Water Quality Management - Wastewater & Sludge Treatment

Sewage

• Sewage is 99.9% water.
• The strength of sewage is related to the total amount of organic material, measured by its Biochemical Oxygen Demand (BOD).
• The average BOD5 of domestic sewage is approximately 200 mg/L.
• Sewage nutrient content is around 35 mg/L Nitrogen (N) and 10 mg/L Phosphorus (P).
• Sewage typically has indicator counts of:
  • Total Coliform: 10^7 to 10^9 per 100 mL
  • Fecal Coliform: 10^6 to 10^8 per 100 mL
• Likely pathogens present include: Shigella, Clostridium perfringens, Salmonella, Giardia, Cryptosporidium, Helminth ova (eggs), Enteric virus.

Federal Water Pollution Control Act (FWPCA)

• Also known as the Clean Water Act.
• Requires the reduction of pollution from point sources.
• Regulates point sources by NPDES (National Pollutant Discharge Elimination System) permits.
• Requires Best Available Technology (BAT) for the treatment of toxic wastes.
• Requires Best Conventional Technology (BCT) for the treatment of conventional pollutants.

FWPCA - NPDES Permits

• NPDES permits set effluent standards, which are maximum allowable effluent concentrations.
• Examples for secondary treatment "conventional" pollutants:
  • BOD5: 30 mg/L
  • Total Suspended Solids (TSS): 30 mg/L
  • pH: 6.0 to 9.0
  • Fecal Coliforms: 200 per 100 mL

Treatment Efficiency

• Sewage Treatment Plant (STP), also known as Publicly Owned Treatment Works (POTWs).
• U.S. STPs have at least primary and secondary treatment.
• Pollutant Removal Efficiency of the STP is measured with:
  • \% \text{ Removal Efficiency} = \frac{P{IN} – P{EFF}}{P_{IN}} \times 100
  • P_{IN} = pollutant concentration of influent
  • P_{EFF} = pollutant concentration of effluent
  • Example: minimum BOD5 removal efficiency for secondary treatment:
    \frac{200 \text{ mg/L} – 30 \text{ mg/L}}{200 \text{ mg/L}} \times 100 = 85\% \text{ efficiency}

Industrial Wastewater

• May contain contaminants and often must be pretreated prior to entering the sewer/STP.
• If not pre-treated, can cause:
  • Pass-through: Non-biodegradable toxic substances pass through the STP without being treated, enter the stream & threaten aquatic life.
  • Interference: Wastes may interfere with the operation of the STP.
    • Example: toxic waste killing the bacteria needed for the breakdown of organics.
  • Contamination: High levels of toxic metals or organics contaminate sewage sludge, limiting sludge disposal options.
  • Corrosion: Corrosive wastes corrode and damage pipes & equipment in the sewer & STP.
  • Hazards: Industrial wastes may be flammable, explosive, or toxic to humans.

National Pretreatment Program

• Regulates industrial wastes entering STPs by:
  • Categorical Pre-treatment Standards: Industry-specific requirements.
    • Example: the iron & steel industry has limits on ammonia & cyanide discharge.
  • Prohibited Discharge Standards are substance-specific:
    • Any fire or explosion hazard.
    • Corrosive, any discharge with a pH less than 5.0.
    • Solid or viscous pollutants that will obstruct the flow.
    • Any pollutant discharged in quantities sufficient to interfere with POTW operations (i.e., BOD, oil).
    • Discharge reaching STP with a temperature > 104^o F (40^o C) when they reach the treatment plant.
• POTWs must develop a local pretreatment program that is enforced by EPA, the state, or the local POTW.
  • To reduce pollutant levels discharged by industry.
  • To protect the STP treatment process.
  • To prevent excess pollutant in the STP discharge or sludge.

Flow Rate & Flow Measurement

• Q = A \times V
  • Q is the flow rate, ft.3/sec. or m3/sec.
  • A is the cross-sectional area of flow ft.2 or m2
  • V is the velocity of flow i.e. ft. /sec. or m /sec.

Flow Rate & Flow Measurement - Flumes

• Flume = an open channel with a constricted section then free-fall
• Allows correlation between depth of flow & flow rate
• Parshall flume – for permanent sewage flow-metering at STP
• May be used at influent & effluent of STP

Hydraulic Loading Rate

• The flow rate applied per unit area
• Velocity = V = \frac{Q}{A}
  • V = velocity (flow rate) (ft. /sec. or m /sec.)
  • Q = flow (ft.3/sec. or m3/sec.)
  • A = surface area (ft.2 or m2)
• Hydraulic overloading results in "wash-out" of the system (loss of solids).
  • Example: high rainfall causes surcharge due to infiltration & inflow (I and I).

BOD Loading Rate

• lb. BOD / day = BOD (mg/L) \times Q (MGD) \times 8.34 (lbs. /gallon)
• Q = flow in million gallons / day or MGD
• Excess BOD loading causes aerobes to use up all DO
• System goes anaerobic
• Lose secondary treatment since it relies on aerobic bacteria

Typical Sequence of Wastewater Processes

• Raw wastewater influent undergoes:
  • Primary treatment (physical removal of solids)
    • Screening
    • Grit removal
    • Sedimentation
  • Secondary treatment (biological degradation of solids)
  • Tertiary treatment (additional physical, biological, or chemical treatment; not always performed)
  • Disinfection

Primary Treatment

• Raw wastewater flows into the plant, usually by gravity, and enters the STP headworks (beginning).
• First enters a bar screen:
  • Long, narrow metal bars spaced about an inch apart that retain floating debris like wood, rags, and bulky objects.
  • Screenings = the collected debris, is usually taken to a landfill.
• Next flows to the comminutor
  • Slotted cylindrical screen with a moving cutter blade that shreds solids that passed through the bar screen.
• Then to grit removal by the grit chamber
  • A long, narrow tank, with an outlet weir that slows the velocity of wastewater from 2 ft/sec to 1 ft/sec.
  • Grit = small, heavier solids e.g. sand, gravel that settle out on the bottom of the chamber.
  • Grit is conveyed or pumped off the bottom & taken to a landfill.
  • The lighter suspended organic solids move on.
  • Sometimes the grit chamber is aerated
    • Air bubble up-flow keeps more organic matter suspended, less OM settles out
• Wastewater with organic solids enters the primary clarifier.
  • Detains wastewater for 1 to 2 hours, allowing solids to settle out (sedimentation).
  • Clarifiers are usually 8-16 ft deep, circular with influent at the center & treated effluent goes over a V notch weir at the surface of the outside edge.
  • Bottom sludge scraper moves sludge to a hopper to be pumped to a digester.
  • Surface skimmer removes grease & other floating materials.
• Removes about:
  • 60\% of the Total Suspended Solids
  • 35\% of the BOD5
• Prepares the wastewater for secondary treatment.

Secondary Treatment

• Biological treatment using aerobic microorganisms to remove solids (organic matter) by biodegradation.
• Aerobic bacteria need oxygen, food (organic matter, OM), and appropriate temperature.
• Often removes 85\% of the TSS & BOD5.
• Secondary treatment systems may be:
  • Fixed Film (microbes attached to media)
    • Trickling Filter, Rotating Biological Contactor
  • Suspended Growth (microbes suspended & mixed in liquid)
    • Activated Sludge (more in part 8b)
  • Pond / Wetland (more in part 8b)

Trickling Filter

• A bed or tower of media with wastewater sprayed over the surface from a rotary distributor arm.
• Wastewater trickles down over the media; aerobes attach to the media (fixed film of bacterial slime) and consume organic matter from the wastewater.
• An up-flow of air through the media provides oxygen.
• Older TF used ~ 3-inch rock media in a circular bed up to 200 ft wide & 6 ft. deep; most of the bed was under-ground, & if the water table intruded, the bed went anaerobic (loss of treatment).
• Newer TF use plastic corrugated media that provides greater surface area for microbe growth in a 30 ft. tall tower above ground with a continuous up-flow of air.
• An under-drain system carries away the treated wastewater.
• When the bacterial slime layer gets too thick, it sloughs off & washes away with effluent.
• Wastewater may be recirculated to increase the removal of organics or during periods of low sewage flow.
  • Determined by the recirculation ratio: R = \frac{Q_R}{Q}
    • R = recirculation ratio
    • Usually ranging from 0.0 (no flow) to the recirculation rate being 3.0X the rate of the raw sewage.
    • Q_R = recirculated flow rate
    • Q = raw sewage flow rate
• Trickling filter effluent flows to a secondary clarifier that collects the solids (excess slime growth) sloughing off the TF media.

Rotating Biological Contactor, RBC

• A series of large plastic discs mounted on a horizontal shaft, partially submerged in sewage.
• The discs rotate, contacting air then contacting sewage.
• A bacterial slime layer (fixed film) on the discs absorbs organic matter from the wastewater flowing by.
• Sloughed-off slime is caught by a secondary clarifier.
• Speeding up disc rotation, slowing down flow & increasing the number of discs may allow nitrogenous BOD removal (tertiary treatment) as well as carbonaceous BOD removal (secondary treatment).