Sewage treatment plants (STP) are essential for maintaining a healthy and sustainable environment. It is important to treat sewage before releasing it into the environment, as untreated sewage can be a source of various diseases and can pollute the environment.
Here, we will discuss the sewage treatment plant for schools with a case study of a school that has installed a sewage treatment plant. We will also provide design calculations of the sewage treatment plant.
Case Study: Sewage Treatment Plant for a School.
A school in a suburban area had a sewage treatment plant installed to treat the sewage generated by the school. The school had a capacity of 1,000 students and 100 staff members. The average water consumption per person was estimated to be 50 liters per day.
The sewage treatment plant was designed to treat the sewage generated by the school and provide treated water for irrigation and flushing purposes.
Processes:
The treatment plant consisted of the following processes:
1. Screening: The first stage of the treatment process is the screening process. The sewage was screened to remove large debris and solid waste such as plastic bags, leaves, and other materials.
2. Grit chamber: The screened sewage was then passed through a grit chamber where sand and other heavy particles were settled down.
3. Aeration tank: After grit chamber, the sewage was transferred to an aeration tank where air was supplied to promote the growth of aerobic microorganisms. The microorganisms feed on organic matter in the sewage and convert it into sludge.
4. Secondary settling tank: The sludge settled down in the aeration tank was then transferred to a secondary settling tank where it was separated from the treated water.
5. Tertiary treatment: The treated water from the secondary settling tank was then subjected to tertiary treatment processes such as chlorination, sand filtration, and activated carbon filtration to remove any remaining impurities and disinfect the water.
Design Calculations.
The following are the design calculations and drawings of the sewage treatment plant:
· Capacity Calculation:
- Total Waste Water Generated.
The capacity of the sewage treatment plant was calculated based on total waste water generated and is equal to the average water consumption per person and the number of students and staff members in the school and multiplying it by 0.8.
Total Waste Water Generated = Total water Demand x 0.8
TWWG = Average water consumption per person x Number of persons x 0.8
TWWG = 0.8 x 80 liters x 1100 (1,000 students + 100 staff members)
TWWG = 70400 liters per day
- Peak Flow.
Peak flow is the flow rate when the waste water generation is max. Assuming here max peak flow factor of 2.2, we get:
Peak Flow generation rate = 70400 x 2.2 liters per day
Peak Flow generation rate = 154800 liters per day
- Assuming Detention Time:
Detention time is the total time period sewage is kept in settling tank. Assuming here Detention period of 12 hours, we get total capacity of settling tank needed.
- Capacity of Settling Tank:
Capacity of Settling tank needed would be calculated by following formula:
Capacity = Peak Flow Generation x Detention Time
Capacity = 154800 x 0.5 Liters
Capacity = 77400 Liters = 77.4 m3
· Tank Design:
The following are the design specifications for the aeration tank and secondary settling tank:
- Settling Tank:
Volume of Settling tank = 77.4 m3
Assuming Depth of aeration tank = 3 meters
Area of Tank Required = 25.8 m2
Proposing number of tanks = 2
Therefore,
Area of each tank = 25.8 /2 m2 = 12.9 m2
Assuming ,Width of tank = 3 meters
We can get Length of tank from the formulas below:
Length of tank = Area / Width or
Length of tank = Volume / (Depth x Width)
Length of tank = 12.9/3 meters
Length of aeration tank = 4.3 meters
Also giving overhead height of 0.3 meters, we will have
Two tanks of dimensions 4.3x3x3.3 meters would be needed.
- Aeration Tank:
Similarly for aeration tank:
Assuming Retention Time of 6 hours, we get
Volume of aeration tank = 77.4 m3 x 6 / 12
Volume of aeration tank = 38.7 m3
Depth of aeration tank = 3 meters
Area Required = 12.9 m2
Width of aeration tank = 2.42 meters (L/B =2.2)
Length of aeration tank = Volume / (Depth x Width)
Length of aeration tank = 12.9/2.42 m
Length of aeration tank = 5.34 meters
Providing 0.3 m of overhead space, we get
Dimensions of Aeration Tank = 5.34 x 2.42 x 3.3 meters
- Secondary Settling Tank:
Similarly for Settling tank:
Assuming Retention Time of 3 hours, we get
Volume of tank = 77.4 m3 x 3 / 12
Volume of tank =19.35 m3
Assuming,
Recirculated flow of 53%, we get:
Volume of secondary settling tank = 19.35 + 0.53(19.35) m3
Volume of secondary settling tank =29.6055 m3 ≈ 30 m3
Assuming depth of secondary settling tank = 2.5 meters
Area required = 30 / 2.5 m2 = 12 m2
Width of secondary settling tank = 2.449 ≈ 2.5 meters ( L/B = 2)
Length of secondary settling tank = 12/2.5 meters
Length of secondary settling tank ≈ 5 meters
Dimensions of Secondary Settling Tank = 5 x 2.5 x 2.5 meters
Process Flow Diagram:
The following is the process flow diagram for the sewage treatment plant:
Conclusion.
The sewage treatment plant is an essential requirement for schools to maintain a healthy environment.
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