Chapter-I
1.0 Introduction
The manufacture of pulp, paper, and paper products is one of the largest industries in
the world; the major producing countries being the United States of America, Canada, Japan,
Sweden, Finland and China. Presently USA is the largest producer of paper material (90
million tonne/year) followed by China (78 million tonne/year). More than 83% of US annual
pulp output is produced by the kraft pulping or sulfate process (USEPA, 2002). The pulp and
paper manufacturing is one of the core industrial sectors in India. India produces about 8
million tonne of paper /year (Dugal H.S., 2009; Industry guide, 2011). It plays a vital role in
socio-economic development, while it is associated with significant environmental concerns
due to its large footprints on environmental resources. While manufacturing pulp from wood,
bleaching is carried out which generates toxic substances. Aerobic biological treatment by
activated sludge process has widely been used to treat pulp and paper mill wastewater.
During biological treatment, microorganisms oxidize dissolved and particulate organic matter
into simple end products with generation of biomass. The excess biomass is separated from
the treated wastewater and disposed of in concentrated form called excess sludge or waste
activated sludge (WAS). Due to the very nature and operation of the industry which are
described in the following sections, the biosludge originating from pulp and paper industry
contains various organic, inorganic and microbiological contaminants. Organochlorines viz.
chlorophenols, dioxins and dibenzofurans are some of the toxicants. Due to the presence of
organochlorine compounds, sludge from pulp and paper industry has been classified as
hazardous waste (MoEF, 2008). Being slimy in nature, it is difficult to dewater. The disposal
of biosludge is a challenging issue in pulp and paper industry.
1.1 Scientific aspects of pulp and paper manufacturing process
Processes of the manufacture of paper in an integrated kraft pulp and paper mill can
be divided into four stages viz. pulping, bleaching, papermaking and chemical recovery.
1
1.1.1 Pulping process
At the pulping stage, wood or other fiber material is treated chemically to separate out
the fiber fraction from other constituents of wood. Chemical pulping using kraft process is
the most prevalent throughout the world. It uses sodium sulfide (Na2S) and sodium hydroxide
(NaOH) as pulping chemicals. The liquor (white liquor) comprising of these two chemicals is
mixed with the wood chips in a reaction vessel (digester), the products consist of wood fibers
(pulp) and liquor that contains the dissolved lignin; typically termed as black liquor. The
chemical process results in approximately 50 percent production of pulp.
1.1.2 Bleaching process
The bleaching of unbleached chemical pulp is carried out with elemental chlorine or
its derivatives like chlorine dioxide (ClO2) or hypochlorite (NaOCl, and Ca(OCl)2). Upto the
end of 20th century, nearly every chemical pulp mill was using elemental chlorine as the first
bleaching stage in India. Because of environmental concerns arising out of formation of
organochlorine compounds, pulp mills now use partially ClO2 substituted chlorination or
elemental chlorine free bleaching technologies. During bleaching, pulp is processed through
three to five stages of chemical reaction and water washing. Bleaching stages generally
alternate between acid and alkaline conditions. Chemical reactions take place with lignin
during the acid stage. The alkaline stages extract the reaction products of lignin through
dissolution. At the washing stages, reaction products are removed. The most common
bleaching sequences followed by mills in India are CEOPD1D2 or CDEOPD1D2 or D0EOPD1D2.
1.1.3 Paper making
At the final stage, the stock is prepared by refining the pulp, and the paper is
manufactured. Stock preparation includes dispersion of pulp in water, refining to increase
surface area and addition of wet end chemicals. Wet-end operation includes the formation of
paper sheet from wet pulp stock, whereas dry-end operation includes drying of paper, surface
treatment and spooling for storage.
2
1.1.4 Kraft chemical recovery process
During chemical recovery, process chemicals from the spent cooking liquor (black
liquor) are recovered for reuse. The chemical recovery process has important financial and
environmental benefits for pulp and paper mills. The black liquor generated during pulping
contains 15-17% solids, which is concentrated to 65-80% solid level which is burnt in a
recovery boiler; the recovered chemicals are dissolved in weak white liquor. The resultant
liquor is called green liquor. The impurities like dregs are separated and dissolved sodium
carbonate is converted into active sodium hydroxide (white liquor) during causticization
process. White liquor is again used in the pulping process.
1.2 Wastewater characteristics
In comparison to the global best specific water consumption of 28.7 m3/ tonne for
wood based pulp and paper mill, water consumption in pulp and paper mills in India is 43-
150 m3/ tonne of product (NPC, 2006; MoEF, 2010). High consumption of water is largely
attributed to the use of old technology/ equipment and poor water management practices.
Most of the pulp and paper mills are onshore and use surface water. The discharge of
wastewater with additional evaporation loss is generally used to give a fair picture of water
consumption. Each pulp and paper making process utilizes large amounts of water, which
reappears in the form of wastewater.
Among the processes, pulping generates a high strength wastewater especially in
chemical pulping. Pulp bleaching process generates toxic substances as it utilizes chlorine
and its derivatives for brightening the pulp. Various substrates like resin acids, unsaturated
fatty acids, diterpene alcohols, chlorinated resin acids, and others are generated depending
upon the pulping process used. The wastewater, which contains filler, fines and starch, from
paper machine is generally recycled to process. The pollutants generated at various stages of
the pulping and paper making process are given in Table 1.1.
Wastewater from the pulp and paper mill contains a broad spectrum of organic and
inorganic substances. It typically consists of fibrous suspended solids and dissolved organic
compounds in high concentration. Both low and high molecular weight compounds are
present in dissolved form. Generally small organic molecules exert biochemical oxygen
3
demand (BOD), whereas lignin and its derivatives cause a chemical oxygen demand and
attribute colour. Characteristics of wastewater from Indian pulp and paper mills are given in
Table 1.2. The time bound reduction of water use as per CREP has resulted in reduction in
water consumption in paper production. The average water consumption for large paper mills
(wood based), agro residue based mills and waste paper based mills is 40-120, 75-100 and
35-50 m3/ tonne of product respectively (Chinnaraj et al., 2011; Endlay et al., 2011). The
proposed norms for chemical pulp and waste paper based pulp and paper mills are 80 and 20
m3/ tonne of product (CPCB, 2011).
Table 1.1: Potential water pollutants from pulp and paper making processes
Source Pollutant
Wood handling/debarking and chip Solids, BOD, COD, color
washing
Chip digester and liquor evaporator BOD, COD, colour, reduced sulfur
condensate compounds
Wastewater from pulp screening, Suspended solids, BOD, COD
thickening, and cleaning
Bleach plant filtrates BOD, COD, color, TDS, organochlorine
compounds
Paper machine water Suspended solids, TDS, BOD, COD
Fiber and liquor spills Solids, BOD, COD, color
USEPA, 2002
Formation of organochlorine compounds in natural eco-system is well documented.
More than 1500 organohalogen compounds have been identified (Biester et al., 2004). The
pulp and paper mill is one of the artificial or manmade sources of organochlorine compounds
in recipient waterways (Zheng and Allen, 1996; Ali and Sreekrishnan, 2000). Organochlorine
compounds are formed during the bleaching of wood pulp with chlorine (Cl2) and chlorine
derivatives such as hypochlorite and chlorine dioxide (ClO2) (Roy et al., 2004). The reactions
between chlorine and lignin are substitution, oxidation and addition.
4
Table 1.2: Characteristics of wastewater from Indian pulp and paper mills
Small paper mill
Parameter Large paper mill
Agro mill Waste paper mill
Flow (m3/t paper) 197-280 187-383 72-159
pH 6.6-10 6.0-8.5 7.1-7.7
TSS (mg/l) 620-1120 600-1115 350-885
BOD5 (mg/l) 240-380 220-1067 100-273
COD (mg/l) 840-1660 2120-4763 472-876
COD/BOD ratio 2.95-4.37 2.49-5.40 2.7-5.7
Colour (Pt-Co unit) 300-655 15000-24000 -
Lignin (mg/l) - 320-700 -
SAR 2.0-6.3 4.7-7.6 -
Ray, 2006
Organochlorine compounds in water and wastewater can be monitored by several
techniques; among them, the one based on adsorbable organic halogens (AOX) is the most
commonly used (Zheng and Allen, 1996; Barroca et al., 2001). During the production of one
tonne of paper, 100 kg of colour imparting substances and 2-4 kg of organochlorines are
generated in the bleach plant effluent (Kansal et al., 2008). A physical-chemical classification
of this chlorinated organic material from conventionally pulped and bleached softwood kraft
pulp is shown in Figure 1.1. Among the organochlorine compounds derived from lignin, 80%
are of high molecular weight and rest 20% compounds are of low molecular weight
(MW<1000). A tiny fraction (1-3% of the total organochlorines) is lipophilic and is the cause
of environmental concern. Collectively these are termed as extractable organic halogen
(EOX) compounds. Chlorophenolic compounds, dioxins and dibenzofurans fall in this
category (Berry et al., 1991; Bajpai and Bajpai, 1997).
5
~80% High MW
material
Relatively hydrophilic
(Water soluble)
Mainly nonaromatic
(Does not permeate cell
AOX walls)
100 % <10% chlorine by weight ~19%
Relatively hydrophilic
includes compounds which
can easily be hydrolyzed or
~20% Low MW metabolized (e.g. Trichloro
material acetic acid)
~1% ~0.1%
Relatively lipophilic LogPow >3
(fat soluble) Highly lipophilic
Potentially toxic and Bioaccumulable
bioaccumulable (e.g., dioxin:
44% chlorine by weight)
Figure 1.1: Characteristics of AOX compounds in the effluent (Berry et al., 1991)
Chlorophenolic compounds include phenols, guaiacols, catechols, syringol and
vanillins substituted with one to five chlorine atoms per molecule. Typically, bleaching
processes that result in the formation of 2,3,7,8-tetrachlorodibenzodioxin (TCDD) and
2,3,7,8-tetrachlorodibenzofuran (TCDF) also generate the higher substituted tri-, tetra-, and
penta-chlorinated compounds (Freire et al., 2003; Roy et al., 2004).
USEPA has identified 12 organochlorine compounds for regulation in the effluent of
pulp and paper mills and has established effluent limitations guidelines and pretreatment
standards (USEPA, 1999) for these chlorinated phenolic compounds. This is in response to
environmental concerns and government regulations on the limits of these discharges
following recognition of their potential adverse biological effects. From the point of
compliance, maximum permissible concentration of 12 chlorophenolics is given in Table 1.3
(USEPA, 2000).
6
Table 1.3: Chlorophenolic compounds under regulation
Compound Maximum concentration (µg/l)
2,4,6-trichlorophenol 2.5
2,4,5-trichlorophenol 2.5
2,3,4,5-tetrachlorophenol 2.5
3,4,6-trichloroguaiacol 2.5
3,4,5-trichloroguaiacol 2.5
4,5,6-trichloroguaiacol 2.5
3,4,6-trichlorocatecol 5.0
Pentachlorophenol 5.0
3,4,5-trichlorocatecol 5.0
Tetrachloroguaiacol 5.0
Trichlorosyringol 2.5
Tetrachlorocatecol 5.0
USEPA, 2000
In both scientific and industrial communities, there has been a growing interest in the
best available technologies for the bleaching of chemical pulps in order to reduce the
discharge of organochlorine compounds in the liquid effluents.
1.3 Wastewater treatment
Improved fiber retention, better in-plant utilization of raw materials, and use of
efficient and environment friendly processes/ technologies are effective means of reducing
pollution at site. In last decade, the amount of water consumption in pulp and paper mills
has been drastically reduced. External treatment of effluent is usually carried out by means
of screening and sedimentation to remove suspended solids (i.e. primary treatment)
followed by biological oxidation to remove suspended and dissolved organic material (i.e.
secondary treatment). Any treatment beyond primary and secondary treatment is usually
termed as tertiary treatment. The sludge generated during different stages is thickened and
dewatered prior to disposal. The schematic diagram of typical effluent treatment process is
illustrated in Figure 1.2.
7
Flocculant,
Neutralizing
Chemicals
Nutrients
Raw Treated
Effluent Primary Secondary
Bar Aeration Effluent
Clarifier Clarifier
Screens Tank
Returned Sludge
Waste Activated
Sludge
Primary Sludge Secondary Sludge
Thickener Thickener
Mixer
Tank ETP Sludge
Belt Press
Figure 1.2: Schematic diagram of typical ETP in pulp and paper mills
1.3.1 Primary treatment
Primary treatment generally refers to the methodology for removing suspended
solids from effluents. In the pulp and paper mill, solids removal is always accompanied by
some reduction in COD, BOD and toxicity. Screening is often used as a preliminary step to
remove relatively large floating or suspended particles from an effluent stream. Generally
screening is performed by bar screens.
Sedimentation or gravity settling is the most common process used to separate the fibrous
material from raw wastewater. Sedimentation is carried out in a holding pond or basin with
sufficient retention time for settling the solid particles. Ideal sedimentation unit provides
quiescent flow to allow the settlable solids to move to the bottom. Solids are pushed to the
center sump with a sludge scraper. Depending on the characteristics of the solids, the
concentration of underflow may vary between 1.5 to 6% (Smook, 1992). In addition to the
8
basic function of separation of solids, primary clarifier acts as an equalization basin cum
shock absorber.
1.3.2 Secondary treatment
Secondary or biological treatment is the heart of the wastewater purification process.
Under aerobic conditions, microorganisms (mostly bacteria) consume oxygen to convert
organic waste into the ultimate end products, carbon dioxide and water. An important aspect
of biological oxidation process is to provide adequate aeration and mixing for intimate
contact between large concentration of microorganisms and the substrate. Aerobic biological
oxidation can be accomplished by various means, depending on the characteristics of the
wastewater, the area available for external treatment, and the required degree of BOD
removal. Aerobic process is divided into three categories viz. suspended growth process,
attached growth process and hybrid process.
The activated sludge process (ASP) is the most popular, well adapted, high-rate
suspended growth biological process for treatment of industrial and municipal wastewaters.
The process was named activated sludge by Ardern and Lockett, 1913 as it involved the
production of an activated mass of microorganisms capable of aerobic stabilization of organic
material in wastewater. ASP is differentiated based on mixing arrangement as ‘plug flow’
and ‘completely mixed’ (Tchobanoglous et al., 2003; Seviour & Nelsen, 2010).
Plug flow system
The plug flow system generates less filamentous bacterial mass and produces settling
sludge than completely mixed one. This configuration often runs inefficiently from uneven
load distribution along the reactors. Thus, demand of oxygen at the inlet of the aeration basin
is high. Here DO remains near to zero, whereas the same remains higher at discharge end.
The mixed liquor is kept in suspension by aeration only (Figure 1.3a).
Completely mixed system
The aeration deficiency in the plug flow system is overcome through the completely
mixed system. However, this configuration is susceptible to bulking by filamentous bacteria.
Returned activated sludge (RAS) and incoming wastewater are mixed rapidly with the
biomass, reducing the risk of toxic shock (Figure 1.3b).
9
Influent
Effluent Influent Effluent
Secondary Secondary
Aeration basin clarifier Agitation clarifier
Diffused aeration Aeration basin
Waste Waste
Air Air
activated activated
sludge sludge
Recycled sludge Recycled sludge
1.3a: Plug flow ASP 1.3b: Completely mixed ASP
Figure 1.3: Flow diagram of plug flow and completely mixed activated sludge process
The essential feature in the suspended growth configurations is the development of a
microbial floc in suspension in the aeration tank. Clarified wastewater is fed continuously in
this tank and the organisms multiply as dissolved organic waste is metabolized. After the
treatment, wastewater is passed through a clarifier where separation of solid–liquid takes
place. A certain portion of this biomass is recycled to the aeration tank to maintain desired
microorganism concentration and food to microorganism ratio. The excess biomass is
concentrated and disposed of. Compared to the aeration lagoon, the activated sludge process
has some disadvantages; it is sensitive to changes in the characteristics of the wastewater, the
requirement of nutrients is relatively higher, and settling aids are sometimes required for
proper clarification.
Activated sludge process is widely used to treat pulp and paper mill effluent. The
treatment can generally achieve relatively higher reduction of BOD and toxicity (Diez et al.,
2002). Partial removal of AOX compounds in the biological system has also been recognized
(Reeve, 1991; Taghipour and Evans, 1996). The removal of these compounds is achieved
through volatilization, biosorption, and biological dechlorination (Leuenberger et al., 1985).
Microorganisms hydrolyze the dissolved and particulate organic matter into simple end
products and subsequently oxidize those with generation of additional biomass.
Cell yield or sludge yield coefficient is dependent on the nature of substrate, various
environmental and process conditions viz. pH, temperature, dissolved oxygen, nutrients,
hydraulic retention time (HRT), sludge retention time (SRT), food to microorganism (F/M)
ratio etc. which influence the metabolization of the organic carbon (Gaudy & Gaudy, 1981).
Excess cells are separated from the purified water in a concentrated form called waste
10
activated sludge which is contaminated with various substances spilled from or generated in
the pulp and paper making process.
1.4 Solid waste handling in the activated sludge process
Wastewater treatment in the pulp and paper mill generates huge solid waste (Monte et
al., 2009). Amount of sludge generation vary widely among mills. For bleached kraft mills,
sludge generation ranged from 14 to 140 kg of sludge per tonne of pulp (USEPA, 2002).
Corresponding figure for biological sludge in the large and small paper mills in India are 35
and 105 kg/t of paper (Ray, 2006). Treatment and disposal of WAS in the case of pulp and
paper industry pose challenges for environmental and regulatory factors. It has been
classified as hazardous waste in India (MoEF, 2008) due to presence of the organochlorine
compounds. The prevalent disposal methods of WAS such as land filling, incineration and
beneficial uses have come under watch and criticism by the public and regulatory agencies
around the world. Because of various environmental and technical reasons, the disposal of
WAS becomes a costly affair (Low and Chase, 1999b; Wood et al., 2009). Many researchers
have reported sludge management costs as high as 40-60% of operating costs of wastewater
treatment. It is to the economic advantage to reduce the sludge generation (Wei et al., 2003;
Yang et al., 2003; Chakrabarti, 2005; Yoon and Lee, 2005; Mahmood and Elliott, 2006).
Increased attention has been given on minimization of WAS generation from activated
sludge process. Different techniques are in use for generation of lesser amount of sludge
based on chemical or biological principles. The reduction of WAS are achieved with the
following techniques (Figure 1.4):
a) Process changes during biological treatment
b) Post treatment of waste activated sludge
11
Reduction of sludge
Process modification Post treatment
Operational control RAS treatment Heat treatment Chemical oxidation Sludge digestion
Extended aeration UV irradiation Incineration Super critical water oxidation Aerobic
Membrane bioreactor Anaerobic/ anoxic conditioning Carbonization Wet air oxidation Anaerobic
Improved aeration Acid bleach effluent Vitrification Alkali digestion
Low sludge process Ozonation Gasification
Additive dosing Mechanical destruction Pyrolysis
Figure 1.4: Outline of sludge reduction technologies (Mahmood and Elliott, 2006)
Perez-Elvira et al., 2006 have categorized the sludge minimization techniques
considering the places in the plant where the treatment is applied viz., i) in the
wastewater line, ii) in the sludge line, and iii) in the final waste line (Table 1.4).
12
Table 1.4: Sludge minimization techniques
Chemical oxidation Chlorination FS
Ozonation FS
Reduction of yield coefficient
Lysis cryptic growth Integration of chemical and heat treatment
In the wastewater line
FS
Pure oxygen process FS
Enzymatic reaction FS
Maintenance
metabolism Membrane bioreactor LS, IN
Uncoupling Chemical uncoupler LS
metabolism Oxic-settling-anaerobic process FS
Anaerobic/aerobic system LS
Ecosystem change Two-stage system LS
Oligochaetes LS
Cavitation High pressure homogenizer LS
Ultrasonic homogenizer LS
Thermal hydrolysis FS
Pretreatment prior to anaerobic digestion
Thermal
Freezing and thawing LS
Physical
Impact grinding LS
Mechanical Stirred ball mill LS
In the sludge line
High performance pulse
LS
technique
Lysat-centrifugal technique LS
Radiation Gamma-irradiation LS
Acid or alkaline hydrolysis LS
Chemical Pretreatment with ozone LS
Enzymatic pretreatment LS
Biological
Combination of thermal, decompression
Combined FS
and shear forces
Chemically enhanced thermal hydrolysis LS
Two-stage anaerobic digestion IN
Modified anaerobic Temperature phased anaerobic digestion IN
digestion
Anoxic gas flotation FS
In the final
Incineration FS
waste line
Gasification and pyrolysis FS
Wet air oxidation FS
Super critical water oxidation FS
FS: full-scaled, LS: laboratory scale, IN: innovative
Each of the four processes applied in the wastewater line for reduction of sludge yield
coefficient is briefly described in the following section:
1.4.1 Lysis-cryptic growth
Cell lysis releases its constituents into the medium, and provides an autochthonous
substrate which is reused in microbial metabolism. A portion of the carbon is liberated as
13
products of respiration resulting in a lower biomass generation. There are two stages in
lysis cryptic growth; lysis and biodegradation. The rate limiting step is lysis and an increase
of the lysis efficiency is directly linked with reduction in sludge generation.
The lysis of sludge can be achieved in several ways (Muller, 2000; Wei et al., 2003;
Bougrier et al., 2006):
i. Thermal treatment in the temperature range from 40 to 180 °C
ii. Chemical treatment using acids or alkali
iii. Mechanical disintegration using ultrasound/ mill/ homogenizer
iv. Freezing and thawing
v. Hydrolysis with enzyme
vi. Pure oxygen
vii. Advanced oxidation processes with H2O2 and ozone
viii. Combination of two or more techniques
1.4.2 Maintenance metabolism
Microorganisms satisfy first the maintenance energy and then produce new cells. By
increasing biomass concentration (e.g. membrane bioreactor) it would theoretically be
possible to reach a situation where the amount of energy supplied equals the maintenance
demand (Low and Chase, 1999a; Wei et al., 2003).
1.4.3 Uncoupling metabolism
Catabolism is the reaction that reduces the complexity of organic compounds and
produces free energy. Anabolic pathways use the free energy to build the molecules required
by cell. Anabolism is coupled with catabolism through the rate limiting respiration. Uncoupled
metabolism would occur if respiratory control does not exist and instead the biosynthetic
processes are rate limiting. Excess free energy would be directed away from anabolism so
that the production of biomass is reduced. The uncoupling metabolism is achieved with
chemical uncoupler, high initial substrate to biomass ratio, oxic-settling-anaerobic process (Liu
et al., 1998; Low et al., 2000; Yang et al., 2003).
14
1.4.4 Ecosystem change
Activated sludge is an ideal habitat for several organisms other than bacteria. One
way to reduce sludge production is to exploit higher organisms such as protozoa and metazoan,
in the activated sludge process, that predate on the bacteria while unaffecting the
decomposition of substrate (Wei et al., 2003).
1.5 Ozone properties and oxidation treatment
Ozonation is one of the most effective treatments for reducing the production of
activated sludge in wastewater treatment plant (Dziurla et al., 2005). The following section
will depict in brief different aspects of ozone, its chemistry and reaction with different
substrates:
Ozone is a molecule that consists of three oxygen atoms. The ozone molecule is very
unstable and has a short half-live, causing it to fall back into oxygen after a while. The
properties of ozone are given in Table 1.5.
Table 1.5: Properties of pure ozone
Parameter Data
Melting point (°C) -192.5±0.4
Boiling point (°C) -111.1±0.3
Critical temperature (°C) -12.1
Critical pressure (atm.) 54.6
Critical volume (cm3/mole) 111
Density of ozone gas (g/cm3) @ NTP 1.95
Ozone is more soluble in water than oxygen (Table 1.6), but because of very low
partial pressure, it is difficult to obtain a high concentration of ozone dissolved in water at
NTP.
15
Table 1.6: Solubility of oxygen and ozone in water at different temperatures
Solubility in water (mg/l) at different temperature
Gas
0 °C 10 °C 20 °C 30 °C
Oxygen
100% 70.5 54.9 44.9 38.2
21% 14.8 11.5 9.4 8
Ozone
100% 1374.3 1114.9 789 499.6
4% 55 44.6 31.6 20
(Wang et al., 2007)
Ozone is a strong chemical oxidant, allotropic, highly reactive and has the properties
of a dipole. As a result, it has a great capacity to attack organic compounds and metals,
with the exception of gold, platinum and iridium. It reacts in two different ways: (1) direct
reactions of ozone and (2) indirect reaction of secondary oxidators, such as free OH
radicals. Both reactions occur simultaneously, the indirect reaction is based on the high
reactivity of hydroxyl radical, which does not react specifically, whereas the direct reaction
depends more on the structure of the reactant. The way of the reaction of ozone depends on
various factors, such as temperature, pH and chemical composition. This is consequential to
the disintegration of ozone into OH radical in water (Cesbron et al., 2003; Gunten, 2003a, b;
Salsabil, 2008).
Considering the necessity of reducing the disposable sludge from pulp and paper mill
and the complexity of both the wastewater and sludge, the oxidation power of ozone has
been exploited in the current research to see how ozone reacts with a complex substrate like
waste activated sludge of pulp and paper mill. Following chapter will present an overview of
the work that different researchers have reported on minimization of sludge including that of
pulp and paper mill.
16
