How Does Wastewater Sludge Treatment Work?
By: Tom Frankel
Post Date: Luglio 10th 2024
How Does Wastewater Sludge Treatment Work?
In the United States, wastewater undergoes treatment as mandated by the U.S. Environmental Protection Agency. Wastewater contains organic and inorganic materials, chemicals, plant nutrients and even pathogens, which can be toxic to humans and the environment. The wastewater treatment process includes sludge treatment and the observance of proper sludge disposal methods.
Wastewater treatment is necessary to remove pollutants from the wastewater and prevent the contamination of natural resources. It also makes treated sludge viable for other uses, ranging from fertilizers to alternative fuels. Learn about what sludge is and how it is treated to protect the planet and allow for the reuse of this substance.
What Is Sludge?
Also known as sewage sludge, municipal sludge is a by-product of treating wastewater. It typically consists of food waste particles, human waste, microorganisms, residual chemicals from everyday medicines and products, and water bound to these materials. This substance can be in solid, semisolid or slurry form and is divided into two main classifications:
- Primary: Primary sludge comes from the primary sedimentation treatment of wastewater.
- Secondary: Secondary sludge is activated waste that results from biological treatments.
Sludge is referred to as biosolids after undergoing significant treatments to stabilize it and decrease its pathogens. Stabilized sludge has no unpleasant smell and can be handled without harming human health or causing other issues. This term helps distinguish treated sewage sludge from its raw counterpart, which contains high levels of pollutants.
What Is Sewage Sludge Treatment?
Like wastewater, sludge undergoes a treatment process. The sludge treatment process reduces sludge volume and weight and ultimately minimizes the costs of transportation and disposal for wastewater treatment plants. As mentioned, it also destroys pathogens that may carry potential health risks. Let’s take a closer look at the principal aims of this process:
- Reduction of hazardous microorganisms: Reducing these microorganisms decreases the health risk for humans handling the material.
- Stabilization: Without stabilization, the organic matter found in sludge would naturally transform into toxic gases in the Earth’s atmosphere. By treating sludge, it is safer for the environment.
- Reduction of volume: As the final volume is lower, the handling expenses, such as pumping and storage, usually decrease correspondingly.
- Collection of products and by-products: Water companies can use or sell these to offset the expenses of sludge treatment.
Reducing the volume and counterbalancing costs can make a notable difference for treatment plants. While sludge might make up a small percentage of these facilities’ wastewater flow, treating this substance can account for a large portion of their operating expenditures.
How Is Sludge Treated?
Sludge treatment typically consists of four main processes — thickening, digestion, dewatering and disposal.
1. Thickening
This is usually the first step in the sludge treatment process, where sludge is stirred with clarifying agents to create larger, solid aggregates. Even with most of the water removed in the early stages of wastewater treatment, raw sludge is still highly liquid, making it impractical to handle. Thickening further separates the water from the solid components and can reduce the sludge’s volume to less than half its original volume.
2. Digestion
Digestion is a biological process that involves reducing the amount of microorganisms and organic matter in solid sludges. In addition to destroying pathogens, this process decreases the total mass of these solids by converting them into liquids and gases. It also makes the following process of dewatering much easier. The end product of digestion is inoffensive, with a similar look and texture to potting soil.
The digestion process is further classified into three methods:
- Anaerobic digestion: Many large treatment plants use this method, consisting of a two-step digestion system that metabolizes bacteria anaerobically — without oxygen. First, the sludge is heated and mixed in a closed tank for a few days, where acid-forming bacteria cause large molecules to break down into smaller molecules before fermenting these into a range of fatty acids. Next, the sludge moves into another tank, where other bacteria convert the dissolved matter into biogas, a fuel source consisting of a blend of methane and carbon dioxide that can be used in cooking and heating.
- Aerobic digestion: With this method, digestion occurs aerobically — with oxygen. The sludge is placed in an open tank and thoroughly aerated for around 20 days. Unlike anaerobic processes, methane is not a by-product of aerobic digestion.
- Composting: This is an aerobic digestion process that involves mixing sludge with other forms of waste, including straw, green waste or wood chips, to create a pasteurized substance that can be composted. Once composted, thermophilic and mesophilic degradation occurs naturally, converting solids into carbon dioxide.
3. Dewatering
Digested sludge is usually dewatered before being disposed of to reduce its water content. The result is a nonliquid material called a cake. Although sludge still contains 50% to 75% water after dewatering, it can be handled as a solid material. Sludge with more water undergoes further treatment.
The dewatering process can be done in one of the following three ways:
- Evaporation: One of the simplest dewatering methods involves placing digested sludge into sand-filled drying beds. The sludge is dried via a combination of gravity drainage and evaporation, and the process can take up to six weeks.
- Centrifugation: Centrifugal dewatering is a quick process that rapidly rotates the sludge in a cylindrical bowl, drying the substance by separating the wastewater solids from the liquids.
- Filtration: Sludge can also be dewatered through filtration techniques such as the belt filter press (BFP) or the rotary drum vacuum filter (RDVF). The BFP dries sludge by pressing the sludge to remove water, whereas the RDVF rotates the sludge in a drum and pulls the liquid through a filter.
4. Disposal
As the final step of sludge management in water treatment, the dewatered sludge gets transported to designated disposal sites. Landfills, where sludge is usually buried underground, are the most common sites for disposal. Other sludge disposal methods include:
- Land application: Biosolids are often used in agriculture, forestry and gardening as fertilizers or soil conditioners. However, as treated sludge can still contain potentially toxic chemicals, it is not spread over crops grown for human consumption.
- Incineration: Biosolids may be incinerated if an appropriate site isn’t available for land disposal. This method involves evaporating all the moisture in the sludge before transforming the solids into inert ash, which is then disposed of.
- Reclamation: Treated sludge can be used in reclamation sites to cultivate vegetation sustainably, lessen the availability of toxic compounds in soils, limit soil erosion and restore layers of soil in areas where they are damaged.
The Importance of Wastewater Sludge Treatment
In the United States, wastewater undergoes treatment as mandated by the U.S. Environmental Protection Agency. This process includes sludge treatment and the observance of proper sludge disposal methods.
Wastewater treatment is necessary to remove pollutants from the wastewater and prevent the contamination of natural resources. It also makes treated sludge viable for other uses, such as fertilizer for gardens and parks, soil erosion control, landscaping, heating and cooking, and generating energy in the form of alternative fuels.
Explore Sludge Treatment Equipment for Diverse Applications
SSI Aeration, Inc. develops sludge control systems for wastewater treatment plants. Our innovative solutions ensure maximum efficiency in your treatment processes and help you adhere to environmental standards. Contact us today to learn more about our products and services.
To learn more about our products and services, contact us at 845-454-8171 or fill out our contact form.
Mr. Frankel co-founded SSI in 1995 with experience in design and distribution of engineered systems. He is in charge of sales, marketing and operations in the company. Mr. Frankel holds multiple US patents related to diffusers. He is a graduate of Washington University in St. Louis.