to sufficiently stir up the pollution load introduced after every rainfall event so that residual biomass is reliably removed.
4x HRS11/4-230-204 EX
Waste water and sewage systems
It is commonly held that agitators are used to clean circular stormwater overflow tanks and injector jets are used to clean rectangular ones. This received wisdom can be found in all current specialist literature on the topic of tank cleaning and is also true of many installations. Two injector jets, measuring 24 x 7 m, were also installed in the stormwater overflow tank at the Schmitt sewage treatment plant for the municipality of Ulmen in Germany. However, they were repeatedly causing problems, and as a result, an increasing number of emergency interventions were required in the sewage treatment plant as a whole. Ultimately, to overcome the problem, the system operator decided on an unusual proposal by HOMA Pumpenfabrik GmbH: to use agitators in the rectangular tank. The municipality of Ulmen was surprised by the result.
“Each time there was heavy rain, even at the end of the working week, we knew that the system would be reporting a fault at the weekend. Emergency interventions during on-call duty – sometimes at 2 o’clock in the morning – had become the norm,” explained Frank Jäger, Senior Waste Water Technician at the Ulmen municipal waste water works. The installed injector jets were extremely susceptible to failure. Firstly, the impellers of the connected pumps became clogged regularly due to the high proportion of solids in the mixed water, and secondly, cleaning was inadequate. The jets had long runtimes without achieving satisfactory cleaning. The high quantity of deposits took a considerable amount of manpower to remove. The even greater disadvantage of this manual cleaning, however, was the impact load on the system. Whilst it is possible to clean the tank in this way, the high quantities of solids introduced impede the downstream processes in the sewage treatment plant. Consequently, either the bar screens or pumps would report faults. A solution needed to be found to avoid this unnecessary and unpleasant extra workload for employees.
The task was clear. An energy efficient tank cleaning system less susceptible to failure needed to be found. “The idea of installing four HOMA agitators sounded risky at first. To be honest, if HOMA hadn’t given us the opportunity to install this as a trial system, we wouldn’t have agreed,” commented Torsten Steffgen, Deputy Works Manager at the Ulmen municipal waste water works. The process was overseen by HOMA employees Alfred Kleu, Head of Application Management, and Florian Steiner, Waste Water Systems Consulting & Sales/Distribution. “As we have confidence in the performance of our agitators, we were certain that they would achieve the required effect in precisely this type of rectangular tank and would minimise deposits. If it hadn’t worked, we would have simply removed the agitators again. However, we were positive that this would not be necessary and the result would be convincing,” explained Mr Kleu.
A partner in this project, in addition to the Ulmen municipal works, was a flow simulation company. On the digital drawing board, the performance of the agitators was impressive, but would they achieve the same results under real operating conditions?
As with all systems that use agitators, a quick break-up time of less than 30 minutes is ideal so that the solids and sediment do not get a chance to accumulate. Switchover between interval and continuous operation is also extremely important. Particular attention was paid to both these aspects in the trial system. During installation, the agitators were therefore positioned with millimetre accuracy. Flow-optimised alignment was also ensured. A further issue was that the moving medium with a heavy pollution load resulted in increased adhesion to the agitator cable. A cable inlet at the top solved this problem.
When installation was complete, all those involved waited expectantly for the first real test in the form of a significant rain event. Even the weather gods were apparently curious and didn’t keep them waiting long. After the first heavy rainfall, everyone was amazed at the result. Only very small sand deposits were evident on the floor of the tank. “We initially left them in the tank and then they too were flushed away the next time it rained,” recalled Robert Schüller from Ulmen municipal waste water works.
Since the installation in January 2019, the employees responsible at the sewage treatment plant can once again relax at the weekends. Not one single fault has been reported since the agitators were installed. “At first, I kept checking that my mobile had reception when I was on call, as we were so used to faults being reported. But there was nothing – good reception, and no faults. We are extremely satisfied with the new solution and glad that the experiment was a definite success,” said Robert Schüller.
The figures speak for themselves. Zero reported faults, as the supply of medium with a high proportion of solids now flows constantly and the system is not overloaded. Cleaning of the installed agitators is also optimal with a significantly lower total output: output of injector jets 2 x P1: 7.45 kW; output of agitators 4 x P1: 1.7 kW.