Step 1: Biological sulfate reduction. To begin the process, sulfate reducing bacteria (SRB) reduce sulfate (SO4) to hydrogen sulfide (H2S or HS-) with the Clearwater BioLogic floating modular bioreactor patented under US-10.597.318-B2 and US-11.104.596. High-sulfate influent water, with added electron donor and nutrients, flows into the 4,000-gallon floating bioreactor vessel. Here billions of SRB affix themselves to over 26 acres of recycled, fine fiber, non-biodegradable, attachment media. The water flows vertically through this 90% open void volume media, requiring very low pressure drop and without forming preferential flow paths. As the water flows past the attached SRB, they biologically convert the sulfate to sulfide, using the electron donor, nutrients, and the oxygen stripped from the sulfate. Each module can reduce thousands of mg/L of sulfate to sulfide in a single pass. Since it floats with the active volume of flowing water below the frost line, it can operate year-round in cold climates, and multiple modules can be rafted together to handle any amount of water flow.
Step 2: Sulfide conversion to sulfur. Now the hydrogen sulfide generated in Step 1 is converted to elemental sulfur with the proven, commercially available PRISC™ system of USP Technologies under patents US-6.773.604 and US-7.147.783. This system uses ferrous chloride to produce iron sulfide from each hydrogen sulfide molecule in the outlet pipe from Step 1. Further down the pipe, hydrogen peroxide is added to regenerate the iron and form insoluble elemental sulfur. The regenerated iron is then synergistically used to react with further hydrogen sulfide to form additional iron sulfide. With repeated use of hydrogen peroxide, the original iron introduced at the beginning of Step 2 can be regenerated and reused several times with no new addition of chloride. This process effectively converts high concentrations of hydrogen sulfide to elemental sulfur using the iron repeatedly, with a minimal introduction of ferrous chloride.
Step 3: Collection and removal of sulfur. The elemental sulfur from Step 2 is now precipitated or filtered out along with the remaining iron hydroxide. This material is collected as a wet slurry that can be pumped off for disposal or for use as a soil amendment where sulfur and iron are needed (such as corn cultivation). The removal of the sulfur from the water treatment system avoids any regeneration of sulfate downstream. For many purposes, Step 3 completes the remediation process.
Optional Step 4: Lime softening. Where further reduction of hardness and specific conductance is needed, a fourth step may be required using traditional lime softening— for example, to meet the Class 3 and 4 water quality standards specified by the Minnesota Pollution Control Agency in 2020. Still, many applications will not require lime softening to meet hardness and specific conductance standards.