Field of the invention The invention relates to a device and process for recovering surfactants, especially in the form of a paste from water, particularly wastewater.

Background of the invention Water is necessary for a variety of activities that we do at home, such as bathing and washing clothes. The utilised water is generally discarded as waste. Industries also generate large amount of wastewater. Its composition varies according to the nature of activities of the concerned industry. In manufacturing sites where personal care products such as shampoos and shower gels and home care products such as laundry and dish wash liquids are made, the wastewater usually contains substantial amount of surfactants.

Industries that produce or utilize surfactants usually purify their wastewater by some means, albeit, with possibly different objectives. It could be to comply with

environmental regulations or to recycle/reuse a portion of the water. Some

technologies were developed to treat water to make at least a part of it reusable for secondary uses. The main technology available for treating surfactant-rich waste water is based on aeration in the presence of microorganisms, which are typically present in human and animal feces. These microorganisms degrade the surfactants and other organic ingredients present in wastewater by utilizing the dissolved oxygen which is replenished by aeration. During the above process the mass of microorganisms grows significantly. It is allowed to settle outside in a settling tank and separated as sludge. This sludge is dewatered and disposed. Therefore, in such technologies, the foam and thereby the surfactant(s) present in the wastewater, is either destroyed or discarded as waste.

An article published in Journal Of Environmental Chemical Engineering 5 (2017) 1586- 1596 (Srinet et.al.) discloses that foam fractionation is an effective method for the removal and recovery of surfactants from water. In this work, removal of an anionic surfactant (i.e. sodium dodecyl benzenesulfonate) is studied at high concentrations in a batch foam fractionation column. The anionic surfactant is separated in solid/paste form near the top of the column and the recovery is about 50 to 60%.

WO2015/52036 A1 (Unilever) discloses a device for purification of grey water. The device comprises an airtight container for holding grey water which has an outlet port for discharging the foam. The container has spargers for introducing a gas. A tube, called foam conveyor connects the outlet port to a foam-breaking device. This device has an inverted cup with the open mouth of the cup positioned in a collection tray, a disc connected to a motor to facilitate rotation of the disc, placed inside the cup and rotatably connected to base of the cup. The foam conveyor establishes fluid communication between the outlet port and the disc of the foam-breaking device. The surface of this disc is rough. The foam is substantially dry and has very little liquid content when it reaches the foam-breaking device. The surfactants get destroyed as a result of the process.

W016033648 A1 (Nexus Ewater Pty Ltd) discloses an apparatus for removing surfactants from wastewater. It has a vessel with inlet for wastewater containing air bubbles and an outlet for treated water. The vessel has at least one tubular member having a lower open end into which the flow of wastewater from the inlet may be directed in use. The tubular member extends from the inlet towards a top of the vessel where an upper exit from the tubular member is defined. In use, most un-aerated water entering the vessel through the inlet can exit the tubular member at the lower open end. Foam travels up the tubular member to the upper exit.

In use, greywater passes into the system and is aerated by venturi effect which vigorously introduces large quantities of air into the greywater before it enters the vessel. The air entering the wastewater forms bubbles which are then sheared into smaller bubbles by static mixer. The surfaces of these bubbles get coated by a mono layer of surfactant molecules arranged so that their hydrophobic portions generally face the air inside the bubble and their hydrophilic portions face the body of the water. Organic material and particulate matter in the grey water also adsorb to the surface of those bubbles. The mixture of wastewater and air then enters the bubble separator into the lower portion of the tubular member via the narrow inlet tube. As the inlet tube is considerably narrower than the tubular member, the velocity of the water drops as it enters the tubular member and most of the un-aerated water being denser than the aerated water escapes under gravity though the opening at the lower end of the tubular member. The foam gradually collapses as it exits the bubble separator.

There is another type of disclosures in which attempt is made to recover at least a part of surfactant(s) present in the wastewater. While the recovered surfactant(s) may not be fit for reuse for the very same purpose (like use in shampoos or personal wash products), the surfactants nevertheless are fit for ancillary applications where purity or origin of the surfactant(s) is not a serious concern e.g. floor cleaning compositions.

US2013319947 A (Nexus Ewater Pty Ltd, 2013) discloses a process and apparatus that treats waste water containing surfactants. The process comprises collecting waste water in a reservoir, injecting gas into waste water to form bubbles, separating the bubbles along with entrained contaminants and drying the separated bubbles followed by returning any water recovered from the dried bubbles to the waste water. JP6226248 A (Matsushita Electric Ind Co Ltd. 1994) discloses a surfactant separator for a washing machine having a tank for storing waste water liquid in which air bubbles are created by aeration. The surfactants in the waste water adsorb on the surface of the air bubbles and float. The air bubbles are introduced to a bubble rupturing means where the surfactants are recovered into a treating tank by liquefying the air bubbles.

JP6178985 A (Matsushita Electric Ind Co Ltd. 1994) discloses a surfactant recycling device for recovering a surfactant from waste water. It has an aerator which generates the foam to adsorb the surfactant in the waste washing water, a foam stagnating device which stagnates this foam a foam rupturing device which crushes the foam and a storage tank which stores the recovered surfactant. About 50% of the initial surfactant content is recovered. The state of the art does not disclose a process or a device for recovering significantly more than 50% of the total surfactant(s) in paste form.

However, there is need for such a device and method.

Summary of the invention

It has been determined that a solution lies in the device and the method in accordance with this invention.

In accordance with a first aspect is disclosed an apparatus (1 ) for recovering surfactants present in water, comprising:

(i) a container (2) having inlet for water containing surfactants and an outlet for foam;

(ii) provision (8) comprising a sensor to stop admittance of water when said container is filled to about 1 to about 80 % of its capacity;

(iii) Provision (3, 9, 10) to introduce bubbles of a gas into said water to generate said foam;

characterized in that,

said outlet comprises plurality of perforations (6) for said foam to pass through leaving behind residue of recovered surfactants.

In accordance with a second aspect is disclosed a method of recovering surfactants present in water (1 1 ), comprising the steps of:

(i) admitting said water (1 1 ) into a container (2) as claimed in any of claims 1 to 11 to about 1 to about 80 % of its capacity using a sensor;

(ii) introducing bubbles of a gas into said water under pressure of 10 ³ to about 10 ⁷ Pa and flow rate of 0.1 to 10000 litres per minute to generate foam;

(iii) allowing said foam (12) to pass through said plurality of perforations (6) leaving behind residue of recovered surfactant (13) on said perforations (6). Detailed description of the invention

The features and advantages of the invention will become apparent to those of ordinary skill in the art from a reading of the following detailed description and the appended claims. For the avoidance of doubt, any feature of one aspect of the present invention may be utilized in any other aspect of the invention. The word "comprising" is intended to mean "including" but not necessarily "consisting of" or "composed of." In other words, the listed steps or options need not be exhaustive. It is noted that the examples, embodiment and figures given in the description below are intended to clarify the invention and are not intended to limit the invention to those examples per se.

Unless otherwise specified, numerical ranges expressed in the format "from x to y" are understood to include x and y. In specifying any range of values or amounts, any particular upper value or amount can be associated with any particular lower value or amount. Except in the examples and comparative experiments, or where otherwise explicitly indicated, all numbers are to be understood as modified by the word "about". All percentages and ratios contained herein are calculated by weight unless otherwise indicated. As used herein, the indefinite article "a" or "an" and its corresponding definite article "the" means at least one, or one or more, unless specified otherwise. The various features of the present invention referred to in individual sections above apply, as appropriate, to other sections mutatis mutandis. Consequently features specified in one section may be combined with features specified in other sections as appropriate. Any section headings are added for convenience only, and are not intended to limit the disclosure in any way. The invention is not limited to the embodiments illustrated in the drawings. Accordingly it should be understood that where features mentioned in the claims are followed by reference numerals, such numerals are included solely for the purpose of enhancing the intelligibility of the claims and are in no way limiting to the scope of the claims. The examples are intended to illustrate the invention and are not intended to limit the invention to those examples per se.

The apparatus of the invention The apparatus is meant for recovering surfactants from water, especially wastewater. The water may be from any known source so long as it contains one or more surfactants in appreciable quantities which renders it fit for being subjected to the method of the present invention. The apparatus of the invention allows recovery of surfactants in a non- destructive manner.

In one aspect the water is industrial wastewater containing one or more surfactants. The concerned industry may be any industry engaged in the manufacture of cleaning products which contain surfactants.

Alternatively, the water is greywater, also sometimes spelt as graywater. The waste water generated at home as a result of washing and rinsing for domestic laundry is generally referred to as greywater. In order to reuse such household grey water for various activities it is essential to remove all the particulate matter and the surfactants.

The container can be of any suitable shape and size. However, it is preferred that the container is vertical and tubular. It is further preferred that cross-section of the container is circular, oval or polygonal. Alternatively, but less preferably, the chamber is of an irregular shape. The term polygonal includes triangular, rectangular, square and other commonly known shapes. It is preferred that the walls of the container rise upwardly and outwardly. The container would then resemble the shape of a household bucket.

The container comprises an inlet for admittance of surfactant-rich water. This inlet is preferably in the form of a pipe/tube but it could be any feature that permits entry of such water into the container. This container comprises a means to close the inlet when the chamber is filled to the extent of 1 to 80 % of its capacity. The purpose is to stop the admittance of surfactant-rich water as per the need and/or operating conditions. This feature is necessary so that the container has enough empty space to hold the foam and to provide the necessary residence time to the foam. The empty space required is in proportion with the quantity of water for treatment and the amount or concentration of surfactants in it. The inlet could be located at any suitable position. However, it is preferred that the inlet is located at or above the halfway mark of the container considering the total height of the container. There may be more than one inlets. If there were not enough space for foam, then it would affect the wetness or water content of the foam, which in turn could adversely affect the method of the invention.

The container comprises a provision for introducing bubbles of a gas to generate foam. The means is in the form of plurality of nozzles to introduce a gas inside the container, i.e. in the surfactant-rich water, under pressure of 0.01 to 1000 psig and flow rate of 0.1 to 10000 litres per minute. The gas is bubbled to generate foam inside the container. It happens due to interaction of the gas with surfactants. Known alternatives of nozzles include spargers or manifold with holes in it. It is preferred that the nozzles (or spargers) are located in such a way that in use, they always remain submerged in the surfactant-rich water. Therefore, it is preferred that the nozzles or spargers are located at or near the floor or bed of the chamber. It is further preferred that the nozzles are configured such that the gas bubbles are distributed uniformly across the floor of the container. It is also preferred that the pore size of the gas spargers are in the range of 0.01 to 1 mm. The gas could be introduced by means of any pressurizing device such as a compressor, air-pump or air-blower, which may form part of the apparatus of the invention. For this purpose, use may be made of a miniature compressor or a gas pump capable of generating and sustaining the required pressure, which allows the gas to form bubbles leading to the generation of stable foam. Preferably, the gas is admitted into the container by connecting the pump and the nozzles with a strong yet flexible pipe/hose of any suitable material.

The container has an outlet for foam. The outlet comprises plurality of perforations to allow foam to pass through them leaving behind residue of recovered surfactants thereon. Preferably the plurality of perforations are located at or near an end of the container which, in use, is the designated upper end of the said vertical container. Further preferably the perforations are circular, oval or polygonal. Further preferably the longest dimension of each perforation is from about 0.1 mm to 20 mm. When the perforations are circular, the longest dimension is its diameter.

It is preferred that the outlet is in the form of an opening sealable with a lid, where said lid comprises said plurality of perforations. In other words, the lid with openings (also known as holes or orifices) is like a sieve. The lid can be of any shape but it preferably is shaped corresponding to the shape of the outlet. The container is airtight, except for the provision of the perforations which allows for the gas to pass through. When the outlet is in the form of a lid, the lid ipso facto also serves as an inlet which can be accessed by opening it. In such a case, the inlet is the same opening as the outlet and the outlet/inlet is closed in operation by a lid which has the perforations.

It is preferred that the capacity of said container is 10 to 100,000 litres but it may be designed as per need. As indicated earlier, in use, the total available volume is not occupied by the water containing surfactants. Some space needs to be kept for the foam.

The container is made of a material which is capable of withstanding low to moderate pressures of upto 6895 X 10 ³ Pa. For this reason, the container is rigid. By rigid is meant that it is made of a material that can withstand the pressure. Suitable materials of construction include metals like iron and copper, or alloys like steel, or even plastics, glass, reinforced plastics and laminates. Alternatively, the container is made of concrete or similar building material. It is preferable to have a means to accelerate drainage of water from inside said container. Such means include at least one of substantially vertical walls, pipes, channels or substantially horizontal perforated trays and separators.

It is preferable to have a discharge port, at or near the bottom of the container for discharging the surfactant-free (on a relative/comparative scale) purified wastewater.

It is further preferred that the apparatus comprises a mechanism to continuously or intermittently scrape-off the recovered pasty surfactants from the perforations so as to allow for smooth and unobstructed functioning of the device. The mechanism could be in the form of a rotating arm having a knife-like edge. The above system can be provided by means to ensure that the upper surface of the perforated lid allowed to remain substantially dry, depending on the ambient humidity. The method of the invention

In accordance with a second aspect the invention provides a method of recovering surfactants present in water, comprising the steps of:

(i) admitting said water into a container as claimed in any of claims 1 to 1 1 to about 1 to about 80 % of its capacity using a sensor;

(ii) introducing bubbles of a gas into said water under pressure of 10 ³ to about 10 ⁷ Pa and flow rate of 0.1 to 10000 litres per minute to generate foam;

(iii) allowing said foam to pass through said plurality of perforations leaving behind residue of recovered surfactant on said perforations.

Preferably the concentration of surfactants in said water is from 100 to 50000 ppm. This is the initial concentration present in the water. Upon following the method of the invention, about 50 to about 95 % of the initial surfactant content is recovered in about 50 to about 65 hours.

It is preferred that size of at least 50 % of the gas bubbles is in the range of 0.01 to 1 mm. It is preferred that the gas is one or more of air, oxygen, nitrogen, or carbon dioxide. Air the preferred gas. The gas could be introduced by means of any pressurizing device such as a compressor, air-pump or air-blower. For this purpose, use may be made of a miniature compressor or a gas pump capable of generating and sustaining the required pressure which allows the gas to form bubbles leading to the generation of stable foam. Preferably, the gas is admitted into the container by connecting the pump and the nozzles with a strong yet flexible pipe/hose of any suitable material.

Foam Residence Time: Foam Residence time is the average time spent by foam inside the container before it exits through the perforations.

Residence time can be calculated by applying the following formula. Residence time = Head space volume of the container/flow rate of the gas

It is preferred that the apparatus is operated by controlling the concerned parameters such that the residence time is 1 to 300 minutes, more preferably 30 to 120 minutes.

The method may be carried out for as long a period as desired. This period will depend on several factors, primarily the volume of water to be purified and concentration of the surface-active agent. It is preferred that flow-rate of said gas is maintained in the range of 0.1 to 1000 litres per minute during the entire duration of the method. Further preferably ratio between flow rate of the gas to the surface area of the foam is maintained such that the rate of entry of gas into the foam column matches the rate of exit of the gas from the foam column from the outlet.

A pressurized gas (e.g. air) is introduced into the wastewater. This leads to creation of foam inside the container. The foam is allowed to rise (preferably as a uniform column) by controlling the gas-flow rate. As the gas inflow continues, the foam rises towards the top end of the container. The top end has an outlet comprising plurality of perforations. The foam passes through them and while doing so, it collapses at the perforations. This occurs only when the foam residence time is in the desired zone, so as to ensure that the foam is sufficiently dry when it reaches the outlet with perforations. On collapsing, the foam leaves behind the surfactants in substantially dry paste form. The "dryness" of the recovered surfactant depends on ambient humidity but with precise process ambience control measures, it is possible to get substantially dry surfactant in paste form which is fit for reuse. It is possible to ensure dry conditions at the top surface of the perforated lid by means to ensure "dryness" known in the art.

The gas entering the bulk of water at the bottom of the foam column moves upwards and away from the body of water towards the top or ceiling as the column of foam rises. The foam carries the detergents upwards away from the bulk of the waste water towards the opening with perforations. Surprisingly it has been found that this foam also carries particles upwards away from the bulk of the water. The ratio of volumetric flow rate of the gas to cross-sectional area in this mid-body section of the container provides the upward linear superficial velocity of the foam body. Thus, for any particular rate of gas being introduced, there is an effective foam residence time that allows drainage of the foam column. If the foam residence time is too much, then the foam may collapse before reaching the outlet and if the foam residence time is too little then the foam would not collapse on the outlet but extend well above it. For a domestic- sized system, it is preferred that gas flow rate is maintained in the range of 0.1 and 20 litres per minute, preferably about between 0.1 and 10 litres per minute and most preferably between about 0.1 to 4 litres per minute. This results in foam residence time of about 60 minutes. The optimal foam residence time will however depend upon numerous other factors.

The said method is operated in continuous or batch mode. The surfactant could be any surfactant or surface active agent known in the art but it preferably is anionic, zwitterionic or nonionic.

It is preferred that temperature of the water is maintained in the range of 20 to 40 °C. It is preferred that concentration of the surfactants and introduction of gas is stopped when it falls below 100 ppm. Further preferably, the recovered surfactant is removed by scraping it off the perforations. Preferably the surfactant is scraped off continuously throughout the duration of the method.

The invention will now be described with reference to the following figures.

Brief description of the figures

Fig.1 is a sectional view of a device in accordance with the invention. Detailed description of the figures

Fig.1 is isometric view of the device in accordance with the invention. By way of example, the present invention is illustrated with reference to the following figure, in which:

Figure 1 is a sectional view of an apparatus in accordance with the invention.

The apparatus (1 ) comprises a container (2) and a compressor (3) for introducing air or any other suitable gas into the container.

The container (2) is elongated and has circular cross section. The walls of the container rise upwardly and outwardly, as a result, the cross section of the container (2) progressively increases from a narrower bottom end (3) to it's wider top end (4). At the top end (4) is a lid (5) with plurality of perforations (6). The lid (5) ipso facto serves as an inlet for water but the container (2) has a separate inlet (7) in the form of a pipe. The lid (5) seals the opening except for the provision of plurality of perforations (6) which serve as multiple/plurality of outlets for foam. The container (2) is provided with a sensor (8) to sense the volume of water getting admitted inside, and it which serves as a provision to stop the admittance of water when the container is filled from 1 to 80 % of it's capacity.

Operatively connected with the compressor (3) is a sparger (9) with plurality of nozzles (all labelled as 10) to introduce a gas into the water.

In use, water (1 1 ) is first admitted into the container (2). The water contains surfactant(s), Bubbles of the gas is introduced under pressure of 10 ³ to about 10 ⁷ Pa and flow rate of 0.1 to 10000 litres per minute to generate foam (12) which rises upwards as a uniform coloumn towards the top end (4) of the container and the lid (5) with perforation (6). The foam (12) passes through the perforations (6) leaving behind residue of recovered surfactant (all labelled as 13) on the perforations.

The invention will now be further described with reference to the non-limiting examples. Examples

Example 1 : A plastic drum of capacity of 50 litres with a plastic lid was used for the experiments. The drum served as the container.

The lid had plurality of circular perforations each of diameter 1 mm. The drum was provided with a water inlet port near the mid-section of the drum and an outlet port near it's bottom. Aerators (spargers) were provided at the floor of the drum. The aerators were connected to an air-inlet port provided on the walls of the drum. A small air-pump was placed nearby to supply pressurised air to the spargers inside the drum. The lid with plurality of perforations was put on the drum and the lid was sealed.

5 Litres of water containing the sodium lauryl ether sulphate (an anionic surfactant, SLES) at a concentration of 10000 ppm was introduced into the container, from which the air was displaced through the above mentioned perforations. Aeration was started by switching on the air pump and the air-flow-rate was maintained at 0.5 litres at minute. The foam reached the lid and rose above it after two hours of continuous operation. On continuation, the foam collapsed leaving residue of the surfactant in paste form on the perforations. Upon chemical analysis, the above mentioned residue was found to contain mainly SLES. The starting conditions are shown in Table 1.

Table 1

The method was continued overnight, after which a sample of the water was collected from inside the drum. It was analysed for the anionic surfactant and the concentration was recorded. Mass of the residue collected from the perforated lid was recorded. It was found that around ~ 23 % of the initial surfactant present in the water was recovered within 23 hours of operation. As foam generation continued to occur, the experiment was allowed to continue. Each day a sample of water was collected from inside the drum and the concentration of surfactant was determined. Collection of the paste (recovered surfactant) was continued. Table 2 shows the time mass and concentration of the surfactant from zero time until about 200 hours of continuous operation.

Table 2

The data in Table 2 indicates that the concentration of the surfactant in the treated water reduced from 9384 ppm to 516 ppm in about 200 hours. This amounts to about 95 % reduction in the surfactant content without using conventional methods like heat and chemicals. As the paste contained a bit of moisture due to ambient weather conditions, the mass of recovered surfactant was a somewhat greater than the initial mass of surfactant.

Table 3 shows the volume of treated water collected at the end of 200 hours of continuous operation. About 4.4 litres of treated water was recovered. This amounts to recovery of 88 %. The rest of the water was lost due to evaporation or was trapped as moisture in the recovered surfactant (paste).

Table 3

Litres of water recovered after treatment 4.4

Weight of surfactant (wet)/g 52

Weight of surfactant (dry)/g 45 Example 2:

The procedure followed for Example 1 was repeated. This time the surfactant was another anionic surfactant called sodium dodecyl benzene sulphonate, also called as Sodium LAS. The conditions and important process parameters are summarised in Table 4. The observations are summarised in Table 5.

Table 4

The illustrated examples indicate that the apparatus and method of the present invention allows for recovery of significantly more than 50% of the total surfactant(s) in paste form.