wastewater-treatment-plant

Municipal Wastewater Treatment Plant Operations Briefing #

Key points #

  • The wastewater treatment process follows a multi-stage "process train": Preliminary → Primary → Secondary → Tertiary (as needed) → Disinfection → Solids Handling.
  • The plant must meet EPA and state permit targets for BOD (Biochemical Oxygen Demand), TSS (Total Suspended Solids), nutrient removal (nitrogen, phosphorus), and disinfection.
  • Biological treatment, primarily using activated sludge in aeration basins, is central to removing organic matter.
  • Solids handling involves thickening, anaerobic digestion (producing biogas for cogeneration), dewatering, and final disposal as Class A/B biosolids.
  • Key operational parameters include Dissolved Oxygen (DO), Mixed Liquor Suspended Solids (MLSS), sludge age, F/M ratio, UV dose, flow, and transmittance.

Context and explanations #

This briefing provided an overview of the municipal wastewater treatment plant's operational process, detailing each major stage from raw wastewater inlet to treated effluent outfall and solids disposal.

The process train begins with preliminary treatment, involving bar screens to remove large debris (rags, plastics) and grit removal to separate sand and other heavy inert materials, protecting downstream pumps and equipment.

Next, primary clarifiers use gravity to separate settleable solids (forming sludge) and floatable materials (scum) from the wastewater.

The heart of the biological treatment is secondary treatment, specifically the activated sludge process. Here, wastewater enters aeration basins where microorganisms consume organic matter. Operators monitor parameters like dissolved oxygen (DO), mixed liquor suspended solids (MLSS), and sludge age. Following aeration, secondary clarifiers separate the treated water from the activated sludge. A portion of this sludge, Return Activated Sludge (RAS), is recycled back to the aeration basins to maintain the microbial population, while Waste Activated Sludge (WAS) is sent for solids handling. Challenges like "sludge bulking" due to filamentous bacteria require operator adjustments to the F/M (Food-to-Microorganism) ratio, DO levels, or selective chlorine dosing.

Tertiary treatment is applied as needed for further polishing. At this plant, it includes filters for additional suspended solids removal and chemical phosphorus removal, often using coagulants like alum or ferric salts, especially when influent phosphorus loads are high. Nitrogen removal involves managing conditions for nitrification (ammonia to nitrate) and sometimes denitrification (nitrate to nitrogen gas) within the secondary process.

Disinfection is the final step for the liquid stream, using UV (ultraviolet) light to inactivate pathogens before the treated effluent is discharged via the outfall to the river. UV dose, flow, and transmittance are logged to ensure effectiveness.

Solids handling is a parallel process for the sludge and scum collected from primary and secondary treatment. This involves thickening to reduce volume, followed by anaerobic digesters where microorganisms break down organic matter, producing biogas that can be used for cogeneration. The digested sludge then undergoes dewatering using belt presses to further reduce volume before being disposed of as biosolids (Class A/B), either through land application or haul-off according to contracts.

Key terms relevant for operator certifications were highlighted:

  • BOD: Biochemical Oxygen Demand, a measure of organic pollution.
  • TSS: Total Suspended Solids, an indicator of turbidity and suspended matter.
  • NPDES: National Pollutant Discharge Elimination System permit, setting discharge limits.
  • SRT: Solids Retention Time.
  • HRT: Hydraulic Retention Time.
  • SVI: Sludge Volume Index.
  • RAS: Return Activated Sludge.
  • WAS: Waste Activated Sludge.

Diagrams # 

flowchart LR
    inlet[Raw Wastewater Inlet] --> prelim[Preliminary Treatment (Screens, Grit)]
    prelim --> primary_clar[Primary Clarifiers]
    primary_clar --> aeration[Aeration Basins (Activated Sludge)]
    aeration --> secondary_clar[Secondary Clarifiers]
    secondary_clar --> tertiary[Tertiary Treatment (Filters, P Removal)]
    tertiary --> disinfection[UV Disinfection]
    disinfection --> outfall[Outfall to River]

    primary_clar --> sludge_scum[Sludge & Scum]
    secondary_clar --> was[Waste Activated Sludge (WAS)]
    sludge_scum --> thickening[Thickening]
    was --> thickening
    thickening --> digesters[Anaerobic Digesters]
    digesters --> dewatering[Dewatering (Belt Presses)]
    dewatering --> biosolids[Biosolids (Land App/Haul-off)]

    secondary_clar --> ras[Return Activated Sludge (RAS)]
    ras --> aeration
    digesters --> supernatant[Supernatant]
    supernatant --> inlet