bjornsson,effects of support material on the pattern of volatile fatty acid accumulation at overload...

8/8/2019 Bjornsson,Effects of Support Material on the Pattern of Volatile Fatty Acid Accumulation at Overload in Anaerobic Dig

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O R I G I N A L P A P E R

L. Bjornsson B. Mattiasson T. Henrysson

Effects of support material on the patternof volatile fatty acid accumulation at overload

in anaerobic digestion of semi-solid waste

Received: 3 December 1996 / Received revision: 7 February 1997 / Accepted: 14 February 1997

Abstract Anaerobic degradation of a semi-solid wastewith a total solids content of 4% particulate matter,much of it insoluble, was investigated in four laboratory-scale reactors. Two of the reactors were equipped withdierent textile materials for immobilisation of micro-organisms, while the other two were used as continu-ously-stirred-tank reactor references. A constant organicloading rate and hydraulic retention time were used inthe start-up period; the hydraulic retention time wasthen decreased and the eects of this change weremonitored. Volatile fatty acid (VFA) concentration andpH were chosen as indicators of the microbial status inthe reactors. The reactors with support material showed

a greater resistance to overload than did the continu-ously-stirred-tank reactors. This is in agreement withmany studies undertaken on the anaerobic treatment ofwastewater. However, no problems with clogging oc-curred, showing that a support material is also appli-cable in systems treating waste containing large amountsof insoluble, particulate matter. The pH was compara-ble to VFA for indicating an approaching process fail-ure. However, the pattern of VFA accumulation wasqualitatively dierent between the reactors with andwithout support material. Obviously the metabolic pat-tern of mixed cultures changes when the microorganismsare immobilised.

Introduction

Anaerobic digestion is recognised as a process that canbe used to treat a broad range of highly concentratedorganic wastes eectively and, simultaneously, recoverenergy in the form of biogas (Ghosh et al. 1975).

However, there are problems with instability at start-upand in the operation of the anaerobic process (Gijzenet al. 1988; Huysman et al. 1983). This depends mainly onthe low growth rates and the sensitivity to pH variationsof anaerobic bacteria (Rozzi 1991). Much research hasbeen carried out on increasing the stability of the processthrough improved process design. It has been shownthat this can be done by using a support material toretain the slow-growing microorganisms within theprocess. This approach has been successfully applied tothe anaerobic treatment of waste water containing highamounts of soluble organic matter (Anderson et al.1994; Defour et al. 1994; van den Berg and Kennedy

1981; Yee et al. 1992). However, the use of supportmaterial for anaerobic digestion of wastes with highamounts of particulate matter has not previously beenstudied extensively. The combination of support mate-rial and particulate matter leads to a potential risk ofclogging, and, thus, the continuously-stirred-tank reac-tor (CSTR) is still the most commonly used processconguration for treatment of this kind of waste.

Another important factor for ecient operation ofthe anaerobic digestion processes, extensively studied inrecent years, is the identification of parameters thatcould be used for monitoring the process (Cobb and Hill1991). The parameter monitored should reect the cur-

rent metabolic status of the active organisms in thesystem. There are several suggestions in the literatureregarding the choice of parameter to measure to indicateinstability before the total failure of the process.

One frequently investigated indicator is the amountof volatile fatty acids (VFA). VFA are intermediates inthe anaerobic degradation process. If the substrate iseasily hydrolysed, further acidogenic fermentation is arapid process (Bryant 1979). However, methanogensgrow more slowly than the acidogens upstream in thefood chain and, therefore, an organic overload will in-duce a build-up of VFA (Rozzi 1991). It is commonlyagreed that VFA build-up is the result of unbalanced

digestion conditions. The concentration of individualVFA, especially acetic, propionic and butyric acid, can

Appl Microbiol Biotechnol (1997) 47: 640644 Springer-Verlag 1997

L. Bjo rnsson (8) B. Mattiasson T. HenryssonDepartment of Biotechnology,Centre for Chemistry and Chemical Engineering,

Lund University, P.O. Box 124, S-221 00Lund, Sweden


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be considered the best control parameters in the liquidphase. This is because they indicate the metabolic stateof the most delicate microbial groups in the anaerobicsystem: the obligate hydrogen-producing acetogens andthe acetoclastic methanogens (Rozzi 1991). Earlier in-vestigations have, however, given contradictory infor-mation on the pattern of VFA build-up at overload. Ithas been shown, by several researchers, that build-up ofpropionic acid is the rst sign of reactor overload in theCSTR process (Renard 1991; Marchaim and Krause1993; Hill et al. 1987), whereas others report that aceticacid accumulates rst at the overload of both CSTR andreactors with attached growth (Cobb and Hill 1991).

Another important indicator is alkalinity (Jenkinset al. 1991; Ripley et al. 1986). If the amounts of VFAare low, the buer capacity in the pH range optimal foranaerobic digestion is almost totally dependent on thecarbonate weak acid/base buering subsystem (Pre-torius 1994). Wastewater containing carbohydrates,VFA, and lipids would not generate bicarbonate alka-linity at all when anaerobically degraded (Anderson andYang 1992). If the substrate is dominated by thesecompounds, the alkalinity needed to maintain the pH isdependent on the prevailing partial pressure of carbondioxide in the reactor (Pretorius 1994). At low initialalkalinity in the process, the decrease in pH will belarger at the increased VFA concentrations caused byoverload. Therefore, pH is more suited to monitoringdigesters fed on wastes which produce low bicarbonatealkalinities during treatment (Rozzi 1991).

In this investigation, VFA concentration and

pH were chosen as indicators of reactor status. Thepurpose was to compare the response of these parame-ters in systems with and without support material to theeect of a sudden change in hydraulic retention time.

Materials and methods

Substrate and inoculum

A semi-solid waste from a potato-processing plant was used as asubstrate. The waste had a total solids content of about 4%, ofwhich 85% were volatile solids (APHA, AWWA, WPCF 1985).The substrate contained a large amount of insoluble, particulatematter. Approximately half of the volatile solids was starch. In

addition, the substrate contained on average 5 g/l lactic acid, 1 g/lacetic acid, 0.5 g/l propionic acid, 0.5 g/l n-butyric acid and 2 g/lethanol. Long-term storage was at A20 C and the substrate waskept at 4 C just before use. The inoculum used was taken from alarge-scale biogas plant operating on this potato waste.

Support material

The most common process for anaerobic treatment of semi-solidwaste is the CSTR. The support material used in this investigationwas chosen to be suitable for upgrading to such a process, that is,the material had to have an open structure to prevent clogging. Thematerial also needed to be both available at low cost and suitablefor the immobilisation of slow-growing microorganisms. Twosynthetic-bre materials were chosen: a band with a width of15 mm and a 5-mm-thick surface of small loops, and a 10-mm-wide band with a bushy fringe; this was originally 200 mm long

but was cut to 100 mm for this application. Both materials arecommercially available and were supplied by Anox AB (IdeonScience Park, Lund, Sweden).

Reactor design

Two of the four reactors were used as CSTR references with vol-umes of 550 ml and 600 ml respectively. The third had 85 cm ofthe band support material and a volume of 600 ml. The fourth hada volume of 500 ml and 35 cm of the fringed support material.

The experimental set-up (Fig. 1) consisted of glass reactorsmaintained at a temperature of 35 2 C and sealed with rubberstoppers, each with in- and outlets for liquid and an outlet for gas.The gas outlet of each reactor was connected to a gas meter, wherethe biogas was collected using the water-displacement principle,and was where gas samples were taken. The liquid in- and outletshad two ports each, one connected to a peristaltic pump with atimer for automatic feeding or withdrawal, and the other used formanual sampling and feeding.

Experimental procedure

The potato waste was dispersed in order to get a pumpable slurry,and then diluted to obtain a constant organic loading rate at de-creased hydraulic retention time (tHR). Volatile solids were used toprovide a rough approximation of the amount of organic materialin the substrate, and the organic loading rate was reported as gvolatile solids lA1 dayA1. Substrate was added in a semi-continuousway once a day, either manually or automatically. Over a start-upperiod of 120 days, an organic loading rate of 1.7 0.3 g volatilesolids lA1 dayA1 and a tHR of 20 2 days was applied to the sys-tem. This is the normal operating conditions for the CSTR process(Ghosh 1984). At day 120, tHR was decreased from 20 to 15 dayswith the organic loading rate remaining constant.

Analytical methods

Volatile fatty acids were monitored with HPLC, with a BioRad125-0115 column for fermentation monitoring. The column tem-perature was 65 C. Sulphuric acid (5 mM) was used as a mobilephase and the liquid ow was 0.8 ml/min. The UV absorbance at208 nm was used for peak detection.

Total solids and volatile solids were determined according tostandard methods (APHA, AWWA, WPCF 1985).

Measurement of pH was made o-line with a pH electrode(Schott-Gera te). Analysis of the substrate starch content was car-ried out using a modied version of an analysis developed byLyckeby Sta rkelsen (Kristianstad, Sweden). The method includesthe enzymatic degradation of starch to glucose with thermostablea-amylase and amyloglucosidase. The amount of glucose was thenanalysed on HPLC, with the same conditions as above, but with arefractive-index detector.

Fig. 1 Schematic outline of the experimental set-up

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Results

During the start up-period, that is, days 1120, the pH inthe CSTR references was 7.1 0.3. In the reactors withsupport material, the pH was higher: 7.2 0.2 for the

reactor with the band support and 7.3 0.2 for thereactor with the fringe. At day 120, the tHR was decreasedfrom 20 days to 15 days with the organic loading ratekept constant. For the CSTR references, this resulted inan immediate drop in pH. In the reactors with the sup-port materials, the pH drop was delayed about 15 daysafter the tHR change (Fig. 2).

The two CSTR references showed similar behaviourin the accumulation of VFA after the tHR was decreased(Fig. 3). The rst response to overload was an increasedconcentration of propionic acid.

The two reactors with support material had higherconcentrations of acetic acid and propionic acid duringstart-up. The pH in the reactor with the band carrierdropped earlier than in the one with the fringed material,and the accumulation of VFA also started more rapidly.The rst indication of overload was an increase of acetic

acid concentration in both the reactors with supportmaterial (Fig. 4).

Discussion

The reactors with support material showed a greaterresistance to overload than did the CSTR. This wasexpected and is consistent with many studies on theanaerobic treatment of waste water (Hickey et al. 1991).The new nding in this study is that a solid support canbe applicable in a process for anaerobic digestion ofwastes with a high content of particulate matter without

causing clogging problems. The reason behind the in-creased stability is the improved retention of slow-growing microorganisms in the process.

There was a small dierence in the resistance tofailure between the two reactors with carrier materials,which can be explained by their diering ability to retainthe microorganisms in the system. Observation under amicroscope showed that the biomass was not growingattached to the surface, but was entrapped in voids ofthe material. The fringed material, which gave the bestresistance to failure, had long porous bres to enclosecell colonies, whereas the band had a smaller surfacearea for cell immobilisation.

The reactors with support material had higherVFA concentrations during the start-up period. High

Fig. 2 Changes in pH in the four reactors after decreased hydraulicretention time

Fig. 3 Accumulation of volatile fatty acids for the continuously-stirred-tank reactor references 1 (A) and 2 (B)

Fig. 4 Accumulation of volatile fatty acids for the reactors with theband support material (A) and the fringe support material (B)

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concentrations of propionic acid at low organic load andlong retention time have been reported previously(Mosey and Fernandes 1989). However, after a start-upperiod of 120 days, the VFA level would have been ex-pected to stabilise at a lower level. In spite of the higherconcentration of VFA at the start-up level, the pH washigher in the reactors with growth-support material thanin the CSTR. This indicates that, apart from a higherconcentration of microorganisms, the buering capacityis greater in the reactors with support material.

The rise in the VFA concentration at overload gave acorresponding drop in pH because the system had a lowbuering capacity. In this system, pH is as good as VFAfor indicating an approaching process failure, but theuse of pH for monitoring can only be recommended insystems where the buering capacity is low. However, itis interesting to note that the pattern of VFA accumu-lation is qualitatively dierent in the reactors with and

without support material.In the CSTR the accumulation of propionic acid wasthe rst indication of overload, while an increased con-centration of acetic acid was the rst sign in the reactorswith support material. Increased amounts of propionicacid have been reported as a consequence of reactoroverload (Archer 1983; Hill et al. 1987; Marchaim andKrause 1993; Renard 1991), which is in agreement withthe results from the CSTR. However, the reactors withsupport material show increased acetic acid concentra-tions as a rst response to overload. These results showthe importance of careful investigations before choosingan indicator parameter for monitoring reactor status,

since the metabolic pattern of mixed cultures obviouslychanges when the microorganisms are immobilised.The most slow-growing and delicate microbial groups

in the anaerobic system are the propionate-assimilating,obligate hydrogen-producing acetogens and the aceto-clastic methanogens (Mosey and Fernandes 1989; Rozzi1991). It is known that the growth rate of the propionate-assimilating organisms is very slow since the free-energygain, and therefore the yield, from the conversion ofpropionate to acetate is low (Barthakur et al. 1991;Bryant 1979). There is also a critical eect of the partialpressure of hydrogen (pH

2) on the formation and degra-

dation of propionate. Glucose is fermented rst to

pyruvic acid, via the Embden-Meyerhof pathway, and ithas been suggested that thereafter the pathways dependon the pH

2conditions (Moosbrugger et al. 1993; Mosey

and Fernandes 1989). Under low-pH2

conditions pyruvicacid is suggested to be converted to acetic acid only,whereas under high-pH

2conditions both acetic acid and

propionic acid are formed (Moosbrugger et al. 1993).Further, high pH

2in the reactor inhibits the propionate-

assimilating organisms, giving an accumulation ofpropionic acid. This could explain why propionic acidaccumulates in the CSTR, suggesting that, in these sys-tems, the stress of overload causes increased pH

2. In the

systems with support material, there is probably a relative

advantage for the hydrogen-assimilating methanogens,and a low pH

2is maintained. This would direct the fer-

mentation towards the formation of acetic acid. Thelimiting step instead becomes the acetoclastic meth-anogens causing a build up of acetic acid at overload.

It should be stressed that these results are only validfor digestion of easily hydrolysable semi-solid wastessince the digestion of poorly hydrolysable wastes islimited by the hydrolysis and not by methanogenesis(Kaspar and Wuhrmann 1978).

Acknowledgements The support from Swedish InternationalDevelopment Corporation Agency (Sida) and valuable technicaladvice from Lyckeby Sta rkelsen is gratefully acknowledged.

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