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Cell Yield (Y) in wastewater systems

5/19/2015

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:

Aerobic Yield

With no information on F/M ratio or influent makeup, I use 0.5 g cells/g BOD5 in the influent.
With starch and other readily degradable influents, you can increase this to 0.6 g cells/g BOD5.
High F/M rations (less biomass, more substrate) - you have a higher cell yield which decreases gradually as the F/M lowers - N.B. it is not a linear relationship between F/M and Y. The lower cell yield is actually do to endogenous respiration (cell death) that occurs in the confines of the treatment system so it does have some advantages & disadvantages (I'll post more on this later).
In practice you see cell yields anywhere from 0.4 - 0.6 in most cases.

Anaerobic Yield

From an energy generation standpoint, anaerobic respiration results in less "energy" for cell growth than aerobic systems. The result is less cell yield per unit of substrate.
Normal cell yield (Y) is 0.06 g cells/g substrate - which is approximately 10x less than yields fro aerobic systems. This is why high strength wastewaters are often treated with anaerobic digestion.

Now for a general table of Cell Yield

Microbial Ecology

5/18/2015

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:

What microorganisms are present - we can classify using temperature, pH, oxygen, redox potential, nutrients, organic compounds, and numerous other information to determine the general microbial populations.
What metabolic reactions could the existing microbes carryout.
What metabolic reactions are actually occurring in the system.
How do the microbes interact with each other and the surrounding environment.

In designing and operating a biological waste treatment system, we develop and environment designed to encourage specific reactions to meet treatment goals. This includes adjusting nutrients, pH, temperature, oxygen, redox potential, retention time, suspended or fixed film growth conditions, and varying these conditions inside the system.

Fungi applications in bioremediation and wastewater treatment

5/5/2015

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.

Given a lower surface area to organism size, the fungi are able to deal with nutrient (N,P) scarcity better than may bacteria. They are very efficient in the use of nutrients to build new cell mass.

Fungi are also able to grow in a wide pH range at levels below ideal for most common soil bacteria.
Fungi can grow at lower levels of moisture than bacteria.
Degradation of lignin - many fungi have adapted to fast grown on wood and other plant debris. Many of the enzymes used in degcrading the most recalcitrant portion of plant waste (lignin) also makes these enzymes able to initiate decomposition of complex organics such as DDT, TNT, and may other chlorinated or polyaromatic hydroarbons (PAHs). This non-specific use of the fungal enzymes is done via cometabolism rather than fungal direct action on the waste compounds. The most commonly studies fungi for waste degradation is the white rot wood fungus (Phanerochaete chyrsosporium).

It is this last portion that gives the weakness of fungi in waste treatment.

Fungi typically grow in attached mode which is used for both support and as a substrate for growth. For example, the P. chyrsosporium grows on wood chips when used in bioremediation.
Most fungi do not do well in suspended growth as found in the water column. This is where bacteria are very efficient at uptake of nutrients, carbon, and oxygen.

So when you have high levels of lignin or other wastes resistant to bacterial remediation, it could be worth investigating fungal cultures for biological treatment. In this case, you cannot simply drop a fungal spore or culture on the waste and expect good results. Instead, it is common practice to mix wood chips with active fungal cultures with soil or water contaminated with the waste. By keeping the blend mixed, aerated, and at proper moisture levels - the fungi can degrade the wastes.

Spring Rains & Storm Water Treatment

5/1/2015

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.

Cell Yield (Y) in wastewater systems

Microbial Ecology

Fungi applications in bioremediation and wastewater treatment

Spring Rains & Storm Water Treatment

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