Wastewater Engineering 3

Methods of Expressing Pollutant Concentrations

• mg/L (milligrams per liter)
• ppm (parts per million)
• PPD (parts per billion)
• Change of mass over time is a minor factor in reactions.
• Common in wastewater treatment; reactions are non-constant.

Effluent Stream Calculation

• Q{in}C{in} = Q{out}C{out} + KVC
• (10 \frac{m^3}{s})(20) + (15 \frac{m^3}{s})(40) = (15 \frac{m^3}{s})(C)
• C = 26.67

Mass Accumulation Rate

• Q{in}C{in} = Q{out}C{out} + KVC
• (1 \frac{m^3}{s})(5 \frac{mg}{L}) + (0.25 \frac{m^3}{s})(10.01 \frac{mg}{L}) = 0.26 \frac{m^3}{s} (C)
• C = 3.35 \frac{mg}{L}

Chemical Reaction Rate/Half Life

• t_{1/2} = \frac{0.693}{k}
• For t_{1/2} = 3: k = \frac{0.693}{3} = 0.23
• For t_{1/2} = 12: k = \frac{0.693}{12} = 0.058

Plug Flow Reactor (PFR)

• \frac{C}{C_0} = e^{-kt}

Complete Mix Process

• t = \frac{V}{Q}
• \frac{C}{C_0} = \frac{1}{1+kt}
• 0.30 = \frac{C0e^{-kt}}{C0} \implies 0.30 = e^{-kt}
• 0.5 = e^{-\frac{kt}{Q}}
• t_{1/2}= \frac{0.693}{k}
• 0.5 = e^{\frac{-V}{Q}} \implies \frac{V}{Q} = \frac{0.693}{0.5} = 0.30

Wastewater Engineering

• Scum: Consists of grease, oil, plastic, leaves, rags, hair, and other floatable material.
• Raw sludge or primary sludge: The mass of settled solids.

Primary Treatment Removal Efficiency

Sedimentation

• Oldest and most widely used unit operation in wastewater treatment.
• Process used in both primary and secondary wastewater treatment where gravity pulls particles to the bottom of a tank (settling).

Primary Sedimentation

• Objective: To remove readily available settleable solids and floating materials, reducing the suspended solids content.
• Settling tanks receiving raw wastewater before biological/chemical treatment are called primary tanks or clarifiers.

Short-Circuiting

• Hydraulic condition in a tank where the travel time is less than the flow-through time.

Types of Sedimentation Tanks

1. Septic Tanks
2. Two-Story (Imhoff) Tanks
3. Plain Settling Tanks (Clarifiers)

• All three can be used for primary treatment; plain settling tanks are normally used for secondary or advanced wastewater treatment processes.

Septic Tanks

• Prefabricated tanks that combine settling, skimming, and anaerobic digestion.
• Provide long settling times (6 to 8 hours or more) but do not separate decomposing solids from the wastewater flow.
• Solids are discharged with the flow when the tank becomes full.
• Suitable for small facilities only.

Two-Story (Imhoff) Tank

• Similar to a septic tank in removing settleable solids and anaerobic digestion.
• Consists of a settling compartment, a lower compartment for digestion, and gas vents.

Plain Settling Tanks (Clarifiers)

• Optimizes the settling process.
• Sludge is removed for processing in downstream treatment units.
• Flow enters, slows, and distributes evenly, then leaves over the effluent weir.
• Detention time: 1 to 3 hours, with a 2-hour average (Spellman).

Rectangular Clarifier

• Solids are scraped to one end.
• Favored when space is a constraint; economical piping arrangements.
• Multiple units should be designed for independent operation.

Circular Tanks

• Theoretical flow pattern is radial.
• Wastewater is introduced in the center or around the periphery (center-feed is more common).
• Larger tanks have a central pier that supports equipment.
• Typical center feed tank has a vertical inlet pipe with ports that transmit flow to the feed well (flocculation center well).
• Energy dissipating inlets (EDI) distribute flow within the feedwell and increase flocculation.
• Flow-splitting device is placed between redundant clarifiers.

Accessories and Parts of Circular Clarifier

Primary Tank's Detention Time

• 1-2 hours

Factors Affecting Settling

1. Time: Must be long enough for particles to settle.
   • Long detention times cause:
     • Solids to become septic
     • Solids to float to the surface
     • High SS levels in primary effluent
     • Odors in primary effluent
2. Volume: Must not exceed tank capacity.
3. Types of solids and characteristics of wastewater.
4. Flow rate
5. Temperature

Design Criteria of Primary Sedimentation Tanks

Types of Settling Phenomena in Wastewater

1. Discrete: Particles settle without interaction, occurring under low solids concentration (e.g., sand removal).
2. Flocculant: Particles initially settle independently but flocculate, increasing settling velocity as they aggregate.

General Design Criteria

Accessories

• Scum Collector
• Sludge Collector
• Primary Sludge Pumps

Additional Design Criteria

Tank Dimensions

Coagulant(+)

• Used to remove charged particles, enabling them to attract and settle.

Calculations

• Circular Surcharge:
  Surcharge = \frac{15,071,000}{3,989} \approx 3,778
• Waste Charge: 5,000 = 15,0719 to
  \
• 190 mg/L BOD
• 225 mg/L BOD
• 210 mg/L SS
• 365 mg/L SS
• Average Flow
• Peak Flow

SOR

• SOR = \frac{77.98 m^3/day}{44.56 m^2} = 1.75 \frac{m}{day}
• BOD removal = \frac{2100}{0} = 89.5 \%.
• SS Removal = (X-X)(%) \implies \frac{(365-3d)}{8207.5 kg/day} \implies 8207.5 kg/day \implies 5:1
• A = \frac{E}{V} = 709.83 m^3

Continuous-Flow Stirred Tank Reactor (CFSTR)

• Also known as Completely-Mix Reactor.
• Reactants entering the reactor are dispersed immediately, and products flowing out are continuous.
• C_o
• Q_o
• V
• C_e

Arbitrary Flow Reactor (AFR)

• Any degree of partial mixing between plug-flow and complete-mix.

Packed Bed Reactor (PBR)

• Filled with inert packing media for biomass growth.
• Wastewater flows continuously in and out.
• Used for attached growth treatment systems.
• Influent C(2)
• Effluent.

Fluidized Bed Reactor

• Filled with packing material that expands and gets fluidized when wastewater moves upward.
• FBR or FBBR (Fluidized Bed Biofilm Reactor): Combination of attached growth and suspended growth.
• Biological slime is developed and maintained on a solid support medium.

Types of FBR:

1. Captor:
   • Biomass is grown inside polyester foam pads; pads can be periodically removed and biomass squeezed.
   • Oxygenation is achieved using a simple sparger.
   • Developed by Simon Hartley Captor.
2. Oxitron:
   • Sand particles are used as a support medium; sand is allowed to overflow, cleaned, and recycled.
   • Uses high-purity oxygen dissolved into the influent stream.
   • Developed by Dorr Oliver.

Modification of Activated Sludge Process (Secondary Treatment)