When dissolved oxygen drops and the water takes on a gray color, everyone uses the word "septicity" to describe the situation. Today I want to review what septicity actually is and how to describe it by more than a color which is hard to tell in darker influents.

Under anaerobic conditions - redox potential less than -200 mV is a good starting point - bacteria begin to use organic compounds as the final electron acceptor. This biochemical process called fermentation results in the production of short chain fatty acids. In the context of an anaerobic digester, this is a vital step to producing methane. In a collection system, it can be a source of volatile organic acid odors. If you have sulfate present at low ORP, sulfur reducing bacteria (SRB) begin to form sulfides which at lower pH become hydrogen sulfide gas. The sulfides combining with iron in the water produce iron sulfides which give septic water the dark gray color.

So here is what we have in a true septic influent:

• Low ORP (redox potential) - usually < -200 mV
• Sulfides & iron sulfide 
  
• Organic acids - primarily acetic, propionic, & butyric acids
• Microscopic exam will have spirochete form bacteria that move by a "corkscrew like propulsion" - these organisms are favored by organic acids and low D.O.
• With long run septicity, you may see increased low DO and organic acid filaments
  

What can be done to prevent septicity (long run not short term)

• Use upstream collection system programs to prevent anaerobic zones (boost redox)
• Increase aeration near the influent to account for higher oxygen demand
• Add mixing/aeration to equalization tanks upstream of WWTP
  

Grease builds up in lift stations, gravity mains, and headworks. In severe cases, you may be tempted to use d-limonene or a solvent products to remove hard grease deposits. While these solvents work very well to mobilize grease, they do not actually remove the grease - it just passes into the water phase as a grease/water emulsion. This emulsion is the problem. 

After using the solvent product to clean the lift-station, the grease moves through the force mains down to the wastewater treatment plant. Suddenly, WWTP operators receive a slug loading of grease/long chain fatty acids. Bacteria in the plant can degrade grease, but it takes time. Let's detail why:

1. Grease even in an emulsion does not immediately cross the bacterial cell wall for metabolism. Exocellular enzymes must first cut the fatty acids into smaller units that can cross the cell wall.
2. Not all bacteria can degrade long chain fatty acids - the most common pathway in wastewater is via Beta-Oxidation which shortens the fatty acids by 2 carbons each pass through the metabolic "wheel".
3. Slug loads of grease attach to the MLSS or biosolids via adsorption. This just means that small grease chunks are stuck in the biomass EPS. This makes the sludge less dense - which can cause floating MLSS and scum on the clarifiers.
4. If the grease continues to enter the system, you will favor hydrophobic organisms that attach to grease particles. This means Nocardia forms. They thrive on long chain fatty acids but are slow growing.

If solvent products are bad, what can you do to remove grease in collection systems? First, never let the problem get so severe that solvents are required. I have worked for over 20 years with various treatments that use organisms with grease degradation pathways dosed upstream of problem grease areas. This is a maintenance rather than instant solution. With biological treatment, the grease deposits tend to be lighter and easily removed with water jets. This is because the bacteria have started the degradation process and the grease/fatty acids are becoming more water soluble and shorter in length. This means less chance of upset down stream in the WWTP from either grease slugs or Nocardia.​

Moving Bed Biofilm Reactor (MBBR) media has potential to expand wastewater treatment plant capacity, but as with any technology there are some tradeoffs compared to conventional suspended growth systems. Usually constructed of HDPE (polyethylene) with a density near that of water - 0.95 g/cm3. The HDPE is extruded into high surface area shapes which provide a surface for bacterial biofilm development. Biofilm treatment systems go back as far as trickling filters - it is just plastic gives us more options for supporting the biofilm. The MBBR media is kept in the aeration basin by screens with excess biosolids soughing off during movement.

MBBR Benefits

• Allows higher levels of biological solids than suspended growth activated sludge systems. Meaning that you can run high MCRT & lower F/M for a given aeration basin/secondary clarifier.
• Lower F/M means less solids production - older biomass but this does require sufficient D.O. to prevent filamentous bacteria development.
• Helps if your clarifiers are not able to support high MLSS due to hydraulic constraints.

So why don't we see every system converted to MBBR technology? There are a few challenges:

• Media can be expensive
• Heavy FOG (grease) loadings can foul the media
• Running further into the growth curve - lower F/M - requires oxygen normally seen in aerobic digesters. Do you have the aeration capacity for more biomass?

Almost every wastewater plant runs the SV30 test. Running the test is great and coupled with MLSS/MLVSS numbers gives the Sludge Volume Index (SVI) which converts settled volume to a more standardized number based on MLSS in grams. The test can also give you other valuable information. Make a few observations over 30 minutes and you can see:

• Information of where you are on the growth curve
  Look at how fast the MLSS settles. Is the supernatant turbid? Are there "fines" floating on top? This can be related to biomass growth curve .
  
• If allowed to sit over 12 - 20 hours - denitrification potential
  When allowed to sit after the 30 minute test. If the sample has nitrate present and denitrifying cultures, you will see small bubbles and floating sludge. If you see the same action in the secondary clarifiers, you may want to lower bed depth or solids residence time in the clarifier.
  Bulking - filamentous vs non-filamentous
• SV30 shows bulking but it can also give you an idea of what type. Filaments make for a rough water/MLSS layer. Non-filamentous bulking has a more "gel" appearance when settling. 
  
• Toxic shock loadings
  Acute toxicity hits settling tests earlier than most other indicator tests. If you see turbidity caused by deflocculation,  you need to start looking for spills that upset the EPS production and create conditions where cells enter lag/log phase growth.
  
• Hints on adjusting wasting rates
  Sludge that settles too fast or leaves turbid supernatant can benefit from adjusting wasting rates. Confirm hints from SV30 with other tests and always remember to waste sludge. It does cost $ to keep solids in the wastewater system.