Moving Bed Biofilm Reactor (MBBR) media has potential to expand wastewater treatment plant capacity, but as with any technology there are some tradeoffs compared to conventional suspended growth systems. Usually constructed of HDPE (polyethylene) with a density near that of water - 0.95 g/cm3. The HDPE is extruded into high surface area shapes which provide a surface for bacterial biofilm development. Biofilm treatment systems go back as far as trickling filters - it is just plastic gives us more options for supporting the biofilm. The MBBR media is kept in the aeration basin by screens with excess biosolids soughing off during movement.
Almost every wastewater plant runs the SV30 test. Running the test is great and coupled with MLSS/MLVSS numbers gives the Sludge Volume Index (SVI) which converts settled volume to a more standardized number based on MLSS in grams. The test can also give you other valuable information. Make a few observations over 30 minutes and you can see:
The easiest way to gauge EPS in activated sludge floc is to use the India Ink test with your existing microscope. In addition to being easy, India Ink requires no specific equipment or skills - just a bottle of $10 India Ink that will last for years. Unlike most inks, India Ink contains finely ground carbon particles that are suspended in water. You can find India Ink in many craft stores or online - Amazon India Ink with dropper.
To use India Ink with your microscope follow these steps:
Oxygen Uptake Rate testing is often done without much thought and we report a number, making sure it is within a specific range. Let's step back for minute and understand what the OUR number really means and how it changes in response to influent quality.
We run OUR using simple equipment - a DO meter and BOD bottle which is present in most labs. No reagents or special techniques are needed. A PDF of the standard OUR & SOUR protocol is at the bottom of the post.
OUR is simply how much oxygen is consumed over a short-time period in a stirred BOD bottle. The OUR gives microbial respiration rates for the MLSS (living bacteria in the bottle). Like humans, bacteria use more oxygen when they are rapidly growing and dividing. What causes rapid growth and cell division? High levels of soluble organics that are a source of food for the microbes. So if we were to spike MLSS with sugars, we would see a rapid increase in respiration rates. A rapid decrease in OUR can be equally problematic. For instance, a shock loading of phenol, solvents, or other biocidal influent can kill many of the living organisms. There is a temporary drop in OUR due to the biocidal action. This is followed by a rapid increase in OUR as the toxicity decreases and the microbes return to steady-state populations. On the growth curve this represents low to decline phase growth where biological wastewater systems are in optimal health.
Molecular testing, including qPCR, allows for direct, rapid analysis of microbes present in a sample. We can look for important good organisms such as ammonia oxidizing bacteria, or bad organisms such as Nocardia foaming bacteria. How does it work?
Genetic material from foam samples is extracted and we identify the specific organisms responsible for Nocardia foam outbreaks. From testing, we have found most Nocardia foams are caused by a bloom of Gordonia species. We use Gordonia specific primers, qPCR gives quantitive data within a few hours of starting the test.
The qPCR advantage stems from its sensitivity and low per test cost. Unlike older microscopic or plate count techniques, qPCR can detect foaming microbes at much lower levels before they become a problem. You can adjust wasting and take corrective actions before the heavy, problem foaming.
Looking at filaments and floc structure is difficult with standard light microscopes. While adding phase contrast helps improve filament and floc former visibility, it is still not as effective as using stains to make cell walls jump out.
Gram staining is a common method to identify bacteria with a microscope. The stain differentiates bacteria into gram-positive or gram-negative based on their cell wall composition. Often used in medical or laboratory settings, it can also be useful in wastewater samples. You need to realize in wastewater with particulates and EPS, gram stain is often variable - so unless you have an non-floc organism such as Nocardia that stains strongly gram positive, you do not have large amounts of contrast.
Using a Gram Stain kit is easy. I'll walk through how I use it in wastewater samples.
As microbial cells develop into floc or biofilm, they are bound tegether by Extracellular Polymeric Substances (EPS). Consisting of a mixture of polysaccharides, proteins, enzymes, and DNA, EPS provides several benefits to the microbial community:
Ammonia, nitrite, and nitrate removal across biological wastewater treatment systems gives many operators fits. Why? Unlike organic compound degradation, ammonia nitrogen goes through several distinct steps to detoxify and finally remove the nitrogen from the water. Today I am going to cover the various groups of microbes involved in the wastewater nitrogen cycle.
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.
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.
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:
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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