What Is the Daily Water Treatment Capacity of a Typical STP?
Sewage Treatment Plants (STPs) are vital for modern sanitation infrastructure, enabling the safe treatment of wastewater generated from domestic, commercial, and industrial activities. Among the most critical parameters when designing, selecting, or evaluating an STP is its daily water treatment capacity—commonly expressed in kilolitres per day (KLD). This metric represents how much wastewater a plant can process effectively within a 24-hour cycle.
Understanding this capacity is essential, whether you’re planning for a residential complex, industrial facility, hospital, school, or municipal township. This article will explore the daily treatment capacities of typical STPs, factors that influence them, their operational implications, and how correctly sizing a plant leads to efficient, economical, and environmentally sustainable wastewater management.
Defining Daily Water Treatment Capacity in STPs
The daily water treatment capacity refers to the volume of wastewater a sewage treatment plant can handle and treat in one day. It is the design benchmark upon which the size, structure, and operation of the plant are based. This capacity is crucial for ensuring the plant can operate efficiently, without being overloaded or underutilized.
The treatment capacity of STPs is generally classified based on their intended application. Let’s look at the common classifications:
1. Small-Scale STPs (10 KLD to 50 KLD)
These are compact plants, ideal for:
- Small residential buildings or gated communities
- Boutique hotels
- Individual industrial units
- Offices and small commercial spaces
2. Medium-Scale STPs (50 KLD to 250 KLD)
Typically used for:
- Large apartment complexes
- Educational institutions
- Hospitals and healthcare centres
- Malls and commercial establishments
- Medium-sized factories
3. Large-Scale STPs (500 KLD and Above)
These systems cater to:
- Townships and urban colonies
- Industrial parks and large manufacturing units
- Airport terminals
- IT parks and large-scale public infrastructure
In massive municipal or city-scale treatment plants, capacities can reach tens of millions of litres per day (MLD), supporting populations of hundreds of thousands.
Importance of Knowing Treatment Capacity
Understanding the treatment capacity is more than a technical detail—it has real-world consequences:
- Environmental Safety: An overloaded STP may discharge untreated or partially treated wastewater, polluting rivers, lakes, or groundwater.
- Regulatory Compliance: Regulatory authorities enforce effluent discharge norms. Undersized STPs often violate these limits, resulting in fines or shutdowns.
- Sustainability: Adequate capacity ensures that treated wastewater can be reused for non-potable applications like gardening, flushing, or industrial processes, reducing dependency on fresh water.
Factors Influencing STP Capacity
Several variables affect the daily capacity required and achievable by an STP:
1. Population or Occupancy Load
The most direct determinant is the number of people generating wastewater. On average:
- A person generates 100–135 litres of sewage per day.
- Commercial establishments may produce higher wastewater per capita due to kitchens, laundries, and restrooms.
Hence, for a 200-person apartment complex:
- Wastewater generated = 200 x 120 L/day = 24,000 L/day = 24 KLD
2. Type and Source of Wastewater
Not all wastewater is equal. Domestic sewage is easier to treat compared to industrial effluent which may contain oils, heavy metals, and chemicals. A plant treating both types may require:
- Additional treatment stages
- Longer retention times
- More sophisticated technologies
3. Treatment Technology Used
The type of treatment technology impacts the efficiency and volume the STP can handle:
| Technology | Description | Impact on Capacity |
|---|---|---|
| ASP (Activated Sludge Process) | Biological treatment using aeration tanks | Cost-effective, moderate footprint |
| MBBR (Moving Bed Biofilm Reactor) | Uses bio-media for higher biomass retention | High capacity, compact design |
| SBR (Sequential Batch Reactor) | Batch-wise treatment, automated | Energy-intensive but flexible |
| MBR (Membrane Bioreactor) | Combines membrane filtration with biological treatment | High quality output, space-saving |
4. Flow Variation and Peak Loads
Wastewater generation isn’t uniform throughout the day. Peak hours—usually morning and evening—can stress the system. STPs are designed with buffer tanks and holding capacities to accommodate such variations.
5. Inflow and Infiltration
Leaks and stormwater intrusion into the sewer system can significantly increase the wastewater volume entering an STP, which should be accounted for during capacity planning.
6. Reuse Intent
If treated water is to be reused for landscaping, flushing, or industrial cooling, the STP may need additional tertiary treatment (filtration, disinfection), which impacts overall throughput.
Operational Implications of Treatment Capacity
1. Overloading the STP
When an STP is forced to handle more than its rated capacity:
- Retention time decreases
- Microbial activity is compromised
- Output quality degrades
- Equipment experiences wear and breakdowns
- Treated water fails to meet regulatory norms
2. Underutilization
An STP operating significantly below its design capacity:
- Leads to energy and chemical wastage
- Increases per-litre treatment cost
- Can result in microbial inefficiency due to lack of nutrients
3. Need for Modular Expansion
STPs in developing areas or expanding facilities should be designed for modular scalability—where capacity can be enhanced in stages as demand increases without disrupting ongoing operations.
How to Calculate the Required STP Capacity
To determine the required capacity of an STP:
- Estimate population or industrial load
- Use average sewage generation figures:
- Residential: 120 L/person/day
- Commercial: 45–90 L/person/day depending on operations
- Add 10–20% safety margin
- Factor in seasonal variations and inflow/infiltration
Example Calculation:
A housing complex with 400 residents:
- Expected sewage = 400 x 120 = 48,000 L = 48 KLD
- With 20% margin = 48 x 1.2 = 57.6 KLD
So, an STP with 60 KLD capacity is recommended.
Benefits of Appropriately Sized STPs
1. Enhanced Treatment Efficiency
Plants operating within their optimal load function better. Biological treatment is more effective, and mechanical components are under less stress.
2. Cost Optimization
- Over-sized STPs incur high installation and operation costs without proportional benefits.
- Undersized systems require frequent maintenance and costly upgrades.
3. Regulatory Compliance
A well-sized STP ensures consistent compliance with local Pollution Control Board (PCB) norms for effluent discharge.
4. Water Reuse and Sustainability
Properly treated water can be reused for:
- Garden and landscape irrigation
- Toilet flushing
- Cooling tower makeup
- Construction activities
This supports water conservation goals, especially in water-scarce regions.
5. Public Health and Environmental Protection
Efficient treatment avoids groundwater contamination, prevents waterborne diseases, and maintains the ecological health of receiving water bodies.
Practical Considerations for STP Installation
- Space Availability: Compact designs like MBBR and MBR are ideal for urban spaces.
- Energy Requirements: Energy-efficient motors, blowers, and pumps reduce operating costs.
- Automation: Smart controls and sensors optimize performance and reduce manpower.
- Maintenance Schedule: Periodic desludging, mechanical inspection, and lab testing are essential.
- Odour and Noise Control: Enclosures and scrubbers improve acceptance in residential areas.
Real-Life Application: STP in a Residential Township
Consider a township with 1,000 households and an average of 4 members each.
- Sewage generated = 1,000 x 4 x 120 = 480,000 L/day = 480 KLD
- Recommended STP capacity (with 15% buffer) = 552 KLD
The township opts for a 600 KLD MBBR-based STP with tertiary UV treatment to enable reuse for garden irrigation and flushing. Result:
- 80% reduction in freshwater dependency
- Zero discharge outside premises
- Full compliance with PCB norms
Conclusion
The daily water treatment capacity of an STP is more than just a number—it’s a key design and operational criterion that determines the plant’s effectiveness, cost-efficiency, and environmental compliance. Whether for a small building or a sprawling industrial estate, selecting the right STP capacity ensures optimized wastewater treatment, promotes water reuse, and supports a cleaner ecosystem.
Proper planning, assessment, and consultation with wastewater experts can ensure that your STP delivers long-term benefits while complying with national and local standards.
Need Help with STP Selection or Installation?
At 3D Aqua, we offer expert consultation and turnkey solutions for all types of water and wastewater treatment systems. Whether you’re planning a new project or upgrading an existing setup, our team ensures you get the most efficient, compliant, and cost-effective solution.
📞 Call us at +91-6262629090
📧 Email: info@3daqua.in
Let us handle your water treatment challenges—efficiently, sustainably, and reliably