Quaternary Ammonium Compounds (Quats) and their impact on wastewater microbial populations

From Wikipedia - The chemical structure of a QAC.

Quaternary Ammonium Compounds (QACs), often called quats, are cationic surfactants (positive charge). There are many QACs in use, but among the most discussed are the disinfectant QACs. The positive charge makes them bind to negatively charged cell walls in microbes where the QAC damages the cell walls/membranes. At concentrations between 200 – 400 mg/L, QACs are used to disinfect surfaces in food processing operations.  As cationic surfactants, QACs are also used in fabric softeners and hair conditioners to neutralize anionic charges (static). These QACs typically are not inhibitory to microbial growth and readily degrade in wastewater treatment plants.

With the number of facilities expressing concerns about QAC toxicity issues, I have been reading many academic papers about QAC impacts on wastewater facilities and in lab testing. So you don't have to read all these papers, I summarized some of the observations:

QAC wastewater impact

• 2 options – biodegradation and adsorption to anionic surfaces (soil, anionic surfactants, etc)
• Biofilms are much more resistant than free bacterial cells
• Most domestic WWTP have influent QAC in the 1 – 5 mg/L range
• QACs are a big group of compounds - not uniform in toxicity
  • Short chains are antimicrobial
  • Longer chains are fabric softeners etc
  • SC are more biodegradable
• Anionic surfactants complex with QAC and inactivate with respect to biocidal action these are also present in influent and will react with the QACs present. Anionic surfactants are in most cleaning products, laundry detergents, and liquid "soaps"
• EPS response to QAC increased MBR fouling

Key findings

• QAC – not a uniform toxicity across the board – certain ones are more harmful
• Anionic surfactants and other compounds present usually bind with the QAC and change/reduce their ability to damage bacterial cell walls and associated enzymes
• A variety of QAC neutralizers can be used to bind QACs – useful when influent QAC concentrations are above normal ranges – 1 – 5 mg/L range in the influent
• QAC toxicity usually impacts AOB/NOB first – 2 mg/L can cause significant inhibition. However, biofilm/floc reduces the ability to QAC to inhibit AOB/NOB.
• QACs reduce microbial diversity & result in changes in EPS (big for MBR fouling)
• Many common heterotrophs an degrade QAC – including Pseudomonas.
• Systems adapt to QAC by changing organism mix and EPS for increased tolerance and neutralization capability

What to do if you suspect QAC Problem
 
Confirm that QAC is the problem

• Do you have AOB/NOB? – test with MCA or qPCR (as the most sensitive organisms, their continued presence in MLSS indicates that QAC is not the problem)
• Quick lab kit tests don’t distinguish among QACs and can have interferences with common wastewater components. For “complete” testing send to an outside lab with HPLC capabilities.
• MCA tests can show changes in microbial diversity – a strong indicator of QAC at sub lethal concentrations
  
  

If there is QAC what can be done

• Find sources of QACs and make sure the users are using the products correctly and if they have neutralization capabilities – make sure they are being used!
• Use QAC binding chemistry to lower toxicity – multiple options are available.

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