The present invention relates to a method of reducing the chemical oxygen demand of water obtained by reverse osmosis, said method comprising the steps of

- subjecting water containing a sulphur compound to reverse osmosis resulting in

- a retentate, and

- a permeate comprising at least some of the sulphur compound; and

- subjecting the permeate to a further purification step.

In the art it is known to process water using reverse osmosis for various reasons. An important field of application is the purification of effluent water before it can be safely discharged. The effluent typically contains organic molecules, for example bio-organic

molecules such as proteins, amino acids or degradation products thereof. They may be capable of passing through the reverse osmosis membrane and thus could end up in the environment if the permeate were discharged. However, discharge is problematic because the chemical oxygen demand (COD) of the permeate is too high. Thus it is known in the art to subject the permeate to a further treatment to lower the COD, but achieving the required low COD level adds considerably to the expense. In addition, introducing oxygen into the oxygen-depleted water results into sulphur-containing precipitates causing maintenance problems .

The object of the present invention is to provide a simple and cost-effective method of reducing the chemical oxygen demand of permeate obtained by reverse osmosis .

To this end, a method according to the preamble is characterized in that with the permeate comprising at least some of the sulphur compound having

- a pH of 3.0 to 6.5, and

- a conductivity of less than 75 pSiemens/cm;

the further purification step involves spraying the permeate

comprising at least some of the sulphur compound in air.

Without wishing to be bound by any particular theory, it is believed that the presence of the sulphur-comprising compound is the explanation for the difficulty experienced to reduce the chemical oxygen demand to the desired extent. Oxygenating permeate by

introducing air according to the state of the art results in an inconvenient sulphur-comprising precipitate. The inventor has found that by spraying in accordance with the method according to the present invention, volatile sulphur compounds, such as H ₂ S, are released from the spray droplets into the atmosphere. This reduces the sulphur concentration to a level where the formation of

sulphur-comprising precipitate is reduced. It should be noted that its concentration in air is sufficiently minor to be environmentally acceptable. Also, the quick rise in oxygen concentration may

contribute to the prevention of inconvenient precipitate.

In the method according to the present invention, the permeate will typically be subjected to a pre-treatment to adjust at least one of the pH and the conductivity, for example by cation-exchange. It is preferred that ammonium is reduced to a concentration of less than 1 mg/L before spraying the permeate, more preferably to a concentration of less than 0.5 mg/L

The nozzle used for spraying is preferably a so called "hollow cone" nozzle.

The nozzle used for spraying is preferable a nozzle capable of spraying with a spraying angle of at least 90°. It is possible to spray downward, sideways or upward. Upward spraying can adversely affect the efficient collection of sprayed water, in particular in case of wind.

Typically, the droplets will travel at least 2 meters through the air, preferably at least 3 meters, before they are collected.

Spraying in air is preferably done outdoors.

According to a favourable embodiment, the permeate comprising at least some of the sulphur compound contains ammonium-ions and is subjected to cation-exchange before the step of spraying the permeate.

This allows for the water to contain a reduced amount of

ammonium, which has been found to help to reduce the sulphur-related problems .

According to a favourable embodiment, the cation-exchange is performed using a cation-exchange resin, wherein said cation-exchange resin is equilibrated with an acid having a pKa of 2 or less,

preferably 1 or less. Such a cation-exchange material is more efficient at removing ammonia from the liquid.

According to a favourable embodiment, spraying in air involves spraying at a height of at least 4 m above a collection area, the height being preferably at least 6 m.

This has been found to be a cheap and effective way of spraying and of collecting the water after spraying. Nozzles will be preferably be pointing downward to reduce the collection area. The collection area may be a pool.

According to a favourable embodiment, the pH of the permeate comprising at least some of the sulphur compound is between pH of 3.5 to 5.5, preferably between 4 and 5.

It has been found that this promotes the reduction of the sulphur concentration .

According to a favourable embodiment, the permeate comprising at least some of the sulphur compound has a conductivity of less than 35 pSiemens/cm, preferably less than 20 pSiemens/cm, and more preferably less than 15 pSiemens/cm.

It has been found that this promotes the reduction of the sulphur concentration .

According to a favourable embodiment, water comprising material of biological origin is subjected to the reverse osmosis to result in the water containing a sulphur compound.

The biological origin may for example be biological material in effluent from the food industry.

According to a favourable embodiment, the water comprising material of biological origin is derived from excrement from animals .

Treatment of liquid manure or water from solid or semi-solid manure is an important field of application for the present invention, serving as a source of water comprising a sulphur-comprising compound. Typically, for semi-liquid or solid manure, solid material is

separated using a filter belt press.

According to a favourable embodiment, at least one cation chosen from calcium and magnesium is added.

This allows for discharge of the purified water into the

environment. It has been found that when the purified water is discharged into water of a river or creek, the presence of the cation prevents the formation of a haze (precipitate) caused by the high level of dissolved oxygen present in the purified water according to the present invention. Preferably both cations are added.

According to a favourable embodiment, the purified water is remineralised by passing the purified water over rock material, preferably dolomite.

Thus the remineralisation is performed at very low cost and without any complicated means for process control necessary. Dolomite has been found to be well suited. The rock material comprises for example pieces of rock, which may be present in the collection area. Alternatively, the rock material may be present in a tank into which water from the collection area is passed.

The present invention will now be illustrated with reference to the drawing where

Fig. 1 depicts a flow diagram of an embodiment of the method according to the present invention.

Fig. 1 shows a flow diagram of an embodiment of the method according to the present invention.

To allow discharge of water obtained from liquid pig manure onto the surface water, the method involves the following steps

1} Reverse osmosis with a first reverse osmosis unit 110 essentially as disclosed in NL2007041. It comprises a first reverse osmosis membrane 111 that is a 99% salt rejection membrane.

II) Reverse osmosis of the permeate of step I) with a second reverse osmosis unit 120. It comprises a second reverse osmosis membrane 121 that is a 99% salt rejection membrane. The retentate was passed back to the first reverse osmosis unit 110 of step I) .

III) Reverse osmosis of the permeate of step II) with a third reverse osmosis unit 130. It comprises a third reverse osmosis membrane 131 that is a 99% salt rejection membrane. The retentate was passed back to the second reverse osmosis unit 120 of step II) .

IV) Ion-exchange of the permeate of step III) using a

cation-exchange unit 140 containing cationic resin, equilibrated with HC1 as a strong acid to remove ammonium.

V) Spraying of the liquid that passed the cation-exchange material of the cation-exchange unit 140 in air using a spray device 150 comprising downwardly facing nozzles on a pole and at a height of 6 meters. The droplet size was smaller than typical rain drops but a fine spray is not desirable because it is taken away with the wind to _c

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easily .

VI) Contacting water collected after spraying in step V) in a reminerali zat ion unit 160 comprising a tank filled with lumps of dolomite .

Several experiments were performed using the above steps. The results of one of these experiments are given below, demonstrating the reduction in the chemical oxygen demand (COD) .