By Jigar Patel
What is in sewage sludge?
Sewage sludge is a byproduct of treated wastewater. It is composed of both organic and inorganic materials, a large concentration of plant nutrients, organic chemicals, as well as pathogens. Therefore, it is extremely important to properly treat such sludge in order to minimize its environmental repercussions. Here is a brief overview of the sludge treatment process to help you gain a better understanding of the treatment techniques and process requirements:
Step 1 – Sludge Thickening
The first step in the sewage sludge treatment plan is called thickening. In this step, the sewage sludge is thickened in a gravity thickener to reduce its overall volume, thus enabling the easy handling of the sludge. Dissolved air flotation is another alternative that can be used to effectively to thicken the sludge by using air bubbles to allow the solid mass to float to the top.
Step 2 – Sludge Digestion
After amassing all the solids from the sewage sludge begins the sludge digestion process. This is a biological process in which the organic solids present in the sludge are decomposed into stable substances. This process also helps reduce the total mass of solids, while destroying any present pathogens to enable easy dewatering. The sludge digestion process is a two-phase process. In the first stage, the dry solid sludge is heated and mixed in a closed tank to enable anaerobic digestion by acid-forming bacteria. These bacteria hydrolyze the large molecules of proteins and lipids present in the sludge and break them down into smaller water-soluble molecules, which they then ferment into various fatty acids. The sludge then flows into the second tank where it is converted by other bacteria to produce a mixture of carbon dioxide (CO2) and methane, after which the methane is collected and reused to power the digestion tank and generate power (depending on the quantity retrieved).
Step 3 – Dewatering
After retrieving useful gases and other byproducts, the remaining sludge is then dewatered before final disposal. In most cases, dewatered sludge usually contains a significant amount of water, as much as 70 percent, in spite of its solidified state. Therefore, it is important to dry and dewater the sludge beforehand. While using sludge-drying beds is the most common way to carry out this process, it is extremely time-consuming and may take weeks before the process is complete. In order to quicken these processes, waste management plans are also employing solid-liquid separation devices to carry out this process. In fact, centrifugation is slowly becoming one of the most preferred methods of dewatering sludge. By passing the sludge through a centrifuge, it becomes easier to retrieve all the water and enable easier handling of the solid waste in shorter durations at reduced costs. Other alternatives include the rotary drum vacuum filter and the belt filter press.
Step 4 – Disposal
Once the sludge has been effectively dewatered, it can be buried underground in a sanitary landfill or can be used as a fertilizer, depending on its chemical composition. In cases where the sludge is too toxic to be reused or buried, you can simply incinerate the sludge and convert it into ash.
While sewage sludge is usually treated using a standard plan of action, it is extremely important to factor in aspects like the origin of the sewage, the treatment process used to reduce the sewage to sludge, as well as the possible byproducts that can be retrieved from it for further use before choosing a sludge treatment plan. This will not only help you optimize your overall output, but will also help you reduce costs by salvaging useful materials for secondary use before ultimate disposal.
Jigar Patel is the director of Oriental Manufacturers. He believes in the power of good, functional design and its ability to boost productivity and drive growth. Fueled by his passion for innovative designs and all things EPC (engineering, procurement, and construction), Jigar took to blogging about all the information and technical know-how he has amassed over the years. He writes on topics related to process machinery production, turnkey solutions, best industry practices, and his personal insights.
Image credit: "Sludge Treatment," Klearchos Kapoutsis © 2008, used under an Attribution 2.0 Generic license: https://creativecommons.org/licenses/by/2.0/