How to design an STP plant for 500 students for a school in India?

Maintaining a clean and safe environment requires designing a sewage treatment plant (STP) in accordance with Central Pollution Control Board (CPCB) standards for an Indian school with 500 kids. The steps and calculations required to construct such a facility will be covered in this guide.

Step 1: Understanding CPCB Norms:

Understanding the CPCB standards and guidelines is essential before beginning the design process. These laws establish the maximum allowable concentrations of a number of characteristics in treated sewage, including Total Suspended Solids (TSS), Chemical Oxygen Demand (COD), and Biochemical Oxygen Demand (BOD). To ensure adherence to current standards, be sure to consult the most recent CPCB recommendations.

Step 2: Estimating Sewage Generation:

Calculate the daily sewage generation depending on the number of students and their typical water usage in order to construct an efficient STP. You can perform the following calculations for a school with 500 students:

· Average Water Consumption per student (as per CPCB guidelines): Typically around 135-150 liters per day.

· Daily Sewage Generation = Number of Students × Average Water Consumption per Student.

Daily Sewage Generation = 500 students × 150 liters/student/day = 75,000 liters/day.

Step 3: Treatment Process Selection:

It is essential to choose the right sewage treatment method. The decision will be made in light of the effluent quality criteria, budget, and available space. For schools, typical alternatives include:

Extended Aeration System: This is appropriate for smaller communities, such schools. It operates reasonably easily and effectively removes BOD.

Sequential Batch Reactors (SBRs): SBRs have a variety of uses and are able to manage varying wastewater loads. They efficiently meet CPCB standards and provide good nutrient removal.

Membrane Bioreactors (MBRs): MBRs ensure high-quality effluent by combining biological treatment and membrane filtration. They are appropriate for universities with strict effluent quality requirements.

Step 4: Preliminary Design Parameters:

For a school with 500 students, an Extended Aeration System is a suitable choice. Here are some preliminary design parameters:

· Daily sewage generation: 75,000 liters/day.

· BOD Load (as per CPCB norms): Typically, this is around 30-100 g/student/day. Let's assume 70 g/student/day:

• Total BOD Load = Number of Students × BOD Load per Student.
• Total BOD Load = 500 students × 70 g/student/day = 35,000 g/day or 35 kg/day.

· Hydraulic Retention Time (HRT): This is a crucial parameter for biological treatment processes. HRT is generally around 24 hours for extended aeration systems.

· Sludge Production: The amount of sludge produced during treatment depends on wastewater characteristics and treatment efficiency. It's approximately 0.5 to 1.0 kg of sludge per kg of BOD removed. Let's assume 0.7 kg of sludge per kg of BOD removed.

• Sludge Production = BOD Load / (BOD Removal Efficiency × Sludge Yield).
• Sludge Production = 35 kg/day / (0.7 × 100) = 0.5 kg/day.

Step 5: Detailed Design of the Extended Aeration System:

The following elements commonly make up the extended aeration system:

Aeration Tank: This is where the biological treatment takes place. The hydraulic retention time (HRT) and average daily flow rate should be used to calculate the tank capacity. The recommended HRT for this system is 24 hours, which equates to an aeration tank volume of 75,000 litres per day divided by 24 hours as 3,125 litres, or 3.125 cubic metres. The real tank should be a little bit bigger to allow for sludge buildup.

Clarifier: Wastewater needs to settle after being treated in the aeration tank to get rid of suspended solids. The clarifier needs to have the right size to allow for effective solids settling. The clarifier's design flow rate typically ranges between 1 and 2 litres per second per square metre (L/s/m2). The required clarifier surface area, given the flow rate of 75,000 litres per day, would be 75,000 / (2 86,400) = 0.434 m2. Round up to at least 1 m2 in order to ensure efficient settling.

Return Sludge: To keep the microbial population alive, some of the clarifier's activated sludge needs to be transferred back to the aeration tank. Typically, the influent flow rate is 20% to 30% higher than the return sludge flow rate. It would be roughly 15,000 to 22,500 liters/day in this situation.

Disinfection: To ensure pathogen elimination, treated effluent should go through a disinfection procedure after the clarifier. Chlorination or ultraviolet (UV) disinfection are frequent techniques.

Handling Sludge: The sludge generated throughout the process needs to be controlled. According to municipal restrictions, it can be dewatered and disposed of.

Step 6: Compliance with CPCB Norms and Effluent Quality:

The system must be regularly monitored and maintained to ensure compliance with CPCB standards. The extended aeration system's effluent should be within the permitted ranges for variables such BOD, COD, TSS, and faecal coliform.

Step 7: Power and Electrical Requirements

Electricity will be needed for the extended aeration system's aeration and other components. The size and efficiency of the system will determine the power requirements. To ensure ongoing functioning, it's essential to have a dependable power source and backup systems.

Step 8: Environmental and Safety Considerations:

To safeguard the school's environment and the wellbeing of its pupils and staff, safety measures should be put in place. Environmental issues like odour management and noise reduction should also be taken into consideration. These worries might be reduced with the help of adequate landscaping and fencing.

Step 9: Funding and Budgeting

Costs are involved in designing, constructing, and maintaining a STP. Government funds, the school administration, or a mix of sources may provide funding. It's crucial to create a thorough budget that accounts for continuing operations and maintenance expenditures as well as capital costs for building.

Step 10: Operation and Maintenance:

An STP must be regularly maintained and operated in order to work properly. The system should be monitored and maintained by qualified employees. Best practises must be followed when performing routine inspections, equipment servicing, and sludge removal.

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