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Improvement of activated sludge stabilisation and filterability during anaerobic

digestion by fruit and vegetable waste addition

Lahdheb Habiba *, Bouallagui Hassib, Hamdi Moktar

Laboratory of Microbial Ecology and Technology, National Institute of Applied Science and Technology, BP 676, Tunis, Tunisie 1080, Tunisia

a r t i c l e i n f o

Article history:

Received 16 June 2008Received in revised form 9 September 2008

Accepted 10 September 2008

Available online 5 November 2008

Keywords:

Anaerobic co-digestion

Activated sludge

Fruit and vegetable waste

Sequencing batch reactor

Filterability

a b s t r a c t

Anaerobic co-digestion of fruit and vegetable waste (FVW) and activated sludge (AS) was investigated

using anaerobic sequencing batch reactors (ASBRs). The effects of AS:FVW ratio and the organic loading

rate (OLR) on digesters performances were examined. The mixtures having AS:FVW ratios of 100:00,

65:35, 35:65, by a total solid (TS) basis were operated at an hydraulic retention time (HRT) of 20 d. How-

ever, 30:70, 20:80, 15:85, 10:90 and 0:100 ratios were tested at an HRT of 10 d. To investigate effects of

aerobic and anaerobic digestion on the sludge filterability, specific resistance to filtration (R) was also

determined. Increasing FVW proportions in the feedstock significantly improved the biogas production

yield. The reactor that was fed with a 30:70 ratio showed the highest VS removal and biogas production

yield of 88% and 0.57 L gÀ1 VS added, respectively. The filterability results showed that the anaerobic

effluent was characterised by a slightly better filterability efficiency of 1.6 Â 1016 m kgÀ1 than 1.74Â

1016 m kgÀ1 of aerobic effluent. However, FVW addition improved the anaerobic co-digestion effluent fil-

terability (5.52Â 1014 m kgÀ1).

Ó 2008 Elsevier Ltd. All rights reserved.

1. Introduction

In the last few years, the number of municipal wastewater

treatment plants (MWWTP) in Tunisia was increased significantly

which results in the production of large quantities of activated

sludge (AS) that should undergo stabilisation. Aerobic stabilisation

of AS has been shown to cause poorer dewatering properties and at

the same time, increases the biopolymer content in solution

(Murthy, 1998) . For this reason, anaerobic digestion process of

sludge is often employed to reduce the mass of solids, reduce their

pathogen content and lead to an energy recovery bonus in the form

of methane gas production (Dinsdale et al., 2000). The rate-limiting

step for anaerobic digestion of AS is the hydrolysis step (Bougrier

et al., 2007). Anaerobic digestion of AS is both slow and incomplete

because the individual cell membranes are not significantly de-graded in conventional mesophilic anaerobic digestion (Borowski

and Szopa, 2007). Except for the resistant to biodegradation, the

low C/N ratio of AS in order of 6/1–16/1 is also regarded as a

serious problem to the anaerobic digestion (Stroot et al., 2001). It

should range from 20 to 30 in order to ensure sufficient nitrogen

supply for cell production and the degradation of the carbon pres-

ent in the process, and in order to avoid at the same time excess

nitrogen, which could lead to toxic ammonium concentrations

(Fricke et al., 2007).

The co-digestion of AS with a substrates containing high level of

C/N, like FVW, to overcome the difficulties of treating AS and to ad-

just its unbalanced nutrients constitutes an interesting solution. In

fact, the large quantities of FVW generated from markets are an-

other type of residue that is characterised by easy biodegradable

organic matter content with high C/N ratio and water content

(>80%) (Bouallagui et al., 2003). Treatment of this organic fraction

is currently carried out through aerobic composting or anaerobic

digestion. But anaerobic digestion seems to be a more attractive

method for the treatment of this waste (Sharma et al., 2000). How-

ever, one of the problems most frequently found during biological

processing of the FVW is the high C/N ratio of these residues. So,

co-digestion of AS and FVW reduces also the FVW anaerobic diges-

tion limitations.

Co-digestion is the term used to describe the combined treat-ment of several wastes with complementary characteristics, being

one of the main advantages of anaerobic technology ( Agdag and

Sponza, 2005). Co-digestion of organic wastes is a technology that

is increasingly being applied for simultaneous treatment of several

solid and liquid organic wastes. The main advantages of this tech-

nology are improved methane yield because of the supply of addi-

tional nutrients from the co-digestates and more efficient use of

equipment and cost-sharing by processing multiple waste streams

in a single facility (Alatriste et al., 2006).

This paper reports the results of the single-stage anaerobic co-

digestion of AS and FVW. The effect of AS:FVW ratio on the reactors

performances and the anaerobic sludge filterability was examined.

0960-8524/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.doi:10.1016/j.biortech.2008.09.019

* Corresponding author. Tel.: +216 22524406; fax: +216 71704329.

E-mail address: [email protected] (L. Habiba).

Bioresource Technology 100 (2009) 1555–1560

Contents lists available at ScienceDirect

Bioresource Technology

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / b i o r t e c h

mailto:[email protected]

http://www.sciencedirect.com/science/journal/09608524

http://www.elsevier.com/locate/biortech

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mailto:[email protected]



2. Methods

2.1. Characteristics of the substrates used

The AS was taken from a local wastewater treatment plant of

Cherguia (Tunis) treating domestic and industrial wastewaters. It

is composed of settled suspended biomass. The FVW were col-

lected from the group market of Tunis. Before being mixed withAS, FVW must undergo some pre-treatments. They were shredded

to small particles and homogenised to facilitate digestion. Both

types of wastes as collected were analyzed for various parameters

such as: pH, TS, total volatile solids (VS), Total Suspended Solids

(TSS), Total Carbon (TC), total Chemical Oxygen Demand (CODt)

and Total Nitrogen (TN). The tested mixtures ratios of AS:FVW

were 100:0, 65:35, 35:65, 30:70, 20:80, 15:85, 10:90 and 0:100

on TS basis. These mixtures were analyzed for the most of the

parameters mentioned below. The characteristics are summarized

in Table 1.

Table 1 showed that the TS and VS content were the lowest in

pure AS and the highest in the FVW. Total COD values in all mix-

tures indicated that organic content were very high. As the FVW

addition increased, the C/N of the feed mixture gradually increased

from 10 to 30 and ranged within the C/N ratio (20–30) required for

stable and better biological conversions reported by others on the

anaerobic digestion (Hong-Wie and David, 2007). The low C/N of

the AS implies a large source of nitrogen, mainly in the form of pro-

teins from lysed cells. The carbon and nitrogen content analysis

show a complementary of the tow types of wastes.

2.2. Digesters design

Experiments were carried out in high anaerobic sequencing

batch reactors (ASBR) (R1, R2, R3, R4, R5, R6, R7 and R8) with a 2 li-

tres capacity (Fig. 1). The reactors were operated under mesophilic

condition (35 °C). The temperature was controlled by a thermo-

statically regulated water bath. Mixing in the reactors was done

by a system of magnetic stirring. Each digester was initially inocu-lated with anaerobic sludge obtained from an active mesophilic di-

gester of fruit and vegetable wastes treatment plant (Bouallagui

et al, 2007). The ASBR was operated with cycles including the fol-

lowing four discrete steps: (i) fill (15 min): 100 mL or 200 mL of

different mixtures of wastes were added to the reactors at the

beginning of a cycle, (ii) react (21 h): during this phase, the reac-

tors were stirred and organic matter was converted to energy

and new cells, (iii) settle (2 h and 30 min): settling started when

the react phase was finished, (iv) draw off (15 min): at the end of

the settling period, the volume of liquid added at the beginning

of the cycle was drawn off from the reactors.

2.3. Analytical methods

TS, VS, TSS, pH, COD, pH, alkalinity and total volatile fatty acids

(TVFA) were determined according to the APHA standard methods

(1995). The volume of biogas product was measured daily using a

gas-meter and its composition was estimated using an ORSAT

apparatus (Bouallagui et al, 2003). The biogas volume was ex-

pressed at Standard Temperature and Pressure (STP). TC was mea-

sured by catalytic oxidation on a TOC Euroglace analyser. TN was

estimated by Kjeldhal method.

Specific resistance to filtration (R) test known as the Buchner

funnel test is one of the most commonly employed test for the

evaluation of wastewater sludge dewaterability or filterability

(Movahedian Attar et al., 2005; Chang et al., 2001; Lee and Liu,

2000). The (R) test was performed using an 8 cm diameter what-

man filter paper at an applied vacuum pressure of 0.8 bar. For fil-

tration studies, the entire suspension from the sample was

carefully transferred to the Buchner funnel and the experimentswere carried out at a constant vacuum pressure. The volume of fil-

trate collected in the measuring cylinder was monitored visually

and recorded every 1 min. The experiments were stopped after

the liquid disappeared from the top surface of the filter cake. The

volume of filtrate was recorded as a function of time. Specific resis-

tance to filtration (R) was determined using a plot of filtration

time/filtrate volume (t/V ) vs. filtrate volume (V ). Using the slope

of the line, R was calculated from the following formula (Pollice

et al., 2007):

t =V ¼ ðlRw =2 A2P ÞV þ lRm= AP ð1Þ

where ‘‘R” is the specific resistance to filtration (m kgÀ1), ‘‘P” is the

pressure of filtration (bar), ‘‘l” is the viscosity of filtrate (Pa s), ‘‘V ”

the volume of filtrate (m3

), ‘‘t ” the filtration time (s), ‘‘w” the weightof dry solids per volume of filtrate (kg mÀ3), ‘‘ A” the area of the filter

paper (m2), and ‘‘Rm” the resistance on the medium (mÀ1). For com-

Table 1

Characteristics of raw substrates and co-substrates mixtures

AS 100% Mixtures ratios FVW 100%

65:35 35:65 30:70 20:80 15:85 0:90

pH 6.98 ± 0.1 6.25 ± 0.12 5.66 ± 0.11 5.32 ± 0.05 4.87 ± 0.14 4.64 ± 0.06 4.64 ± 0.1 5.04 ± 0.15

TS (%) 0.57 ± 0.01 0.67 ± 0.01 1.1 ± 0.03 1.2 ± 0.02 1.8 ± 0.05 2.3 ± 0.04 3.5 ± 0.13 6.8 ± 0.1

VS (g L À1) 4.49 ± 0.1 5.87 ± 0.17 8.42 ± 0.3 9.71 ± 0.24 14.85 ± 0.29 17.83 ± 0.41 27.39 ± 0.68 56.1 ± 0.5

TSS (g L À1) 4.4 ± 0.13 5.44 ± 0.2 6.4 ± 0.3 6.6 ± 0.2 11.6 ± 0.2 16.1 ± 0.4 28.7 ± 1 47.7 ± 0.9

CODt (g L À1) 27 ± 0.8 40.3 ± 0.7 51.7 ± 1.6 53.6 ± 1.8 57.4 ± 2.1 59.3 ± 1.6 61.2 ± 1.9 65 ± 1.6

TC (% MS) 52 ± 2 59 ± 1.5 65 ± 1.8 66 ± 2.1 68 ± 2 69 ± 2.3 70 ± 2.1 72 ± 2

TN (%MS) 5.4 ± 0.2 4.21 ± 0.1 3.2 ± 0.1 3 ± 0.1 2.68 ± 0.1 2.51 ± 0.1 2.34 ± 0.1 2 ± 0.1

C/N 10 ± 0.3 14 ± 0.3 20.5 ± 0.7 22 ± 0.4 25.5 ± 1 27.5 ± 0.8 30 ± 0.8 36 ± 1.2

(4) (3)

(5)(6)

(1)

(7)

(9)(8)

(2)

(4) (3)

(5)(6)

(1)

(7)

(9)(8)

(2)

Fig. 1. Schematic of the experimental ASBR system: (1) ASBR, (2) water bath and

heating recirculation, (3) magnetic stirrer, (4) feedstock, (5) feeding pump, (6)

discharge pump, (7) effluent stock, (8) sampling valve and (9) biogas collector.

1556 L. Habiba et al. / Bioresource Technology 100 (2009) 1555–1560



pressible sludge, ignoring ‘‘Rm” which is very small as compared to

the resistance on the sludge cake, Eq. (1) was reduced to:

t =V ¼ ðlRw =2 A2P ÞV ¼ bV ð2Þ

Taking the slope of the line as ‘‘b”, R was calculated from the

formula:

R ¼ ð2 A2P =lw Þb ð3Þ

In order to study the effect of the aerobic digestion of AS on the

effluent filterability and compare this filterability with the effluent

stemming from anaerobic digestion and co-digestion, the aerobic

digestion of AS was carried out during 48 h in a reactor of 2 litres

capacity. The reactor was operated with an aeration rate of

0.5 vvm and a biomass concentration in order of 4.8 g L À1.

2.4. Statistical analysis

In order to determine whether the observed differences be-

tween digesters performances were significantly different, data

were subjected to the ANOVA tests (StatSoft Inc, 1997). Differences

between AS/FVW ratios effects (p and p1) were compared with

0.05.

3. Results and discussion

3.1. Biogas production at different AS:FVW ratios

As shown in Fig. 2, daily biogas production by digestions of AS

alone and the co-digested wastes varied between 0.131 L dÀ1 and

2.4 L dÀ1 (STP) depending to the substrate composition. The extent

of biogas production increased with the addition of FVW. From all

mixtures ratios, the 00:100 (AS:FVW) ratio yields the maximum

biogas rate because it contains high organic content. The methane

content was relatively the same for the different applied AS:FVW

ratios, it varied between 58% and 60% (Table 2).

The average specific biogas production (Litre of biogas producedper g of volatile solids added orLitre ofbiogasproducedper g of vol-

atile solids removal) was determined at the steady state (4th HRT)

for all digestion processes. During all the experiments, when pure

AS was used, only 0.29 L gÀ1VS added of specific biogas production

with a methane content of 58% was generated. However, this pro-

duction canbe considered as importantbecause anaerobic digestion

of AS alonewas characterised bya problem of lowbiogas production

that was generally low than 0.2 L gÀ1 VS added (Bolzonella et al.,

2005). Bolzonella et al. (2005) examined the mesophilic anaerobic

digestion of AS worked with an HRT in a range of 20–40 days and

an organic loading rate of 1 kg VS.mÀ3 dÀ1. The specific biogas

production was in the range of 0.07–0.18 L gÀ1 VS added.

By addition of FVW to AS, the specific biogas production in-

creased considerably from 0.29 (specific methane production of 0.16 L CH4 gÀ1VS added) to 0.57 L gÀ1VS added (specific methane

production of 0.34 L CH4 gÀ1VS added) corresponding to both

AS:FVW ratios of 35:65 and 30:70. However, the increase of the

FVW fraction more than 70% in the feedstock leads to a slight

decreasing of the specific biogas production. The ANOVA analysis

of the data indicated that digesters performances enhancement

were statistically significant ( p < 0.05) (Table 2).

The addition of the FVW to AS improves the specific biogas

yield. This could be due to the better nutrients balance in feed-

stock, positive synergism and the correct activity of micro-organ-

ism in the digestion medium. This data for the co-digestion may

be compared with earlier works. For example, the methane yield

obtained by Dinsdale et al. (2000) from AS and FVW anaerobic

co-digestion operated in the two-stage tubular digesters with anAS:FVW ratio of 75:25 was 0.25 L gÀ1VS added. Heo et al. (2003)

examined the co-digestion of AS with food waste (FW). A maxi-

mum specific methane production of 0.37 L CH4 gÀ1VS added were

obtained with a methane content of 63% and a AS:FW ratio of

50:50. Fu et al. (2006) have also reported the similar results for

the anaerobic co-digestion of AS with the kitchen garbage (KG).

The specific methane productions of 0.35–0.37 L CH4 gÀ1VS added

were found with a methane content of 61.8%–67.4% with an AS:KG

ratio of 50:50.

3.2. VS reduction at different AS:FVW ratios

VS is an important parameter for measuring biodegradation,which directly indicates the metabolic status of some of the most

delicate microbial groups in the anaerobic system (Elango et al.,

2007). The VS destruction results for the various co-digested sub-

strates showed that the lowest VS removal of 55% was obtained

with the digester treating 100% sludge. Considering the character-

istics of this substrate and its poor biodegradability it is possible to

explain this result (Borowski and Szopa, 2007). However, this

reduction can be considered significant. In fact, Ghosh (1991) re-

ported that anaerobic digestion of AS showed only 30–45% reduc-

tion in VS during anaerobic digestion.

The total VS removal was about 65–88% for the different applied

AS:FVW ratios, reflecting the positive effect of the addition of FVW.

The vegetable wastes showed excellent digestibility; they seemed

to accelerate the digestion process as well as to increase the degreeof the anaerobic degradation of the sludge (Edelmann et al., 2000).

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

D a i l y b i o g a s p r o d u c t i o n r a t e ( L d - 1 )

D a i l y b i o g a s p r o d u c t i o n r

a t e ( L d - 1 )

0

0.5

1

1.5

2

2.5

3

3.5

4

0 10 20 30 40 50 60 70

Time (days)

0 10 20 30 40 50 60 70

Time (days)

a

b

Fig. 2. Biogas production vs. different AS:FVW mixtures: (a): at HRT 20 d, (–Ç–) [R1

(100:0)], (–h–) [R2 (65:35)] and (..N..) [R3 (35:65)], (b): at HRT 10 d, (..s..) [R4

(30:70)], (– –) [R5 (20:80)], (–d–) [R6 (15:85)], (–4–) [R7 (10:90)] and (-j-) [R8

(00:100)].

L. Habiba et al. / Bioresource Technology 100 (2009) 1555–1560 1557



The higher degradation efficiency of VS of 88% was obtained for the

30:70 ratio. It was associated with the higher specific biogas pro-

duction. The material balances in the systems presented in the Ta-

ble 3 showed that biogas production represents between 53% and

88.8% of removed VS. The better mass balances were obtained with

R3.

It is very likely that the high degradation efficiency in the co-

digestion was due to an improved ratio of nutrients and better

availability of the organic substances, which facilate their assimila-

tion by anaerobic flora and increases the degree of degradation(Krupp et al, 2005). The results of VS reduction in this work are bet-

ter than those obtained by Fu et al. (2006) (51.1%–56.4%); Heo et al.

(2003) (53.7%) and Dinsdale et al. (2000) (40%).

3.3. pH, total VFA and alkalinity

As showing in Table 2, the measured parameters reflected the

changing conditions in the reactors as the composition of the sub-

strate was changed and the OLR was increased. Due to the acidity

of FVW, the pH of the influent dropped from 6.98 of pure AS to 4.64

in the 10:90 (AS:FVW) mixture (Table 1). However, the pH in the

effluent varied between 7.13 and 7.51 indicating the process stabil-

ity and the optimal activity of methanogenic bacteria. This result

showed that the co-digestion systems were well buffered.

The concentration of TVFA has been found to be a very good

indicator of the metabolic status of an anaerobic degradation pro-

cess (Fernández et al., 2005; Björnsson et al., 2000). The average of

TVFA level increases from 480 to 2400 mg L À1 as organic feedingrates is increased, which is significantly influenced by the addition

of FVW. Throughout the all co-digestion process, levels of TVFA at

steady-state were very low. These values indicated a high stability

of the reactors after an initial transitory increase in TVFA which is

typical for start-up of an anaerobic process when the balance of the

hydrolytic bacteria, fermentative bacteria and methanogens has

not stabilised yet (Hartmann and Ahring, 2005). This stability

was confirmed by the presence of stable pH and alkalinity of the

reactors. The average values of TVFA reported in this work were

lowest than those reported by Dinsdale et al. (2000), between

1800–1330 mg L À1, in the effluent of a successful methanogenic

reactor treating FVW with AS as a co-substrate.

The average values of partial alkalinity ranged between 1488

and 12031 mg L À1

. Previously, laboratory studies on mesophilicand thermophilic anaerobic organic wastes digestion reported a

range of 2000–4000 mg L À1 partial alkalinity as being typical for

properly operating digesters (Sharma et al., 2000; Chen et al.,

2007). Except the values obtained for the digesters R2 and R3

which fall within this range, all values are higher than the reported

values. Higher levels of partial alkalinity were also found by

Mshandete et al. (2004) indicating that higher levels of partial

alkalinity are possible.

One of the criteria for judging digester stability is the VFA:alka-

linity ratio. There are three critical values for this (Callaghan et al.,

2002): <0.4 digester should be stable; 0.4–0.8 some instability will

occur; >0.8 significant instability. As showing in Fig. 3, When the

AS or the FVW were being digested separately, the ratio was often

in the 0.4–0.8 range. This implying there was the potential forinstability which can more explain the lowest results for the biogas

Table 2

Digesters operated conditions and performance

R1 R2 R3 R4 R5 R6 R7 R8 P

Mixture ratios

(AS:FVW)

100:00 65:35 35:65 30:70 20:80 15:85 10:90 0:100 –

OLR (g L À1 dÀ1) 0.26 ± 0.05 0.3 ± 0.07 0.43 ± 0.01 1.03 ± 0.05 1.55 ± 0.06 1.87 ± 0.08 2.86 ± 0.02 3.45 ± 0.1 –

HRT (d) 20 20 20 10 10 10 10 10 –

VS inlet (g L À1) 4.49 ± 0.01 5.87 ± 0.03 8.42 ± 0.09 9.71 ± 0.09 14.85 ± 0.2 17.83 ± 0.3 27.39 ± 0.4 34.3 ± 0.5 –

VS outlet (g L À

1) 2 ± 0.08 1.98 ± 0.02 1.36 ± 0.11 1.17 ± 0.03 2.11 ± 0.08 2.75 ± 0.1 4.07 ± 0.16 6.44 ± 0.4 –VS removal (%) 55.4 ± 1.6 65.1 ± 1.6

p1 = 0.002

83.8 ± 1.5

p1 = 0.000032

88 ± 1.6

p1 = 0.000037

85.7 ± 1.3

p1 = 0.000037

83.7 ± 1.7

p1 = 0.000024

85.1 ± 1.8

p1 = 0.000022

81.2 ± 1.4

p1 = 0.00002

0.000

Biogas production

rate (L dÀ1)

0.131 ± 0.001 0.283 ± 0.02

p1 = 0.000

0.480 ± 0.03

p1 = 0.000

1.11 ± 0.05

p1 = 0.000

1.33 ± 0.05

p1 = 0.000

1.76 ± 0.08

p1 = 0.000

2.28 ± 0.03

p1 = 0.000

2.4 ± 0.05

p1 = 0.000

0.000

Biogas yield (L gÀ1

added VS)

0. 29 ± 0.03 0.47 ± 0.03

p1 = 0.002

0.569 ± 0.02

p1 = 0.0002

0.57 ± 0.02

p1 = 0.0009

0.44 ± 0.01

p1 = 0.0011

0.49 ± 0.03

p1 = 0.0017

0.41 ± 0.02

p1 = 0.0044

0. 35 ± 0 .0 3 0. 000

Biogas yield (L gÀ1

removal VS)

0.52 ± 0.04 0.72 ± 0.02

p1 = 0.0022

0.67 ± 0.02

p1 = 0.0065

0.64 ± 0.02

p1 = 0.014

0.52 ± 0.01

p1 = 1.000

0.57 ± 0.03

p1 = 0.17

0.48 ± 0.02

p1 = 0.23

0. 43 ± 0 .0 2 0. 000

Methane (%) 58 ± 1 59 ± 0.5 60 ± 1 59 ± 1.5 60 ± 1.5 59 ± 1 60 ± 1 58 ± 0.5 –

TVFA (mg L À1) 480 ± 7.2 560 ± 30 640 ± 20 690 ± 40 800 ± 40 850 ± 30 928 ± 30 2400 ± 50 –

Alkalinity (mg L À1) 1488 ± 56 2698 ± 60 3864 ± 60 4871 ± 140 8309 ± 210 8906 ± 150 12031 ± 180 5800 ± 200 –

VFA/ Alkalinit y 0. 32 ± 0. 02 0. 2 ± 0. 02

p1 = 0.0022

0.16 ± 0.01

p1 = 0.0065

0.14 ± 0.01

p1 = 0.014

0.09 ± 0.01

p1 = 1.000

0.09 ± 0.006

p1 = 0.17

0.07 ± 0.005

p1 = 0.23

0. 41 ± 0 .0 4 0. 00 0

pH 7.51 ± 0.13 7.39 ± 0.11 7.31 ± 0.1 7.27 ± 0.14 7.25 ± 0.03 7.2 ± 0.09 7.13±0.13 6.95 ± 0.05 –

TSS in effluent

(g L À1)

1.49 ± 0.03 0.85 ± 0.01 1.12 ± 0.04 0.62 ± 0.01 1.03 ± 0.05 2.65 ± 0.13 2.9 ± 0.1 3.2 ± 0.2 –

TSS in reactor(g L À1) 11.31 ± 0.4 8.35 ± 0.4 9.35 ± 0.2 11.68 ± 0.2 11.09 ± 0.4 15.11 ± 0.3 16.03 ± 0.4 16.5 ± 0.4 –

p: Indicated the statistical difference between all digesters performances (R1–R7).

p1: Indicated the statistical difference between digesters R1 and one of other digesters (R2–R7).

Table 3

Material balances in the digesters

R1 R2 R3 R4 R5 R6 R7 R8

HRT (d) 20 20 20 10 10 10 10 10

VS removal/litre

of digester

(g L À1 dÀ1)

0.144 0.195 0.36 0.906 1.328 1.565 2.433 2.8

Weight of CH4

(g L À1 dÀ1)

0. 027 0 .0 59 0. 102 0 .234 0. 285 0. 371 0. 489 0. 49

Weight of CO2

(g L À1 dÀ1)

0. 054 0 .113 0. 188 0 .448 0. 523 0. 709 0. 896 0. 99

Weight of biogas

(g L À1 dÀ1)

0. 081 0 .173 0. 291 0 .683 0. 809 1.08 1.385 1.48

Weight of VFA

(g L À1 dÀ1)

0. 024 0 .0 28 0. 032 0 .0 69 0. 08 0. 085 0. 092 0. 24

Weight of

biogas + VFA

(g dÀ1)

0. 105 0 .201 0. 323 0 .752 0. 889 1.165 1.477 1.72

Mass balance (%) 73.2 103.2 89.7 82.9 66.9 74.4 60.7 61.6

Mass balance

Biogas/VS (%)

56.5 88.8 80.9 75.3 60.9 69 56.9 53




yield. When FVW was being added to the feed, the VFA:alkalinity

ratio did not rise above the critical value of 0.4 even during the ini-

tial transitory period witch characterised by a high VFA:alkalinity

ratio. So, co-digestion of AS and organic waste is beneficial not justfor increasing gas production but also for stabilising the digestion

process.

3.4. Sludge production and filterability

As illustrate in the Fig. 4, generally at the beginning of the

experiments, the highest and fluctuating TSS values in the effluent

are observed. This should be attributed to transitory deficient reac-

tors operation accompanied by leaching of suspended solids, or-

ganic matter and even of bacteria to the effluent. In fact, when

biomass settling was incomplete, there was a gradient in solids

concentration along the reactor height. TSS content could thus

have been higher in the effluent. However, at the steady-state

thanks to the biomass acclimation and stability, TSS values de-creased and tended to be stable. The average TSS concentration

in the effluents, at the steady-state, ranged between 0.62 g L À1

and 3.21 g L À1 (Table 2). This data indicates that the additions of

FVW with a ratio ranged from 65:35 to 20:80 improved the sludge

settling and than lowest TSS were recorded in the digesters efflu-

ent. Above this FVW content (15:85–00:100) the TSS tended to in-

crease again, showing a poor sludge settling at high OLR. This can

be explained by the high quantity of fibrous prevent from FVW.

The average TSS in the digesters ranged from 8.35 to 16.5 g L À1

(Table 2). These TSS values were almost lowest for the reactors R1,

R2, R3, R4 and R5. This data confirm the good settling sludge men-

tioned above for these reactors. However, the TSS concentration in

the reactors R6–R8 were higher. This was mainly caused by the

higher OLR. This also explains the few large solids losses observedduring effluent drawdown and the poor settling in these reactors.

Different biomass concentrations in the sludge bed at differentmixture ratios seen to affect effluent quality. So, they had an im-

pact on the reactors performances.

To investigate the effects of aerobic and anaerobic digestion on

the sludge filterability, specific resistance to filtration (R) was

determined. Results showed that the lowest specific resistance

was obtained with the raw waste in the average value of

8.45 Â 1013 m kgÀ1. Sürücü and Cetin (1989) reported that specific

resistance of AS varies generally between 0.98Â 1013À12Â 1013

m kgÀ1. Higher specific resistances of 1.74Â 1016 and 1.6Â 1016

m kgÀ1 are obtained for sludge stemming from the aerobic and

anaerobic stabilisation of the AS alone, respectively. However,

the anaerobic co-digestion of the AS with the FVS improves the fil-

terability by reducing the specific resistance to 5.18 Â 1014–5.52 Â

1014

m kgÀ1

.Particle size distribution is known as one of the parameters

describing the filterability behaviour of sludge (Mikkelsen,

2001).The specific resistance is mainly affected by the presence

of bacterial extacellular polymers substances (EPS) (Sürücü and Ce-

tin, 1989). EPS in activated sludge occur as a capsule surrounding

the bacterial cell wall which enhances flocculation with larger flocs

(Rosenberger and Kraume, 2002). The flocs arrange themselves so

as to leave large pores that would offer little resistance to flow dur-

ing filtration.

After aerobic and anaerobic treatment, a deterioration of the

sludge filterability can be explained mostly by the reduction of

the particle size. During, both aerobic and anaerobic digestion,

the changes in the structure of flocs have apparently made the flocs

more reduced and the content of fine particles increases. As theparticle size got smaller and dispersed solids concentration in-

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Time (days)

V F A / A l c a l i n i t y r a t i o

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 10 20 30 40 50 60

0 10 20 30 40 50 60

Time (days)

V F A / A l c a l i n i t y r a t i o

a

b

Fig. 3. VFA/Alkalinity ratio variation for different AS:FVW mixtures: (a): at HRT

20 d, (–Ç–) [R1(100:0)], (–h–) [R2 (65:35)] and (..N..) [R3 (35:65)], (b): at HRT 10 d,

(..s..) [R4 (30:70)], (– –) [R5 (20:80)], (–d–) [R6 (15:85)], (–4–) [R7 (10:90)] and

(-j-) [R8 (00:100)].

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

0

2

4

6

8

10

12

14

16

18

20

T S S

i n e

f f l u e n t ( g

L - 1 )

T S S

i n

e f f l u e

n t ( g

L - 1 )

0 10 20 30 40 50 60

Time (days)

Time (days)

0 10 20 30 40 50 60 70

a

b

Fig. 4. TSS in the effluents (a): at HRT 20 d, (–Ç–) [R1(100:0)], (–h–) [R2 (65:35)]

and (..N..) [R3 (35:65)], (b): at HRT 10 d, (..s..) [R4 (30:70)], (– –) [R5 (20:80)],

(–d–) [R6 (15:85)], (–4–) [R7 (10:90)] and (-j-) [R8 (00:100)].

L. Habiba et al. / Bioresource Technology 100 (2009) 1555–1560 1559



creased, the passage ways of water through the cake and filter

medium during filtration was clogged, the resistance to the flow

of water increased and so the specific resistance to filtration in-

creased (Sürücü and Cetin, 1989). Deterioration of sludge dewater-

ability is greater in the case of aerobic processes because of much

higher bacterial growth following decay as well as mechanical

stress, which leads to greater disintegration and formation of fine

particles. On the contrary, the disintegration of sludge duringanaerobic digestion can be explained mostly by the degradation

of EPS responsible for floc formation (Borowski and Szopa, 2007).

After anaerobic co-digestion with the FVW, specific resistance

was lower than after anaerobic digestion of AS alone. So, the pres-

ence of organic waste residues improves the filterability measured

of AS in case of fibres presence in this substrate forming a protec-

tive layer to the filter medium reducing so the plugging problem.

4. Conclusion

It is concluded that the anaerobic co-digestion of AS with the

FVW is beneficial and constitutes an interesting solution to over-

come the low biodegradability and the low C/N ratio of AS. This re-

sulted to better biogas yield and a highly buffered system. Different

mixture ratios were investigated. The optimum operating condi-

tions of all the digesters were found to be a mixture of 30:70

(AS:FVW) in terms of the stability and performance. The VS re-

moval efficiency and specific biogas production in this condition

achieved 88% and 0.63 L gÀ1VS added, respectively, corresponding

to an HRT of 10 d and an organic loading rate of 1.03 g VS L À1 dÀ1.

The high performance and stability obtained with this mixture

digestion could be due to positive synergism and an optimal bal-

ance of nutrients in the digester medium.

The effluent filterability efficiency was also investigated. The re-

sults show that the an anaerobic sludge obtained from anaerobic

co-digestion of AS with FVW present a good filterability than that

obtained from anaerobic digestion of AS alone. The presence of or-

ganic waste residues improves the filterability in case of fibres

presence in this substrate.

Acknowledgement

The authors wish to acknowledge the Ministry of Superior Edu-

cation and Scientific Research and Technology, which has facili-

tated the carried work.

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