Interview with the technical director of the AZV Heidelberg about the cooperation

From practical experience: IGB concept for sewage sludge disposal realised in the large dimension

The high-load process for sewage sludge disposal developed at the Fraunhofer IGB was integrated in 2001 into the existing disposal line of the municipal sewage treatment plant in Heidelberg, consisting of three egg-shaped digestion towers.

The result: Today, the Heidelberg wastewater treatment plant converts sewage sludge into biogas in a much smaller space and much more quickly and effectively. This enables the Heidelberg company to save costs and generate energy while at the same time having to dispose of less digested sludge.

On the occasion of the 25th anniversary of the Abwasserzweckverband (AZV) Heidelberg on September 24, 2002, the sewage treatment plant invited the interested public to visit the new upstream high-load digestion plant.

On the occasion of this anniversary, the Fraunhofer IGB held a meeting with the Technical Director of the AZV Heidelberg, Dipl.-Ing. Jürgen Weber. Jürgen Weber studied engineering sciences in Karlsruhe with focus on residential water management.

IGB: Mr. Weber, since April 2001 there is a cooperation between the Abwasserzweckverband Heidelberg and the Fraunhofer IGB. What was the initial situation and what challenges were involved?

Weber: The initial situation was very simple. We have a wastewater treatment plant that was built in 1982 from the water side, which works according to the latest state of the art, i.e. it also biologically eliminates the nutrients phosphorus and nitrogen. On the sludge side, we had and still have three old digestion towers, each with a capacity of two and a half thousand cubic meters, which were still from the old sewage treatment plant built in the 1960s. These digestion towers were no longer able to process the sewage sludge produced, i.e. to stabilise and gasify it.

IGB: In a situation like this, you usually hire an engineering firm to find a solution.

Weber: Correct. We had to react in order to get the matter under control and then also announced an engineering competition with the requirement to develop a concept for expanding the sewage sludge treatment to 360,000 PE, taking into account the latest aspects, i.e. dewatering, digestion, energy recovery from digester gas, etc. Three offices took part in the tender. The proposed concept provided for a two-stage project, first of all the dewatering was to be redesigned, and in a second step afterwards the sludge digestion was to be optimized. Furthermore, the concept of the engineering office envisaged simply extending the existing digestion towers of the old type by a fourth digestion tower and thus the problem of digestion should be solved. We were not sure - and I was convinced that it was not necessarily the best solution to add another large concrete digestion tower. We then considered how we could test this. And then we simply operated a digestion tower at exactly the same load as it would have been subjected to if the sludge digestion had been remediated according to the engineering company's specifications. We reduced the load and operated the digestion tower in this way for six months. But the reduction of the load did not affect the digestion tower at all, it had the same problems. It was foaming very strongly, overboiling and had a really insufficient degree of degradation.

IGB: And then...?

Weber: ... then by chance I read an invitation from the Fraunhofer Institute. It was about the annual seminar that the institute holds on municipal wastewater technology. I went there and listened to the philosophy of the Fraunhofer Institute. At that time there was a research project "Energy from Sewage Sludge", which was carried out by the Fraunhofer Institute and funded by the Ministry of the Environment of Baden-Württemberg, and we simply joined it. We took our sludge to the Fraunhofer Institute, and there it was tested in the pilot plant, in the semi-technical plant, for degradability and for feasibility. The result was that a very high degree of degradation can be achieved in a high-load digestion plant. That was ultimately the decision for us to cooperate with the Fraunhofer Institute and so the project, tailored to Heidelberg conditions, was jointly thought through, jointly developed and finally implemented and completed by 2002.

IGB: What has been your experience with this procedure since then?

Weber: You have to admit that it is a completely new procedure and one is a little suspicious at first. That's how I felt personally. But I have not seen any other way. And the concerns we had, or which I had, were completely dispelled. The plant, since it has been in operation, has been stable, has a very high degree of degradation, a really very high degree of degradation. It does not foam - in other words, everything we had hoped for, which was also guaranteed to us by the Fraunhofer Institute and by the supplier, has been achieved.

IGB: What prospects do you think this technology has?

Weber: This technology can certainly be applied to any wastewater treatment plant, the question is what is already available and how can I integrate it. We in Heidelberg, together with Professor Trösch, have designed this in such a way that we have designed the plant to a certain value, the experts say to the 85% value that the plant can always be operated in the optimal range with 10-12 kilograms of organic matter per cubic meter / day and if peaks occur, - these peaks are automatically bypassed around the high-load digestion in the normal process and the downstream digestion is initiated. We have done very well with this concept so far, and I think that this solution is also suitable for sewage treatment plants that have the same problems as we do: non-functioning digestion towers with unfavourable geometry, and here I also see a point of application for the upstream high-load digestion to achieve a better degradation rate and also to ensure the process stability of a downstream digestion.
For sewage treatment plants that do not yet have digestion and have a population equivalent of 10, 15, 20,000 and a wet sludge disposal or stabilisation plant, I would prefer a two-stage high-load digestion system such as the one in Leonberg. For them, it may well be economically viable to set up a high-load digestion plant. The sludge reduction, the quantity reduction and the energy recovery of the sewage gas must be compared with the operating costs of a current operation. So for sewage treatment plants which do not yet have any digestion at all, and these are the smaller sewage treatment plants, I see an interesting and cost-effective application, also with regard to the new requirements which are coming up for sewage sludge disposal. I think you have to keep in mind that there will be a lot of changes with regard to sewage sludge disposal.

ITUC: Has there already been an echo among experts on the new technology as it is used here in Heidelberg?

Weber: Yes, so the plant is running and we have already had many visits. We have had the ATV here, the sewage treatment plant neighbours who have looked at the sewage treatment plant. And as I said before, it is a slightly different philosophy that the Fraunhofer Institute pursues. At first the colleagues and the specialist companies and the specialist offices that were there were a little sceptical. When they saw the plant, how it works, how it is constructed, how it is designed, and when they saw the operating values and spoke to our operating personnel, this scepticism was overcome by most of them.

ITUC: And as Fraunhofer IGB, we naturally hope that even more sceptics will be convinced by our technology here in Heidelberg. Mr. Weber, thank you for this interview.