eThekwini Water and Sanitation (EWS) in Durban operates around 270 wastewater pumping stations. Despite rainwater flow that can reach high levels, the city has no stormwater pump stations; it relies on open drainage channels to discharge rainwater.
Wastewater pump stations in Durban have technical equipment and standards that are comparable to those in Bremen. Due to their varying ages, the pump stations exhibit a broad range of technical designs. Vandalism and theft lead to shutdowns and are responsible for considerable economic losses.
Pumping wastewater through Durban’s hilly landscape necessitates a higher static delivery head than is needed in Bremen. Consequently the adjustable frequency converters require continuous adjustments. Bremen also employs pumping stations with a high dynamic delivery head; variable frequency converters ensure that power consumption is reduced.
In Durban, most pumping stations are connected to a central control room via a telemetry system that enables the archiving of process data. New and recently renovated pump stations have specialised technology for measuring energy consumption. In the future this measurement data will be integrated into a central database that can be used to analyse the energy efficiency of the pump stations.
Energy consumption is also reduced via the soft starters and dry-weather pumps (jockey pumps) that have been installed in new and recently renovated pumping stations.
Compared to Bremen, EWS employs more staff to operate its pumping stations. This means that more on-site pump inspections are performed in order to check flow rate and pump power and determine the status of the pumps. These inspections are not only for keeping the station running; they are also tools for managing energy efficiency. For example, when inspections reveal worn pumps, operators can react by purchasing new impellers that reduce energy consumption.
As in Bremen, EWS wastewater treatment plants are equipped for all treatment stages, with the exception of energy production from biogas. This energy potential is present in all systems with a secondary fermenter, but combined heat and power (CHP) units are not common in South Africa. One reason for this is the need to meet electric grid operators’ requirements for grid stability. Nevertheless, CHP holds great potential for energy generation in Durban.
The biggest difference in the process technology implemented in Bremen and Durban is the aeration method used in the biological treatment stage. Bremen uses turbo compressors for fine-bubble aeration, whereas Durban uses surface aeration, which is a stable but typically energy-inefficient process. Aeration tanks with an average depth of at least four meters can be equipped with fine-bubble diffusers, then the dissolved oxygen level must be precisely controlled and regularly checked.
Another difference between implementations in Bremen and Durban is the degree of automation. Bremen’s wastewater treatment plants are almost completely automated, which reduces labor costs and increases process stability. Lower levels of automation in Durban’s treatment plants translate to more employees and more time-consuming procedures for tracking energy consumption. To analyse energy efficiency, plant operators rely on widely accepted specific values and the data they collect on the engines’ net output and running hours. A database is being set up to aid in automating this process, allowing for a more detailed analysis of the energy consumption balance.