Field of invention

The present invention relates to a treatment process for treating an aqueous effluent from a platinum group metal refining process using microalgae. The present invention also relates to a system for treating an aqueous effluent from a platinum group metal refining process using microalgae. The present invention also relates to the use of a population of microalgae for the treatment of an effluent from a platinum group metal refining process. The present invention provides for the treatment of effluent to provide a treated effluent stream and an algal biomass containing platinum group metals.

Background

Platinum group metals (PGMs) are commonly used in catalysts in a range of important industrial processes, for example in the pharmaceutical and agrochemical industries. These PGMs typically have a low abundance in the earth’s crust and are expensive to obtain. It is therefore highly desirable to recover PGMs from any spent PGM catalysts.

The PGM refining processes used for recovery of spent catalysts are usually based on chemistries that necessitate use of strong mineral and organic acids, along with some organic compounds. These PGM refining processes generate liquid effluents (waste streams) with extreme pH, high organic carbon and nitrogen content, and high levels of dissolved salts and chlorides. The discharge of such contaminated and untreated effluents to natural water bodies is considered a major environmental issue. It is therefore desirable to treat such effluents prior to discharge in order to meet environmental standards.

The amount of contaminants present in the effluent is typically quantified by measuring the chemical oxygen demand (COD) of the effluent. The COD indicates the amount of oxygen that can be consumed by reaction within the effluent, i.e. the amount of oxygen needed to chemically oxidize the contaminants in the effluent. It is often a requirement to mitigate COD levels in effluents. For example, pollution control boards can mandate that the COD of any effluents discharged from a site cannot exceed 250 mg/L.

Traditional treatment technologies for COD mitigation include the Fenton process, ozone-based oxidation, electrochemical oxidation and Advanced Oxidation Processes (AOP), which rely on oxidation through reactions with hydroxyl radicals. The Fenton process, ozone-based oxidation and electrochemical oxidation have a working capacity between 600-26, OOOmg/L and have been shown to reduce the COD in the range of 15- 90%. However, these techniques suffer from the drawback of an increase in the level of toxicity in the treated water. Hence, discharging the effluent to natural water bodies can still remain a challenge. Another method is electrocoagulation, wherein high levels of COD removal have been observed through use of Cu (anode) and Al (cathode). While the rate of COD reduction using electrocoagulation can be high, the cost of the equipment is also high meaning that it is not practical for all applications.

A study conducted by Azbar N.U.R.I et al entitled “Comparison of various advanced oxidation processes and chemical treatment methods for COD and color removal from a polyester and acetate fiber dyeing effluent” (Azbar, N. II. R. L, Yonar, T., & Kestioglu, K. (2004) Chemosphere, 55(1), 35-43), shows a comparison between various chemical COD reduction methods and Advanced Oxidation Process (AOP) on textile wastewater. The chemical methods reduced 50-60 % of the COD while it was observed using AOP resulted in up to 99% COD removal. However, similar to most methods, toxic treated water was generated from all these processes.

A further wastewater treatment technology for COD reduction is a multi-effect evaporator (MEE). However, this method often requires high energy and has a high cost meaning that it is not practical for all applications.

Biological methods have also been shown to provide COD reduction for certain effluents, but can result in biological matter which could be a hazard in its own right and does not serve any practical purpose.

PGM refining processes can also suffer from a further drawback, namely the extent to which the platinum group metals can be recovered from the spent PGM catalysts. Specifically, the processes do not typically yield complete recovery of the platinum group metals. The steadily increasing use of platinum group metals in various industrial processes has therefore resulted in generation of large quantities of waste that contain PGMs. This means that within the overall recycling process of the catalyst, a significant quantity of new PGMs needs to be introduced.

Conventional methods to recover PGM from waste streams include solvent extraction, chemical precipitation and ion exchange, but these suffer from significant disadvantages such as high capital costs and the use of stoichiometric amounts of hazardous reagents, leading to the generation of toxic sludge or other waste products that require disposal.

It is an object of the present invention to obviate or mitigate one or more disadvantages of the current systems, whether identified herein or otherwise.

Summary of the invention

In a first aspect of the present invention, there is provided a treatment process for treating an effluent from a platinum group metal refining process, the treatment process comprising supplying an aqueous effluent comprising platinum group metals to a medium comprising a population of microalgae; culturing the microalgae; withdrawing a portion of the medium containing the microalgae; and separating the withdrawn portion of the medium into an algal biomass and a treated effluent stream, wherein the treated effluent stream has a reduced chemical oxygen demand (COD) compared to the effluent from the PGM refining process.

It is desirable to develop a wastewater treatment process that reduces the chemical oxygen demand (COD) of a platinum group metal effluent using a cost effective and energy efficient process. It is also particularly desirable if the process combines a reduction in chemical oxygen demand with recovery of at least some of the PGMs present in the refining effluent.

The present invention provides a method that utilizes a population of microalgae to reduce the COD of an effluent stream from a platinum group metal refining process. The population of algae is able to both reduce the COD of the effluent and to absorb PGMs from the effluent stream, which allows the PGMs to subsequently be recovered. The treated effluent stream therefore preferably has a reduced concentration of PGMs compared to the effluent from the PGM refining process.

Chemical oxygen demand (COD) refers to the amount of oxygen required to chemically oxidize contaminants in a wastewater stream. Methods for determining COD are well known in the art and the present invention is not particularly limited by the method by which COD is determined, but can be measured in accordance with the method described in ASTM D1252 - 06(2020), Standard Test Methods for Chemical Oxygen Demand (Dichromate Oxygen Demand) of Water, ASTM International, West Conshohocken, PA, 2020. Similarly, it is possible to routinely determine the concentration of PGMs in a sample and it is therefore routinely possible to compare the concentration of PGMs in a sample before and after treatment.

The term “microalgae” is intended to refer to all microscopic algae organisms including blue-green algae (Cyanobacteria).

The process of the present invention provides a method for treating an effluent from a platinum group metal refining process to obtain an output stream, namely a treated effluent stream with a significantly reduced COD. For example, in embodiments the COD of the treated effluent stream is 250 mg/L or less, allowing it to meet the typical regulations for the discharge of an effluent. Without wishing to be bound by scientific theory, it is believed that this is achieved by converting the pollutants such as organic carbon, nitrogen, and other metals/ salts into algal biomass. The method also avoids the generation of a toxic treated water, as is the case for methods which rely on chemical treatment only. Thus, unlike most chemical methods for reduction of COD, the treated effluent provided by the present invention may be directly dischargeable to natural water bodies without any further treatment and does not require the use of hazardous chemicals. Further, the employment of microalgae over chemical methods allows for mitigation of COD without the need for extremely high capital and operational expenses, which are typically associated with chemical alternatives.

Whilst microorganisms have been used to mitigate COD in other industrial processes, such as bacteria used in sewage treatment, biological COD mitigation which relies on cultivating the algae on the effluent is unexpectedly suitable for a platinum group metal refining process where the effluent provides harsh conditions such as extreme pH and high levels of dissolved salts and chlorides. Such environments are generally inhospitable to microorganisms.

The method also provides a greener solution and a reduction in Greenhouse Gas (GHG) emissions compared to a thermal effluent mitigation process. For example, in embodiments, the microalgae are photoautotrophic organisms which capture photons to acquire energy. No external energy is therefore required and oxygen is actively generated during photosynthesis, thus reducing industrial air pollution. The algal biomass obtained contains platinum group metals (PGMs) extracted from the aqueous effluent from the PGM refining process. The algal biomass preferably includes PGMs at a higher concentration than found in the original effluent. The process may further comprise recovering at least some of the PGMs from the algal biomass.

In this embodiment, the process provides the combined effect of reducing the COD and the recovery of at least some of the PGMs which would otherwise be lost in the effluent. Thus, the algal biomass contains valuable metals that have been removed from the effluent stream.

Previously the recovery of platinum group metal refining process resulted in the formation of a toxic sludge or other waste products that required disposal and therefore had little value, if any. A process such as that of the present invention which both reduces the COD and recovers the metals at the same time is therefore highly unexpected, particularly one which allows the recovery of PGMs from a wastewater stream. Whilst heavy metals such as mercury, lead, and cadmium have been removed from wastewater streams using microorganisms, it is not automatically the case that such microorganisms would be able to tolerate and remediate effluent from a PGM refining process, but also to extract PGMs therefrom to have a dual effect of cleaning the effluent and allowing recovery of the valuable PGMs.

The recovery of the PGMs from the algal biomass may comprise supplying the algal biomass to a PGM refining process. The algal biomass may be incinerated prior to supplying the algal biomass to the PGM refining process. The algal biomass, or the incinerated algal biomass, may be combined with other PGM residues present in the PGM refining process. Metal-containing residue recovered from the algal biomass, such as via incineration of the recovered algal biomass, can be provided to a refining pool of metal-containing materials, which can subsequently be processed to extract the metals contained therein.

Integrating the recovery of the PGMs into the PGM refining process allows for the recovery of the biomass using pre-existing apparatus thereby increasing the efficiency of the process. The additional PGMs present in the algal biomass also improves the economics of the overall PGM refining process. In embodiments, the recovery of the PGMs from the algal biomass comprises incinerating the algal biomass followed by conducting a chemical refining process on the incinerated algal biomass, wherein the chemical refining process comprises a chemical reduction step followed by a purification step. The purification step may involve dissolution, filtration and/or precipitation reactions. The present invention is not particularly limited by the way in which metals are recovered from the algal biomass.

In embodiments, the medium is an aqueous medium. An aqueous medium is highly suited for the culturing of the microalgae. Microalgae are able to grow in water and the effluent from the PGM refining process is aqueous, so when the effluent is added to the medium containing the microalgae, it mixes with the medium containing the microalgae and replaces the water lost when a portion of the medium containing the microalgae is taken off for separation. The process may include providing a top-up water stream to maintain the amount of medium and/or the concentration of algae. The top-up water stream may be provided to dilute high concentrations of salts or other compounds and/or to control the pH of the medium. The top-up water stream may be another effluent stream that has lower salinity, lower COD, and/or different pH. By providing an effluent stream with lower salinity, lower COD, and/or different pH, it is possible to treat a wider range of effluents and can avoid the need for the use of fresh water. Of course, it will be appreciated that fresh water may be provided as the top-up stream.

The population of microalgae may comprise algae selected from green algae and bluegreen algae (cyanobacteria) or a mixture thereof.

The population of microalgae may comprise algae selected from the class of Trebouxiophyceae, Chlorophyceae, Cyanophyceae, and Bacillariophyceae, or a mixture of two or more thereof.

It has been surprisingly found that members of these classes of algae are effective at surviving in the extreme conditions produced by introducing the effluent from the PGM refining process into the medium. The have also been found to be tolerant to significant variability in effluent quality, such as changes in COD, salinity and pH, while still remaining effective at using the effluent as a feedstock. In addition, these classes of algae have been found to be able to tolerate variability in outdoor weather conditions within an open cultivation pond.

The population of microalgae may comprise green algae. The green algae may be of class Trebouxiophyceae. The green algae may be of order Chlorellales. The green algae may be of family Chlorellaceae. The green algae may be of genus Chlorella. The green algae may be of species Chlorella vulgaris. The Chlorella vulgaris may be of variety vulgaris Beijerinck.

Green algae of class Trebouxiophyceae, and particularly of the genus Chlorella, have been found to be particularly beneficial within the present invention. Specially, they have been found to be highly tolerant to the extreme conditions produced by introduction of effluent from the PGM refining process and are able to use the effluent as a feedstock. They have also been found to be able to take in PGMs from the effluent, thereby further treating the contaminated water and also allowing for the recovery of such metals from the algal biomass.

The population of microalgae may comprise multiple classes of algae. The population of microalgae may comprise multiple strains of algae within a single class. The population of microalgae may be a wild population. The population may be obtained from a naturally occurring water sample. The population may be obtained from a water body proximal to the site conducting the PGM refining process.

It has been found that the process can remain effective when using a mixed population of algae. It has also been found to be particularly beneficial to obtain a population of microalgae by harvesting a naturally occurring algae culture. Specifically, the use of such a wild population avoids environmental issues related to the introduction of new strains into natural water bodies.

The microalgae may be phototrophic. The phototrophic microalgae may use sunlight as the primary energy source. Phototrophic algae do not require external energy other than sunlight and accomplish oxygenation of ambient atmosphere during their cultivation. The process is therefore capable of mitigating COD in the effluent streams while also providing other environmental benefits, in particular reduced carbon emissions and the release of oxygen.

The microalgae may be salt-tolerant. This allows the microalgae to survive within the medium containing the effluent from the PGM process. The microalgae may be capable of being cultured in a medium that has a salinity between 0% and 10%, optionally between 0% and 5%. The microalgae may be tolerant to changes in pH. In embodiments, the microalgae may be cultured in a medium with a pH between 4 and 10. That is to say that the microalgae may be able to be cultivated in a medium having a pH of between 4 and 10.

The medium comprising the population of algae may be provided in a vessel. The vessel of the present invention may be any vessel suitable for cultivating algae, such as a bioreactor or a cultivation pond. In embodiments, the vessel is a cultivation pond. Cultivation ponds provide a cost effective method for cultivating algae on a large scale.

The medium may comprise other microorganisms. For example, the medium further comprise zooplanktons and/or diatoms.

In embodiments, the PGMs are selected from Platinum, Palladium, Rhodium, Iridium, Osmium and Ruthenium, or combinations thereof. In embodiments, the PGMs are selected from Platinum and Palladium, or a mixture thereof. Platinum and Palladium are commonly used in catalysts and are thus common metals to be refined within a platinum group metal refining process.

The aqueous effluent from the PGM refining process may comprises organic compounds, dissolved salts, and/or chlorides in addition to PGMs. These components, which are typically present in the effluent of a PGM refining process, may act as the feedstock for the microalgae.

The aqueous effluent from the PGM refining process may have a pH between 0 and 9. The amount of aqueous effluent added may also be adjusted dependant on its pH.

In embodiments, the medium may maintain a pH between 4 and 10.

In embodiments, the effluent from the platinum group metal refining process has a COD of from 500 to 10,000 mg/L. In embodiments, the treated effluent stream has a COD of 250 mg/L or less. A reduction of COD to 250mg/L or less may allow the treated effluent to be dischargeable to natural water bodies without further treatment.

In embodiments, separating the withdrawn portion of the medium comprises coagulation, flocculation, and/or sedimentation.

In embodiments, separating the withdrawn portion of the medium comprises centrifugation and/or microfiltration. Use of chemical free separations may lead to increased throughput and allow for automation of the separation process. In embodiments, the algal biomass is obtained in the form of a slurry.

In embodiments, the salinity of the medium is between 0.1% and 7% (w/v), optionally 1% to 5%, optionally 3% to 4%. The salinity medium may affect the ability of the microalgae to survive and to be cultured using the effluent as a feedstock. The process may further comprise supplying a low salinity or top-up stream to the vessel to control the salinity of the medium. This allows the salinity of the medium to be adjusted such that the microalgae can continue to be cultured within the medium. The salinity of the effluent from the PGM refining process may be between 3% and 20% (w/v), optionally 4% to 15%, i.e. greater than in the medium. The top-up stream may also have a lower COD or a different pH, such that the addition of the top-up stream to the medium lowers the COD or alters the pH of the medium.

In embodiments, the only feedstock used in culturing the microalgae is the aqueous effluent from the platinum group metal refining process. Advantageously, the nutritional requirements of algae cultivation can be met solely through utilization of organic carbon, nitrogen and other minerals from the refining effluent, leading to an efficient and balanced process.

In embodiments, the aqueous effluent supplied to the medium forms between 1% and 25% of the volume of the medium, preferably between 5% and 10% of the volume of the medium. Controlling the input of the effluent can avoid large fluctuations in the environment used to culture the algae while still providing a sufficient feedstock for the population of microalgae.

In embodiments, withdrawing a portion of the medium involves withdrawing between 5% and 40% (by volume) of the medium present in the vessel. This ensures that large proportion of the culture remains in the vessel to maintain a continuing cycle of treatment.

In embodiments, the process further comprises the step of withdrawing a sample of the medium, preferably prior to the step of withdrawing a portion of the medium, and:

(i) measuring the pH of the sample;

(ii) measuring the salinity of the sample;

(iii) observing the sample under a microscope; and/or (iv) measuring the optical density of the sample, preferably wherein the measurement is at 750nm.

Withdrawing a sample and conducting analysis on said sample allows for the adjustment of the parameters of the process. For example, the amount of medium withdrawn in the portion may be determined based on the measured optical density of the sample.

In embodiments, the process further comprises supplying the algal biomass to a system for the generation of phytochemicals and/or biofuels. This may add further value to the biomass generated and increase the efficiency of the overall process.

In a second aspect of the invention, there is provided a system for treating an aqueous effluent comprising:

(A) a vessel comprising: i. a medium containing a population of microalgae; ii. an inlet for receipt of an effluent from a platinum group metal (PGM) refining process; and iii. an outlet for withdrawal of a portion of the medium; and

(B) a separation unit configured to separate the withdrawn portion of the medium to obtain an algal biomass and a treated effluent stream; wherein the population of microalgae is selected to reduce the chemical oxygen demand (COD) of the medium such that the treated effluent stream has a reduced chemical oxygen demand compared to the effluent.

The system allows for the advantageous process of the first aspect of the invention to be conducted. The algae may be any of the algae described in respect of the first aspect of the present invention.

In embodiments of the second aspect of the invention, the vessel is a cultivation pond. In embodiments, the system further comprises a platinum metal recovery unit configured to recover platinum group metals (PGMs) from the algal biomass. In embodiments, the vessel further comprises an inlet for receipt of a low salinity or top up stream. The advantageous of these features are described above in relation to the first aspect of the invention. In embodiments, the system further comprises a monitoring device to measure the COD of the treated effluent. This allows the treated effluent to be checked to confirm it meets environmental standards prior to discharge to a natural water body.

In embodiments, the system further comprises a mechanism for enabling movement of the medium within the vessel. This ensure a good mixing of the effluent within the medium ensuring that the microalgae are supplied with a feedstock. The mechanism for enabling movement of the medium within the vessel may be a paddlewheel.

In a third aspect of the invention there is provided the use of a population of microalgae for treatment of an effluent from a platinum group metal (PGM) process.

In embodiments of the third aspect of the invention, the population of microalgae comprises green algae. The green algae may be selected from the class of Trebouxiophyceae, Chlorophyceae, Cyanophyceae, and Bacillariophyceae, or a mixture of two or more thereof. The population of microalgae comprises algae may be selected from the order of Chlorellales, optionally selected from the family Chlorellaceae, optionally selected from the genus Chlorella, optionally selected from the species Chlorella vulgaris, optionally of the variety vulgaris Beijerinck.

It will be appreciated, that where applicable, any embodiments of the invention described in relation to the first aspect of the invention also apply to the second and third aspects of the invention. All combinations of features of the first, second and third aspects of the present invention are expressly considered and disclosed.

Brief Description of Figures

Aspects of the invention will now be described, by way of example only, with reference to the accompanying figures, in which:

Figure 1 is a schematic depiction of a system for treating an aqueous effluent according to the present invention;

Figure 2 is a graph showing the COD reduction performance achieved with a 32 m ² raceway pond using the process of the present invention; Figure 3 is a graph showing the COD reduction performance achieved with a 55 m ² raceway pond using the process of the present invention;

Figure 4 is a graph showing the algae growth performance achieved with a 32 m ² raceway pond using the process of the present invention; and

Figure 5 is a graph showing the algae growth performance achieved with a 55 m ² raceway pond using the process of the present invention.

The features and advantages of the present invention will become apparent from the detailed description set forth below when taken in conjunction with the drawings and graphs.

Detailed Description

Figure 1 depicts a system 1 for treating an effluent according to the present invention. The system 1 includes a vessel 3 that contains a medium 4 containing a population of microalgae. The system 1 includes an inlet 2 for receipt of an effluent from a platinum group metal refining process. The system 1 includes an optional mixing apparatus 5, which may be a paddle wheel. The system 1 also includes an optional inlet 6 which is configured to provide top-up water to the vessel 3. The top-up water may be fresh water, but is preferably another stream of effluent with a lower salinity, lower COD, and/or different pH as compared to the effluent stream provided via inlet 2. The system also includes an outlet 7 for withdrawal of a portion of the medium 4. The outlet ? is connected to a separation unit 8 which is configured to separate the withdrawn portion of the medium 4 into an algal biomass 9 and a treated effluent stream 10.

In use, an aqueous effluent from a platinum group metal refining process is provided to a vessel 3 which contains a medium, preferably water, containing a population of microalgae 4. The vessel 3 may be any vessel capable of containing water, and in some embodiments is a pond. The aqueous effluent supplied to the vessel 3 is mixed with the medium contained therein 4 via mixing apparatus 5. The population of microalgae present in the vessel 4 is able to grow and reduce the COD of the effluent. The population of microalgae are also able to absorb platinum group metals within the effluent. A portion of the medium containing the microalgae 4 is taken off via outlet 7 and provided to a separator 8. The separator separates algal biomass from water to provide an algal biomass 9 and a treated effluent stream 10. Since the algae have been able to absorb PGMs within the effluent, the PGMs are relatively concentrated in the algal biomass, which makes recovery of such PGMs possible. In addition, the treated effluent has a reduced COD as well as reduced concentration of PGMs. Where water is lost via evaporation, removal for separation, or where the salinity, pH, or COD of the medium 4 within the vessel 3 needs to be adjusted, a top up stream can be provided via inlet 6. The top up stream can comprise fresh water or effluent with a lower COD, salinity, or different pH than the effluent stream provided via inlet 2.

In order to demonstrate the effectiveness of the present invention, results from conducting the disclosed process in both a 32m ² and 55m ² raceway pond are described below. Both ponds were made from fibre-reinforced plastic and fitted with a paddlewheel to enable water movement along the raceway pond track.

The initial population of microalgae (i.e. an algae culture) was obtained from a naturally occurring algae-containing water sample in the dike of a water-softening unit close to a PGM refining plant. The presence and identify of the algae was confirmed via morphological identification using an optical microscope.

Effluent from a PGM refining process was added to each pond daily. The composition of the effluent changed day-to-day depending on the operation of the PGM refining plant. The COD of this incoming effluent was measured in accordance with ASTM D1252 - 06(2020).

A portion of the contents of the pond (the medium) was also withdrawn daily. The withdrawn stream, also known as a harvested stream, was then subjected to coagulation, flocculation, and sedimentation processes, through the use of ferric chloride [Merck, CAS Number - 7705-08-0], polymeric flocculants [Chemexcel Industrial Chemicals, HSN Number 38249022], and gravity settling in open tanks or drums, respectively. The settled algal biomass slurry was recycled for PGM recovery, while the clear supernatant was sampled for COD content prior to discharge.

In both the 32 m ² and the 55 m ² pond experiments, representative samples of the pond contents (medium) were taken twice a day and optical analysis conducted to monitor the state of the algae. Throughout the experiments, green algae was always observed in both ponds. Cyanobateria of class Cyanophyceae were sometimes observed in both ponds, but was not always present, and Bacillariophyceae (diatoms) were also sometimes observed in both ponds but not always present.

Representative samples of the algae in the ponds were taken and taxonomic identification was conducted to more accurately characterise the microalgae present.

All four samples contained identical material of green algal genus namely Chlorella. The particular variant was identified as vulgaris Beijerinck.

Throughout the experiments, a range of other parameters were also measured including the pond depth, pond volume, optical density (turbidity) at 750nm, salinity of the pond and the salinity of the effluent. These parameters were considered when determining the amount of the medium to harvest from the pond each day.

The results of the 32m ² and 55m ² experiments are summarised in Table 1 and Table 2 respectively. Table 1 - Summary of the effluent treatment performance using a 32 m ² algae pond Table 2 - Summary of the effluent treatment performance using a 55m ² algae pond

Table 1 and Table 2 show that process of the present invention is capable of treating large amounts of effluent with an average COD of around 2,500 - 3,000 mg/L while producing a treated effluent with an average COD of around 150 mg/L. The method is effective despite the average salinity of the effluent being 10% or greater and the pH of the pond fluctuating between 5 and 9.

This data is further shown in the figures provided. Figures 2 and 3 show the COD reduction performance of the 32 m ² and 55 m ² ponds, respectively. The solid vertical bars show the COD removal rate for each day in the measured period. The dashed line shows the cumulative COD removal. In the graph, the removal rate can be seen to fluctuate day to day. This is not surprising given the outdoor conditions of the pond and the potential for the composition of the incoming effluent to change, however, the cumulative COD removal generally increases steadily over the period. Figures 4 and 5 show the amount of algae produced by the 32 m ² and 55 m ² pond experiments, respectively. The solid vertical bars show the amount of algae produced for each day in the measured period. The dashed line shows the cumulative algae produced. Comparing Figures 2 and 4, or indeed Figures 3 and 5, it is clear that the removal of COD is highly correlated to the production of the algae.

In addition to the above results, further treatment of the algae produced in the 32 m ² pond, through recycling into the PGM refining process has led to the recovery of 300 g of PG Ms from the process.

Experimental Methods

The pH of the pond was measured using a Hanna instruments model HI98107 handheld pH meter.

The salinity of the pond and the salinity of the effluent can be measured using a salinometer, e.g. a Divinext Model Number: ATC, 0-100% Salinity. The measurement of the salinity can be used to determine if an additional low salinity feed needs to be introduced.

The optical density can be measured using a spectrophotometer e.g. a Thermo Fisher, Model Number: 840210600. The measurement of the optical density can provide an indication of the amount of algae present and can be used to determine the amount of medium to withdraw.

In summary, the present invention provides a way by which effluent from a PGM refining process can be remediated to provide an algal biomass containing valuable PGMs and a treated effluent stream with a reduced COD. Surprisingly, the use of a population of algae can both survive the harsh conditions provided by the effluent and achieve the dual function of reducing COD of the effluent and absorbing PGMs from the effluent. The present invention provides for an improved way of treating such effluents.