The Sulfur Cycle in Wastewater Systems

Being both deadly in enclosed spaces and a nuisance at even low levels, hydrogen sulfide is among the most problematic of compounds in wastewater treatment. Instead of covering the whole geochemical sulfur cycle, I want to look into the wastewater specific cycle that converts benign sulfates and sulfur into the problematic reduced sulfide species. Instead of graphics, I want to detail each form of sulfur found in wastewater.

• Sulfate - one of the most common forms of sulfur found in the environment. Sulfate is used by plants and is an important micronutrient for life.
• Sulfur - the yellow powder or rock form of sulfur. 
• Reduced Sulfides - often seen as S= or in analytical tests as Total Reduce Sulfides. This form of sulfide can bind with iron and give the dark gray to black color seen in septic wastewater. Sulfides are more soluble in water with a higher pH. Reduced sulfides in wastewater have a high chemical oxygen demand and are toxic to Ammonia Oxidizing Bacteria (AOB) that are required for ammonia removal.
• Hydrogen Sulfide - a corrosive, highly toxic gas, H2S is released from water into the atmosphere as pH drops in wastewater. (This is why one of the collection system odor control methods is to increase sewer line pH).
  
  

Now for the microbes involved in wastewater sulfur cycles:

• Sulfate Reducing Bacteria - these bacteria under anoxic conditions use sulfate as electron acceptor to degrade organics. If you have oxygen or nitrate in the system, sulfate reducing bacteria are outcompeted by harmless bacteria that obtain more energy using oxygen or nitrate as an electron acceptor.
• Sulfur Reducing Bacteria - these chemotrophic bacteria obtain energy by reducing sulfur into sulfide. In most wastewater, these are less common than the Sulfate Reducing Bacteria that use sulfur as an alternative electron acceptor. You would find these organisms in mines, subsea volcanic vents, hot springs, and oil formations.
• Sulfur Oxidizing Bacteria - chemotrophic bacteria with the metabolic ability to convert reduced sulfur species into sulfur and sulfate. While some species can use nitrate as the electron acceptor, most sulfur oxidizers require oxyen as the terminal electron acceptor. If taken to conclusion, you produce sulfuric acid (H2SO4). These are the well documented sewer corrosion bacteria that can form if a vapor phase H2S concentration reaches sufficient levels. In wastewater, Sulfur Oxidizing Bacteria (SOB) are vital to removing reduce sulfides. This does require alkalinity to buffer the system from pH drop. Common genera of wastewater SOB include Thiobacillus, Anwoodia, Paracoccus, and Thiosphaera.
  
  

Solutions to sulfide generation in collection systems, equalization tanks, and headworks.

• Sulfides can be kept in water phase by increasing pH, adding ferric or sulfide scavengers to the collection system. This is a quick fix, but does not solve generation of sulfides in anoxic/anaerobic zones.
• Adding alternative electron acceptors to prevent growth of sulfate reducing bacteria - common additions include nitrate, liquid oxygen, and hydrogen peroxide. All work by giving a higher energy electron acceptor than sulfate/sulfide for bacteria in the system.
• Using a comprehensive control program to prevent sulfide formation. This uses monitoring/testing to find zones where redox potential favors the growth of SRB. To combat the SRB, you add the best alternative electron acceptor based on the location and environmental conditions. We have also found it beneficial to add microbes capable of both using the alternative electron acceptor and oxidizing reduced sulfides. In Aster Bio's products we favor Paracoccus and Thiosphaera as they grow well from 5.5 - 8.5 pH - unlike most Thiobacillus that thrive at pH <5.0 where you get significant H2S formation.

Sulfides are a problem in wastewater. Unless you have an influent source of reduced sulfides, most sulfide is formed in water under anoxic/anaerobic conditions that favor SRB growth. The best solution for sulfides forming in the system is to disfavor the growth of SRB. How to accomplish this requires a system survey and determination of which solution best fits your situation. But before selecting a solution or choosing a single solution for a complex system, do a full survey of Redox (ORP), pH, Total Reduced Sulfides, and H2S.