Water is a precious commodity. To protect our natural waters and to support drinking water production, all wastewater is first freed from pollutants and pollutants before it is fed back into the water cycle. Various processes are used to treat the water and to achieve the best possible natural water quality. In general, wastewater treatment can be divided into two basic types. On the one hand, problematic substances are removed from the water. This is done through cleaning, iron removal, manganese removal, sterilization, desalination or softening. On the other hand, substances are specifically added to improve quality and influence parameters such as pH value or conductivity.
THE STAGES OF WATER TREATMENT
There are several water treatment methods available to implement the various treatment stages :
- Physical processes for mechanical processing such as ventilation, sedimentation or thermal effects. This also includes the use of rakes, filters and sieves.
- Biological processes such as anaerobic wastewater treatment, biochemical oxidation or sludge digestion
- Chemical processes such as neutralization, disinfection, flocculation and precipitation
- Membrane nanofiltration processes such as filtration, osmosis, and
The highest volume of wastewater to be treated is recorded in municipal sewage treatment plants, which is why a combination of diverse and effective processes is necessary here. The processes used differ depending on the type of sewage treatment plant.
The processes of wastewater treatment in sewage treatment plants can be divided into different stages.
STAGE 1: MECHANICAL WATER TREATMENT
The still completely untreated wastewater is mechanically cleaned in a first step, which removes around 20-30% of the solids it contains. For this purpose, the wastewater is fed into a rake system, where a rake or a sieve drum filters out coarse dirt such as leaves, paper or textiles. Various rakes, from coarse screens with a gap of several centimeters to fine screens with a gap of a few millimeters, through which the water flows at different speeds, gradually filter out the coarse material. The screenings collected by machine are dewatered and disposed of in an incinerator.
The pre-cleaned water then enters a so-called sand trap. In sewage technology, this is the name given to a sedimentation basin, which has the task of removing coarse particles such as stones, glass splinters or sand as well as coarse organic material that has not been sorted out by the rake. This happens with a relatively high flow velocity of around 0.3 m / s. A distinction is made between the non-ventilated long sand trap , the ventilated long sand trap , also called roller sand trap , and a round sand trap .
With the ventilated sand trap , fats and oils are also removed from the wastewater, and this is how it happens: the process air that is introduced causes the water to move in a roller, which carries the lighter substances such as oils and fats to the surface. Here they can be easily cleared from the water.
With a round sand trap the substances are separated from the wastewater with centrifugal force and sucked off. After cleaning in the sand trap, the sand traps are washed and freed from organic substances. This achieves better drainage of the collected inorganic material, which can be used in road construction, for example. If no further use is possible, the sand traps must be properly disposed of: it is dumped or destroyed in waste incineration plants.
The primary clarifier is the next stage of wastewater treatment. The speed of the sewage is around 1.5 cm / s significantly slower than in the sand trap. The flow velocity is reduced by widening the basin. A low flow rate is necessary so that the finer dirt particles can settle on the ground or on the surface of the water, depending on their properties. The sludge that is created here by sedimentation (settling on the bottom) is called primary sludge. It mostly consists of organic material. The primary sludge is pushed from the floor into a fresh sludge funnel with a clearing bar. The floating substances are passed on to a floating sludge shaft. A pump conveys the fresh sludge into the so-called digestion tower.
Methane gas is produce in four stages (hydrolysis, oxidation, acetone genesis and methanogenic stage) and is converted into electricity at the CHP plant and can be used to power the plant. The decomposition process in the digester is completed after about four weeks. This leaves an odorless sludge that is dewatered on centrifuges or filters and is usually used in agriculture.
The mechanical cleaning stage ends here. On average, 30% to 40 percent of the pollution is removed from the wastewater in this phase. The wastewater now reaches the next stage of wastewater treatment on its way through the sewage treatment plant.
STAGE 2: BIOLOGICAL CLEANING
In most sewage treatment plants, the water that has been pre-cleaned in the mechanical cleaning stage now enters the so-called aeration basins , which are often designed as circulation basins. This is where the biological cleaning takes place.
The water is set in circulation by the supply of oxygen and with the help of propellers. There are more or less ventilate areas in which different environmental conditions are create for bacteria and microorganisms. These microorganisms feed on the remaining organic pollution in the water and convert them into inorganic substances. The bacteria from activate sludge flakes that float freely in the water. The availability of oxygen stimulates the reproduction of bacteria, which in turn promotes the formation of activate sludge. For this reason, this biological wastewater treatment process is also known as the activate sludge method.
The wastewater with the activate sludge goes into the secondary clarifier. Here the flow rate of the wastewater stream is reduce again. Sedimentation takes place: the activate sludge settles on the bottom of the purified water, where it can be separated from the clear water by mechanical clearing devices on the bottom. Part of this is pass on to the digestion tower as additional biomass. The other part of the sludge, which is also call “return sludge”, is fed back into the aeration tank to ensure that there are enough microorganisms in the aeration tank to break down dirt. After the biological treatment, approx. 90% of the wastewater is clean of biodegradable substances. The biological cleaning stage is the most energetic phase in the entire cleaning process due to the oxygen supply by compressors.Once the water has reach the legally require quality, it can be fed back into the water cycle, for example into a river.
In many other cases, biological cleaning is not enough. Then further wastewater treatment processes are necessary, for example processing in the form of chemical treatment. Chemical additives are also use.
STAGE 3: CHEMICAL WASTEWATER TREATMENT
In this stage of wastewater treatment, chemical processes are use for wastewater treatment. To this end, chemical compounds are use to produce the legally prescribe standard water values. Chemical wastewater treatment in sewage treatment plants includes neutralization , disinfection , phosphate precipitation , nitrogen elimination , de-icing and manganese removal .
The neutralization is use to produce the prescribe pH value, which is achieve by adding an acid, for example HCl or a base, for example milk or lime.
The disinfection process destroys pathogens by adding chlorine or chlorine dioxide. Instead of adding chemicals, irradiating the wastewater with UV light is a good alternative, but it is use less often. Phosphate elimination: Our wastewater is often contaminate with phosphates from detergents, fertilizers, food additives and faeces. If they remain in the wastewater, they lead to over-fertilization of the waters and an enrichment with nutrients, which can lead to useless plant growth that is harmful to the ecosystem (eutrophication).
The phosphates are remove using a chemical precipitation or flocculation process. The phosphate precipitation is initiate by the addition of aluminum or iron salts partly already in the sand trap or in the secondary clarifier. The metal phosphate flakes that settle during this secondary clarification are then remove from the wastewater together with the activate sludge. Depending on the operating mode, the phosphate can also be “fish” out of the wastewater with the help of microorganisms. In this case, one speaks of biological phosphorus elimination, which is, however, still rarely use.
Chemical water purification also includes nitrogen elimination: it is carried out in order to remove nitrogen compounds that are harmful to water, such as ammonia and ammonium, from the wastewater. Nitrogen compounds remove the vital oxygen from the water and can even cause fish to die if discharge into water. Nitrogen is eliminate through nitrification and denitrification: In nitrification, ammonium is converted into nitrite with the addition of anaerobic bacteria and oxygen – and in a second step it is convert into nitrate. The subsequent denitrification is also trigger by the addition of anaerobic microorganisms. These decompose the nitrate through enzymatic activities into nitrogen gas, which is return to the atmosphere. Iron
removal : In order to reduce the iron content of the wastewater to the prescribe level, iron (II) cations are oxidized by adding oxygen. In order to trigger the oxidation process, caustic soda must also be add to the wastewater.
Manganese Manganese is mostly in the wastewater before Manganhydrogencarbonat. By supplying oxygen, poorly soluble manganese IV compounds are form, which can easily be remove from the water.
4TH STAGE: MEMBRANE PROCESS / NANOFILTRATION
Membrane and filter processes are use in the fourth and final cleaning stage. This cleaning stage is partly combine with the chemical processes of precipitation and flocculation. This is how, for example, the flocculation filtration method is create. Precipitants and flocculants are add to the wastewater, which leads to flocculation of the substances to be separate. The wastewater with the flocculate material is then pass through a cloth or sand filter. It slowly seeps through the filter sheet. Even the smallest organic suspend matter is remove.
The nanofiltrationworks in a very similar way. In contrast to normal filtration, the water is pass through a membrane under pressure, which also retains the smallest particles such as molecules or heavy metal ions. The same thing happens with reverse osmosis , here even higher working pressures and finer membranes are use.
The pollutants that are retain during filtration, nanofiltration and reverse osmosis are transfer to the sludge treatment via the primary clarifier as filter sludge.
The water now reaches the last area of the sewage treatment plant, the clear water storage tank. Here, water samples are taken again and the water quality checked. The purified water is only fed back into the water cycle when the legally prescribe parameters ha