7/23/2019 Lecture+15+-Primary+and+Preliminary+Treatment.markup

1/17

CEE 3413 Environmental EngineeringLecture 15 Wastewater TreatmentPRELIMINARY AND PRIMARY TREATMENT

INSTRUCTOR:

Dr. Tania DattaOffice: Prescott Hall, Room No. 333

Email: tdatta@tntech.edu

7/23/2019 Lecture+15+-Primary+and+Preliminary+Treatment.markup

2/17

Preliminary

Treatment

Purpose of PRELIMINARY TREATMENT of wastewater is to remove largeand abrasive wastewater constituents in the raw influent that can cause

operational problems in downstream processes or increase maintenancerequirement in downstream equipments

Constituents include:

Large solids

Rags

Floating debris

Abrasive inert material

Oil/Grease

Wastewater Treatment - Introduction

7/23/2019 Lecture+15+-Primary+and+Preliminary+Treatment.markup

3/17

Unit Processes Include:

SCREENING GRIT REMOVAL

FLOW EQUALIZATION

HEADWORKS

HeadworksSecondary

TreatmentDisinfectionInfluent

Effluentto

surface

water

dischargeor

Reclaimed

waterto

reuse

ReturnActivatedSludge(RAS)

Waste

Activated

Sludge(WAS)

PRELIMINARY

TREATMENT

Where

are

they

located

in

a

WWTP?

Preliminary

Treatment

Wastewater Treatment - Introduction

7/23/2019 Lecture+15+-Primary+and+Preliminary+Treatment.markup

4/17

Screens

Wastewater Treatment - Introduction

Removes large solid objects that damage pumps or

Or can remove finer objects to protect downstreamsensitive process equipments

7/23/2019 Lecture+15+-Primary+and+Preliminary+Treatment.markup

5/17

Screens

Screen Classification (based on screen opening sizes)

Coarse or Bar Screens

6 mm to 150 mmFine Screens

0.5 m to 6 mm

Micro Screens

7/23/2019 Lecture+15+-Primary+and+Preliminary+Treatment.markup

6/17

Location: Coarse/Bar screens always placed before grit removal chamber

Finer screens always placed after grit removal chambers

Enclosed or Not Enclosed:Depends on odor control, climate and equipment design

Screen opening size already discussed

Approach Velocity:Design engineer must ensure that wastewater approach velocity is not too

slow and not too fast. Approach velocity of at least 1.25 ft/s is recommended to avoid solids

settling

But during peak flow events, approach velocity should not increase3.0 ft/s

Screens

Design

Considerations

Wastewater Treatment - Introduction

7/23/2019 Lecture+15+-Primary+and+Preliminary+Treatment.markup

7/17

Velocity of flow through the openings

Good designs allow velocities of 0.6 to 1.2 m/s for mechanically

bar screens

Should obtain this data from screen vendor/manufacturer

Headloss through screens

HL

Velocity

thru

opening (V)

Approach

Velocity (v)

g

vV

CHL

2

1 22

C = Empirical coefficient to account for

turbulence and eddy losses

0.7 Clean screen

0.6 Clogged screens

Headloss through bar/coarse screens:

Wastewater Treatment - Introduction

Screens

Design

Considerations

7/23/2019 Lecture+15+-Primary+and+Preliminary+Treatment.markup

8/17

Allowable head loss is

6 for manually cleaned bar racks

6-24 for mechanically cleaned bar racks

Racks should be cleaned when headloss is

greater than the allowable values.

Wastewater Treatment - Introduction

Screens

Design

Considerations

7/23/2019 Lecture+15+-Primary+and+Preliminary+Treatment.markup

9/17

Screenings Handling, Processing and Disposal

Screenings are the materials that are retained on the screens. Bar/Coarse screenings Removed from the screens, grinded,

compacted and hauled to landfill for disposal.

Fine screenings Either processed with bar/coarse screening

or sometimes treated as primary sludge, depending on screensize.

Operational Issues with Wastewater Screening

Odorous screenings requires proper storage and disposal ofscreenings

Mechanical failure always plan for a redundant system

Screening

Wastewater Treatment - Introduction

7/23/2019 Lecture+15+-Primary+and+Preliminary+Treatment.markup

10/17

Purpose: to remove inorganic suspended materialreferred to as grit to prevent abrasion of pumps and to

reduce deposits in pipe lines and open channels.

Grit includes sand, gravel, eggshells, bone chips, coffeegrounds, etc.

Grit Removal follows the concept of Type I settling

Grit

Removal

Wastewater Treatment - Introduction

7/23/2019 Lecture+15+-Primary+and+Preliminary+Treatment.markup

11/17

Grit

Removal

Wastewater Treatment - Introduction

Most common designs for Grit removal:

Horizontal Flow Grit Chambers (square or rectangular

configuration)

Aerated Grit Chambers

Vortex Grit Chambers

7/23/2019 Lecture+15+-Primary+and+Preliminary+Treatment.markup

12/17

TYPICAL DESIGNINFORMATION:

Grit

Removal

Aerated

Grit

Chambers

Wastewater Treatment - Introduction

7/23/2019 Lecture+15+-Primary+and+Preliminary+Treatment.markup

13/17

Uses a vortex flow pattern to allow heavier particles to settle down

and lighter particles to remain floated

Mechanical Vortex Relies on a mechanically enhanced vortex

flow, using a mechanical turbine

Grit

Removal

Vortex

Grit

Chambers

Wastewater Treatment - Introduction

7/23/2019 Lecture+15+-Primary+and+Preliminary+Treatment.markup

14/17

Purpose: Removes readilysettable solids and floating

materials (scum) and thus

reduces TSS, particulate BOD

and bound phosphorus

Removal Rate: 50-70% of totalsuspended solids and 25-40%

of BOD (in particulate form)

Based on: Settling Theory,Ideal Settling Basin Theory

Primary

Treatment

Wastewater Treatment - Introduction

7/23/2019 Lecture+15+-Primary+and+Preliminary+Treatment.markup

15/17

Primary

Treatment

Wastewater Treatment - Introduction

7/23/2019 Lecture+15+-Primary+and+Preliminary+Treatment.markup

16/17

Typical Design Information (Table 5-20 & 5-21 Metcalf and Eddy)Primary

Treatment

Wastewater Treatment - Introduction

7/23/2019 Lecture+15+-Primary+and+Preliminary+Treatment.markup

17/17

Additional

Reading

Assignment

Wastewater Treatment - Introduction

Textbook Chapter 8: Wastewater Treatment

Section 8-1: Introduction

Section 8-2: Characteristics of Wastewater

Section 8-4: Municipal Wastewater TreatmentSystems

Section 8-5: Unit Operation of Pre-treatment

Section 8-6: Primary Treatment