A wastewater treatment plant is a process of removing contaminants from raw sewage before discharging it. Sewage treatment plants are designed to reduce the risk of water pollution by preventing the release of raw sewage into waterways. Depending on the treatment process chosen, wastewater may be treated at a variety of stages, from primary trickling filter to secondary clarifier. Read on to learn about the various stages of a wastewater treatment plant and how they work together.
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Primary trickling filter
A biological trickling filter has many advantages over other types of filters, but its main drawback is clogging. This occurs when solids lodge within the filter media, distributor mechanism, or underdrain system, reducing the filter’s capacity and compromising the biofilm’s performance. To minimize this problem, facilities often implement a primary treatment process before installing a biological trickling filter, including screening or sedimentation.
The process of a primary trickling filter involves the application of wastewater to the filter media, and hydraulic loading, or BOD, is applied to the filter media at a low rate. Flow is applied intermittently to the filter, with rest periods of approximately five minutes. Proper loading of a trickling filter should remove 80 to 85 percent of applied BOD. Major unloadings occur a few times a year, usually for short periods.
The biofilm formed on the trickling filter provides a growing medium for bacteria. The biofilm is capable of maintaining a high load without becoming depleted of oxygen. Almost a century ago, biofilms on support media were the first continuous flow bioprocesses. The biofilm is formed by a layer of media, and wastewater flows over the layer. The wastewater carries ammonia, organic carbon, and dissolved oxygen into the biofilm layer. The bacteria consume oxygen before it reaches the depth of the media, and an anaerobic environment is established near the surface of the medium.
The trickling filter is a specialized treatment device, and its function is to transform non-settleable solids into stable organic matter. The attached material is then carried away in the filter effluent. Ideally, a trickling filter is followed by secondary sedimentation tanks, which remove sloughed-off solids and provide clear effluent. When the primary trickling filter fails to deliver its goal, the treatment system should implement a secondary trickling filter.
Secondary clarifier
The secondary clarifier in your treatment plant is designed to handle a large volume of mixed liquor suspended solids (MLSS), which are the solids that remain after the activation sludge process. Typical MLSS concentrations in municipal wastewater treatment plants range between 1,800 mg/L and 4,000 mg/L. However, the SLR can increase significantly. Here are some tips for optimizing the performance of your secondary clarifier.
Primary clarifiers remove ammonia and organic matter and aid in the flocculation of solids. The clarifiers also reduce the amount of sewage, resulting in a higher quality effluent. The secondary clarifier’s sludge is either waste activated sludge or return activated sludge. In either case, the sludge from the secondary clarifier needs to be disposed in a waste container or returned to the aeration basin.
The design of the suction header is crucial to the operation of the secondary clarifier. Proper header geometry, orifice placement, and sizing are essential for the operation of the clarifier. Monroe Environmental engineers meticulously engineer the mechanism of the secondary clarifier. The sludge is collected in a hopper at the edge of the clarifier through a lever. Once it is collected, the sludge is sent to a secondary thickener for further treatment.
Using a secondary clarifier in a treatment plant will help separate solids from liquid waste. Like the primary clarifier, the secondary clarifier uses air to break down organic materials and aid the growth of bacteria. The return activated sludge is sent back to the aeration tank. The resulting clean water will then go on to disinfection. The clarifier will remove 99.8% of nitrates, and 99% of other organic matter.
Final settlement tank
The final settlement tank at a treatment plant is similar in design to the primary settlement tank. It is an integral part of the activated sludge process, which removes biological matter and yields a low-suspended-matter supernatant. This final step collects microorganisms produced during treatment, which are used to seed settled sewage at the front end of the process. The remaining flow from the treatment plant is known as Returned Activated Sludge, or RA-S.
The final settlement tank at a treatment plant is where treated liquid exits the treatment plant. Here, suspended solids settle to the bottom of the tank, while the treated water flows over radial weirs. The treated water then leaves the treatment plant, and eventually flows back into the river or other water source. This cycle continues until the wastewater reaches its final destination: a river or lake. This wastewater treatment plant is located on the Sauk River.
In 2009, the Final Settlement Tanks Project was completed, which added nitrogen removal capacity. Initially, nitrogen was not a major concern when the treatment plant expanded, but now that nitrogen removal is important to protect the waters of Long Island Sound and the Connecticut River, it was time to address the issue. More tanks means more microorganisms, which translate to more nitrogen removed. Similarly, more tank capacity translates to more profits.
A final settlement tank at a treatment plant should meet the requirements for fine suspended solids. Its surface area should be large enough to accommodate three x Dry Weather Flow. For example, a tank with three x DWF will require an upward flow velocity of 0.9 m/hr. To be effective, the final settlement tank must meet minimum standards for surface area. In addition to that, the treatment plant must have a sufficient number of secondary settlement tanks, including two.
Disinfection in a treatment plant
For decades, water treatment was based on the dilution principle, but the rise of infectious disease has made the need for more effective methods more important than ever. Now, new science and technology are helping identify the public health threats and developing more effective solutions. In the United States, the Environmental Protection Agency has developed a guide for municipal wastewater treatment plants called the Municipal Wastewater Disinfection Design Manual (MWDDM).
During disinfection, chemical agents affect the cell walls, protoplasm, and enzyme activity of microorganisms. By disrupting the cell activity of the organisms, they die. The chemical agents used to disinfect water also destroy organic matter and suppress the growth of microorganisms. They do this by oxidizing the water, which depletes it of its nutrients. A water system that uses disinfectants to reduce microbial growth can save money by reducing the demand for microbial-killing chemicals.
There are many ways to measure the efficacy of a water disinfection process. For example, bacteria are present in the fecal material of warm-blooded animals, and they are also present in soil, plants, and water. Whether or not these organisms are present in a treatment plant depends on how well it can remove coliform bacteria. In a treatment plant, chlorine is a great disinfectant because it can kill virtually all microbial pathogens. A basic chlorine disinfection system consists of a chlorine cylinder, a cylinder-mounted chlorine gas vacuum regulator, a contact tank, and a chlorine injector.
Disinfection in a treatment plant is a crucial step in ensuring that drinking water is safe for human consumption. It can also help prevent a person from becoming ill after drinking contaminated water. In fact, if the water is contaminated with microorganisms, it can be very dangerous to the health of the people who consume it. Disinfection is the first step in the water treatment process, but it is not always required.
Impacts of tertiary treatment
After secondary treatment, effluent from a wastewater treatment facility will require a third stage of biological processing, called tertiary treatment. This step can remove nutrients and pathogenic organisms from the wastewater. Additionally, tertiary treatment can remove heavy metals and remaining inorganic dissolved solids and suspended matter. In some instances, tertiary treatment can be used for irrigation, recreational purposes, and drinking water.
The tertiary treatment stage can take several forms, depending on the final effluent quality. For example, UV irradiation is often used in regions with bathing water to reduce viable bacteria. In the past, chlorine was used to reduce bacteria, but this chemical is toxic and results in bleaching. It can also be detrimental to receiving water, so UV is increasingly used.
Untreated wastewater discharge into freshwater bodies is a major environmental problem. Untreated wastewater can eutrophize freshwater ecosystems, induce anoxia, and decrease water quality. According to the World Health Organization (WHO), half of the world’s population lives in urban areas, and this figure is expected to increase to 66% by the year 2050. The rapid urbanization of cities is increasing nutrient-rich urban wastewater. In order to protect aquatic ecosystems, tertiary treatment is an important step toward achieving these goals.
A wastewater treatment plant that offers advanced tertiary treatment is also beneficial to rivers downstream. It is essential for the environment because wastewater treatment effluent can reduce the abundance and diversity of benthic bacterial communities in urban and suburban waterways. As wastewater treatment advances, applied treatment technology has increasingly moved towards advanced tertiary treatment. The benefits of advanced wastewater treatment will be felt far into the future.
