ORIGINAL PAPER

Inßuence of sludge disintegration by high pressure homogenizeron microbial growth in sewage sludge: an approach for excesssludge reduction

Chockalingam Lajapathi Rai ÆParuchuri Gangadhar Rao

Received: 30 June 2008 / Accepted: 22 January 2009� Springer-Verlag 2009

Abstract Excess sludge treatment and disposal is cur-rently a challenge in wastewater treatment plant due tosocio-economic and environmental regulation factors.Therefore, it is necessary to explore and develop technol-ogy for reduction of excess sludge in wastewater treatmentplants rather than treating the generated sludge. This paperdiscusses technologies for excess sludge growth reductionusing high pressure homogenizer treatment which causescell lysis. The cell lysis and the microbial cellular main-tenance energy requirement increases as a result of appliedstress. This causes the energy required for non-growthreactions to increase with simultaneous increase in sub-strate conversion into carbon dioxide rather than microbialcell multiplication.

Keywords High pressure homogenizer�Maintenance energy� Conversion yield� Excess sludge�Microbial growth reduction

Introduction

The activated sludge process is the most widely used bio-logical wastewater treatment for both domestic andindustrial plants. The major drawback in activated sludge

processes is high excess sludge production. Treatment anddisposal of excess sludge from wastewater treatment plant(WWTP) accounts for about half or even up to 60% of thetotal cost of wastewater treatment. Urban wastewatertreatment processes in the developed world requires moreextensive wastewater treatment methodologies so that theenormous production of dry solids from WWTP (approx-imately 6.5 million tonnes a year) can be avoided(http://themes.eea.europa.eu/Environmental_issues/waste/indicators/sewage/w5_sludge.pdf). Furthermore, many newtreatment plants are in the commissioning stage and thevolume of sewage sludge is expected to increase by at least50%, leading to 10.1 million tonnes of dry solids per year,may increase by nearly 300% in some countries(http://themes.eea.europa.eu; 91/171 European EconomicCommunity report). The expected increase is itself achallenge for waste management and the choices of treat-ment and disposal methods will have large economic andenvironmental implications. The landÞll of sewage sludgeis designed to prevent health risks to humans and livestockdue to potentially toxic elements in the sewage sludge fromheavy metals, pathogens and organic pollutants (Munnichet al. 2006). Decline in available land space coupled withincreasing stringent regulations governing the design andoperation of new landÞlls, have pushed the cost of siting,building and operating new landÞlls to rise sharply.Alternative technology such as incineration of sewagesludge is increasing. This reduces the sludge to ash thatrequires less landÞll capacity. The particulates producedduring incineration are discharged into the air settle out asdust on city streets and farm lands. Therefore, incinerationgas needs an extensive cleanup system. Generally, incin-eration systems have much higher costs and longer pay-back periods due to high capital investment still produce asolid waste residue which requires management. Recycling

C. L. Rai (&)Chemical Engineering Division,Central Leather Research Institute,Adyar, Chennai 600020, Indiae-mail: clrai@hotmail.com

P. G. RaoNorth East Institute of Science and Technology,P.O. Jorhat, Assam 785006, Indiae-mail: pgrao24@hotmail.com

123

Clean Techn Environ PolicyDOI 10.1007/s10098-009-0202-y

of sewage sludge mainly by spreading on soil as fertilizerfor agriculture or forestry, is yet another methodology butgrowing attention to produce clean and non-contaminatedfood for human consumption will make this option lessattractive. On one hand, sludge recycling programmes areencouraged while on the other the demand for clean andhealthy food is growing in the society. The results of asurvey in European Union countries on contamination ofsewage sludge showed a development in the right directioni.e. less use of heavy metals led to less contaminated sludgeand thereby to more recycling (Waste treatment and dis-posal2005) However, the increased use of sewage sludgeas fertilizer needs to be balanced so that the contaminationof agricultural land will neither affect the quality of foodnor lead to damage of the environment (http://themes.eea.europa.eu).

Ultimately, it can be concluded that there are limitedpractical possibilities of disposal of sludge currentlyavailable. Wastes discharged into the air settle out again asdust on city streets and farmlands. LandÞlls leach into theground water or spill into the rivers and sewage sludgedumped into the ocean can wash up on beaches (Young andRichardson1998). Most pathogenic bacteria probably dieoff relatively quickly and some toxic substances can bedegraded. However, heavy metals and other toxins persistand both pathogens and toxins may be taken up by Þsh andshellÞsh. With extensive dumping there can be even moredramatic environmental damage (Hope1986).

Therefore, an ideal way to solve sludge-associatedproblems is to reduce sludge production from wastewatertreatment processing rather than devote time, money andenergy in the post treatment of the sludge. Indeed majorefforts have been carried out in the past (Wei et al.2003;Marjoleine et al. 1998) in order to degrade the sludgegenerated. Prevention of biomass formation in the waste-water treatment train is therefore the best method throughwhich the excess biomass formation can be prevented.

Microbial metabolism liberates a portion of the carbonfrom organic substrates in respiration and assimilates aportion into biomass. To reduce the biomass production,wastewater processing must be engineered such that sub-strate is diverted from biomass synthesis to fuelexothermic, non-growth activities.

Therefore, in the present investigation, an attempt hasbeen made to study the disintegration of the sewage sludgeby employing high pressure homogenizer (HPH) as pre-treatment unit. Also explored in the present study weremethods to control the growth by modifying the microbesthrough HPH. This process is intended to assist decon-struction and resist microbial multiplication. This newapproach not only disintegrates the ßoc but simultaneouslycauses either damage or completely rupture the microbialcells (lysis) by giving moderate speciÞc energy inputs. In

order to quantify the sludge microbial growth reductionrespirometric studies have been carried out.

Past studies on excess sludge reduction by mechanicaldisintegration

A recent study investigated the mechanism of mechanicaldisintegration of thickened surplus activated sludge by adeßaker technology for the production of organic matterfor biological nutrient removal (Kampas et al.2007). Theimprovement of anaerobic digestion was investigatedthrough four different methods of mechanical cell disinte-gration (Mueller et al.1998; Kopp et al. 1997). It wasfound that the best results were obtained using stirred ballmill disintegration and high pressure homogenizer.Mechanical disintegration was shown to physically disruptthe cellular material by breaking up microbial cell wallintracellular carbon sources as well releasing nutrients tostimulate further anaerobic degradation (Baier andSchmidheiny 1997). Besides mechanical disintegrationother methods viz., ozonation, wet oxidation, enzymatic(Barjenbruch and Kopplow2003) thermal, chemical andthermo-chemical pretreatment methods are also used forimproving sludge digestibility (Dewil et al.2007; Bougrieret al. 2007; Sikkim et al.2003).

Respirometric literature sources

A respirometric method was used to study the kinetics ofbiological reactions involved in the treatment of organicwaste matter. The organic matter compositions werecharacterized as three types of biomass; biodegradable,slowly biodegradable and non biodegradable organic mat-ter (Tremier et al.2005). The same respirometric methodwas applied to monitor the degradation process with acti-vated sludge (Krist Gernaey et al.2001). A procedure waspresented to estimate bio-kinetic parameters for hetero-trophic and autotrophic process models and to estimatewastewater characteristics in the context of the ActivatedSludge Models No. 1 and No. 2 (Henze et al.1987, 1995).The procedure is based on respirometric measurements atlow substrate to biomass ratio (S/X) (Spanjers andVanrolleghem 1995). To promote the conversion oforganic pollutants to respiration products with concomitantincrease in aeration requirements (Low and Chase1999a,b), a respirometric procedure was introduced to estimatethe heterotrophic conversion yield (YH) of the sludge. Thisrespirometry technique was also employed to assess thestorage yield for different substrates, to predict and controlthe activated sludge process, as well as for measurement ofsubstrate concentration in wastewater (Karahan-Gull et al.

C. L. Rai, P. G. Rao

123

2002). To model and upgrade the biological treatmentprocess a new respirometric method was proposed in orderto estimate readily biodegradable COD, slowly biode-gradable COD and the active heterotrophic biomass presentin wastewater (Mathieu and Etienne2000).

Materials and methods

The sewage sludge was collected from a wastewater treat-ment plant aeration tank outlet in Braunschweig, Germany.Sludge disintegration experiments were carried out in 2 lbatch size using high pressure homogenizer (HPH) obtainedfrom APV Gaulin, model LAB60/15TBSI, Germany withdisk valve from 150 to 750 bars. The schematic conÞgu-ration of high pressure homogenizer is shown in Fig.1. Thesludge that was fed to the HPH contained total solids (TS) of4.2 g/kg. Before disintegration, studies the sludge wassieved (mesh size\ 500 lm) to remove sand and wastematerials to prevent clogging. After sieving the sludge wasrefrigerated to bring down to initial feed temperature of 9Ð10� C. Sludge samples were collected at different operatingpressures to determine the degree of disintegration, particlesize reduction and microscopic analysis. After disintegra-tion, the outlet temperature of the sludge was found to bebetween 25 and 27� C.

Analysis

Degree of disintegration

The rate of disintegration can be determined by two bio-chemical parameters. Degree of inactivation and degree ofsoluble COD release.

Degree of inactivation (DDO)

The microorganisms in the sludge are aerobic or facultativeanaerobic bacteria. They use up oxygen in the metabolicprocesses; therefore, the activity of such sludge can bedetermined by measuring oxygen uptake rate. The experi-mental setup used for this measurement is shown in Fig.2.The mechanical stress could cause damage to microor-ganisms, resulting in loss of their ability to consumeoxygen. By measuring the oxygen uptake rate of the treatedand the untreated sludge the degree of inactivation could becalculated as follows.

DDo½%� ¼ 1 �OURtr

OURut

� �� 100 ð1Þ

where OURut and OURtr were the oxygen uptake rates ofuntreated and treated sludge.

The measurements of OUR was done according to thereported procedure (DIN 38414 S6, Germany). Well pre-aerated sludge (untreated and treated) was placed in an airfree vessel, the sludge diluted with synthetic wastewater(Peptone from Casein 3.20 g/l; Meat extract from casein2.20 g/l; CO(NH2)2 0.60 g/l; K2HPO4 0.56 g/l; NaCl0.14 g/l; CaCl 2 H2O 0.08 g/l and MgSO4 7 H2O 0.40 g/l)and mixed vigorously to ensure oxygen saturation. Theoxygen concentration within the sludge was measured withoxygen probe (WTW Tri-Oximatic 300 with printer). Theratio was adjusted such that the oxygen uptake rate wasbetween 0.5 and 2 mg/l/min. After dilution, the effectiveOUR of the sludge was determined according to the dilu-tion factor. The synthetic wastewater employed in thisstudy was 20 times more concentrated than the speciÞedlimits mentioned in the procedure DIN 38414 S6, and thiswas done to ensure that the OUR of the microorganismswas not limited by lack of substrates.

Fig. 1 Schematic diagram ofhigh pressure homogenizer

Inßuence of sludge disintegration by high pressure homogenizer

123

Degree of soluble COD release (DDCOD)

DDCOD can be determined by normal COD analysis. Thesludge samples were centrifuged at 10,0009g and thesupernatant is Þltered (under vacuum) using cellulosenitrate membrane of pore size 0.45lm. The Þltered liquidis subjected to COD analysis as per the standard procedure(ALPHA and AWWA 1999). The degree of disintegrationby soluble COD release (DDCOD) can be determined by thefollowing equation.

DDCOD½%� ¼CODtr � CODut

CODNaoH � CODut

� �� 100 ð2Þ

where CODNaOH is the maximum release of COD wasdetermined by alkali digestion method and CODut, CODtr

are the COD of untreated and treated sludge.

Description of respirometer for continuous OURmeasurements

The respirometric setup was used in these studies, which issimilar to the setup reported previously by Vollertsen andHvitved-Jacobsen (1998). The experimental set up tomonitor the continuous oxygen uptake rate measurementswere shown in Fig.3. The procedures adopted are as fol-lows: The experiments were performed in 2.5 l capacitycylindrical aerated reactor (100 mm diameter, 355 mmheight) and the working volume of the reactor is 1.8 l. Thereactors are stirred at 300 rpm by means of a motor (IKAEurostar 130W) equipped with pitched blades, 60 mmdiameter. The reactor is jacketed vessel and the tempera-ture of the sludge is maintained at 20� C (±2� C). pHcontrollers are installed in both reactors to maintain pHbetween 7 and 7.5 The sludge from the reactor is pumpedinto a respirometric chamber (cylindrical, capacity of0.08 l, stirred and air tight vessel) for oxygen uptake rate

measurement. The dissolved oxygen concentrations weremeasured in the respirometric chamber by means of sensor(model cell-ox 325, WTW make, Germany) attached to anoxygen meter (model oxi 340 i). The oxygen meters areconnected to a computer for continuous monitoring as wellas data logging. SufÞcient aeration was given through aquapump and the oxygen concentration was maintained athigher side ([5 mg/l) so that the microorganisms will notsuffer lack of oxygen during substrate degradation(Mathieu and Etienne2000; Abbassi et al.2000). Duringmeasurement of OUR the pump circulation is stopped, noingression of air occurred. OUR can be calculated bymeasuring the decrease in dissolved oxygen concentrationover time. The equation is.

OUR ¼ �d½o2�

dtð3Þ

The vessel is automatically re-aerated for each newOUR measurements.

Biomass estimation

The samples collected were centrifuged at 10,000 rpm for5 min and the supernatant was decanted and the biomasswere separated. The harvested biomass were freed fromsoluble salts, nutrients and waste products by washing themthrice with sterile water. Dry weights were recorded after24 h drying at 80� C and difference between dry and emptyweights yielded the amount of biomass produced/reduced.

OUR Measurements

The sludge containing TS of 3.1Ð3.5 g/kg was taken fromthe outlet of the aeration tank in wastewater treatment plantfor OUR measurement studies. Prior to measuring theinitial OUR proÞle, 20 mg/l of Allyl thio urea (Merck,

Magnetic stirrer

aeration

Oxygen measuring cellwith sensor

Oxygen measuring instrumentprinter

Sludge sample

Coolant out

Coolant in

Thermostat

Fig. 2 Experimental set up todetermine the Degree ofinactivation (DDO)

C. L. Rai, P. G. Rao

123

Germany) is added to the sludge to inhibit nitriÞcation (toarrest oxygen consumption as a result of nitriÞcation). Thesludge sample is aerated for 12 h to degrade all degradablesubstrates which are already present in the sludge. In orderto control foaming, antifoaming agent 0.7 ml/l (AGITAN301, Munzing chemicals, Germany) is added to the reactorsand the respiration rate is measured. This respiration rate istermed as OURendogenous. Then the sludge is removed fromof the reactor and treated with HPH. Subsequent to HPHtreatment, the samples for DDO, COD release analyseswere taken.

The treated sludge was charged back into the reactor(ensuring that the sludge volume is equal in both thereactors) and a fresh synthetic wastewater, according tothe modiÞed speciÞcations laid out by OECD (Ginestet2000, Method for assessing process efÞciency, CentreInternational de Recherche Sur IÕ Eau et IÕEnvironnement(CIRSEE), Suez-Lyonnaise des Eaux: Le PecQ Surseine,France) is added as a substrate in a concentrated solutionof 25 ml in both reactors. The composition of the syn-thetic wastewater comprises of the components sodiumacetate 0.79 g, peptone 0.79 g, meat extract 0.5 g (NH4)2

SO4 0.9 g, NaCl 0.09 g, CaCl2 0.05 g, MgSO4�7H2O0.03 g and K2HPO4 0.35 g. These items are mixedhomogeneously in 65 ml of de-ionized water. The

substrate COD concentration is found to vary between400 and 448 mg/l from batch to batch. After the additionof substrate the respiration were measured for the deg-radation process.

The time, at which the substrate added to both thereactors is referred ast1, and the time at the end of thedegradation process (i.e., endogenous phase) is referred toas t2. The COD of the sludge liquid was analyzed beforethe addition of substrate and also after reaching theendogenous phase. Thus from the difference in CODÕs, theamount of degraded or removed COD can be calculated(i.e. CODinitial ? CODsubstrate- CODend). In addition,sludge total solids (TS) and organic total solids (oTS)before t1 and t2, after OUR measurements (20Ð22 h) areweighed for both treated and untreated sludge to estimatethe difference in solid contents. This was done based onstandards methods for the examination of water andwastewater, ALPHA and AWWA (1999).

SpeciÞc mechanical energy (Espec)

The speciÞc energy provides information about the energyinput for the disintegration process to achieve degree ofdisintegration. The deÞnition of speciÞc energy is theenergy input related to treated solid mass. This can be

Signals to Hcl peristaltic pumpSignals to NaOH peristaltic pump

pH Indicator/Controller

To Air pump To Air pump

Oxygen meter

Pump Pump

Rea

ctor

Computer

Motor /stirrer Motor/stirrer

Rea

ctor

Res

pira

tion

cham

ber

Res

pira

tion

cham

ber

Fig. 3 Experimental set ofsequenced respirometers forcontinuous oxygen uptake ratemeasurements

Inßuence of sludge disintegration by high pressure homogenizer

123

found out by measuring the volume of the sludge sample Vand the solid content TS. SpeciÞc energy can be deter-mined by employing the equation.

Espec¼Dp

TSð4Þ

where DP is difference in pressure (i.e.Pabsolute-

Patmospheric).

Estimation of heterotrophic conversion yield (YH)

Heterotrophic conversion yield (YH) is an importantparameter for assessing the potential effect of stressapplication on cellular maintenance. Hypothetically, if themicrobial cell maintenance is affected, it is expected thatthe energy required for non-growth reactions will increasewith simultaneous increase in substrate conversion intocarbon dioxide (in association with oxygen). Therefore,assessing the conversion of a known amount of biode-gradable substrate into carbon dioxide, will allow theestimation of the conversion yield (YH). This method relieson evaluating the area of the curve under the OUR,excluding the fraction associated with endogenous respi-ration. The similar procedure is adapted by the previousresearchers (Orhon and Okutman2003). Using Eq. (13),the conversion yield is calculated.

YH ¼ 1 �

Rt2t1

ðOURtotal � OURendoÞdt

CODremovedð5Þ

Sludge growth reduction (DYH)

The sludge growth reduction can be calculated based onconversion yields and done by the following equation.

DYH½%� ¼ 1 �YHðtrÞ

YHðutÞ

� �ð6Þ

subscripts (tr) and (ut) stands for treated and untreatedsludge.

Result and discussions

Inßuence of energy input on particle size

The extent of disintegration on particle size and its distri-bution are shown in Figs.4 and 5. It can be seen fromFigures as the energy increases the size reduces. It isobserved that the major particle size reduction taken placeat lower energy input of 3,563 kJ/kg and further increase inenergy input from 14,252 to 17,815 kJ/kg results marginalchanges in size as well in distribution.

Degree of disintegration with high pressurehomogenizer

Figure6 shows the inactivation efÞciency (DDO) of HPHdisintegration. It is seen that the inactivation increases asthe energy input increases. It is also observed that thesludge inactivation efÞciency increased sharply from 53 to77% for the speciÞc energy of 3,563Ð7,126 kJ/kg, andfurther increase in energy from 1,0689 to 17,815 kJ/kgresults 84Ð98% inactivation. Similarly soluble CODincreased steadily as the input increases. At the lowerenergy input of 3,563 kJ/kg 8% of COD release and furtherincrease in energy from 7,126 to 17,815 kJ/kg 18Ð29% ofCOD release is observed. These might be due to release ofextracellular polymeric substances (EPS) during ßoc

1 10 100 1000-20

0

20

40

60

80

100

Cum

mul

ativ

e di

strib

utio

n [%

]

Particle size [µm]

Operating pressure [bar] Untreated 150 300 450 600 750

Fig. 4 Particle size distribution as a function of speciÞc energy

0 2000 4000 6000 8000 10000120001400016000180002000040

60

80

100

120

140

160

180

200

220

240

Med

ian

part

icle

siz

e (X

50,3)

[µm

]

Specific energy [kJ/kg]

Fig. 5 Median particle size as a function of speciÞc energy of HPH

C. L. Rai, P. G. Rao

123

disintegration and the release of cell components duringcell lysis.

Sludge microbial growth reduction

A typical respirograms of untreated and HPH treatedsludge is compared in Figs.7, 8, 9 and 10 at differentspeciÞc energy inputs/different operating pressures (4,365,6,926, 8,553, 10,698 kJ/kg). It is observed that as thedegradation process proceeds, oxygen consumption rate(OUR) Þrst increases and later decreases to the initial valuein untreated sludge whereas in treated sludge (200, 300 400and 500 bars) the enhancement of OUR starts at from 100to 400 min. This time difference at different pressures isprobably due to time required for the microorganisms toacclimate to stress. Finally, in HPH treated sludge, moreOUR is registered when compared to that of untreated

sludge. This can be conÞrmed by estimating the area ofOURendogenousfrom OURtotal. It is found that the area ofOURexogenousfor the treated sludge is higher when com-pared to untreated sludge. The conversion yield iscalculated employing Eq. (5) and the data is summarized inTable1. Table shows that on increasing speciÞc energy theyield coefÞcient (YH) for the treated sludge decreases from0.05 to 0.027 for the speciÞc energies 4,365Ð10,698 kJ/kg.Further increase in speciÞc energy leads to excessivefoaming and OUR proÞle could not be studied.

It can be stated at this stage that reduction in sludgegrowth is primarily due to maintenance energy require-ments of the microbial cells due to HPH disintegration.During the treatment, the sludge microbial ßoc has to passthrough the narrow gap and due to pressure difference(hydrodynamic cavitation) stress acts on the microbial cellsresulting ßoc rupture, increase in soluble COD release and

2000 4000 6000 8000 10000 12000 14000 16000 18000

10

20

30

40

50

60

70

80

90

100D

egre

e of

dis

inte

grat

ion

[%]

Specific energy [kJ/kg]

Inactivation Soluble COD release

Fig. 6 Degree of disintegration with respect to speciÞc energy

0 200 400 600 800 1000 1200 1400

0.5

1.0

1.5

2.0

2.5

OU

R [m

g/l/m

in]

Time [min]

HPH Untreated Treated [200 bar]

Fig. 7 OUR proÞle for the sludge treated at 4,365 kJ/kg

0 200 400 600 800 1000 1200 1400

0.5

1.0

1.5

2.0

2.5

OU

R [m

g/l/m

in]

Time [min]

HPH Untreated Treated [300 bar]

Fig. 8 OUR proÞle for the sludge treated at 6,926 kJ/kg

0 200 400 600 800 1000 1200 1400

0.5

1.0

1.5

2.0

2.5

OU

R [m

g/l/m

in]

Time [min]

HPH Untreated Treated [400 bar]

Fig. 9 OUR proÞle for the sludge treated at 8,553 kJ/kg

Inßuence of sludge disintegration by high pressure homogenizer

123

inactivation occurs and these phenomenons stimulate toenhance the aerobic oxidation process. The enhancement inthe aerobic oxidation is mainly due to increase in mainte-nance energy requirements, as a result of which substratetakes place competitively for its energy maintenance,rather than for reproduction. Thus, HPH appears to be notas the most efÞcient but also an energy efÞcient process asstated by Save et al.1997. At a lower speciÞc energy inputi.e. 1,927 kJ/kg, a maximum sludge growth reduction of91% is observed. The respirometric study of HPH treat-ment method shows that on increasing the speciÞc energy,the YH of treated sludge decreases leading to an increasingsludge growth reduction.

Figure11 shows the sludge microbial growth reduction(calculated by Eq. (6)) as a function of speciÞc energy.Biomass growth retardation is observed in all the cases.This conÞrms that the sludge growth reduction mechanismunder high pressure homogenizer disintegration is mainlydue to increase in microbial maintenance energy require-ments. Also, it may be stated, decreased particle size of themicrobial ßoc can facilitate microorganism participation in

the metabolic activity. Though, increasing the maintenanceenergy requirements leads to reduction in sludge microbialgrowth was postulated before (Low and Chase1999a, b), ithas been proved in this study.

A growth reduction of 94% could be achieved (Table1)at a speciÞc energy of 10,698 kJ/kg. Thus, it has beenproved HPH pretreatment in WWTP would reduce thesludge growth to an extent.

Inßuence of DDO and soluble COD releaseon microbial sludge growth

Sludge biomass inactivation is one of the main parametersfor sludge growth reduction which is done by ßoc ruptureand stressing the organisms by damaging or total rupture ofthe cell.

Figure12 shows the reduction of microbial growth as afunction of inactivation efÞciency. It was observed thatincrease in the degree of inactivation leads to increase ingrowth reduction. At 45% inactivation a growth reductionto an extent of 92% is achieved. This may be due to eachmicroorganism undergo stress (cavitation and turbulence)by passing through the narrow gap and impacts in HPH.Further increase in inactivation up to 86% results only amarginal change. At this stage it may be concluded that, inthis case, even 45% inactivation is enough to achieve majorgrowth reduction.

In sludge microbial growth reduction sludge microbialinactivation alone is not sufÞcient but also release of sol-uble COD (i.e. cell rupture), also plays a major role. InFig. 12 growth reduction as a function of soluble CODrelease is shown. It is observed that 92% of sludge growthreduction occurs at 4.5% COD release. Further, release of10.7Ð11.6% COD enhances the growth reduction to max-imum value of 94%. Also, it was observed that at 12.5% ofCOD release there is a decrease in sludge growth reduction.This may be due to excess rupture of microbial cells andthereby release of more COD or it may not be easilydegradable within the respirometric experimental time. In

0 200 400 600 800 1000 1200 1400

0.5

1.0

1.5

2.0

2.5 O

UR

[mg/

l/min

]

Time [min]

HPH Untreated Treated [500 bar]

Fig. 10 OUR proÞle for the sludge treated at 10,698 kJ/kg

Table 1 Treatment parameters of the sludge by HPH

Operatingpressure (bar)

SpeciÞc energy,Espec(kJ/kg)

Sludgecategory

CODinitial

(mg/l)DDO

(%)DDCOD

(%)CODÞnal

(mg/l)COD removed(mg/l)

YH

(-)Growth reduction,DYH (%)

200 4,365 Untreated 73 Ð Ð 72 399 0.55 91

Treated 199 45 4.5 173 424 0.052

300 6,752 Untreated 73 Ð Ð 76 440 0.48 92

Treated 329 78 10.7 237 552 0.038

400 8,553 Untreated 74 Ð Ð 68 446 0.55 84

Treated 397 75.8 12.5 212 608 0.087

500 10,698 Untreated 66 Ð Ð 62 422 0.45 94

Treated 358 86.3 11.6 226 596 0.027

C. L. Rai, P. G. Rao

123

this study, it shows that at moderate energy input of 6,926Ð10,698 kJ/kg, with the soluble COD release of 10.7Ð11.6%is required for achieving maximum sludge microbialgrowth reduction of 94% by HPH treatment. From thisÞgure a comparison of soluble COD release and degree ofinactivation suggests that COD release is sought for sludgemicrobial growth reduction rather than inactivation.

Reduction of organic total solids

The untreated and treated sludge samples of OUR studies(before and after) were analyzed for oTS employinggravimetric method. In Fig.13 the ratio of the organic totalsolids is plotted against speciÞc energy input. It is observedfrom the Þgure that there is a mass reduction by breakdownof the organic matters that gets degraded and changes of

cell viabilities by disintegration. That may be the reason,decrease in oTS is observed with increase in speciÞcenergy inputs.

Commercial viability of the process

The study with HPH as a disintegrating device to enhancethe sludge microbial growth reduction was expected andappears to be a viable commercial process. Figure14depicts the inclusion of HPH as an operational unit in thewastewater treatment plant for sludge disintegration. Thesludge that will be accumulated in the clariÞer can bereturned through HPH in the aerobic basin. The operationcan be routinely carried out to minimize the excess sludgebiomass production. Incorporation of HPH is a onetimeinvestment, which has long term beneÞts in the overalltreatment of the wastewater and the sludge.

Conclusions

Through the present investigation, it has been demonstratedthat HPH can be incorporated as one of the unit operation

4000 5000 6000 7000 8000 9000 10000 11000

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86

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90

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100M

icro

bial

gro

wth

red

uctio

n [%

]

Specific energy [kJ/kg]

Fig. 11 Microbial growth reduction as a function of speciÞc energy

4 50 55 60 65 70 75 80 85 90

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Degree of soluble COD release [%]

Mic

robi

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row

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[%]

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th r

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Growth reduction by Degree of inactivation

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Soluble COD release

Fig. 12 Inßuence of degree of inactivation and soluble COD releaseon microbial growth reduction

4000 5000 6000 7000 8000 9000 10000 11000

0.954

0.956

0.958

0.960

0.962

0.964

0.966

0.968

0.970

Org

anic

tota

l sol

ids

[oT

Str/o

TS

ut]

Specific energy [kJ/kg]

Fig. 13 Ratio of organic solids as a function of speciÞc energy

BiologicalWastewater treatmentPre-sedimentation Clarifier

Effluent

High pressure homogenizerdisintegration

Treated effluent

Return sludge

Excess sludge

Fig. 14 Incorporation of HPH as an operational unit in the waste-water treatment plant for sludge disintegration

Inßuence of sludge disintegration by high pressure homogenizer

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on activated sludge treatment processes for reducing excessmicrobial growth. The sludge microbial growth reductionhas been substantiated by estimating the conversion yieldand OUR. At a highest value of speciÞc energy input10,698 kJ/kg, the ßoc rupture, cell damage and cell ruptureoccurred simultaneously leading to the reduction ofmicrobial growth. With increased applied energy/pressureincreased release of organic matter was observed withfewer surviving cells, thus extend the lag time andincreasing the maximum oxygen consumption. It ishypothesized that the above speciÞc manipulation results inincreasing the cellular maintenance of micro-organismsand hence the overall oxidative capacity. In addition par-ticle size was also measured to determine the extent of ßocrupture.

Acknowledgments The author wish to thank Dr.-Ing. JohannesMueller for his help and suggestions during this work. Thanks are due toDr. A.B. Mandal, Director, CLRI for permission to publish this work,Head, Chemical Engineering Division and Dr. M. Surianarayanan fortheir useful discussions.

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