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What is septicity in wastewater?

3/31/2015

Spirochete from ucmp.berkeley.edu

We often hear that wastewater has septicity when the water has black color, high sulfides, odors, and spirochetes/spirillum when viewed under the microscope. Today I want to define the conditions that cause septicity and how it can/should be addressed.

Conditions to create septicity:

Soluble organics (BOD5)
Depleted free oxygen and nitrate - pushing to highly negative redox conditions
Sulfate - is used as an electron acceptor under anaerobic conditions producing H2S
Soluble organics become volatile organic acids that cause odors

Problems with septicity in collection systems

Dangerous H2S levels
Odors
Corrosion from H2S and sulfide oxidation
Filament blooms from sulfides and short chain organic acids in wastewater plant influent

Controlling septicity

Add aeration/mixing in EQ tanks
Reduce primary clarifier residence time
Discharge lifts-station more frequently (reduce collection system residence time)
Pre-aeration before the treatment system
Add nitrate to the collection system to increase redox and prevent true anaerobic activity

Sulfur oxidizing microbes and H2S removal

3/23/2015

Reduced Sulfur is converted back to sulfur and sulfate by microbial action. The most common bacteria found in this process are:

Photosynthetic Sulfur Oxidizers - Purple sulfur bacteria & green sulfur bacteria
Obtain energy for growth from the following reaction:

H2S --> So + 2H+ + 2e-.

Both purple and green sulfur organisms require substantial quantities of reduced sulfur (sulfide), anoxic/anaerobic water conditions (that generate sulfides), and sunlight exposure to be a substantial portion of the biomass. You can find photosynthetic sulfur oxidizers in lagoons systems such as sugar mills and paper mills during long daylight hour seasons. In cases the bacteria can even turn the water pink or purple. One problem with these organisms in wastewater is their tendency to float causing high effluent solids.

Chemolithotrophic Sulfur Oxidizers

Two main groups

Thiobacillus & Starkeya sp. – obtain energy by the following

H2S + 1/2O2 --> So + H2O + energy
So + 1½ O2 + H2O --> SO4 + 2H+ + energy

Of notes is the production of sulfuric acid as the final product of Thiobacilllus metabolism. As a group the Thiobacillus are found from pH near neutral down to as low as pH 2. Some of the lower pH Thiobacillus are partially responsible for acid mine waters found near mine tailing piles.

While most Thiobacillus and Starkeya are obligate aerobic microbes, one described species Thiobacillus denitrificans uses nitrate instead of oxygen.

Paracoccus denitrificans and Thiosphaera Pantotropha

Among the most

unique sulfur oxidizing microbes are Paracoccus denitrificans and Thiosphera pantotropha. These microbes have the ability to convert reduced sulfur into oxidized forms with the same chemical reactions as Thiobacillus. They also have the ability to grow using chemotrophic metabolism on odorous short chain volatile organics. This includes odor causing acetic, propionic and butyric acids.

Sulfur Cycle including Hydrogen Sulfide (H2S)

3/21/2015

From textbookofbacteriology.net

In waste treatment we often only talk about sulfur when we have odor and corrosion problems related to hydrogen sulfide or H2S. While sulfur can be a problematic compound, it is also a vital micronutrient for life and is cycled throughout the environment by biological processes. An understanding of these processes can help waste treatment operators keep sulfur in its most “benign” forms and manage to prevent both excessive odors and corrosion.

Key Points

Sulfur is used in every cell as a micronutrient.

Sulfate is used as an alternative electron acceptor by certain species of bacteria (ex. Desulvibrio sp.) under reducing environment which produces sulfides an H2S (pH dependent as to which forms are present).
Sulfate reducing microbe populations are significant when dissolved oxygen is depleted, there are simple organics and the waste stream contains sulfate. We know sulfate reducing bacteria are favored under low redox < -150 mV @ pH 7.0.
Nitrate, if present, yield more energy if used as an electron acceptor (different bacteria) which means that by adding nitrate we can have faster growing cultures degrade available organics preventing the formation of reduced sulfur (S=, H2S).
Reduced Sulfur is converted back to sulfur and sulfate by microbial action.

As I often field requests on microbes that convert reduced sulfur and H2S back into harmless forms, the next post will contain a summary of these microbes and their metabolic capabilities.

Tracer studies and their importance

3/12/2015

A tracer study gives vital information on system hydraulic residence time (HRT), flow & mixing patterns, and information on potential short-circuiting. They are useful in a wide variety of applications including aerated lagoons, polishing ponds, and storm water treatment systems. While most people only focus on the discovery of HRT, some of the most vital treatment information comes from information on mixing and potential short-circuiting. In most cases, the basin volume is a “fixed” condition which is not going to change in the short-term. What we can change based on tracer study information is aeration/mixer placement and prove if curtains are needed or effective in their current placement.

A tracer study is done using a “conservative” material that passes through the system without significant adsorption or degradation. In most cases we use a lithium chloride salt or fluorescent dye. The presence and concentration of which can be readily measured using accurate lab procedures. Dosing is based upon generating a target initial concentration and them measuring concentrations at sampling points at set times for two theoretical residence times.

The sample data is then plotted to determine:

1.       Time at first appearance
2.       Peak concentration time
3.       Peak concentration magnitude (amount of tracer)
4.       Decrease in tracer concentration (time & pattern)

Three Environmental Conditions & How They Apply to Nutrient Removal

3/3/2015

What is septicity in wastewater?

Sulfur oxidizing microbes and H2S removal

Sulfur Cycle including Hydrogen Sulfide (H2S)

Tracer studies and their importance

Three Environmental Conditions & How They Apply to Nutrient Removal

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