If modernizations in wastewater treatment plants are due, as a rule the permanently reducing limit values of ammonium, nitrate and phosphate are pushing the investment decisions. If it concerns the budget, in view of the climate protection and the finances it pays off to include the energy efficiency in the plans. In view of the fact that just the air supply of the aeration often accounts for more than 70 per cent of the operating costs in this field, it soon becomes clear why efficiency improvements pay off especially here - and this with high return on investment periods. This is exactly how wastewater treatment plant Rheda-Wiedenbrück (326,000 population equivalents) planned. They are currently testing a brand-new control concept for the AERZEN blower technology, the AERsmart control combination.