Saving energy at a recirculation plant

A sewage treatment plant in Denmark needed to rebuild four process tanks to reduce energy consumption. A cost-effective solution was chosen which involved the installation of one propeller mixer in each tank. After two months of operation, anticipated energy savings at the plant were met and exceeded.

Holstebro Sewage Treatment Plant in Denmark is a recirculation plant of around 200,000 PE. The plant is equipped with a preliminary treatment plant for reject water and wastewater from a milk condensing factory. It also contains a biological section with four process tanks (aeration tanks) and four clarifiers.

In 1998 Purac carried out a rebuild of the aeration tank in the preliminary treatment plant. Two propeller mixers were used for recirculation, and this formed the basis of rebuilding the four process tanks. Purac was chosen again for the job and the company's first suggestion was to replace the three pumps with two propeller pumps and a non-return valve in each of the two pipes. That solution was rather expensive. They came up with another solution with just one propeller mixer in each tank and no non-return valve. This much cheaper solution was chosen. Furthermore, the sewage treatment plant decided to buy a spare propeller mixer so that they would be able switch them immediately if the pump should break down.

During the summer of 2008 two of the process tanks were rebuilt, and in the summer of 2009 the rebuild of the last two tanks took place. In total, three 15 kW pumps with an output of 1,040 m3 were replaced with a frequency controlled Landia 4 kW propeller mixer, with a 1,500 m3 output. A flow measurer was mounted onto the pipe to control the recirculation level.

Energy savings

In order to ensure the energy savings were correct, the power consumption of the old pumps was measured. The 3.1 kW pumps were found to have a power consumption of 3.0 kW, and the 8.8 kW pumps had a power consumption of 8.5 kW. A combined annual energy saving of approximately 192,000 kWh per year was calculated.

After two months of operation with the new propeller pumps in all four tanks, the result was even greater than expected – the annual saving of energy turned out to be about 224,000 kWh and the control of the recirculation was improved. The total invested amount was approximately €121,300 and the annual saving was € 24,300, i.e. a payback time of almost five years.

Holstebro's energy initiative extends to the anaerobic digester system, where gas is produced and reused in the CHP engine to produce both electricity and heat. The electricity is subsequently sold to the grid and the heat is used primarily for heating the buildings at the plant, with any excess heat sold to the municipality of Holstebro. Any remaining sludge is spread onto farmland.

Throughout the Holstebro plant, extended online instrumentation is installed to optimise the treatment process and ensure minimum electrical consumption. By continuously monitoring all installations, the lightest possible carbon footprint is left behind, and money is also saved due to correct dosage of chemicals, giving the bacteria optimum treatment conditions.