This one is more for industrial wastewater treatment systems, but the information could pertain to other biodegradable compounds that have potential to upset a biological treatment system.

Phenol shocks are one of the most common sources of upset in industrial wastewater treatment systems. While phenol is biodegradable, it was also the first antiseptic used in the 19th century. In industrial wastewaters, spikes in phenol often cause problems maintaining ammonia oxidation and in larger spikes can cause deflocculation. Among the marketed solutions are the use of oxidant pretreatment step and adding activated carbon to adsorb phenol. Another solution is to use equalization tanks to store the high phenol wastewater and then meter the strong wastewater back into the biological unit. Often the limiting factor for phenol treatment is the high oxygen demand for aerobic decompostion of phenol. Otherwise, bacteria including Pseudomonas and Rhodococcus species are very tolerant of high phenol concentrations and able to completely degrade phenol as a sole carbon source.

So if you have high phenol or other quasi-toxid organic released to your treatment system, here is the best protocol for protecting the biological treatment unit.

• Divert the high strength water into storage or EQ tanks.
• Ensure both N & P in the biological treatment unit is sufficient to maintain C:N:P ratios.
• D.O. needs to be maintained above 2.0 mg/L to ensure needed biological activity. I have also used hydrogen peroxide metertedin the system to boost dissolved oxygen during high loading events. Bacterial catalase releases oxygen from peroxide. Be careful to not overdose peroxide which can damage biomass.
• If spill has reached the system or not all the wastewater can be diverted, you can use powdered activated carbon to adsorb phenol.
• If biomass has been damaged and fast recovery is important, adding bioaugmentation cultures can help speed up recovery and blunt the impact of high strength influent. Just make sure the cultures that you are adding are selected for the specific waste compounds. For example, what works on cellulose/starches is probably not the culture for phenol or solvents.
• Monitor closely - this is extremely important. Use microscopic exam, oxygen uptake rates, ATP (if done), effluent turbidity/TSS, and ammonia residuals. If you see warning signs of moving backwards on the growth curve (out of decline phase growth), then stop forward flow from the storage tanks.

Wastewater plants would be much easier to operate if we just didn't have to worry about effluent ammonia or nitrite. However, ammonia and nitrite are both major pollutants and need to be removed before discharge. But why when compared to other wastewater bacteria are the nitrifiers (AOB/NOB) so hard to keep active in wastewater?  Instead of going system specific ranges for growth, I want to give a overhead view of why AOB & NOB are so slow to grow, easy to upset, and difficult to bring back once lost.

1. Nitrifiers are slow growers
   Compared to most wastewater heterotrophs, nitrifiers are slow growers. Even in the best conditions, AOB take up to 12 hours for cell division. Remember that many common heterotrophs can divide every 30 - 60 minutes.
2. Nitrifiers obtain less energy from their metabolism
   A fancy way to say that both AOB & NOB don't get as much food from oxidizing ammonia and nitrite when compared to the BOD (organic) degrading microbes.
3. Requirements for abundant oxygen 
   Many wastewater microbes can thrive at D.O. residuals below 1 mg/L. But oxidizing ammonia into nitrate requires a lot of oxygen. Therefore, AOB & NOB do best when D.O. is abundant. Depending upon your MLSS and EPS levels, this can be a D.O. residual between 2 - 4 mg/L  (it all depends upon the system).
4. Easy to inhibit or kill
   Nitrifiers are inhibited by many common compounds. While more of a problem in industrial systems, nitrifiers are easily damaged by amines, solvents, sulfides, and phenol. Even high organic loadings lower D.O. which results in inhibition.
5. Like a narrow range of environments for growth
   Growth is best in pH between 7.5 - 8.2. Temperatures 20 - 38 Deg C. Outside this range, growth rates decrease to critical levels.

Given all the challenges for maintaining ammonia and nitrite oxidation, how can you make sure that the nitrifiers are well cultivated?

1. Check your D.O. meters often. Remember D.O. residuals at the probe are not the same as seen by flocculated bacteria.
2. Is your sludge age long enough to prevent nitrifier washout. If sludge age < reproduction time, the nitrifer populations will decrease eventually causing effluent ammonia and nitrite breakthrough.
3. Do you have acute toxicity (easier to find) or chronic toxicity (requires more detective work). 
4. If ammonia and nitrite are a consistent issue, we can improve monitoring of the AOB/NOB populations using a combination of molecular (DNA) testing. MCA finds which nitrifer organisms are present. Once we know what is there, we can use qPCR (a fast, quantitative test) to track your system specific nitrifier populations. This way, you can take corretive steps before effluent problems arise.

Highly biodegradable organics are easily consumed by wastewater treatment plant bacteria, but can still cause problems maintaining needed effluent quality. If you have high influent soluble BOD, often organic acids produced by fermentative respiration in collection systems, the system will have a tendency to expeience filamentous bacteria blooms and associated high effluent TSS. So what about having an readily biodegradable influent causes filaments to increase?

Here is what happens to a biomass when the equivalent of microbial candy is introduced to a system:

• When floc which is low F/M comes in contact with high soluble organic influent, bacteria immediately enter log growth on the floc surface
• The  high respiration seen in log growth results in low DO in this area of the treatment plant. With respect to aerobic microbes, low DO favors those with high surfaca area. And, filaments have greater surface area than typical floc formers.
• Other organisms react to the abundant soluble BOD by trapping organics in extracellular polymeric materials (EPS). The "food" will be used later when F/M drops by the microbe. If you form EPS to excess, you call it non-filamentous or Zoogleal bulking.
• Another issue with high soluble organics relates to potential unbalanced C:N:P ratios. With low N or P, bacteria with better N & P scavaging strategies grow faster than other microbes. As with low DO, filamentous bacteria are often favored by low C:N:P ratios.

Control options for systems with high soluble influent organics:

• Step-feed influent - this is simply changing from a single influent to multiple influent points along the basin. This works for plug flow or mulitple complete mix systems. Using step-feed, you reduce the DO depression and excess organics seen in single point influent systems. Again this is not effective if you have a single complete mix basin.
• Selectors - this is an anoxic or anaerobic basin where influent and return MLSS mix. If sized approprately, the selector allows for uptake of soluble organics by floc forming organisms (includes the EPS Zoooglea) to the exclusion of obligate aerobic filamentous bacteria. Again residence time and conditions in the selector make for a fine balance between preventing filaments to encouraging their growth.
• Increasing oxygen levels near the influent. The most obvious solution to low DO caused by soluble organics is to increase DO by adding more aeration. If you cannot move more aeration to the influent zone, some systems can find a solution using conentrate or liquid oxygen addition. I have even used hydrogen peroxide feed to increase DO in a activated sludge plant (not to chemically oxidize organics).