Biological wastewater treatment

Date: 15/01/20 | In: wastewater management

There are typically three types of wastewater. ‘Blackwater’ is heavily polluted wastewater; ‘Greywater’ is a lesser polluted wastewater and ‘Stormwater’ is the rainwater runoff collected from roofs and concreted surfaces.

When treating wastewater that contains soluble organic impurities, biological wastewater treatment will be an integral and important process. Increasingly, wastewater discharge standards have become more stringent. This has led to biological treatment processes becoming more advanced. This increase in standards has been aided by the fact that biological treatment has many economic benefits. This includes operating costs and equipment investment, for instance, thermal and chemical oxidation

Aerobic and Anaerobic Biological Treatment

Aerobic biological treatment is a process carried out using the ambient air, or oxygen. The anaerobic process does not use oxygen.

Both processes are related directly to the type of microorganisms or bacteria that are involved in the treatment of organic impurities in wastewater and the operating circumstances of the bioreactor used in the process.

Consequently, the aerobic treatment process takes place in the presence of air and employs those microorganisms, otherwise known as aerobes. These utilise molecular-free oxygen to convert impurities into carbon dioxide, water and biomass.

Conversely, the anaerobic treatment process takes place in a vacuum.  It uses microorganisms, otherwise known as anaerobes, that do not need air to integrate organic impurities. The resulting product of the organic process in anaerobic treatment are carbon dioxide, methane and biomass.

Although the number of different aerobic biological treatment processes is myriad, there are four main technologies, each with different benefits.

Conventional Activated Sludge Process (ASP)

The oldest and most common process is the Conventional Activated Sludge Process (ASP). It is typically used to treat industrial and municipal wastewater. ASP removes impurities using an aeration tank and a secondary clarifier. Our Detectronic ORAKEL System can include multiple sensors to measure multiple parameters. This includes Dissolved Oxygen (DO) und Chemical Oxygen Demand (COD) for effective monitoring of this treatment process.

 Cyclic Activated Sludge System (CASSTM)

The Cyclic Activated Sludge System (CASSTM) treats wastewater from municipal, petrochemical plants and oil refineries by using the sequencing batch reactor (SBR) process. This method carries out all the functions that a conventional activated sludge plant would employ but in a way that eliminates the need for final clarifiers and the need for high return activated sludge pumping.

Integrated Fixed Film Activated Sludge (IFAS)

The Integrated Fixed Film Activated Sludge (IFAS) system is used where a two-stage biological treatment that comprises a stone or plastic media trickling filter followed by a secondary clarifier has been in operation. This has been used in more recent industrial wastewater treatment installations where the original wastewater treatment process used the usual single-stage activated sludge process. Benefits of the IFAS include the a wider surface area for biofilm to cultivate and remove organic impurities that might be toxic or resistant to biodegradation.

Membrane Bioreactor (MBR)

The final method highlighted here is the Membrane Bioreactor (MBR). MBR is extensively used for the biological degradation of soluble organic impurities in the treatment of domestic sewage. It is used much less in the treatment of industrial wastewater. It is very similar to the standard activated sludge, as both processes have mixed liquor solids in suspension in an aeration tank. The way that the bio solids are separated is the key difference between the MBR process and the conventional activated sludge process.


 Aeration is a treatment process that brings air and water together to remove dissolved gases, for instance, carbon dioxide, and will oxidise dissolved metals like hydrogen sulphide, iron and volatile organic chemicals (VOCs).

The first significant process within a water treatment plant is often Aeration.  It works by introducing small air bubbles to the water and letting them rise through the water, or exposing water droplets to air. The turbulence of this aeration process then removes dissolved gases from the liquid, allowing them to escape to the surrounding atmosphere.

Dissolved metals are removed through oxidation, which is achieved through the chemical combination of oxygen from the air with the unwanted metals contaminating the water. Once oxidised, the unwanted chemicals are removed from the liquor to become particles in the water. They are then removed by filtration or flotation.

The amount of surface contact between the water and air dictates the efficiency of the aeration.

Three basic control tests are required for aeration:

  • Dissolved oxygen – establish when water is over or under aerated by checking the concentration of dissolved oxygen
  • The pH test will give an indication of the amount of carbon dioxide.  As carbon dioxide is removed through aeration pH increases.
  • When temperature decreases, the saturation point of oxygen increases. As water temperature drops, the operator must adjust the aeration process. This is to maintain the correct dissolved oxygen level as the water temperature increases.

Detectronic monitoring solutions

Our ORAKEL System provides modular solutions for monitoring flow, pH, conductivity, TDS, SS and COD/BOD with just one control unit.

Biological Treatment

The ORAKEL System monitors parameters such as organic load, dissolved oxygen, conductivity and pH, as well as total dissolved and suspended solids, to help monitor and control the efficiency of the biological treatment.

Total Dissolved and Suspended Solids

Monitoring dissolved and suspended solids independently at the entrance to the plant provides a whole background of information on the upstream process and how efficiently operations are performing.

More information can be found in our ‘Water Flow And Quality Measurement Equipment And Services’ – download your copy here.