Wastewater treatment is the process of removing contaminants from sewage and stormwater to make it safe for discharge. There are two types of treatment plants: biological and physical/chemical. Sludge is treated and can be used as fertilizer or sent to landfill/incineration. Tertiary treatment includes disinfection before discharge. Physicochemical treatment involves the removal of suspended solids and the addition of chemicals to precipitate dissolved materials. Chemicals such as chlorine and ultraviolet light can also be used. Municipal treatment plants may use chemical treatments to remove materials such as phosphorus.
Wastewater treatment is the process of taking wastewater and making it suitable for discharge into the environment. Sewage can be formed from a variety of activities, including washing, bathing and using the toilet. Stormwater runoff is also considered wastewater. No matter where it comes from, this water is full of bacteria, chemicals and other contaminants. Wastewater treatment reduces contaminants to acceptable levels so that they are safe for discharge into the environment.
In general, there are two types of wastewater treatment systems: a biological treatment plant and a physical/chemical treatment plant. Most households and businesses create waste that can be broken down by natural means. Biological treatment plants use bacteria and other biological materials to break down waste. Industrial wastewater can contain chemicals that can harm the environment, so a chemical plant is needed to treat this waste. Physical/chemical wastewater treatment plants use both physical processes and chemical reactions to treat wastewater.
A biological wastewater treatment plant, such as a municipal treatment plant, uses several tanks to treat the wastewater that enters it. First, the wastewater is screened for easily removed objects, some of which could ruin the treatment plant’s machinery. Subsequently the waste water is conveyed to a primary settling tank where the matter can float or sink in the tank. The remaining water is then sent to the secondary treatment tank where biological matter, such as bacteria, removes much of the remaining suspended matter.
The substances that are removed during water treatment are called sludge. This sludge is treated and can be used as fertilizer or in reclamation, or it will be sent to landfill or incineration. To treat sludge, waste management professionals can use anaerobic digestion, aerobic digestion or composting. The difference between anaerobic and aerobic digestion is that aerobic digestion occurs in the presence of oxygen while anaerobic digestion does not. In composting, sludge is mixed with carbon before being fed into the bacteria for digestion.
After secondary treatment, the water is then sent to tertiary treatment. This treatment is the last stage before the water can be released into the environment and usually ends with a disinfection stage. This step is actually a chemical treatment in a biological treatment environment. The treated water, called effluent, is then discharged into the environment. This recovered water can also be used in fountains and to irrigate lawns.
Physicochemical wastewater treatment begins with the removal of suspended solids from the wastewater. The water is pumped into large tanks where matter settles or sinks, just like in a biological treatment plant. Just like in a biological treatment plant, this process can be aided by agitators that mix the water causing the small particles to join the larger ones. In a physicochemical plant, this process can be further facilitated by the addition of flocculants, a chemical that forms larger particles. The dissolved air can also be used to assist in particle removal.
Chemical processes include chemicals added to precipitate dissolved materials. Chemicals such as chlorine can also be used to convert cyanides into carbon dioxide and nitrogen. Organic chemicals can also be oxidized by adding ozone or hydrogen peroxide. Chemicals in wastewater can also be broken down using ultraviolet light. Municipal treatment plants may also use chemical treatments to remove materials such as phosphorus from wastewater.
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