SE464520B - Process for removing phosphorus and nitrogen from waste water - Google Patents

Abstract

The invention relates to a process for removing phosphorus and nitrogen from waste water. Waste water from a conventional sedimentation step is subjected to the action of anaerobic bacteria in an anaerobic reactor, with phosphorus being released in the form of phosphate ions and nitrogen being released in the form of ammonium ions, after which the water from the anaerobic reactor is treated with magnesium ions and phosphate ions in a precipitation step in order to precipitate out both the ammonium ions and the phosphate ions in the form of struvite. <IMAGE>

Description

464 520 2 anaerobic and finally aerobic environment. This makes the process very difficult to handle. Even if the denitrification step is placed after the first sedimentation step and thereby an aeration step is eliminated, the process becomes sensitive to environmental changes.

Municipal wastewater contains phosphorus and nitrogen, which are important nutrients for the biomass in the recipient. Excessive emissions of these nutrients lead to large bioproduction, which means that lakes and watercourses are eutrophied and, among other things, become unusable for human recreational and outdoor activities. The object of the present invention is to eliminate phosphorus and nitrogen in the wastewater and to reduce the sensitivity of the purification process to variations in water quantity and water quality.

The invention thus relates to a process for removing phosphorus and nitrogen from wastewater, and this process is characterized in that wastewater from a conventional sedimentation step is subjected to the action of anaerobic, sulphate-reducing bacteria in an anaerobic reactor for releasing phosphorus in the form of phosphate and form ions. of ammonium ions, whereby sulphate is introduced into the anaerobic reactor, and that the water from the anaerobic reactor in a precipitation step is treated with magnesium ions and phosphate ions to precipitate both the ammonium ions and the phosphate ions in the form of struvite.

A preferred embodiment of the invention is described below with reference to the accompanying drawing.

The first step (1) is, as in existing plants, a sedimentation step for easily sedimented material.

In the second step (2), the anaerobic environment already formed in the wastewater during its journey to the treatment plant is utilized. In the anaerobic reactor used, organic sludge will be deposited on the bottom and subjected to the decomposition of anaerobic, sulphate-reducing bacteria, for example of the Desulfovibrio type. In this anaerobic environment, phosphorus in the form of phosphate ions and nitrogen in the form of ammonium ions are released. Sulfate in the form of, for example, sodium sulphate is introduced into the anaerobic reactor.

The sludge at the bottom of the reactor is successively separated for further treatment in a sludge thickener (4). The sludge is then transported to a methane fermentation reactor (5), or it is deposited or incinerated. The water from the sludge thickener goes to an aeration stage (6). The reaction in this anaerobic reactor is less sensitive to environmental changes than a denitrification step or an aerobic bacterial aeration step, since the bacteria in the anaerobic reactor are not in suspension but are attached to the sedimented sludge.

The water from the anaerobic reactor is passed to a third stage (3) comprising a reactor for chemical precipitation of struvite (MgNH4PO4-6H2O), which is a mineral with low solubility product. In general, there is an excess of nitrogen in relation to phosphorus and magnesium, so an addition of both magnesium ions and phosphate ions will be needed. This can be done by additions of eg MgCl2, MgSO4 or MgCO3 and eg K2HPO4 or KHZPO4. The amounts added are determined by analyzes of the levels of phosphate and ammonium as well as by chemical equilibrium calculations. After sedimentation, possibly by adding flocculants, the water is allowed to proceed to a final aeration step (6). The struvite sludge can be used as a fertilizer but can also be further treated together with the sludge from the anaerobreactor in a reactor for methane fermentation (5).

The reactor for methane fermentation (5) can be designed in the usual way, as is the case at several treatment plants already today. The water from this reactor may conveniently be wholly or partly recirculated to the anaerobic reactor (2). The anaerobic environment in this reactor is thereby improved, whereby a more stable system is obtained. A methane fermentation step also entails a reduction in the amounts of sludge that must go to landfill.

Claims (4)

'¿E4 520 4 P a t e n t k r a v A process for removing phosphorus and nitrogen from wastewater, characterized in that wastewater from a conventional sedimentation step is subjected to the action of anaerobic, sulphate-reducing bacteria in an anaerobic reactor for releasing phosphorus in the form of phosphate ions and sulphide in which ammonium in the anaerobic reactor, and that the water from the anaerobic reactor in a precipitation step is treated with magnesium ions and phosphate ions to precipitate both the ammonium ions and the phosphate ions in the form of struvite. Method according to claim 1, characterized in that the water from the precipitation step is aerated in an aeration step before it is discharged into a recipient. 3. A method according to claim 1 or 2, characterized in that flocculant is added in the precipitation step. 4. A process according to any one of claims 1-3, characterized in that the sludge from the anaerobic reactor and the struvite sludge from the precipitation step are treated in a methane fermentation reactor, and that the water from this reactor is recycled to the anaerobic reactor.