Anaerobic pre-treatment of slaughterhouse wastewater using®xed-®lm reactors

R. del Pozo, V. Diez*, S. Beltr�an

Department of Biotechnology and Food Science, Faculty of Food Science and Technology and Chemistry Science, University of Burgos,

Pza. Misael Ba~nuelos s/n, 09001 Burgos, Spain

Received 30 December 1998; received in revised form 14 April 1999; accepted 21 April 1999

Abstract

The aim of this work was to study the performance of anaerobic ®xed-®lm reactors with non-random support, for poultry

slaughterhouse wastewater pre-treatment, including the in¯uence of operating conditions. The work was carried out with two lab-

scale reactors, one up¯ow and the other down¯ow, both equipped with vertical corrugated PVC tubes as support and a recirculation

circuit. Both reactors were operated at 35°C.

COD removal e�ciencies ranging from 85% to 95% were achieved for organic loading rates of 8 kg COD mÿ3 dÿ1, while the

highest organic loading rates (35 kg COD mÿ3 dÿ1) led to e�ciencies of 55±75%. The reactors did not show destabilization after 12 h

shock loads of 50 kg COD mÿ3 dÿ1 .

Reactor stability was easily achieved under intermittent operation, with weekend breaks, after which the reactors rapidly re-

turned to their optimal performance. The in¯uences of the hydraulic retention time, temperature, the recirculation ratio and ¯ow

direction were also studied. Ó 1999 Elsevier Science Ltd. All rights reserved.

Keywords: Anaerobic pre-treatment; Slaughterhouse wastewater; Fixed-®lm reactors

1. Introduction

The slaughterhouse industry is a very old humanactivity and, although it is still a relatively small-scaleindustrial sector, its environmental impact has grownconsiderably due to the increase in plant production.The main pollutant in slaughterhouse e�uents is bio-degradable organic matter in such a high concentrationthat aerobic treatment processes are limited by the highenergy consumption needed for aeration, the oxygentransfer capacity and the high sludge production. Thistype of wastewater also has a high grease concentration,which makes suspended bio-mass anaerobic reactors,such as the Up¯ow Anaerobic Sludge Bed (UASB), alsounsuitable. This is due to lipid accumulation over the¯ocs (Chen and Shyu, 1998) with the consequent loss ofe�ciency, and because of desgasi®cation limitationswhich can cause ¯otation and biomass wash-out (Sayedet al., 1987).

Anaerobic Filters (AF) with random support havebeen successfully used for slaughterhouse wastewater

treatment obtaining Chemical Oxygen Demand (COD)removal e�ciencies between 80% and 90% for organicloads up to 20±25 kg COD mÿ3 dÿ1, and a quick start-up of two or three weeks (Henze and Hamerro�es,1983; Tritt, 1992; Borja et al., 1993). In addition, ad-equate performance under acidifying operating condi-tions has already been reported for this type of reactor(Borja and Banks, 1994) which have also shown sat-isfactory stability when treating highly loaded waste-water under sudden changes in the operating conditionssuch as temperature shocks, organic and hydraulicoverloads and feed interruptions (Metzner and Temper,1990). In spite of this, random-support ®lters requireperiodical wash-outs to avoid reactor clogging. Forthis reason di�erent support options should be con-sidered.

The aim of this work was to evaluate the performanceof Anaerobic Fixed-Film Reactors (AFFR) with non-random support in the digestion of slaughterhousewastewater. The experiments carried out were designedto study the in¯uence of the organic loading rate (Bv),the hydraulic retention time (HRT), the in¯uent chem-ical oxygen demand (COD), temperature, recirculation,¯ow direction and intermittent operation.

Bioresource Technology 71 (2000) 143±149

* Corresponding author. Tel.: +34-947-258810; fax: +34-947-258831;

e-mail: vdiezb@ubu.es

0960-8524/00/$ ± see front matter Ó 1999 Elsevier Science Ltd. All rights reserved.

PII: S 0 9 6 0 - 8 5 2 4 ( 9 9 ) 0 0 0 6 5 - 6

2. Methods

2.1. Anaerobic ®xed ®lm reactors

Two lab-scale tubular-shaped reactors, built inPlexiglas, 80 cm high and 8 cm in diameter were used inthis work. PVC corrugated tubes; 70 cm high and 2.5 cmin diameter were employed as non-random supportingmaterial. They provided a super®cial area for biomassattachment of approximately 250 m2 mÿ3 and were ar-ranged vertically in order to avoid reactor plugging dueto the high solids concentration of the slaughterhousewastewater (950 mg lÿ1). The only di�erence betweenboth reactors was the direction of ¯ow. Water circulatedupwards in the U-AFFR and downwards in the D-AFFR, as shown in Fig. 1.

Both reactors had a recirculation circuit in order tocontrol super®cial water velocity and mixing conditions.Temperature was maintained at 35°C with a thermo-static bath. Biogas production was measured with anelectromechanical gas-meter device described in detailelsewhere (Diez, 1991). Periodical re-calibration was

needed due to electrode corrosion caused by the H2Scontent of the biogas.

2.2. Slaughterhouse wastewater

The wastewater used in this work was taken from thepoultry slaughterhouse of Cooperativa Av�õcola yGanadera de Burgos, a local poultry processing plant.Chicken blood was used to increase feed COD concen-tration when needed. The feed tank was stirred with aninternal recirculation system and the input to the reactorwas placed so as to avoid collection of the settled solidsand to provide a homogeneous suspended solids con-centration in the in¯uent. Table 1 presents the usualcharacteristics of poultry slaughterhouse wastewater.

2.3. Reactor operation

Both reactors were operated for 13 months, stoppedfor 4 months and then operated again for 4 moremonths. Despite such a long stop, the reactors werequickly and satisfactorily re-started. After the in¯uence

Fig. 1. Flowchart of the anaerobic ®xed-®lm reactors: (1) up¯ow-AFFR; (2) down¯ow-AFFR; (3) feed; (4) e�uent; (5) recirculation; (6) gas-meter

device (L.C.: level control, C.: counter); (7) thermostatic jacket.

144 R. del Pozo et al. / Bioresource Technology 71 (2000) 143±149

of ¯ow direction was studied, the D-AFFR was operatedwith a week-end break program, a very common oper-ating procedure in small slaughterhouses, while the U-AFFR was fed continuously. Table 2 shows the organicloading rate (Bv) and the in¯uent organic matter con-centration (CODf ) as well as the reactor employed forthe study of each operation parameter.

2.4. Analytical procedures

Feed and e�uent samples were analyzed daily forchemical oxygen demand (COD) and pH. Alkalinityratio (IA:PA), de®ned as the intermediate alkalinity (IA;titration from pH 5.75 to 4.3) divided by partial alka-linity (PA; from original pH to 5.75) (Ripley et al.,1986), was also measured daily. Total and volatile sus-pended solids (TSS, VSS), total kjeldhal nitrogen (TKN)and ammoniacal nitrogen (NHx±N) concentrations weredetermined weekly, increasing the frequency when re-quired. All analyses were developed following StandardMethod's procedures (APHA, 1998). Finally, two bio-gas production measurements were taken daily.

3. Results and discussion

3.1. In¯uence of the direction of ¯ow

Both reactors (U-AFFR and D-AFFR) were oper-ated for 130 days at a hydraulic retention time (HRT) of6 h and moderate organic loading rates (Bv) between 2

and 12 kg COD mÿ3 dÿ1. The COD removal achievedfor the two reactors employed is presented in Fig. 2 as afunction of the organic loading rates. A statistical t-testwith a con®dence level (a) of 0.05, showed that there wasno di�erence in COD removal e�ciency between bothreactors.

Total suspended solids (TSS) values for the two re-actors ¯uctuated within a wide interval, between 60 and220 mg lÿ1, although both reactors had about the sameaverage TSS concentration; 137 for the U-AFFR and146 for the D-AFFR. No relationship was found be-tween this ¯uctuation and any other operating condi-tion, therefore, it was attributed to the design of theoutput pipes in such small reactors as those employed inthis work.

Accumulation of suspended biomass was observed inboth reactors during the complete period of study pre-sented in Table 2. After 17 months of operation, volatilesuspended solids (VSS) concentrations were 6800 mg lÿ1

for the U-AFFR and 5000 mg lÿ1 for the D-AFFR. Inboth cases, suspended biomass formed easily settling¯ocks.

3.2. In¯uence of the organic loading rate (Bv)

3.2.1. In¯uence of a progressive load increaseIn order to study the in¯uence of the organic loading

rate on process e�ciency, values between 5 and 35 kgCOD mÿ3 dÿ1 were applied. Fig. 3 shows that a decreaseof COD removal e�ciency, calculated with e�uentcentrifuged COD, occurred when the organic loadingrate in the up¯ow reactor increased, in other words,when Bv rose from 8 to 30 kg COD mÿ3 dÿ1, COD re-moval decreased from 85±95% to 55±75%. An increasein the organic loading rate slightly a�ected theammoni®cation rate of the anaerobic reactor, loweringthe NHx±N/TKN ratio of the e�uent from 96% to 85%.

Organic loads used in this work were considerablyhigher than those found in the literature for this type ofreactor, the highest of which was 25 kg COD mÿ3 dÿ1

(Borja et al., 1995). Under such conditions and despitethe decrease in removal e�ciency, the pH was above 7.4and the alkalinity ratio (IA:PA) below 0.54, which in-

Table 2

Operating conditions

Operation parameter studied Reactor Bv (kg COD mÿ3 dÿ1) CODf (mg lÿ1)

Direction of ¯ow U, D-AFFR 7.7 1800

Organic loading rate U-AFFR 5±35 4000 and 11800

Shock organic load U, D-AFFR 50 6000

Feed COD concentration U-AFFR 9 2560±11300

Hydraulic retention time D-AFFRa 27 2500

Intermittent operation D-AFFRa 7.7 1600

Recirculation D-AFFRa 9.6 2060

Temperature D-AFFRa 9.4 2160

aIntermittent operation with a break program of 60 h per week.

Table 1

Poultry slaughterhouse wastewater composition

Interval Average

Q (m3 tonÿ1)a 3.4±15 8

BOD5 (mg lÿ1) 600±1700 1200

COD (mg lÿ1) 1100±3400 2100

TSS (mg lÿ1) 600±1900 950

TKN (mg lÿ1) 100±300 220

TP (mg lÿ1) ± 70

Grease (mg lÿ1) ± 110

aTon of broiler.

R. del Pozo et al. / Bioresource Technology 71 (2000) 143±149 145

dicated a tolerable accumulation of Volatile Fatty Acids(VFA). According to Ripley et al. (1986), although op-timal anaerobic digestion occurs at IA:PA values from0.25 to 0.3, methanogenic inhibition does not start until0.8.

3.2.2. E�ect of transient organic overloadsShock loads are common in all kinds of industries

and they may have an important harmful e�ect on an-

aerobic biological processes, causing destabilization ofthe microbial populations. This leads to VFA accumu-lation that can acidify the reactor and therefore inhibitmethanogenic microorganisms. In this case the reactorenters an accumulation±inhibition cycle which impliesits total collapse.

A shock load of 50 kg COD mÿ3 dÿ1 maintained for12 h, with a hydraulic retention time around 3 h wasapplied to the U-AFFR. Fig. 4 shows the evolution of

Fig. 3. COD removal in the up¯ow-AFFR versus organic loading rate (Bv) (calculated with e�uent centrifuged COD concentration).

Fig. 2. COD removal in the up¯ow and down¯ow AFFR with moderate organic loads (calculated with e�uent total COD concentration).

146 R. del Pozo et al. / Bioresource Technology 71 (2000) 143±149

the e�uent centrifuged COD concentration because ofthe shock load. E�uent COD increased from 1550 to2250 because of the Bv increase, which corresponded toa decrease in the removal e�ciency from 72% to 58%.After the shock load, eight days were required for thereactor to recover its normal performance. However, nosigns of destabilization were detected, the reactor pHwas maintained around 7.4 by the bu�ering capacity ofthe system and the alkalinity ratio (IA:PA) was alwaysbelow 0.56. A similar overload was applied to thedown¯ow reactor (D-AFFR). This showed the samestability, but a shorter recovery time after the shockbecause of the usual weekend break.

3.3. In¯uence of feed COD concentration

The reactors used were tubular-shaped, thereforetheir performance could be in¯uenced not only by theorganic loading rate but also by the feed COD concen-tration. In fact, Borja et al. (1995) detected VFA accu-

mulation and an important removal e�ciency decreasedue to the inhibition of the digestion process, whenworking with in¯uent abattoir wastewater COD con-centrations of 29 000 mg lÿ1 in a down-¯ow ®xed-bedreactor packed with clay rings.

In order to study the e�ect of feed organic matterconcentration, three di�erent feeds with average CODconcentrations of 2560, 5080 and 11 300 mg lÿ1 respec-tively, were applied at a constant loading rate about 9 kgCOD mÿ3 dÿ1. To achieve an increase in the concen-tration of the poultry slaughterhouse wastewater,chicken blood without clot, with an average COD of75 000 mg lÿ1, was added.

Table 3 shows the COD concentration inside the U-AFFR reactor (CODe) and the COD removal e�ciency(% COD) for the di�erent feed concentrations (CODf)used, the corresponding hydraulic retention times andthe organic loading rates. Under the operating condi-tions of this experiment, feed concentrations rangingfrom 2560 to 11 300 mg lÿ1 had no in¯uence on COD

Table 3

Results obtained with di�erent feed COD concentrations and ®xed Bv

CODf (mg lÿ1) HRT (h) Bv (kg COD mÿ3 dÿ1) CODe (mg lÿ1) % COD

Average r Average r

2560 6 9.4 1 340 87 4

5080 15 8.0 1 650 87 3

11 300 31 8.3 4 1200 89 1

Fig. 4. Evolution of e�uent centrifuged COD during a shock load in the U-AFFR with a hydraulic retention time longer than 3 h.

R. del Pozo et al. / Bioresource Technology 71 (2000) 143±149 147

removal e�ciency, which remained constant at a ®xedorganic loading rate, regardless of the organic matterconcentration inside the reactor.

3.4. In¯uence of the hydraulic retention time

From the results discussed in the previous section andsummarized in Table 3, it was deduced that, at a ®xedBv around 9 kg COD mÿ3 dÿ1, the process e�ciency didnot depend on HRT when it is longer than 6 h. Butwhen a considerably lower HRT (1.7 h) and a high or-ganic loading rate (23±35 kg COD mÿ3 dÿ1) were ap-plied, removal e�ciency decreased sharply to 38% andbiogas production fell quickly down to 30% of thenormal level. In spite of this, the alkalinity ratio (IA:PA)was maintained at an acceptable value of 0.49 meaningthat methanogenic inhibition did not occur. However,with this low HRT (1.7 h), a 50 kg COD mÿ3 dÿ1 shockload produced a quick increase in the alkalinity ratio toa dangerous value of 0.81. As indicated previously in thesection about transient organic overloads and in Fig. 4,the application of shock loads when HRT was higherthan 2.8 h did not cause such destabilization, a factwhich shows the great in¯uence of very low HRT overprocess stability.

Once operating conditions were relaxed by loweringorganic loading rate from 50 to 17 kg COD mÿ3dÿ1 andraising HRT from 1.7 to 5.5 h, the reactor reached itsusual biological activity in one week.

3.5. Intermittent operation

In order to study the reactor performance under realslaughterhouse operating conditions, the in¯uence ofintermittent operation was determined, with weeklybreaks of 60 h over the weekend. The reactors were keptat 35°C during the breaks.

The behavior of suspended biomass anaerobic reac-tors (UASB) for dairy wastewater treatment under in-termittent operation was studied by Carozzi (1993). Hefound that weekend breaks cause biological unbalancethat drives to ¯oc disgregation and biomass loss. How-ever, experience of anaerobic ®lters indicates that theycan tolerate frequent short breaks (Vincent, 1993).

In this work, satisfactory stability and removal e�-ciency were observed for more than a year of intermit-tent operation. Although slight mineralization of thesuspended solids and the organic nitrogen occurredduring the break, endogenous decomposition of thebiomass during these periods was not signi®cant sincenormal removal e�ciencies were reached within the ®rst24 h after resuming feeding the reactor.

The stability observed under intermittent operationwas maintained even when the HRT was only 1.7 h andalso at a low temperature (20°C). For the highest or-ganic loading rate fed (30 kg COD mÿ3 dÿ1), COD re-

moval e�ciency on the continuously operated U-AFFRwas 66%, while under intermittent operation (D-AFFR)a slightly better removal of 71% was obtained, becauseduring the breaks VFA accumulation was mitigated.

3.6. In¯uence of recirculation

Recirculation can either increase or decrease removale�ciency of ®xed-®lm reactors depending on the extentof the recirculation ¯ow and feed COD concentration(Tritt, 1992). The presence of suspended biomass mayalso be in¯uenced by super®cial velocity.

The reactors used in this work were operated withrecirculation ratios between 0 and 22, which impliessuper®cial water velocities of up to 0.2 m hÿ1, and withinan organic loading rate interval ranging from 7.0 to 15kg COD mÿ3 dÿ1. Each operating condition was main-tained from 1 to 3 months. A statistical t-test showed,with a con®dence level (a) of 0.05, that there was noin¯uence of recirculation over COD removal. Conse-quently, in the operation range studied, ®xed-®lm reac-tors could be successfully operated withoutrecirculation, which implied minimum operating costs.However, the relationship between recirculation andsuspended biomass inside the reactor should be con-sidered in long-term studies.

3.7. In¯uence of temperature

In order to evaluate the feasibility of treatingslaughterhouse wastewater at the temperature it is pro-duced; without any pre-heating, the anaerobic down¯owreactor was operated for 21 days at 20°C and a mod-erate organic loading rate of 6.0 kg COD mÿ3 dÿ1. Thee�ciencies obtained at 20°C and 35°C were comparedand it was observed that the COD removal e�ciencydecreased from 80% to 60%, which agrees with the re-sults presented by other authors under similar condi-tions (Viraraghavan and Kikkeri, 1990). In any case,COD removal e�ciencies from 2160 to 860 mg lÿ1 jus-tify wastewater pre-treatment without any externalheating.

4. Conclusions

The anaerobic ®xed-®lm reactor with non-randomsupport is a very appropriate system for pre-treatment ofwastewater with high organic load and high solids con-centration, such as slaughterhouse e�uents. It providessatisfactory organic matter removal e�ciencies, evenwhen using high organic loading rates, under intermittentoperation, and with or without recirculation. Moreover,under stressed operating conditions, such as shock loads,very low hydraulic retention time or low temperature, thereactor shows a quite stable performance.

148 R. del Pozo et al. / Bioresource Technology 71 (2000) 143±149

Acknowledgements

The authors gratefully acknowledge ®nancial supportprovided by the Universidad de Burgos, Junta de Castillay Le�on (EB01/98) and Cooperativa Av�õcola y Ganaderade Burgos. R del Pozo is grateful to the Spanish MECfor a fellowship.

References

American Public Health Association, 1998. Standard Methods for the

Examination of Water and Wastewater, 20th edn. APHA, Wash-

ington, DC, USA.

Borja, R., Banks, Ch., Mart�õn, A., 1995. In¯uence of the organic

volumetric loading rate on soluble chemical oxygen demand

removal in a down-¯ow ®xed-bed reactor treating abattoir waste-

water. J. Chem. Tech. Biotechnol. 64, 361±366.

Borja, R., Banks, Ch., 1994. Treatment of palm oil mill e�uent by

up¯ow anaerobic ®ltration. J. Chem. Tech. Biotechnol. 61, 103±

109.

Borja, R., Duran, M.M., Martin, A., 1993. In¯uence of the support on

the kinetics of anaerobic puri®cation of slaughterhouse wastewater.

Biores. Technol. 44, 57±60.

Carozzi, A., 1993. Pretratamiento de las aguas residuales de la

industria lechera. In Proceedings of the ®fth Symposium on

Wastewater Anaerobic Treatment. University of Valladolid,

Spain.

Chen, T.H., Shyu, W.H., 1998. Chemical characterisation of anaerobic

digestion treatment of poultry mortalities. Biores. Technol. 63, 37±

48.

Diez, V., 1991. Evoluci�on y comportamiento hidrodin�amico de un

bioreactor anaerobio de lecho ¯uidizado. Ph.D. thesis, Faculty of

Science, University of Valladolid. Spain.

Henze, M., Hamerro�es, P., 1983. Anaerobic treatment of wastewater in

®xed ®lm reactors ± A literature review. Water Sci. Technol. 15, 1±

101.

Metzner, G., Temper, U., 1990. Operation and optimization of a full-

scale ®xed-bed reactor for anaerobic digestion of animal rendering

waste water. Water Sci. Technol. 22, 373±384.

Ripley, L.E., Boyle, W.C., Converse, J.C., 1986. Improved alkalimetric

monitoring for anaerobic digestion of high-strength wastes. J.

WPCF 58, 406±411.

Sayed, S.J., van Campen, L., Lettinga, G., 1987. Anaerobic treatment

of slaughterhouse waste using a granular sludge UASB reactor.

Biol. Wastes 21, 11±28.

Tritt, W.P., 1992. The anaerobic treatment of slaughterhouse waste-

water in ®xed-bed reactors. Biores. Technol. 41, 201±207.

Vincent, M.T., 1993. Digesti�on anaerobia de purines de cerdo. In

Proceedings of the ®fth Symposium on Wastewater Anaerobic

Treatment. University of Valladolid, Spain.

Viraraghavan, T., Kikkeri, S.R., 1990. E�ect of temperature on

anaerobic ®lter treatment of dairy wastewater. Water. Sci. Technol.

22, 191±198.

R. del Pozo et al. / Bioresource Technology 71 (2000) 143±149 149