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Effect of disintegration pretreatment of sewage sludge for enhanced anaerobic digestion

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A sludge disintegration technology, presented in this paper, consists of pretreatment of the sludge before its anaerobic digestion. Disintegration of sludge has been recently incorporated into the wastewater treatment plant's (WWTPs) process scheme in some countries to improve a hydrolysis phase of digestion. Better energy recovery is of crucial importance in the overall sludge disposal. This technology intensifies biogas production and improves the quality of digested sludge. The authors present results of their experiments, performed on sludge samples taken from the Gdańsk WWTP, Poland. During experiments mixture of primary and waste activated sludge (WAS) taken from the aerobic bioreactor was used. The sludge was disintegrated mechanically in a laboratory scale by using a ultra-sound generator Hielsher, Germany (the frequency was 24 kHz). The concentration of chemical oxygen demand (COD) was obtained according to standard methods and the degree of sludge disintegration (DD) in supernatant was calculated. The anaerobic digestion experiment was carried out in laboratory scale using two reactors (volume ca. 30 dm 3 of each), with hydraulic retention time (HRT) of 15 days and under mesophilic conditions. The quantity and quality of produced biogas was measured. Results showed 20% increase of biogas and 10% increase of methane pro-duced generated during anaerobic digestion process of disintegrated sludge compare to untreated one.
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ENVIRONMENTAL ENGINEERING
The 8
th
International Conference
May 19–20, 2011, Vilnius, Lithuania
Selected papers
ISSN 2029-7106 print / ISSN 2029-7092 online
ISBN 978-9955-28-828-2 (2 Volume)
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© Vilnius Gediminas Technical University, 2011
679
EFFECT OF DISINTEGRATION PRETREATMENT OF SEWAGE SLUDGE FOR
ENHANCED ANAEROBIC DIGESTION
Renata Tomczak-Wandzel
1
, Svetlana Ofverstrom
2
, Regimantas Dauknys
3
, Krystyna Mędrzycka
4
1, 4
Gdansk University of Technology, Narutowicza str. 11/12, 80-233 Gdansk, Poland.
2, 3
Vilnius Gediminas technical university, Saulėtekio ave. 11, LT-10223 Vilnius, Lithuania.
E-mails:
1
nata@chem.pg.gda.pl;
2
svetlana.ofverstrom@vgtu.lt;
3
regimantas.dauknys@vgtu.lt;
4
krystyna@chem.pg.gda.pl
Abstract. A sludge disintegration technology, presented in this paper, consists of pretreatment of the sludge before its
anaerobic digestion. Disintegration of sludge has been recently incorporated into the wastewater treatment plant’s
(WWTPs) process scheme in some countries to improve a hydrolysis phase of digestion. Better energy recovery is of
crucial importance in the overall sludge disposal. This technology intensifies biogas production and improves the
quality of digested sludge. The authors present results of their experiments, performed on sludge samples taken from
the Gdańsk WWTP, Poland. During experiments mixture of primary and waste activated sludge (WAS) taken from
the aerobic bioreactor was used. The sludge was disintegrated mechanically in a laboratory scale by using a ultra-
sound generator Hielsher, Germany (the frequency was 24 kHz). The concentration of chemical oxygen demand
(COD) was obtained according to standard methods and the degree of sludge disintegration (DD) in supernatant was
calculated. The anaerobic digestion experiment was carried out in laboratory scale using two reactors (volume ca. 30
dm
3
of each), with hydraulic retention time (HRT) of 15 days and under mesophilic conditions. The quantity and
quality of produced biogas was measured. Results showed 20% increase of biogas and 10% increase of methane pro-
duced generated during anaerobic digestion process of disintegrated sludge compare to untreated one.
Keywords: biogas, COD, DD, disintegration, sludge digestion, sludge processing, ultrasound.
1. Introduction
Recently, serious efforts to reduce the volume of
sludge requiring treatment and final disposal have been
undertaken. The anaerobic digestion process has been
extensively studied during the past 20 years and various
methods for process improvement have been explored.
The investigations concentrated on better sludge hydro-
lyzation by, for example, thermal pretreatment, chemical
solubilization by acid or base addition and mechanical
sludge disintegration. During hydrolysis, a substantial
fraction of particulates can be separated and decomposed
to soluble and less complex forms. Municipal wastewater
sludge, particularly waste activated sludge (WAS), is
more difficult to digest than primary solids due to a rate-
limiting cell lysis step. Physical pretreatment, particularly
ultrasonics, is emerging as a popular method for WAS
disintegration (Khanal et al. 2007, Wei et al. 2003, Wang
et al. 2005, Yoon and Lee 2005, Gronroos et al. 2005).
In order to enhance the performance of anaerobic
digesters, ultrasound can be used do disintegrate waste
activated (WAS) sludge before it is fed to the digester
(Neis et al. 2008).
This increases the anaerobic digestion efficiency
and, as a consequence, increases the volume of biogas
produced while at the same time reducing the volume of
residual sludge. Another possible use an organic matter
obtained from a disintegrated WAS as a source of easy
biodegradable carbon for denitrification process. (Müller
2000). The third application is disintegration of filamen-
tous bacteria in the bulked sludge. This method can be a
very useful way of sludge bulking or foaming minimiza-
tion and control (Wünsch et al. 1993). The application of
disintegration is especially useful for excess sludge be-
cause of its high content of micro-organisms (Müller,
2000). Ultrasound of high acoustic intensities causes
cavitation in water bodies, if the energy applied exceeds
the binding energy of the molecular attractive forces
(Suslick 1988). During sound oscillation the local pres-
sure in the aqueous phase falls below the evaporating
pressure resulting in the explosive formation of micro-
scopic bubbles. These bubbles oscillate in the sound field
over several oscillation periods and grow by a process
termed rectified diffusion. The following implosion of the
gas and vapour filled bubbles leads to high mechanical
shear forces which are apt to disintegrate bacterial cell
680
material. Thus ultrasonic treatment is a suitable method to
disintegrate sewage
sludge and to overcome the slow
biological sludge hydrolysis.
For research, ultrasound have widely been applied as
pretreatment of anaerobic digestion; main results of pre-
treatment of mixture of primary and waste activated
sludge are presented in Table 1.
Table 1. Impact of disintegration pretreatment of mixture of
primary and WAS
Reference Treatment
conditions
Anaerobic
digestion
conditions
Results
Tiehm et
al. 1997
31 kHz, 3.6
kW, 64 s
Continuous,
HRT: 22
days, 37°C
Increase of
VS removal
from 45.8%
to 50.3%
(+9%)
Wang et al.
1999
9 kHz, 200
W, 30 min
Batch, 11
days, 36°C
Increase of
CH
4
produc-
tion from
210 to
345mL/gVS
in
(64%)
Bien et al.
2004
20 kHz,
180 W, 60 s
Batch, 28
days, 36°C
Increase of
biogas pro-
duction
(+24%)
Xie et al.
2007
20 kHz
W/cm
2
, 1.5
s
5000 m
3
egg-shape
digester
HRT: 22.5
days, 29-
33°C
Increase of
biogas pro-
duction
(+45%)
Source: adapted from Carrère et al.; 2010
There are two key mechanisms associated with ul-
trasonic treatment; cavitation, which is favored at low
frequencies, and chemical reactions due to the formation
of OH
-
, HO
2
, H
+
radicals at high frequencies. (Carrère et
al.; 2010). Ultrasound frequencies range from 20 kHz to
10 MHz. Better sludge disintegration has been reported at
a lower frequency range of 20 to 40 kHz (Tiehm et al.;
2001, Khanal et al. 2007, Carrère et al.; 2010).
If one considers that sludge disintegration is carried
out mostly to intensify sludge treatment and enhance bio-
gas production during anaerobic digestion, it is very im-
portant to evaluate the disintegration degree.
The sludge disintegration process can be described by
the particle size analysis. An increase of the energy input
leads to a decrease of the floc size (Lehne et al.2001). At
the Fig1 and Fig 2 the changes in floc size before and
after ultrasound disintegration are presented. Ultrasonic
disintegration of activated sludge process resulted more
dispersed and homogenous flocks of activated sludge
(Cimochowicz-Rybicka and Tomczak-Wandzel 2008).
The disintegration degree (DD) of sewage sludge, in-
cluding bacteria cell lysis, can be evaluated using two
methods: analysis of oxygen consumption by bacteria and
analysis of organic compounds concentration in super-
natant (expressed as COD and protein content) (Lehne et
al. 2001).
Fig 1. Activated sludge before ultrasonic disintegration
Fig 2. Activated sludge after ultrasonic disintegration
The calculation of energy-consumption and cost
shows that disintegration processing of sludge could be
realized economically. The excess biogas could be used
for electricity generation. The investment for the disinte-
gration equipment has to be seen in relation to the re-
duced costs for the sludge disposal. However, since 2003-
2004 a number of attempts to introduce sludge disintegra-
tion have been carried out. An example of a full scale
application is the WWTP in Leinetal, Darmstadt-
Eberstadt (Germany), Fullinsdorf (Switzerland), Kec-
skemet and Zalaegerszeg (Hungary), Bamberg (Ger-
many) (Nickel 2005; Eder 2005; Neis et al. 2008).
The objective of the study was to examine the modi-
fication of mixture of primary and waste activated sludge
characteristics due to ultrasonic pretreatment and its ef-
fect on the anaerobic digestion process under mesophilic
conditions. The effect of this pretreatment was evaluated
at lab-scale anaerobic digesters.
681
2. Materials and methods
Sludge characterization
During this research, mixture of sludge (primary and
WAS) was collected from the municipal WWTP Gdańsk
WSCHÓD, Poland. This treatment plant serves about
760.000 population equivalents. Such type of sludge is
directly fed to the anaerobic digestion. At this WWTP a
huge amount of excess sludge is generated and not effi-
cient amount of biogas is produced. Thus, improvement
of the sludge treatment efficiency is necessary. One of the
options is disintegration of the sludge before its anaerobic
digestion.
Utrasound application
The sludge was disintegrated mechanically in labo-
ratory scale by using a 200 W ultrasound generator UP
200S, Hielscher company, Germany (the frequency was
24 kH and the sonification time lasted 5 min).
Examination of sludge disintegration
The DD can be determined by the measuring the spe-
cific oxygen consumption OC
d
by disintegrated sludge in
relation to the specific oxygen consumption OC
0
of the
original sludge. The oxygen consumption is directly de-
pendent on the metabolism of aerobic microorganisms. If
all bacteria in the sludge are disrupted the oxygen con-
sumption of the sludge is zero and the DD reaches 100%
(Kopp et al. 1997). Another method of measuring the DD
uses COD analysis. Based on this method the organic
material released from the cells by their mechanical dis-
ruption can be determined. In the current research, the
COD content was used for determination of disintegration
degree. The selected analytical method is simple and,
based on their results, one can learn about the increase
in
content of easily available substrate for fermentation bac-
teria in the supernatant.
DD was calculated by determining the chemical oxy-
gen demand (COD) in sludge supernatant (equation 1). A
reference value i.e. the 100% disintegration degree was
defined as the COD of supernatant obtained from sludge
treated with 0.5 mol/L sodium hydroxide for 22 hours at
20
o
C (Müller et al.1998, Gonze et al.2003, Benabdallah
El-Hadj et al. 2007).
%100
=
i
COD
a
COD
i
COD
d
COD
DD
(1)
where: COD
d
- COD of the centrate of the disintegrated
sludge sample, mg/L; COD
i
- initial COD of the centrate
of the original sludge sample (untreated sample), mg/L;
COD
a
- the maximum value of COD, which can be ob-
tained in the supernatant after alkaline hydrolysis of the
sludge; (chemical disintegration with NaOH), mg/L.
Anaerobic sludge digestion
The anaerobic digestion process was carried out in a
semi-laboratory scale. The volume of each of two an-
aerobic reactors was 30 L (reactor D - sludge after disin-
tegration, reactor R – untreated sludge - reference sam-
ple), HRT 15 days. The reactors were used in parallel and
were equipped with:
a. Thermal insulation (oil blanket and heater) to
keep the temperature in 37
0
C.
b. Mechanical stirrer – to maintain the good mixing
in whole volume of reactor.
The seeding sludge (inoculum) was collected from
previous anaerobic digestion test. Same volume (ca. 1 L)
of inoculum was added into the both reactors. Reactors
were fed once and in parallel. The disintegrated sludge in
reactor D was mixed with untreated sludge in ratio
50:50%. This ratio was selected according to proportion
of disintegrated and the raw sludge at the real WWTPs
with integrated disintegration process.
Analytical procedures
During anaerobic digestion the volume and compo-
sition (methane (CH
4
), carbon dioxide (CO
2
) and hydro-
gen sulphide (H
2
S)) of generated biogas were measured
daily by using Gas Analyzers GFM400. The chemical
oxygen demand (COD) was determinated according to
Polish and EU standard (PN-ISO 6060…). Determination
of dry mass was made according to Standard Methods.
Samples were dried to constant mass in water bath and
than in a thermostatically controlled oven at (105
0
C ± 5)
with forced air ventilation. Dry mass is expressed in g/kg.
3. Results and discussion
The disintegration degree (DD) of sewage sludge af-
ter sonification reached 27%. The disintegration degree
(DD) enables an evaluation of the maximum level of
sludge solubilization. Increased DD is determined as the
substance that can be readily used to produce methane
during anaerobic digestion (Wang et al. 2005, Erden and
Filibeli, 2010).
Initially the concentration of COD was completely
different due to disintegration process (disintegrated
sludge (reactor D) 2520 mgO
2
/dm
3
; untreated sludge
(rector R) - 530 mgO
2
/dm
3
). Application of ultrasound
disintegration resulted in further destruction of cell walls
and increased release of organic matter. During anaerobic
digestion COD changed significantly. The highest in-
crease of COD (from 530 to 2590 mgO
2
/dm
3
) was ob-
served for the untreated sludge. At the beginning the
COD of disintegrated sludge increased to 3700 mgO
2
/dm
3
on 6th day of the process, than the investigated parameter
decreased to 3100 mgO
2
/dm
3
. In both supernatants the
concentration of COD after 15 days of fermentation was
still very high.
Fig 3 shows COD content in supernatant obtained
from sludge during anaerobic digestion process.
682
Fig 3. Changes of COD in supernatant during anaerobic
anaerobic digestion process (reactor D – sludge after disin-
tegration, reactor R- raw sludge)
The changes of dry mass content during fermenta-
tion process are presented in Fig 4.
Fig 4. Changes of dry mass content during anaerobic di-
gestion process (reactor D – sludge after disintegration,
reactor R- raw sludge)
The decreasing of dry mass is a result of decomposi-
tion of organic substances during fermentation process
and is an effect of biogas production. The course of de-
composing organic matter in both sludges was quite simi-
lar. However, finally after 15 days of the anaerobic proc-
ess the decreasing of dry mass in disintegrated sludge
ranged 49.3%. In untreated sludge reduction of dry mass
was in lesser degree only 37.7%. At the end the differ-
ence between investigated sludges was about 12%. It is
significant amount of residual sludge which still remain
after anaerobic digestion to the final utilization
.
The volume of biogas during 15 days of anaerobic
digestion process is presented in Fig 5. The bigesst
amount of biogas in both reactors was produced in a few
first days of anaerobic digestion. The biogas was more
intensively generated in reactor with disintegrated sludge
due to high content of easy biodegradable substances
after sonification (COD was visibly highest). The total
volume of generated biogas from desintegrated sludge
was 20% (78 dm
3
) higher than from untreated sludge (60
dm
3
). The volume of produced biogas after 15 days of
experiment was very low in both reactors and analyses
were stopped.
Fig 5. The volume of biogas generated during anaerobic
digestion process (reactor D – sludge after disintegration,
reactor R- raw sludge)
The change of the methane content in biogas is pre-
sented in Fig 6. The analysis of biogas composition
shows that the obtained volume of methane from disinte-
grated sludge was higher during the total time of exami-
nation.
Fig 6. The volume of methane generated during anaerobic
digestion process (reactor D – sludge after disintegration,
reactor R- raw sludge)
The maximum content of methane was 62.4% in the
reactor with sludge after sonification, and in untreated
sludge the maximum content range 59.1%. In both cham-
bers this value was obtained after six days of anaerobic
digestion. Totally the volume of methane produced from
disintegrated sludge was about 10% higher than in un-
treated sludge. It is very important from the economic
point of view. It providing higher biogas yields and hence
increased power generation from biogas. The content of
carbon dioxide (CO
2
) from disintegrated sludge was
lower (34.0-43.3%) than from untreated sludge 35-46%).
683
Concentration of hydrogen sulphide (H
2
S) was similar in
both reactors and ranged from 2300 to 2900 ppm.
4. Conclusions
Disintegration of sludge will be an area with major
perspectives in the field of sewage sludge treatment. The
disintegration of sewage sludge by sonification destroys
the flocks’ structure of sludge and disrupts the cell walls
of the micro-organisms. Due to sludge disintegration,
organic compounds were transferred from the sludge
solids into the aqueous phase resulting in an enhanced
biodegradability. Our experiments have demonstrated
that ultrasound sludge disintegration could positively
affect sludge anaerobic digestion.
The advantages resulting from ultrasound disintegra-
tion of the sludge can be listed as follows:
- increase of sludge digestion efficiency;
- increase about 20% the volume of biogas pro-
duced;
- increase about 10% the methane content in bio-
gas;
- reduces the volume of residual sludge (12%).
Therefore disintegration of sewage sludge is a
promising method to enhance anaerobic digestion rates
and lead to reduce the volume of sludge digesters and the
time of process.
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... The direct effects are monitored based on the physicochemical changes of sludge characteristics (parameters) before and after pretreatment, i.a. pH [11,15], concentration of soluble chemical oxygen demand (SCOD) [3,16], biogenic substances [17,18] and extracellular polymeric substances [9,19] in sludge supernatant, capillary suction time (CST) [7,20] or microscopy examination of a flocs disruption [5,21]. In the case of anaerobic digestion the expected effects are: increase in biogas production, total solids (TS) and volatile solids (VS) reduction, as well as dewatering ability improvement [10,13]. ...
... The above results were in good agreement with other researchers, who stated, that ultrasonic pretreatment of sludge, increased VS reduction and biogas production by: 19% and 26%, respectively [46]. Whereas, the results of other investigation, indicated that ultrasonic pretreatment of sludge prior anaerobic digestion, ensured: 12% increase of TS content, compared to the control sample [21]. ...
... The above results were in good agreement with other researchers, who stated, that ultrasonic pretreatment of sludge, increased VS reduction and biogas production by: 19% and 26%, respectively [46]. Whereas, the results of other investigation, indicated that ultrasonic pretreatment of sludge prior anaerobic digestion, ensured: 12% increase of TS content, compared to the control sample [21]. The changes of CST values in the sludge mixtures after anaerobic digestion revealed, that the best effects of sludge dehydration were obtained for samples containing WAS, after pretreatment in the ultrasonic washer. ...
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... However, there are some discussions of how COD increase in AD system. For example, decreasing of TS/TFS such as carbonate, phosphate, iron etc. by chemolithotrophic bacteria such as Betaproteobacteria family for their cell growth [78], the accumulation of microbial cell lysis in the decay phase [79], and desorption of support media could possibly effect to higher COD observation. C/N ratio and organic matter (OM) of shrimp pond sediment were also analyzed and displayed in Table 1. ...
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... Badania wiele autorów[13,18,37,40,[42][43][44] wykazały, że sumaryczna ilość biogazu uzyskana w wyniku beztlenowej stabilizacji osadów nadmiernych, poddanych wstępnej dezintegracji ultradźwiękami wzrasta w zakresie od 20 do 50 [%] w porównaniu z osadami niedezintegrowanymi. Należy jednak podkreślić, że wielkość przyrostu biogazu w procesie stabilizacji jest zależna od typu i parametrów zastosowanego urządzenia oraz warunków prowadzenia procesu, tj. ...
... Wobec tego, można przyjąć, że zmiany struktury osadu (obserwowane pod mikroskopem) zachodzące pod wpływem oddziaływania fali ultradźwiękowej mogą być wykorzystane jako wskaźnik do oceny wielkości efektów bezpośrednich dezintegracji ultradźwiękowej. Przydatność analizy mikroskopowej osadów do oceny stopnia ich dezintegracji opisywali również inni badacze, którzy zaobserwowali, że wraz ze wzrostem czasu nadźwiękawiania lub ilości energii dostarczonej w procesie, wzrasta stopień rozdrobnienia fazy stałej osadu, w wyniku czego dochodzi do rozerwania ściany komórkowej mikroorganizmów[40][41][42][43]. Efekty analizy mikroskopowej osadów nadmiernych i nadmiernych zagęszczonych, w najkorzystniejszych warunkach prowadzenie procesu dezintegracji ultradźwiękowej przedstawiono na rysunkach 15 i 16. ...
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Celem przeprowadzonych badań było określenie wpływu warunków prowadzenia dezintegracji ultradźwiękowej osadów ściekowych na uzyskiwane w procesie efekty tj. zmianę charakterystyki osadów i cieczy osadowej oraz dyspergowanie fazy stałej. Nadźwiękawianiu poddawano osady nadmierne i nadmierne zagęszczone pochodzące z komunalnej oczyszczalni ścieków, znajdującej się na terenie południowej Polski. Proces prowadzono przy użyciu dezintegratora o stałej mocy znamionowej, wyposażonego w głowicę typy „sandwich” ze stożkowym koncentratorem energii (emiterem). Osady ściekowe poddawano dezintegracji falą o częstotliwości f=23 kHz, przy zmiennej geometrii w obszarze nadźwiękawiania. Badania prowadzono dla objętości osadu V1=0,3 i V2= 0,5 L. Odległość emitera od dna naczynia, w którym nadźwiękawiano osady wynosiła odpowiednio: 1 i 2 cm dla objętości próby V1 oraz 2 cm, 3 cm, 4cm dla objętości próby V2. Dezintegrację osadów prowadzono w określonym zakresie gęstości energii (EV), przeliczonej na energię właściwą (ES). Oceny efektów bezpośrednich dezintegracji dokonano w oparciu o wskaźnik zaproponowany przez Müllera oraz na podstawie przyrostu wartości chemicznego zapotrzebowanie na tlen (ChZT - oznaczonego metodą dwuchromianową) w cieczy osadowej po zakończeniu procesu. Analizie poddano również podatności osadów na odwanianie oraz stopień rozdrobnienia fazy stałej. Najkorzystniejsze efekty dezintegracji uzyskano podczas nadźwiękawiania prób osadów o objętości równej 0,3 L i położeniu emitera w odległości 1 cm od dna naczynia.
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The improvement of anaerobic digestion was investigated in an interdisciplinary research group. Using four different methods of mechanical cell disintegration the influence of the degree of disintegration and the digestion parameters on the performance of the anaerobic process was investigated. Analytical methods to describe the degree of cell-disruption had to be developed. The best results were obtained using a stirred ball mill and a high-pressure homogenizer. As a result of disintegration the degradation is accelerated and the digestion time can be reduced, especially when using immobilized micro-organisms. The treatment of digested sludge by ozonization respectively by mechanical disintegration led to an improved biodegradability of residual organic compounds. In a following second anaerobic process the treated sludge reached an even higher degree of degradation. On the other hand the disruption of the particle structure leads to an increase in polymer-demand and no improvement in dewatering results. Sludge water, returned to the aeration tanks, is slightly more polluted, especially the concentration of ammonia increases because of the better anaerobic digestion.
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Mechanical cell disintegration and its influence on anaerobic digestion was investigated using four different methods. Methods to describe the degree of cell-disruption were developed and the release of organic components into the sludge water was measured. The best results were optained using a stirred ball mill and a high-pressure homogenizer. The influence of disintegration rate and digestion time on the performance of the anaerobic process and the dewatering characteristics were investigated. The degradation is accelerated and the digestion time can be reduced, especially when using immobilised microorganisms. It could be shown that the mechanical disintegration results in a disruption of particle structure and an increase of polymer-demand. As a result of better anaerobic degradation the density and dewatering results of disintegrated sludges are improved in comparison to non-treated sludges.
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Municipal wastewater sludge, particularly waste activated sludge (WAS), is more difficult to digest than primary solids due to a rate-limiting cell lysis step. The cell wall and the membrane of prokaryotes are composed of complex organic materials such as peptidoglycan, teichoic acids, and complex polysaccharides, which are not readily biodegradable. Physical pretreatment, particularly ultrasonics, is emerging as a popular method for WAS disintegration. The exposure of the microbial cells to ultrasound energy ruptures the cell wall and membrane and releases the intracellular organics in the bulk solution, which enhances the overall digestibility. This review article summarizes the major findings of ultrasonic application in WAS disintegration, and elucidates the impacts of sonic treatment on both aerobic and anaerobic digestion. This review also touches on some basics of ultrasonics, different methods of quantifying ultrasonic efficacy, and some engineering aspects of ultrasonics as applied to biological sludge disintegration. The review aims to advance the understanding of ultrasound sludge disintegration and outlines the future research direction. There is general agreement that ultrasonic density is more important than sonication time for efficient sludge disintegration. Published studies showed as much as 40% improvement in solubilization of WAS following ultrasonic pretreatment. Based on kinetic models, ultrasonic disintegration was impacted in the order: sludge pH > sludge concentration > ultrasonic intensity > ultrasonic density. Both laboratory and full-scale studies showed that the integration of an ultrasonic system to the anaerobic digester improved the anaerobic digestibility significantly.
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Fields of application of the mechanical sewage sludge disintegration are presented. Various methods of mechanical cell disintegration are described and the obtained results are compared. All methods are able to break up the flocs, but only some of them provide enough energy for the disruption of micro- organisms. The possible improvement of the anaerobic degradation process using raw sludge, excess sludge and digested sludge is shown. Excess sludge proves to be the most suitable. The use of disintegrated sludge as a carbon source for the denitrification is explained as a second field of application. External carbon sources can be substituted and the amount of sewage sludge to be disposed of is reduced at the same time. Foaming in digesters caused by filamentous micro-organisms can be reduced by disintegration of these sludges. Adsorbed gas bubbles are released and the settling properties of the sludge are improved. Another field of application is the solution of organic components of the sludge solids. The increased concentration of nitrogen and phosphorous in the supernatant causes recovery processes to be operated at a higher cost efficiency. A comparison of effort and advantages shows that disintegration is especially useful if there are problems in the sludge treatment process of the WTP. Keywords Pretreatment; mechanical disintegration; cell-disruption; scum-destruction; excess sludge; biodegradability; denitrification; carbon source; recycling