In earlier posts, we learned sources of wastewater, which are domestic wastewater, industrial wastewater, and infiltration/inflow. In this post, I want to share to you how wastewater is treated.
Wastewater is treated at wastewater treatment plants before it is permitted to be discharged to the environment/water bodies. Basic function of wastewater treatment is to speed up the natural processes of water purification.
Conventional wastewater treatment consists of a combination of physical, chemical, and biological processes, to remove solids, organic matters, and sometimes nutrients from wastewater. Wastewater treatment is usually classified according to different degrees of treatment, in order of increasing level of treatment, which are preliminary, primary, secondary, and tertiary, and/or advanced wastewater treatment. In some countries, pathogen removal by disinfection sometimes follow the last treatment step.
Preliminary Treatment
The goal of preliminary treatment is only the removal of coarse solids and other large materials often found in raw wastewater. Removal of these materials is necessary to enhance the operation and maintenance of the next treatment units.
Preliminary treatment typically include coarse screening, grit removal, and pulverization of large objects. In grit chambers, the velocity of water through the chamber is maintained sufficiently high, or air is used to prevent the settling of most organic solids. In most small wastewater treatment plants, grit removal is not included as a preliminary treatment step. Comminutors are sometimes adopted to supplement coarse screening and to reduce the size of large particles so that they will be removed in sludge form in the next treatment processes. Flow measurement units, often standardized flume (e.g. Parshall flume), are always included at the preliminary treatment stage.
The aim of primary treatment is to remove settleable solids and part of the organic matter.
After passing the preliminary treatment units, sewage still contains non-coarse suspended solids, which can be partially removed in sedimentation units. A significant part of these suspended solids is composed of organic matter in suspension. In this way, its removal by simple processes, such as sedimentation, resulted in reduction of BOD (Biological Oxygen Demand) load directed to the secondary treatment, where its removal is more expensive.
Approximately 25% to 50% of the incoming BOD, 50% to 70% of the total suspended solids (SS), and 65% of the oil and grease are removed during primary treatment. Some organic nitrogen, organic phosphorus, and heavy metals associated with solids are also removed during primary sedimentation, but colloidal and dissolved constituents are not affected.
The sedimentation tanks can be circular or rectangular. Suspended solid will slowly settle to the bottom of sedimentation tanks. The mass of solids accumulated in the bottom is called raw primary sludge. Grease and oil will rise to the surface of the sedimentation tanks, where they are collected and removed from the tank for the next treatment.
Secondary Treatment
Secondary treatment removes about 85% of the organic matter and possibly nutrients (nitrogen and phosphorus) in sewage by making use of the bacteria in it. The essence of secondary treatment of domestic sewage is the inclusion of biological stage. While preliminary and primary treatments have predominantly physical mechanisms, the removal of the organic matter in the secondary stage is carried out through biochemical reactions, undertaken by microorganisms. The principal secondary treatment techniques used in secondary treatment are the trickling filters, activated sludge process, and rotating biological reactors (RBC). Combination of these processes in series is sometimes used to treat municipal wastewater containing a high concentration of organic material from industrial sources.
The objective of tertiary treatment is to remove specific pollutants, usually toxic or non-biodegradable compounds, or the complementary removal of pollutants that were not sufficiently removed in the secondary treatment. Tertiary treatment is rare in developing countries.
Disinfection
To complete secondary treatment, effluent from the sedimentation tank is usually disinfected with chlorine before being discharged into receiving waters. Chlorine is fed into the water to kill pathogenic bacteria, and to reduce odor. The chlorine dosage depends upon the strength of the wastewater and other factors, but dosages of 5 to 15 mg/L are common. If done properly, chlorination will kill more than 99% of the harmful bacteria in the effluent.
Ozone and ultraviolet (uv) irradiation can also be used for disinfection, but these methods are not common in used. These alternatives can be used where chlorine in treated sewage effluents may be harmful to fish and other aquatic life.
References:
- Marcos von Sperling, 2007. Wastewater Characteristics, Treatment and Disposal. Iwa Publishing, New Delhi
- EPA, 1998. How Wastewater Works, The Basic.
- Wastewater Treatment – How Wastewater is Treated?
- Wastewater Treatment
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