Wastewater Treatment Processes

The modern technological chain of wastewater treatment plants includes a set of equipment for mechanical, chemical and biological wastewater treatment, as well as, in many cases, equipment for high-tech water recovery before reuse.

A. Mechanical (preliminary) Treatment Equipment

For this stage performs simple mechanical operations (eg filtration and aeration) and enables highly efficient physical processes (sedimentation, flotation) to remove large particles of contaminants from wastewater. The main contaminants that are removed during the mechanical cleaning stage are:

  • Large floating particles of solid waste.
  • Granular particles (sand) with a size of 0.1 mm or more.
  • Easily settling suspensions, the so-called primary sludge.
  • Oils and fats that float to the surface.

Removal of other contaminants from wastewater at this stage of treatment is considered less important. It is also possible at this stage to treat the waste water by aeration or possibly chlorination.

For mechanical wastewater treatment the following devices are used :

  • Sand traps: vertical, horizontal.
  • Sieves of various types, sizes and designs.
  • Sedimentation tanks of various types.

Using modern mechanical treatment, in addition to separating large particles of garbage, sludge and sand from wastewater, enterprises can reduce the suspended solids content by 50-80%, as well as reduce BOD 5 and COD by 30-50% or more. After separation from wastewater, the sludge is washed and compressed, collected in containers and sent to a landfill or special processing. The sand to be separated from the wastewater in the form of a sand slurry is washed and separated to remove organic matter.

B. Biological or Chemical Treatment

Biological wastewater treatment is based on the decomposition of pollutants in biological oxidation processes, which means the use of microorganisms. Almost all biological wastewater treatment processes are aerobic. Wastewater must contain oxygen, which is consumed by bacteria and protozoa in their life processes, and therefore the oxygen concentration must be constantly maintained. During the construction of modern wastewater treatment plants, engineering companies make extensive use of biological treatment equipment such as bioreactors. This technological process takes place under artificial conditions that intensify natural biochemical processes. The intensification of biological processes is achieved by maintaining an optimal amount of active mass of microorganisms and ensuring a constant supply of oxygen. Wastewater treatment is carried out in installations with activated sludge (biological reactors), the configuration of which depends on the type of wastewater and the substances removed from them. During aerobic biodegradation, organic compounds are degraded by enzymes and then used to grow new microorganisms (increase biomass) and are oxidized to simple inorganic compounds such as carbon dioxide, water, nitrates, sulfates or phosphates. A feature of the technology of aerobic decomposition is the release of a significant amount of energy, which contributes to the growth of biomass and a faster course of the process. Excess biomass is removed from the system in the form of sludge, which is sent for further processing.

Biological nitrogen removal includes the following processes:

  • Ammonification.
  • Nitrification under aerobic conditions.
  • Denitrification under anaerobic conditions.

The selection of the most suitable process and equipment for the construction of wastewater treatment plants depends on the initial composition of the wastewater.

Wastewater chemical treatment technology

Chemical wastewater treatment consists in the addition of coagulants, iron compounds (for example, ferrous sulfate) or aluminum (for example, aluminum sulfate), and sometimes flocculants (anionic polymers). In some cases (at low pH) calcium compounds are added to the water. The main purpose of chemical cleaning is additional phosphorus removal. It should be borne in mind that the addition of chemicals to wastewater will not only reduce the concentration of total phosphorus, but also significantly reduce the concentration of BOD5 and COD, the concentration of suspended solids and total nitrogen.

Depending on the order of adding the coagulant to the wastewater treatment process, there are three methods of chemical precipitation:

  • Pre-sedimentation with the introduction of a coagulant into untreated wastewater (before primary sedimentation tanks). This is a typical process that requires rapid mixing, flocculation and settling. Pre-sedimentation can significantly reduce the load on biological reactors and reduce the cost of treatment.
  • Simultaneous sedimentation. The introduction of the coagulant can be carried out directly into the biological reactor, as a result of which the phosphorus is precipitated during the biological wastewater treatment. At the same time, flocculation occurs and the resulting sludge is separated from the wastewater by sedimentation in secondary sedimentation tanks.
  • Final sedimentation. Phosphorus is separated from wastewater after biological treatment in a separate process, including rapid mixing, flocculation and sedimentation (as separate operations). This method makes it possible to reduce the phosphorus concentration in the treated wastewater to a level below 0.1 g / m³ (in the case of filtration instead of sedimentation).

C. Water Recovery For Reuse

Highly efficient wastewater treatment processes are designed to ensure that the resulting water can be reused for domestic or industrial purposes. This is called water regeneration. Water regeneration technologies complement existing wastewater treatment methods. They are based on processes well known in chemical engineering and commonly used in groundwater and surface water treatment. These are processes such as filtration, coagulation, adsorption and disinfection (chlorination, UV lamps).

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