Before the optimization works, nitrification and denitrification took place simultaneously in the aeration tanks, process-controlled by NH4 und NO3. A maximum total of 16 Mammoth Rotors with each 45 KW power load had been operated for the aeration. After 20 years, the operation mode had become outdated and the energy consumption was comparable high. In addition, the oxygen distribution inside the aeration tanks has been less than ideal.
The wastewater treatment plant in the city of Gotha, located in the county of the same name, has been built in 1993 and treats the wastewater of the city of Gotha as well as of surrounding villages. At the time of its construction it was one of the most modern of its kind. The treatment plant is designed for 150,000 PE and currently loaded with a COD equivalent of approx. 82.000 PE. The biological treatment had been realised via two looped aeration tanks with a total volume of 23,000m³, a downstream circular tank for phosphate elimination as well as four secondary sedimentation tanks.
During the concept stage for the optimization of the biological treatment, the projected energy savings were approx. 72,000 €/year. Considering a capital investment of 420,000 €, the amortization time would have been approx. 6 years.
A recent analysis revealed an actual energy conservation of 30% (see diagram), which exceeded the expectations. Based on the actual energy costs of 19.8 ct/kWh and a monthly energy conservation of average 53,500 kWh, the actual savings amount to 127,300 € per year, reducing the amortization time to approx. 3.3 years. The total energy consumption of the biological stage in relation to the load has improved from 26.2 to 20.6 kWh/(PECOD*year). The following diagram demonstrates the savings during the individual optimization stages. The average outlet parameters in terms of overall nitrogen concentration are far below 5 mg/l.
Energy savings with efficiency improvements on the biological treatment
Wasser- und Abwasserzweckverband Gotha und Landkreisgemeinden
150,000 / 82,000 PE
2013 / 2014
Installation of 8 submerged stirrers and 8 guide baffles for the surface aerators, installation of an Aqualogic®-control system and according sensors