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Relating F/M, plate counts, ATP, and DNA total counts in wastewater treatment MLSS

12/10/2018

 
Picture
How many living bacteria are in my MLSS? I get this question often and it is often difficult when people try to use F/M ratios during a system startup. (Often they wonder why a very high F/M (Low MLSS) - that the living biomass can remove 90% COD/BOD from influent).  Understanding the technology and limits behind each test will give you an idea of the pros and cons of each test.
  • F/M - this is not a test but an operational ratio designed for use in suspended growth (activated sludge) systems. It works well for activated sludge but be cautious in very high or low F/M ratio situations. All we are looking at here is BOD (or COD) loading divided by MLSS or MLVSS. Remember MLSS is a measure of "solids" - this is not a measure of living bacteria, biomass activity. 
  • Plate Counts - give results in Colony Forming Units (CFU) per ml. This test measures bacteria that grow on the plate's media. It only tells you numbers of bacteria that can grow on plates or on the specific media. We know that this misses a huge number of organisms present in wastewater and over-represents cultures that grow under plate count conditions.
  • ATP (Free & Cellular) - ATP testing has been around for years but only in the past 15 years has become common in wastewater operations. Adenosine Triphosphate (ATP) is an energy storage molecule used in living organisms. Free ATP indicates cellular division or lysis (ATP does not last long in free solution with living microbes). Bound or cellular ATP is synthesized inside the cells and used in powering cellular activities. ATP functions as a proxy for living microbes in a system and the amount of cellular division/lysis. This number is a proxy and needs to be calibrated for each system - where it functions best over relative narrow band of operational conditions.
  • DNA total counts - using a qPCR total bacteria test gives a number using 16s ribosomal genetic material. While qPCR can quantify specific organisms, it can also give information on total microbial populations. DNA or molecular testing gives numbers for both culturable (plate count) and organisms that do not grow on plates. As a rapid test, qPCR results are available within hours vs the 24 - 48 hours for most plate counts. Like all the tests mentioned, qPCR numbers should be calibrated to the system and correlated to effluent quality.

Operating a system using a single number is not feasible. Each test works great over a certain range of conditions. Outside that range, the test results do not provide an accurate picture of biomass activity or health. So, combine multiple tests and use good judgement in operating your system.

Why wasting is important in activated sludge systems

12/5/2018

 
PictureDon't make your aeration basin an aerobic digester
Handing and disposing of secondary biological solids can be difficult, expensive, and at times limited by equipment. Many of us are taught that by running a longer sludge age, the cell yield will decrease and make fewer solids requiring wasting. Therefore, I have seen a tendency to move toward long sludge ages which goes with very low F/M and high levels of MLSS. You may ask - "What can go wrong with very long sludge ages?" Let me list my top observations:
  • Increased utility costs keeping excess solids aerobic 
  • More polymer use in secondary clarifiers as pin floc forms
  • Non-filamentous bulking - Thauera & Zooglea species can blossom under low F/M
  • Nocardia - if you have long chain fatty acids or grease - slow growing Nocardia can enter the picture
  • Problems with dewatering secondary solids - EPS entrained water can require more polymer to dewater solids
  • Increased chance of metals toxicity - remember metals can build up in the floc which can inhibit nitrification
  • Lower populations of k-rate (fast growing) organisms can result in low activity if a spill or shock loading event occurs. Remember microbes are different & MLSS is composed of many organisms with different ecological niches

    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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