Old sludge - what high MCRT and very low F/M can do to effluent quality

Nocardia, a slower growing organism, can become a problem with long sludge ages.

Lately, my most common recommendation in consulting with wastewater has been to increase wasting rates. We all know that wasting takes time, involves disposal costs, and there is the temptation to use the aeration tank as an aerobic digester to lower solids going to disposal. So why do we waste? Or what happens if we move along the growth curve to the far right (endogenous respiration)?

Here is what I often see:

• Nocardia and other slow growing organisms that exploit a niche can become more of a problem
  
• Effluent turbidity and fines/pin-floc passing into effluent increase
• Beneficial EPS decreases - organisms store excess organics in EPS but as you enter endogenous respiration the bacteria begin to utilize EPS for cell maintenance/growth
• Oxygen requirements - while individual cell respiration rates drop with a mature biomass, the overall oxygen consumed is higher when you add up all the biological solids
• Filaments become more of a bulking agent with decreased floc former EPS
  
  

While you may not have permit issues related to running in endogenous respiration mode, it costs money as supplying oxygen and mixing for additional biomass takes energy.