Sludge Treatment and Stabilization in Wastewater Plants

Posted on Nov 26, 2024 in Geology

Sludge Treatment in Wastewater Plants

The sludge age (days) is decisive for designing activated sludge plants. It’s the average microorganism residence time in the bioreactor, calculated as the ratio of solids in the reactor to the daily excess sludge production. To avoid microorganism “leaking,” the sludge age must match the inverse of the bacterial growth rate, defined as the increase of microorganisms relative to the total daily microorganisms.

Sludge Types

  • Primary Sludge: Consists mainly of minerals, about 60% organic matter (volatile solids) and 40% inorganic matter. After decanting, concentrations of 2-4% can be extracted.
  • Excess Secondary Sludge: More organic, composed of 80% organic matter and 20% inorganic material. Decanted concentrations are 0.8% (drag scrapers) or 0.6% (suction blades).
  • Total or Mixed Sludge: Achieves a 2-3% concentration, advantageous due to lower water content compared to separately decanted sludges. Secondary sludge, about 40-50% of total sludge, has a third of the primary’s concentration but contains significant water. Sending it to the primary settling at 2-3% eliminates much water, requiring fewer facilities.

Fundamental Objective

Reduce volumes to a minimum, to:

  1. Minimize element sizes, saving on tanks, equipment, and pipelines.
  2. Increase retention times with equal capacity.
  3. Reduce energy costs for racking, heating, and handling.

Sludge Thickening

Gravity Thickening

Objective

Increase solids concentration by removing liquid.

Principle

Sedimentation zones theory, where solids settle as blankets. At t=0, concentration is uniform. After time t:

  • Interface: Defines the clear water and solid mantle boundary. Descends at constant speed.
  • Interfacial Zone: Below the interface.
  • Compaction Zone: Solids compact at the bottom, with constant concentration and ascent rate.
  • Transition Zone: Between the interface and compaction zone, with an increasing concentration gradient.

Surface area is key, determined by sediment testing.

Elements

  • Tank: Prefabricated or civil works, usually circular.
  • Mobile Bridge: Centrally driven scrapers (4-6 m/min).
  • Thickening Train: Mixing paddles, bell, perimeter landfill, distribution header, overflow evacuation, odor cover, and elastic cuff valves.

Flotation Thickening

Separates solids from liquid, especially for biological sludge with density close to water and weak compaction. Used in medium to large plants. More complex than gravity, requiring compressed air and often polyelectrolyte conditioning.

Principle

Particles with density similar to or less than water float (natural flotation). Air supply speeds up the process (aerated flotation). Dissolved Air Flotation (DAF) is most effective.

System

  1. Pressurization System: Compressor, pressure and flow measurement.
  2. Pressure Reducing Valve.
  3. Pressurization Tank (dissolution).
  4. Pressurization Pumps.
  5. Reactant Dosing Unit.

Parameters

  • Introduced Air Ratio: Most important, optimal up to a point. Varies between 0.5% and 6% due to sludge type, pressurization system, air mixing, sludge bed depth, solids loading, hydraulic loading, IVF, and polyelectrolyte addition.

Elements

  • Float, Squeegee Train: Variable speed gearmotor, bell deal.
  • Hopper, Landfill.

Anaerobic Digestion

Alkaline fermentation degrades organic matter into methane and CO2. Sludge is stabilized anaerobically by microorganisms. A slow process requiring specific conditions for different microorganisms with varying kinetics. It’s the most common method for obtaining aseptic final sludge with low pathogen content.

Phases

  • Acidogenic Phase: Rapid, facultative bacteria transform dissolved substrate into acids, CO2, and H2O. Products depend on H2 concentration. Low H2 favors acetic acid; high H2 produces propionic, butyric, valeric acids, etc. Removes dissolved oxygen. Rapid growth (30-minute replication).
  • Acetogenic Phase: Syntrophic bacteria use previous phase products, generating acetic acid. Metabolism is regulated by H2O concentration.
  • Methanogenic Phase: Strictly anaerobic bacteria catabolize H2 and acetic acid. Optimal pH near 7.2. Slow growth (3 days at 35°C to 50 days at 10°C). Two types: acetoclastic and H2-using methanogenic bacteria.

Process Control

  • Temperature: Different bacteria thrive at different temperatures (psychrophiles, mesophiles, thermophiles). Affects reactor volume and stabilization. Above 65°C, activity decreases; below 10°C, bacteria encyst.
  • Alkalinity: Related to pH, measures buffering capacity against volatile acids. Expressed as calcium carbonate equivalent. Generated by solubilized inorganic carbon and volatile fatty acids.
  • Volatile Fatty Acids Concentration: Propionic, butyric, acetic, valeric acids in the liquid phase. Found as CO2, HCO3, or CO3.
  • pH: Balance is crucial for different microorganism populations.
  • Agitation: Homogenizes concentration and temperature, prevents short circuits.
  • Volatile Solids Loading: Controls solids input (volatile matter per day per cubic meter of digester). Sudden increases can raise acid levels.

Circuits

  1. Sludge
  2. Gas
  3. Sludge Heating Equipment
  4. Homogenization Equipment
  5. Control, Measurement, and Safety Equipment

Elements

  • Digester: Civil works deposits with fixed or floating cover, watertight, sloped bottom. Features: porthole, manhole, and scum breaker.
  • Equipment:
    • Heating: Sludge Boilers: Use generated methane as fuel, water as heat transport. Burners. Exchangers.
    • Homogenization:
      • Mechanical:
        • Recirculation: Pumping from bottom to top.
        • Mechanical Stirrers: e.g., propeller agitators (require careful operation).
      • Gas: Biogas is compressed and fed through nozzles for bubble agitation.
    • Gas Line Equipment: Gasometer, Torch, Safety Valves (flame arresters, pressure/vacuum relief), Pressure Reducing, Solid/Liquid Traps, Purge.
    • Cogeneration Elements: Sequential electricity and heat production from biogas in WWTP.

Aerobic Stabilization

Aerobic biological process with high sludge ages, achieving autoxidation (endogenous respiration). Bacteria consume reserves, oxidizing cellular tissue to CO2, water, and ammonia.

Benefits

  • Easy control and operation.
  • Better supernatant quality.
  • Almost no odors.
  • Volatile solids reduction similar to anaerobic digestion, but faster.
  • Final residue is a good fertilizer.

Cons

  • High operating cost, high energy consumption.
  • Susceptible to weather temperature changes.

Parameters

  • Number and characteristics.
  • Hydraulic retention time.
  • Volatile solids retention time.
  • Load cell.
  • Volatile solids elimination performance.
  • Temperature requirements.
  • Oxygen requirements.

Pure Oxygen Stabilization

  • Closed Reactors: Stable thermophilic conditions (40-60°C) due to exothermic reactions. Suitable for cold climates or areas with no soil. Short retention times (around 4 days). Effective bacteria and virus removal.
  • Open Reactors: Lower working temperatures, larger reactor volumes.

Chemical Stabilization

Adding reagents to thickened or dried sludge stops bacterial activity without removing organic matter or reducing volume. Creates conditions inhibiting biological degradation and preventing odors. Lime or chlorine can be used; lime is preferred for cost and handling. Enough lime is added to raise the pH.