Effect of organic loading rate on the stability, operational parameters and performance of a...

Bioresource Technology 96 (2005) 335–344

Effect of organic loading rate on the stability, operationalparameters and performance of a secondary upflow anaerobic

sludge bed reactor treating piggery waste

E. S�anchez a, R. Borja a,*, L. Travieso a, A. Mart�ın b, M.F. Colmenarejo c

a Instituto de la Grasa (C.S.I.C.), Avda. Padre, Garc�ıa Tejero 4, E-41012 Sevilla, Spainb Departamento de Ingenier�ıa Qu�ımica, Facultad de Ciencias, Campus Universitario de Rabanales, Ctra. Madrid-C�adiz,

Km. 396, E-14071 C�ordoba, Spainc Centro de Ciencias Medioambientales (C.S.I.C.), Serrano 115-duplicado, 28006 Madrid, Spain

Received in revised form 25 March 2004; accepted 2 April 2004

Available online 14 May 2004

Abstract

A study of anaerobic digestion of piggery wastewater was carried out in a laboratory-scale sludge bed reactor as a secondary

treatment. The effect of organic volumetric loading rates (BV) in the range of 1.0–8.1 gTCOD/l d on the process performance was

evaluated. The best results were obtained at BV equal to or lower than 4 gTCOD/l d. At higher BV values, the removal efficiency of

the process decreased suddenly. A linear relationship was found between the effluent SCOD and the TVFA/alkalinity ratio (P ). Arelationship was found among the different operational variables (BV, removal efficiency, effluent soluble COD, soluble COD re-

moval rate (R), retention factor (/), specific microbial growth rate (l), methane production rate per volume of reactor and per

volume of waste treated––QM and qM, respectively) and the corresponding regression equations were obtained. An increase of BV

determined a decrease of removal efficiency, / and qM and an increase of effluent soluble COD, l, R and QM. The value of the

maximum specific microbial growth rate (lM) determined through the equation that correlated BV and l was found to be 0.19 d�1.

This value was of the same magnitude as those reported in other works of anaerobic digestion of piggery waste.

� 2004 Elsevier Ltd. All rights reserved.

Keywords: Piggery waste; Upflow anaerobic sludge bed reactor; Organic volumetric loading rate (BV); Operational parameters; Correlation

1. Introduction

Piggery waste is characterized by a high content of

organic matter and pathogenic organisms. The disposal

of piggery waste without adequate treatment can cause a

drastic effect on the environment and human health.

This waste is formed by a mixture of manure (feces and

urine) and food wastage such as swill and sugar canemolasses (S�anchez et al., 2001). According to the most

common characteristics of this waste, anaerobic diges-

tion could be considered one of the most promising

treatment alternatives (Kimchie et al., 1988; Hobson

and Shaw, 1973; Hobson, 1981, 1985, 1992; S�anchez

*Corresponding author. Tel.: +34-95-4689654; fax: +34-95-

4691262.

E-mail address: [email protected] (R. Borja).

0960-8524/$ - see front matter � 2004 Elsevier Ltd. All rights reserved.

doi:10.1016/j.biortech.2004.04.003

et al., 1995; Baader, 1990; Yang and Gan, 1998; Parkin

and Owen, 1986; S�anchez and Travieso, 1994).

Piggery waste treatment by anaerobic digestion has

several advantages, the most important are as follows:

1. The process destroys pathogenic and parasitic organ-

isms.

2. Methane as a valuable by-product can be used as a

source of energy, and liquid and sludge effluents

can be used as a soil conditioner.

3. The low biomass production determines smaller vol-

umes of sludge than in the aerobic processes, thuslowering the cost of sludge management.

4. Capacity to stabilize large volumes of diluted organic

slurries at low costs.

5. This process is more economical than the aerobic

process, especially at strengths higher than 4 g/l of

COD.

mail to: [email protected]

336 E. S�anchez et al. / Bioresource Technology 96 (2005) 335–344

Hobson and Shaw (1973, 1974) studied the mecha-

nism and flora of the anaerobic digestion of piggery

waste in a complete mixed digester. They observed that

Enterobacteria streptococci made up about 50% of the

total content of digester organisms. The predominant

groups of organisms were identified as Clostridium bu-

tyricum, Bacteroides and gram negative Coccobacilli, and

the strict anaerobic ones did not predominate over thefacultative ones. Stevens and Schulte (1979) studied the

effect of the temperature at solids retention times of be-

tween 6 and 55 days at organic volumetric loading rates

of between 0.61 and 4.81 kgVS/m3 d, in a complete

mixed anaerobic digester. They concluded that at or-

ganic rates in the range of 0.61–1.80 kgVS/m3 d and

temperatures lower than 25 �C, the operation proceeded

satisfactorily. Low temperature digestion was found torequire twice as long a retention time with satisfactory

production and composition of gas. A previous study

(S�anchez et al., 2001) looked at the influence of tem-

perature and substrate concentration on the anaerobic

batch digestion of piggery wastewater. This study com-

pared the process at mesophilic temperature (35 �C) withtemperatures in the range of 16.8–29.5 �C, and influent

concentrations in the range of 3.3–26.3 gTCOD/l. Theprocess at mesophilic temperature was more stable than

at ambient temperature, obtaining higher values of re-

moval efficiency. Batch or intermittent feeding, plug-

flow, completely-mixed fixed bed and upflow bed

anaerobic digesters have been used for piggery waste

treatment at laboratory, pilot and full-scale plants with

good results (Baader, 1990; Chen, 1983; Hobson, 1992;

I~niguez et al., 1994; Kimchie et al., 1988; Lapp et al.,1995; Lomas, 1999; Montalvo, 1995; Rodr�ıguez and

Lomas, 1999; S�anchez and Travieso, 1994; S�anchez et al.,1995, 2001, 2002; Yang and Gan, 1998). The effect of

different parameters such as temperature, hydraulic

retention time, organic volumetric loading rate, mixing

and other factors have been previously studied. How-

ever, the upflow anaerobic sludge bed (UASB) reactor

has seldom been used for piggery waste treatment be-cause the composition of this waste, rich in nitrogenous

compounds, makes the active biomass densification, the

granulation process, and therefore the microorganisms

retention in the reactor difficult. UASB reactors have

Table 1

Characteristics of the raw supernatant piggery waste after a settling period o

Parameter Average value

Total COD (mg/l) 10,189

Total organic carbon (TOC) (mg/l) 4100

Total solids (TS) (mg/l) 7210

Total volatile solids (TVS) (mg/l) 5122

Total suspended solids (TSS) (mg/l) 1637

Volatile suspended solids (VSS) (mg/l) 1166

Total nitrogen (TN) (mg/l) 341

Phosphates (P) (mg/l) 419

pH 6.0

been applied mainly to carbohydrate type wastewaters

with success and limited work has been done on the

application of this reactor configuration for piggery

waste stabilization. Moreover, pigs in Cuba are fed with

a mixture of swill and molasses (S�anchez et al., 2001).

This could determine different behaviour and perfor-

mance patterns in Anaerobic Sludge Bed Reactors.

Montalvo (1995) studied the separate anaerobic diges-tion of liquid and solid fractions of piggery waste,

respectively, and compared this system to the digestion

of the whole waste in a single sludge bed reactor. He

found that separate digestion was more effective than

that carried out using the single stage reactor. S�anchezet al. (1995) compared the performance of sludge bed

and anaerobic fixed bed reactors treating piggery

wastewaters. The best results were obtained in the latter.The aim of the present work was to evaluate the

behaviour and performance of a UASB reactor treating

piggery wastewater (rich in carbohydrate) after a pre-

liminary screening and primary sedimentation. A study

of the variations of the operational parameters with

organic volumetric loading rates and their interrelations

by means of different equations was also carried out.

2. Methods

2.1. Piggery waste

The piggery waste used in the experiment was ob-

tained from a farm situated 20 km west of Havana City,

Cuba. The waste was screened through a 2 mm sieve.

After screening, the waste settled for 1 h. The average

characteristics and range of the supernatants obtained in

primary sedimentation are presented in Table 1. The

supernatant after settling was diluted to a TCOD

around 8 g/l, value within the range of primary sedi-mentation supernatant effluents, and this final settled

effluent was used to feed the reactor.

2.2. Equipment

The experiments were carried out in a laboratory-

scale upflow anaerobic sludge bed (UASB) reactor,

f 60 min

Standard deviation Number of samples

4911 44

1748 44

2430 70

2178 70

886 70

813 70

135 41

228 41

0.7 60

E. S�anchez et al. / Bioresource Technology 96 (2005) 335–344 337

consisting essentially of a 5 l cylindrical glass flask 15 cm

in diameter and 30 cm tall. The flask was hermetically

sealed at the top by a rubber cap with two holes 5 mm in

diameter required for biogas outlet and effluent collec-

tion, respectively. The biogas was collected in a gas

holder submerged in a saturated solution of water and

sodium chloride to prevent the absorption of CO2. The

lower part of the reactor was fed by means of a pipe 10mm in diameter connected to the centre of the reactor

allowing the influent to be introduced by using a peri-

staltic pump.

2.3. Inoculum

The upflow anaerobic sludge reactor (UASB) was

inoculated with 1 l of anaerobic sludge obtained from a

batch piggery sludge digester after 40 days of digestion

time. The characteristics of the inoculum are summa-

rized in Table 2.

2.4. Experimental procedure

Once the inoculum was added to the UASB reactor,

the operational volume was completed with distilled

water to obtain 5 l of working volume. After this step,

the influent was continuously fed by a peristaltic pump

at a flow-rate of 0.625 l/d (hydraulic retention time––

HRT––of eight days) until steady-state conditions wereobtained. Steady-state was considered after a period

equivalent to three times the HRT (24 days). Continu-

ous-mode experiments were carried out at influent flow-

rates of: 0.625, 0.833, 1.00, 1.25, 1.66, 2.50 and 5.00 l/d,

which corresponded to HRT values of 8, 6, 5, 4, 3, 2 and

1 days, respectively, and organic volumetric loading

rates (BV) of 1.0, 1.4, 1.6, 2.0, 2.7, 4.1 and 8.1 gTCOD/

l d, respectively.The experiment was performed by increasing the

influent flow-rate and consequently, reducing the HRT

at a constant influent concentration.

A total of 20 samples were taken for each operational

condition and average values were determined for each

parameter analysed. Biogas production was determined

daily. The gas accumulated in the measurement flask

Table 2

Characteristics of the inoculum useda

Parameter Value

TCOD (g/l) 47.5

TS (g/l) 50.0

TSS 41.0

VS (g/l) 26.1

VSS (g/l) 21.4

TOC 15.7

pH 7.3

aValues are averages of five determinations. There was virtually no

variation (<3%) among these analyses.

was analysed by an Orsat analyser to determine the

concentration of CO2. The temperature was maintained

at a range of 30–35 �C during the experimental period.

2.5. Analyses

Samples of effluent were taken and analysed three

times a week and influent once a week. Hence, each runhad a duration of seven weeks at steady-state condi-

tions. Analytical determinations of total and soluble

chemical oxygen demand (TCOD and SCOD, respec-

tively), total and soluble organic carbon (TOC and

SOC, respectively), total suspended solids (TSS), volatile

suspended solids (VSS), total volatile fatty acids

(TVFA), alkalinity and pH were carried out according

to Standard Methods for the Examination of Water andWastewater (APHA, 1989).

3. Results and discussion

Table 3 shows the steady-state results obtained under

the different experimental conditions studied. The in-

crease of BV at fixed value of influent TCOD caused a

decrease in the removal efficiency expressed in TCOD,

TOC, SCOD, SOC, TSS and VSS. A sudden decrease in

removal efficiencies was observed at a BV of 4.1

g TCOD/l d (HRT of two days). The reduction of re-moval efficiencies was a consequence of the increase in

total volatile fatty acids (TVFA) and a simultaneous

decrease in the alkalinity causing an increase in the

TVFA/alkalinity ratio (P ) and a reduction in the pH

value. This situation had a strong influence on the bio-

gas quality, increasing the CO2 concentration on all the

range of BV studied. Fig. 1 shows the strong effect of the

HRT on the TVFA/alkalinity ratio and, therefore, onthe process stability. An increase in the HRT value

caused a decrease in P , favouring the process stability.

At hydraulic retention times in the range of 4–8 days the

process was well balanced and removal efficiencies

higher than 70% were obtained in most cases. However,

when the HRT was lower than or equal to two days, the

removal efficiencies suddenly decreased and values of Pincreased over 0.5, producing unbalanced conditions.Fig. 2 illustrates the variation of the CO2 concentration

as a function of the values of P . The concentration of

CO2 increased when the value of P increased from 0.2 to

0.6 and was practically constant at higher P values.

Similar behaviour was observed in the acidogenesis of

dairy and gelatin-rich wastewaters using upflow anaer-

obic reactors (Yu and Fang, 2002, 2003). A decrease in

the HRT produced an increase in the effluent concen-tration because the anaerobic waste stabilization was

less complete and TVFA concentration and P increased.

Fig. 3 illustrates the relationship between the soluble

Table 3

Steady-state results obtained under different experimental conditionsa

HRT (d) 8 6 5 4 3 2 1

BV (gTCOD/l d) 1 1.4 1.6 2 2.7 4.1 8.1

Parameter Influent Effluent E (%) Effluent E (%) Effluent E (%) Effluent E (%) Effluent E (%) Effluent E (%) Effluent E (%)

TCOD 8117 1205 85.4 1726 78.4 2030 75.0 2141 73.6 3512 56.7 4941 39.1 6611 18.6

SCOD 5782 785 86.0 1444 73.5 1583 72.6 1772 69.4 2412 58.3 3187 44.9 4644 19.7

TOC 3272 513 84.3 681 79.2 706 78.4 815 75.1 1030 68.5 1686 48.5 1755 34.2

SOC 2338 378 83.8 590 74.8 639 72.7 696 70.2 792 66.1 1152 50.7 1410 39.7

TSS 1678 306 84.1 368 77.3 540 67.8 692 61.3 804 52.1 990 41.0 1223 27.1

VSS 1309 223 83.4 268 75.6 353 73.0 390 70.8 469 64.2 548 58.1 752 42.6

TVFA 1855 573 595 750 810 844 858 1211

Alkalinity 1397 1986 1720 1838 1660 1435 1386 1030

Pb 1.1 0.24 0.29 0.34 0.41 0.49 0.52 0.98

pH 5.9 7.4 7.3 7.3 7.3 7.2 7.2 6.4

Q (l/l w d) 5.5 4.5 4.1 3.5 3.3 2.9 2.6

CH4 66 63.3 57.8 54.7 54.2 46.6 33.3

CO2 32.2 34.2 40.1 42.0 43.0 61.3 63.0

qM (l/l w d) 3.63 2.85 2.37 1.91 1.79 1.35 0.86

QM (l/l r d) 0.45 0.47 0.47 0.48 0.60 0.68 0.86

aValues are averages of 20 determinations taken over three weeks after the steady-state conditions had been reached. The differences between the observed values were less than 3% in all cases.b TVFA/alkalinity ratio is expressed in equivalents of acetic acid/equivalents of calcium carbonate.

338

E.S�anchez

etal./Bioreso

urce

Tech

nology96(2005)335–344

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10

HRT (d)

E (%

TC

OD

rem

oval

)

0

0.2

0.4

0.6

0.8

1

1.2

P (e

q. a

cetic

aci

d/eq

. cal

cium

car

bona

te)

E % TCOD

P

Fig. 1. Variation of the percentage of TCOD removal and TVFA/

alkalinity ratio (P ) with the HRT.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 0.2 0.4 0.6 0.8 1 1.2

P (eq. acetic acid/eq. calcium carbonate)

Volu

met

ric fr

actio

n of

CO

2 in

the

biog

as

Fig. 2. Variation of the volumetric fraction of CO2 in the biogas with

the TVFA/alkalinity (P ) ratio.

0

0.2

0.4

0.6

0.8

1

1.2

0 1 2 3 4 5 6 7

S (g SCOD/l)

P (e

q. a

cetic

aci

d/eq

. cal

cium

car

bona

te)

Fig. 3. Effect of the effluent SCOD concentration on the TVFA/

alkalinity (P ) ratio.


substrate concentration (SCOD) and the value of P . Astraight line was obtained with the following equation:

P ¼ 0:182ðSÞ þ 0:056 ð1Þthe regression coefficient, R2, being equal to 0.97.

Eq. (1) showed a strong influence of substrate con-

centration on the TVFA/alkalinity ratio and, therefore,

on process stability. As we already know, TVFA/alka-

linity ratio (P ) can be used as a measure of processstability (Fannin, 1987): when this ratio is less than 0.3–

0.4, the process is considered to be operating favourably

without the risk of acidification. As was observed in this

study, unstable conditions (P > 0:5) appeared in sub-

strate concentrations higher than 2.3 g SCOD/l. The

intercept value can be attributed to the presence of

mineral acids that have a slight influence and contribu-

tion on the P value.As regards the alkalinity values obtained in the

effluents of the process, it was observed (Table 3) that

the buffering capacity of the experimental system was

maintained at favourable levels with appropriate total

alkalinity present at BV values in the range of 1.0–4.1

g TCOD/l d. The experimental data obtained in this

study indicate that a total alkalinity in the range of

1390–1990 mg/l as CaCO3 was sufficient to prevent thepH from dropping to below 7.2 at BV values of up to 4.1

g TCOD/l d. The pH in the reactor remained more or

less constant at HRTs in the range of 8.0–2.0 days, with

7.4 and 7.2 as extreme values. This stability can be

attributed to carbonate/bicarbonate buffering. This is

produced by the generation of CO2 in the digestion

process, which is not completely removed from the

reactor as gas (Wheatley, 1990).

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 2 4 6 8 10

Bv (g TCOD/l d)

Frac

tiona

l rem

oval

effi

cien

cy

TCODSCODTOCSOC

Fig. 4. Variation of the fractional removal efficiency with the organic

volumetric loading rate (BV).

0

1

2

3

4

5

6

0 2 4 6 8 10Bv (g TCOD/l d)

S (g

SC

OD

/l)

Fig. 5. Effect of the organic volumetric loading rate (BV) on the

experimental effluent SCOD values and those theoretical ones pre-

dicted by Eq. (3).


The effect of BV on the removal efficiencies, expressed

in TCOD, SCOD, TOC and SOC is given in Fig. 4. As

can be seen, an increase in the BV determined a pro-

gressive decrease in the removal efficiency, the best re-

moval efficiencies were obtained at BV values in the

range of 1.0–4.1 TCOD/l d. At BV of 8.1 g TCOD/l d theremoval efficiencies dropped suddenly, and this was

concomitant with an increase in P ratio, which achieved

values over 0.5. An exponential relationship between BV

and the removal efficiencies was observed. The typical

equation that correlated both variables is given as fol-

lows:

E ¼ e�kBV ð2Þ

where E, is the fractional removal efficiency and k is a

constant rate given in d l/g, its value depending on

parameters selected for expressing the substrate con-centration (TCOD, SCOD, TOC and SOC).

Therefore, the k values obtained for TCOD, SCOD,

TOC and SOC were found to be 0.21, 0.20, 0.14 and

0.13 d l/g, respectively. The correlation coefficients were

0.99, 0.99, 0.96 and 0.94 for TCOD, SCOD, TOC and

SOC, respectively, and the variability coefficients for the

corresponding constants were: 5%, 5%, 11% and 13%,

respectively. The values of the constant obtained dem-onstrated that the rates of diminution of removal effi-

ciencies were decreased with BV in the following order:

SOC>TOC>SCOD>TCOD. The results showed that

the uptakes of soluble organic carbonaceous compounds

were always higher than those obtained for particulate

organic carbonaceous compounds. From the results

obtained, it can also be demonstrated that suspended

COD was previously transformed to soluble COD and

then transformed to methane.

Fig. 5 shows the variation of the experimental effluent

substrate concentrations, S, expressed as g SCOD/l, withthe organic loading rate BV. As can be seen, a hyperbolic

relationship appears to relate both parameters, which

can be described by the following equation:

S ¼ K1½BV=ðK2 þ BVÞ� ð3Þ

where K1 is given in g SCOD/l and represents the theo-

retical maximum SCOD concentration at the effluentwhen BV is too high in comparison to K2. The constant

K2 is expressed in gTCOD/l d and represents the BV

value for which the effluent SCOD concentration is

equivalent to half the value of K1. A plot of the inverse

of S versus the inverse of BV gave a straight line with an

intercept equal to 1=K1 and slope equal to K2=K1 with a

correlation factor (R2) equal to 0.99. So the values of K1

and K2 were found to be 8.7 ± 0.4 g SCOD/l and7.3 ± 0.3 gTCOD/l d, respectively. Fig. 5 also shows the

theoretical S values, obtained by Eq. (3), and its varia-

tion with the BV values. As can be seen, deviations lower

0

0.5

1

1.5

2

2.5

3

3.5

4

0 2 4 6 8 10

Bv (g TCOD/l d)qm

(l/l

w.d

)0

0.2

0.4

0.6

0.8

1

QM

(l/l

r.d)

qm (l/l w.d) QM (l/l r.d)

Fig. 7. Effect of the organic volumetric loading rate (BV) on methane

gas production per litre of influent (qM) and per litre of reactor (QM).


than 5% between the experimental and theoretical values

of S were always observed. For low BV values the second

term of the denominator of this expression can be ne-

glected and Eq. (3) can be transformed into a linear

equation, as follows: S ¼ 1:20BV.

Fig. 6 illustrates the effect of BV on the SCOD re-

moval rate, R. As can be seen, the SCOD removal rate

increased with the BV according to a hyperbolic model,as follows:

R ¼ RM½BV=ðK þ BVÞ� ð4Þ

where RM, is the maximum SCOD removal rate (g

SCOD/l d), and K is a kinetic constant that represents

the value of BV for which the value of R is equal to

1=2RM.The value of the kinetic constants was calculated by

means of the plot of the inverse of BV versus the inverse

of R. A straight line with a correlation coefficient of 0.97

was obtained. From this linearised equation, RM can be

calculated from the intercept on the y-axis and K can be

calculated from the slope of the straight line. From this

regression, the values of the constants RM and K were

estimated to be 2.1 ± 0.2 g SCOD/l d and 2.4 ± 0.3gTCOD/l d, respectively. Substitution of these values

into Eq. (4) allowed the theoretical R values to be

determined. Differences between experimental and theo-

retical values were lower than 10% in all cases.

The biogas production and its composition were also

a function of the BV used. Fig. 7 shows the effect of BV

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

0 1 2 3 4 5 6 7 8 9

Bv (g TCOD/l d)

R (g

SC

OD

/l d)

Fig. 6. Variation of the SCOD removal rate (R) with the organic

volumetric loading rate (BV).

on the methane production rate (qM), expressed in l/l

waste feed. As can be seen, an increase in BV produced

an increase in methane gas production per volume of thereactor (QM) due to the increase in the total amount of

organic matter provided to the reactor at increased BV

values. However, a reduction of the biogas amount

produced per volume of influent added (qM) was ob-

served in a parallel way when BV increased.

The organic volumetric loading rate (BV) has a great

influence on the anaerobic digestion performance and is

an important parameter to the process scale-up. BV isrelated to the HRT and the influent concentration by the

following equation:

BV ¼ S0Q=V ¼ S0=HRT ð5Þ

where S0, is the influent concentration (gTCOD/l); Q, isthe influent flow-rate (l/d); V , is the volume of the

reactor (l); and HRT, is the hydraulic retention time

(days), which is equal to the quotient V =Q. At a fixed

influent concentration, an increase in BV could be only

obtained by an increase in the influent flow-rate (Q)which causes a reduction in HRT. The HRT is related to

the microorganisms or solids retention time (SRT) asfollows:

SRT ¼ ½X0ðHRTÞ�=XE ð6Þ

where X0, is the concentration of microorganisms in thereactor, given in g VSS/l; XE, is the concentration of

microorganisms in the effluent of the reactor, given in g

VSS/l. The ðX0=XEÞ ratio is defined as the biomass

R ¼ 2:99½l=ð0:016þ lÞ� ð14Þ

0

5

10

15

20

25

0 1 2 3 4 5 6 7 8 9

Bv (g TCOD/l d) R

eten

tion

fact

or (φ

)

Fig. 8. Effect of the organic volumetric loading rate (BV) on the

retention factor (/).


retention factor ð/Þ (McCarty and Mossey, 1991). The

increase in / determines the increase in the SRT and a

better adaptation of the microorganisms to the substrate

favouring methanogenesis and improving process per-

formance. The increase in the SRT in the anaerobic

process can be obtained by increasing the HRT or the /value. Although the increase in the HRT can be

achieved by using greater reactor volume or reducingthe influent flow, the increase in / using alternatives to

prevent the biomass to escape from the process such as

sludge recycling or biomass immobilization are the best

options.

Therefore, HRT can also be expressed as

HRT ¼ SRT=/ ð7Þ

from which the following equation can be obtained:

1=HRT ¼ /=SRT ¼ l/ ð8Þ

taking into account that l, defined as the microbial

specific growth rate (d�1), is numerically equal to the

inverse of SRT. Therefore, by combining Eqs. (5) and

(8), the following equation was obtained:

l ¼ BV=S0/ ð9Þ

Eq. (9) demonstrates that the microbial specific growth

rate and the biomass factor retention are inversely

proportional. Therefore, to achieve good operational

conditions, the retention factor must be increased,

improving the microorganisms separation and reducing

the concentration of VSS in the effluent. The maximum

value of l will be obtained for a maximum value of BV

and a minimum value of /. Fig. 8 shows the effect of BV

on the values of /. According to Table 2, the value of X0

was 4.3 g VSS/l, the value of XE being equal to the VSS

concentration in the effluent. The concentration of

microorganisms in the influent was assumed to be neg-

lected.

The value of / decreased for increased BV values, due

to the increase in influent flow-rate which in turn caused

an increase in the upflow velocity in the reactor. Thisfact provoked a higher microorganisms concentration in

suspension, reducing the SRT. Moreover, an increase in

the amount of organic matter added to the reactor

produced the development of non-methanogenic

microorganisms.

As can be seen in Fig. 8, an exponential relationship

can be established between BV and /. Eq. (10) demon-

strates a strong influence of the organic volumetricloading rate on the retention factor:

/ ¼ 17:63B�0:57V ð10Þ

the regression coefficient, R2, being 0.98.Combining Eqs. (9) and (10), a relationship was

found that correlates l with BV for the range of the

organic loading rates studied:

l ¼ 7� 10�3B1:57V ð11Þ

This equation allows the theoretical l values to be cal-

culated. The value of lM for the experimental range

studied was obtained using Eq. (11). This value was 0.18

d�1 and is very close to that obtained by Rodr�ıguez andLomas (1999). Hence, the minimum theoretical values of

/, SRT and HRT obtained were: 5.4, 5.4 and 1.0 days,

respectively.

Fig. 9 illustrates the effect of the parameter / on themethane production per volume of influent added to the

process (qM). As can be seen, a straight line was ob-

tained, which is given by the following equation:

qM ¼ 0:196/� 0:163 ð12Þ

the regression coefficient, R2, being 0.98.

By combining Eqs. (10) and (12), the following

equation is obtained:

qM ¼ 3:455B�0:57V � 0:163 ð13Þ

Finally, Fig. 10 shows the effect of l on the substrate

removal rate given in SCOD. As can be clearly ob-

served, an increase in l produced an increase in the

substrate removal rate. The curve follows a Michaelis–

Menten equation type, given by the following expres-

sion:

0

0.5

1

1.5

2

2.5

3

3.5

4

0 5 10 15 20 25

Retention factor (φ)

qm (l

/l w

.d)

Fig. 9. Effect of the retention factor (/) on the methane production

per litre of waste influent added (qM).

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2

µ (1/d)

R (g

SC

OD

/l d)

Fig. 10. Effect of the specific microbial growth rate (l) on the SCOD

removal rate (R).


the regression coefficient, R2, and standard error being

equal to 0.97 and 10%, respectively.

Acknowledgements

The authors wish to express their gratitude to the

‘‘Ministerio de Ciencia y Tecnolog�ıa’’ (project REN

2001-0472/TECNO), and ‘‘Ministerio de Educaci�on,Cultura y Deportes’’ for providing financial support.

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