How to Remove Mercury From Your Industrial Water and Wastewater

Mercury (Hg) is a well-known heavy metal that both occurs naturally and can be introduced to air and water through human activity. With its high toxicity, mercury has been the subject of stringent restrictions surrounding its use and disposal since the 1960s. Industrial facilities that deal with mercury, such as those within the chlor-alkali and electronics industries, among others, typically must adopt water treatment technologies to reduce mercury levels in their liquid streams and in their emissions.

In this article, we’ll focus on strategies for dealing with mercury contamination in process and wastewater streams. Specifically, we’ll explore common technologies used for removing mercury from industrial water and wastewater, and take a look at some of the advantages and disadvantages of each.  

What are the best water treatment technologies for removing mercury?

The most common processes used for treatment of mercury-contaminated water are precipitation, adsorption, membrane filtration, and biological treatment. Each of these water treatment technologies offers certain advantages that depend heavily upon the process conditions of a given installation.

To achieve the best possible performance, a facility will need to carefully consider which constituents are present in addition to mercury, as well as factors like stream pH, temperature, flow rate, volume, and BOD. Additionally, a facility will need to consider the target mercury concentration, as not all treatment technologies are capable of reducing mercury levels down to meet water quality goals. It is also worth noting that wastewater discharge standards are subject to change, and planning for future compliance is important when investing in water treatment equipment.  

Chemical precipitation

Chemical precipitation is easily the most popular water treatment process for removal of mercury from both groundwater and wastewater, as it is both economical and relatively simple to operate. To remove mercury by means of chemical precipitation, a facility will first add a chemical precipitant to the stream. The precipitant reacts with dissolved constituents to facilitate mercury removal in one of two ways: either by forming insoluble elemental mercury or mercury compounds; or, by forming particulate solids that then adsorb dissolved mercury present in the stream. Following the precipitation reaction, the facility will then deploy some means of physical separation, like clarification or filtration, to remove insoluble solids from the liquid stream. Chemicals used for mercury precipitation typically include sulfides, ferric salts, ferric sulfates, and calcium hydroxide, although there are other less common precipitants in use as well, such as lignin, for example. For optimal function and cost-effectiveness, chemical precipitants must be carefully selected based on process needs, including consideration of which constituents are present in the stream, process conditions, pH, downstream equipment specifications, and other factors.

For all of its advantages, chemical precipitation does have a few drawbacks, the most significant being that chemical precipitation is typically inadequate as a standalone means for reducing mercury concentrations below stringent process or discharge limits. For this reason, chemical precipitation is often used as part of a larger treatment train, where it typically functions as a primary treatment step ahead of other water treatment technologies such as fine filtration. Another potential drawback is that chemical precipitation produces a semi-solid waste byproduct known as sludge, which is typically subject to stringent disposal requirements, especially if it is contaminated with mercury or otherwise considered hazardous waste. Often, a facility will need to invest in sludge dewatering equipment and additional treatment technologies to comply with solid waste disposal regulations, and minimize discharge costs.  

Adsorption

Adsorption is another water treatment technology that is commonly deployed for mercury removal applications. The main advantages of adsorption include no sludge generation, good selectivity for mercury and/or other heavy metals, and flexibility in choice of adsorption media materials. Like precipitation, adsorption is capable of reducing concentrations of mercury below 2 µg/L.

Adsorption is a process where a substance (adsorbate) accumulates on the surface of a solid (adsorbent) based on forces of molecular attraction. In water treatment applications, the process typically consists of passing a liquid stream through a bed of adsorbent media. As the stream flows through, dissolved contaminant molecules attach to the adsorbent media and are thereby separated out from the liquid stream. Over the course of use, the adsorbent media will become saturated and its ability to remove targeted contaminants will decline. Some adsorbent media can be restored for additional use cycles through application of heat or other means to desorb previously captured contaminants. Eventually, however, spent adsorbent material will need to be replaced.

Replacement of adsorbent media is, in fact, the most significant expense of operating an adsorption system. For this reason, it’s crucial to ensure that the system is designed to fit the needs of a facility, including consideration of the type and form of adsorption media used. Activated carbon remains the most common adsorbent material for mercury removal purposes, although there are other materials available for specialized applications. Mercury-selective ion exchange resins are also very effective as polishers and as primary removal technologies for liquid wastes that require ultralow mercury removal or for treatment of brackish streams. To maximize cost-effectiveness, the adsorbent media should offer as much capacity for contaminant capture as possible, while incorporating a design that resists clogging.  When properly designed, however, adsorption offers low operation costs compared to other mercury removal processes. To maximize this benefit, adsorption is ideal for streams with moderate to low concentrations of mercury, and is best used for secondary treatment following some type of pre-treatment.  

Membrane separation

Membrane filtration is a type of physical separation that works based on the principle of size exclusion. The process consists of passing a liquid stream through a semi-permeable membrane that contains precisely-sized pores that block larger particles and molecules while allowing smaller ones to pass through. As such, membrane filtration is classed based on pore size, including, in order from largest to smallest: microfiltration (MF), ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO).  Of these, RO/NF have pores small enough to retain divalent cations like mercury, but their use is typically reserved for secondary or tertiary treatment since their small pore sizes make them susceptible to clogging and fouling. UF is also used for mercury removal applications, however, since UF pores are too large to capture dissolved mercury ions, it is typically used following precipitation, which results in larger molecules or suspended particles.

The main advantage of membrane separation is its high rate of removal for mercury and other contaminants. This benefit makes it a good fit for facilities who wish to treat wastewater or process water for reuse, or for those needing to comply with stringent discharge limits on mercury or other heavy metals. Conversely, drawbacks to membrane separation can include costs associated with membrane cleaning, maintenance, and replacement, energy costs, and flow rate limitations.

Biological treatment

Biological treatment systems leverage living microorganisms to break down and remove organic contaminants. While mercury itself is inorganic, biological treatment can be used to convert hazardous soluble mercury species to less soluble forms that can be removed or retained more easily. The key benefit of biological treatment is its cost-effectiveness, particularly for streams with higher concentrations of mercury.

The main drawbacks of biological treatment stem from the fact that process conditions must be managed carefully to avoid compromising the living biomass. This can mean addition of chemicals to manage pH, addition of organics to maintain adequate nutrient levels, maintenance of an aeration system, and use of heating/cooling elements to manage temperature. Additionally, excess mercury levels can be toxic to microorganisms, and must therefore be limited to protect the biomass. All of these can contribute to the overall cost to operate and maintain the biological system.

Additionally, biological treatment alone is somewhat less capable than other technologies of reducing mercury concentration below stringent limits. For this reason, biological treatment is usually followed by other treatment technologies such as adsorption or precipitation. Lastly, when using biological treatment systems, facilities also need to plan for treatment and disposal of semi-solid or solid waste products, including mercury-contaminated sludge.

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