Anaerobic digestion of malt whisky distillery pot ale using upflow anaerobic sludge blanket reactors
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Bioresource Technology 49 (1994) 75 -81 O 1994 Elsevier Science Limited
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ANAEROBIC DIGESTION OF MALT WHISKY DISTILLERY POT ALE USING UPFLOW ANAEROBIC SLUDGE BLANKET
J. A. S. Goodwin & J. B. Stuart
Department of Chemical and Process Engineering, Heriot-Watt University, Riccarton, Edinburgh, UK, E H14 4AS
(Received 22 August 1993; accepted 11 March 1994)
Abstract The anaerobic treatment of pot ale, a liquid waste- product from the malt whisky industry, was investigated at laboratory scale using UASB (upflow anaerobic sludge blankeO reactors. Chemical oxygen demand reductions of around 90% were achieved after dilution, pH adjustment and settlement of the pot ale. Large amounts of ammonia were produced during treatment and the pH of the waste rose significantly. The maximum space loading rate consistent with stable operation was around 15 kg COD/m 3 per day at a retention time of 2.1 days; higher loading rates produced process instability and failure. Extensive sludge granulation was observed during periods of stable operation.
Key words: Anaerobic digestion, UASB reactors, malt whisky.
The upflow anaerobic sludge blanket (UASB) process was first developed in The Netherlands in the 1970s (Lettinga et al., 1980), and has been described in detail in many previous publications. It has been used suc- cessfully for the treatment of a wide variety of aqueous effluents, including wastes produced by breweries and alcohol distilleries (Hulshoff Pol & Lettinga, 1986; Panlin, 1989; Cheng et al., 1990). However, it has not generally been adopted for the treatment of malt distil- lery wastewaters, because although these have a high chemical oxygen demand (COD) and are readily bio- degradable, making them suitable for anaerobic diges- tion, the relatively small size of traditional distilleries has to date made alternative disposal routes such as cattle-feed production more economically attractive. Little previous research on the anaerobic treatment of malt whisky distillery effluent has been performed, although some work has been published (Boopathy et aL, 1988).
Malt whisky manufacture in a traditional distillery starts with the enzymatic extraction of sugars from
malted barley in water to yield a solution known as wort. This is fermented to produce wash, in essence an unhopped beer, which is passed to the wash still where the first of two batch distillations takes place. The distillate from the wash still, known as low wines, is transferred to the spirit still, where the second distilla- tion is performed. The middle fraction of the distillate from the spirit still, new spirit, is matured in wooden barrels, typically for 10-25 years, to produce the final product, malt whisky. The liquid residues remaining in the wash and spirit stills after distillation are known as pot ale and spent lees, respectively. Pot ale has a bio- logical oxygen demand (BOD) in the range 15 000-45 000 mg/litre, and contains yeast, inorganic salts and a wide variety of organic compounds includ- ing unfermented sugars; spent lees has a BOD of 1500-2000 mg/litre and contains a number of volatile organics.
Pot ale was used in the study described here because it is responsible for most of the COD of distillery wastewaters and has been shown to be the fraction most resistant to degradation. There is evidence for the presence of inhibitory material which reduces the digestion rate when pot ale is treated undiluted (United Distillers, pers. comm.); successful adaptation of diges- ter biomass to undiluted pot ale would, therefore, suggest that any combination of liquid distillery waste products could be treated by anaerobic digestion.
Seed sludge Sludge used to start up the reactors was obtained from the solids digester at the Whitburn sewage treatment works operated by Lothian Regional Council. Large solids were removed from the sludge prior to use by passing it through a plastic sieve of hole size 1 mm.
Pot ale Pot ale was obtained from the Glenkinchie malt whisky distillery situated near Pencaitland (East Lothian, Scot- land, UK).
76 J. A. S. Goodwin, J. B. Stuart
Apparatus The experimental apparatus is illustrated in Fig. 1 and was similar to that described previously (Goodwin et al., 1990a). There were two identical reactors, each with a working volume of 1"05 litres, which were oper- ated in parallel as duplicates. The wastewater feeds to these reactors were pumped through separate channels of a single Watson-Marlow 501M multi-channel peri- staltic pump head fitted to a 503S drive unit. The reactors were constructed from transparent perspex tubes and had internal diameters of 62 mm. Reactor temperatures were maintained at 35C using a Grant FH 15A flow heater and water-jacket system.
Biogas Biogas production rates were measured by displace- ment of 0.05 u aqueous sulphuric acid solution within individual calibrated gas collection vessels. The design of the apparatus allowed measured gas volumes to be standardised to conditions of standard temperature and pressure (0C, 1 atm).
Ltd, Queensferry, Clwyd, UK). The chromatograph was operated in isothermal mode with a constant oven temperature of 140C, while the carrier gas was oxy- gen-free nitrogen (BOC) flowing at 20 ml/min. Com- pounds leaving the column were detected by a flame ionisation unit and the signal produced was analysed by the chromatograph's integration software, which had been pre-calibrated with VFA standards of known concentration.
Effluent samples were prepared for analysis by filtering them through Whatman cellulose nitrate membranes of pore size 0.45/zm then acidifying the fil- trates with formic acid ( 1 part acid to 10 parts filtrate); 1 /zl of each acidified filtrate was injected into the chromatograph. The formic acid used was a BDH pro- duct described as 'low in acetic acid'.
pH Sample pH values were determined using a portable WPA CD60 pH meter and probe (Waldon Precision Apparatus, Saffron Waldon, Essex, UK).
COD Effluent COD was determined by the small-scale closed tube method described by HMSO (1986). Samples were mixed with an acidic dichromate solu- tion and heated at 150C for 2 h using a Hach 16500- 10 dry-block heater, after which residual dichromate was determined by titration with ferrous ammonium sulphate.
Volatile fatty acids Effluent volatile fatty acid (VFA) concentrations were determined by gas chromatography, using a Perkin- Elmer 8500 gas chromatograph fitted with a stainless- steel column of length 2 m and internal diameter 2 mm packed with 12% free-fatty-acid-phase (FFAP) on 80-100 mesh Chromosorb W-HP (Phase Separations
GAS TO COLLECTOR
EFFLUENT I _~ OUTLET . : .~_. :_ :_
::L-.:- ::--.:: l : : ' : - : : : : L :
EFFLUENT ~m~m~ ~ L 0 0 P l~x~
W A T E R i ~ ~ . ~ ' JACKET ~ ~
Anaerobic digestion of malt whisky distillery pot ale 77
Table 1. Initial feed composition
Table 2. Summary of organic loading rates
Pot ale 100 ml/litre NaHCO3 2 g/litre MgSO 4. 7H20 102.5 mg/litre CaC12" 2H20 36"75 mg/litre FeSO4 10 mg/litre
Trace elements NiSO4.6H20 500/~g/litre MnCi2"4H20 500/zg/litre ZnSO 4. 7H20 100/zg/litre H3BO 3 100 tg/litre CoC12' 6H20 50/zg/litre H3PO 4" 12MoO 3" 24H20 40/zg/litre
the study when high levels of free ammonia were recorded in the reactor effluents. Organic loading rate was varied during the study by altering retention time and the proportion of pot ale in the feed, the two reac- tors receiving identical organic loads; these changes are summarised in Table 2. Much of the variation in organic loading rate was due to differences in the COD of pot ale samples obtained at different times.
The undiluted pot ale was a light brown, turbid liquid with a pH below 4, and diluted samples were also acidic. As methanogenic bacteria are inhibited at pH values below 6.6, feed pH was increased by the addition of sodium hydrogen carbonate in solution during the first few weeks of the study. This appeared to cause a light brown particulate material, apparently consisting largely of yeast, to settle to the bottom of the feed vessel, forming a compact layer around 5 mm thick within a few hours. This settled layer was below the liquid take-off point and was, therefore, not pumped to the reactors. Since the effluent from each reactor generally had a pH significantly higher than 7 in the period following start-up (Fig. 2), the quantity of sodium hydrogen carbonate added was gradually reduced during the first 3months of operation with the intention of allowing a greater proportion of the brown material to enter the reactors. On day 99, sodium hydrogen carbonate was omitted from the feed, which was allowed to enter the reactors without any pH adjustment; this produced no immediate ill- effects, although feed turbidity and COD increased significantly (Table 2), and satisfactory performances were recorded for the next 2 weeks. However, both digesters failed around day 113 with sudden drops in pH and biogas production, and the organic loading rate was reduced and slowly increased again to allow recovery. Following this, sodium hydrogen carbonate, pre-dissolved in water, was included in all feeds at 2-5 g/litre for the remainder of the study. This raised feed pH, although not to neutrality, and the brown material settled to leave a clear reddish-brown supernatant. Reactor A performed well thereafter, although Reactor B suffered a partial failure starting around day 238 and was eventually restarted by reducing organic load,
% Pot Feed COD Organic loading Retention ale (mg/litre) rate time
(kg COD/m 3 (days) /day)
1 10 3 526 1"679 2'1 13 10 3818 1"818 2"1 21 20 4672 2"225 2"1 40 60 9 506 2-263 4'2 57 30 4753 2"263 2"1 62 40 10312 4"910 2"1 72 40 16777 7"989 2.1 75 50 19508 9"290 2"1 99 60 19508 9"290 2"1
104 60 32773 15"606 2.1 111 70 38235 18"207 2.1 121 30 11835 5"636 2'1 125 10 3945 1"879 2.1 133 20 11341 5"400 2'1 141 30 17012 8"101 2"1 152 40 25937 12"351 2"1 159 40 12327 5'870 2"1 163 50 21696 10"331 2'1 174 60 27613 13"149 2.1 188 70 32051 15-262 2'1 196 70 30572 14"558 2"1 208 80 33531 15"967 2"1 223 90 47929 22"823 2"1 224 90 47929 9'129 5-25 232 90 47929 13"694 3"5 238 90 47929 22-823 2'1 239 90 37 106 17"670 2"1 251 80 32983 15'706 2"1 265 80 40111 19'100 2"1 271 80 38961 18"553 2-1 277 60 30612 14"577 2.1 296 60 26248 12"499 2"1 299 80 36044 12"873 2"8 306 100 52 126 14"893 3'5 312 60 19778 9"418 2.1 320 100 42883 12"252 3'5 327 100 42883 15"315 2"8
' ' ' ' ' I ' ' ' ' ' ' ' ' I ' ' ' 1 ' I ' ' ' ' ' I '~ ' I ' ' ' ' '
50 100 150 200 150 300 350 Day
Feed and effluent pH values. Reactor A, ( . . . . ) Reactor B, ( - - . ) feed.
78 J. A. S. Goodwin, J. B. Stuart
adding sodium hydrogen carbonate directly to the digester liquor and transferring active sludge from Reactor A (100 ml wet sludge transferred on days 251, 267 and 281; 200 ml transferred on day 292).
The reactor feeds were supplemented with trace elements (Table 1) from the start of the study up to day 174 to guard against possible deficiency during start- up. Trace element addition was then discontinued until day 272 to determine whether the pot ale could be treated without adding additional nutrients. From day 272 to day 306, trace elements were included in the feeds once again in an attempt to accelerate the recovery of Reactor B; they were then omitted until day 315, then included once more from day 315 to the end of the study.
Performance data pH data, biogas production rates and COD removal efficiencies are presented in Figs 2, 3 and 4, respect- ively; effluent acetate and propionate concentrations are presented in Figs 5 and 6. Effluent ammonia con- centrations recorded between day 197 and day 3 24 are presented in Table 3.
Both reactors adapted rapidly to the pot ale feed at the start of the study. During the first 100 days of operation, COD removal efficiencies gradually increased to around 90%, while biogas production rates increased in line with increases in organic load. Effluent propionate concentrations rose to over 1000 mg/litre during the first few days but then fell rapidly to below 200 mg/litre, while acetate concentrations showed a small rise in Reactor A but then fell to below 300 mg/litre in both reactors. Levels of longer-chain fatty acids remained negligible throughout this period.
The failure of both digesters around day 113 was followed by substantial increases in effluent VFA
', +' +'
? ' ' ' ' ' 1 . . . . . I ' ' ' ' ' 1 ' . . . . I . . . . . I ' ' ' ' ' I ' ' '
50 100 150 200 250 300 350 Day
Fig. 4. COD removal. ( ) Reactor A, ( . . . . ) Reactor B.
E 550 W
0 50 100 150 200 250 300 350 Day
Fig. 5. Reactor A volatile fatty acids. ( ) Acetic acid, ( . . . . ) propionic acid.
4.0 ' '
,,+ ' t,,
I I I I ~' I ' ' ' ' ' l ' ' ' ' ' l ' ' ' 'I
0 50 100 160 300 360 300 360 Day
Biogas production. ( ) Reactor A, ( . . . . ) Reactor B.
Table 3. Feed
Anaerobic digestion of malt
and effluent ammonia concentrations (expressed as mg/litre NH3)
Day Feed Reactor Reactor number A B
197 91 ND ~ 529 211 171 793 557 238 ND ~ 567 472 275 169 921 1214 282 541 794 1039 303 253 1001 1077 309 132 1152 888 324 361 1000 963
aND, not determined.
concentrations. The levels of acetate and propionate exceeded 1000 mg/litre and 2000 mg/litre, respect- ively, with even higher concentrations measured for longer-chain VFA such as valeric and butyric acids. Biogas production rates fell to less than 20% of their previous values as effluent pH values dropped to 6.3. A significant washout of sludge solids occurred, so that effluent COD exceeded feed COD for both reactors on day 127. However, reduction in organic load and increase in feed pH allowed recovery of both reactors; by day 156, VFA levels had fallen significantly, effluent pH values had increased to around 7.5, biogas produc- tion rates had risen and COD removal efficiencies had returned to around 90%.
After recovering from failure between days 113 and 156, Reactor A performed well for the rest of the study, with COD removal efficiencies generally higher than 86% and effluent pH values in excess of 7.5 most of the time. Effluent acetate concentration was around 700 mg/litre at the higher organic loadings applied in the latter half of the study, while effluent propionate concentration was variable. Levels of longer-chain VFA were low during this period.
The performance of Reactor B from day 156 to day 238 was similar to that of Reactor A. However, diges- ter failure occurred in the period immediately follow- ing day 238, with very high VFA levels recorded between days 238 and 296, along with significant reductions in biogas production rate and COD rem- oval efficiency. Effluent pH fell to a minimum of 5.2 on day 292, but was generally above 5.8. By day 306 per- formance had been restored to previous levels follow- ing the transfer of active sludge from Reactor A to Reactor B.
Samples of digester liquor were taken from Reactor A on day 335 for estimation of total reactor biomass. Dry suspended solids concentrations at the top and bottom of the sludge bed were 44"07 and 62.89 g/litre, respectively. The total sludge bed volume was around 800 ml and a mixed-sludge sample was found to have an ash content of 49-6% by mass on a dry basis. From these data, total sludge bed volatile suspended solids were estimated to be 21.56 g. Thus, the daily organic load applied to Reactor A near the end of the study
whisky distillery pot ale 79
was approximately 0.46-0.75 kg COD per kg volatile solids.
Sludge granulation The physical appearances of the sludges were moni- tored throughout the study by visual examination through the transparent perspex reactor walls.
The seed sludge used to inoculate the reactors was totally non-granular. Sludge particle formation was evident after a few weeks, and by day 68 extensive granulation was observed in both reactors.
On day 93, the sludges were highly pelletised, with many particles 2 mm or more in diameter. Changes in the appearance of the sludges became apparent after the omission of sodium hydrogen carbonate from the reactor feeds on day 99; material at the bottom of the reactors, close to the feed inlet, had become white and fibrous in appearance by day 100, and although rela- tively large particles were present in both reactors for some time after this, there was a gradual disintegration of granules as the reactors failed, with significant solids losses around day 127. Re-pelletisation commenced soon after recovery, with increases in the volumes of both reactor sludge blankets observed between days 140 and 152. The sludges were highly flocculent by day 163, and had achieved full granulation by day 168. The sludge in Reactor A remained almost completely granulated from this time onwards, with particle diameters in the range 1-5 mm, and during the last few months of the study it was necessary to waste sludge as around 90% of the reactor volume was full of granular sludge solids. The sludge in Reactor B was granular but paler and more gelatinous in appearance between days 215 and 238, and significant breakup of granules occurred during the failure period from day 238 to day 296. Some degree of granulation, albeit less complete than that in Reactor A, was observed in Reactor B from day 301 onwards, although this may have been the result of reseeding with Reactor A sludge.
The reactors showed gradual improvements in per- formance and ability to accept increased organic loads during the start-up period before day 100. Propionate degradation in particular seems to have improved during the first few weeks. This is consistent with data published previously by De Zeeuw and Lettinga (1980), who used digested sewage sludge, similar to that used in the present study, to inoculate a labora- tory-scale UASB digester treating a synthetic waste based on acetate and propionate. They found that the sewage sludge had a low initial activity, particularly with respect to propionate utilisation, but had deve- loped a substantially increased rate of VFA degrada- tion after 2 weeks.
The failure of both digesters around day 113 pre- sumably resulted from the removal of sodium hydro- gen carbonate from the reactor feed on day 99, the immediate consequences of which were an increase in
80 J. A. S. Goodwin, J. B. Smart
organic loading rate (a significant COD was associated with the pot ale solids, which did not enter the digesters until this time) with a probable reduction in feed pH. However, failure did not occur until 2 weeks after this, although there were changes in the physical appear- ance of the sludges as described earlier; indeed, gas productions and COD removal efficiencies actually increased between day 99 and day 113. The most likely cause of failure is severe inhibition of methano- genic activity by reduced digester pH, although it is also possible that the particulate material which settled out on the addition of sodium hydrogen carbonate con- tained inhibitory material which first entered the reactors in significant amounts after day 99. The time delay between the change in feed composition and eventual reactor failure may be explained if it is postu- lated that little mixing took place at the bottom of each sludge bed, so conditions close to plug flow pre- vailed. Under these conditions, vertical pH, VFA and COD profiles would be established across each bed, with high VFA concentrations and lowered pH near the bottom as described by previous investigators (Russo & Dold, 1989; Sam-Soon et al., 1991 ). The pH would rise with bed height because of VFA degrada- tion and the general pH increase, discussed later, observed on anaerobic digestion of pot ale in this and in previous work (Boopathy et al., 1988). Thus, it is possible that, shortly after day 99, conditions at the bottom of each bed were sufficiently acidic to inhibit or render non-viable the methanogenic population, while higher pH values farther up permitted satisfactory activity, giving acceptable VFA degradation and COD removal overall. This loss of methanogenic activity could lead to the zone of low pH spreading slowly upwards over a period of several days until complete failure occurred. A similar mechanism for digester failure was proposed by Yan et al. (1990) to explain the results of a laboratory study in which a UASB reactor was used to treat cheese whey; the acidogenic zone of low pH observed at the base of their digester spread upwards through the sludge bed when an excessive organic load was applied, and it was postulated that this would eventually lead to process failure.
The failure of Reactor B around day 238 was also associated with a fall in digester pH. This seems to have been caused by organic overload coupled with low influent pH, as the organic loading rate was increased significantly just before failure occurred. However, Reactor A tolerated this loading increase with no apparent ill-effects. It may be that Reactor A had deve- loped a higher methanogenic activity than Reactor B, preventing catastrophic VFA accumulation; it is also possible that Reactor A was mixed to a greater extent, so that localised formation of low pH zones was less significant.
All of the digester failures were accompanied by large increases in effluent VFA concentrations, longer- chain VFA being particularly prevalent. Analysis of data from the present study indicated that most of the effluent COD was present as VFA during failure condi-
tions, showing that overall treatment efficiency was limited primarily by the rate of VFA degradation.
During the stable period between day 156 and day 238, COD removal efficiencies of around 90% were achieved at space loading rates ranging from 10 to 15 kg COD/m 3 per day, although higher loading rates pro- duced instability and failure. This is consistent with results obtained in other studies of the anaerobic treat- ment of distillery wastes using UASB digesters. Panlin (1989) reported satisfactory treatment of rice wine and spirits wastewater at loading rates in excess of 7 kg COD/m 3 per day, while Boopathy et al. (1988), using an anaerobic baffled reactor to treat whisky distillery effluent, achieved over 90% removal of COD at the comparatively low loading rate of 3.5 kg COD/m 3 per day. The estimated sludge loading rate applied towards the end of the study described here, 0"46-0.75 kg COD/kg MLVSS (mixed liquor volatile suspended solids) per day, is similar to that reported to produce granulation in previous studies. For example, Hulshoff Pol et al. (1983) found that granulation occurred only at loading rates above 0"6 kg COD/kg MLVSS per day, with rapid granulation observed at 0"9 kg COD/kg MLVSS per day and poor granulation observed at 0.3 kg COD/kg MLVSS per day.
Pot ale typically contains high levels of protein and lactic acid (United Distillers, pers. comm.), so the increases in ammonia concentration observed during the digestion process (Table 3) were probably due to protein degradation. Although ammonia is used as a nitrogen source by the organisms involved in anaer- obic digestion, it can cause serious inhibition at high concentrations, NH 3 being significantly more toxic than NH 4. Total ammonia concentrations in the range 900-1200 mg/litre were recorded between day 275 and day 324, these being below the inhibition thresh- old of 1500 mg/litre ammonia proposed by McCarty (1964) for digesters operating at pH values above 7"4, but close to the figure of 1000 mg/litre ammoniacal nitrogen reported to inhibit propionate degradation by Hulshoff Pol et al. (1983). Calculation of free (NH3) ammonia concentrations using the Henderson-Hassel- balch equation gives values in the range 40-70 mg/ hire, below the inhibition threshold of 80 mg/litre sug- gested by De Baere et al. (1984). Thus, it seems probable that ammonia toxicity was not a factor which limited digester performance. The release of ammonia by protein degradation, in conjunction with the remo- val of organic acids, is a plausible explanation for the fact that effluent pH values were always higher than the feed pH during periods of stable operation (Fig. 2).
Identification of specific trace element deficiencies in the pot ale was not one of the objectives of this study, but the fact that the reactors performed well between days 174 and 272 with no added trace elements suggests that adequate quantities of these micro- nutrients were present. Trace element limitation in UASB reactors has been shown to result in VFA accu- mulation, implying inhibition of methanogenic activity (Goodwin et al., 1990b), but significant increases in
Anaerobic digestion of malt whisky distillery pot ale 81
VFA concentrations were not observed following the omission of trace elements from the feeds from day 174 onwards. More recent work has shown that diluted pot ale can be treated successfully by UASB reactors with no trace element supplementation whatsoever (Goodwin, Haddow & Low, unpubl, data).
The performance data indicate that diluted pot ale is readily biodegradable using UASB, while a reasonably high COD removal rate can be attained with undiluted pot ale. However, the total liquid effluent from a malt whisky distillery contains not only pot ale from the first distillation stage, but also spent lees from the second, the latter having a BOD of around 2000 mg/litre. The two waste streams are produced in a volumetric ratio of around 72 parts pot ale to 28 parts spent lees (United Distillers, pers. comm.), so a mixed effluent feed to a practical digestion system would have a COD of around 35 000 mg/litre, comparable with some of the feeds used in the latter half of the present study. Hence, it is feasible that UASB reactors operated with retention times of 2-3 days could be used to treat malt distillery effluent at industrial scale, although in view of the process instability observed at higher loadings, longer retention times or higher levels of alkalinity supplementation would be advisable in practice. The degradation of the yeast fraction was not investigated and it is likely that longer retention times would be required for satisfactory digestion of this material.
The assistance of United Distillers staff at Glenkinchie Distillery and Glenochil Research Station is greatly appreciated. Thanks are also due to Eileen McEvoy for her help with analytical work.
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