Wastewater Management in Durban and Bremen - a comparison
put together by hanseWasser
The Ethekwini Water and Sanitation (EWS) in Durban operates around 270 sewage pumping stations for wastewater. Stormwater pumping stations do not exist in Durban. However, rain water flow can be very high in Durban, so that rainwater drains into the exposed surface runoff channels.
In the city of Durban, the pumping stations have similar technical equipment and similar standards as in Bremen. Due to the age of the pump stations a wide range of technical designs can be found. Vandalism and theft are, however, a significant problem.
The static head is higher in Durban due to the hilly landscape. As a result, the use of inverters with variable frequency must be carefully examined in Durban. In Bremen pumping stations with a high dynamic head are in use and variable frequency converters reduce power consumption.
In Durban most pumping stations are connected to a central control room via a telemetry system. In this way, process data can be archived. In new or recently renovated pumping stations special measurement technology for the energy consumption is installed. In the future, this data will be integrated into a central database. Thus, the database can be used to analyze the energy efficiency of the pump stations.
Soft starters for pumps and pump dry weather (jockey pumps) are implemented in new or recently renovated pumping stations. This leads to a reduction of energy consumption.
Compared to Bremen in Durban more personnel is employed for the operation of the pumping stations. This means that more on-site inspections of the pumps are made. This can be used to determine the status of the pump on-site (if the flow rate and the pump output during the test are in good condition). If the machine operator has information about worn pumps, he can react, for example by buying new wheels for the pump and thus reduce the energy consumption of the pump station.
The EWS-treatment plants are equipped with all treatment stages, which are also used in Bremen, except for the production of energy from biogas. This energy potential is present in all systems with a secondary fermenter, but cogeneration is not a commonly used technique in South Africa. One reason is the need to meet the requirements of the grid operator for grid stability. Nevertheless, CHP has great potential to generate energy for Durban.
The biggest difference in the implemented process technology between Bremen and Durban is the ventilation of the biological treatment stage. In Bremen, this is implemented by fine bubble aeration, using turbo compressors. In Durban, a surface aeration is used. Surface aeration is a stable, but usually energy-inefficient process engineering. Aeration basins with an average depth of at least 4 meters can be equipped with fine bubble aeration. Apart from that, the amount of dissolved oxygen must be precisely and accurately controlled and monitored regularly.
Another difference is the degree of automation. Treatment plants in Bremen are almost completely automated, which reduces labor costs and increases process stability. At the sites in Durban a lot more people are employed. As a result of the lower degree of automation, the data base for a detailed energy balance of the treatment plants is only now being established. Nevertheless, an energy analysis of the wastewater treatment plants in Durban is possible.