Wastewater treatment represents the process used to turn the water that can no longer be used for its initial purpose (also called wastewater) into an effluent that can be returned into the water cycle of the environment.
The wastewater treatment process aims to remove or fall apart the pollutants contained by the wastewater in order to create a water that can be reused or returned into the environment in a form that will not harm the nature.
The process of wastewater treatment is made in wastewater treatment plants (WWTP), while the municipal wastewater produced by households and small companies is processed by sewage treatment plants.
The objective of the wastewater treatment is to turn the wastewater into a water that can be reused or disposed properly in a selected location.
The impurity concentration in the treated wastewater may vary with the type of use or with the location of the disposal.
Disposing wastewater into the ocean is ruled by the London Convention and London Protocol and is in force since 1975.
In 1996, the “London Protocol” was updated and under the new Protocol all dumping was considered prohibited, except for possibly acceptable wastes.
The acceptable wastes were mentioned in a so-called “reverse list”.
The Protocol currently consists of 50 Parties and has entered into force on 24 March 2006.
Types of Wastewater Treatment Plants
The process of wastewater treatment is made in different types of plants depending on the type of wastewater that will be treated.
We have this way wastewater that comes from sewage, industrial wastewater that is produced during the different industrial processes that require water, agricultural wastewater, which is the water produced during the operations of producing milk and eggs, and leachate that is wastewater originating from landfills.
1. Sewage Treatment Plants
A sewage treatment plant usually serves a municipality and the wastewater treatment process will include a primary treatment that removes the solid materials, a secondary treatment that can digest suspended and dissolved organic materials and also nutrients such as phosphorus and nitrogen, a disinfection process that kills pathogenic bacteria (if needed), a tertiary treatment process that is increasingly applied today in industrialized countries, and involves micro filtration or the use of synthetic membranes to clean the wastewater even more.
The sewage treatment plant will produce a sewage sludge during the wastewater treatment process.
The sewage sludge contains a raw primary sludge and a secondary sludge.
The sewage sludge will be treated and after that will be disposed in landfills, applied on land as fertilizer or dumped into the ocean.
Nowadays, the ‘sewage treatment plants’ are often called ‘wastewater treatment plants’.
2. Industrial Wastewater Treatment Plants
Disposing industrial wastewater is an expensive and difficult process, and this is the reason why almost all chemical and petrochemical plants and petroleum refineries of today have their own facilities to treat the wastewater produced in the production process.
The on-site facilities will reduce the level of the pollutant concentration in the wastewater down to a level that complies with the local and/or the national regulations in that country.
The industrial wastewater treated this way will then be disposed into rivers, lakes or oceans.
3. Agricultural Wastewater Treatment Plants
Disposing agricultural wastewater is almost similar with the disposal of industrial wastewater, but if there is a pond, a settling basin or a lagoon available, the disposing process will become easier and less costly.
4. Leachate Treatment Plants
Leachate treatment plants are used to dispose leachate from landfills.
The treatment process will include a biological treatment, a mechanical treatment that involves ultrafiltration of the leachate, a treatment using reverse osmosis made with disc tube module technology and a treatment with active carbon filters.
Wastewater Treatment Process
The wastewater treatment involves several processes made step by step in the following sequence.
1. Collecting Wastewater
Wastewater produced by households and small companies in a city is usually collected through a piping system (sewage system) that has the role to transport the wastewater to the treatment plant.
The piping system uses gravity to transport the wastewater to the treatment plant.
If the treatment plant is located uphill, the plant will use pumps to force the wastewater uphill.
The wastewater produced by households that are not connected to the sewage system is usually collected and transported at the treatment plant or discharged into the sewage system using sewage trucks.
Once wastewater will reach the treatment plant, the first process called “Pretreatment” will start.
In this first process the wastewater will flow into a building called “Headworks” where any trash will be filtered out from the water.
A machine called “Barscreen” will screen and remove any large trash contained by the wastewater.
All the trash removed from the wastewater in the first process will fall into a hopper where it will be compacted and bagged up for disposal in the landfill.
3. Removing the Grit
Any inorganic solids such as egg shells, glass and coffee grounds (called grit) will then be removed from the wastewater by slowing down the flow of water and using a conveyor screw.
The grit collected this way will be collected into a trash can and disposed into the landfill.
4. Primary Treatment
In this process, the wastewater will flow into a tank and will make its way into the aeration basin for the aerobic treatment called “Oxygen”.
After aeration, the wastewater will go into anoxic mode, also called “Oxygen depletion”.
5. Removing the Sludge
The sludge (solid materials) will fall into the bottom of the basin and the clean water will remain on top.
A decanter will be used for a further treatment of the water and the water resulted this way will be called effluent.
The effluent will then fall into another basin called “Post Equalization” where the water will be aerated again to reach a specific level of dissolved oxygen (D.O.).
6. UV Disinfection
The effluent will then be pumped into a new basin for disinfection.
The disinfection will kill any “pathogens” that have not been removed in the previous processes.
The disinfection process is made using ultraviolet bulbs and is called UV Disinfection.
7. Recycling Wastewater
After a few hours of disinfection the effluent (clean water) will be returned to nature as recycled water.
The sludge collected in the basin will be pumped into a large storage tank and mixed with a polymer for solid/liquid separation.
Under the effect of the polymer, the sludge will coagulate into a ‘cake’ that will be squeezed into a press machine to further remove water from the sludge.
When the sludge will escape the press machine, it will still contain about 80% water and only about 20% solids.
The sludge will then be mixed with hydrated lime and put into dump trucks using a conveyer.
Being high in nutrients, the sludge will be recycled as fertilizer and spread on hay fields.
How Important are Wastewater Treatment Plants?
Freshwater is a very important resource for the survival of the mankind and by using the wastewater treatment plants we can recycle wastewater and use the sludge as fertilizer for the land.