What happens in Sewage Treatment Plants?

Sewage Treatment Plants

What happens in Sewage Treatment Plants?

In sewage treatment plants, wastewater is purified in such a way that it leaves a clear water that contains only a small amount of substances that adversely affect the waters (the receiving waters) into which they are discharged.

1. First Cleaning Stage

The first step of cleaning is the mechanical separation of coarse materials such as sand (sand trap), fats and oils (fat catch) and settleable substances (primary clarifier) or absorbable substances (fine screening). The trend in modern wastewater treatment plants is increasingly moving from primary clarification to fine screening, as otherwise the ratio of nitrogen to organic cargo will deteriorate.

2. Second (= biological) Cleaning Stage

The biological purification takes place in a large bioreactor, in which bacteria are bred in the “nutrient solution wastewater”. The bacteria absorb the dissolved and colloidal contaminants. With the help of oxygen (mostly from the air) these pollutants are inhaled and the other half converted into new bacteria. Thus, there is always an excess of bacteria (excess sludge), which is removed as sewage sludge from the system.
This sludge can be reduced by digestion at 30-45 ° C in its amount and z. T. in fermentation gas (biogas, about 75% methane) are implemented. Very high polluted wastewater, z. B. from animal carcasses or sugar factories, can also be anaerobically digested. No air is needed for this, which otherwise would have to be injected with a lot of energy. After digestion, however, an aerobic post-purification is necessary.

3. Third Purification Stage

Although the amount of oxygen-consuming substances is reduced during biological purification, the “fertilizers” nitrate and phosphate remain in the wastewater.
They cause an increase in growth (eutrophication) in the receiving waters. Oxygen-free treatment of the second stage effluent with the addition of fresh effluent may denitrify the nitrate to nitrogen gas. Phosphate is partly bound in sludge (biological phosphate elimination), partly it has to be precipitated by the addition of iron and aluminum salts or by addition of lime.

4. Fining

As a final step, filtration can be done with a gravel or sand filter that retains even the finest suspended particles. In part, the purified wastewater is not discharged directly into a body of water, but previously passed through a fining pond with a few days of residence, in which a biological post-purification can take place. However, mass algae growth can also lead to secondary pollution.

5. Technical Design of Sewage Treatment Plants

The aerobic cleaning happens today for the most part in activated sludge plants. Bacteria in the form of biological flakes (100-500 μ diameter) are held in suspension in a basin and ventilated at the same time.
It flows constantly to wastewater and to the same extent wastewater activated sludge mixture from. The wastewater activated sludge mixture separates in a downstream secondary clarifier. The supernatant water is fed either to the third purification stage or directly to the receiving water. The sludge is pumped back to the aeration tank to maintain a uniform bacterial density. The resulting excess sludge is withdrawn.
In a second version, the wastewater is pumped through a growth material (lava stone, plastic lattice) on which bacteria settle as a biological lawn. This is called a trickling filter method.
Air flows from below. If the fouling becomes too thick, it is rinsed off and separated via a secondary sedimentation tank. A mud return is not necessary here.
There are also combined systems with growth bodies in activated sludge tanks.
The anaerobic treatment of sludge or concentrated wastewater takes place in a thermally insulated container, the digestion tower, in which an anaerobic activated sludge forms. Mud will be rotted out within 20-30 days. Concentrated waste water requires a much shorter time, especially if at the same time growing bodies for the anaerobic activated sludge are offered.

6. The determining Parameters for Biological Wastewater Treatment

Activated sludge process

The pollution load of a waste water is expressed by the amount of oxygen-consuming substances under standardized conditions as Biochemical Oxygen Demand (BOD5). Per inhabitant, one calculates with 60 g of BOD5 per day, the so-called population equivalent (EC). Domestic wastewater has BOD5 contents of 150-300 mg / l.
Domestic wastewater has BOD5 contents of 150-300 mg / l.
The most important parameter for the cleaning performance of an activated sludge plant is the ratio between the contaminated load (kg BOD5 per day) and the total biomass in the activated sludge tank (expressed in kg dry substance (kg DM)). Normally, aeration plants have a TS content of between 3-5 g / l. This ratio is called the sludge load (BTS) expressed as kg BOD5 / kg TS.d. Highly loaded plants have worse discharge values than low-loaded ones. Below BTS = 0.15 kg BOD5 / kg TS.d, an activated sludge plant is able to nitrify nitrogen to nitrate if the temperature is above 10 ° C.
The second important parameter is the calculated mud age, i. H. the ratio of the amount of sludge (kg DM) in the activated sludge tank to the amount of excess sludge withdrawn per day (in kg DM). For heavily loaded plants, the sludge age can be a few hours, in low-load plants it is 20 days or more. The bacterial species that cause nitrification grow quite slowly. The mud age must therefore be over 10 days.
The third important parameter is the oxygen content in the aeration tank. It must always be above 0.5 mg / l. For nitrification 1,5-2,0 mg / l are necessary.
For denitrification, the main parameters are the ratio between aerated and non-aerated volume (20-45% non-aerated volume), the return ratio of nitrified effluent to the non-aerated zone (50-300%) and the relationship between the BOD5 load and the Nitrogen load (BSB5at least three times that of nitrogen).

Trickling Filter Process

The most important parameter for the trickling filter is the volume load (kg BOD5 per m³ of trickling filter volume). A highly loaded trickling filter is at 1.2 kg BOD5 / m³.d, a low loaded one at 250 g BOD5 / m³.d.
The sludge age can be very high, which is why trickling filters always nitrify if they are not stressed too much. Denitrification with trickling filters is only possible to a very limited extent.

The biocenosis of sewage sludge bacteria

The activated sludge flake or the biological lawn consists of living and dead bacteria, which stick together by excreted mucilage to flake each other. In low-loaded plants, the majority of bacteria (90-95%) are dead. It includes organic and inorganic particles down to small grains of sand (10-50%). Since oxygen is diffused for breathing from the outside and consumed quickly, oxygen-free to oxygen-free states prevail in the interior.
In the absence of oxygen (highly loaded sludge) changes the flake shape. Instead of more or less round (smallest surface per volume) star-like flakes often develop with filamentous bacteria (large surface per volume), the so-called bulking sludge, which settles badly.
A bacterial flake always consists of a multitude of different bacteria that form a biocenosis. The composition depends on both the waste water composition (domestic waste water, brewery waste water, etc.), as well as the mud load and the sludge age.
There are specialists and omnivores. But there are also bacteria with very different generation times.
The growth rate is also dependent on the oxygen content, the temperature and the pH. The result of competition for oxygen and food is the composition of the mud flake, which can also change quickly.
Most bacteria can only perform certain degradation steps and are dependent on the presence of the bacteria taking the next step (assembly line principle). If an intermediate product accumulates, this often causes a feedback inhibition of the degradation step from which it originated (end product inhibition). Such intermediates (organic acids, fatty acids, ammonium, amines) are often already recognizable by the (unpleasant) smell.
In addition to this positive cooperation (synergism), there are also bacteria that compete with each other (antagonists). The “winners” in this competition are not always the bacteria that are technically desirable (bulking mud, scum).
The activated sludge flake is also able to adapt to very “exotic” wastewater constituents such as phenols, hydrocarbons, cyanides, etc. over longer periods of time, if these substances are constantly present (otherwise the specialists quickly disappear). As research has shown, the composition of an activated sludge biocenosis can be influenced not only by environmental parameters (oxygen, pH, temperature, sludge loading, sludge age) but also by membrane effectors.
All nutrients that are respired and rebuilt by bacteria must be actively taken up by the cell membrane or passively diffused. The excretion of mucus envelopes around the bacteria into which enzymes (exoenzymes) are released for the degradation of substances outside the cell is influenced by such membrane effectors as well as the release of these enzymes. Without this extracellular degradation further transport and further processing of (smaller) fragments in the cell is not possible.


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