Sludge processing
After treatment, we are left with sewage sludge. Because waste is supplied constantly, the bacteria (activated sludge) multiply and this creates a surplus of bacteria. This surplus is called 'simple sludge' and is processed further at a central site.
All sewage sludge produced is digested first before it is dewatered.
Sludge digestion
The Garmerwolde wastewater treatment plant has a sludge digestion plant. In a sludge digestion plant, organic substances that are still present in the sludge are broken down. If we do not do this, these substances will rot. This causes odours and disrupts sludge dewatering. Sludge digestion reduces the amount of sludge. This results in lower costs for final processing.
In addition to the sludge from Garmerwolde itself, all sludge from our other wastewater treatment plants is also digested at the Garmerwolde wastewater treatment plant. Digesting sludge releases biogas, which is then used at the treatment plant itself. Two large engines use the biogas as fuel. The engines drive generators that produce electricity. We use the heat released during electricity production to maintain the temperature of the sludge digestion process and to heat the building.
Sludge dewatering and sludge drying
We have a central sludge dewatering site. The Garmerwolde site has five chamber filter presses. The sewage sludge that has been produced at the various wastewater treatment plants and that has been digested first, is transported here for further dewatering. Removing water from the sludge reduces its volume. This is called sludge dewatering.
The steps of sludge dewatering
Step 1: Pre-treatment
Before sludge can be pressed, it must be pretreated. This is necessary in order to be able to press it. Chemicals are added to make it easier for the water to flow away and to make the sludge flocs pressure-resistant. This allows as much water as possible to be removed from the sludge in the next step.
Step 2: Sludge pressing
The sludge is subjected to high pressure in the chamber filter presses. A chamber filter press is a device for mechanically dewatering sludge by applying pressure in a filter chamber. Because the sludge cannot be pressed through the filters, only water is removed. After a while, a solid sludge cake remains between the filter plates (chambers).
The extracted water is returned to the wastewater treatment plant, where it is treated again. The filter plates are automatically pulled apart one by one, after which the sludge cakes fall onto a conveyor belt. The sludge cakes end up in containers. From there, they are transported by lorry to the adjacent sludge drying installation.
Step 3: Drying sludge
The dewatered sludge from both locations is dried in the sludge drying installation of an external company. Heated air is directed over the sludge in rotating drums. The dried solid material is discharged as fine granules and used as fuel for power stations and in cement kilns, for example.
Collaborate
The Hunze en Aa's and Noorderzijlvest water authorities have drawn up a cooperation agreement for sludge processing. Noorderzijlvest also dewaters all the sludge that comes from Hunze en Aa's, and in the end it is all dried together.