Method for cleaning wastewater

Abstract

A method (100) for cleaning wastewater using a mixing volume (200) and an additive with a flocculant The method has the steps of: a) Mixing (110) wastewater, additive and gas within the mixing volume (200) with sufficient energy to break up agglomerates and bubbles to average sizes less than 0.1 mm; b) Avoiding (120) mixing for a separation time long enough to allow flocs to float into scum (201) and sink into grout (202); c) Removing (130) scum (201) and / or grout (202) from a water effluent (203); and d) Repeating (140) steps a) – c) until a desired volume of wastewater greater than the mixing volume (200) is cleaned. The method (100) significantly increases settling speeds relative to traditional flocculation, precipitation and flotation applications. This enables smaller, compact cleaning facilities.

Description

BACKGROUND Field of the invention

[0001] This invention concerns a method for cleaning industrial wastewater, particularly for applications much smaller than municipal plants for water treatment. Prior and related art

[0002] There is a general need to remove contaminants from wastewater, here e.g. washing water or bilgewater. In the following, contaminants include surfactants, solid particles, oils, fats, unwanted ions, and bacteria. ‘Solid particles’ include colloids (sizes < 0.1 µm), which tend to have negative charges and thus repel each other and stay suspended in water.

[0003] The next paragraphs specify more common terms for later use.

[0004] We divide output water quality into three categories with increasing levels of treatment: I) ‘Reusable water’ that is fit for reuse, e.g. in washing applications or as non- corrosive cooling water. For example, it may be cheaper to recycle freshwater than to desalinate seawater. Reusable water may contain some surfactants, heavy metal ions and possibly some bacteria; II) ‘Pool quality water’ fit for use in a swimming pool. This is clear water without bacteria. Typical municipal regulations require that water released to nature or a municipal net for surface water has near pool water quality; III) ‘Drinking water’ without bacteria and with concentrations of harmful substances below defined limits. Due to taste and smell, drinking water has lower concentrations of chlorine than levels allowed in pool water.

[0005] Common processes in wastewater treatment include flocculation, precipitation, flotation, ion exchange, and biotreatment (usually with bacteria).

[0006] ‘Flocculation’ brings dispersed particles (with sizes < 0.1 µm and negative charges) out of suspension in ‘flocs’ or flakes. Some sources divide this process into ‘coagulation’ = ‘collecting smaller particles into flocs’ and ‘flocculation’ = ‘sedimentation of flocs’. In this document, ‘flocculation’ = ‘coagulation’ = ‘agglomeration’ = ‘coalescence’, and flocs may either sink into ‘grout’ or float into ‘scum’. Furthermore, ‘sludge’ is a mixture of grout and scum, and ‘raw sludge’ contains much water.

[0007] On a finer scale, ‘adhesion’ binds particles with different properties, e.g. flocculants to colloids, and ‘cohesion’ binds particles with similar properties, e.g. into flocs.

[0008] For flocculation, a ‘flocculant’ = a ‘coagulant’ is mixed into the wastewater. The mixing is usually gentle to promote formation of flocs. Opposite, violent mixing may break up larger flocs and cause re-entrainment, thereby inhibiting flocculation.

[0009] Flocculation as defined above has been utilised to clarify wines and beers for more than 150 years. For example, wines and pilsner-type beers have been and are clarified by binding dead yeast cells etc. in flocs that sink, whereas ales and stouts are clarified with flocs that float. Traditional ‘fining agents’ or ‘clarifying agents’ (flocculants) typically include ‘biopolymers’ such as gelatine (e.g. from animal bones), chitosan (from chitin in mushrooms or shells of crustaceans), isinglass (collagen from swim bladders of fish), egg whites (albumin), etc. Clay-particles, often from bentonite, have been and are widely used to weigh down flocs and thereby increase sedimentation speeds during clarification of wines.

[0010] Modern treatment of wastewater may employ multivalent metal-ions, e.g. Al3+or Fe2+, as effective inorganic coagulants for colloids with negative charge. Ions are supplied as soluble salts, and papermaker’s alum Al2(SO4)3∙nH2O (n ≈ 14) is a widely utilised coagulant.

[0011] The biopolymers briefly mentioned as clarifying agents are ‘natural biopolymers’ derived from living, as opposed to fossil, organisms. Natural biopolymers have fixed properties. In contrast, ‘synthetic biopolymers’ may be tailored for specific applications. Both natural and synthetic biopolymers are ‘biodegradable’ to components without direct detrimental effects on living organisms or the natural environment. However, synthetic biopolymers are usually petroleum-based (aliphatic or aromatic polyesters or copolyesters), and add carbon from fossil sources to the atmosphere.

[0012] In today’s chemical terms, natural biopolymers of interest herein may be classified as ‘polysaccharides’ with mainly linear (aliphatic) chains of carbon atoms, ‘polyphenols’ with aromatic rings, and ‘polypeptides’ = ‘proteins’ with relatively short chains of amino-acids. Polysaccharides may be further subdivided into ‘neutral’, e.g. starch or cellulose, ‘anionic’, e.g. alginates, and ‘cationic’, e.g. chitosan. Polyphenols include lignin that ‘glues cellulose together’ in trees. Polypeptides include gelatine, collagen, and albumin.

[0013] Summarised so far, flocculants = coagulants collect particles into flocs by well- known mechanisms such as adding cations that prevent negatively charged particles from repelling each other, and by adding cohesive substances that bind larger particles in even larger flocs. A skilled person knowing the application at hand should obviously search for existing flocculants, e.g. on the internet. If no suitable flocculant is found, designing a synthetic biopolymer may be an alternative.

[0014] ‘Precipitation' removes ions rather than particles from wastewater, and thus differs from flocculation. Precipitation can be achieved by adjusting pH or by adding soluble salt(s), both of which produce insoluble salt(s) that sink. Some sources use the term ‘precipitate’ for the grout at the bottom of a settling tank.

[0015] Here, ‘flotation’ means forming films of hydrophobic substances such as oils, grease and fats around gas bubbles that raise through water. Gas, e.g. air, N2or CO2, may be mixed into the water in several ways. In ‘dissolved air flotation’ (DAF), gas is added under pressure. A subsequent pressure-drop releases gas, which rapidly forms bubbles according to Henry’s law. Anyone who has opened a bottle of beer knows that it may take hours before all gas bubbles have left the beer, i.e. such that equilibrium according to Henry’s law is achieved.

[0016] In flotation tanks, gas may be gently mixed into liquid to promote coalescence and prevent re-entrainment. ‘Gentle mixing’ is also known as ‘avoiding turbulence’, ‘maintaining laminar flow’, ‘keeping flow in the Stokes’ regime’, etc.

[0017] It is a first objective to increase sedimentation speeds of flocs and precipitates to grout in an efficient manner. In particular, sand may be utilised to weigh down flocs. The sand is typically recycled to limit costs for new sand, transportation etc. However, recycling sand requires a hydro cyclone and energy that increase costs of operation and maintenance.

[0018] A second objective is to increase flotation speeds of flocs that float into scum. Greater sedimentation and flotation speeds reduce treatment times and operational costs.

[0019] ‘Ion exchange’ may replace potentially harmful ions in wastewater with harmless ions of the same charge. For example, all nitrates (salts of nitrate NOିଷ) are soluble in water, so nitrate (anions NOିଷ) cannot be removed by precipitation. Instead, water with nitrate may be filtered through a porous solid medium loaded with hydroxide (OHି) on its surface. In this example, nitrate ions bind to the solid surface and replace hydroxide ions that are released to the water. Ion exchange is reversible, so here the porous medium may be ‘reloaded’ by flushing it with a strong solution of NaOH in water. In general, the flushing water has a relatively high concentration of unwanted ions, here nitrate, which facilitate further treatment.

[0020] Typical porous media include ‘ion-exchange resins’ in the form of porous beads of gel polymers, clays of ‘flake forming minerals’ such as bentonite, and sintered porous glass beads. Ca-bentonite and sintered glass beads have surfaces suitable for removing divalent cations of heavy metals, e.g. Cu2+and Ni2+, that are not removed by precipitation in earlier stages of the water treatment. Some metal ions, e.g. Fe2+and Zn2+, are not considered harmful for living organisms or the natural environment.

[0021] Common examples of ions that can bind to ion-exchangers: Monovalent ions (H+, OH-, Na+, K+, Cl-); divalent cations such as Ca2+, Mg2+, Cu2+etc.; polyatomic inorganic anions such as NOିଷ, SOଶସିand POଷସି; organic bases and acids; and biomolecules that can be ionised such as amino acids, peptides, polypeptides (= proteins), etc.

[0022] Additives with ion-exchange properties may bind unwanted ions in grout or scum, and thereby reduce the need for columns or tanks with ion-exchangers in later ‘polishing’.

[0023] ‘Biotreatment’ is widely used in municipal (large scale) water treatment. The purpose is to bind unwanted ions (heavy metals, nitrate, …) in ‘bio-sludge’ that, in contrast to ions, is possible to filter. Bacteria from biotreatment and / or sewage must be removed before water is released to nature or a municipal net. This may be done by sedimentation followed by removing residual turbidity (cloudiness) from water effluent, and finally irradiation with UV- light through the clear water. Some of these techniques may be useful in smaller facilities.

[0024] Biotreatment to remove unwanted ions may be superfluous in smaller applications, However, bacteria, algae and other organisms may thrive in tanks or pipes, e.g. causing foul smells in washing applications. In maritime applications, ‘fouling’ = growth of bacteria, algae, barnacles etc. may clog pipes, valves and other equipment.

[0025] It is an objective of the present invention to produce water with sufficiently low turbidity to permit UV-treatment without intermediate steps, e.g. for biotreatment or for removing turbidity.

[0026] The purpose of the present invention is to satisfy at least one of the objectives above while retaining benefits from prior art. SUMMARY

[0027] This is achieved by a method according to independent claim 1. Further features and benefits appear in the dependent claims. In the claims and elsewhere, articles ‘a’, ‘an’ and ‘the’ mean ‘(the) at least one’, whereas ‘one’ means exactly one. Furthermore, ‘for’ means ‘suitable for’.

[0028] More particularly, the invention concerns a method for cleaning wastewater using a mixing volume and an additive with a flocculant. The method is distinguished by the steps of: a) Mixing wastewater, coagulant and gas within the mixing volume with sufficient energy to break up agglomerates and bubbles to average sizes less than 0.1 mm; b) Avoiding mixing for a separation time long enough to allow flocs to float into scum and sink into grout; c) Removing scum and / or grout from a water effluent; and d) Repeating steps a) – c) until a desired volume of wastewater greater than the mixing volume is cleaned.

[0029] The additive may contain clay or sand. The flocculant must have cohesive properties to form flocs. Many polymers have this property.

[0030] In step a), violent mixing creates a homogenous mixture where flocculants have short mean paths to adhere to particles and need short times to encapsule micro-bubbles. In step b), short mean paths between flocculant particles promote cohesion into flocs. Some flocs contain evenly distributed gas, e.g. air, and have significant buoyancy. The mixing does not break down solid particles such as clay or fine sand, so some flocs sink. Experiments show that violent mixing increases separation speed significantly compared to speeds obtained with traditional gentle mixing, all other factors equal.

[0031] The additive may further comprise a soluble salt for precipitating an insoluble salt. The precipitate sinks into the grout together with some flocs.

[0032] The flocculant is preferably a natural biopolymer derived from a living organism as opposed to from a fossil source. ‘Biopolymer’ is short for ‘biodegradable polymer’, and may be petroleum based.

[0033] In a batchwise mode of operation, steps a) – c) are separated in time and take place in a reactor tank with volume less than 1.3 times the mixing volume. In this mode of operation, the separation time may be determined by means of a sensor measuring a physical property in a group consisting of turbidity, electrical resistivity and electrical capacitance. The sensor(s) implicitly detect(s) i) when the liquid level corresponds to the mixing volume, and ii) when the scum and / or grout is / are sufficiently separated from the water effluent to be removed, for example by a skimmer and / or a screw conveyor.

[0034] In a preferred embodiment of the batchwise mode of operation, step c) involves removing the water effluent such that the scum sinks into the grout and form a wet raw sludge. This eliminates the need for a skimmer. The raw sludge may be removed from a conical bottom of the reactor tank, e.g. by gravity through a controlled butterfly valve.

[0035] In a continuous mode of operation, steps a) – c) are separated in space by means of a flow directed from the mixing volume toward a settling volume. The settling volume may be located in a reactor tank together with the mixing volume, or in a separate tank downstream from the mixing volume.

[0036] In both modes of operation, steps b) and c) may take place in a lamella clarifier downstream from the mixing volume. A lamella clarifier provides a long separation time in a relatively small tank, but may require pretreatment in an upstream settling volume.

[0037] All embodiments may include a step of removing scum by means of a skimmer. This does not necessarily imply a set liquid level.BRIEF DESCRIPTION OF THE DRAWINGS

[0038] The invention will be described in greater detail with reference to the accompanying drawings, in which: Fig.1 illustrates a context for the method according to the invention: Figs.2a – d illustrate a batch version of the proposed method; Figs.3 – 5 illustrate a continuous version of the proposed method; and Fig.6 is a flowchart summarising the main steps in the proposed method. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0039] The drawings are schematic and not to scale. Furthermore, details known to those skilled in the art are omitted.

[0040] Fig.1 is a simplified process flow diagram of a cleaning facility 90 using the method according to the invention. The facility 90 takes in clean water 1, and produces sludge 2 and pool quality water 3.

[0041] A site 10 receives clean water 1 mixed with additives and contaminants from a general source 11, and the resulting wastewater is collected in a storage tank 15.

[0042] Alternative sites 10 include a hall for washing vehicles, a rig for washing road tunnels and a collector of bilge water. For example, it may be cheaper to clean bilge water to non-corrosive cooling water than to desalinate sea water by evaporation or reverse osmosis.

[0043] The storage tank 15 represents any (large) or continuous source of wastewater. A transfer pump 17 moves wastewater to a wastewater inlet 19 on a reactor tank 20.

[0044] At least one additive source(s) 21 supplies / supply additives, e.g. flocculants, clays, possibly with ion-exchange properties, etc., to the reactor tank 20 through additive inlet(s) 22. A raw sludge line 24 leads raw sludge from the reactor tank 20 to a raw sludge tank 25. A water effluent line 28 conveys water effluent from the reactor tank 20 to a buffer tank 30.

[0045] The reactor tank 20 accelerates settling and flotation, and thereby separates most contaminants from water faster than traditional flocculation, precipitation and flotation. This enables a smaller and more compact facility 90.

[0046] A sieve or coarse filter may remove much of the water from the raw sludge. A sludge water line 29 returns sludge water from raw sludge to the storage tank 15. Outside Fig.1 and the present invention, sludge 2 with much less volume than the raw sludge receives further treatment, for example dewatering in a band filter press.

[0047] The buffer tank 30 is primarily a source of reusable water for the site 10, thereby reducing the need for clean water 1 and the net water consumption of the cleaning facility 90. As indicated in the introduction, reusable water many contain substances not allowed in pool quality water 3, e.g. some surfactants in washing water. Still, water effluent from line 28 may require some treatment to be reusable. In Fig.1, the buffer tank 30 has the standard symbol of a settling tank to indicate that some further treatment may be required for reusable water and / or pool quality water 3. The descriptions of Figs.3 -5 below have more details.

[0048] A UV-source 37 on a reusable-water line 39 illustrates a possible need to remove living organisms before water is reused on the site 10. As indicated, this may be bacteria that cause foul smells in a car wash or organisms that may cause fouling in equipment exposed to seawater. The skilled person knows polishing steps, e.g. filtering, ion-exchange and UV- irradiation to achieve pool quality water 3. These steps are not part of the invention.

[0049] Figs.2a – 2d illustrate batchwise operation adapted to the size of the reactor tank 20. Arrows indicate motion and open lines, whereas line segments indicate closed lines / pipes.

[0050] In Fig.2a, the reactor tank 20 is being filled with wastewater through the wastewater inlet 19. In real embodiments, the wastewater inlet 19 may supply wastewater in a splashing manner from the top of the tank to mix some air into the water. Flocculant(s) and other additives from the additive source(s) 21 enter the reactor tank 20 through additive inlet(s) 22.

[0051] An accelerator 23 for providing ‘sufficient turbulence’ is inactive in Fig.2a. Here, ‘accelerator’ means a device able to accelerate settling and flotation processes.

[0052] In Fig.2b, the mixing volume 200 determines the water level. All inlets to and outlets from the reactor tank 20 are closed. Energy E, e.g. electric energy for a submerged pump or hydraulic energy from a pump outside the reactor tank 20, is supplied to the accelerator 23 during a ‘mixing time’, typically less than a minute. Violent mixing creates a homogenous mixture with short average distances between contaminants and coagulants. Furthermore, the time required to encapsulate a bubble decrease with bubble size. Both these factors accelerate separation of contaminants from water.

[0053] In Fig.2c, the accelerator 23 has stopped and enough time has elapsed to allow flocs and / or bubbles to float into scum 201 or sink into grout 202. The water effluent line 28 conveys relatively clear and clean water toward the buffer tank 30 such that the scum 201 sinks toward the grout 202. Thus, there is no need for a skimmer or other apparatus to move scum from the water surface.

[0054] In Fig.2d, raw sludge 203 = scum 201 + grout 202 + some water leaves the reactor tank 20 through the raw sludge line 24, e.g. by means of gravity through a controlled butterflyvalve (not shown). Once the raw sludge 203 has left the reactor tank 20, the batchwise version returns to the state in Fig.2a for a new batch.

[0055] In this document, a ‘batch’ fits in the reactor tank 20 and is unrelated to the size of the storage tank(s) 15 and any other source of wastewater. ‘Batchwise’ operation means that the mixing, settling etc. in Figs.2a – 2d are at separate and consecutive time intervals.

[0056] Figs.3 – 5 illustrate ‘continuous’ operation where mixing, settling etc. take place at separate and consecutive volumes in one or more tanks. On the scale of such volumes, the volume flow rate Q (in m3 / s) for a liquid remains constant through all areas A1, A2, … perpendicular to the mean large-scale flow. That is: Q = A1v1 = A2v2 = constant (1) where A1, A2are cross-sectional areas (in m2) of a ‘large-scale’ volume, e.g. a tank, and v1, v2 are the respective mean flow speeds (in m / s) through the areas A1, A2

[0057] Fig.3 depicts a tank with a mixing volume 200 and a settling volume 210. Reference numerals 15 – 22 are explained with reference to Fig.1. The accelerator 23 in Figs.2a – 2d) is replaced with numerals 231 – 233 in Fig.3.

[0058] Specifically, the pump 17 provides a flow rate (in m3 / s, l / s or l / min) equal to a mean flow rate indicated with arrow 209 that leaves the mixing volume 200. The pump 17 also provides a pressure, which, by definition, is equal to energy per volume unit. The input energy increases turbulence, i,e, increases the speeds of fluid particles in eddies much smaller than the mixing volume.

[0059] An inline mixer 231 mixes wastewater from the pump 17 with additive from the additive source 21. The inline mixer 231 may be any known static mixer or mixing eductor suitable for mixing a powder or a liquid into wastewater with concentrations and flow rates that depend on the application. Some inline mixers 231 may provide sufficient turbulence to work as an accelerator.

[0060] In Fig.3, an optional curved pipe 232 with one or more tank-mixing eductors 233 adds energy to turbulence in the mixing volume 200. A tank-mixing eductor 233 has an open entrance, pulls in ambient water, adds turbulence and discharges homogenised water. In this example, the pump 17 provides the energy required to increase turbulence.

[0061] In this example, the size of the mixing volume 200 depends on the range and orientation of the tank-mixing eductor(s) and there is no distinct boundary between the mixing volume 200 and the settling volume 210. Obviously, separate tanks for the volumes 200 and 210 would provide a distinct boundary.

[0062] A water outlet in the lower party of the end wall 212 and an overflow over an upper rim of the end wall 212 both lead to the water effluent line 28. The overflow determines a common water surface 205 in the mixing volume 200 and the settling volume 210.

[0063] A retainer 213 for retaining scum 201 on the water surface 205 extends into the water from above and perpendicular to the paper plane. A shovel 214 is oriented to push scum 201 out of the paper plane, for example toward the raw sludge line 24 shown in Fig.1. Water that flows under the retainer 213 and over the rim of wall 212 may contain some scum 201.

[0064] A screw conveyor 215 removes wet grout 202 from the lowest part of the settling volume 210 out of the paper plane for further treatment. Several alternative devices for moving scum 201, grout 203 and / or sludge 203 are commercially available.

[0065] According to equation (1), the average horizontal speed of a particle in the settling volume 210 decreases with increasing cross-sectional area. Thus, a ‘wide and deep’ settling volume 210 would give flocs etc. a ‘long’ time to sink or float. However, flocs etc. must float or sink greater distances in a deep vessel than in a shallow vessel, so many municipal floccu- lation basins are wide, long and shallow. The present invention increases settling speeds significantly compared to traditional methods. Greater vertical speeds reduce settling times and thereby the need for large settling volumes 210.

[0066] Fig.4 illustrates an optional lamella clarifier 310 as an example of known technology that may be included in the symbolic buffer tank 30 shown in Fig.1.

[0067] A stack 311 of inclined lamellas (thin, flat plates) provides short paths for sediments and floating particles to reach the nearest lamella. The total settling area is equal to the total area of lamellas projected on the horizontal plane. Thus, a lamella clarifier with a relatively small volume may provide the function of a long, shallow settling tank.

[0068] Lamella clarifiers usually require pre-treated water, in this example provided through the water effluent line 28 from the settling volume 210. In accordance with common practice, input water hits the stack 311 about 20% from the bottom of the lamella stack 311. This reduces the effect of inflowing water on sediments sliding down over the lamellas. Similarly, a water outlet 319 is located about 20% from the top of the lamella stack 311. This reduces the effect of outflowing water on flocs and bubbles floating up under the lamellas. An arrow 314 indicates an average fluid flow through the lamella clarifier 310.

[0069] In this example, the clarifier 310 has an inclined floor 315 that collects sediments or grout at a lowest level. Furthermore, the clarifier 310 has an inclined roof 316 that may double as the floor for a settling tank in a compact design with a vertical water line 28.

[0070] In Fig.4, a level sensor 316 measures a liquid level 31 that must be kept over the lamella stack 311 during operation. A pump 317 on the water outlet 319 stops and starts to maintain the liquid level 31. The skilled person will know that this is an example of a general control loop with a sensor, a controller, an activator and feedback.

[0071] Connection numerals 34 correspond to the line 34 to the raw sludge tank 25 in Fig.1. Devices to remove scum and grout from the lamella clarifier 310 are implied, commercially available and not shown explicitly in Fig.4.

[0072] Fig.5 illustrates a buffer tank 320 for reusable water, that is, water fit for reuse in or on the site 10 described with reference to Fig.1. In Fig.5, the buffer tank 320 receives water treated in the settling volume 210 and / or the lamella clarifier 310. Either way, a water level 32 varies depending on the need for water at the site 10.

[0073] In Fig.5, the UV-source 37 is located within the buffer tank 320 rather than on the reusable-water line 39 as in Fig.1. Accordingly, we assume that the water in tank 320 is clear enough to let UV-radiation pass through.

[0074] Connection numeral 3 indicates that some water from the buffer tank 320 may be treated to near pool quality water 3 fit for release to nature or a municipal net for surface water. Equipment for such treatment is outside the present invention, and not shown in Fig.5.

[0075] Fig.6 is a flowchart illustrating the method 100 according to the invention. Step 101 ‘Start’ includes all obvious preparations such as designing and deploying a suitable apparatus.

[0076] In step 102, ‘additives’ include one or more mandatory flocculant(s) and possibly one or more other substances, e.g. for weighing down flocs and / or for ion-exchange. Additives and their concentration may be determined by well-known jar tests. The mixing time is the time needed to break down agglomerates and gas bubbles to sizes less than 0.1 mm. It depends on the wastewater, the additives and the chosen accelerator. If the mixing time is over 1 minute, a more ‘powerful’ accelerator should be utilised.

[0077] Here, ‘powerful’ may be explained by means of the Reynolds number (Re), which represents turbulence. Re is the ratio of inertial to viscous forces, and thus depends on both liquid speed in eddies (supplied kinetic energy) and viscosity, which depends on the type and concentration of additive(s). A ‘great’ Reynolds number implies ‘great’ shear forces suitable for breaking down agglomerates and gas bubbles. ‘Powerful’ means ‘supplied energy per time unit’ and is related to ‘pressure’ ≡ ‘energy per volume unit’.

[0078] Step 110 specifies ‘violent mixing’ as opposed to the ‘gentle mixing’ in traditional flocculation, precipitation and floating applications. This implies a sufficiently great Reynolds number as described in the previous paragraph.

[0079] In step 111, the delay for the mixing time may be set on a timer.

[0080] Step 119, stop mixing, applies to the batchwise mode of operation and not to the continuous mode of operation.

[0081] In the batchwise mode of operation, step 120 - ‘wait separation time’ – may be 5 implemented by a timer set to ‘separation time’ or by sensors detecting ‘clear water’ between scum and grout. In the continuous mode of operation, the settling speed plus the shape and type of a settling tank determines the separation time.

[0082] Step 130, ‘remove scum and grout’, involves known devices. We note that further treatment of scum implies simple release of gas, e.g. air, and thereby cheaper recycling than10recycling sand or clay from grout in a hydro-cyclone. Hence, the method 100 may advantage- ously be optimised for floating flocs and flotation.

[0083] Step 139 determines whether there is more water to be treated, or if the process should halt temporarily for some other reason, e.g. for maintenance.

[0084] If not finished, step 140 returns control to a point before step 110 – ‘violent mixing’. 15 If the process is finished, step 150 ‘Stop’ contains any procedures for stopping the operation.

[0085] While the invention has been described by means of examples, the scope of the invention is determined by the following claims.

Claims

CLAIMS 1. A method (100) for cleaning wastewater using a mixing volume (200) and an additive with a flocculant, characterised by the steps of: a) Mixing (110) wastewater, additive and gas within the mixing volume (200) with sufficient energy to break up agglomerates and bubbles to average sizes less than 0.1 mm; b) Avoiding (120) mixing for a separation time long enough to allow flocs to float into scum (201) and sink into grout (202); c) Removing (130) scum (201) and / or grout (202) from a water effluent (203); and d) Repeating (140) steps a) – c) until a desired volume of wastewater greater than the mixing volume (200) is cleaned.

2. The method (100) according to claim 1, wherein the additive further comprises a soluble salt for precipitating an insoluble salt.

3. The method (100) according to claim 1 or 2, wherein the flocculant is a natural bio- polymer derived from a living organism.

4. The method (100) according to any preceding claim, wherein steps a) – c) are separated in time and take place in a reactor tank (20) with volume less than 1.3 times the mixing volume (200).

5. The method (100) according to claim 4, wherein the separation time is determined by means of a sensor measuring a physical property in a group consisting of turbidity, electrical resistivity and electrical capacitance.

6. The method (100) according to claim 4 or 5, wherein step c) involves removing the water effluent (203) such that the scum (201) sinks into the grout (202) and form a wet raw sludge (204).

7. The method (100) according to any claim 1 – 3, wherein steps a) – c) are separated in space by means of a flow (209) directed from the mixing volume (200) toward a settling volume (210).

8. The method (100) according to claim 7, wherein the settling volume (210) is confined in a separate settling tank downstream from the mixing volume (200). 5 9. The method (100) according to any preceding claim, wherein steps b) and c) take place in a lamella clarifier (310) downstream from the mixing volume (210).

10. The method (100) according to any preceding claim, further including a step of removing scum (201) by means of a skimmer (214).

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