Collection systems with long residence will have anaerobic zones. Under highly negative redox conditions (no oxygen or nitrate/nitrite present), microorganisms that can utilize sulfate and organics as electron acceptors begin to grow on influent organics. We call these organisms sulfate reducing bacteria (SRB) and fermentative bacteria. The SRB are responsible for generation of sulfides and H2S, while the fermentative bacteria produce short chain volatile fatty acids. Both cause odors with the sulfide smelling of rotten eggs and the volatile fatty acids having sharp, rancid odors. I will list the most common control methods used in collection systems.

• Increase pH to neutralize acids and increase sulfide solubility in the water phase.
• Using iron (FeCl) to bind with sulfides
• Adjust redox upward (nitrate, hydrogen peroxides, ozone, oxygen diffusers)
• Sulfide scavengers (such as amine solutions)
• Odor masking agents

Each option has areas where they are most effective. Often it is best to combine technologies to achieve best odor control at the lowest cost. Before undertaking wide scale odor control, you should sample lift stations and lines to isolate the problem areas (use ORP meter), H2S monitors, and finally organic acid analysis. Once the problem areas are mapped, you can evaluate which technology best suits the area.

Redox potential or ORP is a measure of the tendency of a chemical species to acquire electrons and thereby be reduced. In wastewater biology, redox potential's importance concerns electron donors (often organic compounds measured as BOD5) and electron acceptors. This movement of electrons generates the energy used by the organisms. The redox potential in the water is directly related to the available electron acceptors. The most common wastewater electron acceptors are listed below:

• Oxygen - when oxygen is the electron acceptor we have aerobic conditions. At neutral pH, this is represented by ORP > 0. Aerobic metabolism provides more energy for microbial growth and reproduction.
• Nitrate/nitrite - many organisms have the ability to use nitrate/nitrite as an alternative electron acceptor when dissolved oxygen is not present (although some will continue to use nitrate/nitrite even under aerobic conditions). Nitrate/nitrite as a terminal electron acceptor provides more energy than acceptors lower on the "electron tower". 
• Sulfate - as redox potentials drop below -125 mV at pH of 7, microbes capable of utilizing sulfate as a terminal electron acceptor start to appear. The sulfate is converted to reduced S= and increasingly to H2S as pH drops. Energy from sulfate reduction is lower than for Oxygen, Nitrate, and Iron.
• Organics - when the electron acceptor is an organic compound, you have fermentative respiration. This is the first step in anaerobic digester microbial processes, but also occurs in anoxic systems with low redox potential. Fermentative respiration produces short chain fatty acids (also odorous) including acetic, butyric, and propionic acids.
• Methane production happens when organic acids and hydrogen are used by archaea organisms into methane. In this case, the low redox potential <-400 mV, allows for energy production by combining CO2, H+, and COOH (organic acids) with methane being the final product.

I am a big proponent of using quick microscopic exams in wastewater treatment systems. The basic exam includes the following:

• Floc size & density
• Free bacteria cells in solution (at 400x magnification this looks like small rods or circles bouncing around)
• If septicity is a common problem note the number of spirochetes
• Protozoa - note numbers of amoeba, flagellates, free swimming ciliates, crawling ciliates, and stalk ciliates
• Metazoa - any rotifers or worms should be noted

Once you have baseline microscopic information for light, normal and heavy loadings; microscopic exams become much more powerful for influent quality diagnostics. Within hours of a heavy or shock loading event, you will see an increase in free bacteria (this means you have moved back on the growth curve to log phase growth). Flagellates that feed on the free bacteria often increase shortly after the free bacteria increase. If the loading is depressing D.O. or toxic to organisms, you will lose the stalk and some of the free swimming ciliates. Also, rotifers tend to encyst or disappear if the loading is longer term.

In the SV30 test, an high loading increases turbidity before anything else happens. Turbidity is due to free bacterial cells in the solution.

As a wastewater treatment system "matures" (decrease in soluble BOD5), dissolved oxygen concentrations eventually rise to levels that can support multicellular lifeforms. Unlike bacteria (prokaryotes) and protozoa (eukaryotes) which are all single cellular organisms, metazoa are more complex organisms with differentiated cells. These organisms feed on microbial floc and protozoa present. While seen with excellent water quality, an abundance of metazoa indicates an older sludge. The biggest problem with older sludges (very low F/M) is the increase in effluent turbidity and pin floc carryover in the secondary clarifiers. Common wastewater metazoa include:

• Rotifers
• Nematodes (worms)
• Bristle worms
• Tardigrades (among the most complex metazoa in wastewater)

The SV30 is possibly the easiest test done by wastewater treatment system operators. Fill a wide cylinder or settleometer with aeration basin MLSS and allow to settle for 30 minutes. This test is designed to give you settling rates for the MLSS and an estimate of how well secondary clarification will work. If all things are in good shape, just reading the 30 minute number is enough. However if you have problems with filamentous or non-filamentous bulking, secondary clarifiers have become undersized (either by hydraulic or solids carrying rate), denitrification, or have problems with pin-floc, fines, or turbidity - you should do more than just read the number after 30 minutes. Here is how to take advantage of settling tests using SV30 equipment.

• Monitor settling rate every 5 minutes during the SV30 test. You want the MLSS to settle but not too fast. Old sludge rapidly drops, but leaves turbidity and pin floc in the supernatant. Younger sludge settles more slowly, yet captures more of the small solids that contribute to pin floc or turbidity.
• Don't dump the sample after 30 minutes. Allow to settle for several hours and monitor compaction (secondary clarifiers run in the hours not minutes. If after time you see floating sludge and small bubbles, you can have denitrification. If a secondary clarifier is denitrifying, you can find the maximum solids inventory time before you start to see problems.
• If the MLSS does not compact well, you can confirm filaments or non-filamentous bulking with a microscopic exam. The amount of compaction can be correlated to what you see under the microscope which is less subjective that other filament abundance estimate methods.
• Rapid deflocculation - mainly an industrial WW phenomenon - spills happen in industrial WW. The first sign of problems is an increase in turbidity, free-bacteria in solution, and floating solids. The SV30 test can help detect spills - just look at supernatant turbidity and floating solids.
• Filamentous bulking often makes a rough layer on top of the MLSS line in the SV30 test. If you have rough MLSS with a steel-wool or mesh appearance, you have filamentous organisms extending from the floc.
• MLSS color - color helps indicate sludge age and microbial makeup. Note the MLSS color!

Above are the most common SV30 test extensions that I use. Do you have any other observations that relate to effluent quality or clarifier performance? If so, please comment.

Molecular testing includes qPCR or quantitative PCR which our lab at Aster Bio is developing into a powerful wastewater monitoring tool. After discovering organisms of interest using metagenomic testing or system surveys, custom test kits make qPCR a rapid, quantitive test for target microbial populations. What can be monitored with qPCR?

• AOB/NOB (Nitrifier) populations - any changes can impact these slower growing microbes, qPCR tracking is more sensitive and accurate than any other nitrifier monitoring technology.
• Filamentous organisms - after identifying the most problematic filaments, a custom qPCR test can monitor MLSS with accuracy and objectivity that is difficult with current microscopic methods.
• Sulfur reducing and oxidizing microbes - SRB and SOX microbes are important in both collection and wastewater treatment systems. Instead of relying on plate counts or corrosion/odors, qPCR monitors the population even at extremely low concentrations. Thereby, operators can correct the problems before problems arise.
• Phosphate accumulating organisms - a key microbial group for biological phosphate removal, PAO bacteria can be monitored to improve operational controls.
• Other - any key organism or group of organisms can be monitored by building a custom qPCR test.