UNIT II : PRIMARY TREATMENT

Process Flow Sheet-I

Process Flow Sheet-II

Process Flow Sheet-III

Unit operation Efficiencies

WWTP: Basic Design Considerations

1. Influent characteristics and strength
2. Effluent Quality
3. Design Loading
4. Design parameters

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Design Considerations

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Influent Strength and characteristics

(Taught in Unit-I)

Design Considerations: Effluent Quality

• Based upon ultimate disposal/reuse
• Basis for selection of treatment process, unit operations and their design
• Efficiency of treatment
• Disinfection vs no-disinfection

Design Considerations: Design Loading

1. Hydraulic Loading
1. In terms of volumes
2. Mass Loading
• In terms of mass of BOD or SS

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• Average, Maximum, Minimum loading???
• Daily, Monthly, yearly??
• Dry weather runoff (no stormwater runoff) or wet weather runoff (stormwater in rainy season)??

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Design loading (contd..)

• Hydraulic loading: pump, pipes and hydraulically limited equipment design
• Mass loading: aeration, sludge digestion equipment

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• Peak flow factor= Max flow/avg flow = 1.5 – 3.0
• Minimum flow factor= Min flow/avg flow = 0.1 – 0.3

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• Check for max., min and average conditions. Units may be shut off during minimum flow. Scouring to be avoided during min. flow.
• Max design: capacity underutilization, Min design: process overloading

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• Design multiple units

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Design Considerations: Design Parameters

• Typically published criteria/standards (govn. Bodies-CPHEEO, India) for unit process design.
• Obtained from research and laboratory scale model studies and field operational experience

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• Hydraulic criteria (flow Rates and detention time)
• Sedimentation tank: weir loading rate, overflow rate, hydraulic detention time
• Mass loading criteria:
• Mass of BOD per volume of aeration tank
• Geometric ratio (L:B:H)
• Dead pockets/ Hydraulic short-circuiting due to incorrect L:B ratio
• Inefficient hydraulic performance due to improper L:H ratio.

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Typical Design Loading criteria

Principles of Reactor design

• Reactors: The units or vessels that hold wastewater for treatment by chemical or biological process.

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• Basin/tank: units used for separation of solids from liquids by settling or floatation.

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• Reactors or tanks are generally rectangular or circular.

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Chemical/Biological Reactions

• Homogeneous reaction: all reactants have same phase (solid, liquid or gas)
• Heterogeneous reaction: reactants have more than one phase (e.g., reactions at interphases)
• Reaction orders: 0, 1^st and 2^nd
• Biological processes: Heterogeneous and 1^st order
• Biological reaction rate doubles for 10°C temperature rise.

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Reactor sizing

• L:B and L:H for reactors or diameter of reactors
• Influent and effluent conduits
• Inlet and outlet chambers
• Distribution and collection boxes
• Hopper bottom sludge pockets
• Scum removal mechanism
• Surface/diffuse aeration system
• Mixers/mechanical equipments
• Pumps, etc.

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Actual/Net reactor size also includes provisions for appurtenances- inlet/outlet, channels, sludge collection, etc.

Types of reactors

• Batch reactor
• Plug-flow reactor (PFR)
• Continuous-flow stirred tank reactor (CSTR)
• Arbitrary flow reactor (AFR)
• Fluidized bed reactor (FBR)
• Packed bed reactor (PBR)
• Sequencing batch reactor (SBR)

Reactors: Batch reactor

• Flow neither enters nor leaves the reactor during detention time, e.g. BOD bottle… L:B ~1
• Assumptions: 1^st order kinetics (BOD) and uniform reactant concentration throughout the reactor. Homogeneous reactions (reactants have same phase)
• Application: bench-scale lab studies and sludge digestion.

Reactors: Plug-flow reactor

• FIFO (First-in-first-out) principle
• Particles exit in the same order in which they entered.
• No Longitudinal mixing. Each “plug” moves independently.
• Reactions along the length, reactant concentration decreases and product’s increases.
• Long and narrow reactors of large length and small width
• May be made serpentine.
• Homogeneous reactions
• Grit channel or sedimentation tanks

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Effluent, Q_out, C_ao

Influent, Q_in, C_ai

Q, C_a

Plug Flow reactor

• Continuous flow.
• No variation with time (at a given space), but with space/position (at a given time).
• With partial/complete recycling can mimic a CSTR
• More efficient than a CSTR of same dimensions
• Conservative dye (no reaction): [Influent]=[effluent]
• 0 order reaction: removal rate constant, [reactant] decreases uniformly along length.
• 1^st order reaction: removal rate decreases along the reactor length, [reactant] decreases exponentially

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Theoretically Ideal as longitudinal mixing is always there.

Plug Flow Reactor

Reaction kinetics and concentration graphs for

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• Conservative tracer
• 0 order reaction
• 1^st order reaction

CSTR (continuous-stirred tank reactor)

• The reactant is instantaneously dispersed in the reactor and is completely mixed.
• Contents are continuously and uniformly redistributed.
• Continuous flow and L:B~1
• Homogeneous reaction
• Concentration within reactor is same at all points
• Effluent concentration is same as that inside reactor
• No spatial (in space, given time) or temporal (in time, given space/position) variation (At steady state).

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CSTR

• Conservative tracer: [influent]=[effluent]
• Zero order reaction: same as Plug-flow, for same detention time and conditions
• 1^st order reaction: concentration same everywhere in reactor and is less than PFR.
• Since, removal rate is function of concentration, CSTR is less efficient than PFR for 1^st and higher orders.

CSTR

Reaction kinetics and concentration graphs for

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• Conservative tracer
• 0 order reaction
• 1^st order reaction

CSTR vs. PFR

• CSTR and PFR have same efficiency for conservative tracer and 0 order reaction.

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• PFR is more efficient than a CSTR for 1^st and higher reaction orders.

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• PFR will act as a CSTR if sufficient recycling of the reactants is done.

Arbitrary Flow Reactor

• Plug-flow reactor designed with flow dispersion (intermixing).
• Non-ideal reactor, intermediate between Plug-flow and CSTR
• Not used in Wastewater treatment, since Biological processes are not designed with intermediate mixing.

Fluidized Bed Reactor

• Filled packing material expands and gets fluidized when influent wastewater moves upwards.
• Air is also introduced along with influent flow at inlet.

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• Used in aerobic or anaerobic biological oxidation.
• Also for sludge treatment and removal of dissolved gases.

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Packed Bed Reactor

• Filled inert packing material for biomass growth is kept packed (or fixed).
• Flow may be upward or downward
• Packing material: slag, rock or ceramic
• Can be made anaerobic, by completely filling closed reactor with media (UASB)

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Sequencing Batch Reactor

• SBR is an activated sludge process and works on fill-and-draw principle.
• Reactions for aeration, waste conversion and effluent clarification occur in same reactor, in time-sequence.
• Steps:
• Anoxic fill: reactor is filled with sewage up to desired volume
• React: aeration & mixing for designated time period
• Settle: clarification or sedimentation of biomass
• Decant: clarified effluent is withdrawn to separate the sludge
• Idle:

Process Flow Sheet

• Sequence of processes
• (general) Order of appearance in STP
• Already shown at the beginning of the unit

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• Summary of process function
• What does the process do?
• E.g., skimming tank (oil and grease removal), BOD/Solids/Bacteria removal (secondary trt.)
• See the Table (slide#05)

Primary Treatment

• Screens
• Oil and Grease removal
• Grit Chamber
• Primary Sedimentation Tank (PST)

Screening

• Removal of floating matter (pieces of clothes, woods, paper, etc.) by bars spaced (generally) at an angle
• Protects pumps and other following units from debris
• Coarse (or racks, spacing>50 mm) and Medium screens (6-40 mm bar spacing)
• Fine screens not generally used with sewage
• Bar screens: bars arranged on a rectangular steel/RCC frame at designed spacing and inclined at 30-60°
• Collected debris is cleaned by manual or mechanical rakes.
• Screen cleaning frequency is governed by head loss through screens.
• Screenings (material collected) is disposed by incineration or composting.

Coarse screen

Mechanical Fine Bar screen

Skimming Tanks

Skimming Tank

• Oil/grease removal unit, placed before sedimentation
• Oil/grease forms scum on sedimentation tank and prevents biological activity in secondary treatment units
• Compressed air blown from bottom causes the grease to coagulate and congeal (solidify) and rises to surface from where it is removed.
• Skimming (removed oil/grease) is disposed off by burning
• Hot weather may prevent coagulation of oil,
• Provide oil/grease trap near source, before sewers
• Oil/grease floats on top, outlet is near the middle or bottom of chamber.
• Periodic cleaning of trap.

Grit Chamber

• Grit: Inorganics solids (e.g., pebbles, sand, egg shells, glass, metal fragments, ) and largers/heavier organics (bone chips, seeds, coffee/tea grounds, etc.)
• Size> 0.2 mm, Specific gravity~ 2.65
• Inorganic and/or putresciable (undergoes putrefaction)
• Need:
• Grit constituents are abrasive; causes accelerated wear on pumps and sludge handling equipments.
• Absorbs grease and may solidify on pumps, sumps, clarifiers
• Non-biodegradable (inorganics) occupy and waste space in sludge digestors.
• Type I (gravity) sedimentation,
• Enlarged channel area for reduced velocity & grit settling, grit removed by mechanical scrapper.
• Grit disposed by landfilling and incineration

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Grit Chamber

• Quantity:
• 0.004-0.037 m³/1000 m³ (separate sewage system)
• 0.004-0.18 m³/1000 m³ (combined sewage system)
• Constant horizontal velocity (flow-through) of 0.3 m/s
• 25% velocity increase: grit washout
• 25% velocity decrease: retention of non-target organics
• Velocity controlled by velocity control section, proportioning flow/sutro weir at effluent end or parabolic shaped channel

Grit Chamber

• Horizontal flow type:
• Horizontal flow with constant velocity,

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Grit Chamber: Parshall Flume

• Parshall Flumes added at the end of Parabolic (or V-shaped) grit chamber.
• Has negligible head loss, provides smooth hydraulic flow and prevents solid deposition.

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Aerated Grit Chamber

• Consists of spiral flow aeration tank, spiral velocity is controlled by dimensions/shape of chamber and air quantity
• Also aerates anaerobic sewage
• Detention period: 3 min (typical), 2-5 min (range)

2 chambered aerated grit chamber

single chambered aerated grit chamber

Vortex type grit chamber

• Cylindrical tank, flow enters tangentially creating vortex flow pattern,
• centrifugal (flotation of organics, lighter particles) and gravitational (grit settling) forces causes grit separation
• Detention time = 20-30 sec
• Central turbine maintains flow velocity

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Grit Chamber: Detritus Tank

• Rectangular chambers, smaller flow velocity (0.09 m/s) and longer detention (3-4 min), organics and fine suspended solids also removed, rising air bubbles separates organics and can be returned to flow

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Primary Sedimentation

• Follows the screens, grit chamber and skimming units.
• Removes suspended organic particles
• Typically Type II settling occurs, without the application of coagulants
• Sludge particles are sticky in nature and flocculate naturally
• Flocculated particles settle down by gravity.
• Sometimes, may also be clubbed with ASP process
• Part of return sludge may be fed back to PST

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• May also be combined with skimming, which can be done at surface of sewage using scum removal mechanism

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PST

• Rectangular or circular made of RCC
• Long narrow rectangular tanks with horizontal flow or circular tanks with radial or spiral flow
• Long detention period decreases flow velocity causing sedimentation of suspended organics, which are removed from sludge hopper situated at base of PST.
• Clean effluent is collected using effluent weirs
• Efficiency: 60-65% suspended solids & 30-35% BOD removal
• Removal efficiency is function of overflow rate

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PST: Design criteria

• Detention Time: 1.5-2.5 h; typical: 2 h
• Overflow rate (m³/m².d): average: 32-48, peak flow: 80-120
• Weir loading rate (m³/m².d): 125-500
• Dimensions, m:
• Rectangular: Length: 15-90, Depth: 3-5, width: 3-24
• Circular: depth: 3-5, diameter: 3.6-60

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Rectangular PST

Circular PST

Sedimentation

• Particles with S.G.>1 settle down when flow velocity reduces/turbulence is retarded.
• Gravity or chemical-aided (coagulation-flocculation).
• Settling tank/sedimentation tank/sedimentation basin/clarifier: Rectangular or circular

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• Gravity/Plain/Type-1 sedimentation
• Specific Gravity > 1.
• Sedimentation by gravity alone.

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• Discrete particles: size/shape/S.G. constant with time
• Flocculent particles: properties change due to aggregation..

Sedimentation theory

• Flow velocity: slower velocity, better settling.
• Flow/water Viscosity: low viscosity (at high temperature), better settling
• Size, shape & specific gravity:
• Higher specific gravity, better settling
• Small sized particles settle slowly.

Stoke’s Law

Vs= (g/18). (G-1). (d²/ ) d< 0.1mm

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Vs: settling velocity (m/s) for spherical particle (dia, d, in m)

G: specific gravity of particle

v: kinematic viscosity (m²/sec)

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Settling velocity

Vs = 1.8 {gd(G-1)}^1/2 d> 1 mm

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Vs = 418 (G-1)d (3T+70)/100

0.1 mm < d < 1 mm

Circular Clarifier

Schematic: Circular Clarifier

Schematics: Rectangular Clarifier

Plain Sedimentation (Type-I)

• Flow velocity control by extended detention period.
• Use of a long tank, (large area)…

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• Sludge continuously removed by mechanical scrapper.

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• Mainly horizontal flow type.

Sedimentation (Type-I)

• Rectangular Tank
• Equal velocity at all points on each vertical line
• Circular Tank
• Uniform radial flow with decreasing velocity towards periphery.
• Particle removal is independent of tank depth.

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Design Concept

• Overflow Rate/Surface Overflow rate/Surface Loading:
• Design velocity: theoretical time for which the particle stays in the tank.
• Settling Velocity (Vs) vs. Loading Rate (Vo) determines the particle removal.
• Units: m³/d/m²
• Discharge per unit surface area.
• Q/A_s
• Typical Rates: 12-18 m³/d/m²

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Settling Column Analysis (Discrete Particles)

• To determine the theoretical settling/removal efficiency of a given suspension.
• Column of 2 m height is used.

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• Samples withdrawn regularly.

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• Theoretical % of particles removed
• (100-X) + ʃ(V_s/V₀).100.dX
• X: fraction with V_s < V₀

PST: Issues

• Short-circuiting:
• Portion of flow escapes the tank before designed detention period.
• Causes: local disturbances at inlet and outlet.
• Prevented by using long-narrow channels, properly designed inlet and outlets.

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• Coagulant-aided sedimentation
• Increases cost, obsolete/unnecessary since secondary processes are more efficient, skilled operation needed.
• Not used in PSTs for STPs.

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PST Design

Q: A municipal wastewater treatment plant processes and average flow of 5000 m³/d, with peak flow up to 12500 m³/d. Design a primary clarifier assuming appropriate detention period and overflow rate.

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Solution:

Assuming overflow rate= 35 m³/m²/d.

Surface area= 5000/35 m² =143 m²

Circular tank: diameter = (4xarea/π)^1/2 = 13.5 m

Assuming side wall depth of 3m,

tank volume = 143x4 m³

= 429 m³

Detention time = V/Q = 429/5000 d ~ 2 hrs.

PST design: Numerical II

Q: Design a rectangular sedimentation tank for sewage treatment of a city, with a maximum daily water demand of 12 million lt. State the assumptions made.

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Solution:

Assuming 80% of water supplied is converted to sewage.

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Sewage volume = 12x0.8 = 9.6 million lt/day

Assuming overflow rate of 40,000 lt/m²/d,

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Surface area = 9600000lt/40000 lt m² = 240 m²

PST Numerical II contd.

Assuming 30 m length, width = 8 m

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Assuming depth = 3m, tank volume = 240x3 = 720 m³

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Detention time = volume/discharge = 720/9600 d = 1.8 hrs

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Provide a tank of 30 m x 8m x 3 m with a detention period of 1.8 hrs. (This does not include freeboard and space for sludge hopper at the base of PST)

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Settling column

• Particles with V_s > V₀ are removed 100%.
• Particles with V_s < V₀ are removed in ratio V_s/V_0.

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• Steps:
• Find particle size by sieve analysis.
• Find settling velocity.
• Do column analysis and prepare settling curve.
• Use settling equation to find theoretical removal efficiency.