GB2308590A - Removing phosphorus from waste water - Google Patents

Description

ELECTRICAL METHOD FOR REMOVING PHOSPHATE FROM A WASTEWATER This invention relates to sewage and industrial wastewater treatment.

The European Union's Urban Waste Water Treatment Directive, 1991, requires phosphorus levels to be reduced to less than 2 mg Total P/l for some small wastewater treatment plants in areas where eutrophication of rivers and lakes may occur. It is possible to do this by dosing chemicals such as ferric sulphate, ferrous sulphate or aluminium sulphate into sewage or industrial wastewaters. However, these chemicals are strongly acidic and most of these sites are not manned and hence there is a concern that they may present a possible danger to children and the public in general. A more passive method is desirable.

Ferrous (foe2+) and Ferric (foe=) ions will precipitate phosphorus as ferric phosphate, however iron in a solid state is only slightly soluble. The dissolution may be greatly enhanced by passing an electrical current between two electrodes with one of the electrodes being formed from iron or an iron alloy, e.g mild steel. This iron electrode will be the anode (positively charged) and will gradually dissolve. The second electrode, the cathode, (negatively charged) may be made from another material, e.g stainless steel. The cathode does not dissolve but may become coated with a white deposit (basically calcium carbonate) in some waters. The process generates hydrogen at a low rate.

The ferric phosphate that is precipitated is mixed with the biological sludge and leaves the system as a waste product.

Where it is possible to reverse the electrical current flow (and hence reverse the polarity of the electrodes) both may be made of iron or iron alloy and each can be dissolved.

Example 1 Figure 1 demonstrates the system when applied to the activated sludge process. The wastewater to be treated (1) is pumped into the open channel tank (2). The liquid is circulated around the channel and aerated by the paddle aerators (3) and (4). The wastewater comes into contact with a suspension of active microbial biomass (called activated sludge "mixed liquor"). During its passage around the aeration channel the liquid comes into contact with electrode plates (5) which are alternately spaced anodes and cathodes. A current is applied to these electrodes. Iron is dissolved from the anodes and reacts with the phosphorus in the wastewater to form a precipitate of ferric phosphate. A proportion of the mixed liquor is taken off to the settlement tank (6) where the microbial biomass is settled to form (a) a clear liquid effluent (7) which discharges from the top of the tank and (b) a settled sludge, part of which is retumed to the aeration channel via (8) (called "returned sludge") and some of which is wasted (9) (called "waste sludge"). The precipitated ferric phosphate leaves with the slurry of microbial biomass via pipe (9) and is wasted with the waste sludge.

Example 2 Figure 2 shows the system for use with a biological filter (1). The filter contains a bed of medium (2) on which biomass grows. The wastewater (3) sprays onto the top of the bed of medium where biological treatment takes place using the biomass growing there and the oxygen present in air which ventilates through channels between the elements of medium (4). The treated effluent and any biomass which breaks off the media leave via the outlet pipe (6) and pass through an open chamber (7) containing the alternately spaced anodes and cathodes (5). An electrical current is applied to these electrodes. Iron is dissolved from the anodes and reacts with the phosphorus in the wastewater to produce ferric phosphate.

The mixture passes to the settlement tank (8) where the effluent comes off as a clear liquid (9) and sludge containing the biomass and the precipitated ferric phosphate settle to the bottom of the tank from where it is wasted as a sludge (10).

Claims (9)

1 An electrically-operated process which can dissolve iron and thereby precipitate phosphate in a wastewater treatment process.

2 An electrically-operated process which can dissolve iron according to Claim 1 and thereby precipitate phosphate in the sludge of an activated sludge treatment process.

3 An electrically-operated process which can dissolve iron according to Claim 1 and thereby precipitate phosphate between a biological filter or biological aerated filter and a settlement tank.

4 A process which allows the current electrical usage to be accurately calculated to meet a set phosphorus concentration in the effluent.

5 A process which allows the re-use of scrap iron or iron alloys (e.g mild steel).

6 A process which allows the electrodes to be made of the same material and then by reversing the electrical current fiow allows equal consumption of iron from each electrode.

7 A process according to Claim 1 where the electrical power is supplied from the mains power supply.

8 A process according to Claim 1 whereby the electrical power is supplied by the use of wind or solar power for small un-manned sites.

9 A process whereby it is possible to reduce the effluent concentration to less than 1.0 mg Total P/litre.