Water Quality Management: Wastewater & Sludge Treatment

Secondary Treatment Systems

• Secondary treatment systems can be categorized as:
  • Fixed Film: Microbes are attached to a media.
    • Examples: Trickling Filter, Rotating Biological Contactor
  • Suspended Growth: Microbes are suspended and mixed within the liquid.
    • Examples: Activated Sludge, Pond/Wetland

Activated Sludge (AS)

• Definition:
  • An aeration tank is used to mix oxygen with incoming wastewater and a suspended-growth of microbes growing on sludge particles.
  • The microbes growing on sludge particles are referred to as activated sludge.
  • The combination of wastewater and activated sludge is called Mixed Liquor Suspended Solids (MLSS).
• Aeration and Mixing Methods:
  • Compressed air is injected through diffusers along the bottom of the tank for continuous mixing.
  • Rotary blades or paddles spin at the top of the tank to provide continuous mixing.
• Process:
  • Microbes absorb and biodegrade the suspended organic matter from incoming wastewater.
  • Conventional AS involves approximately 6 hours of aeration.
  • The mixed liquor then flows into a secondary clarifier where the sludge particles (floc) settle out.
• Clarifier Outputs:
  • Supernatant: Clear water at the top of the clarifier, discharged over an effluent weir.
  • Sludge: Pumped out from the clarifier bottom.
    • Approximately 30% of the sludge is returned to the aeration tank influent as Return Activated Sludge (RAS).
    • RAS is further biodegraded and provides an inoculum of bacteria to the incoming wastewater.
    • Approximately 70% of the sludge is disposed of as Waste Activated Sludge (WAS).
• Food to Microorganism Ratio (F/M):
  • Indicates the organic load on the system.
  • Typically ranges from 0.2 to 0.5 for conventional AS.
  • "Food" represents the flow of organic matter into the system.
  • "Microorganisms" are represented by the suspended solids in the system.
  • Formula: F/M = (Q * BOD) / (MLSS * V)
    • Q = raw sewage flow rate, MGD
    • BOD = applied 5-day BOD, ppm
    • MLSS = mixed liquor suspended solids, ppm
    • V = volume of aeration tank, MG
• Microorganism Composition:
  • Requires a balanced ecosystem of microorganisms for effective treatment.
    • Approximately 95% bacteria: feed on soluble nutrients.
    • Approximately 5% higher-level organisms.
      • Protozoa
      • Metazoa
• Protozoa:
  • Single-celled organisms.
    • Flagellates & amoeba:
      • Dominate in young sludge.
      • Feed mostly on soluble nutrients.
      • Small contribution to AS treatment.
    • Stalked, crawling & free-swimming ciliates:
      • Indicators of a stable, good treatment process.
      • Feed mostly on suspended bacteria and improve clarity.
      • Dominate after most soluble nutrients are taken up.
• Metazoa:
  • Multicellular organisms with little impact on AS treatment.
    • Rotifers: top predator of bacteria & protozoa, will dominate only in old sludge.
    • Nematodes & Fungi: indicators of old sludge.
• Settling of AS:
  • Solids settle out from the clarified water in the secondary (final) clarifier.

Settling of AS - Sludge Bulking

• Occurs when excess filamentous bacteria (e.g., Sphaerotilus natans) make the sludge "fluffy."
  • Too light to settle.
  • Supernatant is not clear.
  • Much of the sludge flows out over the effluent weir.

Settling of AS - Sludge Volume Index (SVI)

• Evaluates the settleability of the activated sludge.
  • Sample mixed liquor for total suspended solids (TSS).
  • Allow mixed liquor to settle for 30 minutes in a 1-L graduated cylinder, then read the settled sludge volume in mL.
  • Formula: SVI = (settled sludge volume (mL/L) * 1000) / MLSS (mg/L)
    • SVI < 100 = sludge with good settleability.
    • SVI > 100 = settleability decreasing.
    • SVI > 200 = sludge bulking occurs.

Activated Sludge Modifications

• Extended Aeration:
  • Used by small systems; often consists of a prefabricated steel tank (package plant).
  • Contains both an aeration tank & a secondary clarifier.
  • Differs from a conventional AS system:
    • Primary treatment is screening only (no settling first).
    • Aeration time is approximately 30 hours (compared to 6 hours in conventional AS).
    • Operates at a very low F/M ratio (e.g., 0.05).
• Step Aeration:
  • Multiple feed points of the incoming wastewater into the aeration tank (not just one as in conventional AS).
  • Spreads the oxygen demand more uniformly across the tank.
  • May decrease aeration time down to 3 hours.
• Pure Oxygen Aeration:
  • High-purity oxygen is injected into the aeration tank.
  • Aeration time = 2 hours.
  • F/M ratios can be very high (e.g., 1.0 to 1.5).
  • Less tank volume is needed compared to other systems.
  • In AS systems, the more efficient the input of oxygen, the greater the F/M ratio may be & less tank volume is needed.
• Oxidation Ditch:
  • Uses horizontal rotor-brush aerators in oval-shaped basins ("race tracks").
  • Brush aerators at the surface must propel the mixed liquor with sufficient velocity to prevent settling.
  • Usually operate as extended aeration (i.e., 12-hour aeration time).
• Sequencing Batch Reactor (SBR):
  • 5 operational steps in one reactor tank over time:
    • Fill reactor with sewage.
    • React: create activated sludge with aeration.
    • Settle solids; clarify the upper portion.
    • Draw off clarified liquid (effluent).
    • Idle without aeration, remove waste sludge, leave some return sludge for the next “batch” in the reactor.
  • Typically need 2 reactors.
  • Advantages over conventional AS:
    • No additional secondary clarifier tank is needed.
    • The “batch” can be sampled prior to release & treated further if needed.
• Membrane Bioreactor (MBR):
  • Replaces the secondary settling basin (clarifier) of a conventional AS process.
  • Uses membrane microfiltration to retain the AS (solids & microbes) and pass clear water.
  • Microbe retention allows more organic degradation; may reduce BOD_5 by up to 99%.
  • Membrane fouling (clogging) is reduced by bubble aeration & periodic backwashing.

Sewage Lagoon or Stabilization Pond

• Diked to prevent inflow of surface runoff.
• Approximately 2 meters water depth.
• Relies on:
  • algae (not rooted plants) & aerobic bacteria in the aerobic water column.
  • anaerobic bacteria in the anaerobic bottom sludge layer.
• Solids settle out on the bottom (primary treatment) & undergo anaerobic digestion with by-products of methane, hydrogen sulfide & organic acids.
• Aerobic bacteria in the water finish decomposition (stabilization) of the wastewater organics.
• Facultative bacteria may grow in either the aerobic upper or anaerobic bottom layers.
• Oxygen is naturally added by wind action & photosynthesis by algae.
• Detention time is usually 60 days or longer.
• Low maintenance cost but requires a very large surface area; often only cost-effective for small rural towns.
• Often exceeds discharge limits for TSS in the summer due to algal growth (i.e., may need a higher TSS limit of 90 mg/L).
• Adding additional cells (surface area) or mechanical aeration may allow greater BOD loading.

Constructed Wetlands

• Shallow basins over low permeability soil or a liner, diked to prevent inflow of surface runoff, containing hydrophytes (water-loving plants).
• Types:
  • Surface Flow:
    • Mimics natural wetlands with emergent rooted plants in water exposed to sunlight & wind.
    • Degradation processes are similar to the stabilization pond.
  • Sub-surface Flow:
    • Wastewater flows across emergent plant roots in media (e.g., gravel, sand).
    • Greater surface area for microbe growth; can treat more wastewater per surface area than surface flow.
  • Floating Plant Systems
• May act as both primary & secondary treatment to remove BOD, TSS & nutrients.
• Optimum wastewater detention time = 5 - 14 days.
• Performance:
  • Surface Flow:
    • Up to 80% BOD removal; up to 90% TSS removal.
  • Sub-surface Flow:
    • More efficient than surface flow at BOD & TSS removal.
    • The majority of solids settle out in the first 10-20% of media.
    • Inlet zone may clog & need solids removal.
• Floating Perennial Aquatic Plant Systems:
  • Long, shallow ponds with a top growth of either water hyacinths or duckweed.
  • Harvesting the plants removes some N & P from the system.
  • Water Hyacinth:
    • 20-47 inches from flowers to root tip.
    • Rapid growth: 10 plants can cover 1 acre in 8 months.
    • Considered an invasive species that can crowd out other plants and clog waterways.
    • Plant transports some oxygen to the roots.
    • Requires wastewater flow over the roots for BOD removal by the bacterial growth on the roots, need relatively shallow pond
    • Complete die-off at extended 1°C; only used in most southern U.S.
  • Duckweed (Lemna sp.):
    • 30% faster growth than water hyacinth; forms a surface mat that can double its area in 4 days.
    • High in protein; used in animal feed.
    • More cold-tolerant than water hyacinth; grows at least 6 months of the year in most of the U.S.
    • No oxygen transfer to roots; may need pre-aeration.
    • Duckweed mat stills the waters beneath & allows better sedimentation of suspended solids; deeper ponds allow greater TSS removal.
    • Duckweed mats tend to be wind-blown; may need surface baffles to retain the mat in place.