More safety on thermal drying plants of sewage sludge

The amount of sewage produced has dramatically increased worldwide over the last years due to population growth and industrialization. Water treatment produces a large quantity of waste that has to be disposed with the least environmental impact. In order to solve this issue, new wastewater treatments and reuse are being developing. Today, the thermal drying of sludge is the most widely used since the dry sludge are considered alternative solid fuels.

The thermal process involves a moisture decrease from 90% up to 5-10%. This factor, along with the great dryness of the product, is responsible for the presence of large amounts of dust in these types of buildings causing diverse problems. Therefore, it is needed to take into account both, the pollution problems and flammability properties of this kind of dust. We need to pay special attention to the second factor, the flammability properties are essential parameters for safe operations in this types of plants. These properties are classified into four groups: ignition sensitivity, explosion severity, sensitivity and thermal stability.

In order to identify these parameters, researchers started a R&D project at the Madariaga Lab (LOM) from the Universidad Politécnica de Madrid and funded by the Lloyd's Register Educational Trust (LRET) in which they collected 20 samples of thermally dried sewage sludge in eight Spanish plants and in different seasons of the year.

They found diverse explosibility parameters in the groups mentioned before. They also studied the variability of these parameters compared to their origin and the season of the year when the samples were collected and they observed similarities due to these two factors. In addition to the flammability properties, researchers also found diverse chemical and physical properties in the samples and detected significant links between composition parameters and flammability properties of thermally dried sludge.

Thanks to these links, researchers are able to the reduce explosion risk in normal operation through the control of simple parameters. For example, explosion risk can be reduced by adding a percentage of inert materials to the samples. This preventive measure would reduce the volatile content. Finally, the properties related to the sewage sludge spontaneous combustion were identified. These parameters are crucial to develop a safe design of the processes and machineries for storage and transportation of these materials.

In addition to the results obtained in this project, there are some practical preventive measurements than can be easily implemented and have a great value when it comes to improving safety in this types of industrial plants.

More information (in Spanish) can be found at: http://minas.upm.es/images/PDF/Gua%20Lodos%20LRET-LOM_Versin%20digital.pdf