Wastewater treatment

Abstract

Quality of life on Earth in the future will largely depend on the amount of safe water. As the most fundamental source of life, water is relentlessly consumed and polluted. To halt this trend, many countries are taking extensive measures and investing substantial resources in order to stop the contamination of water and return at least tolerably good water quality to nature. The goal of water purification is to obtain clean water with the sewage sludge as a by-product. Clean water is returned to nature, and further treatment of sludge may be subject to other procedures. The conclusion of this paper is simple. The procedure with purified water is easily achievable, purified water is discharged into rivers, lakes and seas, but the problem of further treatment of sludge remains. This paper presents the basic methods of wastewater treatment and procedures for processing the products from contaminated water. The paper can serve as a basis for further elaboration.

In order to ensure normal life of living creatures, the water in which they live or the water they use must have a natural chemical composition and natural features. When, as a result of human activities, the chemical composition of water and the ratio of its chemical elements significantly change, we say that water is polluted. When the pollutants come from industrial plants, we are talking about industrial wastewater, and when they come from households and urban areas, we are talking about municipal wastewater. Both contain a huge amount of pollutants that eventually end up in rivers. Then, thousands of defenseless birds, fish and other animals suffer, and environmental consequences become immeasurable. In addition, the waste fed to the water often ends up in the bodies of marine animals, so they can return to us as food. Thermal water pollution also has multiple effects on the changes in the wildlife composition of aquatic ecosystems. Polluted water can be purified by mechanical, chemical and biological agents. Mechanical methods are based on the effect of physical forces. Chemical agents are based on chemical processes. Biological measures are based on natural laws and activities of living beings. Water saving and its rational use are some of the most effective ways of saving water from pollution.

Water treatment

Water treatment is done in two ways: by sedimentation and filtration. Dirt falling on the bottom is called deposition. The passage of clean water through the material is called filtering. Water containing dissolved substances is purified by distillation. To improve the taste of distilled water, aeration should be performed. The sun’s ultraviolet rays destroy biological pollutants. Mechanical, biological and chemical methods are used for water purification.

Mechanical methods

Mechanical methods are based on the removal of physical impurities from water and the action of natural forces. For this purpose we use: grids and sieves, sedimentation, flotation, filtration, centrifugation, sand sedimentation tanks, grease traps, primary sedimentation tanks and flow equalization tanks. Wastewater aeration equipment is also used within these facilities.

Grids and Sieves

Larger, insoluble and floating substances in wastewater are removed with grids and sieves.

Sedimentation

The application of grids and sieves as well as sand sedimentation tanks and grease traps can be viewed as a process of deposition using certain infrastructure facilities intended for this type of separation of impurities. Infrastructure facilities are sedimentation tanks. There are vertical, horizontal and radial flow sedimentation tanks.

Flotation

Particle resurfacing with bubbles of air is called flotation. The best effect is achieved by aeration of bubbles of smaller diameters in a larger area.

Filtration

Filtration is a process used in water conditioning to remove insoluble substances. During filtration, water passes through a layer of granular material placed on a perforated surface. Some other processes can also take place during filtration.

Centrifugation

Centrifugation is a method of separating particles from a suspension due to the effect of the centrifugal force that is many times greater than the force of gravity. For larger water quantities, it is not realistic to apply centrifugation as a water purification technique.

Sand sedimentation tanks

Substances with specific gravity higher than water, such as sand, gravel, stone, earth, slag, mineral particles, etc., accumulate in the plants called sand traps.

Grease traps

Substances lighter than water: oil, grease, soap flakes, bits of wood, cork, etc., are removed by means of grease traps.

Flow equalization tanks

They provide conditions for the slow and steady movement of water and retain wastewater long enough to provide gravity sedimentation of suspended particles.

Chemical methods

Chemical purification methods are the processes in which the treatment is carried out by means of certain chemical reactions, or certain physical and chemical phenomena. Basic operations of the chemical wastewater treatment are the removal of suspended and colloidal substances: by coagulation and flocculation, and the removal of some dissolved substances: by chemical deposition, ion exchange, oxidation,   gas blowing and adsorption.

Chemical deposition

The process of chemical deposition is based on the conversion of dissolved substances in wastewater to insoluble compounds, using suitable reagents.

Ion exchange

Ion exchange is much more widely used as a water preparation process to remove water hardness than as a wastewater purification process.

Oxidation

Oxidation can convert some organic and inorganic substances in industrial wastewater into compounds that are far less polluting. The most often used oxidant is chlorine.

Gas blowing

Some dissolved substances can be removed from the wastewater by means of blowing gas, ordinary air or steam.

Adsorption

Adsorption is a process of accumulating fluid substances on the surface of the solid phase. A substance being adsorbed is  adsorbate, and the phase upon which the adsorption is carried out is called the adsorbent.

Biological methods

Biological purification processes are based on the activities of a complex microflora, which is in the course of their life cycle adopted by organic and parts of inorganic materials causing wastewater pollution, using them to maintain life activities and to create new cells. During a biological treatment, stabilized sludge is precipitated,and removed from the water in secondary sedimentation tanks. Biological purification processes can be aerobic and anaerobic, with the help of aerobic or anaerobic microorganisms. Aerobic processes with suspended microflora are divided into: the processes with activated sludge in bioaeration tanks, processes in aerated lagoons (biological lagoons) and processes in aerobic (shallow) lakes (biological artificial lakes).

Activated sludge process

This is an aerobic process of a biological wastewater treatment, as it occurs due to aerobic microbial population. Microorganisms are found in aeration basins, where, with the help of oxygen in the metabolic process, substrate degradation is provided. Microorganisms oxidize the dissolved substrate into carbon dioxide and water. A part of the organic material is synthesized into new cells or used for the growth of existing ones and the rest consists of waste and excess sludge. A part of sludge is returned into the process (activated sludge) where it has a role of an activator of the biological process. The rest of sludge is discharged into the sludge treatment device or disposed of in a proper way. Clear purified water (effluent) is discharged into the recipient, or, if necessary, taken to additional processing.

Process in aerobic lagoons

Lagoons and shallow basins, as an aerobic biological wastewater treatment method, provide a sufficient amount of oxygen through photosynthesis.

Process in shallow lakes

Aerobic lakes are large and shallow earthen basins in which wastewater treatment takes place with minimum regulation, practically as natural self-purification.

Anaerobic processes

The anaerobic treatment process is based on the methane fermentation of organic wastewater pollution, where organic material is converted into a mixture of gases. The use of microorganisms in the absence of oxygen is called anaerobic digestion.

Processing products from the wastewater treatment process

Waste materials from the wastewater treatment process should have a high proportion of isolated components and a small proportion of the remaining moisture. Sludge treatment methods may include: thickening, stabilization, conditioning, dewatering, drying or oxidation and disposal. In accordance with the applied technology, sludge treatment processes may be biological, technical and mechanical ones. Biological sludge treatment processes are performed for compost production, to improve sludge residue for disposal, for biological stabilization of sludge residue. Composting is the simplest way of processing biodegradable sludge (humus). Compost can be used in agriculture, to revitalize the soil laid bare by fire and to improve the quality of land next to roads. Mechanical-biological treatment of sludge is based on a process of mechanical treatment of sludge, where valuable components are separated from sludge. These components are metal, paper, plastic, nonmetal or harmful substances that cannot be disposed of in landfills. The thermal treatment of sludge is carried out using the following methods: combustion, pyrolysis and gasification.