Edition 01/16

When it was decided that the 23-year-old oil heating system in the operations building required renovation, the main consideration was to arrange its replacement. However, in parallel with this decision process, they had the idea of using the heat coming from the three positive displacement blowers to heat the room and the warm water. “The awareness developed continuously”, reflects Harald Gerbeth, head of the wastewater treatment plant. Physically related waste heat originating from the compression of air, mainly during the warm summer months, was recognised as having unexploited potential. It is important to understand that Wertach is one of 15 award-winning communities in Bavaria which have been particularly successful in their commitment to dealing with climate change as pilot communities under the European SEAP project (Sustainable Energy Action Plan).

Utilise heat from the process air

Conversion of the wastewater treatment plant was translated into action during the three-year SEAP-project phase. The objective was to extract heat from the blower air and use this to supply the hot water for the heating system boiler. With this system, the air coming from the AERZEN positive displacement blowers flows into the air-water plate heat exchanger with a temperature of 68 degrees C and leaves it in direction to the aeration basin with a temperature of below 30 degrees C. “Thus, we are utilising a delta of about 40 degrees”, underlines Gerbeth.

The system works simply but effectively: in case of low heating requirements in the operations buildings, a mechanical flap in the piping can be set so that the air coming from the blowers is fed into the basins without any detours. When the requirement for heating is high, the volume flow can be channelled through the heat exchanger completely or partially before the air reaches the basin. To ensure that the heat recovery does not affect the operational costs of the blowers, flow-optimised profiles are installed in the plate heat exchanger. The pressure losses are only about 0.05 bar with a delta p of about 0.5 bar. “After all, the Delta Blowers are primarily destined to supply air for aeration of the basins and not to generate heat”, wastewater manager Gerbeth, who heads the wastewater treatment plant of the community Wertach as independent contractor, makes clear.

Simple and efficient technology

The wastewater treatment plant, located in the Allgaeu region, has a total of three Delta Blowers (GM 4 S, Vmin 46 m³/h, Vmax 342 m³/h) with an installed power of 7.5 kW each. Combined into a cascaded group, two Delta Blowers with fixed speed cover the permanent base load. The third positive displacement blower works with variable speed, to adjust the volume flow energy-efficiently to requirements. 

All in all, the technology installed in Wertach is nearly maintenance-free, as the construction with only two pumps, a small control system and a buffer storage with feelers and the self-cleaning plate heat exchanger has been designed effectively and simple. And the linkage of the positive displacement blowers including heat exchanger installed was simple. For transport of the recovered heat they only had to recess an isolated pipe including control cable in the floor. The connection to the preserved heater network is accomplished via a used buffer storage, which was not needed anymore at the Wertach primary school. The overall costs for the modifications amounted to about EUR 13,400. The annual savings of fuel oil, of 1,850 litres on average, correspond to about 1,000 Euros and five tons of CO₂.

The example at Wertach shows how energy efficiency can be increased easily and with short ROI periods with currently available technology. Gerbeth sees the highest potential in linking positive displacement blowers and plate heat exchangers mainly for smaller wastewater treatment companies. So, heat recovery from process technology is becoming ever more important. AERZEN is closely tracking this trend and providing engineering support, so that positive displacement blowers and heat recovery are perfectly in tune with each other. Moreover, in the medium-term we recognize that in blower technology heat recovery will have an even greater influence on the design of the complete solution.