Sewage treatment

A sewage treatment plant in Switzerland and Austria also called ARA ( sewage treatment plant ), the purification of waste water, which is collected from the sewers and transported there is.

To clean the waters contaminating components of the wastewater are mechanical ( also called physical ) used biological and chemical processes. Since these types of procedures are successively used in various purification steps, called modern sewage treatment plants " three-step ".

The first sewage treatment plant on the continent was taken in 1882 in Frankfurt am Main in operation.

  • 2.3.1 aeration tank
  • 2.3.2 secondary clarifier
  • 2.3.3 fixed bed process
  • 4.1 volume of wastewater
  • 4.2 Degree of pollution 4.2.1 BOD5
  • 4.2.2 Chemical Oxygen Demand
  • 4.2.3 nitrogen
  • 4.2.4 phosphorus


Plant parts

Rain relief

When rain and dirty water are fed into one channel of the wastewater treatment plant ( mixing system ), the sewer system usually needs to be relieved by a rain -relief system, by a combined sewer overflow and / or by a rain overflow basins, so that the treatment plant is not overloaded. This can be done already in the sewer network or only in the treatment plant. When no such means are provided, the treatment plant must have a higher performance. This contrasts with the separation system. Here the dirty water is supplied in a separate pipeline to the treatment plant, while the rain water is passed through a separate channel, possibly after cleaning in a sedimentation tanks, directly into surface waters.

Mechanical pre-


In the computer system, the waste water is passed through a rake or a sieve drum. In computing gross contamination, such as items of sanitary protection, condoms, stones, but also leaves and dead animals stuck. These coarse materials would both clog the pumps the wastewater treatment plant as well as degrade the biological cleaning result. The smaller the passage for the effluent, the less coarse material containing waste after the rake. A distinction is fine screens with a few millimeters and coarse screens with several centimeters gap width. The screenings are washed to remove the fecal matter mechanically, by means of screenings dehydrated ( weight saving ) and then incinerated, composted (fertilizer) or deposited in a landfill.

Sand trap

A sand trap is a settling tank with the task to remove coarse deductible contaminants from wastewater, such as sand, small stones or pieces of glass. These substances would lead to operational faults in the system ( wear, constipation ). As design is a

  • Long sand trap, a
  • Ventilated long sand trap in which both fats and oils are deposited on the surface, a
  • Circular grit trap or a
  • Deep sand trap possible.

The ventilation of the sand trap ( attached to the pelvic floor ) creates a vortex flow. By the injected and dissolved air, the density of the waste water is reduced. Due to these two effects, the heavy, mainly mineral solids (mainly sand) settle to the pool floor. In deep sand trap, the waste water from above flows into the basin and the depth achieved by a relatively high residence time, causing the heavier sand on bottom (sand hopper ) settles. In modern systems, the grit trap is washed after removal from the sand trap, so very largely freed from organic impurities to allow better drainage and subsequent usability ( for example, in road ).

Primary clarifiers

The dirty water slowly flows through the primary clarifiers. Suspended solids (faeces, paper, etc.) are deposited ( settleable solids ) or float on the surface. Approximately 30 percent of the organic matter can thus be removed. It is produced primary sludge, which is used in most water treatment plants in the so-called pre-thickener ( see diagram above). Together with the excess sludge from the aerobic activated sludge plant it is thickened there. The mud settles and the excess water ( turbid water ) is removed and returned to the further purification process of the sewage treatment plant. The thickened sludge is pumped for further treatment in the anaerobic digester.

In modern plants with nitrogen removal this part of the system eliminates many or small-sized, because the organic matter of the waste water as reducing agent for removal of nitrogen by denitrification (reduction of NO3- to N2 ) are required in the anoxic section or in the anoxic phase of the biological stage.

Also, this piece of equipment is not used in waste water treatment plants with simultaneous aerobic sludge stabilization in the biological stage, otherwise continue non-stabilized primary sludge would be incurred.

Biological treatment

The processes occur as in the self-purification of water bodies. In the aeration basin aerobic micro-organisms (bacteria, yeasts) can degrade the biological contaminants still present in the wastewater under constant air supply. In this process, part of the organic matter in the wastewater are degraded by microorganisms and partially oxidized inorganic substances. To this end, and air is pumped. For this purpose, many methods have been developed ( for example, the activated sludge process, trickling filter process, the, the immersion state process, the fixed bed reactor process ).

Aeration tank

The majority of municipal wastewater treatment plants in Central Europe is operated by the activated sludge process. Thus, the wastewater constituents of the fresh wastewater are degraded biotic oxidation in so-called aeration tank by aeration of the activated sludge (mass of flaky aggregate bacteria) offset wastewater. In this case (e.g., yeast ) can be aerobic (oxygen consuming ) bacteria and other microorganisms carbon compounds is oxidized with oxygen to nitrates mostly decomposed to carbon dioxide, and partially converted to biomass and nitrogen initially removed from organic compounds by bacteria other than ammonia, and this ( nitrification). The activated sludge process is usually operated with continuous flow, that is, continually running into the aeration tank wastewater and runs continuously at the same rate activated sludge containing water. By the addition of precipitating agents nutrient phosphorus can be removed by chemical reactions also optionally substituted by simultaneous precipitation. This also improves the settling characteristics of the activated sludge in the secondary clarifier.

There are various technical variants of the activated sludge process, especially upstream, downstream and simultaneous denitrification. Here, find nitrification and denitrification either in the same or in separate basins, here then in a different arrangement each other instead. A special form represents (SBR = Sequence Batch Reactor ), the SBR process, the biological purification and - combined secondary clarification in a single pool - described below. Here, the waste water does not run continuously, but only in a limited amount, which is then purified by the activated sludge process (nitrification and denitrification), then (ie, the secondary sedimentation ) takes place in the same tank, the settling phase, and finally the activated sludge is conveyed away. Then the next thrust wastewater is introduced and purified. When SBR process at least two basins are required, either a second aeration tank or a buffer tank in which the taper during the cleaning process wastewater volume is cleaned or cached.

Secondary clarifier

The secondary clarifier is a process unit with the aeration basin. In it, the activated sludge is separated by settling from the waste water. A part of the sludge in the aeration tank is returned ( return sludge ) to obtain the concentration of microorganisms in the aeration tank is sufficiently high. Otherwise, the degradation performance was in too low. The surplus ( increase in biomass, excess sludge ) is removed for further treatment along with the mud of the primary settlement tank in the pre-thickener.

The activated sludge must have good settling properties. If this is not the case, for example, mass -wise growth of filamentous microorganisms, resulting in the formation of bulking sludge, drives the activated sludge from the settling tank into the water, into which the treated effluent (so-called receiving water ). This not only affects the waters. Since then enough mud in the system aeration tank / secondary clarifiers can not be maintained, the cleaning performance and the sludge age ( the mean residence time of the biomass in the system) decreases decreases. First, therefore, are of such a failure, the slow-growing bacteria (such as the nitrifying bacteria which oxidize ammonia to nitrate ) affected. Especially wastewaters with easily degradable organic substances ( for example from the food industry) tend to sludge bulking. The prescreening small, with little or no aerated basin in front of the activated sludge tank ( selectors ) can avoid the sludge bulking. A special form of the settler is the funnel-shaped Dortmund tank.

Fixed-bed process

In the fixed-bed process differently shaped solids serve as a basis for the nursery of microorganisms that degrade the contaminants. These solids are immersed alternately in waste water and air so that the microorganisms come in contact with both the contaminants as well as with the oxygen needed for their oxidative degradation. Biochemically, is happening here is basically the same as the above activated sludge process.


The resulting from the reduction of wastewater constituents of microbial biomass is removed as sludge, but mostly in the so-called digesters under anaerobic ( oxygen-free ) conditions (essentially a mixture of methane and carbon dioxide ) decomposed by anaerobic bacterial strains to digested sludge and combustible biogas.

There are four phases in the digester degradation: hydrolysis phase, acidification, acetogenic phase and methanogenic phase.

In practice, the raw sludge contained in about 70 percent organic matter decreased in the anaerobic conversion to about 50 percent organic matter in the sludge in the sludge. Theoretically, the reduction would even further, but verliefe much slower, since the residual organic substance is not readily degradable. In the technical sense, the sludge is considered at a loss on ignition of about 50 percent as very good ausgefault and stable.

As a degradation product formed during the digestion biogas. There is a gas mixture which is composed as follows:

  • Methane: 60 to 70 percent
  • Carbon dioxide: 26 to 36 percent
  • Small amounts of hydrogen and hydrogen sulfide.

This process corresponds to the production of biogas in a biogas plant. The digesters are often ovoid and then be referred to as digester ( see Figure ).

The digester gas is often used in purified form ( for example, removal of hydrogen sulfide ) gas engines and cogeneration power for captive consumption of electricity and heat.

The digested sludge is then the so-called post-thickener (see above diagram ) passed. There, it is concentrated by settling to reduce the volume and the water content further. With special height-adjustable trigger devices, the turbid water is deliberately withdrawn.

The resulting sludge may, if he is free of pollutants and toxins, in agriculture as organic fertilizer ( Klärdünger ) are used. Otherwise, it is dewatered in filter presses or decanters further and incinerated in incinerators or power plants. The sludge can not be stored in landfills, because he is still regarded as waste and not as waste. In addition, organic waste must not be disposed of in landfills since 2004.

Cleaning processes

Stage 1: Mechanical methods usually form the first purification step. Here about 20 to 30 percent of the solid ( undissolved ) swimming pools and suspended solids are removed. In the advanced wastewater treatment and industrial water management, among others, adsorption, filtration and stripping are used.

2nd stage: Biological processes are used in the second cleaning stage to municipal sewage treatment plants and the reduction organically highly polluted wastewater in the aerobic and anaerobic wastewater treatment. Using microbiological degradation processes. This degradable organic wastewater constituents are possible completely mineralized, ie in the aerobic wastewater treatment up to the inorganic end products of water, carbon dioxide, nitrate, phosphate and sulfate reduced. In the anaerobic wastewater treatment they are converted to organic acids, methane and carbon dioxide. Usually therefore, the carbon compounds are removed from the wastewater. Similarly, the removal of organically bound nitrogen and ammonia by bacterial nitrification and denitrification. Increasingly, the phosphorus is eliminated by bacteria in medium and large wastewater treatment plants.

3rd stage: Abiotic and chemical processes use chemical reactions such as oxidation and precipitation without the participation of microorganisms. They are used in municipal wastewater treatment, especially the removal of phosphorus by precipitation reactions ( phosphorus removal ). This process is of great importance for the prevention of eutrophication of the receiving water. In addition, abiotic chemical processes for precipitation in industrial water management and advanced wastewater treatment (for example, flocculation / precipitation / filtration) used.

4th stage: Since about the end of the 1980s, partial more stringent cleaning procedures have been developed which are indeed already ready to go, but because of their sometimes very high operating costs could not yet prevail. Here, for example, the wastewater filtration and disinfection to call.

The processes in wastewater treatment plants can mathematically by their reaction kinetics ( macrokinetics ) are described.

Load characteristics

The burden of wastewater treatment plants is determined by population equivalent (pe ). It is the sum of the actual population (population, EZ ) and the population equivalent ( pe). The population equivalent is the size of the agreement for a " standard population " to be applied in emission of waste water. For commercial, industrial and agricultural production related to production sizes loads (eg 10 EW BOD5 per ha vineyard area ) specified. Note, however, that the ratios between the individual parameters can move. Waste water can be more concentrated (less wastewater volume for the same contaminant load ), or they can be for example, nutrient- rich in organic carbon compounds and. The content of biotic degradable materials is the sum parameter biochemical oxygen demand, BOD abbreviated quantified. In general, it is measured by the biochemical oxygen demand in milligrams within five days under standard conditions and referred to as BOD5 ( see below). For the biotic degradation has a nutrient ratio of BOD5: N: P of about 100:5:1 be given to supply the microorganisms with adequate nitrogen and phosphorus. This is based on the assumption that about half of the degraded organic material is used for biomass growth and biomass in the dry substance consists of about twelve per cent of nitrogen and approximately two percent of phosphorus.

A population equivalent, abbreviated EW corresponds to the following sizes:

Amount of wastewater

As load on the wastewater treatment plant with a sewage wastewater flow of 150 to 200 liters per capita per day was previously scheduled. The wastewater flow corresponds approximately to the water consumption. For new plans or advance planning now the site-specific water consumption is determined and attempts an estimate for the future. Usually amounts of dirty water are carried around 130 liters per capita per day.

This value takes into account the usual in Central Europe with dense channel networks values ​​. However, an allowance for extraneous water ( leaky sewers, discharges from drainage and the like) is usually taken into account for the design of wastewater treatment plant. This may amount to 100 percent of the sewage flow. The foreign amount of water is based on the connected and sealed area should not exceed 0.15 l / ( s · ha ) amount.

In combined sewer systems ( storm water and waste water in a channel ) corresponding surcharges for processing the rainwater to be considered, which are carried mostly with 100 percent of the daily peak during dry weather.

For the hydraulic analysis ( number and size of the feed pumps ) of the sewage treatment plant is also the diurnal variation of the load of meaning. The average daily load is therefore to share for the maximum hourly value for the design not through 24 hours, but by a smaller number (10 to 14).

Degree of pollution


When BOD5 value, the biochemical oxygen demand during a measurement time of five days at 20 ° C, that oxygen demand is detected, caused by the oxidation of organic matter by aerobic micro-organisms. He belongs to the so-called sum parameters, since it can not be determined the degradation of individual compounds.

Called The bacterial oxidation of ammonia (NH3 ), ammonium (NH4 ) and nitrite ( NO2 ) to nitrate ( NO3- ), nitrification, should not be recorded and will in the measurement by an inhibitor, such as allyl thiourea ( ATH) or sodium hydroxide cookies prevented.

As usual value for BOD5 60 grams per inhabitant and day value are recognized. Of these, about 20 grams can be removed in the primary treatment by sedimentation. For determination of the BOD following requirements are necessary:

Chemical Oxygen Demand

The chemical oxygen demand, also abbreviated as COD, also belongs to the so-called sum parameters, since it does not single compounds can be quantified. It is determined by means of the oxidation of the waste water content of potassium, and detects the oxygen required for the oxidation of most of the organic substances. Are in the wastewater also oxidizable inorganic compounds such as sulfites contain, they are also recognized as chemical oxygen demand. This parameter is also used to balance the system.

For the chemical oxygen demand, a value of 120 grams per inhabitant and day value is recognized.


Nitrogen is in the raw wastewater mainly organically bound (for example, in proteins, nucleic acids, urea) and in the form of ammonium ions ( NH4 ), and in small amounts in the form of nitrate ( NO3- ) and nitrite ions (NO2 - ) before.

Be appropriate to begin here about ten to twelve grams per inhabitant and day value.


Phosphorus is organically bound as phosphate group and present as free phosphate. Here about 1.8 grams per inhabitant and day value will be accepted.

Energy consumption

Electricity costs for sewage treatment plants are usually the highest position of all the facilities for communities in Germany. On average, there are about 20 percent of municipal energy consumption. Be Nationwide for 4400 gigawatt hours of energy consumed in the year ( 2009).

Of this usually requires the pressurization system in the aeration tank by far the largest energy expense of all process steps of a municipal wastewater treatment plant. The energy consumption for the ventilation is on average about 50 percent of total energy demand. Then the continuously running pump and the third largest consumer group follow generally form the continuous mixers. These three main components consume in normal run plants more than 80 percent of the energy.

In Germany there are 10,000 wastewater treatment plants, which produced per cubic meter of pure water are consumed 0.58 kilowatt hours for raw water treatment, filters and pressure increase on average, as a survey of 422 utilities revealed.

The following measures to increase energy efficiency are proposed, among other things:

  • By exchanging, optimization and control of the aerator aeration energy can be saved by about 50 percent.
  • Improving operational procedures to avoid pressure losses.
  • Use of modern pump with the highest efficiency class (EFF 1).
  • Transparent monitoring (monitoring). More electricity meter. Detecting a pressure loss in the ventilation equipment. Clear arrangement of the measuring instruments.
  • Mechanical dehydration prior to introduction into the digester to reduce the heating energy.
  • Efficient use of digester gas to generate electricity.
  • Drying processes on solar energy or waste heat.
  • Using a modern cogeneration plant for biogas generation. In order to reach a self -sufficiency rate of about 33 percent of the electricity demand (as of 2009 ).