Chromium (Cr) is a chemical element that is used in the production of a broad range of goods and materials, including refined metals and alloys, leathers, paints, pigments, wood preservatives, electronics, and chemicals. Because of its anti-corrosive properties, it’s also the main additive in stainless steel. Wastewater resulting from these and other industrial activities can contain excess levels of chromium that must be carefully monitored and managed. In this article, we’ll take a look at which technologies are the best at removing chromium from industrial water and wastewater and explore the key benefits and drawbacks of each as they relate to various industries and processes.
(Note: There are many compounds and oxidation states of chromium that occur in nature and are created in the industry, however trivalent chromium (Cr(III)) and hexavalent chromium (Cr(VI)) are the only oxidation states that are water-soluble (even if slightly). For this reason, they are the forms of chromium most industrial facilities will deal with and which ones this article focuses on.)
Removing chromium with chemical precipitation
Chemical precipitation is a method of separation where chemical precipitants are added to a liquid stream in order to drive contaminant ions out of the solution so they can then be removed through some means of physical separation. The precipitants used for chromium separation include relatively affordable chemicals, such as calcium hydroxide, sodium hydroxide, magnesium oxide, or calcium magnesium carbonate—making chemical precipitation a generally economical, simple, and popular treatment option for chromium removal.
While it is true that chemical precipitation is very effective for removing trivalent chromium (Cr(III)), the process is a bit more complex for streams with hexavalent chromium (Cr(VI)), which requires an additional reduction step. This generally consists of adding a reduction agent of some kind (typically sodium bisulfate, sodium metabisulfite, iron sulfate, or gaseous sulfur dioxide) which acts on Cr(VI) to reduce it to Cr(III).
Additionally, facilities need to consider a number of other factors such as stream pH, the type and quantity of precipitating agent needed, and whether batch or continuous flow is needed and assess how these might affect system performance. While pH is almost always an important consideration for chemical precipitation systems, it is of particular importance where reduction of Cr(VI) is needed. This is because the chromium reduction reaction is sensitive to pH, and will proceed more quickly under acidic conditions. Unfortunately, the converse is also true, so for streams with a pH over 5, the reduction reaction is too slow to be practical for most industrial settings. Thus, chemical precipitation may not be a good fit for facilities with slightly acidic, neutral, or alkaline wastewater streams.
Removing chromium with ion-exchange
Ion exchange (IX) is a physical-chemical process that selectively removes contaminants from a solution by effectively swapping out ions of similar electrical charges. IX offers advantages such as cost-effectiveness, convenience, and reversibility, and can be an excellent choice for facilities looking to recover and reuse chromium from its waste streams.
The work of an IX unit is carried out by an IX resin substrate that must be carefully selected depending upon process conditions and stream characteristics. Removal of trivalent chromium can be achieved with the use of strong acid cation (SAC) resins, weak acid cation (WAC) resins, or chelating resins. Hexavalent chromium removal, on the other hand, can be achieved with strong base anion (SBA) resins. In either case, facilities need to plan for regular maintenance and resin regeneration cycles to ensure consistent and adequate chromium removal. Additionally, facilities may also need to consider a brine waste minimization strategy to limit the use of regenerant chemicals, liquid waste volumes, and to otherwise optimize system performance from an operational, economic, and environmental standpoint.
Removing chromium with coagulation and flocculation
Coagulation and flocculation are wastewater treatment technologies used for removing contaminants from colloidal streams or streams with fine particulates. The process consists of adding chemical coagulants to the stream along with gentle mixing to encourage agglomeration (or “flocculation”) of fine particles into large particles that then settle out of the liquid stream. Coagulation and flocculation can be used for a variety of separation needs, including the removal of organic materials, color, odor, suspended particles, and heavy metals, including chromium. Typically, coagulation and flocculation are used as a primary wastewater treatment step to reduce the overall load of chromium or other heavy metals, which can inhibit the performance of biological treatment such as that which is often used in municipal wastewater treatment plants.
Facilities looking to implement coagulation and flocculation as a means of wastewater treatment have some flexibility when it comes to system design, including the ability to choose conventional two-step plants that utilize separate units for the coagulation and flocculation phases, or combined system designs. Other factors to consider include the type of coagulant chemicals used (typically ferric chloride or ferric sulfate for chromium removal), as well as unit size and capacity, particle size and concentration, and contact time.
Removing chromium with adsorption
Adsorption is a treatment technology that harnesses molecular forces of attraction to capture contaminants from a liquid stream. The adsorption process consists of passing a liquid stream through some type of porous adsorbent media. Since the soluble contaminants are more attracted to the adsorbent media than they are to the water in the stream, the contaminants bind to the surface of the media while the liquid effluent flows through. Adsorption is effective for the separation and removal of chromium and other heavy metals in streams with relatively low concentrations of heavy metal contaminants. Chromium separation applications can use many different kinds of adsorbent media, including activated carbon, clay, zeolite, peat moss, and even agricultural wastes, such as walnut shells or rice husks.
The main benefits of adsorption for chromium removal and separation applications are its low operational costs, relative technical simplicity, and wide availability of adsorbent media. Adsorption does have its disadvantages, though, with the most significant being limited capacity and limited regeneration potential. As such, higher contaminant concentrations generally will mean higher costs due to more frequent media replacement. Adsorption is, therefore, best used for streams with relatively low concentrations of chromium.
Removing chromium with electrochemical reduction
The electrochemical reduction is an increasingly common treatment approach for streams that contain Cr(VI). This process applies an electrical current to metal electrodes, causing them to dissolve and release ions into the solution. The ions then oxidize and, in the process, reduce Cr(VI) to a more easily dissoluble Cr(III). The electrochemical reduction may be followed by a separate precipitation reaction, or precipitation may occur concurrently, depending upon system design. The reduction process is usually followed by a clarification step.
There are numerous benefits to electrochemical reduction, including the ability for the reaction to proceed at a neutral pH, as well as minimal use of chemicals, which can be beneficial from both a cost and an environmental perspective. Potential drawbacks can include compromised performance due to poor reactor design, inadequate reaction time, and mixing speed. Additionally, both the metal electrodes’ surface area and material choice have a significant impact on the system’s performance, with iron and aluminum generally showing the greatest effectiveness for chromium removal.
Removing chromium with membrane filtration
Membrane filtration is a physical separation process where a liquid stream is passed through a semi-permeable filtration membrane that separates materials based on the principle of size exclusion. The membrane has specially sized pores that retain targeted contaminants while allowing the liquid stream and smaller particles to flow through. The types of membrane filtration commonly used for chromium separation include ultrafiltration (UF), nanofiltration (NF), reverse osmosis (RO), and electrodialysis. In some cases, multiple types of membrane filtration are used in sequence to maximize the performance and longevity of the membranes, while also achieving adequate removal of chromium. Membrane filtration is also a good choice for facilities looking to reclaim chromium, as both NF and RO can produce a chromium-rich retentate that can be reused in process streams.
Despite these benefits, facilities should consider the potential shortcomings of membrane filtration as well. These can include high operational costs for cleaning, maintenance, and membrane replacement, as well as potentially high energy consumption and flow rate limitations.
Which chromium-removing technologies should your facility use?
There are a number of wastewater treatment technologies that are effective for chromium removal, each of which has its own merits. In short, there’s no single best strategy for chromium separation and removal. Rather, the best chromium treatment system is one that has been carefully matched to the specific needs and priorities of a facility.
Perhaps the biggest factor that influences the selection of wastewater treatment systems for removing chromium is the type of chromium present. This is because chromium exists in different oxidation states in water, each having its own properties and recommended removal strategies.
For example, Cr(III) is naturally occurring and may also be present in process and wastewater streams stemming from a variety of industrial production activities. It is only slightly water-soluble, and it can be effectively separated through adsorption, ion exchange (IX), and membrane filtration. Cr(VI), on the other hand, is most known for its presence in wastewater streams generated by the leather tanning industry, but it poses a problem for other industries as well. Usually present as chromate or dichromate, Cr(VI) is highly toxic and binds easily to suspended particles. Because of these properties, most removal strategies utilize an additional reduction step where the Cr(VI) is first reduced to Cr(III), then one or more separation steps are implemented for removal and/or recovery. Common removal technologies for Cr(VI) include chemical precipitation, coagulation, chemical reduction, electrochemical reduction, and biological treatment.
As with any wastewater treatment system, planning should also entail the careful review of process and stream characteristics that can determine the viability of various water treatment technologies for a specific facility. Details such as which constituents are present in the waste stream and at what concentrations, as well as pH, temperature, and flow rate can all narrow down the field of appropriate solutions. Additionally, choosing the best water treatment technologies means that you’ll need to consider the relative importance of various priorities, like material reclamation and reuse, regulatory compliance, available space, operational skill requirements, capital investment, and long-term operations costs.
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