Understanding Wastewater Treatment Processes and Their Importance
Pretreatment: Its phases are known as operations. It seeks the removal of sand, thick materials, and fat. To protect the channel.
- A) Remove very thick solids through the Well of Fat.
- B) Dispose of solid mass of thickness (30 to 80 mm) in two stages using thick bars (with 30-50 mm spacing between bars) and plates from 12 to 15 mm, followed by a second phase with a fine grid (with a span of 10-15 mm between plates).
For protection of the plant: Operations include aerated grit removal, which removes particles larger than 0.20 mm, while simultaneously removing fat. The air serves two purposes: to cool the water and to provide oxygen, creating a flow that allows sedimentation of sand while avoiding the deposition of organic matter. These operations are performed in the sand trap.
Primary Treatment: The phases are referred to as processes. This stage aims to treat putrescible materials. The organic content of these materials generally consists of carbohydrates or nitrogen substances involved in the process: the settling mechanism by primary settling. The solids ratio comes in at 300 mg/l (concentration of 0.03%) and extracted decanted sludge (primary sludge or slurry) is at 2%. This process yields ?SS ?DBO = 70% = 30-35%.
Secondary or Biological Treatment: This consists of two processes: the biological reactor or aeration tank, which operates with the activated sludge process. It involves the digestion of organic matter through aerobic biological mass that produces CO2, requiring flocs and oxygen for the secondary settler digestion. The phase of separation follows, thus removing the flakes formed in the reactor. These flocs are secondary sludge or effluent from the secondary biological settler. The yields are approximately ?SS ?DBO = 95% = 90-93%.
Sterilization: Optionally, after the secondary settler, sterilization can be performed (killing bacteria and viruses). In some cases, chlorine is used, which can be harmful to humans.
Sludge: The primary sludge has a maximum concentration of 2 to 4%. It is better to concentrate sedimenting. The secondary sludge has a maximum concentration of 0.6 to 0.8%. It is preferable to concentrate floating.
Thickening: To concentrate the sludge, a gravity thickener is used: it mixes the mud, facilitating the separation of liquid. The effluent comes out at a concentration of 8 to 10% in primary sludge. This is a mechanical process that produces a significant amount of supernatant to be sent to the head of the plant.
Flotation thickener or float: This device blows air to favor the coagulation of flocs. The supernatant (or rather sub-supernatant) is sent to the head of the plant, while the mud floats out through anaerobic stabilization.
Aerobic Digestion or Aerobic Stabilization: This process is carried out with heterotrophic bacteria, along with the addition of oxygen and agitation. Prior to this, the mixture is thickened to facilitate oxidation of the mud. Digestion takes about 15 days. The sludge is then sent for dehydration, which can be done using a band filter, filter press, or centrifugal methods. The mud comes out with a concentration of 20 to 22% solids (similar to plasticine).
Anaerobic Digestion: Aerobic stabilization requires a significant amount of energy and oxygen; therefore, large plants utilize anaerobic digestion. This process produces digestion gas (methane or biogas) used to heat the digester to the required temperature of 30 °C and for energy recovery. The sludge is then sent to the gravity thickener, reducing it to 6%, before being led to mechanical dehydration, achieving up to 25% concentration.
Capacity: Samples should be taken for 24 hours to locate the ends. The peak flows in urban areas typically occur between 14 to 16 hours in the afternoon and 6-9 hours at night. In cases of industrial components in wastewater, the flow curve may deform, causing changes in the plant (digesters typically have a retention time of 4-8 hours). The design is usually made for an urban population, meaning a curve with two points. In the absence of real measured data, flow estimates are made. An industrial component may transform these flows into equivalent population flows. These flows discharged by industries are compared with other less relevant data regarding suspended solids (SS): everything that is not dissolved or in colloidal form in the wastewater can be removed by sedimentation over an extended period. Suspended solids are approximately 70% and may settle in about one hour. The primary settling aims to eliminate 95% of settleable suspended solids, meaning 70 x 95 = 65% of total suspended solids (SS performance of primary settling).
Biological Oxygen Demand (BOD5): A sealed sample of wastewater consumes oxygen. If water has no dissolved oxygen, it is termed septic. When the dissolved oxygen in wastewater is consumed, an anaerobic process begins, producing odors. All design parameters are based on the BOD5. The variation of oxygen consumption over time follows an asymptotic curve. The oxygen demand over 5 days (BOD5) represents 75-80% of the asymptotic demand, meaning this consumption is necessary to stabilize the water. Typical values in urban areas can be considered normal at 60 g/person/day in shantytowns, 65 g/person/day in middle-class areas, and 75 g/person/day in residential areas.
Chemical Oxygen Demand (COD): This measures the quantity of dissolved oxygen consumed by wastewater during oxidation via a strong chemical oxidant. Its determination is faster than for BOD, indicating results in 1 or 2 hours if oxidation is performed in cold conditions, or 20 to 30 minutes if carried out with hot dichromate. The determination depends on the oxidizing reagent and the organic and inorganic materials present. While there is no direct relationship between COD and BOD5, it is possible in certain types of wastewater to obtain valid correlation curves for control.
Nitrogen: Nitrogen in wastewater primarily comes as ammonia. Nutrient control is crucial in channels used for supply, as it can lead to eutrophication. Nitrogen must be limited in any channel, as ammonia can become toxic to wildlife.
Limitations of the Project: Topography of the site in relation to the piezometric height loss. Geotechnics: The loads transmitted are around 3-4 t/m² for anaerobic digesters, except for Pits: which are usually very close. This can lead to foundation problems, causing equipment to float and necessitating ballast. Points of connection for electricity and drinking water must be checked. The wastewater system in one or more collectors is important. Maximum flow, sections, and geometric characteristics of the collector, including slope on the last leg and final ground elevation, must be considered. Hydraulic capacity of the total collector is essential for sizing the spillway. With the capacity curve of the collector and peak flows, the water levels can be determined for the dimensions of the sheet at the plant entrance. The maximum water level in the channel is usually considered to be the height reached during the return period of a 25-year flood. Number of Lines: A single device is never designed for each process; instead, multiple cleanings and arrangements are provided. Typically, in towns with fewer than 50,000 inhabitants, two lines are planned. For populations between 50,000 and 100,000, three lines are made, and older populations may require even more lines.