What is Biological wastewater treatment?

Biological wastewater treatment

The biological treatment of wastewater is performed through a series of important processes that have in common the use of microorganisms (including bacteria note) to effect removal of soluble components in water. These processes take advantage of the ability of microorganisms to assimilate organic matter and nutrients (nitrogen and phosphorus) dissolved in wastewater for their own growth. When they reproduce, they aggregate together and form macroscopic flakes with enough critical mass to settle out in a reasonable time.

The traditional application consists in the elimination of biodegradable organic matter, whether it is soluble or colloidal, as well as the elimination of compounds containing nitrogen and phosphorus. It is one of the most widespread treatments, not only in the case of urban wastewater but also in large part of industrial water, due to its simplicity and low economic cost of operation.

In most cases, organic matter is the source of carbon energy needed by microorganisms for their growth. It is also necessary to count on the presence of nutrients contained by the elements essential for growth, particularly nitrogen and phosphorus, and finally, in the case of aerobic systems, the presence of dissolved oxygen in the water. Oxygen is not essential, because microorganisms are capable of degrading organic matter also under anaerobic conditions. This will be essential when choosing the most suitable biological process. In cellular metabolism, the final electron acceptor plays a fundamental role in the processes of oxidation of organic matter. This aspect also influences the possibilities of application to wastewater treatment. Concerning what is called, the final electron acceptor, we distinguish three cases:

- Aerobic systems

oxygen is the final electron acceptor preferred by all cells. If there is oxygen in the environment, this will be the final electron acceptor, which implies obtaining high energy yields and a large production of vases, because of the significant growth of bacteria under aerobic conditions.

- Anaerobic systems

In this case, the final electron acceptor is the organic matter itself, which acts as a source of carbon. As a result of this metabolism, most of the carbon is intended for the formation of growth byproducts (biogas, i.e., CO2 and methane). While the fraction of carbon is used for cell synthesis is low. In terms of treatment, this has two advantages i.e., a small amount of sludge occurs at the same time as biogas is produced, which can be exploited. It is generally used to produce electrical energy, which is consumed by the installation itself.

- Anoxic systems

This is what we call systems whose final electron acceptor is neither oxygen nor organic matter. Under anoxic conditions, the final electron acceptor is generally nitrogen, sulfate, hydrogen, etc. When the final electron acceptor is a nitrate, as a result of the metabolic process, the nitrogen in the nitrate molecule is transformed into nitrogen gas. Thus, this metabolism allows the biological elimination of nitrogen from the wastewater.

The biological wastewater treatment process

The use of pure oxygen with an appropriate design makes it possible to work very efficiently. All companies working with biological water treatment can benefit from the use of pure oxygen. Its use has several advantages:

- Energy efficiency due to the mass transfer of pure oxygen (the active molecule), with less consumption in kWh for the transfer of oxygen necessary for bacteria.

- Optimal settling: a compact floc allows good settling and increases the dewatering capacity of the sludge.

- Few noise emissions, among other things, due to the immersion of the Ventoxol injection equipment.

- Low emission of volatile organic components due to the absence of nitrogen from the air injected into the basins. The odors are almost nonexistent.

- Absence of foaming because pure oxygen, unlike air, does not contain nitrogen, which generally participates in the formation of this foam.

- Pure oxygen avoids reducing media and thus prevents corrosion, especially in concrete structures.

- An existing air treatment plant can be "doped" to increase its nominal capacity by 50% by converting it to pure oxygen.

Advantages of each system according to criteria

Taking all these aspects into account, there is a wide variety of operating modes, depending on the characteristics of the water, as well as the organic load to be treated. Whether it is better to choose an aerobic process or if an anaerobic process would be more profitable, are the concentration of organic matter to be eliminated. It is necessary to eliminate nitrogen, the availability of physical space, and the relationship between the OPEX and CAPEX of the project. In the following table, we can observe how, based on certain criteria, what type of process (aerobic or anaerobic) is best?

Aerobic Anaerobic

Organic matter concentration DQO <3000 mg / L DQO <3000 mg / L

Space required Very large Small

Nutrient removal Possible impossible

CAPEX (investment costs) Low High

OPEX (operating costs) High Low

On the other hand, the biomass can grow freely, in suspension inside the bioreactor, or by adhering to support (fixed biomass). In the conventional process, it grows in suspension, as in the case of sequential reactors (SBR) and in biomembrane reactors (MBR). In bio-disc, biofilter, percolator, or moving bed (MBBR) reactors, the biomass grows in adhering to the surface of plastic support or with sand. If the biomass relies on the suspension or fixed to a support, it entails a series of practical consequences that should be taken into account when selecting which technology is the most appropriate. They are summarized in the following table:

Fixed biomass Biomass in suspension

Space required Small Tall

CAPEX (investment costs) High Low

OPEX (operating costs) Low High

Nutrient removal Low High

Flexible operation Medium-low High

Response to toxic and / or inhibitors Medium-low High

Consequently, the selection of the most appropriate biological process can be made after analyzing the characteristics of the effluent, the type of industrial process that generates it, the degree of purification required, and the general needs of the client. 

The charcoal filters adsorb, and serves as a support for the bio particle, in addition to feeding the microorganisms with minerals and trace elements. On the other hand, the adsorption process makes a double contribution to the process by stratifying the loading peaks of contaminants. This way, the dwelling time of the contaminants inside the reactor increasesBiological wastewater treatment, which makes possible the degradation of organic compounds. 

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Jun 16, 2020

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