The regulations governing biosolids process are so strict that treatment plants have to regularly study the processes they use in order to get the necessary permits and licenses from governing authorities. But these rules and regulations are essential to assure that the treated sewage sludge can be applied on land used for food production.
The most critical stage of treating biosolids is the dewatering and drying process. These two processes will ensure that either the biosolids will contain almost no trace of pathogens and other harmful contaminants or it can be transformed into a source of renewable energy.
It is important to note that before biosolids are produced, they are first pre-treated to remove the harmful contaminants it contains. The solids are removed from the wastewater and pH levels are increased to remove the odor. The sanitization process will then control the pathogens.
Dewatering refers to the removal of the liquid sludge from the wastewater and thus converting it into a solid product. Once the water has been removed, the drying procedure will commence. The drying technologies used will elevate the temperature of the wet solids to remove the remaining water by evaporation. These high temperatures are also intended to kill remaining pathogens as required by regulatory agencies.
Drying biosolids have no effect on the nutrient content of the organic matter. After the process, the material will remain to be a nutrient-rich organic fertilizer that can be used for agricultural purposes.
Once the dewatering and the drying process has been completed, the biosolids pellets will be cooled down to a thermally stable temperature before storing, packaging, and transporting them. There is danger to biosolids that have not been properly cooled. They could be at risk of auto-ignition, fires, and explosions.
There are two types of drying technologies used on biosolids: direct dryers and solar dryers. Direct dryers use hot air or gas in direct contact with the product to increase the temperature of the wet solid.
There are two prime equipment used for this process: rotary drums and belt dryers. Rotary drums will be fed by a heat supply (like a furnace) that produces an exhaust gas. The heat comes in direct contact with the product. In this scenario, the exhaust gas can be recycled and used in the process again.
On the other hand, belt dryers are when the dewatered solid flows along a slow-moving belt that is exposed to hot air produced by the furnace.
Solar drying is a slow drying process that uses the heat produced by the sun. The dewatered solids are placed inside a greenhouse enclosure to heat and dry the moisture from the wet solids. Elements such as cloud cover, humidity, temperature, and wind speed can slow or delay the process.