Process Biochemistry 34 (1999) 653–657

Studies on the use of an anaerobic baffled reactor for thecontinuous anaerobic digestion of pulp and paper mill black

liquors

R. Grover a, S.S. Marwaha a, J.F. Kennedy b,*a Department of Biotechnology, Punjabi Uni6ersity, Patiala, India

b Birmingham Carbohydrate and Protein Technology Group, School of Chemistry, The Uni6ersity of Birmingham, Birmingham B15 2TT, UK

Received 8 May 1998; received in revised form 13 October 1998; accepted 24 October 1998

Abstract

This paper reports the use of an anaerobic baffled reactor (ABR) and effect of different pH, temperatures, hydraulic retentiontimes and organic loading rates on continuous anaerobic digestion of black liquor from pulp and paper mills. A maximum CODreduction of about 60% at an organic loading rate (OLR) of 5 kg m−3 d−1 at hydraulic retention time of 2 d, pH 8.0 and 35°Cwas recorded. The OLR above 6 kg m−3 d− l were toxic and destabilised the reactor system. © 1999 Elsevier Science Ltd. Allrights reserved.

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1. Introduction

The alkaline black liquor released by paper millsconstitutes only 10–15% of the total wastewater, butcontributes approximately 95% of the total pollutionload of pulp and paper mill effluents [1]. The blackliquor has characteristically high biochemical oxygendemand (BOD), chemical oxygen demand (COD), andtotal solids (TS) along with slowly degradable lignincompounds, which make it significantly toxic to theenvironment. To date, the most effective method is thecombustion sodium recovery process in which the blackliquor is first concentrated, then incinerated. In thisway, organic substances in the wastewater may bemineralised and sodium simultaneously recovered.However, this treatment process is unsuitable for smallscale mills because of high capital cost which makes theprocess ineffective and the operational cost very high.

Thus, a more practical treatment process for smallpulping mills is needed [2].

The anaerobic baffled reactor (ABR) comprises aseries of vertical baffles to force the wastewater to flowunder and over them and therefore, the wastewatercomes into contact with a large active biological mass.This reactor system has been reported to have manyadvantages over other well established reactor systems.It is simple in design and requires no gas separationsystem. Moreover, the over and underflow of liquidreduces bacterial washout and enables it to retain activebiological solids without the use of any fixed media [3].The unique structure of the ABR brings about thepartial separation of acidogenesis and methanogenesis[4] and prevents most of the biomass being exposed tolow pH during shock loads and enhances reactor stabil-ity [5]. The ABR configuration has potential in treatingindustrial waste that vary in both flow and concentra-tion and still enable high removal rates to be achieved[6]. Although attention has been paid to the use of thisreactor system for the anaerobic treatment of industrialwastewaters such as high strength molasses wastewater[7] and distillery wastewaters [8], a thorough literaturesurvey revealed no application of ABR for the anaero-bic treatment of black liquors of the pulp and paperindustry.

Abbre6iations: ABR, anaerobic baffled reactor; HRT, hydraulicretention time; COD, chemical oxygen demand; OLR, organic load-ing rate; VFA, volatile fatty acids.

* Corresponding author. Tel.: +44-121-4144385; fax: +44-121-4144384.

E-mail address: jfkennedy@chemistry.bham.ac.uk (J.F. Kennedy)

0032-9592/99/$ - see front matter © 1999 Elsevier Science Ltd. All rights reserved.

PII: S 0 0 3 2 -9592 (98 )00138 -1

R. Gro6er et al. / Process Biochemistry 34 (1999) 653–657654

2. Materials and methods

2.1. Substrate

The black liquor used in the experiment was collectedfrom M/s Shreyans Pulp and Paper Mill, Ahmedgarh(Pb.), which used cereal straw as the raw material anda soda process for pulping the straw. The black liquorwas collected in plastic cans and stored frozen at −4°Cfor further use. The black liquor thus collected wasanalysed for its physico-chemical characteristics.

2.2. Seed material

The digested sewage sludge (DSS) used as seed cul-ture was procured from the Municipal Sewage WasteWater Treatment Plant, Okhla, New Delhi andscreened through a 2 mm sieve before use. The DSShad 69 g l−1 total suspended solids (TSS) and 22 g l−1

volatile suspended solids (VSS). The acclimatisation ofDSS was carried out in the reactor itself. For thispurpose, the diluted black liquor was mixed with DSSin the ratio of 3:1 (v/v) so as to have a resultantdissolved COD of 2 g l−1. The final pH was adjusted to7.2 using HCl (12 N). The reactor was sealed afterfilling and its gas outlets were clipped. These wereloosened everyday to observe the gas formation whichwas also indicated by the frequency of bubbling inwater. Little or no gas formation indicated the comple-tion of digestion of the organic matter by the anaerobicmicrobes. A part of the supernatant was removed at thecompletion of each digestion and was replaced with anequal amount of fresh diluted black liquor. This processwas repeated until a constant COD removal of approx-imately 65% was achieved at a retention time of 5 days.The whole process of acclimatisation was completed in50 days. This DSS was further used for continuousanaerobic digestion of black liquor.

2.3. Fabrication of anaerobic baffled reactor

A laboratory scale anaerobic baffled reactor (ABR)system used in the study was fabricated using plexi-glass. The ABR consisted of three chambers and eachchamber was separated by a vertical baffle. The work-ing volume of the reactor was 10 l (length, 38 cm;breadth, 12 cm; height, 29.5 cm). The reactor was filledwith digested sewage sludge and diluted black liquor inthe ratio of 3:1 (v/v) so as to have a resultant solubleCOD of 2 g l−1. The DSS was acclimatised for about50 days in batch condition to have fast startup of thereactor. The ABR was run continuously by feeding theblack liquor at a constant flow rate using a peristalticpump (Miclins PP10).

2.4. Analysis

The untreated and anaerobically digested blackliquor samples were analysed as per the APHA stan-dard method [9].The volatile fatty acids (VFA) andmethane content of the biogas were analysed using gasliquid chromatography (Nucon 5700) FID with chro-mosorb 101 glass and Porapack Q stainless steelcolumns, respectively. The respective temperatures foroven, injector and detector were 180, 200 and 210°C.Nitrogen was used as carrier gas in the column andhydrogen was used for burning of the flame. Thepressure for nitrogen and hydrogen was maintained at 3kg cm−2 and 1.2 kg cm−2, respectively. The samplesfor analysis of volatile fatty acids were pre-treated toseparate out solids with orthophosphoric acid and cen-trifuged at 10 000 rpm for 30 min. The supernatant wasused for estimation of volatile fatty acids by injecting 1ml in the glass column with the help of glass syringe(Hamilton, USA). The different volatile fatty acidswere calculated in mg l−1 from the respective areas ofpeaks of standard fatty acids with known concentra-tion. Biogas production was quantified by upward dis-placement of acidified solution into a graduated gasholder connected to a displacement reservoir. Forbiogas analysis, a 10 ml sample was injected with a gastight syringe into the GLC with a Porapack Q stainlesssteel column using the same conditions as for volatilefatty acids.

3. Results And Discussion

3.1. Characterisation of black liquor

The black liquor periodically collected from theShreyans Pulp and Paper Mills Ltd., Ahmedgarh (Pb.)was characterised for its various physico-chemical con-stituents (Table 1). The black liquor samples had highamounts of total solids, total suspended solids, total

Table 1Physico-chemical characterisation of black liquor*

pH 9.2–10.240 000–50 000Total solids (TS)

Total suspended solids (TSS) 11 000–15 00029 000–35 000Total dissolved solids (TDS)32 000–40 000Chemical oxygen demand (COD)

Biochemical oxygen demand (BOD) 12 000–16 000Total carbohydrates 13 000–15 000

6400–8000Lignin and related compounds6000–7500Alkalinity1500–2000Volatile fatty acids (VFAs)

Ammonia nitrogen 750–1000Phosphates 400–500

700–900Sulphates

* All parameters except pH are in mg l−1.

R. Gro6er et al. / Process Biochemistry 34 (1999) 653–657 655

Table 2Methanogenesis of black liquor using anaerobic baffled reactor at different hydraulic retention timesa

Influent COD (mg l−1) Biogas (l d−1)HRT (d) COD removal (%)Effluent COD (mg l−1) Methane content (% v/v)

4031922 12359285 69.490.3 1.0790.01 50.390.612229204 52.490.51.3690.0169.790.44032926

3 12409144022928 69.290.4 1.8790.02 54.790.52 4027917 1306922 67.690.5 2.8890.02 59.190.3

a Each value represents the mean 9S.D. from five observations in 10 days. Operation conditions: temperature 3092°C, pH 7.2.

dissolved solids, pH, lignins and related compoundsand volatile fatty acids. The high alkalinity of the blackliquor is attributed to the use of the soda process forpulping of straw (agroresidue) for paper making. Thevariation in the physical and chemical characteristics ofthe black liquor at different time intervals may beattributed to the extent of back water reuse and to thevariation in pulping size of the mill. The high value ofBOD in the black liquor suggested that it contained alarge amount of biodegradable organics. In addition,black liquor contained fairly good amounts of ionicnutrients such as ammonia nitrogen, phosphates andsulphates which have been reported to be essential foranaerobic digestion [10].

3.2. Start-up of anaerobic baffled reactor at differenthydraulic retention times (using suspended growthsystems)

The continuous operation of the ABR was startedusing an initial COD concentration of 4 g l−1 athydraulic retention time (HRT) of 5 d. The ABR wasrun continuously and observations were made for 10days at a particular HRT. When there were no morefluctuations in different parameters such as COD re-moval, methane content and biogas production thenthe HRT was decreased further. The results in Table 2show that there was little variation in the percent CODremoval with decrease in retention time, but there wasan increase in methane content and biogas production.This suggested that at higher retention times there waslimited availability of substrate for methanogenesis asmost of the COD was utilised for new cell synthesis.However, the decrease in retention time resulted inmore availability of substrate which could be converted

to biogas. The increase in methane content may beattributed to the increase in the growth of anaerobesparticularly methanogens in the newly synthesisedbiomass. The methane yield values ranged between0.096 and 0.125 m3 kg−1 COD removed. No volatilefatty acid was detected in the effluent of the reactor atdifferent tested HRT which indicated limited substrateavailability for large active biomass. Partial hydrolysisfollowed by complete conversion of the volatile fattyacids to methane may be attributed to the presence oflignins which are considered to be the major recalci-trant in anaerobic digestion. Similar results have alsobeen observed in cases of water hyacinth-cattle dungwaste [11].

3.3. Effect of pH on methanogenesis of black liquorusing ABR system

The black liquor is alkaline in nature and to test itssuitability for methanogenesis a pH range from 6.5 to9.5 was investigated in a continuous system. Informa-tion on the effect of different pH values on variousparameters (Table 3) shows that the black liquor couldbe successfully anaerobically digested over the pHrange of 7.5–8.5. However, pH value higher than 8.5were observed to be highly toxic to the anaerobes. Theresults are in contradiction to those of Dangcong andQiting [2] who observed that black liquor could besuccessfully treated effectively over the pH range of9.5–10.6. These workers explained that firstly, anaero-bic digestion of black liquor was acidogenic and hadneutralising ability, and secondly the bulk of the reac-tor had high alkalinity which could provide a signifi-cant buffering capability. The present studynevertheless was made at low organic loads which may

Table 3Methanogenesis of black liquor using anaerobic baffled reactor at different pH valuesa

Influent COD (mg l−1) Effluent COD (mg l−1)pH COD removal (%) Biogas (l d−1) Methane content (% v/v)

2.4190.0259.790.41614920 52.590.540029166.568.190.31280918 59.490.440199107.5 2.9590.02

3998917 13339228.5 66.790.4 2.7990.03 55.890.348.490.456.490.4175892040299179.5 2.1890.02

a Each value represents the mean 9S.D. from five observations in 10 days. Operation conditions: temperature 3092°C, HRT 48 h.

R. Gro6er et al. / Process Biochemistry 34 (1999) 653–657656

Table 4Methanogenesis of black liquor using anaerobic baffled reactor at different temperaturesa

Temperature (°C) Influent COD (mg l−1) Effluent COD (mg l−1) COD removal Methane content (% v/v)Biogas (l d−1)(%)

64.090.41448927402792025 58.490.52.5690.014028915 59.090.62.9190.0268.490.61273913304023916 120591835 70.090.5 3.3590.02 59.490.94042939 1216925 70.090.540 3.3790.02 59.290.5

a Each value represents the mean 9S.D. from five observations in 10 days. Operation conditions: pH 8.090.2, HRT 48 h.

have resulted in a small amount of acid production andthus had little neutralising ability for alkaline blackliquor and showed poor performance at high pH. Theresults are however, supported by work of Prasad [12]on bagasse based paper mill wastewater.

3.4. Effect of temperature on methanogenesis of blackliquor using ABR system

The effect of temperature (25–40°C) on methanogen-esis of black liquor in the continuous system (Table 4)shows an increase in biogas production and COD re-moval rates with an increase in temperature from 25–35°C. The increase in temperature beyond 35°C showedno marked improvement in the performance parame-ters. A similar trend has also been observed by manyworkers on different wastes [11,13]. The use of a tem-perature lower than the optimum for carrying outanaerobic digestion has been suggested [14] as this hasthe advantage of low energy input. The low ambient airtemperature has been reported to result in decrease inthe biogas production due to decrease in anaerobicactivity [15]. The methane yield increased from 0.114 to0.147 m3 kg−1 COD removed as the temperature wasincreased from 25 to 35°C. Therefore, a temperature ofaround 35°C was maintained in all further experiments.

3.5. Effect of organic loading rate using suspendedgrowth in the ABR system

The effect of different organic loading rates (OLR)was studied in a continuous system by varying theCOD of the influent substrate. The increase of organic

loading rates (Table 5) resulted in a decrease in CODremoval. This may be explained on the basis of obser-vations made by several workers [16–18] who reportedthat at low organic loading rates the influent COD wascorrespondingly low. As a result the concentration ofthe lignins present in the black liquor were lower thanthe inhibitory concentrations, whereas the increase ininfluent COD resulted in an increase in lignin concen-tration and subsequently, inhibited the anaerobicbacteria.

An increase in biogas production was observed withan increase in organic loading rate despite the decreasein percent COD removal. This may be attributed to thefact that although there was a decrease in COD re-moval at higher loading rates; even then the removal ofinfluent COD was more at higher OLR than at lowerOLR The increase in OLR resulted in a decrease inmethane content and this can be attributed to increas-ing rate of acidogenesis and non proportional growthof methanogens (which consumes CO2 as substrate toproduce methane). The results (Table 5) show that withincrease in OLR there was an accumulation of volatilefatty acids in the reactor as indicated by effluent VFA;however, the reactor showed stable operation at anOLR of 5 kg m−3 d−1. The reactor lost its stability atan OLR of 6 kg m−3 d−1 which was apparent by thedecrease in biogas production rate and its methanecontent. The methane yield varied between 0.141 and0.178 m3 kg−1 COD removed. The low methane yieldvalues are attributed to the toxic effect of black liquoron methanogenesis. This was also accompanied bylarge washout of the biomass (VSS 1.7 g l−1). Theresults obtained were in agreement with the observa-

Table 5Methanogenesis of black liquor using anaerobic baffled reactor at different organic loading ratesa

OLR BiogasCOD removal Methane content Effluent VFAEffluent CODInfluent COD(L D−1) (% v/v)(mg l−1) (%)(kg m−3 d−1) (mg l−1)(mg l−1)

–59.490.93.3590.0270.090.5120591840239162.05.0690.0466.890.5199293460029353.0 183920.766.590.4

10003969 3966954 60.390.35.0 8.2590.06 354923.865.090.454.090.2557992412118958 448923.36.0 58.590.58.9090.05

a Each value represents the mean 9S.D. from five observations in 10 days. Operation conditions: temperature 3592°C, pH 8.090.2, HRT48 h.

R. Gro6er et al. / Process Biochemistry 34 (1999) 653–657 657

tions made by other workers [18,19] as far as toxiceffect of black liquor, but differed with respect to OLR.The poor performance of the reactor at high OLR maybe attributed to the poor settlement of the sludge. Theactive biomass in terms of VSS in the reactor at the endof the study was 43 g l−1.

The present study concludes that black liquor is toxicto anaerobic treatment at high concentration and needsto be diluted to a subtoxic level, with the waste streamsgenerated at other processing stages of the mills, beforeit is subjected to anaerobic treatment. Further studiesare suggested on the detoxification of black liquor forcommercial application of the anaerobic treatment inpulp and paper mills.

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