Field of the Invention

The present invention relates to a manure, landfill leachate, sewage and/or wastewater treatment plant, treatment equipment or downstream conveying equipment.

The present invention also relates to a method for making an article with a coated surface.

Background of the Invention

Struvite scaling impairs the function of various articles, such as, for example, pipes and pumps. Struvite scaling may grow over time, blocking the flow and involving costly cleaning operations.

Struvite is known to deposit especially in manure, sewage or wastewater treatment plants or equipment and in particular after treatment of liquid manure, sewage or wastewater in anaerobic digesters. Struvite scaling may also occur in the treatment of landfill leachate or in manure treatment plants, in particular liquid manure storing, treatment of processing plants, for example lagoons or equipment for spreading liquid manure in the fields.

The theoretical chemical composition of struvite is H ₄ MgP0 ₄ -6H ₂ 0, it is thus also referred to as ammonium magnesium phosphate (AMP) or magnesium ammonium phosphate (MAP). It comprises by weight 6.57% H, 5.71% N, 65.20% O, 9.90% Mg, and 12.62% P, which corresponds to 7.35% ¾ ⁺ , 9.90% Mg ²⁺ , 38.7% P0 ₄ ^3" and 44.05% H ₂ 0.

When referring to struvite scaling, struvite describes a class of materials rather than an exact chemical composition. In observed, real struvite scales in the respective plants, struvite deviates from the exact sum formula. Furthermore, such scales also contain inorganic and organic impurities that are not struvite. However, all scales where am- monium and magnesium cations, phosphate anions and water are the dominant components and represent together the main part of the scale are commonly referred to as struvite. Herein, struvite refers to this class of materials. Various compositions of observed struvite scales are herein provided as non- restricting examples:

Prater (Improved Production of Magnesium Ammonium Phosphate (Report), University of Wisconsin System, Stevens Point, 2014, pp. 66-67, available at https://www.wisconsin.edu) treated landfill leachate to precipitate struvite comprising by weight 3.2% N and 8.7% P.

Li & Zhao (Ecological Engineering, 20 (2003), 171-181) treated landfill leachate to precipitate struvite comprising: 4.9% N, 8.6% Mg and 16% P.

Borgerding et al. (Journal of the Water Pollution Control Federation 44 (1972), 813- 819) experienced undesired struvite formation of wastewater treated by anaerobic digesters with a composition of 7.3% H ₄ ⁺ , 9.8% Mg ²⁺ , 38.8% P0 ₄ ^3" , and 44.1% H ₂ 0 and organic compounds.

Mohajit et al. (Biological Wastes 30 (1989), 133-147) precipitated struvite when treating wastewater from a pig farm with a composition of 5.01% H ₄ ⁺ , 10.25%) Mg ²⁺ , 38.99% P0 ₄ ^3" , and 44.66% H ₂ 0. Miinch & Barr (Water Research 35 (2001), 151-159) precipitated struvite in waste water treated by anaerobic digesters comprising 5.1% N, 9.1% Mg, 12.4% P and 39% crystalline water. Furthermore, the consistency of struvite scale can according to Miinch & Barr differ widely and occur in the form of large crystals, small crystals, large curds, or a gelatinous mass.

The precipitation of struvite occurs, likewise any precipitation, from a supersaturated solution, for example a slightly basic aqueous solution at a pH of 7-1 1, for example according to the general scheme: Mg ²⁺ + H ₄ ⁺ + HP0 ₄ ^2" + OH ^" + 5H ₂ 0 → Mg H ₄ P0 ₄ 6H ₂ 0. According to Bhuiyan et al. (Environmental Technology 28 (2007), 1015-1026), the pKsp value for the solubility product of struvite in water at 25 °C is 13.36±0.07.

Besides direct precipitation, which is expected to be the main cause of struvite scaling, struvite scale can also occur by agglomeration of struvite particles that are already precipitated and thus from a struvite suspension. Such suspensions may optionally be colloidal suspensions of struvite nanoparticles.

Typically involved process steps are further outlined in the following. Anaerobic di- gesters are usually mesophilic or thermophilic digesters in which organic carbon compounds are digested under formation of biogas. Biogas is a mixture of gases, with the main component being Methane. A mesophilic digester runs at elevated temperatures of typically approx. 35-40 °C, while thermophilic digesters typically run at temperatures of 45-55 °C.

After the treated liquid manure, sewage or waste water leaves the anaerobic mesophilic or thermophilic digester, the temperature is usually lowered, e.g. passively to adapt to a lower temperature of the surroundings, such as ambient air or soil/ground (when using underground pipes for transporting treated waste water), or actively, e.g. by using heat exchangers or heat pumps, to transfer heat from the outlet of the anaerobic digester to another part of the plant or to other processing steps, e.g. to the inlet to the anaerobic digester for preheating thereof prior to entry into the anaerobic digesters. Struvite scale deposits may be removed by mechanical cleaning, e.g. using abrasive tools and/or mechanical impact, e.g. by hammer and chisel, a jackhammer or similar tools. Such mechanical tools may cause damage to the surfaces of the component or equipment where the struvite build-up is removed leading to increased repair costs or even that the component, e.g. a pump, becomes so damaged that it needs replacement of the component by a new one. Alternatively, struvite scale deposits may be removed by dissolving the deposits using an aqueous acidic cleaning solution.

These methods are time consuming and require significant down time of the treatment plant or equipment during struvite scale removal. Scale removal is usually repeated regularly and cause regular down time periods. This leads to less effective utilisation of the treatment plant or equipment and may cause increased production costs due to increased working hours by staff and/or by increased costs due to the use of chemicals, i.e. acids and/or acidic detergents. Further, struvite scale removal by chemical or mechanical methods may require hard manual labour and/or risk of potential hazardous situations if staff comes into physical contact with acidic cleaning solutions acidic cleaning

It is also known to remove struvite e.g. by precipitation in so called crystallizers, usually fluid bed or up-flow crystallizers containing seed crystals to induce or enhance struvite precipitation at one or more desired and well defined positions in the processing plant or equipment. This solution provides the possibility of recovering the struvite, but does not necessarily eliminate struvite scale formation in downstream processing components or equipment.

Several disclosures cover deposits build by nucleation of struvite either in pipes handling wastewater or on pump impellers and test coupons. The following materials were compared, each item is arranged in the order from lowest to highest amount of scaling.

• PMMA (polymethyl methacrylate) < PTFE (polytetrafluoroethylene) < stainless steel. (Doyle et al, Water Research 36 (2002), 3971-3978.). Surfaces were compared at comparable surface roughness. For the same material, a smooth surface leads to lesser scaling than a rough surface.

• PMMA < PVC (polyvinyl chloride) (Mohajit et al., Biological Wastes 30 (1989), 133-147).

• PVC and PE (polyethylene) < metal surfaces (Westerman et al, Proceedings of the 5th International Symposium of Agricultural Wastes. American Society of Engineers, St. Joseph, MI, 1985, pp. 613-623).

• Plastic parts < metal parts (Booram et al, Transactions of the American Society of Agricultural Engineers, 18 (1975), 340-343)

• PVC < Techite < cast iron (Borgerding et al., Journal of the Water Pollution Control Federation 44 (1972), 813-819) Techite is a glass fibre reinforced plastic. Borgerding expects the result is due to surface roughness also being in the order from lowest to highest of PVC < Techite < cast iron. The disclosures apply bulk materials and do not mention a coating to combine the superior mechanical stability of metals with the desired surface properties of the polymer. The proposed polymers have drawbacks such as being rather soft thermoplastic materials with limited scratch and abrasion resistance and, in the case of PVC and PTFE, applying less environmental friendly halogenated polymers.

Thus, there is a need in the art for effective surfaces to mitigate struvite scaling, being applicable as a coating and providing at the same time structural stability, e.g. towards mechanical impact, e.g. from abrasion from particles present in sewage, waste water or manure and/or wear caused by cavitation in pumps, impellers, valves and/or similar components.

The problem of other mineral scaling than struvite has been widely investigated and both general solutions and solutions featuring specific scaling have been proposed. However, the general solutions are in their entirety contradictory. The skilled person will not necessarily expect that a surface that is effective against one type of scaling will also be effective against another type of scaling. This is outlined by the following example. A fundamental study performed by the Massachusetts Institute of Technolo- gy on scale resist surfaces by Azimi et al. (Applied Surface Science 313 (2014), 591- 599), precipitating CaS0 ₄ , concludes that surfaces with low surface energy, especially with a low polar part of the surface energy, inhibit mineral nucleation in general. However, despite a higher surface energy of PMMA, Doyle et al, as mentioned above, found less struvite scale on PMMA compared to PTFE which is contradictory to Azimi et al. Surface energies are disclosed in common literature. For example, Diversified Enterprises (2009, http://www.accudynetest.com/polytable_03.html, accessed 24 Sep 2015) discloses surface tensions of 37.5 mN/m for PMMA and 19.4 mN/m for PTFE. Surface-tension.de (2007, http://www.surface-tension.de/solid- surface-energy.htm, accessed 24 Sep 2015) discloses surface tensions of 41.1 mN/m (polar part 10.3 mN/m) for PMMA and 20 mN/m (polar part 1.6 mN/m) for PTFE. Object of the Invention

The object of the invention is to mitigate struvite scaling on articles which are in con- tact with suspensions containing struvite particulates and/or supersaturated struvite solutions.

It is also an object of the invention to provide a method for mitigating struvite precipitation on an article in contact with suspensions of struvite particulates and/or supersat- urated struvite solutions.

Description of the Invention

The present invention relates to a manure, landfill leachate, sewage and/or waste water treatment plant, treatment equipment or downstream conveying equipment, such as a treatment plant comprising one or more biological treatment reactors, where said treatment plant or equipment comprises at least one component, wherein the at least one component and/or the at least one or more reactors or a drain of the treatment plant or equipment comprise at least one surface, which is exposed to a suspension comprising struvite particulates or colloids or to a supersaturated struvite solution, and where said reactor(s), pipes and/or said component is coated with a coating, said coating being a different material than the underlying component and said coating being applied to at least a part of said surface exposed to the struvite containing suspension or supersaturated solution.

Due to minimal changes in pH, temperature or other chemical and physical parameters, struvite scaling may occur over a longer distance and involve various articles such as, for example pipes, pumps, valves or heat exchangers. Surprisingly, a coating can significantly reduce struvite scale formation.

In addition, the coating can provide a surface from which struvite scale is easily removed. Any formation of struvite scale on the coated surface is easily removed, e.g. by hand cleaning, or by low impact mechanical cleaning, possibly automated, e.g. using a brush or high pressure liquid impact. Thus the inventive coating on the component can be easily cleaned without the use of hard impact tools, such as when using a chisel and a hammer or the like. According to a preferred embodiment, the coated article is used in contact with a supersaturated struvite solution. While struvite scale in principle can be caused by surface agglomeration of struvite particles dispersed in a fluid, the main cause of struvite scaling in manure, landfill leachate, sewage or waste water treatment plants is by direct nucleation of struvite on a surface from a supersaturated solution. Thereafter, the initial nuclei grow by deposition of further struvite.

In the following, a manure, landfill leachate, sewage or waste water treatment plant may be solely referred to as a wastewater treatment plant. Preferably the manure, landfill leachate, sewage and/or waste water treatment plant comprises one or more biological treatment reactors. At least one of the bioreactors is an anaerobic digester or a bioreactor comprising at least an anaerobic step, preferably at least one anaerobic digester provided for biogas generation. The bioreactor is preferably running under mesophilic or thermophilic anaerobic digesting conditions.

Components provided downstream to an anaerobic bioreactor or a reactor comprising at least an anaerobic step are especially prone to struvite scaling caused by e.g. reduction of the temperature or pH in the liquid which is transported or processed in downstream process equipment or components, e.g. pipes or pumps or similar as mentioned below.

According to a preferred embodiment the respective equipment with the inventive coating is a downstream conveying equipment, subsequent to a process step that leads to struvite precipitation. Examples for such equipment are pipes, pumps, valves or heat exchangers. While struvite precipitation in some cases is an undesired side reaction, many treatment plants induce struvite precipitation on purpose by adding solutions of the respective ions to purify the wastewater and to regain nutrients. The precipitation typically occurs in designated equipment or basins. Even when precipitated on purpose, small changes, especially in pH and temperature, but also in other physi- cal and chemical parameters, can trigger a secondary, undesired precipitation in downstream equipment adjacent to the designated precipitation equipment or basin. Surprisingly, this secondary precipitation in downstream components, which is a main cause of costly cleaning operations, is mitigated or at least reduced significantly when providing a coating on the one or more components according to the present invention and as outlined above.

In another embodiment, said component is used in a wastewater treatment plant or at a drain of a waste water treatment plant, or in plants or equipment for treating and/or handling sewage, landfill leachate or manure, the latter refers in particular to liquid and/or liquidized manure.

These environments are especially prone to struvite scaling, which is effectively mitigated by applying the coating according to the present invention.

Examples of components or equipment used in waste water treatment or biogas production facilities are pipes, such as internal pipe connections between processing equipment in the treatment plant, and/or sewers, outlets for purified waste water from the treatment plant. Other examples of components used in waste water treatment plants are channels, valves, pumps, heat exchangers, heat pump surfaces in contact with the liquid from which struvite may precipitate, internal surfaces of reactors, such as aerobic, anoxic and/or anaerobic reactors, struvite crystallizers, etc., because internal surfaces of such processing equipment are particularly prone to struvite scaling. Examples of components which are used in handling and/or treating liquid or liquidized manure are e.g. lagoons, manure spreaders, such as slurry tankers, and/or parts thereof. Examples of parts are pipes, pumps, or manure mixing means, such as impellers or nozzles, which reintroduce slurry or manure into a lagoon after circulating through one or more pipes and/or pumps.

In another embodiment, said article is a pump, a valve, a pipe or a heat exchanger or a part of the mentioned articles in contact with waste water or any fluid produced by processing waste water. These articles are prone to struvite scaling. Depending on their size, pipes can be very difficult to access for cleaning. In a preferred embodiment, said article is a pump or a part of a pump, such as a casing or rotor or impeller. Due to the possible mixing of waste water with air, pumps are especially prone to struvite scaling. It is in particular preferred to coat the rotor or im- peller of a pump. Rotors or impellers of pumps are not suitable for mechanical cleaning as any damage caused during mechanical cleaning of the impeller or rotor may cause the rotor or impeller to rotate in an instable manner resulting in increasing the level of noise from the pump or even damage. The risk thereof is elegantly eliminated or at least reduced significantly.

According to another preferred embodiment, said coating comprises by weight 30% to 70% carbon (C), 10% to 35% oxygen (O), 5% to 40% silicon (Si) and optionally further elements, where the sum of carbon, oxygen, silicon and hydrogen (H) is 70% to 100%). At least 5% silicon is bound to x carbon and y oxygen atoms, where x is 1 or 2 and y is 4 - x. Based on the total carbon content, at least 90% of the carbon atoms are bound to 0 or 1 silicon atoms and to 1 to 4 atoms chosen from the group of hydrogen, boron (B), carbon, nitrogen (N), oxygen, fluorine (F), phosphor (P), sulphur (S), chlorine (CI), bromine (Br), iodine (I) and/or combinations thereof. These intervals and the binding situation of C and Si define the nature of a coating matrix that is especially effective against struvite fouling. Such an organic-inorganic hybrid coating comprises a significant organic part (at least 30% carbon) consisting of carbon based structures (organic chains or networks), optionally including other non- metal atoms, especially H, O, and/or N, but optionally also B, F, P, S, CI, Br and/or I.

Examples are alkyl or aryl groups or networks, optionally halogenated, any common functionalities known from technical organic coatings such as, for example, alcohols, esters, ethers, epoxides, amines, amides, urethanes, thiourethanes, ureas and/or combinations thereof, but also phosphonic and/or boronic esters.

The organic part provides the necessary flexibility and provides the surface with the anti-struvite-scaling properties as compared to inorganic surfaces such as bare metal surfaces. Furthermore, the coating according to this preferred embodiment comprises a significant inorganic part (at least 5% Si bound to at least two oxygen atoms) that forms Si- O chains or networks, which are referred to as siloxane chains or networks. The inorganic part provides chemical and mechanical stability and can, if necessary, additionally provide improved adhesion to metallic surfaces by creating chemical bonds to metal surfaces (Si-O-M), as compared to solely organic polymers.

The organic and inorganic parts of the coating are connected at molecular level through Si-C bonds (at least 5% by weight of Si, which is bound to at least one carbon atom). Thus, the coating performs as a homogenous surface layer and acts as a homogenous material to combine the benefits of organic and inorganic part. Thus, it is clear that the coating does not solely comprise inorganic particles in an organic matrix. Such a coating may lead to a heterogeneous surface without the desired proper- ties.

Due to achieving an optimal compromise of hardness and flexibility, such organic inorganic hybrid coatings are especially robust against abrasion, for example by traces of sand and/or other particulate matter in the waste water, land fill leachate, sewage or liquid/liquidized manure or from cavitation . For example cavitation may be caused by the turbulent flow of water containing gases in a pump housing. The soluble gases, and potentially also water vapour, creates bubbles in the pump housing and causes wear on the rotor or the pump housing or both.

It is known to the skilled person that a paint system can comprise different layers, such as, for example, a primer, a base coat and a top coat. In such case, the present composition refers to the last layer of the coating according to the invention, which is applied on top of the other layers, as this is the layer in contact with the struvite sus- pension.

In a further preferred embodiment, the inventive coating comprises by weight 35% to 60% carbon (C) and 7% to 30% silicon (Si), and a sum of hydrogen, carbon, nitrogen, oxygen, sulphur and silicon of 90-100%). Such coating provides an optimised com- promise of hardness and flexibility and reduces or mitigates struvite fouling on the coated surface.

In an even further preferred embodiment, at least 7% by weight Si is chemically bound to three oxygen atoms and one carbon atom for further improved structural stability by guaranteeing a three-dimensional siloxane network.

In another preferred embodiment, the coating on the article has a thickness of 0.1 to 100 μιη. A minimum thickness is required for structural stability and distinguishes the coating from monolayers or ultrathin films. A thickness above 100 μπι is not advantageous due to the increased possibility of tensions in the coating film. When the coating according to the invention is applied on top of other coating layers, the thickness refers solely to the coating according to the invention. In a further preferred embodiment, the thickness is below 15 μπι to save materials, but above 0.5 μπι to provide sufficient mechanical stability of the coating layer itself.

In another preferred embodiment, an article is coated with a coating providing a receding water contact angle of at least 70°. The receding contact angle is a measure for the work required to remove a substance from a surface. Thus, in addition to the surprising effect against struvite nucleation, such coating with a high receding contact angle also reduces the adhesion, in case struvite deposits anyway, and facilitates cleaning of the coated surface. In a further preferred embodiment, such receding water contact angle as outlined above is achieved by applying a silane with a perfluoroalkyl moiety and/or by applying polydimethylsiloxane (PDMS) that is capable of forming chemical bonds to the coating matrix in the preparation of the coating. Such components can achieve outstanding repellency in respect of struvite scaling and react with the coating matrix to prevent leaching.

Examples for such components are (lH,lH,2H,2H-Perfluorooctyl)triethoxysilane and/or silanol-terminated PDMS. In another preferred embodiment, an article provides a receding water contact angle of at least 70° and is coated with a coating comprising polydimethylsiloxane chains, which on one terminus are chemically bound to the cross-linked coating matrix, and on the other terminus chemically bound to a terminating group that is not chemically bound to other parts of the coating matrix and is thus dangling.

Such type of polydimethylsiloxane can merge to the surface during curing of the coating to provide the hydrophobic surface with the respective receding contact angle as outlined above. For this purpose, the preferred terminating groups of the PDMS are alkyl groups. As compared to other hydrophobic surfaces, such surface will provide further improved reduction of struvite nucleation and struvite adhesion to the coated surface by providing a surface with flexible groups, which show significant movement at ambient temperatures. Such type of PDMS is, for example, disclosed by C. Nagel et al. (European Coatings Journal 2010 (04), 32-39).

The coating preferably comprises at least 0.005% by weight of PDMS, preferably 0.05-5% by weight, or more preferred 0.05-2%) or in particular 0.1-1%. Such low amounts of PDMS additive effectively provide the hydrophobic surface with the respective receding contact angle as outlined above.

In another preferred embodiment, a component is coated with a coating, where said coating comprises, by weight, at least 2% C which is tetravalent and covalently bound to exactly three atoms, where the first atom is N, the second is O, and the third atom is either N, O, or S, such as organic nitrogen containing groups selected from urea, ure- thane, thiourethane, oxazolidinone, biuret, uretdion, cyanurate and/or combinations thereof. Such carbon atoms are part of nitrogen-containing derivates of carbonic acid.

Such groups are usually formed from industrially available isocyanates, for example by reaction with alcohols, thiols, amines, other isocyanates (and optionally water) and/or epoxides and can provide further improved scratch resistance due to strong intramolecular hydrogen bridging. A coating with such groups provides a certain re- flow effect when the surface has been scratched. Thus, the reflow effect re-closes the scratch in the surface, i.e. the coating layer becomes, at least to some extent, "self- repairing". In a preferred embodiment, the precursor for said groups is a commercially available, and thus cost effective isocyanate resin, optionally a blocked isocyanate. Non-limiting examples for such resins are 4,4'-Methylenebis(cyclohexylisocyanate) and trimers of hexamethylene diisocyanate or isophorone diisocyanate, optionally blocked with bu- tanonoxime and/or mixtures thereof.

The objects mentioned above are also met by a method for preparing a coated article for use in an environment where at least part of the article's coated surface is exposed to a struvite suspension, which is optionally a colloidal suspension, or a supersaturated struvite solution, said method comprising the steps of

(i) preparing a liquid coating composition,

(ii) applying said liquid coating composition to at least part of the surface of an article and

(iii) curing to form a solid coating film.

Such a method is an economic way to provide a coating film as compared to, for example, gas phase deposition, and allows to apply thin coatings to save material, as compared to powder coatings, and provides the struvite repellent surface coating.

The liquid coating may be applied by conventional methods, e.g. by dipping, spraying or by using an applicator, such as a brush, a roller or similar conventional application means. In a preferred embodiment, said liquid coating composition used in the method or on a component of the treatment plant as mentioned above comprises by weight based on solids at least 40%, preferably 60% of one or more silanes having at least one silicon atom bound to at least two, preferably at least three atoms chosen from the group of O, N, F, CI Br or combinations thereof, and/or or any hydrolysation and/or condensation product of said silanes.

Such silanes form, when cured, a strong siloxane network that allows to provide mechanical and chemical stability at low film thickness, e.g. below 15 μπι, while at the same time, said silanes provide the effect against struvite scaling. Low film thickness leads to material and cost savings.

The higher silane ratio of at least 60% and/or a higher crosslinking density achieved by three hydrolysable groups improves mechanical and chemical stability even further.

Such silanes have hydrolysable groups such as, for example, F, CI, Br, R ₂ or OR that react with water while forming Si-OH groups and the respective leaving group. Water may be added or may come from atmospheric moisture. The Si-OH groups condense subsequently either with one of the mentioned hydrolysable groups or with a Si-OH group of another silane molecule. In both cases, a siloxane bond is formed.

While the silanes optionally are hydrolysed and partly pre-condensed in the liquid coating composition, the full siloxane (Si-O) network is usually formed upon curing. Such coatings comprise, at least partly, an inorganic siloxane network and provide besides the effect against struvite nucleation high mechanical stability and chemical stability when compared to organic coatings. When referring to a coating or a coating composition's solid content in this description, it is the sum of solids used for preparing the coating composition. This takes not only the evaporation of solvents into account, but also the evaporation of volatile components that are set free during curing, such as blocking agents or hydrolysable groups.

Silanes form condensed siloxane networks during curing illustrated by

≡Si-OR + RO-Si≡ + H ₂ 0→≡Si-0-Si≡ + 2 ROH,

Where≡ represents bonds from the Si atom to three further atoms which are substitu- ents or part of a network.

Two hydrolysable groups (OR) are replaced by one single oxygen atom, which is shared by two Si-atoms. Thus, the solids of a silane molecule are the mass of a molecule, where each hydrolysable group (e.g. -OR) bound to Si is replaced by a half O- atom, i.e. the O atom shared by two Si atoms. In a preferred embodiment, such silanes are commercially available silanes bearing two or three methoxy and/or ethoxy groups per silicon atom, such as, for example, Methyltriethoxysilane, (3 -Aminopropyl)tri ethoxy silane, (3- Aminopropyl)trimethoxysilane, (3-Aminopropyl)methyldiethoxysilane, Bis[3- (triethoxysilyl)propyl]amine, (3-Mercaptopropy)trimethoxysilane or (3- Glycidyloxypropyl)trimethoxysilane and/or mixtures thereof.

In another preferred embodiment of the method, said liquid coating composition is preferably free of particulate, inorganic fillers.

The vast majority of technical coatings contain inorganic fillers, for example as colour pigments, thixotroping agents, or fumed silica or anticorrosive pigments. The particles can be at μιη-size as well as at the size of several nm. There is a risk that such inor- ganic fillers, when being present at the coating surface of the coating according to the present invention, will interfere with the advantageous effect of the coating matrix against struvite scaling and are therefore to be avoided in the coating.

In a further preferred embodiment of the method or the treatment plant as mentioned above, said liquid coating composition comprises, based on solids,

(i) 20-80% of a silane comprising at least one silicon atom, said silicon atom comprising at least two methoxy- or ethoxy substituents or a combination of methoxy and ethoxy substituents, said silicon atom being further connected to at least one organic substructure via a Si-C bond, said organic substructure comprising at least one func- tional group from the group of amine, thiol, epoxide and/or combinations thereof,

(ii) 10-60% of an isocyanate resin having at least two isocyanate groups,

said isocyanate groups may optionally be blocked,

and/or any reaction product of (i) and (ii) and/or any hydrolysis and/or condensation product of (i) or of said reaction product of (i) and (ii).

Preferred embodiments of said organic substructure(s) are derived from commercially available alkoxy silanes, for example 3-(glycidyloxy)propyl, 2(3,4- epoxycyclohexyl)ethyl, 3-mercaptopropyl, 3-aminopropyl, 3-(N-butyl-amino)propyl, 3-[(2-aminoethyl)amino]propyl or p-aminophenyl and/or mixtures thereof. The respective functional group(s) can react with the isocyanate.

Preferred silanes are (3-Glycidyloxypropyl)trimethoxysilane, (3- Aminopropyl)trimethoxysilane, (3-Aminopropyl)methyldimethoxysilane, Bis-[3- (trimethoxysilyl)propyl]amine, [3-(2-Aminoethyl)aminopropyl]trimethoxysilane, (3- Mercaptopropyl)trimethoxysilane and the respective derivates of the mentioned silanes bearing ethoxy groups instead of methoxy groups or mixtures of methoxy- and ethoxy groups.

Preferred isocyanate resins are commercially available isocyanate resins, such as reaction products of hexamethylene diisocyanate, toluene diisocyanate or isophorone diisocyanate, optionally blocked, for example with butanone oxime or caprotlactame. Such coating composition leads to a further optimised combination between a hard and chemical resistant inorganic siloxane network and a flexible organic network based on urethanes, carbamates, thiourethanes, ureas, biurets, uretdions, cyanurates, and/or oxazolidinones and/or combinations thereof. A coating based on reaction products of an isocyanate resin, and thus comprising urethanes, ureas, thiourethanes, oxazolidinones or combinations thereof is especially advantageous to withstand mechanical and chemical impact from wastewater, sewage and/or liquid/liquidized manure streams while also ensuring a surface which reduces or eliminates nucleation of struvite deposits on the surface.

In the following, the invention is described in more details by one particular embodiment and example.

Description of the Drawing

The present invention will be described in detail in relation to the drawing in which Fig. 1 shows a side view, schematic illustration of a sewage pump,

Fig. 2 shows a top view of the lower casing of the pump, Detailed Description of the Invention

The drawing shows a simplified schematic of a sewage pump to illustrate which parts had been coated with the coating according to coating example 1 below. Fig. 1 shows a side view of the pump, and Fig. 2 shows a view into the lower casing of the pump, seen from the top (6, see below). The upper part 3 of the casing contains the motor. A lower surface 4 of the upper casing is in contact with liquid inside the pump housing. This surface 4 was not coated in example 1 below. The pump comprises an impeller 5, which rotates when pumping. After dismantling the pump, the impeller 5 was coated in the example below. 6.

The lower part 6 of the casing is surrounding the impeller 5. The inner side of the lower casing part 6 is also in contact with the pumped liquid. The inner surface of the lower casing 6 was also coated in example 1 below. The inlet 7 to the pump housing is not visible in side view at Fig. 1). The pumped liquid exits the pump through an outlet 8.

Inventive example

3.361 g (14.2 mmol) (3-Glycidyloxypropyl)trimethoxysilane (CAS No. [2530-83-8]) is placed in a bottle. 0.307 g of a 0.1 N hydrochloric acid is added. The mixture is stirred for 16 h at 18-22°C. Subsequently, 2.441 g Desmodur BL 4265 SN (4.70 mmol blocked NCO, ), 9.675 g Butylacetate, 0.026 g Borchi Kat 22 and 0.118 g Byk Silclean 3700 are added. After each addition, the mixture is stirred until all compo- nents are dissolved resulting 15.784 g clear solution. Provided quantities illustrate the relations of the reactants, not the actual batch size.

Borchi Kat 22 is 2-Ethylhexanoic acid, zinc salt, basic, CAS No. [85203-81-2]. Byk Silclean 3700 is a polyacrylate from Byk with OH functions and polydime- thylsiloxane side chains as 25% solution in 2-Methoxy-l-methylethyl acetate. OH equivalent weight, based on solids, -1870 g/mol. Calculations based on XPS data provided by Esmaeilpour et al. (Progress in Organic Coatings 90 (2016), 317-323, online available in 2015) lead to a PDMS content, based on solids, of 32-42%. Desmodur BL 4265 SN is a 65% solution of a blocked aliphatic polyisocyanate based on isophorone diisocyanate in solventnaphtha 100. The blocked NCO equivalent weight: 519, according to DE 10152853 Al, the blocking agent is butanonoxime.

Table 1 : Composition of the organic-inorganic hybrid coating

The approximate chemical composition of a cured film, calculated/estimated from the single components is: 56.9% C, 8.2% H, 4.1% N, 16.8% O, 13.8% Si, 0.1% Zn. Based on total solids after curing, the PDMS content is approximately 0.31%, and the amount of solids contributed from silanes is about 66%.

A new sewage pump was dismantled. The impeller and the inner side of the lower part of the casing were coated. The drawing provides a schematic of the pump to illustrate which parts were coated. These parts are factory-providedly coated with a cathodic epoxy coating. The coating according to this example was applied by spraying on top of the epoxy coating. In addition, a bare aluminium panel was spray coated. All coated objects were cured for 1 h at 185°C to result in a clear, 2-4 μπι thick film. The sewage pump was assembled again. On the aluminium panel, the coating provides a receding water contact angles of about 93°. The pump was installed in a waste water treatment plant in Denmark in a drain containing water that is prone to struvite scaling and was run for three weeks. Thereafter, the test was stopped and the pump was inspected. Impeller and casing of the pump were besides single spots practically free of struvite. These single struvite spots are about 3-4 mm thick, but cover no more than 2% of the surface. In addition, these spots were very easy to remove by rubbing lightly with the fingers only. Furthermore, the coating itself had not shown any loss of adhesion or visible degradation. The pump was reassembled and run for further 9 weeks. Thereafter, the impeller was inspected again. Solely 3% of the impeller surface was covered by 2-4 mm thick struvite scale, the rest was clean.

Comparative example

A pump identical to the pump of the inventive example, but solely with the black factory-applied cathodic epoxy coating, was in a parallel arrangement simultaneously tested in the same water treatment plant, under identical conditions as the pump according to the inventive example. After three weeks, half of the impeller surface and practically the complete inner casing surface were covered by a continuous struvite layer of 3-5 mm thickness. This layer could not be removed with the fingers only. The struvite deposits could be removed with a hammer and a chisel. After running the pump additional 9 weeks, 100% of the impeller surface was covered with 3-5 mm thick struvite scale.