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This article was downloaded by: 10.3.98.104On: 03 Oct 2021Access details: subscription numberPublisher: CRC PressInforma Ltd Registered in England and Wales Registered Number: 1072954 Registered office: 5 Howick Place, London SW1P 1WG, UK
Sludge Management
Bhola R. Gurjar, Vinay Kumar Tyagi
Treatment of sludge
Publication detailshttps://www.routledgehandbooks.com/doi/10.1201/9781315375137-4
Bhola R. Gurjar, Vinay Kumar TyagiPublished online on: 02 Mar 2017
How to cite :- Bhola R. Gurjar, Vinay Kumar Tyagi. 02 Mar 2017, Treatment of sludge from: SludgeManagement CRC PressAccessed on: 03 Oct 2021https://www.routledgehandbooks.com/doi/10.1201/9781315375137-4
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-4Chapter 3
Treatment of sludge
3.1 INTRODUCTION
Generally, the sludges comprises 92–98% moisture, the remaining part being theputrescible organic materials. Due to the high organic content of the sludge, it requiresfurther treatment prior to its final disposal.
The common order of unit operations employed in wastewater treatmentsuggests that some operations, viz. screening, sedimentation, and chemical floccula-tion or precipitation constitute preliminary treatment. Subsequent operations, notablythose associated with trickling filtration and activated sludge treatment constitute‘secondary treatment’. Sludge treatment may include all or a combination of thefollowing unit operations and processes: thickening, digestion, conditioning, dewa-tering, and incineration. Thickening is meant for the reduction of moisture content ofthe sludge. Digestion is a biological method of treatment and is meant for the reduc-tion of organic content of the sludge. Conditioning improves the drainability of thedigested sludge, so that dewatering may be accomplished easily by air-drying in sanddrying beds or by mechanical smears. The ultimate disposal of dewatered sludge orthe ash after incineration of dewatered sludge may be done on to the land or into thesea. The particular combination of unit operations to be employed for sludge treat-ment depends upon the quantity and characteristics of sludge. As shown in Figure 3.1,common combinations of unit operations in the category of solids concentration andstabilisation are:
(i) Digestion of plain-sedimentation sludge followed by air drying,(ii) Concentration and chemical conditioning of activated sludge in advance of
vacuum filtration, and(iii) Incineration of a mixture of trickling-filter humus and plain-sedimentation
sludge after digestion, elutriation, chemical conditioning, and vacuumfiltration.
The waste liquor from wastewater sludges is often putrescible and high in solids.Coagulation and concentration of the removed floc may be necessary in preparing theeffluent for discharge.
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Figure 3.1 Flow chart for sludge handling (Arrows indicate possible flowpaths).
3.2 DEFINITIONS(Coker et al., 1991; Fleming, 1986)
Sludge thickening and stabilisation are the key processes in water and wastewatertreatment facilities practiced before disposal of sludge. Component operations andprocesses include the following.
Thickening: Thickening concentrates sludge by stirring it long enough to form larger,more rapidly settling aggregates with smaller water content. An example is thethickening of activated sludge to increase its solids concentration by 3–6 fold in8–12 hours of stirring, chlorine being added, if necessary, to impede decomposition.Displaced sludge liquor is the by-product.
Centrifuging: This operation concentrates sludge run into a centrifuge intermittentlyor continually to separate solids from the suspending sludge liquor, which becomesa by-product.
Chemical conditioning: This coagulates the sludge and improves its dewatering char-acteristics. An example is the addition of ferric chloride to wastewater sludge to bedewatered on vacuum filters.
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Elutriation: Elutriation washes out the sludge substances that interfere physicallyor economically with chemical conditioning and vacuum filtration. An exampleis the reduction in the alkalinity of digested sludge in the amounts of chemi-cals that need to be added in advance of filtration. The elutriating water is aby-product.
Biological flotation: In this operation sludge solids are lifted to the surface by gasesof decomposition. This concentrates the sludge. An example is the flotation ofprimary sludge in 5 days at 35◦C and the withdrawal of the subnatant, which is aby-product.
Dissolved air flotation: It is similar to the biological flotation, except that compressedair is used in this operation. Dissolved air flotation units are better alternative tothicken the ‘light’ sludge, such as waste activated sludge.
Vacuum filtration: This withdraws moistures from a layer of sludge by suction, thesludge to be dewatered being supported on a porous medium, such as coiled springs,or cloth on screening. An example is the dewatering of chemically conditioned,activated sludge on a continuous, rotary, vacuum drum filter. A sludge paste orcake is produced. The sludge liquor removed is a by-product.
Air drying: Air drying removes moisture from sludge run on to beds of sand or othergranular materials. Included moisture evaporates in the atmosphere and drains inthe drying bed. An example is the air-drying of well-digested sewage sludge on sandbeds, a spread able, friable sludge cake being produced. The by-product is the liquorreaching the under drains.
Heat drying: This drives off moisture by heat. The sludge can be reduced to substantialdryness. An example is the drying of vacuum- filtered, activated sludge in a contin-uous flash drier. If sludge is to be marketed, its moisture content must generally bereduced to less than 10%.
Sludge stabilisation: Sludge stabilisation converts raw sludge into a less offensive formthat has substantially reduced numbers of pathogens and mineralised solids, and issuitable for disposal safely. Traditionally, sludge is biologically stabilised by eitheraerobic or anaerobic digestion process. However, there are non-biological methodsalso to stabilise the sludge.
Sludge digestion: It is the aerobic/anaerobic decomposition of sludge. Digestion isaccompanied by gasification, liquefaction, stabilisation, destruction of colloidalstructure, and concentration, consolidation, or release of moisture. The gases pro-duced generally include, besides carbon dioxide, combustible methane and, morerarely, hydrogen.
Sludge digestion is one of the processes used to stabilise the sludge. Examplesare: (i) aerobic digestion of sludge in aerated reactors under endogenous-phaseconditions of respiration, and (ii) the digestion of settled solids in septic tanks(anaerobic digestion process).
Aerobic process: Aerobic process is a biological-treatment process that occurs in thepresence of oxygen. Certain bacteria that can survive only in the presence of dis-solved oxygen are known as obligate (i.e. restricted to a specified condition in life)aerobes.
Anaerobic process: This is a biological-treatment process that occurs in the absence ofoxygen. Bacteria that can survive only in the absence of any dissolved oxygen areknown as obligate anaerobes.
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Denitrification: Denitrification is the biological process by which nitrate is convertedto nitrogen and other gaseous end products.
Anoxic denitrification: It is the process by which nitrate nitrogen is convertedbiologically to nitrogen gas in the absence of oxygen.
Anoxic sludge digestion: It is nothing but the anoxic denitrification process used tostabilise the sludge. When nitrate is used rather than oxygen, the anoxic biomassstabilisation process similar to the aerobic digestion process takes place.
Dry combustion or incineration: This operation leads to the ignition and incinerationof heat-dried sludge at high temperatures, alone or with added fuel. Examples are(i) the incineration of heat-dried sludge, and (ii) the burning of heat-dried sludgeon the lower hearths of multiple-hearth furnace, on the upper hearths of which thesludge to be incinerated is being dried. The end product of incineration is a mineralash. The stack gases are by-products.
Wet combustion: Wet combustion oxidises wet sludge at temperatures of about 540 of(282◦C) and air pressures of 1200–1800 psig. The effluent suspension and exhaustgases are by-products.
Other unit operations: Other unit operations of sludge treatment include conditioningby heating, freezing, or physical flotation and dewatering by pressure filtration(filter pressing), etc.
3.3 METHODS OF SLUDGE TREATMENT(MWST, 1991)
Table 3.1 enlisted the principal methods used to process and dispose of sludge. Thesludge moisture is remove by thickening (concentration), conditioning, dewatering,and drying, however, aerobic and anaerobic digestion, incineration, and wet oxidationare used to stabilise the organic material in the sludge.
3.4 FLOW-SHEETS OF SLUDGE TREATMENT(Fleming, 1986)
A generalised flow-sheet/flowchart incorporating the unit operations and processesemployed in sludge treatment is presented in Figure 3.2 and 3.3. In practice, the mostcommonly used process flow-sheets for sludge treatment may be divided into twogeneral categories, depending on whether or not biological treatment is involved.
Typical flow-sheets incorporating biological processing are presented in Figure 3.4.Depending on the source of the sludge, either gravity or air flotation thickeners areused. In some cases, both may be used in the same plant. Following biological digestion,any of the three methods shown (i.e. vacuum filtration, centrifugation, drying beds)may be used to dewater the sludge, the choice depending on local conditions.
Because the presence of industrial and other toxic wastes has presented problemsin the operation of biological digesters, a number of plants have been designed withother means for sludge treatment. Three representative process flow-sheets withoutbiological treatment are shown in Figure 3.5.
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Table 3.1 Methods for processing and disposal of sludge.
Unit operation or treatment method Function
(i) Preliminary OperationsSludge grinding Size reductionSludge degritting Grit removalSludge blending BlendingSludge storage Storage
(ii)ThickeningGravity thickening Volume reductionFlotation thickening Volume reductionCentrifugal Volume reduction
(iii) StabilisationChlorine oxidation StabilisationLime stabilization StabilisationHeat treatment StabilisationAnaerobic digestion Stabilisation, mass reductionAerobic digestion Stabilisation, mass reduction
(iv) ConditioningChemical conditioning Sludge conditioningElutriation LeachingHeat treatment Sludge conditioning
(v) DisinfectionPasteurization DisinfectionHigh pH treatment DisinfectionLong term storage DisinfectionChlorination DisinfectionRadiation Disinfection
(vi) DewateringVacuum filter Volume reductionFilter press Volume reductionHorizontal belt filter Volume reductionCentrifuge Volume reductionDrying bed Volume reductionLagoon Storage, volume reduction
(vii) DryingFlash dryer Weight reduction, volume reductionSpray dryer Weight reduction, volume reductionRotary dryer Weight reduction, volume reductionMulti-hearth dryer Weight reduction, volume reductionOil immersion dehydration Weight reduction, volume reduction
(viii) CompostingComposting (Sludge only) Volume reduction, resource recoveryCo-composting with solid wastes Product recovery, volume reduction
(ix)Thermal ReductionMultiple-hearth incineration Volume reduction, resource recoveryFluidised-bed incineration Volume reductionFlash combustion Volume reductionCo-incineration with solid wastes Volume reduction, resource recoveryCo-pyrolysis with solid wastes Volume reduction, resource recoveryWet air oxidation Volume reduction
(x) Ultimate DisposalLandfill Final disposalLand application Final disposalReclamation Final disposal, land reclamationReuse Final disposal, resource recovery
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Figure 3.3 General schematic of a sludge treatment system.
3.5 PRELIMINARY TREATMENTS(MWST, 1991; Rao & Datta, 1987; Bahadori, 2013)
Preliminary sludge treatment operations include grinding, de-gritting, blending, andstorage of sludge, which helps to deliver a homogeneous feed to sludge handling
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Figure 3.4 Typical sludge treatment flow chart with biological digestion and three different sludgedewatering process: (a) vacuum filteration, (b) centrifugal, and (c) drying beds.
facilities. The mixing and storage of sludge can be achieved either in a single unitdesigned to do both or separately in other plant facilities.
Important preliminary operations can briefly be described as below:
3.5.1 Grinding of sludge
Sludge grinding is a process in which large material contained in sludge is cut orsheared into small particles. Sludge grinders use one of two techniques: hammer millpulverizing or cutting. The requirement of this type of unit usually depends on thespecific application as mentioned below:
(a) Sludge grinding is required before heat-treatment-process of sludge to preventclogging of high-pressure pumps and heat exchangers.
(b) Nozzle-disk and solid-bowl centrifuges require sludge grinding as a precedingoperation/process to prevent clogging in nozzles and between disks. Nozzle-diskunits may also require fine screens.
(c) It is a prerequisite to grind sludge before chlorination, so that the chlorine contactwith sludge particles can be enhanced.
(d) Sludge grinding is required before “pumping with progressing-cavity pumps’’too so that it can reduce heat and prevent clogging.
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Figure 3.5 Typical nonbiological sludge treatment flow chart; (a) heat treatment with vacuum-filterdewatering; (b) multiple hearth incineration; and (c) fluidised-bed incineration.
3.5.2 De-gritting of sludge(Bahadori, 2013)
In some plants where separate grit removal facilities are not used ahead of the primarysedimentation tanks, or where the grit removal facilities are not adequate to handlepeak flows and peak grit loads, it may be necessary to remove the grit before furtherprocessing of the sludge. Where further thickening of the primary sludge is desired, itis practical to consider de-gritting the primary sludge. The most effective method ofde-gritting sludge is through the application of centrifugal forces in a flowing system,to achieve separation of the grit particles from the organic sludge. Such separation isachieved through the use of hydro-clones, which have no moving parts. The sludgeis applied tangentially to a cylindrical feed section, thus imparting a centrifugal force.The heavier grit particles move to the outside of the cylinder section and are dischargedthrough a conical feed section. The organic sludge is discharged through a separateoutlet.
The efficiency of the hydro-clone is affected by pressure and by the concentrationof the organics in the sludge. To obtain effective grit separation, the sludge must berelatively dilute. As the sludge concentration increases, the particle size that can beremoved decreases.
3.5.3 Blending of sludge(Bahadori, 2013)
Blending of sludges is necessary to produce a uniform and homogeneous mixtureof sludge. It is particularly important ahead of sludge stabilization and dewatering
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Figure 3.6 Typical sludge mixer and blender used in conjunction with sludge storage.
processes and incineration, Sludge from primary, secondary, and advanced processescan be blended in several ways, viz.:
(i) In primary settling tanks: Secondary or tertiary sludges transferred to theprimary settling tanks, in which settling and mixing with the primary sludgewill takes palce.
(ii) In pipes: The careful control of sludge sources and feed rates is require toensure the appropriate mixture.
(iii) In Sludge-processing facilities with long retention times: The feed sludge can bemix uniformly in Aerobic and anaerobic digesters (continuous-flow stirred-tanktype).
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(iv) In a separate blending tank: The quality of mixed sludge can be controlefficiently by this practice.
In small treatment plants (<162 m3/h), mixing is usually achieved in the primarysettling tanks. At large treatment facilities, optimum efficiency is achieved by separatethickening and mixing of sludges before blending. A typical blending tank with mixingfacilities is shown in Figure 3.6.
3.5.4 Storage of sludge
Sludge storage must be provided to smooth out fluctuations in the rate of sludgeproduction and to allow sludge to accumulate during periods when subsequent sludge-processing facilities are not operating (i.e. night shifts, weekends, and periods ofunscheduled equipment downtime). Sludge storage to provide a uniform feed rateis particularly important-ahead of the following processes:
• Chlorine oxidation• Lime stabilisation• Heat treatment• Mechanical dewatering• Drying, and• Thermal reduction
Short-term sludge storage may be accomplished in wastewater settling tanks or insludge thickening tanks. Long-term sludge storage may be accomplished in sludge-stabilisation processes with long detention times (i.e. aerobic and anaerobic digestion)or in specially designed separate tanks. In small installations, sludge is usually storedin the settling tanks and digesters. In large installations that do not use aerobic andanaerobic digestion, sludge in often stored in separate blending and storage tanks. Suchtanks may be sized to retain the sludge for a period of several hours to several days.Sludge is often aerated to prevent septicity and to promote mixing. Mechanical mixingmay be necessary to assure complete blending of the sludge. Chlorine and hydrogenperoxide are often used to arrest septicity and to control the doors from sludge storageand blend tanks.
QUESTIONS
1 Why sludge stabilisation is necessary before discharge into the environment orreuse?
2 What are the most common methods for reducing the sludge volume?3 What is the difference between sludge dewatering, sludge conditioning and sludge
thickening? Name the different methods used for the abovementioned purposes.
