We often use the word septicity to describe wastewater with malodors from hydrogen sulfide and dark color from the reaction of iron with sulfide (FeS). As with most things wastewater, it is a more complex condition than can be described by hydrogen sulfide or color.
Septic wastewater forms when you have:
When this water hits an aerobic treatment system the following happens:
Septic wastewater forms when you have:
- Soluble organics in abundance (BOD5)
- Absence of dissolved oxygen (what was there is depleted)
- Lack of alternative electron acceptors (in this case usually NO3, NO2, Fe(III))
- Microbes with anaerobic metabolic pathways (usually in abundance in sewage)
- Dissolved oxygen becomes depleted (Redox potential falls below 0 mv @ 7 pH)
- Soluble BOD continues to fuel microbial growth
- Nitrate/nitrite/Fe is used by microbes (Redox continues to fall below -150 mv @ 7 pH)
- Sulfate and sulfur are used by Sulfur reducing bacteria (SRB) as an electron acceptor for organic degradation. (Redox falls into the - 200 mv @ 7.0 pH)
- Non SRB bacteria use organics as the electron acceptor which creates organic acids and hydrogen. Some including propionic, butyric, and acetic have strong odor potential.
- In collection systems, we rarely drop below - 450mv redox where we would start to see methane production.
When this water hits an aerobic treatment system the following happens:
- Highly soluble organic acids and reduced compounds create a biochemical and chemical oxygen demand that depresses D.O. and favor the growth of microbes that grow well on highly soluble organics @ low DO (i.e. filaments)
- Sulfide and many organo-sulfide compounds are toxic to autotrophic nitrifiers that are needed for ammonia removal
- Once aerated or redox is increased back to positive range, the waster is back to "normal".
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