Sulfate Removal Technologies: A Review

Sulfate concentrations in water have come under increasing scrutiny from regulatory authorities over the past two decades.  In contrast to contaminants such as nitrate, arsenic, and heavy metals, sulfate has no primary standard for drinking water or aquatic life.  However, the secondary standard for drinking water in the U.S. is 250 mg/L and concentrations above 600 mg/L may create laxative effects.  In Minnesota, future sulfate discharges may be limited to as low as 10 mg/L (an unenforced standard that is currently under review) to protect wild rice habitat.  Guidelines for sulfate levels around the world are shown in Table 1.

TABLE 1.  RECOMMENDED MAXIMUM SULFATE LEVELS

From:    Ramachandran, 2012

Many treatment technologies have been developed and refined to remove sulfate from water, including chemical, biological, and physical processes.

Chemical Treatment Technologies

Chemical methods for reducing sulfate concentrations include:

1. Lime precipitation
2. Barium precipitation
3. The CESR process
4. The SAVMIN™ process

The simplest technology for reducing high sulfate concentrations is lime precipitation.  Adding calcium as pebble lime, hydrated lime, or limestone can precipitate calcium sulfate (gypsum) and reduce sulfate concentrations to the solubility limit of 1,500-2,000 mg/L.  Concentrations already below this level will generally be unaffected by lime addition.  Typical equipment requirements for this process are a lime silo, lime slaker, or other reagent feed system, reaction tank, and clarifier.  If sulfate must be further reduced (“polished”), an add-on process such as barium, CESR, or SAVMIN is recommended.

As a polishing step for sulfate removal, barium salts can be added to precipitate barium sulfate, which has a very low solubility in water, with the final sulfate concentration limited only by the amount of barium added and reaction time.  Typical salts used are barium chloride and barium carbonate.  The disadvantage of barium addition is the relatively high chemical cost; a recent price for barium chloride was about $2/lb.

The Cost-Effective Sulfate Removal (CESR) process was originally developed as the Walhalla process in Europe in the 1990s.  A specialized powdered cement (reagent) is added to precipitate ettringite, which is a hydrated calcium aluminum sulfate compound.  The CESR process requires lime addition and a pH of about 11.3 for ettringite formation, and can achieve sulfate concentrations far below the gypsum solubility limit (Reinsel, 2001).  Sulfate concentrations are typically limited only by the amount of reagent added and reaction time.  Disadvantages are the large amount of sludge generated, and the fact that high sodium concentrations inhibit the process.  The CESR reagent costs about $0.40/lb.

The SAVMIN process was developed by MINTEK in South Africa in the 1990s to treat acid mine drainage.  Ettringite is precipitated as in the CESR process, in this case by recycling aluminum hydroxide.  Sulfate levels can be reduced to less than 200 mg/L by this process.  MINTEK has signed an agreement with Veolia South Africa to further develop the SAVMIN process (Ramachandran, 2012).  The first pilot evaluation of the improved SAVMIN process using Veolia’s MULTIFLO™ clarifier was recently undertaken.

Biological Treatment Technologies

If metals are present in the water to be treated, biological treatment has the advantage of being able to remove them along with sulfate via metal sulfide precipitation.  Biological processes for removing sulfate include:

1. The THIOPAQ™ process
2. Other packed bed or fluidized bed reactors
3. Passive treatment
4. In situ treatment

In the THIOPAQ process developed by the PAQUES company (Netherlands), sulfide is produced by contacting the sulfate-containing stream with sulfate-reducing bacteria (SRB) in the presence of a carbon source (electron donor) such as hydrogen gas or acetic acid.  The reaction for hydrogen is:

                SO₄^2- + 4 H₂ + SRB à S^2- + 4 H₂O

Excess sulfide can then be converted to elemental sulfur (S^o) with aerobic bacteria as follows:

                HS^- + ½ O₂ + bacteria à S^o + OH^-

The main advantages claimed by this process are:  a) H₂S concentrations are low, b) most of the H₂S present will be dissolved in water rather than in the gas phase, c) the process can be conducted at ambient temperatures, and d) flow rates can be varied.  The first commercial plant for this process was built in 1992 at the Budel Zinc refinery to remove zinc and sulfate from acid plant blowdown (Ramachandran, 2012).  Numerous other plants are in operation using this technology.

Apex Engineering has designed several relatively small-scale treatment systems for sulfate removal from mine water (Table 2).  The first three are packed bed systems with a continuous carbon source feed (methanol or ethanol), while the last is a passive bioreactor.  We are in the process of designing two more passive bioreactors for construction in 2015.

Passive bioreactors or biochemical reactors are another proven technology for sulfate removal.  Biochemical reactors (BCRs) are engineered treatment systems that use an organic substrate to drive microbial and chemical reactions to reduce concentrations of metals, acidity, and sulfate (ITRC, 2013).  BCRs have been used primarily to treat mining-influenced waters over the past two decades.  BCRs may be configured to operate with or without external energy and chemical input, and can often be sustained for months at a time without human intervention (hence the name “passive bioreactors”).  A list of sulfate-reducing BCRs is shown in Table 3.

TABLE 3.  BIOCHEMICAL REACTORS

According to Mattson (2014), standard bioreactors have a performance advantage over BCRs but at increased capital and O&M cost.  Standard bioreactors such as THIOPAQ or packed bed systems are more efficient and can be better adapted to large-scale applications, according to Mattson.

In situ sulfate reduction (ISSR) is an innovative technology that combines biological sulfate reduction with remediation hydrogeology approaches (Gillow et al., 2014).  A carbon source such as lactate is injected to catalyze sulfate reduction via in situ SRB, with sulfur then sequestered as sulfide minerals and/or elemental sulfur.  ISSR was developed by ARCADIS.  Apex Engineering has incorporated ISSR as a component of the treatment systems for Kinross Gold and Veris Gold (Table 2).

Reported advantages are:  a) many choices for low-cost carbon sources, b) low potential for process disruptions, and c) less effort to operate than pump and treat.  However, challenges include managing the precipitates, managing final water quality, distributing the carbon source in the subsurface, and the possibility of sulfate “rebound” after treatment ceases.  Several options are available for injecting the carbon source. 

ARCADIS’s view on the future outlook for ISSR is that:

• It is a viable technology for specific applications.
• It is important to consider depth, saturated thickness and downgradient receptors.
• Iron addition to control dissolved sulfide should not be necessary.
• Hydraulic performance and biogeochemical parameters should be monitored.
• The technology should be scaled from pilot-scale to intermediate/full-scale.

Physical Treatment Technologies

Physical processes for removing sulfate include:

1. Ion exchange processes such as GYP-CIX and Sulf-IX™
2. Nanofiltration
3. Reverse osmosis

GYP-CIX is a fluidized bed ion exchange process developed in South Africa to remove sulfate from water that is close to gypsum saturation, so it could be used as a polishing step after lime precipitation.  It is the historic predecessor to the Sulf-IX process, which maintains the IX resin in the same vessel to minimize attrition from resin handling.

BioteQ Environmental Technologies of Vancouver has developed the Sulf-IX process to remove sulfate from waters high in hardness and at near gypsum saturation levels.  The Sulf-IX process is designed to selectively remove calcium and sulfate from water to achieve effluent compliance with sulfate discharge limits.  It is a two-stage IX using two resins (one cationic and one anionic) to partially demineralize the feed water.  The cationic and anionic resins are regenerated using sulfuric acid and lime, respectively, to generate nontoxic solid gypsum (the only byproduct of the process).  One significant advantage of Sulf-IX over membrane systems is that it produces no brine solution, providing substantial cost savings on brine disposal via storage or evaporation.  The first commercial plant using this technology has been operating in Arizona since 2011, with a capacity of 600 m³/day (110 gpm).

Nanofiltration (NF) is a membrane process that can be used to remove sulfate and other contaminants.  It operates at higher pressures (higher operating costs) than microfiltration or ultrafiltration, but lower pressures than reverse osmosis (RO).  NF will have a high removal (high rejection) of sulfate because it is a divalent ion, but will have lower rejection of monovalent ions such as nitrate and sodium.  For NF and other membrane processes, sulfate and other contaminants are concentrated in a reject stream, which may comprise between 10 and 40 percent of the original flow.  Disposal or treatment of the reject stream is another consideration.

Reverse osmosis for sulfate removal is generally only considered when monovalent contaminants must also be removed.  Otherwise, NF is more cost-effective for sulfate removal than is RO.

Golder Associates presented a recent paper summarizing sulfate removal treatment processes (Golder, 2014).  Their conclusions include:

• Active biological treatment has never become popular despite extensive research and development.
• Passive treatment has advanced and may be cost-competitive.
• Operating costs for IX are sensitive to reagent prices and reagent utilization efficiency.
• Membrane technologies for sulfate removal below gypsum solubility levels are commercially demonstrated and have achieved acceptance.
• The cost and complexity of advanced sulfate removal projects warrants independent peer review.

References

1. Gillow, J., M. Hay and J. Horst, 2014.  In Situ Sulfate Mine Water Treatment – Practical Engineering in the Field.   INAP Sulfate Workshop, Salt Lake City, February 27^th.
2. Golder Associates, 2014.  Established and Innovative Sulfate Removal Treatment Processes.  INAP Sulfate Workshop, Salt Lake City, February 27^th.
3. ITRC, 2013.  Biochemical Reactors for Mining-Influenced Waste.  BCR-1.  Washington, D.C.: Interstate Technology & Regulatory Council, Biochemical Reactors for Mining-Influenced Waste Team.  www.itrcweb.org.
4. Mattson, B., 2014.  Review of Sulfate Treatment Issues:  An Overview of INAP’s Treatment of Sulfate in Mine Effluents.  INAP Sulfate Workshop, Salt Lake City, February 27^th.
5. Ramachandran, V., 2012.  Removal, Control and Management of Total Dissolved Solids rom Process Effluent Streams in the Non-Ferrous Metallurgical Industry – A Review.  Proceedings of the 51^st Conference of Metallurgists, September 30-October 3, 2012, Niagara, Ontario, Canada.  Pp. 101-117.
6. Reinsel, M., 2001.  A New Process for Sulfate Removal from Industrial Waters.  Water Online, June.

For more information, contact Mark Reinsel at http://www.apexengineering.us.