Microorganisms in Wastewater – How They Grow

Large quantities of microorganisms in wastewater slow down the process of treatment. The stored wastewater at the plants, provides an ideal ground for the microbes in wastewater to grow. Keeping tabs on this growth can apprise the plant personnel about how to proceed with the treatment. In this article I discuss a formula that can help measure this growth.

Exponential Growth Of Microorganisms In Wastewater

To identify the types of microbes in wastewater, samples from the plant are cultured in laboratories. In such batch cultures, it is observed that the microorganisms exhibit different phases of growth and ultimately die thus finishing their life-cycle. The first phase is a lean or lag phase with slow growth. This is followed by a fast growth phase known technically as the exponential phase. During this phase the microorganisms in wastewater consume large quantities of nutrients. The next phase is a stabilization phase where the activities of the microorganisms tend to slow down and remain subdued compared to the previous phase. During the next phase large quantities of smaller microorganisms die and are consumed by comparatively larger microorganisms and new forms are observed.

Of all these four phases, the phase that most affects the treatment process is the exponential phase which sees high growth rate of the microbes in wastewater. This growth can be measured by the equation: dx / dt =: x ==> X = Xo e ^ -T

In this equation,
X represents the concentration of the biomass in wastewater at a given time written as g / l or cell number,
: x represents the specific rate of growth written as h ^ -I,
Xo represents the concentration of biomass at time = 0 written as g / l or cell numbers,
T represents the time measured in hours and written as h.

An important measure during this culture test for microorganisms in wastewater is the estimation of "doubling time" or the time it takes the biomass to double in quantity represented as Td. This value is given as the following Td = ln 2 /: = 0.693 /: