CVEN3502 - Screening, grit removal and sedimentation PDF

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CVEN3502 Water & Wastewater Treatment Screening, grit removal and sedimentation

Stuart Khan: [email protected]

WWTP Design flows and loadings • Design of WWTP based upon •

Volumetric flow rate (Q)

•

Mass loading rate (m)

• Average Dry Weather Flow (ADWF) • Peak Dry Weather Flow (PDWF) • Peak Wet Weather Flow (PWWF)

Equalising Storage is approx. 30% of the total daily DWF

Volumetric flow rates (Q) • Volumetric flow rate = volume/time: Not to be confused with velocity (V).

• Volumetric flow rate = Area x flow velocity

Not to be confused with volume (V).

Screening and grit removal at WWTP

Screening at a WTP

The dirty dozen • Condoms • Cotton buds • Sanitary towels • Razor blades • Tampons • Nappies • Bandages/plasters • Syringes/needles • Ladies tights • Dental floss • Incontinence pads • Colostomy bags

Other solid materials •

Silt and grit, •

•

typically particles of 10 – 1000 µm diameter with density 2000-2600 kg/m3

Flocs and biomass, •

typically 100 – 1000 µm diameter with density 1005-1010 kg/m3

Screening • Removal of solids by passing the water through apertures, which are smaller than the solid particles • Used in both water and wastewater treatment • Bar racks • Screens

Typical screening applications •

Removal of solids which might block downstream processes

•

Removal of abrasive solids which might damage pumps, etc

•

Protection of potable water works intakes

•

Removal of algae from potable water

•

“Polishing” of sewage works final effluent

•

Removal of floating material from industrial wastewaters

•

Between unit operations in wastewater treatment.

Screen classification Type

Aperture (mm)

•

Coarse

>50

•

Medium

15 – 50

•

Fine

3 – 15

•

Milli

0.25 – 3

•

Micro

0.025 – 0.25

• Maximum size – To reduce the volume of screenings for disposal – To minimise contamination with faeces

• Minimum size – To ensure removal of material which would disrupt downstream operations – To ensure high quality biosolids i.e. reduce non-degradable materials

Bar racks or coarse screens •

Usually installed at an incline to facilitate debris removal

•

Mechanical cleaning uses automatic rakes that enter the spaces and lift the screenings off, usually onto a conveyor to disposal

Travelling screens

Travelling screens • devices consist of a series of teeth, made of metal or nylon, travelling along a continuous belt • often selected for coarse and medium duties as they are manufactured as standard units

Design • Critical velocity •

At high flows, maximum velocity through bars to prevent screens being pushed up – Vmax < 1.0 m/s

• At low flows, minimum velocity in the approach channel to prevent deposition and accumulation of solids in channel – Vmin > 0.4 m/s

Headloss through bar screens • A function of the flow velocity and the opening in the screens

1  VBS2  V A2   hL   C  2g  hL = head loss through bar screen (m) C = Discharge coefficient, 0.6 for clogged screen, 0.7 for clean screen. vBS = velocity of flow through bar screen (m/s) vA = velocity of approach in upstream channel (m/s) g = acceleration due to gravity (9.81 m/s2)

Exercise

Bar screen design

Rag removal and disposal

0.005 – 0.05 m3/ML of wastewater treated

Comminution • Mechanical maceration of solids material to reduce the size of suspended particles • domestic sewage containing rags, paper and other organic debris • wastewater from the meat processing industry containing meat residues

Comminutor (or grinding pumps) • A typical comminutor will produce particles small enough to pass through a 6mm screen • The units are prone to blockage and mechanical damage and require a high level of maintenance

Grit chambers • Designed to remove sand, gravel, egg shells, and other mineral matter •

Also removes coffee grounds, tea leaves, etc.

•

Particles diameters: ~0.2 mm or greater

• Main types of chambers: • Constant velocity channels • Aerated grit chamber

Constant velocity channels • Channels are either parabolic or trapezoidal in cross section and designed to give a constant flow velocity of about 0.3 m/s under variable flow conditions

Particle settling in constant velocity channels





0.5

[For turbulent (not laminar) flow conditions]

VS = settling velocity (m/s) g = acceleration due to gravity (m/s2) Sp = specific gravity of a particle (dimensionless) = Mass density of particle (kg/m3) / Mass density of liquid (kg/m3)

d = diameter of particle (m) See derivation in Mines (2014), pages 342-343.

Design of constant velocity channels • Design of channel length (L) • based on Vh and Vs with calculated H • Minimum L = [Vh/Vs].H

H

Vs

• e.g. Vh = 0.3 m/s; Vs = 25 mm/s

Vh

L

Min L = 12 H Practical values (F = 1.25, Sydney water) L = 15 H Design removal for ADWF, PDWF, PWWF and 3ADWF (m3/s)

Aerated grit chamber •

Air flow encourages a helical flow pattern (roll) in a tank

•

Allows a smaller footprint than a constant velocity channel

•

Also aerates the sewage

•

Very efficient at grit removal

•

Produces a very clean grit

•

But requires air supply, which adds to the cost

Sedimention

Sedimentation

• Sedimentation is the settlement of particles which have a higher density than the liquid in which they are suspended under the influence of gravity • Typical examples • horizontal settling tanks • vertical settling tanks • radial settling tanks

Horizontal settling tanks •

Simple rectangular tanks typically about 2m deep with a length/width ratio of about 2 – 5

•

Water enters one end, leaves via overflow weir at the other

•

Solids settle to the bottom

•

Sludge is usually mechanically scraped to one end by a chain and flight scraper

Horizontal settling tanks

Drinking water settling basin

Vertical and radial settling tanks • Commonly used for primary settlement of screened sewage and because they take up less space than horizontal tanks • Raw water enters via a diffuser drum in the centre which directs the flow downwards to the bottom of the tank • As the liquid flows upwards in the case of a vertical flow tank or outwards in the case of radial flow, the liquid velocity reduces allowing solids to settle.

Radial settling tanks

Radial settling tanks

Vertical settling tanks

Manual skimmer operation

Stokes’ Law For laminar flow conditions:



2







2



VS = settling velocity (m/s) g = acceleration due to gravity (m/s2) p = mass density of particle (kg/m3) w = mass density of water (kg/m3)  = absolute or dynamic viscosity of water (kg/(m.s))  = Kinematic viscosity of water (m2/s) Sp = specific gravity of a particle (ρp / ρw , dimensionless) d = diameter of particle (m)

Settling behaviour

• Particle type • Discrete • Flocculent

• Particle concentration • Dilute: single particle settling, no interference • Moderate: some particle hindrance • High: particle hindrance results in zone settling

Classes of settling behaviour • Class I – ideal • unhindered settling, discrete particles

• Class II – common • dilute suspension, flocculent particles

• Class III – high concentration • discrete or flocculent particles

• Class IV – consolidation • within the settled sludge layer

Ideal settling basin

Particle settling time and distance travelled Time required to settle a specific particle (min, hours):



 

The same particle will travel distance L in the same time (min, hours):



Critical settling velocity (to achieve settling before outlet zone): 󰇛󰇜 

 

     

See Mines (2014), pages 280-281, 345-346.

Flow rate through the basin (m3/s)

Q= Vh x H x W Vh = flow velocity (m/s) W = tank width (m)

See Mines (2014), pages 280-281, 345-346.

Critical particle settling velocity in terms of flow rate and tank area 

      

󰇛󰇜 



 

 

 

  

The “Overflow rate” (Vo) or “surface loading rate” (m3/(day.m2))

Note: Settling velocity is assumed to be independent of tank depth (applies only for ‘ideal’ settling) See Mines (2014), pages 280-281, 345-346.

Overflow rate or “surface loading rate”

Q

Typical design dimensions and overflow rates

(Tchobanoglous & Schroeder, 1985)

Detention time in basin (min, hours)

V V = volume (m3) Also known as the ‘hydraulic retention time’ (HRT).

See Mines (2014), pages 280-281, 345-346.

Weir loading rate (m3/(d.m)):

q = Q/Weir length

See Mines (2014), pages 280-281, 345-346.

Solids loading rate (SLR)

󰇛.   󰇜 

󰇛/󰇜 Tanksurfacearea Tanksurfacearea󰇛 󰇜

Exercise

Design of rectangular settling basin

Class III sedimentation •

Hindered settling occurs when particles are close enough together for velocity fields to overlap

•

With increasing concentration, the degree of hindrance increases, resulting in deceased settling velocity

•

At high concentrations particle suspensions settle as blankets – zone settling

• Secondary sedimentation tanks are used to •

settle effluents from biological treatment processes such as activated sludge and trickling filters

• Sludge blanket clarifiers •

allow settlement of floc particles following coagulation-flocculation during water treatment

Wastewater treatment system PRIMARY TREATMENT

SECONDARY TREATMENT

primary sedimentation

TERTIARY TREATMENT secondary sedimentation

screened sewage settled sewage

air recirculated biomass raw /primary sludge

secondary sludge

Disinfection

Further reading

Chapter 6: Design of Water Treatment Systems Chapter 7: Design of Wastewater Treatment Systems...