Cell yield (Y) is the amount of new cell mass created per unit of substrate removed. In wastewater, we usually define substrate as BOD5, COD, or TOC. In most cases a true defined substrate is unknown becuase the influent is a mix from multiple streams. So over the years, data has been collected from multiple systems with differing influent makeup, temperatures, F/M ratios, pH, and any number of other environmental factors. I'll distill some of the information below:
Now for a general table of Cell Yield
In a waste treatment system, or even a diverse natural environment, microorganisms are found in a mixed community with genetic variability and ecological niches. With respect to waste treatment, we can explain treatment efficiency and outcomes of changes based on our general understanding of the underlying microbial ecology. To understand the system's ecology, you must know the following:
In most of my earlier posts, I have discussed mostly bacterial action/grown on wastes. while bacteria are the most common organisms in waste treatment, we cannot ignore the impact of various fungi on treating wastes and recycling nutrients.
In the environment, fungi are competing with bacteria for nutrients and carbon sources. This competition has resulted in the ability to exploit various ecological niche by different microbes.
Lately, Houston has experienced many heavy rains. Many local retention and storm water holding ponds are near capacity. With the influx of storm water, the ponds also often pick up pollutants including petroleum, herbicides, pesticides, fertilizers, and almost anything else that people allow to get to storm drains or dump in parking lots.
While screening gets large trash out of the effluent, petrochemicals including motor oil, diesel, gasoline, and lubricants seem to be the most common and problematic offending pollutants. If the hydrocarbon is in a thick layer, it is often skimmed off using vacuum trucks and oil adsorbents (it is then taken for reprocessing as slop oil). In most cases, operators are faced with a slight petroleum sheen that is difficult to remove by physical methods.
The easiest way to deal with light hydrocarbon sheen and water soluble hydrocarbons is to use biological treatment. By adding mixing via an aerator or mixer, the hydrocarbons (food source) are mixed with microbes that are present in the water column.
In cases where the hydrocarbon loading has increased rapidly and the number of microbes utilizing hydrocarbons as food is low, treatment rates can be increased by adding a concentrate of hydrocarbon degrading microbes. This helps remove the problem compounds within 5 - 28 Days (temperature, mixing, and hydrocarbon concentration make up for the variation in time). With treatment completed, the water can be safely discharged to receiving streams.
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.
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