Results of the OptDigest research project

Foam in digesters

Due to the necessary conversion of the energy supply to renewable energy sources and the current shortage of natural gas, biogas production is becoming increasingly important. Sewage treatment plants are present throughout South Tyrol and have a considerable potential for the production of renewable energy in the form of biogas, they can therefore make an interesting contribution to the climate plan. The anaerobic process for the production of biogas from sewage sludge is well known and technically established, but can and must be optimized in some aspects. One of them is the formation of foam in digesters. This phenomenon is well known in the literature and causes problems for process engineering and plant maintenance. In the industrial research project "OptDigest", several companies in this field are collaborating to study this phenomenon and propose effective strategies to reduce foam in the fermentation process. As part of the project, the technical know-how of Atzwanger AG engineers was made available for the construction of a pilot plant for the pre-treatment of sewage sludge in order to minimize foaming. After initial laboratory tests, the pilot plant was installed at the Unteres Pustertal wastewater treatment plant.

Detection of effects is often a challenge

Large-scale trials are often carried out at wastewater treatment plants to investigate the effect of processes or process agents. These trials are often carried out in cooperation with the relevant suppliers.

There are seldom ideal test conditions and in real operation many parameters play a role, so that it can easily come to the fact that effects are wrongly estimated. If the tests are not carried out with the necessary care and in compliance with the rules of good technical/scientific practice, incorrect conclusions can be drawn and, as a result, possibly ineffective process media and processes can be used.

Therefore, in this project, emphasis was placed on controlling the process well in order to obtain meaningful metrics. The first rule in such experiments is to be neutral about the result.

Large scale test

Already 10 weeks before the commissioning of the test plant at the ARA Unteres Pustertal, the series of measurements to determine the actual condition was started at the wastewater treatment plant.
The effect of the thermal pretreatment was to be comprehensively investigated, not only the foaming behavior, but also the degradation behavior, the dewatering behavior, etc. were to be recorded.
Subsequently, the test with thermal digestion of the ÜSS was carried out, but had to be stopped in the fifth week of the test due to failure of a pump. Due to long delivery times of the replacement part, it did not make sense to restart the trial for a few weeks afterwards.
Simultaneously with the large-scale test, the laboratory tests were run to investigate the effect of thermal pretreatment under controlled conditions.

The results

If one tries to summarize all results and experiences from literature and the present project, an inconsistent picture emerges!

In the laboratory, the COD digestion degree of the thermal pretreatment can be reproduced well.
However, an increase in biogas production due to thermal hydrolysis is usually only detectable in the case of digestion of ÜSS, while the effect remains insignificant in the case of digestion of mixed sludges (ÜSS+PS).

For the dewatering properties, no significant change could be detected in the present experiments, contrary to the experiments of J. Kopp, where significant change of the dewatering parameter TR(A) was obtained.

Contrary to all expectations, a significant change (increase!) of the maximum foam height by pretreatment was measured in the laboratory fed batch test.
This would therefore mean a negative effect on the foam behavior, which, however, does not fit with literature references and experience of ARA Tobl.

Because of the premature termination, the large-scale test ran too short overall, but the test gave similar results as in the laboratory: stable process, no influence on the dewatering behavior and no improvement of the foaming behavior.
But: the degree of degradation was reduced during the treatment, possibly due to larger amounts of precipitant dosed in the digestion tower during the trial.Thus, overall, no clear conclusions could be drawn yet and open questions remain:

To what extent do the results vary from wastewater treatment plant to wastewater treatment plant?
To what extent does the composition of the excess sludge influence the change in gas yield?

on the change of the gas yield
on the change of the dewatering properties
on the foaming behavior

Further investigations are necessary!

Optigest Results (German)