With oxygen being less soluble in warmer water and higher temperatures triggering microbial growth, summer months can bring an explosion in biological activity in both collection systems and wastewater treatment plants. While microbial growth is good for reducing pollutants, it also has unwanted side effects including:

• Corrosion from hydrogen sulfide and acids
• Nuisance odors from volatile organic acids and hydrogen sulfide
• Potential worker exposure to hydrogen sulfide
• Organic acids and sulfides can trigger filamentous bulking in some activated sludge units

Anaerobic conditions (low redox potentials) trigger biological growth that creates the problem compounds. We often think of anaerobic conditions as any Dissolved Oxygen (DO) below zero. However, there is a number of electron acceptors (oxygen alternatives) that will not result in hydrogen sulfide or other odor causing microbial byproducts. Potential alternative electron acceptors include:

• Hydrogen peroxide 
• Nitrate
• Ferric iron

What we know as problem anaerobic activity occurs when microbes use sulfate as an electron acceptor releasing sulfides into solution. At the same time as sulfate is used as an electron acceptor, other bacteria begin fermentative respiration producing volatile organic acids. While volatile fatty acids are great in anaerobic digesters as methanogen food, in open wastewater plants and collection systems organic acids can cause odor complaints. 

The different electron acceptors and different "levels" of anaerobic conditions are why I always ask for pH and redox information when doing odor control projects. As pH increases, sulfides and volatile organic acids remain more soluble and stay in the water column. This is why some odor control methods include adding caustic or other base solutions. ORP or Redox Potential gives us information on what electron acceptor is being used by microbes. At a pH of 7, when we are above -150 mV ORP - you will be using non-sulfate electron acceptors and odors will be minimal. Control strategies with ORP include adding peroxide, nitrate, or other electron accepting alternative.

With all the options, I like to pick a solution that best fits the system and problem. Do a system survey to see where problem originate and then think of various options. I have written on system using lift-station aeration systems (blowers), pure oxygen injection with nano-bubbles, nitrate solutions, and even targeted hydrogen peroxide additions. All have merits and ways to improve their effectiveness. The key is to not pick a solution until you do the survey.

We have a list of "good" and "bad" microbes in wastewater plants. Filamentous bulking occurs when microbes grow into high surface area filaments preventing proper settling and solids compaction. In addition to filaments, we also have non-filamentous or Zoogleal bulking where excessive biological polymers create a gel that traps water in the floc - this creates compaction problems, floating scum, and sometimes billowing clarifier beds. As we see more MBR systems, non-filamentous bulking interferes with normal membrane operations due to pore blinding by biological polymers.

Aster Bio's Environmental Genomics (genetic testing of MLSS) has found multiple examples of "bulking" organism DNA being present when the system is not yet bulking. In these cases, the organisms are not yet at a bulking threshold or conditions such as sufficient D.O. keep the microbes in their non-filamentous form. An example of sufficient D.O. preventing filament growth happens with S. natans. While often a severe filament, S. natans also effectively removes organics from wastewater. When it grows under aerobic conditions with supporting microbes, S. natans works within the floc as single cells.

As for non-filamentous bulking, conditions that promote excessive EPS production include high soluble BOD (organic acids), low nutrients, and low F/M  or food scarcity.

Unlike microscopic or settling tests, genetic testing monitors these organisms before they exhibit problem behaviors. This is due to genetic testing being for specific genotypes vs microscopic exam where we look for phenotypes (appearance or form). 

Biological wastewater treatment usually works best in a pH range from 7.0 - 8.0. Remember that this is the "best" range in a general sense. In making bioaugmentation products, we have used strains with pH ranges from 3.0 (Thiobacillus) to 11.0 (alkanophilic Bacillus). 

The most pH sensitive process tends to be ammonia removal or nitrification. Ammonia Oxidizing Bacteria (AOB) do best at a pH of 7.2 - 8.2 where the free ammonia (NH3-N) is present but is still soluble in water.

Now for pH adjustment - most systems use strong chemistry:

• To lower pH acids such as sulfuric acid (H2SO4) can be used. A newer technology is to use carbon dioxide to adjust pH without the chance to over shoot your target. The correct choice all stems from how much adjustment is required.
• Raising pH is usually done using sodium hydroxide (caustic soda) (NaOH). As with sulfuric acid, sodium hydroxide immediately raises pH. It does not buffer the solution & buffering is a key concept that we need to consider in biological systems.

Buffering refers to how the pH tends to remain stable once adjusted. Organisms such as AOB & Nitrite Oxidizing Bacteria (NOB) like a slightly alkaline pH while also consuming significant alkalinity (usually expressed as calcium carbonate). Additional alkalinity is required to buffer against organic acids, carbon dioxides (from respiration), and other biochemical processes. This is why we often run alkalinity tests on influent during initial characterization to see if we have enough buffering at the inlet for desired biological activity. To add alkalinity we have several choices:

• Lime (calcium carbonate) - the oldest method but is highly insoluble and somewhat messy
• Magnesium hydroxide - effective but also has solubility issues
• Sodium carbonate - much more soluble, but can be "too strong" a base
• Sodium bicarbonate - soluble, tends to max out pH at 8.3 - so low overdosing potential.

So remember that you have choices in adjusting pH and buffering the system. Alkalinity or buffering capacity is a key consideration in wastewater treatment especially if you require ammonia oxidation. AOB/NOB activity consumes approximately 7.1 mg alkalinity as CaCO3 per mg ammonia fully oxidized. Other processes in the nitrogen cycle can release alkalinity back into the water.

Almost every textbook and lecture on wastewater discusses nitrification (the conversion of ammonia into nitrate) is due to the action of two separate species of bacteria. 

Nitrosomonas sp. - are a bacterial species that oxidizes ammonia into nitrite. Nitrosomonas is a common ammonia oxidizing bacteria or (AOB). Nitrosomonas is an obligate aerobe that thrives in mesophilic waters with pH from 7.5 - 8.2 as it requires ammonia (NH3-N) form for growth. Lower pH decreases nitrogen in NH3 or free ammonia form, thereby decreasing Nitrosomonas growth rates. Other AOB include Nitrosococcus  and Nitrospira.

Nitrobacter sp. - are bacteria that oxidize nitrite into nitrate (the second step mentioned in textbooks). These organisms are also called nitrite oxidizing bacteria (NOB) Other bacteria known for nitrite oxidation include Nitrobacter, Nitrococcus, Nitrospina, and Nitrospira.

Now there is another ammonia oxidation step:

Ammonia Oxidizing Archaea (AOA) - are less well known ammonia oxidizers, but are very important in marine environments and in areas not exploited by AOB. In wastewater systems, you would find AOA in systems with lower pH as many AOA can also utilize nitrogen in the NH4+ form. Researchers are still exploring the diversity of the AOA and how they impact ammonia oxidation in wastewater treatment plants.

In addition to discovery of new organisms responsible of ammonia and nitrite removal, research and system design is moving into new directions based on ecological niche and some unusual organisms. Below are the newer ideas in ammonia and nitrite removal in wastewater treatment.

Anaerobic Ammonia Oxidation (ANAMMOX) - involves the aerobic conversion of some influent ammonia into nitrate (the action of AOB & AOA). Under anaerobic conditions, an unusual group of bacteria can use NH4+ + NO2− → N2 + 2H2O. With ANAMMOX technology, oxygen requirement for ammonia & nitrite removal are lowered. However, ANAMMOX cultures are slow growing and require monitoring to ensure proper growth conditions exist.

Complete Ammonia Oxidation (COMAMMOX) - with recent genetic testing, researchers have found Nitrospira organisms that have the ability to oxidize both ammonia to nitrate - which is both steps in one organism. At Aster Bio, we have found significantly higher concentrations of Nitrospira in industrial wastewater treatment plants than any other AOB or NOB. Our molecular testing using both 16s Microbial Community Analysis and qPCR testing has found most aerobic nitrification activity is from a combination of Nitrospira and Nitrosomonas activity. 

Your wastewater treatment plant has great quality effluent. Warm temperatures during summer helps by increasing bacteria activity. Once water temperatures reach the 20 - 35 degree C range, bacterial activity increases significantly as temperatures reach the ideal range for most organisms. The warm weather also can bring problems with insect pests including red worms. Tiny midge flies lay their eggs in wastewater treatment plants. Once the eggs hatch, midge larvae live on your microbial biomass or MLSS. 

In normal waters, midge larvae feed on decaying plant materials and associated decomposing bacterial/fungi. While this helps recycle nutrients in nature, the larvae finding an abundant food source in with wastewater MLSS can cause a major loss of total solids. Faster growing heterotrophic bacteria can usually keep up with the problem. It is the slower growing AOB/NOB and PAO organisms that are most effected during a midge fly outbreak.

What can be done? The EPA has approved two options for combatting midge fly larvae.

• Bacillus thuringiensis spores (Bt) are a natural insecticide to the midge larvae. Brandnames include VectoBac and AquaBac XT.
• Strike is an approved insect growth regulator that stops larvae development. (It is similar to growth regulators used for flea control in pets).