Application of rumen microorganisms for a high rate anaerobic digestion of papermill sludge
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Biological Wastes 32 (1990) 169-179
Application of Rumen Microorganisms for a High Rate Anaerobic Digestion of Papermill Sludge
Huub J. Gijzen,* Piet J. L. Derikx & Godfried D. Vogels~
Department of Microbiology, Faculty of Science, University of Nijmegen, Toernooiveld, NL-6525 ED Nijmegen, The Netherlands
(Received 8 October 1988; revised version received 18 August 1989; accepted 23 August 1989)
The anaerobic digestion of papermill sludge containing a high amount of inorganic matter was studied. Despite the presence of about 58% inorganic matter in the substrate, a high rate of hydrolysis and subsequent acid formation could be achieved in an acidogenic reactor which was inoculated with rumen microorganisms. Degradation efficiency of neutral detergent fibre amounted to 62% at high loading rate (34.2 g volatile solids per litre per day) and short solid retention time (51 h). In order to study the effect of accumulation of inorganic matter in the reactor, degradation efficiency was studied at various loading rates and solid retention times. An increase of solid retention time to 74 h resulted in a decreased degradation efficiency, probably due to an increased ash content in the reactor under these conditions. The effect of inorganic matter accumulation was also studied after coupling of the rumen derived acidogenic reactor to an upflow anaerobic sludge blanket-type methanogenic reactor. By using this two-stage digestion process an overall conversion of papermill sludge into biogas could be realized. The operation over about three months of the two-stage process was studied in terms of process stability, nutrient recycling and accumulation of inorganic matter.
* Present address: Department of Botany, Microbiology Unit, University of Dares Salaam, PO Box 36050, Tanzania. :~ To whom correspondence should be addressed.
169 Biological Wastes 0269-7483/90/$03"50 1990 Elsevier Science Publishers Ltd, England. Printed in Great Britain
170 Huub J. Gijzen, Piet J. L. Derikx & Godfried D. Vogels
Pulp and paper mills discharge large quantities of wastewater and sludge which mainly consist of cellulose and inorganic material. The sludge produced is usually dewatered up to a solids concentration of 30-45% and subsequently dumped at landfill sites, which results in high disposal costs. On the other hand, the organic part of the waste material may be considered as a potential source for the production of renewable energy in the form of methane through anaerobic digestion. Recent advances in the microbiology and technology of anaerobic digestion have led to a broad application of this process to the treatment of industrial wastewaters (Sahm, 1984; van den Berg, 1984).
On the contrary, rather low degradation efficiencies and biogas production rates are observed in conventional stirred-tank reactors which are currently being used for the digestion of some solid organic wastes. The major factor that would lead to significant improvement of the economics of anaerobic digestion would be a decrease in the retention time necessary to obtain effective breakdown of solid organic wastes.
Since the hydrolysis of cellulose has been reported to be the rate-limiting step in overall degradation of complex organic material (Noike et al., 1985), the rate of decomposition might be improved by increasing cellulase activity in the reactors. Therefore, the development of a process with increased cellulolytic activity is of particular interest.
In a recent series of papers we have demonstrated that the use of rumen microorganisms in an 'artificial rumen' reactor results in an enhanced conversion efficiency of cellulosic substrates, including papermill sludge (Gijzen et al., 1986, 1987, 1988b). The improved conversion rates resulted from the high cellulolytic activities of the rumen microorganisms, which are specialized in the digestion of plant fibres. In a subsequent study we demonstrated the overall conversion of papermill sludge into biogas by using a hybrid-polyurethane carrier reactor (Gijzen et al., 1988a). However, the fact that a major part of the papermill sludge is composed of inorganic matter might hamper digestion efficiency and process stability, especially during long-term operation. In this study various conditions of solids retention time (SRT) and loading rate (LR) were applied during the hydrolysis of papermill sludge in order to determine the effect of inorganic matter accumulation.
The operation of a two-phase process consisting of an 'artificial rumen' reactor coupled to a high-rate methane reactor of the upflow anaerobic sludge blanket-type (UASB), was studied in terms of stability, nutrient recycling and accumulation of inorganic matter.
Anaerobic digestion of papermill sludge 171
Acidogenesis of the substrates was performed in a 3 litre (working volume 1.5 litres) 'artificial rumen' reactor (39C) operated with differential removal rates of solids and liquids as was described previously (Gijzen et al., 1986). Experiments were started by inoculating reactors with 250-ml strained rumen fluid which was obtained from a fistulated sheep. After inoculation the reactor was filled with pre-heated (39C) fermentation medium (Rufener et al., 1963) modified by the addition of 0.2mllitre -1 trace elements (Vishniac & Santer, 1957) and NH4C1 (28 mM). Desired hydraulic- (HRT) and solid-retention times (SRT) were established by means of peristaltic pumps which continuously supplied fresh fermentation medium and
Fig. 1. Schematic diagram of the artificial rumen reactor (1A) and the two-phase digestion process (1B). (A) acidogenic reactor; (B) UASB-type methanogenic reactor; (C) fermentation medium reservoir; (D) fermentation medium supply pump; (E) 30/~m pore size filter; (F)
filtered effluent removal pump; (G) methanogenic reactor effluent; (H) rotary shaker.
172 Huub J. Gijzen, Piet J. L. Derikx & Godfried D. Vogels
removed filtered (30#m pore size) liquid effluent, while homogeneous reactor-contents were removed once every day shortly before substrate addition. The difference adjusted between the rate of buffer supply and filtered-effluent removal caused an increase in the reactor working volume. This increase was daily removed as homogeneous reactor-contents, thus leading to the indicated SRT.
The reactor contents were mixed every 15 min for a period of 20 s by means of a laboratory rotary shaker. A schematic diagram of the 'artificial rumen' reactor is presented in Fig. 1A. The 'artificial rumen' reactor is referred to as R1 in the second series of experiments.
During one experiment the acidogenic reactor (R1) was connected to a UASB-type methane reactor (R2; 39C, 2"5 litre volume). The methane reactor was started by the addition of about 1 litre settled granular sludge obtained from a full-scale UASB plant (AVEBE de Krim, The Netherlands). Liquid effluent from R1, containing dissolved organic matter, was continuously fed to R2. During the first part of the experiment the effluent of R2 was discharged. To test the performance of the two-phase process and possible accumulation of inorganic matter with a closed fluid circuit, R2 effluent was recirculated to R1 from day 25 on. At the same time the supply of fresh fermentation medium was stopped. The liquid from the homogeneous effluent was also fed back to the system after separation of the solids. A schematic diagram of the two-phase process is shown in Fig. 1B.
Papermill sludge produced by a papermill (PAGE, Gennep, The Nether- lands) using waste paper as a raw material for tissue production was used as a predominant digester feed. The sludge (about 40% total solids) was separated from the waste water (about 1% total solids) by means of a decanter centrifuge. A small amount of dried and ground (1-2 mm) alfalfa (van Heeswijk, Veghel, The Netherlands) was added as an additional substrate (3% of total substrate dry weight).
The substrate was added to R1 at desired loading rates once every day, while the culture medium was added continuously. The compositions of the two substrates are shown in Table 1.
Sampling and analyses
Biogas productions from R1 and R2 were monitored individually by means of 10 litre mariotte flasks containing acidified tap water (about 0.02% HC1).
Anaerobic digestion of papermill sludge 173
TABLE 1 Composit ion of the Substrates"
Determination Papermill sludge Alfalfa
TS b (%) 39.8 + 2.1 93.0 _ 0.6 VS c (% of TS) 42.6 _ 1.8 90.9 _ 1.9 NDF d (% of TS) 38-8 + 3-8 45-9 + 1-4 Total N (% of TS) 0.16 _ 0.02 2'3 _ 0.2 COD (gO2 per gTS) 0.78 _ 0.10 1.1 _ 0.1 Lignin (% of TS) 5.8 + 1.6 8.9 + 0.7 Hemicellulose (% of TS) 1-4 13.5 Cellulose (% of TS) 31"6 _+ 3.4 23"5 _ 1"6
a Means _ SD. b Dry weight. c Volatile solids. a NDF = neutral detergent fibre = cellulose + hemicellulose + lignin (Goering & van Soest, 1970).
Methane content of the biogas was determined gas chromatographically. Samples for determination of pH, volatile fatty acids (VFA) and neutral
detergent fibre (NDF) were taken from the homogeneous acidogenic reactor effluent.
Ash content in R1 was analysed every day during steady-state operation of the reactor (week 2) on 50-ml samples taken from the daily homogeneous reactor effluents. Ash content of the sludge in R2 was analysed once every week in duplicate. To check the performance of R2, pH and VFA were also analysed in the R2 effluent. All analytical methods were as described previously (Gijzen et al., 1986, 1987).
RESULTS AND DISCUSSION
Acidogenesis of papermill sludge at different loading rates (LR) and solid retention times (SRT)
Performance of the acidogenic reactor was tested in three short-term experiments (2-3 weeks) at different loading rates and retention times of the solid substrate. Hydraulic retention time (HRT) was kept within the range 10.9-13.3h for all experiments. The conditions of Experiment l are summarized in Table 2.
The results of this experiment show that with papermill sludge as a major reactor feed, a high degradation efficiency could be obtained at relatively short SRT (51h) and high LR (34.2g VS litre-1 day-,). This result is
174 Huub J. Gijzen, Piet J. L. Derikx & Godfried D. Vogels
TABLE 2 Fermentat ion of Papermill Sludge in an Acidogenic Reactor at Different Loading Rates (LR)
and Solid Retention Times (SRT)
1 a 2 b 3 c
LR (g VS l itre- 1 day- i) 34"2 35.0 20"4 Ash- input (g TS l itre- 1 day- 1) 39-3 39.4 20.0 HRT (h) 10"9 + 1.1 11"4 13'3 SRT (h) 51 ___ 1 74 83 NDF a digestion (%) 62 ___ 2 26 58 Ash contenff (g TS per litre) 63 ___ 5 93 51
a Means + SD (n = 3 experiments). b (n = 2 experiments). c (n = 1 experiment). a NDF = neutral detergent fibre = cellulose + hemicellulose + lignin (Goering & van Soest,
1970). e g dry weight per litre reactor content.
comparable to results obtained previously with papermill sludge at LR and SRT of 29.4 g VS liter- ~ day- 1 and 40 h, respectively (Gijzen et al., 1987).
Recently we reported that, using a cellulosic fraction of domestic refuse, degradation efficiency could be increased by applying longer SRT (Gijzen et al., 1988b). Only at very long SRT (144 h) was a slightly lower degradation observed, which was due to difficulties in mixing because of the increased TS content in the reactor. In order to test whether an increased degradation efficiency could also be obtained with papermill sludge as a predominant substrate, Experiment 2 was conducted at a prolonged SRT. Surprisingly, degradation efficiency appeared to be markedly lower under these conditions (Table 2). If, on the other hand, simultaneously with an increase of SRT, LR was lowered, degradation efficiency increased again to a value of about 60% (Experiment 3). The sharp drop in degradation efficiency at longer SRT is not consistent with our previous findings on the degradation of a cellulosic fraction of domestic refuse (Gijzen et al., 1988b). During all three experiments no problems with mixing of the reactor contents were observed at any time, thus the lower degradation efficiency observed during Experiment 2 could not be attributed to incomplete mixing. However, the ash content of the papermill sludge appeared to be markedly higher than that of other substrates tested previously (Gijzen et al., 1987, 1988b). Especially under conditions of a high LR and long SRT, the content of inorganic matter in the reactor may increase to inhibiting high levels. Ash content in the reactor during Experiment 2 amounted to 9.3% (w/v)
Anaerobic digestion of papermill sludge 175
(Table 2). Therefore the decreased degradation efficiency during Ex- periment 2 might be caused by the high content of inorganic matter in the reactor. This possibility is supported by the finding that the negative effect of a prolonged SRT could be restored by simultaneously lowering substrate LR (Experiment 3).
Rumen ciliates, characteristic for the rumen-derived reactor, were present in high numbers (about 50 102 ml-1) during Experiments 1 and 3. The exact number of ciliates could not be established because it was difficult to distinguish between ciliates and substrate particles by microscopical enumeration. No ciliates could be observed during Experiment 2, suggesting that ciliates were inhibited by the high ash content in the reactor.
Surprisingly, the degradation efficiency observed during Experiment 3 did not exceed that of Experiment 1, despite the combination of a long SRT and low LR. Maybe a part of the organic matter is not accessible to the hydrolytic enzymes produced by the rumen microorganisms. Light- microscopical observations revealed that cellulose fibres were partly embedded in inorganic material, thereby forming a physical barrier against hydrolysis (results not shown). Therefore, a degradation efficiency higher than about 60% might only be realized after pretreatment of the sludge in order to release the fibres from the inorganic material.
Performance of the two-phase system
In order to study the performance of the two-phase process and the effect of inorganic matter accumulation, a long-term experiment (4) was performed with the acidogenic reactor (R1) coupled to a UASB-type methane reactor (R2). On the basis of the results of the previous experiments, the two-phase system was operated at relatively short SRT (59 h) and a low LR (23.8 g VS litre- 1 day- 1) to ensure stable performance of R1. HRT in the acidogenic reactor was adjusted to 12"1 _0.1 h.
Since the organic part of papermill sludge is essentially composed of polysaccharides, additional nutrients (N and others) were added in the fermentation medium during all experiments in this study. In order to minimize the amount of nutrient addition, the possibility of nutrient recycling was studied. Previously we demonstrated that complete nutrient recycling could be applied during the digestion of filter-paper cellulose in a two-phase process (Gijzen et al., 19...