Septicity conditions - what biological & environments create "septic wastewater"

Experienced operators see gray/black water with high H2S odors, they confidently say the water is "septic". But what is "septic" water and what upstream biochemical conditions form the odorous, high oxygen demand water. 

First septic water usually gets its dark gray color from reduced sulfides (S=) binding with iron to form iron sulfide. The binding into iron sulfide is a good thing and is often used to control H2S gas in collection systems. For quick reading, I'll give the characteristics of "septic water":

Septic Water Characteristics

• If original water had sulfate - there will be H2S, FeS, and S= (depends on pH, and if ironi is present)Mer
• Soluble organic acids (acetic, propionic, butyric) - odorous products of anaerobic microbial activity
• Mercaptans - strong odor compounds formed when you have reduced sulfides and organics
• High organic acids - increase the amount of soluble BOD5 and promote filamentous bacteria growth (from initial oxygen depression at the influent & some grow very well on organic acids)
• Reduced sulfur species have a chemical oxygen demand that also lowers dissolved oxygen in the influent area
• Sulfides can thios can be very toxic to ammonia oxidizing bacteria (nitrifiers) in low concentrations

Biochemistry of Septicity

• Low ORP (redox) potentials in collection system occur as dissolved oxygen is depleted by bacteria growing on wastewater organics.
  
• If nitrate is present - usually the bacteria will move to using nitrate as an alternative oxyen source. This does not create septic water as both sulfides and organic acids do not accumulate.
  
• Once ORP drops sufficiently, microbial forms that can use oxygen from sulfate are favored and start to produce sulfides. Other microbes, without the ability to use sulfate, continue to grow slowly under fermentative respiration - which produces the organic acids. While organic acid production is vital in anaerobic digesters, in collection systems it can be an odor issue.
  

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How to Control Septicity in Collection Systems

• Control redox or ORP - add aeration, peroxides, or nitrate. Objective is to increase ORP above the value where sulfides or organic acids accumulate. Remember, inject redox control agents at points upstream of where you notice the problem.
• Increase lift-station pumping frequency - don't let water sit in non-aerated areas.
• Add aeration/mixing to equalization upstream of the treatment plant - we are not looking for a D.O. residual, we just want to keep ORP up.