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}$$$0.5 \frac{mg}{L}$$ 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$$$CH_3OH$$) 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$$$N_2$$ 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$$$95^o F$$, 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$$$Ca(OH)_2$$) 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$$$mg/kg$$

        • Arsenic: 10 $mg/kg$$$mg/kg$$

        • Cadmium: 16 $mg/kg$$$mg/kg$$

        • Lead: 500 $mg/kg$$$mg/kg$$

        • Chromium: 890 $mg/kg$$$mg/kg$$

        • Zinc: 1740 $mg/kg$$$mg/kg$$

    • 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).