Sewage treatment

FOAM FORMATION DURING TREATMENT OF SEWAGE

The increase of living standards and comsumption of goods has resulted in a growth of waste products. In the field of water supply and sewage treatment this situation has led to a higher direct and indirect water consumption and therefore to an increase of waste water quantities. The amount of industrial sewage produced has considerably increased more than communal sewage. Simultaneously the percentage of purified industrial sewage has increased because of stronger environmental awareness, laws and restrictions. Communal sewage is often mixed with industrial waste water to achieve a better blend of substances contained.

Sewage entering purification plants is often a variety of very different components, particularly the foam active ingredients i.e. detergents, proteins etc. Due to this, the degree of foam generation is very different in sewage treatment plants. Pure communal sewage tends to foam less than industrial sewage. However, communal sewage can also cause foam formation when the detergent content is higher.

The method of sewage treatment has the major influence on foam development. Foam occurs if gas escapes from liquids that contain foam active substances. This especially applies to aerobic sewage treatment according to the activated sludge process. All other chemical, mechanical or biological sewage treatment processses are considerably less foam active due to their structure.


FOAM PREVENTION AND FOAM DESTRUCTION WITH STRUKTOL ANTIFOAM AGENTS

The foam active substances in sewage are very different, therefore antifoam agents have to be adapted to the medium to be defoamed. A careful choice is necessary in order to achieve effective defoaming without any negative influence on the sewage system. Lab pre-trials are recommendable in order to select the most appropriate materials. These trials should only be carried out with fresh sewage i. e. in the purification plant. However, due to international experience we provide recommendations in many cases.

Apart from good defoaming properties the antifoam used should not have a negative impact on the sewage treatment itself. In the case of aerobic sewage treatment, microorganisms have to be supplied with sufficient oxygen for an optimum of contaminants digestion.  Microorganisms can only take up oxygen dissolved in water. 

If the percentage of dissolved oxygen decreases, the digestion will deteriorate. Therefore it is essential, that the antifoam agent does not influence the oxygen transfer as far as possible.

Struktol antifoam agents have a remarkably low effect on the oxygen transfer. And they are low viscosity materials and therefore easy to use. All Struktol antifoam agents listed in the following - with the exception of Struktol SB 2077 - are free from mineral oils or other paraffinic hydrocarbons and conform to OECD recommendations. With the exception of Struktol SB 2077 all Struktol products stated in the following tables are almost completely degradable. Struktol SB 2077 is a silicone antifoam agent and silicones are not biodegradable.

The dosing quantities of antifoam agents depend on many conditions e.g. the concentration of foam active substances. In pure communal purification plants the dosing quantity is usually less than 1 ppm calculated on total sewage amount. Industrial sewage treatment plants often have a higher percentage of foam active substances so dosing quantities of 10 ppm or more can be required. Therefore it is essential to find out the necessary quantity in each case.

Furthermore the choice of the best possible dosing point is also important. The dosing point depends on the defoaming procedure i.e. whether already existing foam has to be destroyed or foam generation has to be avoided. Struktol antifoam agents can be used for foam prevention as well as for foam destruction. However, foam prevention is most advantageous. Experience has shown that in this case dosage should be introduced at the inlet to the biological stage.

REDUCTION OF SCUM

In many purification plants the undesired effect of scum during final sedimentation occurs. It is caused by gas bubbles (air, nitogen or methane) entrapped in the sludge agglomerates, that floats them to the surface. Some Struktol types reduce the linkage force between gas bubbles and sludge agglomerates settle again.

To optimize the performance of these scum reducers the choice of the right dosing point is essential. Dosage has to be carried out in that way that sufficient distribution of the scum reducers ia achieved. In many cases the activated sludge stage is the best dosing point. An additional dosage of antifoam agent is normally not neccessary.

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