Strategies for Wastewater Treatment: Physico-Chemical Approach

Strategies for Wastewater

Treatment: Physico-Chemical

ApproachConventional Approach and Recent Trends

Purnendu Bose

Department of Civil Engineering

IIT Kanpur

Strategies for Wastewater Treatment: Physico-Chemical Approach

Introduction

What does Wastewater Contain ??

> 98% water (by weight) approx. 1 – 2 % Contaminants

Wastewater Treatment involves removing the contaminants from water

Cost of removal of contaminants: from Rs. 10/kL to Rs. 400/ kL

Secondary Treatment of DomesticSewage before disposal into rivers

Tertiary Treatment of Industrial Effluent for Recycling

Origin of Wastewater: Domestic/Commercial or Industrial


Introduction

Types of Contaminants in Wastewater

Inorganic: 1) Particulate; 2) Colloidal; 3) Dissolved

Organic: 1) Large Floating Material; 2) Particulate;

3) Colloidal ; 4) Dissolved;

Biodegradable Non-Biodegradable

Biological Treatment Physico-Chemical Treatment


Introduction

Objective of Wastewater Treatment:

Removal of Organic and Inorganic Contaminants

How is it Done?

UO-1 UO-2 UO-3 UO-4 UO-nRaw

Wastewater

Treated

Wastewater

Chemical Addition

Mixing

+

Bacterial Action

Heating / Pressure

Catalyst

Aeration

By Designing a Wastewater Treatment Process Train

UO – Unit Operation


Introduction

‘Unit Processes’ in Wastewater Treatment:

Say, a wastewater treatment train consists of ‘n’ unit operations, UO(1) – UO(n)

The first ‘i’ unit operations, i.e., UO(1) – UO(i) constitute an ‘unit process’ called,

‘Preliminary Treatment’

The unit operations from ‘i + 1’ to j, i.e., UO(i+1) – UO(j) constitute an ‘unit process’ called

‘Primary Treatment’.

The unit operations from ‘j + 1’ to ‘k’, i.e., UO(j+1) - UO(k) constitute an ‘unit process’

called Secondary Treatment’.

The unit operations from ‘k + 1’ to ‘n’, i.e., UO(k+1) – UO(n) constitute an ‘unit process’

called ‘Tertiary Treatment’.

Each ‘Unit Process’ performs a particular function in wastewater treatment


Introduction

Functions of ‘Unit Processes’ in Wastewater Treatment:

Preliminary Treatment: Physical Processes

Unit Operations in Preliminary Treatment removes, 1) large organic floating material and

2) inorganic particulate and matter from wastewater

Primary Treatment: Physical/Chemical Processes

Unit Operations in Primary Treatment removes, 1) organic particulate matter and

inorganic colloidal matter from wastewater and 2) also help (if required) in transforming

non-biodegradable dissolved organic matter in wastewater to a more biodegradable form

Secondary Treatment: Biological/Physical Processes

Unit operations in Secondary Treatment remove colloidal and dissolved organic matter

from wastewater

Tertiary Treatment: Biological/Chemical Physical Processes

Unit operations in Tertiary Treatment remove, 1) residual dissolved organic matter from

wastewater, 2) residual organic/inorganic particulate matter from wastewater, 3) dissolved

inorganic matter from wastewater

Treated wastewater may be disposed, reused or recycled

Domestic sewage after ‘Secondary Treatment’ is generally suitable for disposal into natural surface

water bodies

Industrial effluent may require some ‘Tertiary Treatment’ before being suitable for disposal into

natural water bodies.

Both domestic sewage and industrial effluent can generally be reused for various beneficial purposes

after adequate ‘Tertiary Treatment’.

Industrial effluent can be recycled, generally after adequate ‘Tertiary Treatment’

Disposal, Reuse and Recycling

Treated wastewater may be discharged to natural water bodies provided the relevant discharge

standards are attained through wastewater treatment. This is known as wastewater disposal.

Alternatively, the treated wastewater may be used for certain other beneficial purpose, provided

adequate treatment is given to the wastewater to meet the water quality standards for that

beneficial use. This procedure of using water for multiple beneficial uses is known as reuse.

Alternatively, the treated wastewater may be used for the same beneficial purpose as before,

provided adequate treatment is given to the wastewater such that it meets the standards for the

original beneficial use. This procedure for using water for the same beneficial use is known as

recycling.


Introduction

Grit ChamberProportional Weir

Grit Collection

Sewer

Rectangular Open Channel

Bar Rack / Screens

Front View

Top View


Conventional Unit Operations

Name of UO: Bar Rack / Screen

Type: Physical Process

Unit Process: Preliminary Treatment

Purpose: Removal of Large Floating Material (up to 1 mm size) from Wastewater

Large floating materials are caught inthe screen and removed

Three screens with progressivelysmaller openings are provided in seriesto remove all floating material > 1 mmsize

Sewer

Rectangular Open Channel

Bar Rack / Screens

Front View

Top View



Name of UO: Grit Chamber



Purpose: Removal of Inorganic Particles (up to 1 mm size) from Wastewater

Placed immediately after bar rack /screen

The channel is suddenly widened, sothat horizontal flow velocity is reducedin grit chamber. Particles settle in thechamber and are removed.

Proportional Weir

Grit ChamberProportional Weir

Grit CollectionFront View

Top View



Name of UO: Equilization Tank



Purpose: Flow Equilization

Equalization Tank (Flow Equalization)

Variable Flow(from Grit Chamber)

EqualizationTank Constant Flow

In- Line Equalization

EqualizationTank

Variable Flow Constant Flow

Inflow Outflow

Off - Line Equalization

Placed immediately after GritChamber

Inflow into Equilization tank isvariable, but the outflow isconstant.

Equilization tank is required forefficient operation of thedownstream unit operations.

Two types of Equilization tanksare possible, off-line and on-lineequilization.



SC-1 SC-2 SC-3 GC EQTo Primary Treatment

From Sewer

The wastewater after preliminary treatment has,

1) No floating matter2) No inorganic particles of size > 0.2 mm3) Equalized Flow

The First Unit Process: Preliminary Treatment

Rapid Mix Tank: Vertical Shaft Impeller

Plan

Elevation

Inflow

outflow

Coagulant



Name of UO: Rapid Mix Tank


Unit Process: Primary Treatment

Purpose: Dispersal of Coagulant

Coagulant addition is mostlyrequired for industrialwastewater treatment.

This unit is generally notrequired for domesticwastewater treatment.

The objective of the rapid mixtank is to quickly disperse theadded coagulant into the bulkwastewater

Flocculation Tank:Horizontal Shaft Paddle

Plan

Elevation



Name of UO: Flocculation Tank



Purpose: Flocculation of destabilized particles

Flocculation tank provided immediatedownstream of the rapid-mix tank. It ismostly required for industrialwastewater treatment.

This unit is generally not required fordomestic wastewater treatment.

The objective of the flocculation tank isto facilitate the agglomeration ofparticles destabilized by coagulantaddition.

The larger particles thus formed areremoved in the settling tank providedimmediate downstream of theflocculation tank.

Sludge

Scum RemoverEffluent Launder

Inlet Channel

Elevation of Sedimentation Tank

Baffle

Settling Zone

L

D



Name of UO: Primary Sedimentation Tank



Purpose: Sedimentation of organic particles up to 60 mm size

Organic particles up to60 mm size areremoved in thesedimentation tank



FT PSTTo Secondary Treatment

From Preliminary Treatment

The wastewater after Primary treatment has,

1) No floating matter2) No inorganic particles of size > 0.2 mm3) Equalized Flow

1) All organic and inorganic particles up to 60 mm size removed

The Second Unit Process: Primary Treatment

RMT

Coagulant

Optional

Preliminary Treatment

Primary Treatment

After Primary Treatmentwastewater mainly has colloidal and dissolved organic and inorganic matter



Name of UO: Biological Treatment - Secondary Sedimentation

Type: Biological – Physical Process

Unit Process: Secondary Treatment

Purpose: Removal of colloidal organic and inorganic and dissolved organic matter

SSTTo Tertiary Treatment or Disposal

From Primary

Treatment

The wastewater after Secondary treatment may have,1) Non-Biodegradable organic matter (may impart color)2) Small concentrations of colloidal organic matter (mostly biomass)3) Dissolved inorganic matter (cations and anions)

Domestic wastewater after secondary treatment looks clean

The Third Unit Process: Secondary Treatment

Bioreactor

Dissolved organic matter(assuming biodegradable) isalmost completely removed.Colloidal organic matter ispartially removed.

Colloidal organic and inorganic matter, along with biomass effluent from the bioreactor is removed.


Recent Trends


From Sewer


FT PSTTo Secondary

Treatment


RMT

Coagulant

Optional



From Primary

Treatment


Bioreactor

Summary

Biological Process

Large floating organic matter and inorganic particle up to 0.2 mm size is removedFlow is equalized.

Inorganic and organic particle up to 60 mm size is removed.

Dissolved organic matter and colloidal organic and inorganic matter is removed.


Recent Trends

Recent Trends:

Conversion of Non-Biodegradable organic matter to a more biodegradable form during

primary treatment using physico-chemical methods. This helps more efficient organic

matter removal during secondary treatment.

This is mostly applicable for industrial effluents with large non-biodegradable organic

content.

Physico-Chemical Tertiary Treatment Processes:

1. Chemical precipitation of specific inorganic cations and anions (phosphate, heavy

metals, etc.) followed by removal of residual particulate matter and precipitates by

sand and membrane filtration (micro-, nano-filtration).

2. Removal of residual dissolved organic matter (non-biodegradable, micro-pollutants) by

a) activated carbon adsorption, b) nanofiltration, and c) post-ozonation.

3. Removal of residual inorganic cations/anions by, a) ion exchange, b) reverse osmosis

Tertiary treatment makes the treated wastewater more suitable for recycle/reuse. Degree

of tertiary treatment provided depends on the end-use of the treated effluent.

Aliphatic Compounds are easily biodegradable

Compounds with Aromatic rings and Cross linkages are difficult to Degrade

Biodegradability


Recent Trends

Strong Oxidizing Agents: Advanced Oxidation Processes (AOPs)

Only Ozone (O3):

Ozone (O3) is a strong oxidizing agent. In aqueous solutions O3 often directly reacts with

the target contaminant.

O3 may combine with OH- in water and through a series of reactions form hydroxyl

radical. Hydroxyl radical is also a strong oxidizing agent.

O3/UV:

H2O2/UV:

H2O2/O3:

Fe2+/H2O2 (Fenton’s Reaction):


Recent Trends

2h

23 OOH2OHO

OH2OHh

22

2232

32222

OOOHOHO

OHHOOHOH

]OH[

OHOHFeFeOH 3222

Mechanism of AOP:

Strong oxidizing agents attack electron-rich sites in complex organic molecules, i.e., aromatic

rings and double bonds. This breaks up the molecules into more aliphatic components, which

are more amenable to subsequent biodegradation.


Recent Trends

Name of UO: Pre-Oxidation (AOP)

Type: Chemical Process


Purpose: Enhancing Biodegradability of Non-Biodegradable Organic Matter

FT PST

To Secondary Treatment


RMT

Coagulant

Optional

PO

Optional

Pre-Oxidation (PO) will enhance the biodegradability ofnon-biodegradable organic matter often found inindustrial effluents. This is beneficial for the subsequentbiological treatment


Removal of Specific Inorganic Cations/Anions: Chemical Precipitation

Addition of chemicals to promote precipitation of specific cations/anions in secondary treated

effluent.


Recent Trends

Name of UO: Chemical Precipitation


Unit Process: Tertiary Treatment

Purpose: Removal of specific dissolved inorganic cations/anions

FT

From SecondaryTreatment

RMT

Chemical

Used mainly for removal of phosphate from domestic effluent or in thetreatment of industrial effluent. Chemical Precipitation will precipitatespecific dissolved inorganic cations/anions present in secondary treatedeffluent.

BackwashTank

BackwashTank

Pump

Backwash Rapid Sand Filter Schematic

Plan

Front Sectional View


Recent Trends

Filtration: Removal of Residual Particulates and Precipitates

Filtration can be done either using sand filters, i.e., rapid sand filters or pressure filters,

followed by membrane filters, i.e., micro- or ultra-filtration.

Name of UO: Filtration



Purpose: Removal of residual particulates and precipitates

Pressure Filtration

Operating principle is the same as in a rapid sand filter

Due to higher pressure application, throughput in pressure filters are higher

Compact systems, which can be purchased off the shelf and installed quickly


Recent Trends

Membrane FiltrationWater is passed through membranes of various pore sizes. Depending on the pore-size of the membrane, some impurities in water are ‘rejected’ by the membrane.

Inlet

Outlet

Reject

Hollow Fiber Membranes

Outlet

Membrane Reactor Schematic


Recent Trends

FT


RMT

Chemical

After filtration, the treated effluent will have no particulate or dissolved solids.Hence the turbidity of the effluent should be below 1 NTU.

However, the effluent may still contain residual non-biodegradable organicmatter and salts.

RSF/PF M/U Filt

Chemical Precipitation Residual Solids Removal


Recent Trends

Name of UO: Adsorption



Purpose: Removal of residual dissolved organic matter

Activated Carbon Adsorption

Treated effluent is passed throughcolumns containing activated carbon.Dissolved organic molecules areattached to the activated carbon surfaceand thus removed from the effluent


Recent Trends

Name of UO: Post-Ozonation



Purpose: Removal of residual dissolved organic matter (Color, odor)

Name of UO: Nano-filtration



Purpose: Removal of residual dissolved organic matter

NFFiltered Effluent AC

PostOzonation

Removal of Residual Organic Matter

Removal of Residual Organic Matter. Color and Odor removal.

Sparkling Water.


Recent Trends

Name of UO: Ion Exchange



Purpose: Removal of residual dissolved inorganic matter

Name of UO: Reverse Osmosis



Purpose: Removal of residual dissolved inorganic matter

Filtered Effluent IE/RO

Removal of Residual Inorganic Ions

Recyclable Effluent


From Sewer


FT PSTTo Secondary

Treatment


RMT

Coagulant

Optional

PO

Optional



From Primary

Treatment


Bioreactor

Summary

Biological Process

Large floating organic matter and inorganic particle up to 0.2 mm size is removedFlow is equalized.

Inorganic and organic particle up to 60 mm size is removed. Non-biodegradable organic matter is made biodegradable

Dissolved organic matter and colloidal organic and inorganic matter is removed.

Summary: Tertiary Treatment

FT


RMT

Chemical

NF/DF

Biological ProcessAmmonia/Nitrate

Removal RSF/PF M/U Filt

NFFiltered Effluent AC

PostOzonation IE/RO

Phosphate, heavy metals precipitation and residualcolloidal matter and precipitate removal

Residual dissolved non-biodegradable organicmatter, organic micro-pollutants, color, odorremoval

Residual Inorganiccations and anions,i.e., salt removal

Documents

Strategies for Wastewater Treatment: Physico-Chemical Approach