Process for recovering chemicals from water treatment residuals

Abstract

A process for recovering chemicals from water treatment residuals (WTR) generated during a water treatment process, enabling the recycling of these chemicals into the treatment process. The process comprises the key steps of a) receiving WTR from a water treatment process, wherein the WTR contains at least one coagulant, b) leaching the coagulant metal compounds, from the WTR using an acid solution to produce a leachate, comprising metal ions and co-digested organics, and a thickened sludge, c) feeding the leachate into a Donnan Dialysis process, wherein the metal ions are separated from the organics using and concentrated into an acid sweep solution, and d) processing the acid sweep solution containing the coagulant metal ions through nano-filtration to separate the coagulant from the acid solution, thereby producing reusable coagulant and recovered acid and recycling these chemicals into the water treatment process.

Description

[0001] PROCESS FOR RECOVERING CHEMICALS FROM WATER TREATMENT RESIDUALS

[0002] FIELD OF THE INVENTION

[0003] THIS invention relates to a process of recovering chemicals from water treatment residuals generated in a water treatment process and recycling the recovered chemicals into the water treatment process, thereby reducing the amount of precipitate sludge discarded into evaporation ponds or landfill sites as well as operational expenses associated with the treatment of water and recovery of the chemicals.

[0004] BACKGROUND OF THE INVENTION

[0005] The use of chemical agents known as coagulants and flocculants is often necessary during the process of treating water to meet applicable quality standards, such as those for the potable water industry as well as the Fast-Moving Consumer Goods (FMCG) industry, e.g. for water used in the production of beverages. Coagulants are employed to neutralize the charges on suspended particles, facilitating their aggregation into micro-flocs that are more readily separable from the water through solid-liquid separation techniques. Following coagulation, flocculants are introduced to further aggregate these micro-flocs into larger flocs, enhancing their removal by methods such as filtration, centrifugation, sedimentation, or flotation. This combined use of coagulants and flocculants enables the effective removal of suspended solids, organic loads, and turbidity, resulting in improvements in water quality. Conventional water treatment practices typically involve the consumption of large quantities of coagulants. Chemicals such as aluminium salts, particularly aluminium sulphate (alum), are extensively used as coagulants in large-scale potable water treatment processes. While alum is effective as a coagulant, it also produces substantial sludge residue which is typically disposed of in evaporation ponds or landfill sites. When the resultant precipitate sludge is discharged, in untreated form, into landfills or water bodies, it poses significant risks to both ecosystems and human health, as the metals in the sludge could leach into the ground leading to contamination of underground water sources. Also, the use of evaporation ponds for precipitation of such sludge requires large amounts of land which translates into increased capital expenditure for water treatment plants. Further, the increasing necessity to treat effluent water resulting from the use of coagulants, such as alum, during the pre-treatment phase of water purification, is underscored by the need to meet stringent freshwater discharge limits. All the abovementioned factors, combined with escalating disposal costs, have heightened the need for more advanced and environmentally friendly waste management processes for sludge generated in water treatment processes. Coagulant recovery can reduce both the volume of sludge produced from water treatment processes and the associated costs by enabling the recovery and purification of coagulants from water treatment residuals for reuse in the water treatment process. However, stringent regulatory requirements have limited the adoption of coagulant recovery technologies and led to research directed towards alternatives that present a lower risk but potentially less gain. Therefore, there is needed an effective coagulant recovery process that results in several benefits for water treatment facilities, including reduced environmental impact, optimal use of natural resources. It is equally important for the coagulant recovery process to be cost-effective when compared with the typical methods of handling coagulants in commercial operations.

[0006] Despite prolonged efforts, the recovery of coagulants from sludge has proven to be challenging. Various methods and processes for recovering coagulants from water treatment operations have been proposed and used in the art, encompassing both traditional and innovative research-driven approaches. However, while these have presented unique advantages, there are some challenges that are associated with each. The prior art provides a comprehensive review of the efficacy and limitations of existing technologies for coagulant recovery and identifies significant gaps in that respect.

[0007] One traditional recovery method is leaching or solid-liquid extraction, such as acid treatment and alkalization. While this method is able to achieve significant aluminium recovery rates, it is also generally non-selective in recovering coagulants. The acid or base digestion of water treatment residuals results in the co-digestion of organic materials, thus solubilizing both organics and heavy metals. Thus, the resultant recovered coagulant has a low purity and is unsuitable for direct reuse as a coagulant in the water treatment process. Also, the codigestion of organics can be a precursor to trihalomethanes downstream when chlorine is introduced as a disinfectant in the water treatment process. This process also leads to complications in waste management.

[0008] Another common method is liquid ion exchange, also known as liquid-liquid extraction, which faces setbacks due to the entrainment of solvents, such as kerosene, used during the stripping phase. Exposure to kerosene can be toxic. Additionally, liquid ion exchange typically requires multiple process solutions which escalate the costs of the recovery process. Ion exchange resins, on the other hand, require more expensive regeneration agents like sodium hydroxide. Moreover, these processes experience downtime during regeneration, unless two units are used in parallel, which in turn increases the cost of operation.

[0009] Another technique is electro-dialysis, which uses electricity to achieve separation. This process often results in high operating costs.

[0010] Further, conventional pressure-driven membrane processes have achieved only partial success, primarily due to difficulties in separating small organic compounds from the aluminium solution and significant membrane fouling issues. Further methods like ultrafiltration and nanofiltration for direct separation of can lead to fouling and scaling, increasing both operational costs and causing intermittent operation.

[0011] Lastly, several business-related methods are presently under pilot testing. For example, Aquacritox - a technology that makes use of hydrothermal oxidation - utilizes supercritical water oxidation to treat wet organic wastes. This process involves subjecting the wet residue to temperatures and pressures above the critical point (374°C and 221 bars, respectively), where the addition of oxygen facilitates rapid oxidation and allows for the recovery of significant quantities of aluminium hydroxide. The REAL process - a technology heavily reliant on the use of traditional membrane process to recover coagulants - is another innovative method comprising four steps: dissolution of aluminium hydroxide by adding sulphuric acid, ultrafiltration to remove suspended solids, nanofiltration to concentrate the permeate solution, and precipitation of the concentrate with potassium sulfate to form potassium aluminium sulfate. Donnan Dialysis (DD) on the other hand is a relatively new technology in the water treatment field. DD utilizes a specialized cation-exchange membrane which selectively permits the passage of positively charged ions while obstructing negatively charged and neutral species. This selective permeability is central to DD’s ability to recover metal ions, such as aluminium (Al3+) from waste streams laden with organics and other contaminants without the resultant recovered stream being contaminated by these substances. This method prevents the passage of colloids and organics through the membrane, offering a distinct advantage over other filtration methods such as ultrafiltration, microfiltration, and reverse osmosis, which often suffer from pore fouling.

[0012] The present invention seeks to provide a process to recover coagulants from water treatment residuals and the acid used in the recovery of the coagulants. The present invention further relates to a process of treating water incorporating the process of recovering coagulants, as well as associated process acid. Thus, the process of the present invention reduces the amount of metal-loaded sludge that may need to be discarded in landfills or evaporation and enables cost savings to be realised.

[0013] SUMMARY OF THE INVENTION

[0014] According to a first aspect of the invention there is provided a process for recovering chemicals from water treatment residuals, comprising the steps of: a. receiving water treatment residuals (WTR) from a water treatment process, wherein the WTR comprises at least one coagulant that is targeted for recovery, wherein the coagulant is a metal compound, such as aluminium sulphate or alum, ferric sulphate, ferric chloride, ferrous chloride, or sodium aluminate; b. subjecting at least a portion of the WTR to a leaching process wherein metals compounds, including the at least one coagulant, in the WTR stream, are leached into solution using an acid solution to produce a leachate, comprising at least metal ions of the at least one coagulant, and a thickened sludge; c. feeding the leachate to a Donnan Dialysis process, wherein the leachate is fed on the feed side of the cationic exchange membrane and an acid solution on the sweep side, wherein the metal ions of the at least one coagulant present in the leachate, are recovered through their movement across the cationic exchange membrane and concentrated and into the acid sweep solution, and at least a portion of the organics present in the leachate remain on the feed side and are separated into a solids or sludge by-product; and d. feeding the acid sweep solution containing the at least one coagulant metal ions, or the permeate, into a nano-filtration process, wherein the at least one coagulant metal ions, now combined with sulphate ions are separated from the acid sweep solution to produce metal coagulant and recovered acid; wherein the metal coagulant from the nano-filtration process is returned to the water treatment process for reuse in a process of coagulating suspended solid particles in the water.

[0015] Preferably, the acid recovered from the nanofiltration step is returned to the process for recovering chemicals according to the present invention as described above, and more preferably to the leaching step and to the Dannon Dialysis step.

[0016] In a preferred embodiment of the invention, the acid recovered from the nano-filtration step is fed to a Reverse Osmosis (RO) process wherein it is further concentrated by the removal of water, prior to being returned to the process for recovering chemicals.

[0017] Preferably, the solids or sludge from the various process steps of the present invention are sent to evaporation and / or disposal.

[0018] Preferably, the solids or sludge from the various process steps of the present invention are sent to a neutralization step prior to evaporation and / or disposal.

[0019] Preferably, the coagulant to be recovered is aluminium sulphate or alum. However, the process may be used to recover other types of coagulants such as ferric sulphate, ferric chloride, ferrous chloride or sodium aluminate.

[0020] Preferably, the metal ions of the at least one coagulant are selected from the group consisting of aluminium, ferric, ferrous and sodium ions.

[0021] Preferably, the coagulant is recovered from a water treatment process for drinking or potable water. Preferably, the leaching solution used in the leaching process is an acid, such as sulphuric acid (H2SO4) and hydrochloric acid (HCI). More preferably, H2SO4 is used as a leaching solution in the leaching step. More preferably, the H2SO4 has a concentration of from about 0.1 to about 0.5 M. Preferably, the leaching residence time is from about 1 to about 2 hours. HCI may be used as a leaching solution in the leaching step, in which case it is preferred that the concentration of the acid solution is from about 0.1 to about 0.4 M.

[0022] Preferably, the coagulant (Al3+) recovered across the leaching stage is at least about 60% (mass) and not more than about 90% (mass), more preferably at least 65% (mass) to not more than about 90% (mass), more preferably at least 65% (mass) to not more than about 70% (mass), even more preferably about 67% (mass). In the case of alum being used as a coagulant, the aluminium (Al3+) concentration in the leachate typically ranges from about 300 mg / L to about 600 mg / L.

[0023] In a preferred embodiment of the present invention, the Donnan Dialysis uses cationic exchange membranes, preferably arranged in series. More preferably, the sweep solution used is H2SO4, even though other similar acids such as HCI can be used. Even more preferably, the H2SO4 has a concentration of from about 1 to about 2 M. Where HCI is used as the acid sweep solution, it has a concentration of from about 0.8 M to about 1.7 M. Preferably the membranes used in the Donnan Dialysis are a combination of Dupont’s Nation 115 and Nation 117 membranes.

[0024] Preferably, the nanofiltration process uses Dupont NF 90 and NF 70 membranes, arranged in a crossflow configuration, in a 1 stage, 3 pass configuration. The nanofiltration is preferably conducted at an operating pressure of from about 4 bar to about 8 bar. Preferably, the nanofiltration is conducted at an operating temperature of from about 5 degrees Celsius to about 40 degrees Celsius, even more preferably at an operating temperature of from about 15 degrees Celsius to about 25 degrees Celsius.

[0025] Preferably, the Reverse Osmosis process uses Dupont Brackish Water BW-2540 membranes arranged in a crossflow configuration, in a 2 stage, 2 pass configuration. The Reverse Osmosis is preferably conducted at an operating pressure of from about 6 bar to about 12 bar. Preferably, the Reverse Osmosis is conducted at an operating temperature of from about 5 degrees Celsius to about 40 degrees Celsius, preferably at an operating temperature of from about 15 degrees Celsius to about 25 degrees Celsius.

[0026] In a preferred embodiment of the invention, the leachate from the leaching step is subjected to a solid-liquid separation step, such as a filter press, wherein more solids are filtered out, prior to being fed to the DD process.

[0027] In a preferred embodiment of the invention, at least a position of the filtrate from the solidliquid separation step, such as a filter press, is subjected to an ultrafiltration step, wherein more solids are filtered out, prior to being fed to the nano-filtration step.

[0028] BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 - shows a schematic diagram of a preferred embodiment of the present invention.

[0030] Figure 2 - shows a schematic diagram of another preferred embodiment of the present invention, including additional steps of solid-liquid separation to enhance the recovery and concentration of coagulant and acid.

[0031] DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

[0032] The present invention focuses on a process for recovering chemicals, in particular, coagulants, from water treatment residuals (WTR) generated during water treatment processes, in particular for potable water or water used in the FMGC industry. Water treatment residuals form when suspended solids in the raw water react with chemicals, including coagulants, added in the water treatment processes.

[0033] The process also recovers the acid used in the recovery process for reuse in the process itself. By recycling these recovered chemicals back into the water treatment process, and the recovery process, the invention aims to reduce the disposal of precipitate sludges, resulting from the use of coagulants in the water treatment process, in evaporation ponds or landfills. Additionally, the present invention addresses challenges of operational costs associated water treatment processes and traditional chemical recovery methods, as well as supply chain bottlenecks related to importing raw materials needed for chemical manufacturing in certain countries.

[0034] The present invention relates to a process for the recovery and reuse of chemicals, such as coagulants, from water treatment residuals, comprising the following steps:

[0035] 1. Leaching: Utilizes specific acids, such as sulphuric or hydrochloric acid, at optimized concentrations and contact times to dissolve coagulants from sludges, preparing them for further separation processes.

[0036] 2. Donnan Dialysis (DD): Employs a cation-exchange membrane that selectively allows the passage of positively charged ions from the leachate (feed side) to an acid solution (sweep side) across the cation exchange membrane, driven by an electrochemical potential gradient. This selective permeability enables the efficient recovery of coagulant metal ions, such as Al3+, from the waste stream without contamination from organics and other particulates, reducing fouling and pore blocking issues associated with other membrane technologies.

[0037] 3. Nanofiltration (NF): Recovers and recycles the acid solution used in the DD step, using NF membranes. NF membranes achieve separation not only through size exclusion but also through charged surface interactions influenced by the membrane’s IsoElectric Point (I EP) and the pH of the solution. Variables such as membrane type, operating pressure, and temperature are selected to enhance recovery rates and reduce operational costs.

[0038] Additional steps may include Reverse Osmosis (RO), to further concentrate the process acid recovered from the nano-filtration step by removing water, ultrafiltration and a filter press for further separation and dewatering of the treatment residuals. The unrecovered materials from the various steps of the recovery process, i.e. the solids, are subjected to a neutralisation process prior to disposal or evaporation to minimise the potential harm. These steps ensure the recovery and thorough purification of the coagulants and process acid as well as a reduction in the environmental impact of the residual solids.

[0039] The present invention is described hereinbelow in detail with reference to the accompanying drawings. Figure 1 shows a schematic process diagram illustrating a preferred embodiment of the process of the present invention.

[0040] A stream of Water Treatment Residuals (WTR) (2) is obtained from the coagulation or flocculation step of a water treatment process. The WTR stream (2) typically contains coagulants, including ferric, ferrous, and aluminium-based coagulants, such as aluminium sulphate (alum). The WTR stream (2) is fed into a leaching step (20), wherein a leaching solution (3), such as sulphuric acid (H2SO4) or hydrochloric acid (HCI), is added to the leaching step (20) and used to leach the coagulants in the WTR to produce a leached WTR (leachate) stream (4) and a thickened sludge (14). The concentration of the leaching acid (H2SO4) is preferably kept below about 0.5 M, preferably from about 0.1 to about 0.5 M. In the case that the acid used is HCI, the concentration of the leaching acid is from about 0.1 to about 0.4 M. The thickened sludge (14) is sent to a neutralisation step (26) before being sent to disposal to landfill or evaporation, to minimize its hazardous impact. Due to the non-selective nature of the leaching process, the leachate stream (4) from the leaching step (20) comprises both the metal coagulant, in solution, and co-digested organics. The coagulant (Al3+) recovered across the leaching stage is at least about 60% (mass) and not more than about 90% (mass), more preferably at least 65% (mass) to not more than about 90% (mass), more preferably at least 65% (mass) to not more than about 70% (mass), even more preferably about 67% (mass). In the case of alum being used as a coagulant, the aluminium (Al3+) concentration in the leachate typically ranges from about 300 mg / L to about 600 mg / L. In the leaching step, particle size may also be selected to maximize efficiency.

[0041] The leachate stream (4) from the leaching step (20) is sent to a Donnan Dialysis step (22).

[0042] Donnan Dialysis (DD) is a relatively novel technology in the water treatment field. DD utilizes a specialized cation-exchange membrane which selectively permits the passage of positively charged ions while obstructing negatively charged and neutral species. This selective permeability is central to DD’s ability to recover target metal ions, such as aluminium (Al3+), from waste streams laden with organics and other contaminants without the resultant recovered stream being contaminated by these substances. Fundamentally, DD operates by having the leachate stream, laden with the coagulant metal in solution, flow adjacent to one side of the cation-exchange membrane, referred to as the feed side, while an acid solution flows on the opposite side, known as the sweep side. The process is driven by an electrochemical potential gradient across the cation-exchange membrane, prompting the target metal ions, such as Al3+, to migrate from the feed side to the sweep side, and H+ions to move from the sweep side to the feed side. The driving force of the movement of the target metal ions across the membrane is the electrochemical potential difference across the cation exchange membrane and not pressure difference, which often leads to pore fouling. As such, this method is effective in preventing the passage of colloids and organics through the membrane, offering a distinct advantage over other filtration methods such as ultrafiltration, microfiltration, and reverse osmosis.

[0043] The effectiveness of the Donnan Dialysis process is influenced by several factors, including the quality of the membrane used. Generally, there are four qualities considered in the selection of a membrane for use in the Donnan Dialysis process, namely: a) the diffusivity of the membrane. In general, the higher the diffusivity of the membrane, the larger the flux or rate of ions that can go through a membrane at a time (kinetics). b) ratio of diffusivities of target ions: undesired ions. The higher this ratio is, the more likely the membrane will reject unwanted ions and transfer target ions (function of selectivity). c) the strength of the membrane which is described generally as a function of its Nominal thickness. For larger flows rates and larger pressure drops across systems, thicker membranes are used, but this comes at a detriment of kinetics, thicker slower, thinner faster. d) the “backbone” functional group the membrane is made of. For example, Nation membranes use a per-sulfonated functional group. This functional group determines the membrane’s ability to handle variable operating parameters, such as pH and temperature.

[0044] The Nation membranes were found to meet the specifications of these parameters compared to other membranes considered. These membranes provide a robust range of operating conditions (i.e. pH and temperature) with little sacrifice to product quality. They also are able to provide a very high selection of acid and coagulants. Lastly, they have also been designed to specifically operate under low pressures, thus resulting in reduced energy consumption as well as operating costs. Other factors such as the acid type, acid concentration, as well as run time also influence the kinetics and thermodynamics of the Donnan Dialysis process. Therefore, it is important to ensure that these factors are carefully selected to ensure optimal recovery of the target coagulant metals ions from the leachate.

[0045] Referring again to Figure 1 , the stream of leachate (4) from the leaching step (20), or leached WTR, is fed on the feed side of the Donnan Dialysis step (22). Preferably, an acid solution of sulphuric acid (H2SO4) (5), with a concentration of from 1 to 2 M, is added on the sweep side of the membrane to absorb the target coagulant metal ions, such as Al3+, Fe2+or Fe3+, into solution. Where required due to the type of coagulant being recovered, hydrochloric acid may be used as the sweep acid solution. The target coagulant metal ions selectively migrate across the cationic exchange membranes due to the difference in electrochemical potential across the feed side and the sweep side of the membranes, thus selectively separating the targeted coagulant metal ions from the organics in the leachate feed. In this step, at least 95 % of the organics in the leached WTR stream are rejected, resulting in a solids stream, comprising the organics, and a filtrate stream, comprising mainly the acid and the coagulant ions now bound with the sulphate ions, with minimal contaminants. The solids or sludge (12) from the Donnan Dialysis step (22) is sent to the neutralisation step (26) before being sent to disposal to landfill or evaporation.

[0046] In the DD process, transfer across the cationic exchange membranes occurs as long as there exists an electrochemical potential between the feed and sweep sides of the DD. However, the rate and extent of transfer are influenced by both kinetics and thermodynamics. For instance, using a 0.1M concentration in the leaching step instead of 1M for purposes of increasing the electrochemical potential, may extend reaction times in the Donnan Dialysis step to up to 15 hours. Additionally, the mass of recovered target metal ions increases with the acid concentration in direct proportion. This increase, while enhancing recovery, entails higher costs due to increased acid concentration and capital expenses for construction materials capable of withstanding higher acid concentrations. Moreover, higher acid concentrations accentuate hydrodynamic effects during the process. Although more coagulant mass is recovered, the process may concurrently experience a decrease in concentration due to osmotic water transfer across the membrane, constituting a tradeoff. Therefore, it is preferable to maintain a ratio of Concentration of Acid used in the Donnan Dialysis step: Concentration of Acid used in the Leaching between 3:1 and 10:1 , preferably between 3.5:1 and 10:1 , within the concentration ranges provided above. The filtrate (6), laden with coagulant metal ions, such as Al3+, Fe2+or Fe3+now bound to sulphate, from the Donnan Dialysis step (22) is fed to a nano-filtration step (24) to separate the process acid (H2SO4) from the metal coagulant. The nanofiltration step is selective as it achieves separation through the combination of size exclusion and the membranes charged surface, which have the ability to attract or repel selected ions. The membrane charged surface is achieved as a result of the membrane’s Iso-electric Point (I EP) and the pH of the filtrate from the Donnan Dialysis step. Preferably, the nanofiltration is conducted at an operating temperature of from about 5 degrees Celsius to about 40 degrees Celsius, even more preferably at an operating temperature of from about 15 degrees Celsius to about 25 degrees Celsius. Higher temperatures provide poor rejection characteristics of membranes, less operation of acid and coagulants, whereas very cold temperatures cause more damage and fouling to membranes as feed flow becomes more viscous.

[0047] The metal coagulants (8) recovered from the nano-filtration step (24) are recycled back into the water treatment process for use in the coagulation and flocculation process. They can also be sent to storage until they are required for use in the water treatment process.

[0048] The acid (10) recovered from the nano-filtration step (24) is subjected to Reverse Osmosis step (28) for further concentration through the removal of water from the acid. Thereafter the concentrated acid (19) is recycled back to the recovery process for use in the leaching and / or Donnan Dialysis process. Fresh acid (not shown in the drawing) may be added to the recovered acid as required to meet the specifications of the leaching step and / or the Donnan Dialysis step.

[0049] The process of the present invention results in an overall coagulant selectivity rate of at least 80 %, more specifically at least 90% and even more specifically of at least 95%.

[0050] Figure 2 illustrates another preferred embodiment of the present invention.

[0051] As shown in Figure 2, a stream of Water Treatment Residuals (WTR) (2) as described above is obtained from the coagulation or flocculation step of a water treatment process. The WTR stream (2) is fed into a leaching step (20), wherein a leaching solution (3), such as sulphuric acid (H2SO4) or hydrochloric acid (HCI) is added to the leaching step (20) and used to leach the metal coagulants to produce a leached WTR (leachate) stream (4) and a thickened sludge. The concentration of the leaching should preferably be kept below about 0.5 M, preferably from about 0.1 to about 0.5 M, or in the case that HCI is used as the leaching solution, the concentration should be from about 0.1 to about 0.4 M. The thickened sludge (14) is sent to a neutralisation step before being sent to disposal to landfill or evaporation, in order to minimize the hazardous impact. The leachate stream (4) from the leaching step (20) is separated from the thickened sludge. The leachate comprises both the metal coagulant and co-digested organics, due to the non-selective nature of the leaching process. The aluminium (Al3+) recovered across the leaching stage is at least about 60% (mass) and not more than about 90% (mass), more preferably at least 65% (mass) to not more than about 90% (mass), more preferably at least 65% (mass) to not more than about 70% (mass), even more preferably about 67% (mass). The content of coagulant from the leaching stage typically ranges from about 300 mg / L to about 600 mg / L.

[0052] It may be desirable to subject the leached WTR stream (4) to a further solid separation step prior to or in the alternative to the Donnan Dialysis to remove and further reduce any solids that remain in the stream. As illustrated in Figure 2, such a step may be a filter press, wherein the leachate stream (4) from the leaching step (20) is fed to the filter press (30) and at least a portion of the organics and other solids present in the leachate stream are filtered out prior to feeding the leachate stream to the Donnan Dialysis step. Other forms of solid-liquid separation processes can be used for this purpose, such as screw press or centrifuge. The pressed sludge (15) from the filter press step (30) is sent to the neutralisation step (26) before disposal to landfill or evaporation, and the resulting filtered leachate stream (13) is sent to the Donnan Dialysis step (22).

[0053] In the Donnan Dialysis step (22), the targeted metal coagulant ions, such as Al3+, Fe2+or Fe3+, are selectively separated from the organics in the leached WTR feed, through cationic exchange membrane action and the difference in chemical potential across the feed side and the other side (sweep side) of the membrane. H2SO4 with a concentration of from 1 to 2 M, is added used on the sweep side of the cationic exchange membrane and, with the assistance of the cationic exchange membrane, selectively absorbs the metal coagulants into solution as explained above. The filtrate (6) from the Donnan Dialysis step (22) is fed to a nano-filtration step (24). The solids or sludge (12) from the Donnan Dialysis step is sent to the neutralisation step (26) before being sent to disposal to landfill or evaporation. It may be desired to feed a portion of the filtrate from the filter-press (30) or the leaching step (20) to an ultrafiltration step (32) and then directly to the nanofiltration step (24). The filtrate may first be subjected to an additional nanofiltration step prior to being fed to the nanofiltration step.

[0054] The filtrate (6) from the Donnan Dialysis step (22) as well as the filtrate (7) from the ultrafiltration step (32) are fed to a nano-filtration step (24) to separate the acid from the metal coagulant. The nanofiltration step (24) is selective as it achieves separation through the combination of size exclusion and the membranes charged surface, which have the ability to attract or repel selected ions. The recovered metal coagulant (8) is then returned to the water treatment process, for use in the coagulation process, or taken to storage to be used in the water treatment process when required.

[0055] The acid (10) recovered from the nano-filtration step is subjected to reverse osmosis (28) for the removal of water from the acid. Thereafter the concentrated acid (12) is recycled back into the recovery process for use in the leaching step and / or Donnan Dialysis step. Fresh acid (not shown in the drawing) may be added to the recovered acid as required to meet the specifications of the leaching step and / or the Donnan Dialysis step.

[0056] The process of the present invention results in an overall selectivity rate of the coagulant at least 80 %, more specifically at least 90% and even more specifically of at least 95%. Further, the process does not suffer from solvent entrainment issues, and utilizes a single process solution, generally sulphuric acid, which is also recovered and reused. This method is more cost-effective due to its lower operating temperatures and pressures, reducing the need for specialized construction materials and lowering both operational and capital costs. Additionally, the process offers higher process selectivity and reduced plant downtimes and maintenance requirements, setting a new standard in the field of coagulant recovery. This chemical recovery translates to approximately a 15% reduction in total volumes of potentially hazardous sludge that would be discarded to the environment.

[0057] The process of the present invention not only enables significant cost savings by reducing the amount of metal-loaded sludge requiring disposal but also optimizes the use of natural resources and protects the environment from potential contamination. By integrating these innovative recovery and recycling techniques, water treatment facilities can achieve higher efficiency and sustainability in their operations. EXAMPLE

[0058] The example provided herein illustrates a method of recovering coagulant, more specifically alum, from water treatment residuals (WTR) using the method of the present invention. The method employs a combination of leaching, Donnan Dialysis, ultrafiltration, nanofiltration, and reverse osmosis to recover coagulants effectively. The treatment steps are described in more detail below, including the specific process parameters applicable to each step and the results obtained summarised. a. Leaching

[0059] A sample of Water Treatment Residuals, comprising alum as a coagulant, with the properties set out below, was subject to leaching using sulphuric acid as the leaching solution to extract aluminium from the WTR. The process parameters were as follows:

[0060] • Initial mass of Aluminium: 1800 mg

[0061] • Initial concentration of Aluminium: 600 mg / l

[0062] • Leaching stage time: 45 minutes

[0063] • Leaching stage concentration: 0.1 M sulphuric acid

[0064] • Final concentration of Aluminium: 342 mg / l

[0065] • Final mass of Aluminium: 1200 mg

[0066] This stage successfully leached the aluminium in the WTR, together with some of the organics, thus reducing the aluminium concentration in the thickened sludge, and preparing the leached WTR for subsequent treatment steps. b. Donnan Dialysis

[0067] Donnan Dialysis was utilized to further treat the leachate, comprising aluminium in solution, using sulphuric acid as a sweeping solution. The process parameters in this step were as follows:

[0068] • Feed concentration: 342 mg / l

[0069] • Initial mass of Aluminium: 1200 mg

[0070] • Concentration factor: 3.5

[0071] • Feed to sweep side ratio: 0.33

[0072] • Acid concentration: 1 M sulphuric acid

[0073] • Stage run time: 18 hours • Final concentration: 1197 mg

[0074] • Mass of Aluminium: 1080 mg

[0075] Donnan Dialysis efficiently concentrated the aluminium, enhancing its recovery. The majority (i.e. about 90% of the aluminium in the leachate) was concentrated into the sweep solution and separated from the organics. c. Ultrafiltration

[0076] Ultrafiltration was applied to remove any remaining particulates and further concentrate the aluminium solution. The key parameters were:

[0077] • Cross flow rejection: 95%

[0078] • Pressure: 3 Bar d. Nanofiltration

[0079] The filtrate from the ultrafiltration was fed to a nanofiltration step aimed at the acid used in the recovery process from the aluminium solution. The parameters for this stage were:

[0080] • Recovery: 84%

[0081] • Rejection: 98%

[0082] • Pressure: 6 Bar

[0083] • Final concentration: 4400 mg / l

[0084] Nanofiltration significantly increased the acid concentration, preparing it for the final purification stage. The coagulant recovered from this stage was of a high purity and could readily be used in a water treatment process. e. Reverse Osmosis

[0085] Reverse Osmosis was the final step to achieve high purity and recovery of the acid by used pressure to separate out excess water from the acid. The process parameters were:

[0086] • Recovery: 98%

[0087] • Pressure: 8 Bar

[0088] Results

[0089] The effectiveness of the coagulant recovery process using the aforementioned parameters is summarized in Table A below: Table A: Results from Exemplified Process

[0090] The coagulant recovery process demonstrated high efficiency and effectiveness. The total concentration factor of 11 indicates a significant increase in aluminium concentration through the process. The selectivity of 85% and alum recovery rate of 80% underscore the process's capability to selectively recover aluminium while minimizing losses. Additionally, the high acid recovery percentage (90%) and the purity of recovered acid (98%) highlight the economic and environmental benefits of the process.

[0091] The method of recovering coagulant from WTR as described herein presents a viable and efficient approach for coagulant recovery. Each step of the process, from leaching to reverse osmosis, plays a crucial role in ensuring high recovery rates and purity of the recovered coagulant as well as the acid.

Claims

CLAIMS1. A process for recovering chemicals from water treatment residuals (WTR), comprising: a. receiving WTR from a water treatment process, wherein the WTR comprises at least one coagulant; b. subjecting at least a portion of the WTR to a leaching process wherein metal compounds in the WTR stream, including the at least one coagulant, are leached into solution using an acid solution to produce a leachate, comprising at least metal ions of the at least one coagulant, and a thickened sludge; c. feeding the leachate to a Donnan Dialysis process, wherein the leachate is fed on the feed side of the cationic exchange membrane and an acid solution on the sweep side, wherein the metal ions of the at least one coagulant present in the leachate, are recovered through their movement across the cationic exchange membrane and into the acid sweep solution, and the organics present in the leachate remain on the feed side and are separated into a solids or sludge byproduct; and d. feeding the sweep solution containing the coagulant metal ions, or the permeate, from the Donnan Dialysis step into a nano-filtration process, wherein the coagulant metal ions, now combined with sulphate ions, are separated from the acid sweep solution to produce a metal coagulant and a recovered acid; wherein the metal coagulant from the nano-filtration process is returned to the water treatment process for reuse in a process of coagulating suspended solid particles in the water.

2. The process of claim 1 , wherein the coagulant is selected from the group consisting of aluminium sulphate or alum, ferric sulphate, ferric chloride, ferrous chloride and sodium aluminate.

3. The process of claim 1 or 2, wherein the coagulant is aluminium sulphate or alum.

4. The process of any one of claims 1 to 3, wherein the target coagulant metals ions are selected the group consisting of aluminium, ferric, ferrous and sodium ions.

5. The process of any one of claims 1 to 4, wherein the acid solution used in the leaching process is selected from the group consisting of sulphuric acid and hydrochloric acid.

6. The process of claim 5, wherein the acid solution used in the leaching step is sulphuric acid.

7. The process of claim 5 or 6, wherein the concentration of the sulphuric acid is from about 0.1 to about 0.5 M.

8. The process of claim 5, wherein the acid solution used in the leaching step is hydrochloric acid.

9. The process of claim 5 or 8, wherein the concentration of the hydrochloric acid is from about 0.1 to about 0.4 M.

10. The process of any one of the preceding claims, wherein the residence time of the leaching step is from about 1 hour to about 2 hours.

11. The process of any one of the preceding claims, wherein the coagulant recovered across the leaching stage is at least about 60% (mass) and not more than about 90% (mass).

12. The process of any one of the preceding claims, wherein the coagulant recovered across the leaching stage is at least 65% (mass) and not more than about 90%(mass).

13. The process of any one of the preceding claims, wherein the coagulant recovered across the leaching stage is at least 65% (mass) and not more than about 70%(mass).

14. The process of any one of the preceding claims, wherein the coagulant recovered across the leaching stage is about 67% (mass).

15. The process of any one of claims 3 to 14, wherein the concentration of aluminium ions (AI3+) in the leachate is from about 300 mg / L to about 600 mg / L.

16. The process of any one of the preceding claims, wherein the solution used on the sweep side in the Donnan Dialysis process comprises sulphuric acid.

17. The process of claim 16, wherein the concentration of sulphuric acid used in the Donnan Dialysis step is from about 1 M to about 2 M.

18. The process of any one of claims 1 to 15, wherein the solution used on the sweep side in the Donnan Dialysis process comprises hydrochloric acid.

19. The process of claim 18, wherein the concentration of hydrochloric acid used in the Donnan Dialysis step is from about 0.8 M to about 1.7 M.

20. The process of any one of the preceding claims, wherein the Donnan Dialysis process uses a cation exchange membrane selected from the group consisting of Dupont’s Nation 115 and Nation 117.

21. The process of claim 20, wherein the cation exchange membranes are arranged in series.

22. The process of any one of the preceding claims, wherein the sweep solution from the Donnan Dialysis process is processed through nanofiltration to produce metal coagulant and process acid.

23. The process of claim 22, wherein the nanofiltration is conducted at an operating pressure of from about 4 bar to about 8 bar.

24. The process of claim 22 or 24, wherein the nanofiltration is conducted at an operating temperature of from about 5 degrees Celsius to about 40 degrees Celsius.

25. The process of claim 24, wherein the nanofiltration is conducted at an operating temperature of from about 15 degrees Celsius to about 40 degrees Celsius.

26. The process of claim 25, wherein the nanofiltration is conducted at an operating temperature of from about 15 degrees Celsius to about 25 degrees Celsius.

27. The process of any one of the preceding claims, wherein the metal coagulant recovered from the nanofiltration process is reused in a water treatment process for coagulation.

28. The process of any one of the preceding claims, wherein the acid recovered from the nanofiltration step is returned to the process for recovering chemicals.

29. The process of claim 28, wherein the acid recovered from the nanofiltration step is returned to the leaching step and to the Dannon Dialysis step of the process for recovering chemicals.

30. The process of claim 28 or 29, wherein the acid obtained from the nano-filtration step is fed to a Reverse Osmosis (RO) process wherein it is further concentrated by the removal of water, prior to being returned to the process for recovering chemicals.

31. The process of claim 30, wherein the Reverse Osmosis is conducted at an operating pressure of from about 6 bar to about 12 bar.

32. The process of claim 30 or 31, wherein the Reverse Osmosis is conducted at an operating temperature of from about 5 degrees Celsius to about 40 degrees Celsius.

33. The process of claim 32, wherein the Reverse Osmosis is conducted at an operating temperature of from about 15 degrees Celsius to about 40 degrees Celsius.

34. The process of claim 33, wherein the Reverse Osmosis is conducted at an operating temperature of from about 15 degrees Celsius to about 25 degrees Celsius.

35. The process of any one of the preceding claims, wherein the leachate from the leaching step is subjected to a solid-liquid separation step, wherein more solids are separated out of the leachate, prior to being fed to the DD process.

36. The process of any one of the preceding claims, wherein the solid-liquid separation step is as a filter press.

37. The process of any one of the preceding claims, wherein at least a portion of the filtrate from the filter press is subjected to an ultrafiltration step, wherein more solids are filtered out, prior to being fed to the nano-filtration step.

38. The process of any one of the preceding claims, wherein the solids or sludge from the leaching, Dannon Dialysis, filter press and / or ultrafiltration steps are sent to evaporation and / or disposal.

39. The process of claim 38, wherein the solids or sludge are sent to a neutralization step prior to evaporation and / or disposal.

40. The process of any one of the preceding claims, wherein the coagulant is recovered from a water treatment process used for treating potable water or water used in the FMCG industry.

41. A method of using the process defined in any of claims 1 to 38 in a water treatment process for the purpose of reducing environmental waste and cost associated with chemical coagulants in water treatment facilities.

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