Municipal sludge – known as sewage sludge – is a by-product of wastewater treatment. Sludge can also come from industrial facilities such as food manufacturers, pulp and paper mills, chemical and fuel plants, etc. Waste sludge treatment requires a specific process to ensure safety and not cause pollution to the environment.
1. What is sewage sludge treatment? Why is it important?
Sludge treatment is an important part of the municipal wastewater treatment process. This is essential because volume reduction and sludge treatment cost offset operations are significant at wastewater treatment plants. While sludge typically accounts for only 0.2 – 0.4% of the wastewater flow received by a treatment plant, its treatment typically accounts for up to 50% of the facility’s operating costs.
The main goals of sludge treatment are:
- Reduce harmful microorganisms present in sewage sludge, reduce the risk of adverse effects on human health and the environment.
- Stabilize some organic matter in the sludge that would otherwise naturally turn into harmful gasses in the atmosphere.
- Reduce its final volume, usually reducing the associated handling costs.
- Collect treatment products and by-products, which can be used or sold to offset sludge treatment costs.
2. Four standard steps in the industrial sludge treatment process
Sewage sludge is a by-product of treated wastewater with a composition including organic and inorganic materials, many phytonutrients, organic chemicals, and pathogens. Therefore, proper sludge treatment is extremely important to minimize its negative impact on the environment. The following is an overview of the 4 standard steps in the sludge treatment process to help you better understand the treatment techniques and process requirements:
2.1 The first step in sludge treatment: Thickening the structure of the sludge
The first step in a sludge treatment plan is called “thickening”. In this step, the wastewater sludge is thickened in a gravity thickener to reduce its overall volume, thus allowing for easy sludge handling. Dissolution of floating gas is an alternative that can be used to thicken sludge. This is an effective solution that uses air bubbles to allow the solid to rise.
2.2 The second step in the sludge treatment process: Sludge decomposition
After accumulating all the solids from the sewage sludge, we will begin the sludge decomposition process. This is a biological process in which the organic solids present in the sludge are broken down into stabilizers. This process also helps to reduce the mass of the total solid while killing any existing pathogens to make dehydration easy. The sludge decomposition process consists of two stages:
- The dry solid sludge is heated and mixed in a closed tank for anaerobic decomposition by acid-forming bacteria in the first stage. These bacteria hydrolyze the large molecules of proteins and lipids present in the sludge and break them down into smaller water-soluble molecules, then ferment them into various fatty acids.
- The sludge then flows into a second tank. It is metabolized by other bacteria to create a mixture of carbon dioxide (CO2) and methane, which is then collected and reused to provide energy for the plant. The tank decomposes and generates electricity (depending on the amount taken out).
2.3 The third step in sludge treatment: Drying and dewatering of the sludge
After recovery of valuable gasses and other by-products, the remaining sludge is dewatered before final sludge treatment. In most cases, untreated sludge usually contains significant amounts of water, up to 70%, albeit in a solid-state. Therefore, it is essential to dry and dehydrate the sludge. Using a sludge drying bed is the most common way to accomplish this process. Still, it is incredibly time-consuming and can even take several weeks to complete.
Therefore, you should plan for waste management and use specialized solid-liquid separators to carry out this process. In fact, the centrifugation method is gradually becoming one of the most popular sludge dewatering methods. The slurry through a centrifuge is quicker than getting all the water and allows for more straightforward solid waste disposal in less time at the lowest cost. Other alternatives include rotary drum vacuum filters and belt filter presses.
2.4 The final step in sludge treatment: Selection of disposal method
The final step in the sludge treatment process is sludge disposal after dewatering. You can bury the treated sludge underground in a sanitary landfill or use it as a fertilizer (depending on the chemical composition of the sludge). If the sludge is too toxic and cannot be reused or buried, you just need to burn the sludge and convert it to ash.
Sewage sludge should be treated following industrial processes. In addition, you should be aware of the source of the sludge and the method of sludge treatment. This helps you optimize your overall production and helps cut costs by taking advantage of valuable secondary wastes before complete sludge removal.