Wastewater Treatment Process Steps And Stages
The wastewater treatment process consists of a series of steps and stages that remove solids, organic matter, and sometimes nutrients. The goal is to produce water that is clean enough for drinking and other uses.
Preliminary wastewater treatment is a crucial step in the wastewater treatment process. It not only prevents the main phases of treatment from becoming ineffective, but it also helps the equipment and infrastructure last longer. The primary goal of preliminary treatment is to remove debris, sand and grit from the influent. These large items can clog pumps, settling tanks and other mechanical equipment. They can also cause sludge buildup and damage the pipes in the plant, so it is important to remove them as quickly as possible.
This preliminary stage of wastewater treatment typically involves using a coarse bar screen, two automatic rotating screens and grills to remove the larger items from the influent. This can include rags, branches, bottles, etc. Grit removal is often done using a chamber or tank with flow control devices that direct wastewater velocity. These systems typically need to be at a velocity between 0.7 and 1.4 feet per second to settle grit at the bottom of the tank. Detention time should be between 2 and 5 minutes.
Primary treatment is the first stage of wastewater treatment. This stage consists of a number of physical processes that reduce the total pollution load to ease the flow of wastewater through the rest of the treatment process. The purpose of primary treatment is to settle material by gravity, removing floatable objects and reducing the biochemical oxygen demand (BOD) and total suspended solids (TSS). BOD and TSS are measured in pounds per cubic foot (lbs/cf) or milligrams per liter (mg/L).
Wastewater enters primary treatment through a sewered connection to the main city sewer. It passes through screens made of long, closely spaced narrow metal bars to prevent floating debris such as rags, wood, and food particles from entering the sewage pumping system. Screens may be manually cleaned by a worker or may be mechanically washed. A comminutor may also be used to grind and shred the rags and debris that pass through the screens. This material is then disposed of on the plant grounds. In some plants, the screenings are recycled for a new use.
Biological filtration systems operate on a fixed film of microorganisms that consume organic matter and pollutants in wastewater as it flows through them. These systems include trickling filters, sand bioreactors and rotating biological contactors. These biological filtration systems are designed to remove BOD, TSS, nitrogen and phosphorus and some non-biodegradable organics. The effectiveness of these processes depends on the amount of dissolved oxygen and other factors that promote biological activity.
During secondary treatment, the wastewater is further purified by removing the remaining organic matter and suspended solids from it. Depending on the wastewater plant, this may include aerobic, anaerobic or anoxic treatments. Activated sludge is one of the most common forms of secondary treatment. It is a biological process that uses bacteria to degrade and remove the organic matter, nutrients and other contaminants in water. The microorganisms that are used in this process can be either aerobic (require oxygen) or anaerobic (do not require oxygen). During this process, the organisms will consume the organic matter and other waste in the water. This treatment can also be used to remove certain nutrients in the wastewater, such as nitrogen and phosphorus. These substances can cause problems for aquatic ecosystems if they are present in the wastewater. For this reason, most treatment plants use aerobic systems to treat wastewater during secondary treatment. These systems supply wastewater with oxygen to feed the microorganisms that will then remove the nutrients in the water.
Several different types of aerobic systems are used to treat wastewater during secondary treatment, including aeration tanks and media filters. The aeration tank mixes air with the wastewater, while the media filter systems use thousands of small plastic media pieces that are attached to a biofilm. After this, the water is passed into a clarifier area where it settles to the bottom. Undissolved air bubbles are then collected around the solids and float to the top. Eventually, the water will be clear and clean of any sludge.
Disinfection is the process of killing germs that can still be present in wastewater after the previous treatment steps. This step is important for preventing the spread of viruses and bacteria in wastewater, as it’s piped to homes, schools and businesses. In wastewater treatment, disinfection is a chemical process that destroys disease-causing organisms by using chlorine or chloramine. These are similar to the types of chlorine used in swimming pools, but much stronger. Chlorine can also be combined with a number of other chemicals, including peracetic acid and hydrogen peroxide. These are less common methods of disinfection but are still effective at killing pathogens.
Several physical disinfection methods are available, with the most popular being ozone and ultraviolet (UV) light. These technologies meet a variety of criteria, such as low or no residual, minimal impact on the receiving stream, EPA-compliant effluent and cost-effective. Historically, the chemical agent of preference for municipal wastewater treatment was chlorine. But concerns that low chlorine concentrations are toxic to fish and other wildlife have prompted the use of more physical methods such as ozonation or UV light. These technologies have a range of benefits, such as minimal impact on the receiving water, EPA-compliant effluent, low or no residual, easy to use and handle and cost-effective.
Sludge treatment is the second stage of the wastewater treatment process. It aims to stabilize the sludge; remove water to reduce volume; and disinfect it to kill disease-causing organisms. Sludge can be primary sludge (the sludge collected in primary treatment) or secondary sludge (sludge that has been produced during secondary treatment). It includes the solids and grease that were removed from the wastewater as well as extra microorganisms grown during secondary treatment.
The sludge is typically thickened and sent to digesters, where it will be processed into biosolids (nutrient-rich waste that can be reused or dewatered). In this anaerobic digestion process, bacteria break down the large molecules of proteins and lipids into smaller water-soluble molecules, and then ferment them into various fatty acids. The resulting gas is used as a fuel in engines or to power other stages of the plant. Next, the sludge is dried. This can be done by a variety of methods including sludge drying beds and centrifugation. Drying beds are usually porous and allow the sludge to drain out of them while centrifugation uses centrifugal force to separate the sludge from the water.
Waste-to-energy is a process in which waste that can’t be recycled or composted is turned into energy. It’s a powerful way to reduce waste, save energy, and help the environment. It’s also an effective way to reduce greenhouse gas emissions. Studies have found that waste-to-energy plants emit fewer greenhouse gases than landfilling and recycling combined, which is why they’re used in a number of countries around the world.
One of the primary ways that waste-to-energy works is through incineration, which burns non-hazardous trash – often municipal waste – and converts the heat into steam for electricity production. The ash from the fire is then processed to recover metals for recycling. However, a growing movement is moving away from incineration towards technologies that are more energy-efficient and less environmentally damaging. These include thermochemical, mechanical & thermal, and biochemical processes. For example, hydrothermal carbonization (HTC) is a thermochemical method that turns organic waste into structured carbons that are similar to fossil fuels. HTC is a more efficient option because it’s more cost-effective, it releases dioxins and furans at lower rates, and it produces a product that’s more easily recyclable.