Upflow Anaerobic Sludge Blankets (UASB Reactors)

Q: Our UASB reactor is not producing adequate sludge granulation or biomass. It starts to build biomass and get good COD removal, but then the granules disperse.

We have influent that contains terephthalic acid and acetic acid and conditions that are adequate for methanogens (Temperature at 37.2°C (99°F) and pH 6.8-7.2).

The metals that are entered into the system are aluminum, calcium, cobalt, chromium, copper, iron, potassium, magnesium, manganese, phosphate, and zinc. Can you help?

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A: For readers unfamiliar with upflow anaerobic sludge blanket (UASB) technology, normally referred to as a UASB reactor, it is a form of anaerobic digester that is used in the treatment of wastewater.

To analyze your situation, we would first look at your mixed liquid. We would analyze:

• Suspended solid amounts (SS), which may be responsible for the dissipation of granules.
• Settling rate (SVI).
• Presence of fats, oils, or greases which could inhibit granule formation or cause aggregates to float and be lost.
• Presence of NO3- or NO2- or SO4 2- (Methanogens need some sulfide but not over 200 mg/L).
• Presence of toxic compounds (e.g., p-toluic acid, heavy metal compounds). The range varies depending on the industrial influent.

Next, we would look at the granules:

• • Measure granule size (should be 0.5-2mm diameter)

• • Look at a cross-section to look at morphology and structure:

    

        A cross-section of a granule

    Are there “spaghetti-like” methanogens that hold the structure together (would probably see on outer edge)? Are there concentric layers with channels and pores and polysaccharide plugs (basically, a biofilm stuck to itself)? Is there a core of lysed cells? If the core is hollow it could mean they need to increase their upflow liquid velocity.

• Analyze for polysaccharide plugs and metal ions
• Measure the amount of Ca2+, Fe2+, and PO4 2- in the granules (at high levels of Ca2+, for example, the granules should change color)
• Look at methane production rates. In particular, how they coincide with COD removal.

Finally, we would calculate:

• Ratios of COD/N/P
• Micronutrient amounts: Methanogens will be affected first by high or low levels. For stable granules, methanogens must be cultivated. Aggregates formed during acetogenesis or hydrolysis are not stable and will disrupt quickly when not fed.

Additional factors we would look at:

• How long was the UASB reactor running before the problems arose?
• Upflow liquid velocity: One way to promote granular formation from dispersed growth is to apply mild stress such as increasing the upflow velocity to select for microbes that can attach to each other. At low velocity, the microbes may starve and lyse leaving the middle of the aggregate hollow which could float and be washed out of the reactor.
• g COD/m3 per day feed in
• Hydraulic retention time ((methanogens are slow growers – for terephthalic acid, division rates may be 7-28 days)
• Use of lime or soda ash to raise pH (lime can precipitate out calcium)
• Is the influent being diluted?
• Is effluent being recycled back into the system?
• Is acetic acid being added to the system, or is this a byproduct from methanol degradation or terephthalic acid degradation? If it’s being added, it should be about 10:1 ratio acetic acid: terephthalic acid.

At AQUAFIX, we blend VitaStim anaerobic micronutrient cocktails that are designed specifically for the optimization of upflow anaerobic sludge blankets.