Do you know the difference between adsorption vs absorption when talking about MLSS? This is a case where the English makes it difficult but fortunately the concept is simple.

Adsorption​
Influent organics can come in surges (temporary high strength waste), be in particulate form (insoluble), or contain inorganic/non-biodegradable compounds. In these cases, the MLSS with associated surface area and "sticky" EPS will attract these compounds to the surface. This moves the compounds from the water phase into the floc. We call this process adsorption - I like to think of it as the MLSS acting as a sponge. After attaching to the floc surface, the bacteria use exoenzymes and biosurfactants to make the insoluble compounds into soluble forms that can cross the cell wall. Inorganics and non-biodegradable fractions remain trapped in the floc and are eventually wasted out. The MLSS does not have unlimited capacity to adsorb high strength influent. Limits on adsorption capacity usually show up as high TSS, deflocculation, loss of nitrification, and problems maintaining D.O. 

Absorption
In MLSS the bacteria tend to first consume soluble organics - things such as sugars, alcohols, short-chain fatty acids all readily cross the cell wall and provide food for the bacteria. This is why you see a fast drop in D.O. with an influent containing organic acids, sugars, or highly soluble organics. The larger compounds that don't readily cross the cell wall remain in the EPS layers until exoenzymes and other biochemical processes allow for the compounds to cross the cell wall -- this is the absorption part of the equation. Absorption requires time and conditions allowing the bacteria to generate energy from metabolism of the compounds.

Last week I saw an article on a municipality discovering that a biodegradable detergent was effective in reducing grease buildup in a problem lift station.  according to the article, this "natural" solution was ecologically friendly solution to the grease problem.

Well solvents and detergents can mobilize grease & fatty acids - cleaning the lift station. But to call this a "good" solution to the problem is a mistake.  Surfactants do not degrade or transform grease but instead create oil-water emulsions that move downstream to the wastewater treatment plant. If there is not too much grease, the emulsion and associated grease will be biologically degraded by the wastewater plant bacteria. (I know too much is qualitative and not a hard number, but the exact amount of grease differs for every system). But often using surfactants just pushes the problem to another part of the system and better solutions can be implemented.

If you use surfactant lift station treatment here are the things that can happen:

• Emulsion breaks in the collection system and redeposits
• Additional FOG enters the treatment system promoting Nocardia growth
• FOG can create non-filamentous bulking conditions by increasing water trapped in MLSS - makes biosolid disposal more difficult.
• Can increase organic loading to biological unit (decrease primary treatment efficiency) - which results in more difficulty in keeping the biological system operating in target zone.
  
  

What can be done to treat a lift station instead of surfactants?

• Utilize programs to keep grease out of the sewers - public education
• Encourage the use of grease traps with routine inspection/cleaning for restaurants & commercial buildings
• Use biological pretreatment - grease degrading microbes (like those in the wastewater plant) can initiate biological treatment in the gravity mains and lift-stations
• Mixers/aeration in the lift station can help by promoting grease biological degradation (best used with above biological treatment program)

Periodically systems with facultative or anaerobic holding ponds experience blooms of bacteria that transform the water to a pink of purple color. This color change comes from the growth of purple sulfur bacteria and/or purple non-sulfur bacteria. Both grow in the anoxic/anaerobic zone and are a product of a lagoon without high levels of mixing. 

Purple Sulfur Bacteria
The PSB is an interesting phototrophic organism that uses sunlight to power the conversion of sulfides (including hydrogen sulfide) into sulfur. The carbon source for their growth is usually carbon dioxide, but they can also assimilate carbon from organic acids or alcohols. They are obligate phototrophs, so they tend to be more common in summer months. As they remove H2S and some volatile organic acids, the PSB can be useful in lowering odor problems with lagoons.

Purple Non-Sulfur Bacteria
These are found in lagoons with lower sulfide (H2S) levels and can use organic compounds as an electron donor. They are still found in anoxic environments and use photosynthetic reactions to obtain energy.