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Anaerobic digestion of waste sludges from the alginate extraction process
Bioresource Technology 37 ( 1991 ) 17-24 7Olb~ ¸
ItJl~,' ,(a
Anaerobic Digestion of Waste Sludges from the Alginate Extraction Process Karl N. Kerner,* Jon F. Hanssen~: & Tor A. Pedersen Department of Biotechnological Sciences, Agricultural University of Norway, PO Box 40, 1432 Aas-NLH, Norway (Received 7 February 1990; accepted 15 August 1990)
Abstract
Anaerobic digestion of waste sludges produced during the industrial extraction of alginate from the algal species Laminaria hyperborea (Gunn.) Foslie and Ascophyllum nodosum (L.) Le Jol was studied. Experiments were carried out in bench scale (8-litre) intermittently stirred digesters at 35°C. Sieve and flotation sludges were digested in batch (1 month) and semi-continuous cultures. In the semicontinuous trials, retention times of 23 days and 16 days were tested. Methane production varied from 0.10 to 0"15 litre g-1 VS added during batch; and from 0.07 to 0.28 litre g-1 VS added during semicontinuous fermentation. Specific gas production was significantly higher at 23 days than at 16 days retention time. VS reductions were 20-40% (batch) and 40-50% (semi-continuous). A distinct improvement of the settling qualities of digester effluents was obtained during the anaerobic digestion process. Key words: Alginate extraction, anaerobic digestion, biogas, brown algae, methane, settling qualities, waste treatment. INTRODUCTION Alginate, the major structural polysaccharide in the brown algae, is widely used in the textile and foodstuff industries as a thickener and gel-forming agent. The extraction of alginate includes a series of acid and alkaline treatments, resulting in a heterogeneous solution of dissolved alginate and a cellulosic suspended phase. These two fractions are separated by sieving followed by flotation of *Present address: NorwegianCenter for EcologicalAgriculture. 6630 Tingvoll.Norway. CTowhomcorrespondenceshouldbe addressed.
the algal solution. The large amounts of sieve and flotation sludges produced during these two steps are considered as 'wastes', but because of their organic matter content they also represent a valuable resource. Besides reducing the sludges' polluting load, anaerobic digestion has the advantage of energy recovery in the form of methane gas. Several studies on the anaerobic digestion of residues from the extraction of agar-agar have been carried out (Bird et al., 1981; Goes, 1987), but anaerobic treatment of waste sludges from the alginate extraction process has been reported only by Carpentier (1986), and Carpentier et al. (1988). In this case, the sludges examined were obtained after extraction of alginate from Laminaria digitata. Reduction of organic matter as well as gas production were high. In this study the results from bench-scale trials of the anaerobic digestion of alginate extraction residues are presented. The sludges were obtained after commercial extraction of alginate from the brown algae Laminaria hyperborea and Ascophyllum nodosum. The primary aim of the study was to evaluate the extraction sludges' potential as a substrate for methanogenesis. However, it is equally important to examine how effectively an anaerobic treatment would reduce the effluent's polluting load.
MATERIALS AND METHODS
Digesters The digesters had a working capacity of 8 litres. The temperature was controlled by a thermostatregulated heating coil (Raychem) inside the digester. All experiments were carried out at 35°C. A shaft with two propellers was driven by a
17 Bioresource Technolog)' 0960-8524/91/S03.50 © 1991 Elsevier Science Publishers Ltd, England. Printed in Great Britain
18
Karl N. Kerner, Jon F. Hanssen, Tor A. Pedersen
450 W electric motor. Digester contents were stirred for 30 rain every second hour and 10-litre gas sampling bags (Calibrated Instruments Inc) were connected to the fermentors to collect the produced biogas.
Inoculum Effluent from an active anaerobic digester, using algal sludge as feed, was used as inoculum. Alginate extraction sludges The extraction sludges were delivered by Protan A/S, Haugesund, Norway. Two sludge types from each of the two different production lines were examined. Two were sieve sludges (SH-1 and SH2), the others were flotation sludges (FH-1 and FH-2). In the H-1 production line, only stems of L. hyperborea are utilized, whereas H-2 utilizes a mixture of L. hyperborea and A. nodosurn (whole plants). The sludges were sent in 25-litre containers and arrived at our laboratory within 48 h. Upon arrival, pH was measured and total (TS) and volatile solids (VS) were determined. The sludges were stored at 4°C. The pH of the sludges was normally 9-10 (see Tables 2 and 3), and sank only slightly during transport, with one exception; the FH-1 sludge used in the 23 day retention trial. Due to delays during transport, microbial activity had set in by the time it arrived at our laboratory, and a pH of 6"5 was measured. For the semi-continuous trials, amounts of sludge corresponding to one daily feed ration were frozen separately. These were later taken from the freezer approximately 24 h before feeding. Occasionally, the thawed samples were analyzed to confirm the constancy of the test material. Due to their high content of TS, the sieve sludges were diluted with water ( 1 : 1 ) and homogenized before digestion. This was necessary to achieve comparable loading rates for both sludge types in the semi-continuous trials. The flotation sludges were digested without pretreatment. Batch fermentation All four waste sludges (FH-1, FH-2, SH-1, and SH-2) were digested as batch cultures for about 1 month. The reactors were filled with 1.5 litre inoculum and 6-5 litre substrate. If necessary, the pH was adjusted to pH 7.0 with 4 M NaOH. Samples for volatile fatty acid (VFA) and chemical oxygen demand (COD) measurements were taken twice a week. Gas volume and composition were measured every second day.
Semi-continuous fermentation The two sludges from the H-2 production line were chosen for detailed study in semi-continuous trials, since these represented the larger portion of the plant's total effluent. The digesters were fed manually once a day. Feeding and effluent removal were always carried out while the digester contents were stirred. Sludge characteristics were determined at the start of the experiment. Effluent characteristics were either determined on the current day's sample (pH, VFA) or on a weekly mean sample (TS, VS, COD, alkalinity, and settling qualities). Gas production (volume) and composition were measured every second day. Two retention times, 23 days and 16 days, were tested subsequently on each of the two sludges. No feed was added to the digesters for 4 days prior to reducing the retention time to 16 days. Analytical methods TS were determined by drying at 105°C t o constant weight. VS were obtained by difference after combustion of a dried sample at 550°C for 4 h. Using TS and VS values of feed and effluent, the relative degree of VS reduction was calculated as VS red = VS(in) - VS(out)/VS(in) The volume of biogas produced was measured in a wet gas meter (Ritter), while its concentration of methane and carbon dioxide was determined using an Ill gas-analyzer (Leybold-Heraeus). VFA were determined on a gas chromatograph (Shimadzu GC-9AM) with a flame ionization detector. Column: 6 feet glass, 4 mm diameter. Support: Chromosorb 101, mesh size 80/100. Carrier gas: N 2, 60 ml/min. Temperatures: injector 220°C, column 180°C, detector 220°C. Sample size: 1/zl. The extraction of VF,a/s was carried out as described (SUPELCO Inc., 1985). COD was measured using the dichromate reflux method (APHA, 1975). Alkalinity measurements were carried out as a double-endpoint titration to pH 5.75 and pH 4.3 (Jenkins et al., 1983). The effluent's settling qualities were determined as described in Standard Methods (APHA, 1975). Total nitrogen (TKN) was determined by the Kjeldahl method. The ammonium content was obtained by distillation in an alkaline medium. Nitrate-N was measured with a nitrate electrode (Orion). The total sulphur concentration was determined by adding a 70% MgNO3.6HzO solution
19
Anaerobic digestion of waste sludge from alginate extraction
and concentrated H N O 3 tO the sample,, which was then dried. The residue was burned at 550°C and the ash dissolved in concentrated HC1. After dilution with distilled water, the sulphur content was determined using ICAP (inductive coupled argon plasma). Mercury was analyzed by dissolving the sample in HNO3 and determining the Hg content by tameless atomic absorbtion spectrophotometry (Perkin-Elmer 1100 B). All other macroand micronutrients were determined by ICAP (Jarell-Ash ICAP 1100).
nodosum is also shown. All chemical analyses were carried out on only one sludge delivery. It should be considered that variations in the algae's chemical composition occur naturally. To a large degree, the mineral composition of the sludges resembled the composition of the flotation sludges previously examined by others (Carpentier et aL, 1988). A comparison of the total nitrogen content of whole Laminaria with the two H-1 sludges (which originate from the treatment of only Laminaria stems) shows there was no net nitrogen loss during the extraction process. However, contents of other nutrients (P, K, Ca, and Mg) were lower in the waste sludges than in the whole algae. The large amounts of sodium in the sludges stem from Na-additions during the extraction process. The amount of residual alginate in the sludges still being 6-7% was sufficient to influence the
RESULTS Sludge characteristics The characteristics of the waste sludges are presented in Table 1. For comparison, the chemical composition of whole L. hyperborea and A.
Table 1. C o m p o s i t i o n of the alginate extraction sludges, c o m p a r e d with w h o l e
Laminaria hyperborea
and
Ascophyllum
nodosurn" Analysis
TKN Ammon.-N Nitrate-N P K Ca Mg Na Total S T o t a l CI
Sludges received in July 1988 FH-1
FH-2
SH-1
SH-2
Laminaria
AscophyUum
(g/lOOg TS)
(g/lOOgTS)
(g/lOOg7S)
(g/lOOgTS)
(g/lOOgTS)
(g/lOOgTS)
3-46 0.73 0"18 0"18 1"82 1"82 0"36 7.64 2.03 1-64
4.85 trace 0-09 0"11 0"97 1.36 0"29 5.34 1.46 0.58
(mg/kg TS) Fe Cu Mn Zn
B Ni Cd. Pb, Hg Alginate b Laminaran Mannitol Fucoidan Fat Cellulose Lignin
3.83 0.44 0"09 0.17 1.31 0"78 0"26 4.72 1.57 0.71
(mg/kg TS)
(mg/kg TS)
210 -8'4 19'3
-----
----
trace 25.5 trace
48.5 trace trace
trace
trace
--
9.6
6.7
--
trace
trace
--
-----
(% of TS)
(% of TS)
(% of TS)
(% of TS)
(% of rs)
(% of TS)
6.9
--
20-45 0-20 3-15 2-4 1-2 1-8 3-3
15-30 0-10 5-10 4-10 2-7
(ppm)
(ppm)
--
--
-. . . . . .
. . . . . .
(mg/kg TS)
----2-3 -0"5-1 3-4 ---
261 7"6 8"7 15"7
_
(mg/kg TS)
3.5 --0"80 4-12 3-7 0"5-1 2-6 1.2 --
378 _ d 16"5 21"3
6.0 . . . . . .
(mg/kg TS)
2.69 1.09 0"08 0"17 0.84 1"09 0"25 3.95 1.43 0.42
237 13"7 16"4 25"5
(ppm) Formaldehyde h,
Wholeplants
. . . . . .
(ppm)
(ppm)
24.0
--
(ppm) 127.0
" K e l p characteristics are from the literature (Indergaard, 1 9 8 3 ; F a u c h i l l e , 1 9 8 4 ; Carpentier hMeasured in sludges received in F e b r u a r y 1 9 8 9 . ' A d d e d to harvested algae as a preservative. ' l N o t determined.
et al., 1 9 8 8 ) .
20
Karl N. Kerner, Jon F. Hanssen, Tor A. Pedersen
sludges' consistency. These were slimy and showed poor settling qualitites (see Fig. 3). The laminaran and mannitol contents in the sludges were not determined, but from practical experience it is known that these components are removed to a large degree in the initial steps of the extraction process. Batch fermentations
The pH of digester contents sank sharply the first five days, from pH 9-10 at the start, to around pH 6.0-6.5. There were noticeable differences between the H-1 and H-2 sludges. The pH values in the H-1 sludges sank to 6-0, the H-2 sludges to around 6-6. A NaOH solution of 4 M was added until the pH had stabilized between 7 and 7.5. No NaOH additions were necessary after day 10. The characteristics of the feed used in the batch trials, as well as the main results are presented in Table 2. Except for FH-2, there was a good correlation between specific gas production (litre g- 1 VS added) and VS and soluble COD conversion. Specific methane production varied only slightly between the different sludge types. The results from FH-2 are inconsistent with those of the other 3 sludges. The high VS conversion in spite of a low specific gas production may indicate some disturbances in the fermentation process. One possible explanation may be the formation of volatile organic compounds, leading to a faulty determination of the VS content. Possibly, the appropriate bacterial community had not yet been established during the 30 days, and gas production would have increased after a longer period. Three values for COD removal are presented, depending on whether COD was measured as total or soluble COD. The third alternative is an attempt to estimate the reduction of the waste's polluting load after an anaerobic treatment; the reasoning being that only the liquid phase of the digester effluents would be finally discharged. Figure 1 shows the cumulative gas production of the 4 sludges during batch fermentation. Gas production started after about I week, and it must be noted that the reactors were still producing gas at the end of the 30-day experimental period. Thus, the achieved gas yields do not represent the sludges' maximum energy potential. The earlier mentioned deviation of the FH-2 sludge is clearly shown in Fig. 1. Methane contents averaged 56% in the FH-2 gas, compared to 62-66% in the others. The higher cumulative gas yields in the sieve sludges are due to their higher TS content (Table 2).
Figure 2 illustrates the changes in soluble COD during the batch process. The tendency in all 4 sludges was an increase in soluble COD during the first 14 days before a sharp decrease occurred to values lower than the soluble COD of the feed. This development may be a result of the initial hydrolysis of the anaerobic digestion process, during which particulate organic matter is broken down to smaller, soluble components, leading to the rise in soluble COD. These intermediary metabolites then serve as substrates for the methanogenic bacteria. The resulting CODdecrease corresponds closely with the stabilization of digester pH and the start of vigorous gas production. The settling qualities of digester feed and effluent after 1 month were measured (Fig. 3). The results show that the sedimentation properties were clearly improved after anaerobic digestion of the sludges, and that most of the sludge sedimentation had taken place after only 5 min. The settling qualities for the sludges after 16 days retention are not presented since they are the same as for the 23 day retention trials. S e m i - c o n t i n u o u s culture
The two substrates used in the semi-continuous trials were first run as batch cultures for 1 month Table 2. Feed characteristics and results obtained from the anaerobic digestion of alginate extraction sludges; batch culture
Sludge type
Feed characteristics pH TS (%) VS(%ofTS) COD (g O litre- t)" COD (g O litre- t)h
FH-1 F H - 2 SH-I
SH-2
9.7 0-9 78.7 6"0 2"6
9"8 3"1 84"5 35"2 4'5
9.7 1"1 82-1 9.2 1'7
9.7 3"2 85.4 17'4 3'5
Digesterperformance Specific biogas production litre g-t VS added litre g- t VS removed Specific methane production litre g- t VS added litre g- t VS removed VS conversion (%) a COD removal (%)" COD removal (%)b COD removal (%)'
0"25 0.82
0.17 0.44
0"18 0.89
0-21 1.02
0"15 0-09 0"51 0-24 30.2 38.5 16-4 -51-4 24.6 79.1 86.1
0-11 0.53 20.3 37.1 26.2 85"0
0" 14 0"66 21.1 46.0 46.1 93-1
aCompletely stirred samples. hSoluble COD. ' C O D in effluent supernatant compared to total COD in feed.
Anaerobic digestion of waste sludge from alginate extraction 80
I--~-FH-1 -.-FH-' ""-81"I-1 ---SH-~)
21
I
m i[j
10
i
I
I
0
5
Fig. 1.
10
I~ys
20
25
SO
Cumulative gas production during batch culture.
_
[
I
FH-1
-o-
FH-2
SH-2
SH--1
I
o
[
0
I
I
i
i
i
0
Fig. 2.
1
i
10
i
I
I
Dllyl
l
L
I
I
i
20
80
Soluble COD m al~nate extraction sludge (day 0) and digester effluent during batch culture.
25O
2OO
i o
!loo
i SO
FM-11
814"11
FH.12
8H-12
FH'I 3
1
1114-13
Fig. 3. Settling qualities of feed and effluents from batch and semi-continuous trials. Bars represent settled sludge volume at certain time intervals after filling 250 ml feed or digester effluent into graduated cylinders and stirring. ~= untreated alginate extraction sludge. -"= batch effluent. -~= semi-continuous effluent (23 day retention).
Karl N. Kerner, Jon F. Hanssen, Tor A. Pedersen
22
before regular feeding commenced. By thus starting with already active digesters the lag phase was reduced to only a few days; gas production and digester pH were already fairly stable after 1 week of feeding. The results from the semi-continuous runs are shown in Table 3, along with feed characteristics. Results are presented as mean values achieved during steady-state conditions.
The percentage of methane in the biogas was very stable, averaging 56% and 48% (23 days and 16 days retention time) in FH-1 and 60% and 62% (23 days and 16 days retention) in SH-1. There were considerable variations in the specific gas production during the semi-continuous trials. Specific gas production was higher in both sludges at the 23 day retention time. There was no effect of sludge type on gas production. These results do not correspond to the low VS reductions measured at the longer retention time. A possible explanation was the lower TS content in the feed compared to the TS of the reactor contents at the start of the semi-continuous trials. Thus, when feeding started, digester effluents had a higher content of organic matter than the feed. COD removal is presented as the reduction of soluble COD as well as % reduction calculated from the soluble COD in the effluent and the total COD in the feed (see previous section). Thus, in the semi-continuous trials, a hypothetical reduction of the wastes' polluting load of 80-95% was achieved. The pH remained stable at 6.8-7.0 throughout the experimental period. The use of the LA/PA ratio as a more sensitive tool for monitoring digester stability has been suggested (Ripley et al., 1985). Effluent alkalinity and the corresponding IA/PA ratios are shown in Fig. 4, where PA= bicarbonate alkalinity and IA--volatile acid alkalinity. The observed increase of the IA/PA ratio is due to a relative increase in VFA concentration, a sign of reactor instability. The following drop in the IA/PA ratio and constant alkalinity
Table 3. Feed characteristics and results obtained from the anaerobic digestion of alginate extraction sludges; semicontinuous culture
Sludge type FH-1 SH-I Retention time (days) Loading rate (g VS litre- l digester day- ~)
23 0-15
FH-I SH-I
23 16 16 0-57 0"37 0"81
Feed characteristics pH 6-5 TS (%) 0"5 VS(%ofTS) 74.4 C O D (g O iitre- t) a 5.3 C O D (g O litre- l) b 1.1 Alkalinity (mg CaCO~ litre- ~) 320.0
9"7 1.6 83.3 13.2 2.2 600.0
9.8 8"9 0.7 1"6 81"7 79.4 _ d _ -----
Digester performance Specific biogas production litreg-t VS added Specific methane production litreg-~ V S a d d e d VSconversion(%), C O D removal(%) b C O D removal (%)'
0-50
0.25
0.28 d 57.1 91.5
0-15
0.16
0-15 0.07 0.10 15"6 45"8 46"7 71-8 47.2 60-1 95.2 81.1 91.6
aCompletely stirred samples. hSoluble COD. ' C O D in effluent supernatant compared to total C O D in feed. aNot determined.
'F
f
4
i ,FH-1 o -1 .---'"
.
.
1
t u
............................................... ....................................................... .
..v
.
.
.
.
.
-v ......
- .................
ot
2
0.4
O2
0
5
10
15 Days
20
25
30
Fig. 4. Development of total alkalinity and the IA/PA ratio during semi-continuous culture. Day 0 represents digester alkalinity before feeding started. Solid line = Alkalinity. Dotted line = IA/PA ratio.
Anaerobic digestion of waste sludge from alginate extraction
indicate the return to stable reactor conditions. These changes illustrate the sensitivity of the method, since no significant pH changes were registered within the same period. Digester contents were monitored for volatile fatty acids, but no significant amounts were detected in any of the samples. It is not clear whether this can be explained by low VFA concentrations or technical problems during VFA extraction and analysis. Figure 3 shows the settling qualities of the sludges after digestion at two different retention times. The results compare well with those of the batch fermentations. DISCUSSION Generally, marine algae are considered ideal substrates for anaerobic fermentations due to their high content of easily degradable polysaccharides such as alginate, laminaran, and the sugar-alcohol mannitol (Jerger & Tsao, 1987). Methane yields of 0.20-0.37 litre g-l VS added have been reported for various algal feedstocks (Ghosh et al., 1981; Hanssen et aL, 1987; Jerger & Tsao, 1987). Earlier studies on the anaerobic digestion of algal wastes include the anaerobic treatment of agar extraction residues from Gracilaria tickvahiae (Bird et al., 1981). Those experiments were unsuccessful, mainly due to the nature of the residues. These consisted to a large degree of cellulosic material, low in proteins and amino acids, which led to a strong acidification of the digested liquor during fermentation. The results we obtained with the waste sludges from L. hyperborea and A. nodosum are similar to the results from the sludges from L. digitata (Carpentier et aL, 1988). Even though loading rates in our study were approximately 50% lower, the maximum methane yield achieved of 0.28 litre g-~ VS added is only slightly lower than the 0.29 litre g-~ VS earlier reported (Carpentier et al., 1988). An interesting result of this study is the much higher specific methane yield obtained from the feed with a markedly lower pH (FH-1 at 23 day retention). Since gas production in all other trials was significantly lower, a connection between commencing biological activity in the feed and high gas production can be suspected. Asinari Di San Marzano et al. ( 1981 ) examined the influence of VFA levels in methanogenic substrates on
23
digester stability and methane production. They concluded that an anaerobic digestion system will generally appear more reliable when the substrate contains certain amounts of VF,a/s. The reasoning behind this is that an absence of VFA?s in the substrate could lead to a bacterial community consisting mainly of the syntropy between fermentative and methanogenic bacteria. In this case, even the slightest increase of VF,~/s could be detrimental to digester stability due to the absence of appropriate numbers of obligate proton-reducing bacteria. On the other hand, steady concentrations of VFAgs in the feed will force the bacterial system to include the syntropy between obligate proton reducers and methanogens. An increase of VFA concentrations would be less upsetting than in the first mentioned situation. This study was initiated to examine how effectively an anaerobic treatment would reduce the polluting load of the alginate extraction sludges, expressed as % VS and COD reduction. Anaerobic digestion studies of whole algae have resulted in VS reductions of between 34% and 54% (Klass et al., 1979; Hanssen et al., 1987). The 46% VS reduction measured in the 16 day retention trial was very similar to the 45-56% reduction earlier reported by others (Carpentier et aL, 1988). The soluble fraction of waste effluents is often expressed as soluble COD, and its degree of reduction is an important parameter for treatment efficiency. Soluble COD reductions were 25-51% in the batch fermentations, and 47-71% in the semi-continuous trials. When considering the improvement of the sludges' settling qualities after anaerobic digestion, a possible form of waste treatment would include a short period of sedimentation after its stay in the anaerobic reactor. Ideally, only the fluid decant would finally be discharged to the recipient. A comparison of this effluent to the present untreated alginate extraction sludge results in COD reductions of around 90%. This would represent a significant reduction of the sludges' polluting load. Certain aspects of the alginate extraction process and some sludge characteristics speak in favour of an anaerobic treatment. The extraction process itself includes several steps which resemble proposed pretreatment methods for increased degradability and biogas yield (Tsao, 1987). Furthermore, the extraction sludges leave the plant at around 40-50°C, so that little or no energy input would be necessary to run a meso-
24
Karl N. Kerner, Jon F. Hanssen, Tor A. Pedersen
philic (35°C) fermentation. Finally, alginate production is very stable throughout the year; subsequently there would be a steady and reliable source of substrate.
CONCLUSION
The experimental results from this and earlier studies show that the extraction sludges are definitely suitable as substrates for an anaerobic treatment. Combined with a sedimentation of digester effluents, anaerobic digestion would be expected to sufficiently reduce" the waste sludges' polluting load. As a waste treatment technology, anaerobic digestion has the advantage of generating an energy source (methane) as a by-product. Thus, when considering certain features of the extraction process, as well as the properties of the extraction sludges, this form of treatment could be expected to be economically feasible.
ACKNOWLEDGEMENTS
The authors gratefully acknowledge financial support from the Bioenergy Program of the Norwegian Agricultural Research Council. They would also like to express their appreciation to Protan A/S, Haugesund, for willingly providing substrate samples and to the Chemical Analytical Laboratory (Agricultural University of Norway) and the Agricultural Analytical Center for performing chemical analyses.
REFERENCES APHA (1975). Standard Methods for the Examination of Water and Wastewater. 14th edition, American Public Health Association, American Water Works Association, Water Pollution Control Federation, Washington DC. Asinari Di San Marzano, C. M., Binot, R., Bol, T., Fripiat, J. L., Hutschemakers, J., Melchior, J. L., Perez, 1., Naveau, H.
& Nyns, E. J. (1981). Volatile fatty acids, an important state parameter for the control of the reliability and the productivities of methane anaerobic digestions. Biomass, 1,47-59. Bird, K. T., Hanisak, M. D. & Ryther, J. H. (1981). Changes in agar and other chemical constituents of the seaweed Gracila.ria tikvahiae when used as a substrate in methane digesters. Resour. Conserv., 6, 321-7. Carpentier, B. (1986). Digestion ana6robie de la biomasse algale: les r6sidus de rextraction de l'acid alginique. Les ulves de mar~e vert. Th~se de docteur de 3 eme cycle. Universit6 Pierre et Marie Curie, Paris, France. Carpentier, B., Festino, C. & Aubart, C. (1988). Anaerobic digestion of flotation sludges from the alginic acid extraction process. BioL Wastes, 23, 269-78. Fauchille, S. (1984). Digestion anaerobie de v6getaux aquatiques. Th6se de docteur ing6nieur. Institut National Polytechnique de Lorraine, Nancy, France. Ghosh, S., Klass, D. L. & Chynoweth, D. P. (1981). Bioconversion of Macrocystis pyrifera to methane. J. Chem. Tech. Biotechnol., 31,791-807. Goes, J. (1987). Methanization of algal residues after extraction of agar-agar from Gelidium. In Aquatic Primary Biomass (Marine Macroalgae): Biomass Conversion, Removal and use of Nutrients, ed. P. Morand & E. H. Schulte. Proc. of the First Workshop of the COST-48 Sub-Group 3, held in L'Houmeau, France, 12-14 February. Commission of the European Communities, pp. 111-13. Hanssen, J. F., Indergaard, M., Ostgaard, K., Baevre, O. A., Pedersen, T. A. & Jensen, A. (1987). Anaerobic digestion of Laminaria spp. and Ascophyllum nodosum and application of end products. Biomass, 14, 1-13. Indergaard, M. (1983). The aquatic resource. In Biomass Utilization, ed. W. A. Cole. Plenum Publishing Corp, pp. 137-68. Jenkins, S. R., Morgan, J. M. & Sawyer, C. L. (1983). Measuring anaerobic sludge digestion and growth by a simple alkalimetric titration. J. Water Poll. Contr. Fed., 55, 448-53. Jerger, D. E. & Tsao, G. T. (1987). Feed composition. In Anaerobic Digestion of Biomass, ed. D. P. Chynoweth & R. Isaacson. Elsevier Applied Science, pp. 65-90. Klass, D. L., Ghosh, S. & Chynoweth, P. (1979). Methane production from aquatic biomass by anaerobic digestion of Giant Brown Kelp. Proc. Biochem., 14 (4), 18-23. Ripley, L. E., Boyle, W. C. & Converse, J. C. (1985). Improved alkalimetric monitoring for anaerobic digestion of poultry manure. In Proceedings of the 40th Industrial Waste Conference, Purdue Univ., Indiana. Butterworth, Boston, pp. 141-9. SUPELCO Inc (1985). Packed column GC analysis of volatile fatty acids from anaerobic fermentation. Supeico GC Bulletin 748 H, Bellefonte, Pennsylvania, USA. Tsao, G. T. (1987). Pre-/Posttreatment. In Anaerobic Digestion of Biomass, ed. D. P. Chynoweth & R. Isaacson. Elsevier Applied Science, pp. 91-107.
ItJl~,' ,(a
Anaerobic Digestion of Waste Sludges from the Alginate Extraction Process Karl N. Kerner,* Jon F. Hanssen~: & Tor A. Pedersen Department of Biotechnological Sciences, Agricultural University of Norway, PO Box 40, 1432 Aas-NLH, Norway (Received 7 February 1990; accepted 15 August 1990)
Abstract
Anaerobic digestion of waste sludges produced during the industrial extraction of alginate from the algal species Laminaria hyperborea (Gunn.) Foslie and Ascophyllum nodosum (L.) Le Jol was studied. Experiments were carried out in bench scale (8-litre) intermittently stirred digesters at 35°C. Sieve and flotation sludges were digested in batch (1 month) and semi-continuous cultures. In the semicontinuous trials, retention times of 23 days and 16 days were tested. Methane production varied from 0.10 to 0"15 litre g-1 VS added during batch; and from 0.07 to 0.28 litre g-1 VS added during semicontinuous fermentation. Specific gas production was significantly higher at 23 days than at 16 days retention time. VS reductions were 20-40% (batch) and 40-50% (semi-continuous). A distinct improvement of the settling qualities of digester effluents was obtained during the anaerobic digestion process. Key words: Alginate extraction, anaerobic digestion, biogas, brown algae, methane, settling qualities, waste treatment. INTRODUCTION Alginate, the major structural polysaccharide in the brown algae, is widely used in the textile and foodstuff industries as a thickener and gel-forming agent. The extraction of alginate includes a series of acid and alkaline treatments, resulting in a heterogeneous solution of dissolved alginate and a cellulosic suspended phase. These two fractions are separated by sieving followed by flotation of *Present address: NorwegianCenter for EcologicalAgriculture. 6630 Tingvoll.Norway. CTowhomcorrespondenceshouldbe addressed.
the algal solution. The large amounts of sieve and flotation sludges produced during these two steps are considered as 'wastes', but because of their organic matter content they also represent a valuable resource. Besides reducing the sludges' polluting load, anaerobic digestion has the advantage of energy recovery in the form of methane gas. Several studies on the anaerobic digestion of residues from the extraction of agar-agar have been carried out (Bird et al., 1981; Goes, 1987), but anaerobic treatment of waste sludges from the alginate extraction process has been reported only by Carpentier (1986), and Carpentier et al. (1988). In this case, the sludges examined were obtained after extraction of alginate from Laminaria digitata. Reduction of organic matter as well as gas production were high. In this study the results from bench-scale trials of the anaerobic digestion of alginate extraction residues are presented. The sludges were obtained after commercial extraction of alginate from the brown algae Laminaria hyperborea and Ascophyllum nodosum. The primary aim of the study was to evaluate the extraction sludges' potential as a substrate for methanogenesis. However, it is equally important to examine how effectively an anaerobic treatment would reduce the effluent's polluting load.
MATERIALS AND METHODS
Digesters The digesters had a working capacity of 8 litres. The temperature was controlled by a thermostatregulated heating coil (Raychem) inside the digester. All experiments were carried out at 35°C. A shaft with two propellers was driven by a
17 Bioresource Technolog)' 0960-8524/91/S03.50 © 1991 Elsevier Science Publishers Ltd, England. Printed in Great Britain
18
Karl N. Kerner, Jon F. Hanssen, Tor A. Pedersen
450 W electric motor. Digester contents were stirred for 30 rain every second hour and 10-litre gas sampling bags (Calibrated Instruments Inc) were connected to the fermentors to collect the produced biogas.
Inoculum Effluent from an active anaerobic digester, using algal sludge as feed, was used as inoculum. Alginate extraction sludges The extraction sludges were delivered by Protan A/S, Haugesund, Norway. Two sludge types from each of the two different production lines were examined. Two were sieve sludges (SH-1 and SH2), the others were flotation sludges (FH-1 and FH-2). In the H-1 production line, only stems of L. hyperborea are utilized, whereas H-2 utilizes a mixture of L. hyperborea and A. nodosurn (whole plants). The sludges were sent in 25-litre containers and arrived at our laboratory within 48 h. Upon arrival, pH was measured and total (TS) and volatile solids (VS) were determined. The sludges were stored at 4°C. The pH of the sludges was normally 9-10 (see Tables 2 and 3), and sank only slightly during transport, with one exception; the FH-1 sludge used in the 23 day retention trial. Due to delays during transport, microbial activity had set in by the time it arrived at our laboratory, and a pH of 6"5 was measured. For the semi-continuous trials, amounts of sludge corresponding to one daily feed ration were frozen separately. These were later taken from the freezer approximately 24 h before feeding. Occasionally, the thawed samples were analyzed to confirm the constancy of the test material. Due to their high content of TS, the sieve sludges were diluted with water ( 1 : 1 ) and homogenized before digestion. This was necessary to achieve comparable loading rates for both sludge types in the semi-continuous trials. The flotation sludges were digested without pretreatment. Batch fermentation All four waste sludges (FH-1, FH-2, SH-1, and SH-2) were digested as batch cultures for about 1 month. The reactors were filled with 1.5 litre inoculum and 6-5 litre substrate. If necessary, the pH was adjusted to pH 7.0 with 4 M NaOH. Samples for volatile fatty acid (VFA) and chemical oxygen demand (COD) measurements were taken twice a week. Gas volume and composition were measured every second day.
Semi-continuous fermentation The two sludges from the H-2 production line were chosen for detailed study in semi-continuous trials, since these represented the larger portion of the plant's total effluent. The digesters were fed manually once a day. Feeding and effluent removal were always carried out while the digester contents were stirred. Sludge characteristics were determined at the start of the experiment. Effluent characteristics were either determined on the current day's sample (pH, VFA) or on a weekly mean sample (TS, VS, COD, alkalinity, and settling qualities). Gas production (volume) and composition were measured every second day. Two retention times, 23 days and 16 days, were tested subsequently on each of the two sludges. No feed was added to the digesters for 4 days prior to reducing the retention time to 16 days. Analytical methods TS were determined by drying at 105°C t o constant weight. VS were obtained by difference after combustion of a dried sample at 550°C for 4 h. Using TS and VS values of feed and effluent, the relative degree of VS reduction was calculated as VS red = VS(in) - VS(out)/VS(in) The volume of biogas produced was measured in a wet gas meter (Ritter), while its concentration of methane and carbon dioxide was determined using an Ill gas-analyzer (Leybold-Heraeus). VFA were determined on a gas chromatograph (Shimadzu GC-9AM) with a flame ionization detector. Column: 6 feet glass, 4 mm diameter. Support: Chromosorb 101, mesh size 80/100. Carrier gas: N 2, 60 ml/min. Temperatures: injector 220°C, column 180°C, detector 220°C. Sample size: 1/zl. The extraction of VF,a/s was carried out as described (SUPELCO Inc., 1985). COD was measured using the dichromate reflux method (APHA, 1975). Alkalinity measurements were carried out as a double-endpoint titration to pH 5.75 and pH 4.3 (Jenkins et al., 1983). The effluent's settling qualities were determined as described in Standard Methods (APHA, 1975). Total nitrogen (TKN) was determined by the Kjeldahl method. The ammonium content was obtained by distillation in an alkaline medium. Nitrate-N was measured with a nitrate electrode (Orion). The total sulphur concentration was determined by adding a 70% MgNO3.6HzO solution
19
Anaerobic digestion of waste sludge from alginate extraction
and concentrated H N O 3 tO the sample,, which was then dried. The residue was burned at 550°C and the ash dissolved in concentrated HC1. After dilution with distilled water, the sulphur content was determined using ICAP (inductive coupled argon plasma). Mercury was analyzed by dissolving the sample in HNO3 and determining the Hg content by tameless atomic absorbtion spectrophotometry (Perkin-Elmer 1100 B). All other macroand micronutrients were determined by ICAP (Jarell-Ash ICAP 1100).
nodosum is also shown. All chemical analyses were carried out on only one sludge delivery. It should be considered that variations in the algae's chemical composition occur naturally. To a large degree, the mineral composition of the sludges resembled the composition of the flotation sludges previously examined by others (Carpentier et aL, 1988). A comparison of the total nitrogen content of whole Laminaria with the two H-1 sludges (which originate from the treatment of only Laminaria stems) shows there was no net nitrogen loss during the extraction process. However, contents of other nutrients (P, K, Ca, and Mg) were lower in the waste sludges than in the whole algae. The large amounts of sodium in the sludges stem from Na-additions during the extraction process. The amount of residual alginate in the sludges still being 6-7% was sufficient to influence the
RESULTS Sludge characteristics The characteristics of the waste sludges are presented in Table 1. For comparison, the chemical composition of whole L. hyperborea and A.
Table 1. C o m p o s i t i o n of the alginate extraction sludges, c o m p a r e d with w h o l e
Laminaria hyperborea
and
Ascophyllum
nodosurn" Analysis
TKN Ammon.-N Nitrate-N P K Ca Mg Na Total S T o t a l CI
Sludges received in July 1988 FH-1
FH-2
SH-1
SH-2
Laminaria
AscophyUum
(g/lOOg TS)
(g/lOOgTS)
(g/lOOg7S)
(g/lOOgTS)
(g/lOOgTS)
(g/lOOgTS)
3-46 0.73 0"18 0"18 1"82 1"82 0"36 7.64 2.03 1-64
4.85 trace 0-09 0"11 0"97 1.36 0"29 5.34 1.46 0.58
(mg/kg TS) Fe Cu Mn Zn
B Ni Cd. Pb, Hg Alginate b Laminaran Mannitol Fucoidan Fat Cellulose Lignin
3.83 0.44 0"09 0.17 1.31 0"78 0"26 4.72 1.57 0.71
(mg/kg TS)
(mg/kg TS)
210 -8'4 19'3
-----
----
trace 25.5 trace
48.5 trace trace
trace
trace
--
9.6
6.7
--
trace
trace
--
-----
(% of TS)
(% of TS)
(% of TS)
(% of TS)
(% of rs)
(% of TS)
6.9
--
20-45 0-20 3-15 2-4 1-2 1-8 3-3
15-30 0-10 5-10 4-10 2-7
(ppm)
(ppm)
--
--
-. . . . . .
. . . . . .
(mg/kg TS)
----2-3 -0"5-1 3-4 ---
261 7"6 8"7 15"7
_
(mg/kg TS)
3.5 --0"80 4-12 3-7 0"5-1 2-6 1.2 --
378 _ d 16"5 21"3
6.0 . . . . . .
(mg/kg TS)
2.69 1.09 0"08 0"17 0.84 1"09 0"25 3.95 1.43 0.42
237 13"7 16"4 25"5
(ppm) Formaldehyde h,
Wholeplants
. . . . . .
(ppm)
(ppm)
24.0
--
(ppm) 127.0
" K e l p characteristics are from the literature (Indergaard, 1 9 8 3 ; F a u c h i l l e , 1 9 8 4 ; Carpentier hMeasured in sludges received in F e b r u a r y 1 9 8 9 . ' A d d e d to harvested algae as a preservative. ' l N o t determined.
et al., 1 9 8 8 ) .
20
Karl N. Kerner, Jon F. Hanssen, Tor A. Pedersen
sludges' consistency. These were slimy and showed poor settling qualitites (see Fig. 3). The laminaran and mannitol contents in the sludges were not determined, but from practical experience it is known that these components are removed to a large degree in the initial steps of the extraction process. Batch fermentations
The pH of digester contents sank sharply the first five days, from pH 9-10 at the start, to around pH 6.0-6.5. There were noticeable differences between the H-1 and H-2 sludges. The pH values in the H-1 sludges sank to 6-0, the H-2 sludges to around 6-6. A NaOH solution of 4 M was added until the pH had stabilized between 7 and 7.5. No NaOH additions were necessary after day 10. The characteristics of the feed used in the batch trials, as well as the main results are presented in Table 2. Except for FH-2, there was a good correlation between specific gas production (litre g- 1 VS added) and VS and soluble COD conversion. Specific methane production varied only slightly between the different sludge types. The results from FH-2 are inconsistent with those of the other 3 sludges. The high VS conversion in spite of a low specific gas production may indicate some disturbances in the fermentation process. One possible explanation may be the formation of volatile organic compounds, leading to a faulty determination of the VS content. Possibly, the appropriate bacterial community had not yet been established during the 30 days, and gas production would have increased after a longer period. Three values for COD removal are presented, depending on whether COD was measured as total or soluble COD. The third alternative is an attempt to estimate the reduction of the waste's polluting load after an anaerobic treatment; the reasoning being that only the liquid phase of the digester effluents would be finally discharged. Figure 1 shows the cumulative gas production of the 4 sludges during batch fermentation. Gas production started after about I week, and it must be noted that the reactors were still producing gas at the end of the 30-day experimental period. Thus, the achieved gas yields do not represent the sludges' maximum energy potential. The earlier mentioned deviation of the FH-2 sludge is clearly shown in Fig. 1. Methane contents averaged 56% in the FH-2 gas, compared to 62-66% in the others. The higher cumulative gas yields in the sieve sludges are due to their higher TS content (Table 2).
Figure 2 illustrates the changes in soluble COD during the batch process. The tendency in all 4 sludges was an increase in soluble COD during the first 14 days before a sharp decrease occurred to values lower than the soluble COD of the feed. This development may be a result of the initial hydrolysis of the anaerobic digestion process, during which particulate organic matter is broken down to smaller, soluble components, leading to the rise in soluble COD. These intermediary metabolites then serve as substrates for the methanogenic bacteria. The resulting CODdecrease corresponds closely with the stabilization of digester pH and the start of vigorous gas production. The settling qualities of digester feed and effluent after 1 month were measured (Fig. 3). The results show that the sedimentation properties were clearly improved after anaerobic digestion of the sludges, and that most of the sludge sedimentation had taken place after only 5 min. The settling qualities for the sludges after 16 days retention are not presented since they are the same as for the 23 day retention trials. S e m i - c o n t i n u o u s culture
The two substrates used in the semi-continuous trials were first run as batch cultures for 1 month Table 2. Feed characteristics and results obtained from the anaerobic digestion of alginate extraction sludges; batch culture
Sludge type
Feed characteristics pH TS (%) VS(%ofTS) COD (g O litre- t)" COD (g O litre- t)h
FH-1 F H - 2 SH-I
SH-2
9.7 0-9 78.7 6"0 2"6
9"8 3"1 84"5 35"2 4'5
9.7 1"1 82-1 9.2 1'7
9.7 3"2 85.4 17'4 3'5
Digesterperformance Specific biogas production litre g-t VS added litre g- t VS removed Specific methane production litre g- t VS added litre g- t VS removed VS conversion (%) a COD removal (%)" COD removal (%)b COD removal (%)'
0"25 0.82
0.17 0.44
0"18 0.89
0-21 1.02
0"15 0-09 0"51 0-24 30.2 38.5 16-4 -51-4 24.6 79.1 86.1
0-11 0.53 20.3 37.1 26.2 85"0
0" 14 0"66 21.1 46.0 46.1 93-1
aCompletely stirred samples. hSoluble COD. ' C O D in effluent supernatant compared to total COD in feed.
Anaerobic digestion of waste sludge from alginate extraction 80
I--~-FH-1 -.-FH-' ""-81"I-1 ---SH-~)
21
I
m i[j
10
i
I
I
0
5
Fig. 1.
10
I~ys
20
25
SO
Cumulative gas production during batch culture.
_
[
I
FH-1
-o-
FH-2
SH-2
SH--1
I
o
[
0
I
I
i
i
i
0
Fig. 2.
1
i
10
i
I
I
Dllyl
l
L
I
I
i
20
80
Soluble COD m al~nate extraction sludge (day 0) and digester effluent during batch culture.
25O
2OO
i o
!loo
i SO
FM-11
814"11
FH.12
8H-12
FH'I 3
1
1114-13
Fig. 3. Settling qualities of feed and effluents from batch and semi-continuous trials. Bars represent settled sludge volume at certain time intervals after filling 250 ml feed or digester effluent into graduated cylinders and stirring. ~= untreated alginate extraction sludge. -"= batch effluent. -~= semi-continuous effluent (23 day retention).
Karl N. Kerner, Jon F. Hanssen, Tor A. Pedersen
22
before regular feeding commenced. By thus starting with already active digesters the lag phase was reduced to only a few days; gas production and digester pH were already fairly stable after 1 week of feeding. The results from the semi-continuous runs are shown in Table 3, along with feed characteristics. Results are presented as mean values achieved during steady-state conditions.
The percentage of methane in the biogas was very stable, averaging 56% and 48% (23 days and 16 days retention time) in FH-1 and 60% and 62% (23 days and 16 days retention) in SH-1. There were considerable variations in the specific gas production during the semi-continuous trials. Specific gas production was higher in both sludges at the 23 day retention time. There was no effect of sludge type on gas production. These results do not correspond to the low VS reductions measured at the longer retention time. A possible explanation was the lower TS content in the feed compared to the TS of the reactor contents at the start of the semi-continuous trials. Thus, when feeding started, digester effluents had a higher content of organic matter than the feed. COD removal is presented as the reduction of soluble COD as well as % reduction calculated from the soluble COD in the effluent and the total COD in the feed (see previous section). Thus, in the semi-continuous trials, a hypothetical reduction of the wastes' polluting load of 80-95% was achieved. The pH remained stable at 6.8-7.0 throughout the experimental period. The use of the LA/PA ratio as a more sensitive tool for monitoring digester stability has been suggested (Ripley et al., 1985). Effluent alkalinity and the corresponding IA/PA ratios are shown in Fig. 4, where PA= bicarbonate alkalinity and IA--volatile acid alkalinity. The observed increase of the IA/PA ratio is due to a relative increase in VFA concentration, a sign of reactor instability. The following drop in the IA/PA ratio and constant alkalinity
Table 3. Feed characteristics and results obtained from the anaerobic digestion of alginate extraction sludges; semicontinuous culture
Sludge type FH-1 SH-I Retention time (days) Loading rate (g VS litre- l digester day- ~)
23 0-15
FH-I SH-I
23 16 16 0-57 0"37 0"81
Feed characteristics pH 6-5 TS (%) 0"5 VS(%ofTS) 74.4 C O D (g O iitre- t) a 5.3 C O D (g O litre- l) b 1.1 Alkalinity (mg CaCO~ litre- ~) 320.0
9"7 1.6 83.3 13.2 2.2 600.0
9.8 8"9 0.7 1"6 81"7 79.4 _ d _ -----
Digester performance Specific biogas production litreg-t VS added Specific methane production litreg-~ V S a d d e d VSconversion(%), C O D removal(%) b C O D removal (%)'
0-50
0.25
0.28 d 57.1 91.5
0-15
0.16
0-15 0.07 0.10 15"6 45"8 46"7 71-8 47.2 60-1 95.2 81.1 91.6
aCompletely stirred samples. hSoluble COD. ' C O D in effluent supernatant compared to total C O D in feed. aNot determined.
'F
f
4
i ,FH-1 o -1 .---'"
.
.
1
t u
............................................... ....................................................... .
..v
.
.
.
.
.
-v ......
- .................
ot
2
0.4
O2
0
5
10
15 Days
20
25
30
Fig. 4. Development of total alkalinity and the IA/PA ratio during semi-continuous culture. Day 0 represents digester alkalinity before feeding started. Solid line = Alkalinity. Dotted line = IA/PA ratio.
Anaerobic digestion of waste sludge from alginate extraction
indicate the return to stable reactor conditions. These changes illustrate the sensitivity of the method, since no significant pH changes were registered within the same period. Digester contents were monitored for volatile fatty acids, but no significant amounts were detected in any of the samples. It is not clear whether this can be explained by low VFA concentrations or technical problems during VFA extraction and analysis. Figure 3 shows the settling qualities of the sludges after digestion at two different retention times. The results compare well with those of the batch fermentations. DISCUSSION Generally, marine algae are considered ideal substrates for anaerobic fermentations due to their high content of easily degradable polysaccharides such as alginate, laminaran, and the sugar-alcohol mannitol (Jerger & Tsao, 1987). Methane yields of 0.20-0.37 litre g-l VS added have been reported for various algal feedstocks (Ghosh et al., 1981; Hanssen et aL, 1987; Jerger & Tsao, 1987). Earlier studies on the anaerobic digestion of algal wastes include the anaerobic treatment of agar extraction residues from Gracilaria tickvahiae (Bird et al., 1981). Those experiments were unsuccessful, mainly due to the nature of the residues. These consisted to a large degree of cellulosic material, low in proteins and amino acids, which led to a strong acidification of the digested liquor during fermentation. The results we obtained with the waste sludges from L. hyperborea and A. nodosum are similar to the results from the sludges from L. digitata (Carpentier et aL, 1988). Even though loading rates in our study were approximately 50% lower, the maximum methane yield achieved of 0.28 litre g-~ VS added is only slightly lower than the 0.29 litre g-~ VS earlier reported (Carpentier et al., 1988). An interesting result of this study is the much higher specific methane yield obtained from the feed with a markedly lower pH (FH-1 at 23 day retention). Since gas production in all other trials was significantly lower, a connection between commencing biological activity in the feed and high gas production can be suspected. Asinari Di San Marzano et al. ( 1981 ) examined the influence of VFA levels in methanogenic substrates on
23
digester stability and methane production. They concluded that an anaerobic digestion system will generally appear more reliable when the substrate contains certain amounts of VF,a/s. The reasoning behind this is that an absence of VFA?s in the substrate could lead to a bacterial community consisting mainly of the syntropy between fermentative and methanogenic bacteria. In this case, even the slightest increase of VF,~/s could be detrimental to digester stability due to the absence of appropriate numbers of obligate proton-reducing bacteria. On the other hand, steady concentrations of VFAgs in the feed will force the bacterial system to include the syntropy between obligate proton reducers and methanogens. An increase of VFA concentrations would be less upsetting than in the first mentioned situation. This study was initiated to examine how effectively an anaerobic treatment would reduce the polluting load of the alginate extraction sludges, expressed as % VS and COD reduction. Anaerobic digestion studies of whole algae have resulted in VS reductions of between 34% and 54% (Klass et al., 1979; Hanssen et al., 1987). The 46% VS reduction measured in the 16 day retention trial was very similar to the 45-56% reduction earlier reported by others (Carpentier et aL, 1988). The soluble fraction of waste effluents is often expressed as soluble COD, and its degree of reduction is an important parameter for treatment efficiency. Soluble COD reductions were 25-51% in the batch fermentations, and 47-71% in the semi-continuous trials. When considering the improvement of the sludges' settling qualities after anaerobic digestion, a possible form of waste treatment would include a short period of sedimentation after its stay in the anaerobic reactor. Ideally, only the fluid decant would finally be discharged to the recipient. A comparison of this effluent to the present untreated alginate extraction sludge results in COD reductions of around 90%. This would represent a significant reduction of the sludges' polluting load. Certain aspects of the alginate extraction process and some sludge characteristics speak in favour of an anaerobic treatment. The extraction process itself includes several steps which resemble proposed pretreatment methods for increased degradability and biogas yield (Tsao, 1987). Furthermore, the extraction sludges leave the plant at around 40-50°C, so that little or no energy input would be necessary to run a meso-
24
Karl N. Kerner, Jon F. Hanssen, Tor A. Pedersen
philic (35°C) fermentation. Finally, alginate production is very stable throughout the year; subsequently there would be a steady and reliable source of substrate.
CONCLUSION
The experimental results from this and earlier studies show that the extraction sludges are definitely suitable as substrates for an anaerobic treatment. Combined with a sedimentation of digester effluents, anaerobic digestion would be expected to sufficiently reduce" the waste sludges' polluting load. As a waste treatment technology, anaerobic digestion has the advantage of generating an energy source (methane) as a by-product. Thus, when considering certain features of the extraction process, as well as the properties of the extraction sludges, this form of treatment could be expected to be economically feasible.
ACKNOWLEDGEMENTS
The authors gratefully acknowledge financial support from the Bioenergy Program of the Norwegian Agricultural Research Council. They would also like to express their appreciation to Protan A/S, Haugesund, for willingly providing substrate samples and to the Chemical Analytical Laboratory (Agricultural University of Norway) and the Agricultural Analytical Center for performing chemical analyses.
REFERENCES APHA (1975). Standard Methods for the Examination of Water and Wastewater. 14th edition, American Public Health Association, American Water Works Association, Water Pollution Control Federation, Washington DC. Asinari Di San Marzano, C. M., Binot, R., Bol, T., Fripiat, J. L., Hutschemakers, J., Melchior, J. L., Perez, 1., Naveau, H.
& Nyns, E. J. (1981). Volatile fatty acids, an important state parameter for the control of the reliability and the productivities of methane anaerobic digestions. Biomass, 1,47-59. Bird, K. T., Hanisak, M. D. & Ryther, J. H. (1981). Changes in agar and other chemical constituents of the seaweed Gracila.ria tikvahiae when used as a substrate in methane digesters. Resour. Conserv., 6, 321-7. Carpentier, B. (1986). Digestion ana6robie de la biomasse algale: les r6sidus de rextraction de l'acid alginique. Les ulves de mar~e vert. Th~se de docteur de 3 eme cycle. Universit6 Pierre et Marie Curie, Paris, France. Carpentier, B., Festino, C. & Aubart, C. (1988). Anaerobic digestion of flotation sludges from the alginic acid extraction process. BioL Wastes, 23, 269-78. Fauchille, S. (1984). Digestion anaerobie de v6getaux aquatiques. Th6se de docteur ing6nieur. Institut National Polytechnique de Lorraine, Nancy, France. Ghosh, S., Klass, D. L. & Chynoweth, D. P. (1981). Bioconversion of Macrocystis pyrifera to methane. J. Chem. Tech. Biotechnol., 31,791-807. Goes, J. (1987). Methanization of algal residues after extraction of agar-agar from Gelidium. In Aquatic Primary Biomass (Marine Macroalgae): Biomass Conversion, Removal and use of Nutrients, ed. P. Morand & E. H. Schulte. Proc. of the First Workshop of the COST-48 Sub-Group 3, held in L'Houmeau, France, 12-14 February. Commission of the European Communities, pp. 111-13. Hanssen, J. F., Indergaard, M., Ostgaard, K., Baevre, O. A., Pedersen, T. A. & Jensen, A. (1987). Anaerobic digestion of Laminaria spp. and Ascophyllum nodosum and application of end products. Biomass, 14, 1-13. Indergaard, M. (1983). The aquatic resource. In Biomass Utilization, ed. W. A. Cole. Plenum Publishing Corp, pp. 137-68. Jenkins, S. R., Morgan, J. M. & Sawyer, C. L. (1983). Measuring anaerobic sludge digestion and growth by a simple alkalimetric titration. J. Water Poll. Contr. Fed., 55, 448-53. Jerger, D. E. & Tsao, G. T. (1987). Feed composition. In Anaerobic Digestion of Biomass, ed. D. P. Chynoweth & R. Isaacson. Elsevier Applied Science, pp. 65-90. Klass, D. L., Ghosh, S. & Chynoweth, P. (1979). Methane production from aquatic biomass by anaerobic digestion of Giant Brown Kelp. Proc. Biochem., 14 (4), 18-23. Ripley, L. E., Boyle, W. C. & Converse, J. C. (1985). Improved alkalimetric monitoring for anaerobic digestion of poultry manure. In Proceedings of the 40th Industrial Waste Conference, Purdue Univ., Indiana. Butterworth, Boston, pp. 141-9. SUPELCO Inc (1985). Packed column GC analysis of volatile fatty acids from anaerobic fermentation. Supeico GC Bulletin 748 H, Bellefonte, Pennsylvania, USA. Tsao, G. T. (1987). Pre-/Posttreatment. In Anaerobic Digestion of Biomass, ed. D. P. Chynoweth & R. Isaacson. Elsevier Applied Science, pp. 91-107.