Surface water, especially from rivers, is the main source of water for domestic and industrial use in most countries. Once the water is used, it is treated and sent back to the river, groundwater or sea. Strictly speaking, a plant that treats wastewater from industries is referred to as an industrial effluent plant although a municipal wastewater treatment plant may also treat some industrial waste. The main purpose of the municipal wastewater treatment plant is to treat wastewater from homes. The plant does the necessary treatment processes to ensure the water is safe for direct discharge into the river.
The municipal wastewater treatment plant processes
The three main types of sewage treatment plants are Biological Filter, Pasveer Ditch, and Activated Sludge.
This type of sewage treatment uses a huge cylindrical tank or a series of tanks that have plastic pieces or stones that are sprayed with the screened sewage via a mechanical rotating arm that is moving over the surface of the bed. The medium should be loose enough to allow the liquid to permeate it and provide easy air access. The bed supports a thin film of microorganisms that help to decompose organic waste. The effluent that leaves the bed then passes on to humus tanks where the residual solids collect. These solids are usually removed periodically for sludge disposal. The main challenge with a biological filter system is the difficulty in controlling the variation in the concentrations as well as the quantiles of the sewage. It is therefore not the most ideal for large scale operations.
This municipal wastewater treatment style was invented in 1914 and has remained popular to date. In this method, a culture of bacteria is developed in a large tank as well as on the lanes that contain settled sewage. Oxygen, either as air or as pure oxygen, is then introduced mechanically into the sewage. This helps to improve the oxidation process. The oxygen is provided by two main methods;
Mechanical aeration: this entails the use of agitators on the tank’s surface. Usually, the aeration rate is controlled by varying the depth of immersion of the agitator.
Air diffusion: air diffusion is when perforated pipes referred to as diffusers are placed at the base of the tank. Air is pumped via the diffusers thereby producing small bubbles that are efficient for oxidation. The rate of oxidation can be controlled by increasing or decreasing the speed of the compressors.
This is not very common in most regions of the world but it is very popular in mainland Europe. This system is made up of an oval-shaped channel that is 2-3 meters deep which is used to convey the sewage after it has undergone primary treatment. The sewage is then aerated and circulated in a ditch with the help of rotors that are mounted at different points all over the ditch. How fast the rotation goes will be determined by the oxygen demand of the sewage.
The sewage treatment plants typically adhere to four stages of treatment namely;
- Primary treatment
- Secondary treatment
- Tertiary treatment
- Sludge treatment.
Let’s look at each of these in more detail.
Primary treatment in wastewater treatment plant
Primary treatment helps in the removal of large solids like silt and road grit. This is usually the first stage in order to avoid blockage and damage of valves, pumps, channels, orifices and other components of the system. The wastewater is first of all taken into a settling tank, also referred to as a Grit Removal unit, after which it goes through screening in order to remove large organic material. It starts with coarse screens made of bars with a spacing of 6mm and then followed with finer screeners and drum filters. Screening is sometimes combined with maceration, which is the shredding of raw sewage and then followed by a process of crushing the solids into tiny particles. The screened sewage is then passed into a sedimentation tank for settling most of the suspended matter. Dissolved and colloidal solids are however not removed because they need further treatment which usually happens at the secondary stage.
Secondary treatment wastewater treatment plant
Sewage waste is comprised of toxic, organic and inorganic waste. The organic waste is usually broken down through biological processes. This entails the making of cultures and other micro-organisms that continually multiply under the right conditions and feed on organic waste and also help in the breaking down of some chemicals. For instance, when ammonia is oxidized, it is converted into nitrogen compounds like nitrate. This is usually referred to as nitrification. Inorganic waste can to a small degree be treated biologically but for the most part, it will need chemical treatment. If the secondary stage is done properly, the outcome will be treated sewage that has very little toxicity.
The growth of the microbial population is facilitated by the nutrients that are provided by the raw sewage. But in addition to the nutrients, the right temperature, pH as well as dissolved oxygen must also be in place in order for the environment to be optimum for the growth of bacteria. On average, the most ideal environment should be approximately 25-32 degrees Celsius, 5.5-9.5pH and 2mgl of DO.
Once sewage water has undergone through the secondary treatment, it is then passed on for a final filtration before it is finally discharged into the river or sea. This is referred to as the tertiary treatment phase and it typically takes place in the clarifier or settling tank. this settling tank is somewhat similar to the one that is used for primary treatment except that it is usually followed by a polishing filter. At this stage, bacteria, viruses, and harmful parasites are also removed from the water. Chlorine is usually introduced in order to remove harmful microorganisms from the water before it is recycled or discharged into the environment.
A sludge treatment plant periodically receives solids that were recovered from screens, grit traps, surplus sludge, and settling tanks. In old treatment plants, the sludge is passed on to huge lakes where the water is allowed to evaporate slowly and thereby return to the water cycle. The solids that remain are then burned, buried or sold as fertilizer. Modern sludge treatment facilities use dewatering of the sludge which is achieved by using filters before passing the sludge to digesters. Anaerobic bacteria flourish in these digesters under the right conditions and they help in the breaking down of the sludge. Methane is produced as one of the main byproducts of this process. This methane can be used to generate electricity for use on the site.
In complex sludge treatment plants, the oxidation stage of the sludge is preceded by nitrate and phosphate reduction stages. Phosphate reduction is done to reduce the phosphates in the final effluent. It is typically achieved through the addition of ferric sulfate to the sewage which forces the phosphates to coagulate with ferric. It also helps to form a sludge that can easily be passed over to the sludge treatment center. Some plants may also add nitrate removal just before the oxidation stage.
Municipal wastewater treatment plants make it possible for most city dwellers to have running water in their houses without having to worry about their wastewater. The wastewater from the kitchen and bathroom leaves the house and is fed into the plant where it is treated to remove toxic substances and pathogens before it is discharged safely into rivers and oceans. This, therefore, helps in sustaining the water cycle.