Sludge is produced as a by-product during the treatment of municipal wastewater. The sludge of the WWTP Harnaschpolder is pumped into a digester, where methane gas is formed which is used for the generation of energy. The digested sludge is dewatered before being transported by trucks to the sludge processor HVC. During the transport of the sludge, odour is being emitted. In order to reduce the emission of odour during transport, HVC initiated the project “Odourless sludge loading”.
Delfluent Services BV (DSBV) wants to eliminate the possibility of odour emission during transport on the premises of the WWTP Harnaschpolder, as much as possible. Besides that, DSBV intends to improve the working environment in the loading station during the loading process and to introduce automatic loading of sludge. DSBV has therefore proposed Harnaschpolder as the pilot location for this project.
In order to minimize the odour emission, the choice was made for a completely closed system, whereby both the loading as well as the transport will take place under airtight conditions. The closed sludge container, which is installed on a truck, only has one filling opening. This opening is provided with a valve, which is closed during transport. A screw jack pushes the sludge to the end of the container, resulting in a maximum filling ratio. The maximum quantity of sludge is limited by the maximum tonnage capacity of the truck. A weight-sensor in the truck signals when this maximum is reached, upon which the loading will stop. The maximum net loading capacity is optimized through a combination of the construction material and the shape of the container.
The container is made of a light synthetic (composite) material, resulting in a higher remaining tonnage for the sludge. On top of that a choice was made for a shape of the container without corners, by which the sticky sludge will no longer remain behind in the container. By maximizing the net loading capacity of the truck, a yearly reduction of 5% of the number of sludge transports can be realized.
In order to make automatic loading at WWTP Harnaschpolder possible, a number of modifications had to be made. The WWTP has two silos for the storage of the dewatered sludge. Originally each silo had its own discharge point, through which the sludge was loaded into the truck by gravity. Because the sludge fell directly from the silo into the truck, the loading speed could not be regulated.
One of the requirements for the design was the possibility to regulate the loading speed. Therefore two screw jacks were mounted under the silos. The loading speed can be adjusted by changing the speed of the screw jacks, if necessary. The second advantage is that the two screw jacks transport the sludge to one discharge point under the silos. An airtight coupling is provided to make odourless sludge loading into the truck possible. Before sludge loading can start, the truck driver connects the truck electronically to the WWTP installation. After giving the start commando, the loading is performed automatically.
The pilot with the adapted installation and the new sludge truck started in February 2011 and was successfully concluded early June 2011. It is expected that starting from early 2012, only closed sludge trucks will take care of the sludge transport from Harnaschpolder.