Water treatment

Abstract

Provided herein is a method for treating water wherein the water comprises an amount of at least one dissolved sulphide and at least one microorganism, the method comprising the steps: i) contacting the water with a source of Fe2+ or Fe3+ ions to reduce the amount of the at least one dissolved sulphide, and ii) contacting the water with a percarboxylic acid to provide disinfection against the at least one microorganism; wherein step ii) is performed after step i). The reduction of the amount of the at least one dissolved sulphide improves the disinfection performance of the percarboxylic acid.

Description

[0001] WATER TREATMENT

[0002] Field of the Invention

[0003] The present disclosure generally relates to a method of treating water. The disclosure relates particularly, though not exclusively, to a method, apparatus and system for optimising disinfection performance of a percarboxylic acid in a water treatment process. The present disclosure further relates to a use of Fe2+ions or Fe3+ions to improve disinfection performance of a percarboxylic acid in water.

[0004] Background of the Invention

[0005] The need for purified water is increasing rapidly around the world. Efforts are being made to purify water using lower concentrations of chemical disinfectants, without, however, considerably raising the cost of the purification process. In addition, there is a need for the use of biodegradable or otherwise less harmful chemicals having fewer detrimental health effects.

[0006] Chlorine-based disinfectants (for example, hypochlorite, chlorine dioxide and chloramines) have traditionally been used to disinfect water, including wastewater. Chlorine-based disinfectants are quite effective against bacteria, but have lower efficiency against viruses, bacterial spores and protozoan cysts. In addition, chlorine-based disinfectants give rise to potentially toxic and mutagenic by-products, making them less desirable for use in disinfection processes.

[0007] Therefore, alternative disinfection methods have been considered. Among those, ultraviolet (UV) irradiation is currently the most widely used alternative disinfection method. It is typically efficient against enteric bacteria, viruses, parasite cysts and bacterial spores, and does not produce harmful by-products. However, if the UV dose is too low, photo-reactivation or dark repair of UV-damaged microorganisms can occur, leading to potential regrowth under favourable conditions Furthermore, UV-disinfection systems are highly dependent on upstream conventional treatment processes: UV is efficient only if the treated water quality is high (i.e. with low turbidity), as suspended solids can shield microorganisms from UV light. In addition, UV disinfection methods are relatively energy-intensive and expensive. Other alternative disinfection methods such as ozonation, ultrasound and membrane filtration have been studied. However, these methods are generally more expensive and have their own drawbacks.

[0008] The organic peroxides peracetic acid and performic acid have more recently been considered as alternative disinfectants.

[0009] Peracetic acid (PAA or CH3COOOH) is a broad-spectrum disinfectant with a high oxidationreduction (redox) potential. PAA is commercially available as an acidic quaternary equilibrium mixture with acetic acid, hydrogen peroxide (H2O2), and water as illustrated in reaction (1) below:

[0010] CH3COOH + H2O2 CH3CO-OOH + H2O (1)

[0011] PAA is active against a wide spectrum of microorganisms. Disinfection mechanisms of PAA are based on the release of highly reactive oxygen species (ROS) such as hydroxyl (HO»), alkoxyl (RO»), hydroperoxyl (HO2*) and superoxide (02* ) radicals. The ROS can alter the metabolism of microbes and damage the structure of microbial cells, which occurs due to chain reactions between the ROS and biomolecules such as enzymes, lipids, structural proteins and DNA. PAA advantageously produces little to no toxic / mutagenic by-products after reaction with organic material, and degrades to acetic acid, hydrogen peroxide and water.

[0012] Performic acid (PFA or HC000H) has been used to disinfect primary and secondary wastewater treatment plant effluents (see below for description of wastewater treatment processes). PFA is normally applied as an equilibrium mixture of PFA, water, hydrogen peroxide and formic acid, as illustrated in reaction (2) below:

[0013] CHO-OH + H2O2 CH0-00H + H20 (2)

[0014] PFA is very unstable and typically needs to be generated on-site, shortly prior to use. The disinfection mechanisms of PFA are thought to be analogous to PAA via generation of ROS. PFA is considered to be more effective in disinfection than PAA (for example, requiring lower doses and / or shorter contact times) for inactivating at least some microorganisms including E. coli and Enterococcus. This may be attributable to the higher redox potential of PFA which provides a greater capacity to oxidise contaminants. Analogously to PAA, PFA produces little to no toxic / mutagenic by-products after reaction with organic materials. PFA is fully biodegradable and degradation products of PF A include carbon dioxide and water (Gehr et al., 2009, Water Sci.

[0015] Technol. 59, 89-96).

[0016] There remains a need to improve the disinfection performance of organic peroxides such as PAA and PF A in order to increase the overall efficiency of water treatment systems and reduce operational costs.

[0017] Summary of the Invention

[0018] Accordingly, in a first aspect, the present invention provides a method of treating water, wherein the water comprises an amount of at least one dissolved sulphide and at least one microorganism, the method comprising the steps: i) contacting the water with a source of Fe2+or Fe3+ions to reduce the amount of the at least one dissolved sulphide, and ii) contacting the water with a percarboxylic acid to provide disinfection against the at least one microorganism; wherein step ii) is performed after step i).

[0019] In a second aspect, the present invention provides an apparatus comprising: at least one processor; and at least one memory including computer program code, the at least one memory and the computer program code being configured, with the at least one processor, to cause the apparatus to perform the above method.

[0020] In a third aspect, the present invention provides a water treatment system comprising the above apparatus, the system comprising: a first dosing device which is configured to feed a source of Fe2+or Fe3+ions to the water, a second dosing device which is configured to feed a percarboxylic acid to the water, and a first measuring device which is configured to measure the level of the at least one dissolved sulphide in the water and generate output data relating to the measured level of dissolved soluble sulphide, wherein the apparatus is constructed and arranged to receive the output data relating to the measured level of the at least one dissolved sulphide from the first measuring device, monitor the measured level of the at least one dissolved sulphide in the water, and to regulate the amount of the source of Fe2+or Fe3+ions that is fed into the water by the first dosing device and / or regulate the amount of the percarboxylic acid that is fed to the water by the second dosing device based on the monitored level of the at least one dissolved sulphide.

[0021] Preferred features of all aspects of the present invention are defined in the dependent claims.

[0022] In a third aspect, the present invention provides a use of Fe2+or Fe3+ions to improve disinfection performance of a percarboxylic acid against at least one microorganism in a method of treating water, wherein the water comprises an amount of at least one dissolved sulphide and the at least one microorganism, and the use comprises reducing the amount of the at least one dissolved sulphide in water by contacting the water with the Fe2+or Fe3+ions.

[0023] The method, apparatus, system and use defined herein are particularly useful in wastewater treatment. The present inventors have found that by reducing the amount of dissolved sulphide in water through contact with Fe2+ions or Fe3+ions, the disinfection efficiency of percarboxylic acids is advantageously increased.

[0024] Brief Description of Figures

[0025] To assist understanding of the present disclosure and to show how embodiments may be put into effect, reference is made, by way of example only, to the accompanying Figures in which:

[0026] Figure 1 is a schematic block diagram illustrating an apparatus according to an example of the invention.

[0027] Figure 2A is a bar chart illustrating PFA efficacy in wastewater samples with a low dissolved sulphide content.

[0028] Figure 2B is a bar chart illustrating PFA efficacy in wastewater samples with a high dissolved sulphide content.

[0029] Figure 3 is a bar chart illustrating PFA efficacy in wastewater samples pre-treated with Fe2+or Fe3+.

[0030] Figure 4A is a photo of wastewater samples treated with 4.5 mg / 1 Fe2+(left) or 7 mg / 1 Fe3+(right). Figure 4B is a photo of wastewater samples treated with 9 mg / 1 Fe2+(left) or 14 Fe3+(right).

[0031] Detailed Description of the Invention

[0032] Regardless of the disinfection technology, disinfection performance is primarily governed by the concentration of residual disinfectant. The term “concentration of residual disinfectant” as used herein refers to the concentration of disinfectant after a period of contact with (or exposure to) water to be treated. Thus, if a threshold concentration of residual disinfectant is dynamically maintained during the disinfection process, then consistent disinfection performance will be met. However, with specific regard to the use of percarboxylic acids as a disinfectant in wastewater treatment systems, variations in water quality and quantity, as well as numerous side reactions between the disinfectant and water contaminants, may reduce the concentration of residual disinfectant, and consequently have an adverse effect on disinfection performance. For example, when added to wastewater, PF A and PAA undergo an initial rapid consumption (i.e. instantaneous disinfectant demand) followed by a more gradual decay. Poor water quality and water contaminants may accelerate the initial consumption and subsequent decay. As a result, dosing strategies which do not take into account the conditions affecting demand and / or decay may result in insufficient disinfectant performance and possible violations of regulatory microbial limits.

[0033] As mentioned above, the present invention provides a method of treating water, wherein the water comprises an amount of at least one dissolved sulphide and at least one microorganism, the method comprising the steps: i) contacting the water with a source of Fe2+or Fe3+ions to reduce the amount of the at least one dissolved sulphide, and ii) contacting the water with a percarboxylic acid to provide disinfection against the at least one microorganism; wherein step ii) is performed after step i).

[0034] By reducing the amount of dissolved sulphides in the water to be treated, disinfection performance of the percarboxylic acid is unexpectedly improved. This, in turn, enables a reduced concentration of the percarboxylic acid to be used to achieve satisfactory disinfection and compliance with microbial reduction targets, with a consequent reduction in operational costs. The term “dissolved sulphide” as used herein may also include any sulphide compound which is soluble or capable of becoming dissolved in water.

[0035] Water to be treated

[0036] The water to be treated is not particularly limited and is any water or aqueous solution in need of disinfection treatment. The water to be treated may include raw water (for example, surface water from a lake, sea or river), drain water, water used in agriculture, or wastewater. In some examples, the water may comprise industrial water. In this context, the term “industrial” may refer to pulp and paper industry, oil industry, mining industry, food industry, or to any other applicable industry. The water to be treated typically includes one or more contaminants such as bacteria, viruses, and other non-living organic matter.

[0037] Wastewater treatment

[0038] In preferred examples, the water to be treated comprises wastewater. Accordingly, the method of treating water according to the invention may be conducted within a wastewater treatment system or plant. The wastewater to be treated may include municipal wastewater, sewage and / or industrial wastewater.

[0039] Municipal wastewater or sewage treatment generally involves the following sequential processes: preliminary, primary, secondary and tertiary treatments. These are well-known to a person skilled in the art of wastewater treatment and water purification, and are further discussed below.

[0040] A preliminary treatment may remove coarse and large suspended materials that can be easily collected from the raw sewage or wastewater, for example, by screening and / or comminution, before they damage or obstruct any pumps and sewage lines of primary treatment apparatuses.

[0041] The primary treatment is designed to remove gross, suspended and floating solids from raw sewage or wastewater. Primary treatment may include screening to trap solid objects and sedimentation by gravity to remove suspended solids (removed and collected as sludge). The sedimentation process may be accelerated by the use of chemicals. The total suspended solids concentration (TSS) is an effective indicator of primary treatment. The TSS represents the weight proportion of fine particulate matter that remains in suspension per unit volume of water. Primary treatment may reduce the TSS concentration to 40 to 50%. After the primary treatment, the wastewater may be directed to a secondary treatment which typically includes biological treatment steps and sedimentation. Specifically, primary effluent may be subjected to an activated sludge technique in which the effluent is aerated, and aerobic microorganisms metabolise organic matter to carbon dioxide and water and reproduce to form a microbial community. Organic nitrogen compounds may be converted to ammonia and subsequently nitrate. A secondary sedimentation tank may allow the microorganisms and solid wastes to agglomerate and settle as sludge. At least some of the collected sludge (activated sludge) may then be recycled for use as an inoculum for biological treatment of further incoming wastewater.

[0042] The secondary treatment may reduce the TSS content to 10 to 15%. The Biochemical Oxygen Demand (BOD) is a further indicator of secondary treatment. The BOD is a measure of the amount of oxygen needed or demanded by aerobic microorganisms to break down the organic matter present in a certain sample of water at a specific temperature and over a given time period. Secondary treatment typically reduces the BOD to 10 to 15%.

[0043] Alternative or additional processes carried out during secondary treatments may include biofiltration and oxidation ponds. Biofiltration requires the use of microorganisms immobilised on filters (e.g. sand filters, contact filters or trickling filters) to decompose organic matter and remove additional sediment. Oxidation ponds involve passing wastewater through large bodies of water (e.g. lagoons) in sunlight for extended periods of time to enable microorganisms to decompose organic matter.

[0044] Primary and secondary treatments are often sufficient for many purposes and not all wastewater treatment plants use tertiary treatment. Those that do use tertiary treatment achieve more stringent levels of cleanliness to meet the exacting standards that govern water reuse, especially in public water supplies. Tertiary treatment is also beneficial when facilities must discharge water into sensitive or fragile ecosystems (for example, estuaries, low-flow rivers, coral reefs, etc). Tertiary treatment may include filtration, disinfection and removal of nitrogen and phosphorus.

[0045] Treatment with a source of Fe2+or Fe3+ions

[0046] Dissolved sulphide compounds in the water to be treated may originate from bacterial metabolism.

[0047] In wastewater systems, dissolved sulphides may be present at various stages of the treatment process. Bacteria in water treatment systems, particularly wastewater treatment systems, may utilise soluble oxygen, soluble nitrate or soluble sulphate as sources of energy. Soluble oxygen is usually present in fresh wastewater, but it is rapidly depleted by biological activity. There is typically also very little nitrate present in wastewater, while sulphate is typically abundant. Therefore, sulphate may be used by bacteria as a source of energy in the absence of oxygen and nitrate. Dissolved sulphides may be generated from reduction of sulphates during bacterial respiration, and may subsequently combine with hydrogen ions to form hydrogen sulphide compounds. Accordingly, in some embodiments, the at least one dissolved sulphide compound comprises H2S, HS' and / or S2'.

[0048] The source of Fe2+or Fe3+ions may be added to the water stream in at least one process location prior to the addition of the percarboxylic acid. In continuously flowing water treatment systems, the source of Fe2+or Fe3+ions may be added to the water stream in at least one process location upstream of the addition of the per carboxy lie acid. It is possible to add the source of Fe2+or Fe3+ions in several different locations. These may include, but are not limited to, any location along the water treatment system where dissolved sulphides are present. Preferably, the source of Fe2+or Fe3+ions is added to the water at one, two or more process locations that are close to locations where dissolved sulphides exist or are generated. In some examples and with specific regard to wastewater treatment, the addition point may be at the influent or effluent of the primary or secondary treatment sections. In other examples, the source of Fe2+or Fe3+ions may be added to at least the influent of the wastewater treatment plant, during primary treatment, and / or during secondary treatment. In preferred examples, the source of Fe2+or Fe3+ions may be added after secondary treatment and before tertiary treatment, for example, at the effluent of secondary treatment. In some examples, water entering a treatment plant may comprise significant amounts of dissolved sulphides. In these examples, the source of Fe2+or Fe3+ions may be added to the water stream prior to entering the treatment plant. Secondary wastewater treatment may also be especially prone to the production of dissolved sulphides. Therefore, in other examples, treatment with the source of Fe2+or Fe3+ions may be more effective on an effluent of secondary treatment than in other stages of the wastewater treatment process. After secondary treatment, a large proportion of the microorganisms may have sedimented and settled as sludge, and there may not be further significant dissolved sulphide production. A first dosing device may be configured to add or feed the source of Fe2+or Fe3+ions into the water stream. The first dosing device may comprise one or more pumps or valves which facilitate delivery of the source of Fe2+or Fe3+ions, optionally via one or more lines, into the water. The first dosing device may be operated manually or automatically, as described in further detail below. The source of Fe2+or Fe3+ions may be added or fed into the water continuously or at regular intervals at a constant rate, or the dosing may be adjusted as described below.

[0049] The rate of dissolved sulphide production may be dependent upon the concentrations of sulphate ions, organic matter, as well as other factors such as pH, temperature, retention time and stream velocity. Accordingly, in some embodiments, it may be desirable to measure the concentration of the dissolved sulphides in water prior to contacting the water with the source of Fe2+or Fe3+ions. The dosing of the Fe2+or Fe3+ions may then be adjusted according to the information obtained about the concentration of the dissolved sulphides. According to another embodiment of the invention, the concentration of the dissolved sulphides may be measured downstream of or after the addition of the source of Fe2+or Fe3+ions. Thus, the concentration of dissolved sulphides may be measured prior to the addition point and / or after the addition point of the source of Fe2+or Fe3+ions.

[0050] The concentration of the dissolved sulphides in the water to be treated may be measured continuously. In this context, by “continuously” it is meant that the level of dissolved sulphides is measured and at regular, repeating intervals without interruption. For example, the level of the dissolved sulphides in water may be measured at regular intervals from 1 minute to 5 minutes, or at regular intervals of 10 minutes, 20 minutes, 30 minutes, or every hour. The dosing of the source of Fe2+or Fe3+ions may be adjusted based on the measured concentrations. This type of system enables an effective concentration of Fe2+or Fe3+ions to be provided for the desired reduction in dissolved sulphide content.

[0051] In order to maintain the concentration of dissolved sulphides at acceptably low levels, the molar ratio of the Fe2+ions or Fe3+ions to the at least one sulphide compound may be from 0.5: 1 to 5: 1, or from 1 : 1 to 3 : 1.

[0052] The concentration of dissolved sulphides may be measured by a first measuring device. In some examples, the concentration of dissolved sulphides is measured by colorimetric methods. In these examples, the sulphide-containing water sample is reacted with DPD (N,N-diethyl-p- phenylenediamine) in the presence of Fe3+ions. The reaction of DPD with sulphide results in an intermediate compound which is ultimately oxidized to methylene blue by Fe3+ions. The amount of methylene blue will be in direct proportion to the amount of dissolved sulphides in the sample and can be quantified via comparison to a standard colour chart or measured via a photometer. The Hach™ LCK653 Sulphide Cuvette Test is one such commercially available absorbance test for measuring dissolved sulphides. In other examples, the measuring device may comprise a sensor for directly determining dissolved sulphide concentration.

[0053] The detection / measuring of dissolved sulphides and the addition or dosing of the source of Fe2+or Fe3+ions through the first dosing device may be automated. Preferably, the measurement of dissolved sulphides is performed online. In other embodiments, the measurement of dissolved sulphides is performed inline. Inline and online measurements are both forms of continuous, in situ measurement. Online measurements are not made directly in the main process line, but rather in a built-in branch or by-pass (for example, a sampling loop) into which samples of water containing dissolved sulphides are automatically fed. Inline measurements are made directly in the main process line which requires placing a probe or sampling interface directly into or in line with the process flow. For dissolved sulphide measurement methods requiring additional reagents to measure dissolved sulphides (for example, colorimetric methods as described above), online measurement configurations are preferred.

[0054] A control apparatus may control the dosing of the source of Fe2+or Fe3+ions from the first dosing device. The control apparatus may comprise a computing apparatus configured to implement at least some of the features described herein. In one example, the invention provides a control apparatus comprising at least one processor, and at least one memory including a computer program code, the at least one memory and the computer code being configured, with the at least one processor, to cause the apparatus to perform any of the methods described herein.

[0055] Figure 1 is a block diagram of control apparatus (18) according to an example of the invention. The control apparatus (18) is suitable for implementing at least some of the operations described herein. With specific reference to Figure 1, the control apparatus (18) may comprise at least one processor (28), at least one memory (29), a communication interface (32) and a user interface (31). The control apparatus may further comprise other internal circuitry and components necessary to perform the tasks described herein. The control apparatus (18) may be constructed and arranged to receive output data from the first measuring device (19) to monitor the level of dissolved sulphides present in water as measured by the first measuring device (19), and to regulate the feeding of the source of Fe2+or Fe3+ions from the first dosing device (16) to the water.

[0056] The control apparatus (18) may comprise a communication interface (32) for connecting the control apparatus to a data communications system and enabling data communications with the apparatus. The communication interface (32) may comprise a wired and / or wireless communication circuitry, such as Ethernet, Wireless LAN, Bluetooth, GSM, CDMA, WCDMA, LTE, 5G circuitry, and / or analog. The communication interface can be integrated in the control apparatus (18) or provided as a part of an adapter, card or the like, that is attachable to the control apparatus (20). The communication interface (32) may support one or more different communication technologies. The control apparatus (18) may also or alternatively comprise more than one communication interface (32).

[0057] The user interface (31) may comprise a circuitry for receiving input from a user of the control apparatus (18), for example, via a keyboard, graphical user interface shown on the display of the apparatus, speech recognition circuitry, or an accessory device, such as a headset, and for providing output to the user via, for example, a graphical user interface or a loudspeaker. The control apparatus may be operated remotely.

[0058] The at least one processor (28) may be coupled to the at least one memory (29). The at least one processor (28) may be configured to execute an appropriate computer program code to implement one or more of the aspects described herein. The at least one processor (28) may be a central processing unit (CPU), a microprocessor, a digital signal processor (DSP), a graphics processing unit, an application specific integrated circuit (ASIC), a field programmable gate array, a microcontroller or a combination of such elements.

[0059] The at least one memory (29) may comprise a work memory (30) and a persistent (non-volatile, N / V) memory (33) configured to store computer program code (34) and data (35). The memory (33) may comprise any one or more of: a read-only memory (ROM), a programmable read-only memory (PROM), an erasable programmable read-only memory (EPROM), a random-access memory (RAM), a flash memory, a data disk, an optical storage, a magnetic storage, a smart card, a solid state drive (SSD), or the like. The control apparatus (18) may comprise other possible components for use in software- and hardware- aided execution of tasks it is designed to perform.

[0060] The control apparatus (18) may comprise a plurality of memories (33). The memory (33) may be constructed as a part of the control apparatus (18) or as an attachment to be inserted into a slot, port, or the like of the apparatus (18) by a user or by another person or by a robot. The memory (33) may serve the sole purpose of storing data, or be constructed as a part of an apparatus (18) serving other purposes, such as processing data.

[0061] The skilled person would understand that in addition to the elements shown in Figure 1, the control apparatus (18) may comprise other elements, such as microphones, displays, as well as additional circuitry such as an input / output (I / O) circuitry, memory chips, application-specific integrated circuits (ASIC), a processing circuitry for specific purposes such as a source coding / decoding circuitry, a channel coding / decoding circuitry, a ciphering / deciphering circuitry, and the like. Additionally, the control apparatus (18) may comprise a disposable or rechargeable battery (not shown) for powering the apparatus (18) if an external power supply is not available. Further, it is noted that only one apparatus (18) is shown in Figure 1, but certain embodiments may equally be implemented in a cluster of shown apparatuses.

[0062] In some embodiments, the control apparatus (18) may be configured to receive input of specific parameters, for example, a pre-defined threshold concentration of dissolved sulphides. The specific parameters may be input through the user interface (31). In these embodiments, based on output data received from the first measuring device (19), the control apparatus (18) may detect an increase in concentration of dissolved sulphides above the pre-defined threshold concentration (for example, when there the water quality is poor), and cause the first dosing device (16) to increase the amount of the source of Fe2+or Fe3+ions that is fed to the water to be treated over a given period of time to restore the concentration of sulphides to below the pre-defined threshold value. This may be effected, for example, by increasing the velocity of a pump or by opening of a valve which facilitates delivery of the source of Fe2+or Fe3+ions to the water to be treated. Conversely, based on output data received from the first measuring device (19), the control apparatus (18) may detect a decrease in concentration of dissolved sulphides below the pre-defined threshold value and cause the dosing device (16) to decrease the amount of the source of Fe2+or Fe3+ions that is fed to the water to be treated over a given period of time to avoid unnecessary depletion of reagents. This may be effected, for example, by decreasing the velocity of a pump or closing of a valve which would otherwise facilitate delivery of the source of Fe2+or Fe3+ions to the water to be treated. An appropriate computer program code (34), as executed by the processor (28) and stored in memory (29), may determine, based on output measurement data received from the first measuring device (19), whether the measured level of dissolved sulphides is above or below the pre-defined value, and the adjustment required in the amount of the source of Fe2+or Fe3+ions added to the water to be treated, as described herein. Accordingly, the control apparatus (18) may be constructed and arranged to compare the measured dissolved sulphide concentration with the pre-defined concentration of sulphide concentration, and may be constructed and arranged to adjust the performance of the first dosing device (16).

[0063] In an example of the invention, the at least one processor (28) may comprise a proportional- integral-derivative (PID) controller. A PID controller is a control loop mechanism employing feedback that is widely used in industrial control systems and in a variety of other applications requiring continuously modulated control. The PID controller may continuously calculate an error value as the difference between the pre-defined set concentration of dissolved sulphides and the measured concentration of dissolved sulphides, and may subsequently apply a correction based on proportional, integral, and derivative terms. The controller may attempt to minimize the error over time by adjustment of its output (for example, by adjustment of the velocity of the one or more pumps delivering the source of Fe2+or Fe3+ions) such that the concentration of dissolved sulphides does not exceed the pre-defined threshold concentration of dissolved sulphides. In another example, a PI (proportional, integral) -based controller is used.

[0064] The pre-defined threshold concentration of dissolved sulphides may be 5 mg / 1, 2 mg / 1, 1 mg / 1, or 0.5 mg / 1. Therefore, a sufficient amount of Fe2+or Fe3+ions may be added to reduce the concentration of the dissolved sulphides in the water to less than 5 mg / 1, less than 2 mg / 1, less than 1 mg / 1, or less than 0.5 mg / 1. In preferred embodiments, a sufficient amount of Fe2+or Fe3+ions is added to reduce the concentration of the dissolved sulphides in the water to less than 0.5 mg / 1. The present inventors have found that percarboxylic acid disinfection performance is optimal when the concentration of the dissolved sulphides in the water is less than 0.5 mg / 1. In some examples, iron (II) chloride or iron (III) chloride provides a source of Fe2+and Fe3+ions, respectively. Fe2+or Fe3+ions may be added to the water at a concentration of at least 5 mg / 1, at least 10 mg / 1 at least 15mg / l or at least 20mg / l. Particularly effective concentrations of Fe2+or Fe3+ions may be from 5 mg / 1 to 15 mg / 1 or from 10 mg / 1 to 15 mg / 1. The source of Fe2+or Fe3+ions may be added to the water at a basal concentration as exemplified above. The concentration of Fe2+or Fe3+ions may subsequently be adjusted according to the level of dissolved sulphides as detailed herein.

[0065] The source of Fe2+or Fe3+ions is not particularly limited and may comprise any ferrous or ferric salt which is soluble in water, such that on contact with water, Fe2+or Fe3+ions are available to react with the dissolved sulphides. In some examples, the source ofFe2+or Fe3+ions comprises one or more of: ferric sulphate (F 62(804)3, ferric chloride (FeCh), ferrous chloride (FeCh), and ferrous sulphate (FeSO4). In preferred examples, the source of Fe2+or Fe3+ions may comprise ferrous chloride and / or ferric chloride..

[0066] It is believed that insoluble sulphide precipitates are formed on binding of Fe2+and Fe3+ions to dissolved sulphide ions, thus reducing the amount of dissolved sulphides. Insoluble elemental sulphur may also be formed during treatment with Fe2+and Fe3+ions. The insoluble sulphide precipitates and sulphur may be removed from the water through settling or sedimentation. In some embodiments, the sulphide precipitates / sulphur are allowed to sediment in a sedimentation tank where they may be removed from the water. In the methods of the invention, Fe2+or Fe3+ions may further neutralize the charge on other suspended particles in the water causing their sedimentation. Furthermore, the precipitates formed may entrap additional particles promoting flocculation and further clarification of the water. In some examples, the pH of the water may be adjusted to optimise precipitation of the dissolved sulphides. In some examples, a pH range of 5 to 11 provides optimal removal of dissolved sulphides. In preferred examples, a pH range of 6 to 7 provides optimal removal of dissolved sulphides.

[0067] The present inventors have unexpectedly found that Fe2+ions are more effective in improving the disinfection performance of percarboxylic acids than Fe3+ions. Without wishing to be bound by theory, Fe2+ions may be more effective in precipitating sulphide ions and in. promoting aggregation of suspended particles. This may result in a reduction in the total suspended solid (TSS) content, with a consequent reduction in microbial growth in water and microbial load in effluent, and an enhancement in disinfection performance of the percarboxylic acid. Disinfection by percarboxylic acid

[0068] Following treatment with the source of Fe2+or Fe3+ions, a disinfectant comprising a percarboxylic acid is brought into contact with or fed into the water to provide disinfection against the at least one microorganism.

[0069] The percarboxylic acid may comprise peracetic acid (PAA), performic acid (PFA) or a combination thereof. In preferred examples, the percarboxylic acid comprises PFA.

[0070] A second dosing device may be configured to add or feed the percarboxylic acid into the water stream. The dosing device may comprise one or more pumps or valves which facilitate delivery of the percarboxylic acid, optionally via one or more lines, into the water. The second dosing device may be operated manually or automatically, as described in further detail below.

[0071] PAA is commercially available as an acidic quaternary equilibrium mixture with acetic acid, hydrogen peroxide (H2O2) and water, as illustrated in reaction (1) below, and accordingly, may be fed into the water in the form of the equilibrium mixture.

[0072] CH3COOH + H2O2 CH3CO-OOH + H2O (1)

[0073] Due to the greater instability and faster decomposition times of PFA, PFA may need to be generated immediately before use. Preferably, PFA is generated in situ (i.e. at the site of water treatment). As such, the second dosing device may comprise a reaction vessel in which PFA is produced. In other embodiments, PFA is produced outside the water treatment system and transferred directly and rapidly to the dosing device for feeding to water. A preferred preparation method of PFA comprises mixing formic acid with hydrogen peroxide according to reaction (2) below optionally, in the presence of an acid catalyst such as sulphuric acid, ascorbic acid, or boric acid. The equilibrium of reaction (2) below may be shifted in favour of PFA formation if the molar ratio of formic acid to hydrogen peroxide is increased, or by removing water from the reaction.

[0074] CHO-OH + H2O2 CHO-OOH + H2O (2)

[0075] The percarboxylic acid may be fed into the water continuously (i.e. without pause) or at regular, pre-determined time intervals. The percarboxylic acid may be fed into the water at a constant rate. Alternatively, the amount of percarboxylic acid fed into the water may be adjusted based on the measured level of dissolved sulphides and / or a measured level of residual percarboxylic acid. As indicated above, the level of residual percarboxylic acid in water provides an indicator of disinfection efficacy.

[0076] In some examples, the feeding of the percarboxylic acid into the water is automated. In these examples, and with further reference to Figure 1, the control apparatus (18) described above may be operatively connected to the second dosing device (16a). The control apparatus (18) may be constructed and arranged to receive output data from the first measuring device (19) relating to the concentration of the dissolved sulphides, and to regulate the feeding of the percarboxylic acid from the second dosing device (16a) to the water based on such output data. This may ensure that the impact of any change in dissolved sulphide concentration on the disinfection performance of the percarboxylic acid is minimised or negated by an adjustment in the amount of percarboxylic acid that is fed to the water for disinfection.

[0077] Thus, based on output data received from the first measuring device (19), the control apparatus (18) may detect an increase in concentration of dissolved sulphides above a pre-defined threshold concentration, and cause the second dosing device (16a) to increase the amount of percarboxylic acid that is fed to the water to be treated over a given period until the concentration of dissolved sulphides is restored to the pre-defined threshold. This may be effected, for example, by increasing the velocity of a pump or opening of a valve which facilitates delivery of the percarboxylic acid from the second dosing device (16a) to the water to be treated. Conversely, based on output data received from the first measuring device (19), the control apparatus (18) may detect a decrease in concentration of dissolved sulphides below the pre-defined threshold value and cause the second dosing device (16b) to decrease the amount of percarboxylic acid that is fed to the water to be treated over a given period of time to avoid unnecessary depletion of the percarboxylic acid and an increase in residual percarboxylic acid concentration above regulatory limits. This may be effected, for example, by decreasing the velocity a pump or closing of a valve which would otherwise facilitate delivery of the percarboxylic acid from the second dosing device (16b) to the water to be treated.

[0078] An appropriate computer program code (34), as executed by the processor (28) and stored in memory (29) as described above, may determine, based on output measurement data received from the first measuring device (19), whether the measured level of dissolved sulphides is above or below the pre-defined threshold, and the adjustment required in the amount of the percarboxylic acid fed to the water to be treated, as described herein. Accordingly, the control apparatus (18) may be constructed and arranged to compare the measured dissolved sulphide concentration with the pre-defined threshold sulphide concentration, and may be constructed and arranged to adjust the performance of the second dosing device (16a).

[0079] In further examples, the dosing of the percarboxylic acid may additionally or alternatively be adjusted based on the concentration of residual percarboxylic acid. In these examples, a second measuring device (19a) may be provided to measure the residual concentration of the percarboxylic acid. The control apparatus (18) may be constructed and arranged to receive output data from the second measuring device (19a) relating to the concentration of the residual percarboxylic acid and to regulate the feeding of the percarboxylic acid from the second dosing device (16a) to the water based on such output data. Thus, for example, based on output data received from the second measuring device (19a), the control apparatus (18) may detect an increase in concentration of residual percarboxylic acid above a pre-defined threshold concentration, and cause the second dosing device (16a) to decrease the amount of percarboxylic acid that is fed to the water to be treated over a given period to avoid unnecessary depletion of the percarboxylic acid and an increase in residual percarboxylic acid concentration above regulatory limits, and to restore the concentration of residual percarboxylic acid to the pre-defined threshold. This may be effected, for example, by decreasing the velocity of a pump or closing a valve which otherwise facilitate delivery of the percarboxylic acid from the second dosing device (16a) to the water to be treated. Conversely, based on output data received from the second measuring device (19a), the control apparatus (18) may detect a decrease in concentration of residual percarboxylic acid to below the pre-defined threshold value and cause the second dosing device (16a) to increase the amount of percarboxylic acid that is fed to the water to be treated over a given period of time to maintain the required disinfection efficacy. This may be effected, for example, by increasing the velocity of a pump or opening a valve which facilitates delivery of the percarboxylic acid from the second dosing device (16a) to the water to be treated. In examples where the percarboxylic acid is synthesised in situ, changes in the dosing of percarboxylic acid may be effected by corresponding changes in the rate of production of percarboxylic acid (e.g. by changing the amount of reactants available to produce the percarboxylic acid). The pre-defined threshold concentration of residual percarboxylic acid may be determined based on relevant regulatory limits governing the area in which the water treatment system is located. In some embodiments, the pre-defined threshold concentration of percarboxylic acid may be from 2 mg / 1 to 5 mg / 1, 0.3 mg / 1 to 1 mg / 1, or from 0.4 mg / 1 to 0.6 mg / 1.

[0080] In some embodiments, the method of treating water of the invention is a continuous method. In an example of the invention, a percarboxy lie acid solution, preferably a PF A solution, is contacted with the water at a basal concentration (i.e. the concentration of per carboxylic acid at the point of addition to water before its consumption) of from 0.5 to 50 mg / 1, or from 0.8 to 25 mg / 1 or from 1 to 10 mg / 1, based on the amount of active percarboxylic acid. . The percarboxylic acid may be continually fed from the second dosing device (16a) in which it may be produced, to the water to be treated, at the above active concentrations In response to changes in dissolved sulphide concentration and / or changes in residual percarboxylic acid concentration, the dosing of the percarboxylic acid may be adjusted as described above.

[0081] A PID controller as referred to previously, may continuously calculate an error value as the difference between the pre-defined set concentration of residual percarboxylic acid and the measured concentration of percarboxylic acid, and may subsequently apply a correction based on proportional, integral, and derivative terms. The controller may attempt to minimize the error over time by adjustment of its output (for example, by adjustment of the velocity of a pump delivering the percarboxylic acid) such that that the concentration of the percarboxylic acid does not exceed the pre-defined threshold concentration.

[0082] The percarboxylic acid may be added to the water in at least one process location, after the addition of the source of Fe2+or Fe3+ions, such that the amount of dissolved sulphides has been reduced at the point of addition of the percarboxylic acid. This ensures the disinfection performance of the percarboxylic acid is minimally affected by the presence of dissolved sulphides. In continuously flowing systems, the percarboxylic acid is preferably added downstream of the process location(s) at which the source of Fe2+or Fe3+ions is added. In the treatment of wastewater, the percarboxylic acid may be added to the water as the last treatment step, for example, as a tertiary treatment following primary and secondary treatments. Measurement of percarboxylic acid

[0083] In the method for treating water according to the present invention, the level of residual percarboxylic acid may preferably be measured and monitored continuously and in real-time in order to detect any fluctuations from a threshold concentration of residual PF A. In this context, by “continuously” it is meant that the level of percarboxylic acid is measured at regular, repeating intervals without interruption. For example, the level of residual percarboxylic acid may be measured and monitored at regular intervals such as every 2 minutes, every 3 minutes, every 4 minutes, every 5 minutes, every 10 minutes, every 30 minutes or every hour. The measurements may be performed online or inline as described above in relation to dissolved sulphide measurements.

[0084] The percarboxylic acid may be measured manually or in an automated fashion. The percarboxylic acid may be measured using standard methods including amperometric techniques and colorimetric methods such as the well-established DPD (N,N-diethyl-p-phenylenediamine) method. DPD kits and photometers are commercially available. Examples of DPD analysers which may be used for percarboxylic acid measurements include Hach® CL- 17 analyser, Hach® CL-17sc analyser, or a Xylem® 3017M analyser. The percarboxylic acid may also be measured using a standard Refl ectoquant Peracetic acid test.

[0085] The level of residual percarboxylic acid may be measured by the second measuring device (19a) at any process location after the percarboxylic acid is fed into the water. Preferably, the measurement is commenced after a sufficient contact time (i.e. time between addition of percarboxylic acid to water and measurement) has elapsed. A contact time of at least 5 minutes, 10 minutes, 20 minutes or 30 minutes is desirable. In some examples, a contact time of at least one hour or two hours is provided. In a continuously flowing system, this means that measurement is performed at a flowing distance of at least 5 minutes, 10 minutes, 20 minutes, 30 minutes, one hour or two hours downstream of the point at which the percarboxylic acid is fed into the water.

[0086] Further provided is a use of Fe2+or Fe3+ions to improve disinfection performance of a percarboxylic acid against at least one microorganism in a method of treating water, wherein the water comprises an amount of at least one dissolved sulphide and the at least one microorganism, and the use comprises reducing the amount of the at least one dissolved sulphide in water by contacting the water with the Fe2+or Fe3+ions.

[0087] The method may be as defined herein.

[0088] Although at least some aspects of the embodiments described herein with reference to the drawings comprise computer processes performed in processing systems or processors, the invention also extends to computer programs, particularly computer programs on or in a carrier, adapted for putting the invention into practice. The program may be in the form of non-transitory source code, object code, a code intermediate source and object code such as in partially compiled form, or in any other non-transitory form suitable for use in the implementation of processes according to the invention. The carrier may be any entity or device capable of carrying the program. For example, the carrier may comprise a storage medium, such as a solid-state drive (SSD) or other semiconductor-based RAM; a ROM, for example a CD ROM or a semiconductor ROM; a magnetic recording medium, for example a floppy disk or hard disk; optical memory devices in general; etc.

[0089] The following are intended as examples only and do not limit the present disclosure.

[0090] EXAMPLES

[0091] Example 1 - Dissolved sulphide content of wastewater

[0092] The sulphide content of various wastewater (WW) samples before and after biological (secondary) treatment was determined using Hach test LCK 653. The results are illustrated in Table 1 below. Dates and times of sample collection before and after secondary (biological) treatment are indicated.

[0093] Table 1 It can be seen from Table 1 that the dissolved sulphide content in incoming WW and in the influent of secondary treatment is significantly lower than the dissolved sulphide content of the effluent of secondary treatment.

[0094] Example 2 - PEA efficacy and dissolved sulphide content

[0095] The disinfection efficacy of PFA against A. coli, total coliform bacteria and total aerobic bacteria was assessed in wastewater (WW) samples. The tests were made on site at a wastewater treatment plant immediately after sulphide content measurement. The first WW sample had a sulphide content of 2.84 mg / ml and the second WW sample had a sulphide content of 5.08 mg / ml. The quality of the WW samples was otherwise similar.

[0096] WW samples were dosed with varying concentrations of PFA to determine disinfection efficacy, after a set contact time of 12 minutes. After this time, E. coli and total coliform bacteria were quantified using Compact Dry EC bacterial cultivation plates (Nissui Pharma, Japan). Total aerobic bacteria colonies were enumerated using 3M Petrifilm aerobic count plates. All bacteria were cultivated at +37°C for 24 hours.

[0097] It can be seen from Figure 2A that when the sulphide content of the WW was 2.84 mg / L, incubation with 20mg / L PFA resulted in a significant decrease in E. coli, total coliform bacteria and total aerobic bacteria. 34mg / L PFA eradicated total coliform bacteria and total aerobic bacteria completely. As seen in Figure 2B, when the sulphide content of the WW was raised (5.08mg / L), PFA efficacy was reduced. Even the highest dose of 34mg / L PFA was not sufficient to kill total coliform bacteria and total aerobic bacteria.

[0098] Example 3 - PFA efficacy after Fe2+ / Fe3+treatment

[0099] WW samples were taken from the effluent of secondary (biological) treatment and incubated with 13.5 mg / L Fe2+(provided as iron (II) chloride solution), 14 mg / L Fe3+(provided as iron (III) chloride solution) or 21 mg / L Fe3+(provided as iron (III) chloride solution) using the following treatment protocol: 30 seconds of fast mixing, 10 minutes of slow mixing and 10 minutes to allow sedimentation. The Fe2+ / Fe3+treatment reduced the dissolved sulphide content in water to less than 0.005 mg / L. The samples were then incubated with various concentrations of PFA for a contact time of 12 minutes, and E. coli and total coliform bacterial counts were subsequently determined. K illustrated in Figure 3, after treatment with 13.5 mg / L Fe2+(iron (II) chloride solution), 14 mg / L Fe3+(iron (III) chloride solution, and 21 mg / L Fe3+(iron (III) chloride solution), 3 mg / L PFA reduced E. coli and total coliform bacteria. 20mg / L PFA totally eliminated E. coli and total coliform bacteria. These data can be broadly compared to Figures 2A and 2B which illustrate that at higher sulphide concentrations (2.84mg / L and 5.08mg / L, respectively), 20mg / L PFA was not sufficient to kill bacteria to acceptable limits.

[0100] Example 4 - Dissolved sulphide removal with Fe2+and Fe3+

[0101] The effects of Fe2+and Fe3+on dissolved sulphide removal from WW samples were assessed visually. WW samples obtained from the effluent of secondary treatment were incubated at room temperature with 50 ppm iron (II) chloride which provides 4.5 mg / 1 Fe2+ions (Figure 4A left), 50ppm iron (III) chloride which provides 7 mg / 1 Fe3+ions (Figure 4A right), 100 ppm iron (II) chloride which provides 9 mg / 1 Fe2+ions (Figure 4B left), or lOOppm iron (III) chloride which provides 14 mg / 1 Fe3+ions (Figure 4B right). A visual assessment was made after 10 minutes of slow mixing and 10 minutes of sedimentation.

[0102] Treatment with 9 mg / 1 Fe2+ions (Figure 4B left) resulted in a dissolved sulphide concentration of <0.005 mg / 1. Treatment with 14 mg / 1 Fe3+ions (Figure 4B right) resulted in a dissolved sulphide concentration of 0.325 mg / 1. This indicates an improved efficiency of Fe2+ions over Fe3+ions in reducing the content of dissolved sulphides. The increased opacity of the water sample treated with 9 mg / 1 Fe2+ions (Figure 4B left) as compared to the water sample treated with 14mg / L Fe3+ions (Figure 4B right) may be attributed to precipitated iron sulphide which has not sedimented to the bottom of the container. Analogous considerations apply to Figure 4 A in which an increased opacity is observed with treatment with 4.5 mg / 1 Fe2+ions (left) as compared to treatment with 7 mg / 1 Fe3+ions (right).

[0103] Other variants or use cases of the disclosed techniques may become apparent to the person skilled in the art once given the disclosure herein. The disclosure is not limited by the described embodiments but only by the accompanying claims. 1

Claims

Claims1. A method of treating water, wherein the water comprises an amount of at least one dissolved sulphide and at least one microorganism, the method comprising the steps: i) contacting the water with a source of Fe2+or Fe3+ions to reduce the amount of the at least one dissolved sulphide, and ii) contacting the water with a percarboxylic acid to provide disinfection against the at least one microorganism; wherein step ii) is performed after step i).

2. The method of claim 1, wherein the percarboxylic acid comprises performic acid and / or peracetic acid.

3. The method of claim 2, wherein the percarboxylic acid comprises performic acid.

4. The method of any of claims 1 to 3, wherein step i) comprises contacting the water with a source of Fe2+ions.

5. The method of any preceding claim, wherein the dissolved sulphide comprises H2S, HS' and / or S2'.

6. The method of any preceding claim, wherein the method comprises measuring the amount of the at least one dissolved sulphide prior to performing step ii), optionally wherein the source of Fe2+or Fe3+ions is contacted with the water in step i) in an amount which is determined based on a measured amount of the at least one dissolved sulphide compound.

7. The method of claim 6, wherein the percarboxylic acid is contacted with the water in step ii) in an amount which is determined based on a measured amount of the at least one dissolved sulphide compound.

8. The method of any preceding claim, wherein the amount of the at least one dissolved sulphide is reduced to less than 5 mg / 1, or less than 2 mg / 1, less than 1 mg / 1, or less than 0.5 mg / 1, prior to step ii).

9. The method of claim 8, wherein the amount of the at least one dissolved sulphide is reduced to less than 0.5 mg / 1.

10. The method of any preceding claim, wherein the percarboxylic acid is contacted with the water in step ii) in an amount of from 0.5 to 50 mg / 1, based on active percarboxylic acid.

11. The method of claim 10, wherein the percarboxylic acid is contacted with the water in step ii) in an amount of from 1 to 10 mg / 1, based on active percarboxylic acid.

12. The method of any preceding claim, wherein the source of Fe2+or Fe3+ions is contacted with the water in step i) to provide Fe2+or Fe3+ions in an amount of from 5 mg / 1 to 15 mg / 1.

13. The method of any preceding claim, wherein the method further comprises measuring the amount of residual percarboxylic acid in the water, optionally wherein the percarboxylic acid is contacted with the water in step ii) in an amount which is determined based on the measured amount of residual percarboxylic acid.

14. The method of any preceding claim, wherein the method further comprises allowing a precipitate formed in step i) to sediment.

15. The method of any preceding claim, wherein the water comprises wastewater.

16. The method of claim 15, wherein the method comprises performing primary treatment and / or secondary treatment of the wastewater, optionally wherein step i) is performed on an influent of secondary sedimentation.

17. The method of claim 15 or claim 16, wherein the method further comprises performing tertiary treatment of the wastewater, optionally wherein step ii) is performed during tertiary treatment.

18. The method of any preceding claim, wherein the source of Fe2+or Fe3+ions comprises ferric sulphate, ferric chloride, ferrous chloride, and ferrous sulphate.

19. The method of claim 18, wherein the source of Fe2+or Fe3+ions comprises ferrous chloride or ferric chloride.

20. The method of any preceding claim, wherein in step i), the source of Fe2+ions or Fe3+ions is brought into contact with the water in a molar ratio of the Fe2+ions or Fe3+ions to the at least one dissolved sulphide compound of from 0.5 : 1 to 5 : 1 , or from 1: 1 to 3 : 1.

21. An apparatus, comprising: at least one processor; and at least one memory including computer program code, the at least one memory and the computer program code being configured, with the at least one processor, to cause the apparatus to perform the method of any of claims 1 to 20.

22. A water treatment system comprising the apparatus of claim 21, the system comprising: a first dosing device which is configured to feed a source of Fe2+or Fe3+ions to the water, a second dosing device which is configured to feed a percarboxylic acid to the water, and a first measuring device which is configured to measure the level of the at least one dissolved sulphide in the water and generate output data relating to the measured level of dissolved sulphide, wherein the apparatus is constructed and arranged to receive the output data relating to the measured level of the at least one dissolved sulphide from the first measuring device, monitor the measured level of the at least one dissolved sulphide in the water, and to regulate the amount of the source of Fe2+or Fe3+ions that is fed into the water by the first dosing device and / or regulate the amount of the per carboxy lie acid that is fed to the water by the second dosing device based on the monitored level of the at least one dissolved sulphide.

23. The system of claim 22, wherein the system further comprises: a second measuring device which is configured to measure the level percarboyxlic acid in the water and generate output data relating to the measured level of percarboxylic acid, and wherein the apparatus is constructed and arranged to receive the output data relating to the measured level of the percarboxylic acid from the second measuring device, monitor the measured level of the percarboxylic acid in the water, and regulate the amount of the percarboxylic acid that is fed to the water by the second dosing device based on the monitored level of the percarboxylic acid.

24. Use of Fe2+ions or Fe3+ions to improve disinfection performance of a percarboxylic acid against at least one microorganism in a method of treating water, wherein the water comprises an amount of at least one dissolved sulphide and the at least one microorganism, and the use comprises reducing the amount of the at least one dissolved sulphide in water by contacting the water with the Fe2+ions or Fe3+ions.

25. The use of claim 24, wherein the method is as defined in any of claims 1 to 20.

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