"Sewage treatment" sounds unglamorous. But standing inside the 8 MLD (Million Litres per Day) Sewage Treatment Plant in Mahendragarh, watching the systematic transformation of raw sewage into treated effluent, I realised I was seeing one of the most important pieces of civil infrastructure in any city — and almost no engineering student ever gets to visit one.
This visit was part of my 6-month internship with MICADA (Minor Irrigation & Command Area Development Authority), Haryana Government. The plant engineers were generous with their time, walking our small group through every stage of the process.
MLD = Million Litres per Day. This plant treats 8 million litres of sewage every single day — approximately the daily wastewater output of around 50,000–60,000 people. For context, Mahendragarh district has a significant urban population that this plant serves.
Why Does Sewage Need Treatment?
Raw sewage contains pathogens (disease-causing bacteria and viruses), organic matter, suspended solids, nutrients like nitrogen and phosphorus, and various chemicals. If discharged untreated into water bodies, it causes:
- Waterborne disease outbreaks in downstream communities
- Eutrophication of rivers — algal blooms deplete oxygen, killing aquatic life
- Groundwater contamination affecting drinking water wells
- Soil degradation if used directly in agriculture
Treatment doesn't just make water look cleaner — it makes the water cycle safe.
The Treatment Process — Stage by Stage
At the Mahendragarh STP, the process follows the conventional activated sludge treatment method. Here is what I observed at each stage:
Civil Engineering Components I Noted
Beyond the process itself, I paid attention to the civil engineering structures that make the STP function:
- Inlet chamber — RCC structure with flow measurement using a Parshall flume or weir
- Aeration tanks — Large rectangular RCC tanks, typically 4–6 metres deep, with diffuser grids at the bottom for air injection
- Circular clarifiers — Reinforced concrete structures with rotating surface scrapers driven by a central mechanism
- Sludge drying beds — Rectangular beds with brick or concrete sides and a sand/gravel filter layer at the base
- Control room and blower room — The mechanical and electrical heart of the plant, housing air blowers, return sludge pumps, and monitoring equipment
The treated effluent looked like clean water. It was odourless. The plant engineer said it met irrigation standards. The transformation from raw sewage to usable effluent over a matter of hours is genuinely remarkable — and it runs continuously, 24 hours a day, 365 days a year. Someone has to design, build, and maintain every one of these plants. That's the civil engineer's job.
Key Parameters and Standards
The plant was designed to meet the Central Pollution Control Board (CPCB) standards for treated effluent discharge. The engineer mentioned the following key parameters they monitor daily:
- BOD (Biochemical Oxygen Demand): Target ≤ 30 mg/L at outlet
- TSS (Total Suspended Solids): Target ≤ 100 mg/L
- pH: Target 5.5 – 9.0
- Faecal Coliforms: Target ≤ 1000 MPN/100 mL for irrigation reuse
- Chlorine residual: 0.5 – 1.0 mg/L after disinfection
Why Civil Engineers Must Understand Water Infrastructure
As civil engineers, we often focus on the structural, highway, or geotechnical aspects of our education. Water infrastructure gets treated as the "environmental" niche. But every city, town, and industrial estate requires water supply and wastewater treatment systems. These are among the most complex and critical infrastructure projects in existence.
Understanding how an STP works — the civil structures involved, the treatment stages, the effluent standards — makes you a better engineer even if you specialize in something else. You understand what happens to the wastewater from the roads you build, the buildings you design, and the sites you excavate.
Final Thoughts
If you're a civil engineering student and you ever get the chance to visit a sewage treatment plant — go. It might not sound glamorous, but it will shift your perspective on infrastructure fundamentally. You'll understand why environmental engineering is not an optional extra in civil education, and why clean water is a directly human-engineered achievement, not a given.
I'm grateful to the MICADA team and the plant engineers at Mahendragarh for their time and openness during our visit. These are the kinds of experiences that transform a student into an engineer.
