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. Multiple modules can be rafted together to handle any amount of water flow.

This graph shows actual sulfate reduction results in labs tests from October, 2023 to March, 2024. The blue line shows the sulfate level of the influent water, taken from the St James Pit Lake near Aurora, MN. It averages about 340 mg/L of sulfate. The red line is the final sulfate concentration after biological reduction. It is consistently below 3 mg/L. This is better than a 99% removal rate of the sulfate.

The sulfate has been converted to hydrogen sulfide.

Step 2: Sulfide conversion to iron sulfide. Now the hydrogen sulfide generated in Step 1 is converted to iron sulfide with the addition of reactive iron. There is no addition of undesirable elements such as chloride, aluminum, or barium. The use of reactive iron in Step 2 dramatically reduces the operational costs of the overall system. The iron can be added passively with reduced iron pellets for adsorption of the hydrogen sulfide on the internal and exterior surface areas. In a more active system the iron can be mobilized from porous iron electrodes using low voltage direct current electricity. With this system the iron ions react with the sulfide ions to form iron sulfide.

This second graph shows the elimination of the hydrogen sulfide generated in Step 1. The red line shows the concentration of the  hydrogen sulfide generated. It runs about 100 mg/L The blue line shows the resultant hydrogen sulfide concentration after iron treatment. Typically over 99% is bonded with iron to form iron sulfide.  Iron sulfide is an insoluble particle that is then removed from the system as a slurry.

 

Step 3: Collection and removal of iron sulfide. The iron sulfide from Step 2 is now removed on the iron pellets in the more passive system or precipitated out as fine flocculated particles for the active electrode. This material is collected in pellet format or as a wet slurry that can be pumped off. Both formats of sulfur collection are safe for disposal or for use in other applications such as a soil amendment (such as corn cultivation) or for ground water remediation where sulfur and iron are useful. The removal of the sulfur from the water treatment system avoids any regeneration of sulfate downstream.