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Biological Treatment

Written for Water & Wastewater Equipment, Treatment & Transport Show. Indiana convention center/February 18, 2016
Copy Right© By Ann Bersbach

Biological treatment involves two components: bacteria and wastewater

There are three types of bacteria:
(1) Aerobic Bacteria: survive only in the presence of oxygen
(2) Anaerobic Bacteria: survive only in the absence of oxygen
(3) Facultative Bacteria: survive in both conditions

Municipal wastewater contains two important food sources for the bacteria. These are carbon and nitrogen which are also pollutants that must be removed from the wastewater before discharge from the wastewater treatment plants.

Two aerobic bacteria will remove carbon and nitrogen according to the following two formulas:

              Heterotrophs

Carbon -----------------------> CO2 (Carbon dioxide) + H2O (Water)

                Autotrophs

Nitrogen ----------------------> NO3 (Nitrates)

Heterotrophs require 1.5 pounds of oxygen to oxidize one pound of carbon. Autotrophs require 4.6 pounds of oxygen to oxidize one pound of nitrogen. The difference between autotrophs and heterotrophs is that heterotrophs feed on organic carbon from wastewater while autotrophs feed on inorganic carbon. To make it clearer, heterotrophs absorb carbon atoms that are already linked together and use it to build its structure resembling someone whose meal was already cooked and ready to eat. Autotrophs absorb one carbon atom, (inorganic carbon), at a time and link these atoms together inside the cell to make organic carbon and use it to build its structure or, in other words, cooking a meal from scratch.

This fundamental difference makes autotrophs harder to raise and to keep in a system. Heterotrophs, on the other hand, are the most abundant bacteria in biological treatment systems and they are the police that prevent any unwanted organisms to predominate. As long as they are strong and healthy, the system runs as expected.

Carbon and nitrogen pollutants in wastewater cause the biochemical oxygen demand or BOD. Biochemical oxygen demand takes place in two stages:

The first stage is the carbonaceous oxygen demand, which takes place when heterotrophs oxidize the carbon to carbon dioxide and water.

The second stage is the nitrogenous oxygen demand, which takes place when autotrophs oxidize the nitrogen to nitrates.

Carbonaceous oxygen demand must take place first and be completed before the second stage or the nitrogenous oxygen demand takes place.

We must pay attention to this fact before we design any biological treatment system involving removal of both carbon and nitrogen. The amount of oxygen supplied and the detention time are two critical factors for successful operation.

Anaerobic treatment is considered for wastes that are hard to treat by aerobic bacteria or resist oxidation by bacteria. Also, it is used to treat sludge generated from aerobic biological processes.

Two types of anaerobic bacteria will go to work under anaerobic conditions or the absence of oxygen according to the following formulas:

              Acid formers

Carbon -----------------------> Organic acids + CO2 (Carbon dioxide)

                  Methane Fermenters

Organic acids ----------------------------> CH4 (Methane gas) + CO2 (Carbon dioxide)

The two reactions above digest the waste and the process itself is called digestion.

Let us go back to what exactly happens in the biological treatment system. Bacteria will feed on and remove anything dissolved in the wastewater. Dissolved solids will be taken by simple diffusion through the cell membrane to the inside of the bacteria cell. Any solids that are not dissolved will not be removed by this process or actively by the bacteria but will be removed later by a passive adhesion process. As previously mentioned, carbon must be removed first and it is the determining factor in how clear the water will look. If most of the carbon is removed within the appropriate stage, the heterotrophs will stop feeding and will start digesting the food and releasing their waste into the water. Their waste is a natural polymer and, as a result, any suspended solids in the waste will adhere to the bacteria cell and the cells will adhere together to form a floc. Solids will settle and the water will be clear and clean.

The above-described operation requires two stages: an aeration stage where the bacteria absorb, oxidize, and digest the carbon and release the waste; and a clarifier where the solids removed settle. This operation may take place in one tank in batch treatment systems or two tanks in continuous flow treatment systems.

Removing nitrogen is more complicated and requires more knowledge and experience to accomplish successful removal. First you must grow autotrophs, which is a challenge in itself because autotrophs require system stability. Systems suffer from upsets and solids washout will not grow autotrophs. It takes a long detention time and high sludge age to grow autotrophs. Also, they are very sensitive to the temperature and the pH of the wastewater. They would rather have it warm than cold and they like a high pH. Another important factor is the alkalinity of the wastewater. Calcium carbonate (CaCO3) or the chemical responsible for the alkalinity of the water is considered food for the autotrophs. This is the inorganic carbon food source mentioned earlier that they use to build the organic carbon inside their cells.

Special attention must be paid to the clarifiers in these systems because the nitrified solids (contain nitrates) are fluffy and do not settle well.

To incorporate removal of both carbon and nitrogen in one system the following important factors must be considered:

(1) The detention time in the aeration tank must be 8 hours or more.
(2) The oxygen level must never drop below 1 mg/L in the nitrification stage.
(3) Sludge age 16 days or older.
(4) Calcium Carbonate must be 50 mg/L or higher.
(5) Pay attention to pH. Compensate for temperature drops with longer detention time.
(6) Do not remove any solids from the wastewater ahead from the aeration tank (no primaries).

Nutrients

Nutrients play important role in biological treatment. Two major nutrients are nitrogen and phosphorus. Other nutrients include calcium and magnesium. These nutrients help the bacteria to transfer the food from the wastewater through their cell membrane to the inside of the cell. If nutrients are deficient in wastewater bacteria canít eat and the pollutants are not removed. A condition known as bulking takes place when nutrients are low in the water being treatment. In bulking the solids stay suspended in the water and continuously are lost with the effluent which causes more bulking. To correct nutrient deficiency spent phosphoric acid and aqueous ammonia may be added to the wastewater. Some plants add dog food on regular bases to their aeration tanks to improve solids settlability in the clarifiers. Kraus control is a procedure used to solve bulking problems by adding anaerobically digest sludge to the aeration tank. It provides inert heavy solids that take the suspended solids down while settling. Digested sludge also provides nitrogen and phosphorus which are two important nutrients that improve solids settleability. Supernatant from anaerobic digesters also has been used to treat bulking problems. It is rich in ammonia, phosphorus and magnesium. However, adding large quantities of supernatant must be avoided because ammonia is toxic to the bacteria.

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