Biological wastewater treatment plants are a mini-ecosystem with multiple interactions between billions of bacteria, protozoa, metazoa and potentially other microbial forms. At any time, the populations shift and adjust to new influents, temperatures, pH, oxygen availability, macronutrients, sunlight and really anything that enters or influences the biomass. Instead of just calling it - the bugs as if it were one single item, consider the ecological chaos of a natural environment. We just have to control the environment to get the treatment results we need. Here are some basic considerations when evaluating a biomass:

• Nutritional requirments of the microbes (Heterotrophs, Chemotrophs, Autotrophs)
• Environmental factors that impact growth (temp, pH, alkalinity, D.O.)
• Metabolism and byproducts/intermediates of metabolism
• Relationship between growth (new cells) and substrate utilization

While my above list is short, it makes for a very complex puzzle when you consider how each organisms responds to the above inputs.

Another blog topic selected from online discussion groups. This one concerns miracle products that digest sludge to a point where wasting or dredging is not necessary.

While systems vary in waste solids makeup and improved solids digestion is possible if there is a limiting factor such as:

• Insufficient mixing
• Missing micronutrients (usually in anaerobic systems with low methane production)
• Temperatures are too low (again usually related to methane production)

However, no additive can defy the laws of nature and solids will require removal. To show my logic, look at the following:

Sludge can be subdivided into categories based on time for degradation. Each system's sludge will be slightly different based on influent makeup and local conditions, but the general categories still hold.

• Readily Digested Organics - usually dead microbial cells, adsorbed isoluble organics (fats, oils, grease), cellulosics - or the difference between BOD20 and BOD5.
• Slowly Digested Organics - More complex natural polymers, longer chain hydrocarbons, and fatty acids
• Recalcitrant Organics - this includes lignins, complex poly aromatic hydrocarbons, asphaltines, chlorinated hydrocarbons
• Insoluble Inorganics - usually silt, sand, gravel

Usually we can only improve degradation rates of Readily Digested and Slowly Digested fractions. With more complex treatment (getting everything correct), it is possible to remove recalcitrant organics but this takes time and effort. And, the inorganic/insoluble fraction must be removed as it will only accumulate in the system.

So, no matter what you do - wasting or removing solids is necessary. You can only enhance digestion rates and the impact of enhancements depends upon sludge makeup.

Recent internet discussions on palm oil mill effluent (POME) wastewater have focused on pressuring mill owners to install the latest in anaerobic digesters and other advanced wastewater systems. While these systems are very effective at treating the water, they are all very capital intesive or more simply, expensive. So what type of treatment system should a palm oil processing facility or pulp/paper mill in a remote area install?

I am going to buck the trend and recommend a lagoon system for POME wastewater. This recommendation is based on the following assumptions:

• Sufficient land exists to install lagoons
• Lagoons can be constructed that only discharge water at the designed effluent point
• Periodic dredging can be done to deal with accumulated solids - inorganic and biological solids can buildup no matter how much mixing is provided
• Advanced systems require more attention, maintenance, and experienced operators. Lagoons once constructed are much more forgiving
• Prior to discharge, the water is aerated to prevent dissolved oxygen depression near the lagoon effluent

Experienced operators see gray/black water with high H2S odors, they confidently say the water is "septic". But what is "septic" water and what upstream biochemical conditions form the odorous, high oxygen demand water. 

First septic water usually gets its dark gray color from reduced sulfides (S=) binding with iron to form iron sulfide. The binding into iron sulfide is a good thing and is often used to control H2S gas in collection systems. For quick reading, I'll give the characteristics of "septic water":

Septic Water Characteristics

• If original water had sulfate - there will be H2S, FeS, and S= (depends on pH, and if ironi is present)Mer
• Soluble organic acids (acetic, propionic, butyric) - odorous products of anaerobic microbial activity
• Mercaptans - strong odor compounds formed when you have reduced sulfides and organics
• High organic acids - increase the amount of soluble BOD5 and promote filamentous bacteria growth (from initial oxygen depression at the influent & some grow very well on organic acids)
• Reduced sulfur species have a chemical oxygen demand that also lowers dissolved oxygen in the influent area
• Sulfides can thios can be very toxic to ammonia oxidizing bacteria (nitrifiers) in low concentrations

Biochemistry of Septicity

• Low ORP (redox) potentials in collection system occur as dissolved oxygen is depleted by bacteria growing on wastewater organics.
  
• If nitrate is present - usually the bacteria will move to using nitrate as an alternative oxyen source. This does not create septic water as both sulfides and organic acids do not accumulate.
  
• Once ORP drops sufficiently, microbial forms that can use oxygen from sulfate are favored and start to produce sulfides. Other microbes, without the ability to use sulfate, continue to grow slowly under fermentative respiration - which produces the organic acids. While organic acid production is vital in anaerobic digesters, in collection systems it can be an odor issue.
  

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How to Control Septicity in Collection Systems

• Control redox or ORP - add aeration, peroxides, or nitrate. Objective is to increase ORP above the value where sulfides or organic acids accumulate. Remember, inject redox control agents at points upstream of where you notice the problem.
• Increase lift-station pumping frequency - don't let water sit in non-aerated areas.
• Add aeration/mixing to equalization upstream of the treatment plant - we are not looking for a D.O. residual, we just want to keep ORP up.