Municipal wastewater treatment is the process of removing harmful pollutants from wastewater. The main sources of pollutants are sewage and wastewater from households, businesses and industries. Plants that treat industrial wastewater are called industrial wastewater plants, and domestic sewage is treated through municipal wastewater treatment plants. The correct water treatment process is used to ensure that water can be safely discharged directly into the river.
Municipal wastewater treatment plant processes
Activated sludge
The activated sludge process is the most widely used method for treating municipal wastewater. It removes from the wastewater biochemical organic matter in dissolved and colloidal state, as well as suspended solids and some other substances that can be adsorbed by activated sludge, and also removes a portion of phosphorus and nitrogen. The bacterial cultures are carried out in a large pond as well as in a lane containing settled effluent. Oxygen is then introduced mechanically into the effluent in the form of air or pure oxygen. This helps to improve the oxidation process. Oxygen is provided by three main methods.
Mechanical aeration: an agitator is used on the surface of the tank. The aeration rate is controlled by varying the depth of immersion of the agitator.
Air diffusion: Air diffusion involves the placement of perforated tubes called diffusers at the bottom of the tank. Air is pumped through the diffuser, resulting in small bubbles that are effective for oxidation. The rate of oxidation can be controlled by increasing or decreasing the speed of the compressor.
Blower aeration: A roots blower is connected through the piping and the rate of aeration is controlled by adjusting the rate of the blower.
Biofiltration
Biofilm treatment is an aerobic biological treatment technology alongside the activated sludge method. It is an artificial enhancement of soil self-cleaning and is a method of purifying microbial communities by attaching them to the surface of other objects in the form of membranes and bringing them into contact with wastewater. It includes biofilter, bio-rotary, biological contact oxidation and other forms. Biofilm regeneration management is relatively complicated. The process does have low operating costs, but the biofilm is easy to fall off and is not easy to cultivate, which increases the management difficulty to some extent.
Oxidation ditch
Oxidation ponds are a wastewater treatment method with simple construction, easy maintenance and management, good effluent purification and energy saving. The process of sewage purification in oxidation ponds is very similar to the natural water self-purification process, in which the sewage is purified by the slow flow and storage in the pond for a long time, and the organic pollutants in the sewage are degraded through the metabolic activities of microorganisms.
Because the operation and management of the sludge return device can not be set, and the oxidation ditch has some of the advantages of oxidation ponds, and overcome the large area of oxidation ponds, treatment effect is not stable and other shortcomings, the application has some development. Combined oxidation ditch is the general name of a series of modifications developed in recent years, they are characterized by the sedimentation tank and oxidation ditch together, the influent and aeration are continuous and unchanged, it has the advantages of other oxidation ditch at the same time, to achieve the infrastructure costs to save, low operating costs, management is simple and convenient. But regardless of what form of oxidation ditch, are subject to the limitations of the water depth can not be too large, in part of the aeration is running at full capacity, etc., resulting in its development is affected.
Municipal water treatment process
Municipal wastewater treatment plants typically go through 4 treatment stages.
- Primary treatment
- Secondary treatment
- Tertiary treatment
- Sludge treatment
Primary treatment
Primary treatment removes large solids, such as grit and silt. This is usually the first stage to avoid clogging and damage to valves, pumps, channels and other components of the treatment facility. Wastewater first enters a settling tank, also known as a grit removal unit (e.g., a grit separator), which is then screened to remove large amounts of organic material. It starts with a coarse screen, followed by finer fine screens, screen compactors and drum filters. Screening is sometimes combined with maceration, in which the raw sewage is chopped and the solids are crushed into tiny particles. The screened effluent is then sent to settling tanks to settle most of the suspended material.
Secondary treatment
Wastewater wastes include toxic, organic and inorganic wastes. Organic wastes are usually decomposed by biological processes. This requires the cultivation of cultures and other microorganisms that continually multiply under the right conditions and feed on organic wastes and help break down certain chemicals. For example, when ammonia is oxidized, it is converted to nitrogen compounds such as nitrates. This is often referred to as nitrification. Inorganic waste can be treated biologically to a small degree, but in most cases it requires chemical treatment.
The nutrients provided by the untreated effluent promote the growth of microbial populations. But in addition to nutrients, there must be the right temperature, pH and dissolved oxygen to make the environment optimal for bacterial growth.
Tertiary treatment
After secondary treatment, the wastewater undergoes final filtration and is finally discharged into a river or ocean. This is known as the tertiary treatment stage and is usually carried out in a clarifier or settling tank. This settling tank is somewhat similar to the settling tank used for primary treatment, with the difference that it is usually followed by a polishing filter. In this stage, bacteria, viruses and harmful parasites are also removed from the water. Harmful microorganisms are usually removed from the water using chlorine (often produced by chlorine dioxide generators or sodium hypochlorite generators).
Sludge treatment
Sludge treatment plants regularly receive solids recovered from screens, grit traps, residual sludge and settling tanks. Sludge treatment facilities use sludge dewatering(accomplished using filter presses or screw presses), which is accomplished by using filtration equipment before the sludge enters the digester. Under the right conditions, anaerobic bacteria proliferate in these digesters and they help to break down the sludge. Methane is one of the main by-products of this process. This methane can be used to generate electricity for on-site use.
In complex sludge treatment plants, the oxidation stage of sludge is preceded by nitrate and phosphate reduction stages. Phosphate reduction is performed to reduce the phosphate in the final effluent. It is typically accomplished by adding iron sulfate (usually with a polymer dosing system) to the effluent, which forces the phosphate to coagulate with the iron. It also helps to form sludge that can be easily passed to the sludge treatment center. Thermal drying using direct or indirect dryers is used to achieve almost complete removal of water from the sludge. Paddle dryers are often a good choice.
Summary
Municipal wastewater treatment plants allow most city residents to use tap water in their homes without having to worry about their sewage. Municipal wastewater is treated by a variety of methods, including physical, chemical and biological processes, and then safely discharged into rivers and oceans.