- Introduction
A wastewater treatment plant is a water treatment plant for the purification of wastewater, whose main objective is to collect the water of a population or industrial sector and remove the pollutants from it in order to return it later to the water cycle, either by discharge to the sea or by direct reuse. The various substances to be removed include residues, oils, sands and various settleable solids, compounds with nitrates, ammonia and phosphates. The problem is that conventional wastewater treatment plants are not designed to remove pollutants of an emergent nature, as will be discussed below.
- types of treatment
2.1 Physical treatments
These are processes that involve physical separation, generally of solids. These processes often depend on the physical properties of the contaminants, such as viscosity, particle size, buoyancy, etc. They include screening, precipitation, separation and filtering of solids.
2.2 Chemical treatments
These are processes that depend on the chemical properties of the impurities or reagents introduced into the water. The iron and oxygen elimination, the phosphate and nitrate elimination, the coagulation, the electrochemical processes, the oxidation, the ion exchange, etc. should be highlighted.
2.3 Biological treatments
DISPLAY
These processes use biological processes that attempt to eliminate colloidal impurities. They are microorganisms that act on suspended solids and convert them into sedimentable solids. They may be aerobic or anaerobic processes, such as active sludge, trickling filters, anaerobic co-digestion, or aerated lagoons.
- stages of treatment
3.1 Pretreatment
This is the preliminary stage of treatment of the wastewater treatment plant. In this process, the wastewater flow entering the station is regulated and measured. In this stage, the larger solids, sand and grease contained in the black water are removed. These compounds are removed by filtration.
Pre-aeration is also usually performed to reduce the volatile organic compounds dissolved in the water, which give off a bad odor and increase the COD value of the water. Devices used include the coarse shaft, coarse roughing, fine roughing, and degreasing/degreasing.
3.2 The initial treatment
This first stage has the task of removing suspended solids, which is done by a gravity sedimentation process or by precipitation, either assisted or by the addition of chemical substances. Solids are removed by screening based on particle size.
Compounds such as aluminum, flocculating polyelectrolytes, and iron salts are then added, and phosphorus dissolved in small colloids or in a very fine suspension is precipitated using hydraulic machinery.
The main methods include the primary decanter (gravity sedimentation), the levitation method (separation of suspended solids by bubbling), and chemical treatment with the addition of reagents to increase the sedimentation of dissolved solids.
3.3 Secondary treatment
The objective of this second stage is to eliminate the dissolved organic matter in colloidal state by biochemical oxidation processes. In addition, biological substances caused by human waste are degraded. In these treatments there are aerobic and anaerobic processes. Aerobic processes are carried out in the presence of oxygen supplied by injection into storage tanks.
Anaerobic processes are carried out in the absence of oxygen. In these processes, fermentation reactions of organic matter take place and are converted into released energy, CO2, CH4, and C. Some of the most commonly used anaerobic and anaerobic processes are active sludge, green filters, aerated lagoons, bacterial beds, and anaerobic digestion. There are also physicochemical processes such as particulate beds. These processes reduce much of the BOD and eliminate the remainder of the settleable solids. Generally, there are combinations between these different processes, resulting in biological processes with two or more stages.
Biological reactors such as moving bed or membrane reactors can also be used, but their construction and operating costs are usually higher than those of a conventional filter treatment plant.
This stage also includes secondary sedimentation, where the treated water and the sludge produced in the biological process are separated.
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3.4 Tertiary treatment
In this final treatment stage, processes are carried out to eliminate pathogens, such as fecal bacteria, in order to raise the quality standards required for return to the water cycle through discharge to the sea, rivers, lakes, aquifers, reservoirs, and other water systems. This type of treatment is carried out only in wastewater treatment plants that discharge into a protected area.
The processes carried out include, first of all, filtration in sand filters, where a large part of the substances is kept in suspension. The remaining activated carbon retains the rest of the toxins.
Lagoon treatment provides the necessary sedimentation and an additional biological benefit. It basically mimics the natural self-purification processes of a river or lake. Not only are lagoons extremely aerobic, resulting in habitual stream growth, but filtering invertebrates aid in the process of removing solids. On the other hand, artificial wetlands consist of a series of reed belts or similar, which cause a process of phytoremediation (decontamination of soils).
During this phase, nutrient decomposition also takes place. Nitrogen is removed by biological oxidation by bacteria such as Nitrobacter or Nitroso monas, which convert NH 3 to nitrates and then to N 2 . In this denitrification process, nitrates and nitrites are utilized by these bacteria under anaerobic conditions, producing nitrogen gas and CO 2 and water as end products.
Phosphorus is removed through the process of enhanced biological phosphorus removal, in which polyphosphate-forming bacteria take up phosphorus from the water. The resulting sludge is complicated to handle, but this is a minor drawback compared to the increased efficiency of the phosphorus removal process.
Finally, the wastewater is disinfected to reduce the number of organisms formed in the intermediate stages. The most common methods are ozonation, UV light treatment or chlorine treatment.Chlorine disinfection is still the most common form of disinfection, especially in Pakistan, while UV disinfection is becoming increasingly important in Pakistan. We also provide mechanical wastewater treatment in Pakistan, so you can check here for water disinfected
- removal of emerging pollutants
Water is a natural resource, scarce and essential for human life, which also enables environmental sustainability. It is both in qualitative and quantitative terms.
