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How fast do bacteria grow in wastewater?

10/18/2016

 
PictureBacillus subtilis
We often hear that bacteria double every 20 minutes. This number is recited in almost all wastewater microbiology classes and technical literature. Is this the right number to use for projecting growth in a wastewater unit during startup or following a loss of activity? 

First, the 20 minute number comes from a typical lab bacteria, E. coli, grown under ideal lab conditions of temperature, pH, sufficient nutrients, and a carbon source (energy) from glucose - which is "bug candy". Unlike my pure lab E. coli  culture, a wastewater unit has many different species of microbes including gram positives, gram negatives, heterotrophic, chemotrophic, obligate aerobic, facultative anaerobic, and even some anaerobic cultures (in some systems). This mix of bacteria forms the MLSS with each organism functioning in various ecological niches as a consortia. We refer to this mix of living bacteria, extracellular materials, and adsorbed particles as MLSS, floc, or biofilm depending upon the system. 

So how fast do these bacteria grow? Well each strain has a different maximum growth rate. Most common heterotrophic (BOD/COD degraders in wastewater) have a doubling time of 30 - 60 minutes. Slower growing organisms appear under adverse conditions (a whole other topic) and are known as r-rate strategists. More delicate cultures such as ammonia oxidizing bacteria have a doubling time measured in hours, which makes them more susceptible to washout and slow to recovery from a toxic shock event.

Also, the numbers above are for growth with ideal medium under lab conditions. In wastewater, we usually operate in much lower F/M conditions which further slows growth. We usually monitor microbial growth/activity using oxygen uptake rates and ATP.  When oxygen uptake rates are high and free ATP is present in the solution, we are experiencing rapid growth. This of course is not ideal for operating a waste treatment plant. As the respiration rates drops and free ATP decreases, we start to see a reduction in cellular growth. Eventually entering the range of ideal F/M conditions where effluent quality is optimal.

So while bacteria can grow very fast under lab conditions, they rarely achieve 20 minute doubling times in wastewater. In fact, we only see max growth rates during startup or following a shock loading. Even then, it takes longer than 20 minutes for the bacteria to double given the nature of wastewater. 

pradip karmur
2/28/2019 04:31:16 am

hello sir
actually i need your advice on particulate matter
in my etp's aeration tank water turn into white color so what can i do to stable my plant and what is reason for aeration tanks water turning into white color. provide your guidance on above mail address
thank u

ADRIAN BUSTAMANTE
3/20/2021 09:29:47 pm

Hello Sir,

May I ask know how on how to start up with growing bacterial culture in aeration tank. I am hardly growth of microbial population during start up. Thank you

Erik Rumbaugh
3/21/2021 08:08:47 am

Startup takes time with filling the aeration tank and allowing bacteria to start growing before starting flow. The slowly start flow and avoid washing our bacteria. To speed things up, you can import sludge from a local plant and/or use bioaugmentation cultures. This can reduce startup time by several weeks.


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    Author

    Erik Rumbaugh has been involved in biological waste treatment for over 20 years. He has worked with industrial and municipal wastewater  facilities to ensure optimal performance of their treatment systems. He is a founder of Aster Bio (www.asterbio.com) specializing in biological waste treatment.

    View my profile on LinkedIn

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