitable. (Ferric oxide, e.g hematite Fe
2
O
3
would be unsuitable, because its mineral structure makes hematite less susceptible to reductive dissolution.
The RID material that is to be placed in the anaerobic conditions, i.e the material that contains the ferric iron, need only, contain a little iron, as an overall percentage. The minimum effective iron content, for the purposes of the invention, may be regarded as being about 1 milli-gram of iron per gram of aggregate material (i.e 0.1%/wt). The iron content referred to here is the content of ferric iron that can be reduced to ferrous iron (which is sometimes called the “acid-extractable” iron). The preferred ferric iron content in the RID material is in the region of 0.3%/wt to 5%/wt.
As to its physical or mechanical character, the RID material should be physically capable of releasing its ferrous ions into the water. The RID material should be porous, and permeable to the passage of a flow of water through the material. The RID material may include a matrix of sand, which is chemically inert, which supplies the grain size for maintaining permeability. The RID material preferably may be supplied as sand-sized grains, the grains being coated with powder-sized particles of the ferric substances.
Ferric hydroxides and oxyhydroxides are available quite readily in many places, in the soil-B horizon; that is to say, in the mineral-enhanced area underneath the top-soil. Suitable ferric hydroxide compounds have the characteristic red-orange-brown colour associated with a high ferric content.
If the ferric content of the locally-available material is inadequate, extra ferric can be provided in the form of e.g Fe(OH)
3
(ferri-hydrite) or FeOOH (geothite). These minerals are readily available in powder form (they are used for making pigmentations), and can be used with bulk filler such as local sand.
Providing RID material with a ferric content of more than about 5%/wt would not be suitable, because the ferric hydroxide and oxyhydroxide materials minerals are generally of a fine-grained nature, and the greater percentage would, or might, diminish permeability too much.
The degree to which the phosphorus contamination in the water is attenuated depends on the amount of iron that is solubilized. In a typical real case, the residence time needed for an effective amount of ferrous ions to enter the water may be regarded as being in the order of a day or two; that is to say, about the same residence time as the water in a well-engineered small system spends in the anaerobic septic tank.
Typically, a system that handles a through-flow of 1000 liters/day of sewage has a septic tank volume of about 3500 liters. In a case where the presence of phosphorus is troublesome enough to warrant being addressed by the system as described herein, the concentration of phosphorus in the water might be, for example, 10 mg-P per liter of the water. Such a system would need to be supplied with fresh ferrous iron entering solution at the rate of about 5 grams Fe per day. Given that the body of RID material contains, for example, about 2%/wt of ferric iron, a mass of RID material of about 1800 kilograms (which would be about 1 cubic meter of material) can be expected to go on releasing ferrous iron into solution at that sort of rate for many months, and possible for many years.
It is recognised
Cheng Tao
Wu Lin-June
Ackerman Stephen B.
Chaudhari Chandra
Pike Rosemary L. S.
Saile George O.
Taiwan Semiconductor Manufacturing Company
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