Water Quality Management: Wastewater & Sludge Treatment

Disinfection

• Secondary Effluent Disinfection
  • Destroys pathogens in the effluent that survive the secondary treatment process.
  • Chlorination:
    • Chlorine (Cl) is added until a free chlorine residual is attained.
    • A chlorine contact basin with baffles slows down water flow to ensure sufficient contact time between the effluent and free Cl.
    • Example: 0.5 \frac{mg}{L} free Cl for 15 minutes.
    • Dechlorination is needed to remove Cl and prevent fish kills.
    • Example: add sodium thiosulfate.
  • UV Light Radiation Disinfection:
    • Leaves no residual.
    • UV lamps need periodic cleaning and replacement.
    • No hazardous Cl gas storage.
    • Cost is comparable to a Cl system.

Tertiary Treatment

• Secondary Treatment removes 85-95% of BOD (Biochemical Oxygen Demand) & TSS (Total Suspended Solids) but may not be sufficient to protect sensitive aquatic ecosystems.
  • Example: eutrophic algal bloom caused by excess nutrients.
• Tertiary Treatment involves taking secondary treated effluent and applying additional treatment that:
  • Removes additional BOD and/or TSS.
    • Additional Activated Sludge (AS) treatment for additional BOD removal (e.g., membrane bioreactor).
    • Rapid mixed media filtration for additional TSS removal.
  • Removes additional nutrients (Nitrogen (N) and/or Phosphorus (P)).
  • It is not used in every STP (Sewage Treatment Plant) and can be very expensive.
• Tertiary Treatment to Remove N
  • Up to 85% N removal by Biological Nitrification-Denitrification.
    • Nitrification (1st step): An aerobic process.
      • Uses methods such as AS, trickling filter, or rotating biological contractor.
      • Encourages the growth of nitrifying bacteria to convert ammonia to nitrite and then nitrite to nitrate:
        • NH3 \rightarrow NO2^- \rightarrow NO_3^-
    • Denitrification (2nd step): An anaerobic process.
      • Encourages the growth of denitrifying bacteria to convert nitrate to N2 (nitrogen gas), which is lost to the atmosphere.
      • Requires a source of Carbon (C) for cell synthesis.
        • Example: methanol (CH_3OH) is often added since most carbonaceous material was oxidized by secondary treatment.
    • Internal recirculation from the nitrification (aerobic) tank to the denitrification (anaerobic) tank can result in loss of N_2 gas to the atmosphere.
• Tertiary Treatment to Remove P
  • Enhanced Biological Phosphorus Removal (EBPR):
    • Uses cycling from an anaerobic tank to an aerated AS tank to enhance P uptake by bacteria.
    • Bacterial cells normally contain 1-2% P.
    • Some bacteria are stimulated by being in an anaerobic environment to take up excess phosphates (= luxury uptake).
    • Those bacteria are then sent to an aerobic phase where they store P as polyphosphate (up to 20% of their mass is P).
    • The cells are settled using a clarifier, and wasting the sludge removes the cells with excess P.
  • Chemical precipitation of P:
    • Alum coagulant is added prior to the aeration tank for floc formation and precipitation to remove 90% of P.

Sludge (Biosolids) Treatment

• Each person generates approximately 4 gallons of sludge per week.
• Accounts for > 1/2 of total typical STP costs.
• Sludge comes from the bottom of clarifiers.
• Sludge is treated prior to disposal or use to:
  • Reduce the volume by increasing the solids concentration (reduce the water content).
    • Reduces costs of handling, treatment, and disposal.
  • Stabilize the organics.
    • Less odor.
    • Less attractive to disease vectors.
    • Fewer pathogens.
    • Safer to handle.
    • Potential beneficial use.
• Sludge Treatment
  • Sludge Thickening:
    • Increases solid concentration of the sludge, preps sludge for dewatering.
    • Gravity settling heavy concentrated sludge or air floating the lighter sludge upward; either way, the lighter sludge is removed from the surface to go back to headworks, and the heavier sludge is dewatered or digested.
  • Sludge dewatering to produce dewatered sludge cake, by:
    • Centrifuge, press, or drying bed.
    • Dried to contain < 70% water.
    • Dewatered sludge may be incinerated, landfilled, or composted.
• 2-stage Anaerobic Sludge Digestion
  • Biodegradation of sludge (organic solids) from clarifiers.
  • Digested sludge may be land applied or dewatered & landfilled.
  • Time = 15 – 40 days.
    • 1st in covered tank, sludge is mixed & heated to 95^o F, has two of three digestion phases:
      • 1st digestion phase = hydrolysis (by bacteria) of polymers.
        • Example: proteins, fats to monomers i.e. amino acids, fatty acids.
      • 2nd digestion phase = acid-forming bacteria metabolize fatty acids to organic acids, flows to 2nd tank.
    • 2nd tank has a floating cover:
      • Sludge settles, and supernatant is pumped back to headworks.
      • 3rd digestion phase = methanogenic-bacteria convert organic acids to methane gas.
      • Methane production raises the floating cover.
      • Stored methane may be flared or burned for heating the 1st tank.
    • Digested sludge is dewatered if composted.
    • May not be dewatered prior to land application
• Lime stabilization of digested sludge
  • Mixing quicklime (CaO) or hydrated lime (Ca(OH)_2) with digested sludge.
    • Raise pH to 12 for at least 2 hours.
    • Kills microorganisms (pathogens).
    • Aids to further stabilize & dewater digested sludge.
    • Digested & limed sludge may be land applied.
• Land Application of Sludge:
  • The primary method of biosolids used in the U.S.
  • Benefits of sludge spread on fields:
    • Macronutrients for plant growth:
      • Typical sludge NPK ratio (% by wt.) = 2.5 : 1.6 : 0.4
      • Typical inorganic fertilizer NPK ratio = 10 : 10 : 10
    • Micronutrients for plant growth:
      • Iron, manganese, copper, chromium, selenium, zinc.
    • Organic matter is added to soil.
      • Improves soil structure, tilth, water-holding capacity, water infiltration, soil aeration, biological diversity in soil.
      • Improves CEC (Cation Exchange Capacity) that allows the soil to retain potassium, calcium, magnesium.
  • Dangers:
    • Over fertilization:
      • With N, potential excess nitrate in groundwater & possible methemoglobinemia (blue-baby syndrome).
      • Runoff to surface waters leads to eutrophication.
    • Pathogens, vector attractiveness & odors must be reduced prior to application.
    • Heavy metal concentrations may accumulate in the soil.
      • Typical sludge heavy metal concentrations:
        • Mercury: 5 mg/kg
        • Arsenic: 10 mg/kg
        • Cadmium: 16 mg/kg
        • Lead: 500 mg/kg
        • Chromium: 890 mg/kg
        • Zinc: 1740 mg/kg
    • Preventing heavy metal pollution is critical because cleaning contaminated soils is extremely expensive and difficult.
    • Applicators of industrial waste or sludge must abide by the regulatory limits set by the U.S. Environmental Protection Agency (EPA).
• Use of Digested Sludge
  • Code of Federal Regulations 40, Part 503, EPA Regulated.
    • Management practices:
      • Bulk sewage sludge cannot be applied:
        • In such a way as it enters a wetland.
        • Within 10 meters of U.S. waters.
        • To harm a threatened or endangered species or adversely modify the species critical habitat.
        • Greater than the agronomic rate (nutrients needed for plant production) for the site.
      • Bagged sewage sludge:
        • May be sold or given away if properly labeled:
          • Name & address of producer.
          • Identifies the annual sludge application rate (that is not to be exceeded).
• Class A Sludge Restrictions & Uses
  • No site restrictions due to pathogens.
    • Less than 1,000 MPN fecal coliforms/g dry solids.
    • Salmonella sp. less than 3 MPN / g dry solids.
  • Is like a fertilizer; may be bagged.
  • Common aerated compost pile process to produce Class A is:
    • Composting with a min. 40°C heat for 5 days, with 4 hours at 55°C.
• Class B Sludge Restrictions & Uses
  • Sludge digestion may produce class B sludge:
    • < 2,000,000 MPN fecal coliforms/g dry solids.
    • Site use restricted due to pathogen content.
    • Site Restrictions for Class B Sludge, after sludge application:
      • Animals not to graze for 30 days.
      • Food or feed crops with edible part not touching sludge are not harvested for 30 days.
      • Land with low public exposure is not accessed for 30 days (i.e., private farmland).
      • Turf grown for high public exposure areas is not harvested for 1 year.
      • Land with high public exposure is not accessed for 1 year (i.e., park, ballpark).