In many areas, wastewater and stormwater go through facultative or moderately aerated lagoon systems prior to discharge. These lagoons remove residual oragnics, nutrients, and solids that can pass through the upstream treatment. In the case of stormwater, the lagoons remove nutrients from fertilizer runoff and hydrocarbons from parking lots/roads. Unlike modern wastewater treatment sytems, the lagoons have low levels of biomass (MLSS) that can range from 30 - 100 mg/L volatile solids. The solids component consists of:

• Residual TSS from wastewater treatment - both living organisms and non-living components
• Facultative bacteria that thrive in the low F/M environment (we actually see AOB thriving in ponds with high sludge levels during summer months)
• Algae - both eukaryotic and cyanobacteria. Often excess phosphorus can trigger a cyanobacteria bloom causing problems with TSS, odors, and pH.

The reason for ammonia appearing and AOB growing during summer months is a phenomenon related to sludge degradation on the pond bottom. While the sludge layer degrades all year, the rate of anaerobic lysis increases with summer temperatures. The pond bottom remains anaerobic/anoxic producing organic acids, methane, and nutrients (N, P). The upper layers have oxygen present from algae photosynthesis, natural diffusion from the surface, and action of mixers/aerators if present. In ponds with more sludge, the problems related to ammonia release and algae blooms tend to cause more problems. So what can be done to control this other than a big mechanical dredging project?

• Add mixers for low velocity, high volume movement. The mixer circulates water from anerobic zone to the upper aerobic regions, allowing for more effective sludge stabilization/nutrient removal. We are also seeing benefits from adding new micro & nanobubble aeration technologies here too.
• If mixers are not enough to control the problem, you can then use algae control methods inclusing sonic systems, copper sulfate, and biological algae control methods.
• If sludge layer is visible at the surface and has produced "islands" - then it is time to evaluate dredging or various biodgredging technologies. If the sluge has a high organic fraction, then biodredging may prove less costly and effective.

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.

On paper denitrification should be easy. All you need is a "food" source, common facultative heterotrophic bacteria, and anoxic conditions (no free D.O.). However, in practice you need to have a balance of conditions to ensure denitrification. If you are having problems maintaining denitrification, it helps to look at the process from an ecological perspective. 

What is required for bacteria to use nitrate/nitrite as electron acceptor (AKA alternative oxygen source)

• Many bacteria can use nitrate but not all can use nitrite. Your goal is to develop cultures that an convert all NOx into N2 gas. This process actually h as multiple steps that include 4 enzyme pathways.
  
  
• Denitrification happens with many substrates (electron donors) - this includes soluble organics (sugars, alcohols, starches) but can also include chemoautotrophic organisms such as sulfur and metal oxidizing bacteria. This last group is being researched as Autotrophic Denitrification (AuDen). However, most systems denitrify best with typical wastewater heterotrophic bacteria including Thauera, Zooglea, & Pseudomonas.
  
  
• Anoxic conditions are necessary to trigger denitrification. When D.O. is present, the bacteria obtain more energy using oxygen as an electron acceptor. Inside dense floc, biofilms, or aerobic granular sludge can have anoxic zones that denitrify inside aerobic basin. This is part of the appeal of MBBR and aerobic granular sludge systems. You do not have to manage the anoxic zone redox potentials (ORP) or residence time as closely as you would in conventional aerobic activated sludge.
  
  
• Always consider residence time, temperature, and availability of suitable electron donor (food) for the bacteria in the denitrification zone. Failure in any variable can be a problem.  Denitrification to N2 gas often takes action by several different organisms (although a few can take it to N2 gas inside a single cell).

Ammonia Oxidizing Bacteria (AOB) and Nitrite Oxidizing Bacteria (NOB) are often grouped into what we call nitrifiers.  These organisms grow by converting ammonia into nitrite and finally nitrate.  Organisms working in the nitrogen cycle are diverse and include the following:

• Heterotrophs that degrade organic nitrogen compounds (proteins, amines, etc) into ammonia. They also use nitrogen to build proteins used in cellular activities and reproduciton. Under anoxic conditions, many heterotrophs use nitrate and nitrite as alternative electron acceptors converting the NO3/NO2 into N2 gas.
• Chemoautotrophs - this includes AOB/NOB, COMAMMOX (funtions as both AOB & NOB in one organism), ANAMMOX (under anoxic conditions takes NO2 + NH4+ directly to N2 gas)
• Heterotrophic ammonia oxidation (slower than obligate AOB/NOB but does take place).

So given such diversity, toxicity screens become a complex undertaking.  So, here is what we are doing (while the COVID19 virus has travel and field work at minimum levels).

• Sampling diverse WW systems to see makeup of their nitrifying populations using Mirobial Community Analysis (MCA)
• Using qPCR tests created from MCA data for individual systems to develop baseline data for operating wastewater treatment plants - this involves desigining and refining oligos used in qPCR tests.
• Testing suspected compounds using concentrated AOB cultures for acute toxicity using Tox-N testing. In this we spike buffered water with suspect compounds and add AOB. We compare ammonia oxidation rates versus a control to get % inhibition. 
• More advanced simulation in pilot activated sludge units where we use a healthy MLSS and subject it to influents spiked with compounds of interest. We then monitor ammonia, nitrite, nitrate, and use molecular testing to see drifts in AOB/NOB populations.