# How Sewage Treatment Plants (STP) Work: A Complete Guide for Industrial Facilities
Executive Summary
An STP is not “just a compliance requirement.” For industrial facilities and large developments, a properly designed STP reduces freshwater demand, stabilizes operations, and enables safe reuse — provided it is sized correctly and built for real-world influent variation.
This guide explains the three treatment stages, the equipment that matters, and the design decisions that usually determine performance and O&M cost.
What an STP does (in one sentence)
An STP converts wastewater into treated water suitable for reuse or compliant discharge by removing solids, organic load, and pathogens through staged physical, biological, and polishing processes.
The 3 treatment stages (simple and practical)
1) Primary treatment (physical removal)
Purpose: remove large solids and settleable material.
- Typical unit operations:
- screening (coarse and fine)
- grit removal (sand/heavy particles)
- primary clarification (settling)
Why it matters: strong primary treatment protects downstream biology and reduces sludge problems.
2) Secondary treatment (biological removal)
Purpose: remove dissolved/colloidal organic matter (BOD/COD) using microorganisms.
- Typical unit operations:
- aeration (oxygen transfer)
- biological reactor (activated sludge or equivalent)
- secondary clarification (biomass separation)
Why it matters: biology is sensitive to shock loads and poor aeration. Stable control reduces energy and improves effluent quality.
3) Tertiary treatment (polishing + disinfection)
Purpose: “polish” the water for reuse targets and ensure microbial safety.
- Common polishing:
- filtration (sand/media or membranes, depending on target)
- disinfection (UV/chlorine as required)
Why it matters: reuse quality is defined here (irrigation vs. utilities vs. higher-grade reuse).
Key equipment (what to pay attention to)
- - Screens: prevent clogging and reduce maintenance headaches.
- Aeration system: often the biggest energy consumer; design and control quality matters.
- Clarifiers: separation quality impacts downstream filters and disinfection.
- Disinfection: selected based on effluent quality and reuse standard.
- Sludge handling: frequently underestimated; impacts uptime and operating cost.
How to size an STP correctly
- Sizing is not only “m³/day.” You should confirm:
- average and peak flow (diurnal peaks, events)
- influent strength (BOD/COD/TSS, fats/oils)
- variability (weekend vs. weekday, seasonality)
- required effluent standard (discharge vs. reuse target)
The best designs are robust to variability — because real influent is never “steady-state.”
O&M: the difference between good and excellent projects
- To keep performance stable over years, plan for:
- monitoring (flow + key quality indicators)
- preventive maintenance schedule
- operator training
- spare parts strategy
- clear escalation and support model
Next step: design review / feasibility
If you’re planning an STP for an industrial facility or a large development, WEST Water can support the feasibility, design, EPC delivery, and O&M — aligned to your reuse targets and compliance requirements.
- - Phone: +201008440407
- Email: hazmalgaml@west-water.com
- Related:
- Services: /services
- Projects: /projects