Process for removing ammonia from waste waters

Details Process for removing ammonia from waste waters Process for removing ammonia from waste waters

1992-07-22

1994-03-15

McCarthy, Neil M.

Liquid purification or separation

Processes

Making an insoluble substance or accreting suspended...

210706, 210903, 210906, 423306, C02F 158, C02F 166

Patent

active

052943488

DESCRIPTION:

BRIEF SUMMARY
The invention relates to a process for removing ammonia from waste waters.
The removal of ammonia from waste waters is generally carried out by a biological treatment. Waters heavily laden with ammonia, containing more than one gram of ammonia per litre, are lethal to living organisms. In order to carry out the biological treatment of these waters, it is necessary to modify the process and to add nitrification and denitrification stages, which leads to high additional costs.
There are also physical processes for removing ammonia.
The stripping of ammonia with air or with steam requires major investment, which can only be borne by large plants.
Ion exchange is only conceivable for low concentrations. In fact, because of difficulties in treating the regeneration eluates, the problem is simply shifted.
Membrane techniques, such as electrodialysis, give goods results but still leave many questions unresolved, concerning the service life of the membranes, blockages, etc. The cost of these techniques is also high.
German Patent Application DE-A-3,732,896 describes a chemical process based on the formation, by successive addition of a phosphate and then of magnesia to waste waters, of a salt, magnesium ammonium phosphate, which is slightly soluble in aqueous media. This process does not allow the complete removal of the ammonia contained in the waste waters. With the aim of improving this process, and thus of increasing the degree of removal of the ammonia, European Patent Application EP-A-335,280, by the same inventor, proposes a three-stage process. This process consists in inserting an acid treatment between the phosphate addition and magnesia addition stages. This three-stage process is more complex and thus costly, and requires the handling of two solid reactants.
We have now found a process for removing ammonia in a single stage, using a single reactant.
One great advantage of this process is the use of a liquid reactant which is easy to handle and to measure.
Another advantage of this process is a degree of removal of ammonia which is markedly superior to that of the prior art.
An advantageous possibility is to follow the process according to the invention with a biological treatment, made possible by the reduction in the ammonia content of the treated waste waters.
Finally, it is very important to note that the process allows ammonia and phosphates to be removed from waste waters simultaneously.
The process according to the invention for removing ammoniacal nitrogen contained in waste waters consists in the precipitation of the ammoniacal nitrogen in the form of a magnesium ammonium phosphate, characterised by the addition to the waste waters of a single liquid reactant containing sources of magnesium ions, phosphate ions and an acid.
The magnesium ammonium phosphate corresponds to the formula NH.sub.4 MgPO.sub.4. It generally crystallises in the hexahydrated form. This salt is very sparingly soluble in water and it is even used in analytical chemistry for the determination of magnesium or of phosphoric acid.
The single liquid reactant preferably contains equimolar quantities of phosphate and magnesium ions. This corresponds to the stoichiometry of magnesium ammonium phosphate. The phosphate/magnesium ratio can however vary between approximately 0.5 and 1.5.
If the waste waters also contain phosphate ions in addition to ammonia, this is taken into account in the formulation of the reactant in order to obtain the desired stoichiometry.
The reactant generally contains approximately 0.8 to 2 moles of magnesium ions and of phosphates per mole of ammonium ion. An excess of magnesium and of phosphate ion in relation to the ammonium ions is generally used.
In general, an excess of 1.1 to 1.5 moles of magnesium ions and of phosphates per mole of ammonium ions is sufficient.
The source of magnesium ions is generally magnesium oxide or magnesia, MgO. It is, however, possible to use magnesium salts, such as the chloride or the sulphate.
The source of phosphate ions is generally phosphoric acid, H.sub.3 PO.sub.4, but

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