I. Safety Precautions

Safety is the absolute priority when dealing with the confined spaces and hazardous atmospheres of an STP.

A. Pre-Inspection Measures

• Permit to Work (PTW): A formal "Confined Space Entry Permit" must be obtained and signed by authorized personnel.
• Risk Assessment: Identify hazards including toxic gases (Methane H2S), oxygen deficiency, electrical risks, and biological contamination.
• Isolation (LOTO):
  • Electrical: De-energize and Lock-Out/Tag-Out (LOTO) all pumps, blowers, and UV units at the main switchboard.
  • Fluid: Close and blank off all inlet/outlet valves to prevent accidental flooding.
• Ventilation: Use portable blowers or eductors to purge gases. Ventilation must remain active throughout the entire procedure.
• Gas Testing: Use a calibrated gas detector to confirm:
  • Oxygen (O2): 20.8% ± 1%.
  • Flammable Gases: Below 1% LEL (Lower Explosive Limit).
  • Toxic Gases: Check for H2S and Carbon Monoxide (CO).
• PPE Requirements: Chemical-resistant coveralls, gloves, safety boots, goggles, and personal multi-gas detectors. A Safety Harness with a lifeline/tripod system is mandatory.
• Standby Personnel: A trained "Standby Man" must remain at the entry point to maintain communication and trigger the Rescue Plan if necessary.
• Cleaning: The unit must be drained, flushed, and cleaned of all sludge/scum before entry to minimize biological risks.

B. During & Post-Inspection

• Continuous Monitoring: Maintain gas testing and constant verbal/visual contact with the standby man.
• Hot Work Ban: No welding or cutting is permitted without additional specific permits.
• Tool Control: Inventory all tools before and after to prevent "Foreign Object Damage" (FOD).
• Closing Up: Confirm all personnel are out before securing hatches, re-energizing equipment, or reopening valves.

II. Inspection Procedure

Phase 1: Preparation

1. Review Documentation: Study the STP manual, previous reports, and schematics.
2. Isolate & Clean: Follow the LOTO procedures and perform a deep clean of internal surfaces for better visibility.
3. Atmospheric Check: Ensure the environment is safe via ventilation and gas testing.

Phase 2: Execution (Entry)

1. Authorized Entry: Enter only after the permit is validated.
2. Systematic Survey: Move through the treatment stages (Primary → Secondary → Tertiary), documenting findings with photos and measurements.

Phase 3: Documentation & Recommissioning

1. Reporting: Create a detailed report of corrosion, wear, and structural integrity.
2. Refilling: Follow manufacturer guidelines to gradually return the plant to service.

III. Components to be Inspected

A. Primary Treatment Section

• Comminutor/Macerator: Check cutters/blades for sharpness, wear, and blockages. Inspect the motor mounting.
• Settling Tanks: Inspect walls and floors for cracks or coating failure. Check that Baffles and Weirs are aligned and free of scale.
• Sludge Hoppers: Ensure hoppers/cones are free of corrosion and blockages.

B. Secondary Treatment (Biological)

• Aeration Tank: * Diffusers: Check for clogging, damage, or uneven air distribution.
  • Bio-media: Look for signs of "channeling" or excessive biomass growth.
  • Mixers: Inspect propeller blades and shaft alignment.
• Secondary Clarifier: * Scrapers: Check for smooth operation and mechanical wear.
  • Launders: Ensure weirs are perfectly level to maintain uniform flow.

  C. Tertiary Treatment & Disinfection

  • Chlorination Unit: Inspect contact tank walls and baffles for scale buildup.
  • UV Disinfection: Check quartz sleeves for fouling/cracks and test automatic wipers.
  • Filters: Inspect filter media for integrity and ensure the underdrain system is not blocked.

  D. General Components (System-Wide)

  • Internal Piping: Check for leaks, internal corrosion, and valve seating integrity.
  • Sensors: Clean and inspect pH, Dissolved Oxygen and level probes.
  • Structural: Inspect all internal supports, brackets, and access manholes/gaskets for signs of fatigue or thinning.