Attempt to optimize a secondary clarifier

Status and state of the art

A secondary clarifier separates activated sludge from biologically treated wastewater by allowing it to settle. The separated sludge is collected in the center of the tank in the sludge hopper, thickened statically, and returned to the biological treatment process, while the treated water flows off the surface via a weir.

Two identical secondary clarifiers, which were designed more than 25 years ago, are in operation at the Unteres Pustertal wastewater treatment plant. Today, these tanks would probably be designed differently, as technical knowledge and understanding of the flow processes in a settling tank has increased enormously since then.

What is striking is that our tanks do not have a guide floor that directs the flow from the inlet structure outwards into the tank.

Status and state of the art — Construction plan of the existing secondary clarifiers, section

Different operating conditions require different inlet openings.

In the past, these basins were designed statically for maximum inflow. However, the actual load is extremely dynamic: inflow volumes can fluctuate greatly throughout the day, and there are also seasonal influences, sludge index, dry matter, and floc structure of the activated sludge.

With modern measurement technology and the enormous computing power of computers, complex flow simulations based on many years of experience are now possible, allowing hydraulic flows, velocity distributions, and density currents to be simulated and flow-optimized basins to be adapted to actual operating conditions.

Here is an excellent publication for anyone who wants to delve into the subject. Although it is almost 20 years old, it provides a good overview:

Institut für Strömungsmechanik
und Elektronisches Rechnen im Bauwesen
der Universität Hannover
Bericht Nr. 73/2006
Sebastian Schumacher
Leistungsbestimmende Prozesse in Nachklärbecken
- Einflussgrößen, Modellbildung und Optimierung -

Today, adjustable inlet structures are also frequently built.

Different operating conditions require different inlet openings. — Flow simulation; Institut für Strömungsmechanik und Elektronisches Rechnen im Bauwesen der Universität Hannover, aus Bericht Nr. 73/2006, Sebastian Schumacher

Issue

Under ‘normal’ operating conditions, our secondary clarifiers functioned without any problems. However, in recent years, there have been frequent problems due to changes in sludge parameters (index): when both tanks were operated in parallel, the return sludge outlet in one tank often became blocked, resulting in uneven sludge distribution. As a result, sludge accumulated in one basin and the other was no longer fed. This in turn caused a sharp drop in RLS-TS and associated problems with thickening and aeration. Although this blockage could be remedied quite easily, it should not have happened in the first place and we were unable to find a satisfactory solution that worked reliably. The safest option was to leave only one basin in operation, but this could lead to overload and associated sludge discharge during heavy rainfall events.
With a high index and associated poor settling properties, a lot of sludge also accumulated in the basin, which was difficult for the scraper to transport into the sludge hopper.

Discussions with various experts confirmed the problem, but no one was able to assess whether an improvement was possible. This would require a survey and flow simulation, which could cost €20,000 or more. After that, the results would still have to be verified in real operation.

Own experimental setup

So, after much deliberation, we decided to take action ourselves: we simply wanted to test whether improvements could be made to the operation. To do this, we covered our own pipe construction made of stainless steel pipes with PVC pond liner as a flow guide and installed it in one of the two secondary clarification tanks. The material costs for this were relatively low, and we carried out all the welding and assembly work ourselves. For reasons of stability and safety, however, we decided not to use the structure in a height-adjustable manner, as originally planned. Nevertheless, we consider the trial installation to have been very successful; the basin could then be refilled and put back into operation.
This means we can now compare the operation of both basins, the original and the converted one.

At the same time, we incorporated a change in the feeding strategy into the software: normally, we now feed the basins alternately, with only one basin remaining in operation and the inlet valve to the other basin remaining closed. The switchover takes place automatically after a configurable time. Only when the inflow volume requires it are both basins operated in parallel. This operating mode has already proven itself, as it prevents the sludge outlet from becoming blocked.

Over a period of at least one year, we will collect data and observe whether our assumptions are confirmed and whether there are more positive than negative effects, or whether it would be better to restore the site to its original condition.

We will be happy to publish the results here again.