thermophilic anaerobic digestion of alcohol distillery wastewaters

Bioresource Technology 43 ( 1993) 131-140

THERMOPHII JC ANAEROBIC DIGESTION OF ALCOHOL DISTILLERY WASTEWATERS

A. Vlissidis

Department of Chemical Engineering, N.T.U. Athens, GR-15773 Zografou, Greece

&

A. I. Zouboulis*

Department of Chemistry, Aristotelian University, GR-54006, Thessaloniki, Greece

(Received 14 September 1991; revised version received 18 April 1992; accepted 25 April 1992)

Abstract Results are presented from a full-scale plant using thermophilic (50-55°C) anaerobic digestion of wastewaters from an alcohol distillery. The distillery processes mainly beet molasses, but also molasses from figs, raisins and wines. A new version of the process is described, where the digester has a feedback of bacteria and undi- gested solids, in order to overcome high sulfate and nitrogen concentrations and the high organic load of the effluent. The best results for a waste feed with 60 g/liter COD were found to be the following: volumetric load up to 6 kg/m J day, biogas production about 2"5 m3/rn~,e~,o, day (76% CH4 content), methane production ~4 rn3/kg COD removed, efficiency in converting organic solids to CH4 70%, and mean C02 content ofbiogas 18%.

Key words: Anaerobic digestion, distillery slops (vinasse), thermophilic, contact process, bacteria feedback, full scale.

INTRODUCTION

The main source of sugar produced in temperate cli- mates is beets. Molasses are byproducts of the sugar- extraction process and are often used as a raw material in alcohol distilleries. In Europe and the USA a large number of such industries are currently operating; in addition to alcohol production, considerable quantities of liquid wastes are produced.

*To whom correspondence should be addressed.

Bioresource Technology 0960-8524/92/S05.00 O 1992 Elsevier Science Publishers Ltd. England. Printed in Great Britain

Ethyl alcohol is the final product of a complex manufacturing process, which includes fermentation, followed by multistage distillation. The primary raw materials for this process in Greece are molasses, mainly from beets (around 70%) but also from figs, raisins and wines (30%). The residue from the fermentation and distillation processes, named vinasse, is the major liquid waste. The alcohol distilleries are typical of traditional Greek agricultural industries, and are of great importance for the national economy. Today, in the time of increasing environmental concern, these industries are facing a difficult problem: how to treat efficiently their liquid effluents.

The nine Greek distilleries have a total production of 170 000 liters alcohol/day; at the same time around 2000 m3/day of liquid wastes are produced. It has been calculated that for 1 t of anhydrous alcohol, 16 m s of vinasses are produced and 60 m 3 of cooling water are necessary (Vlissidis, 1986; Vlissidis et al., 1988). These wastes have a high organic load (25 000-45 000 mg BODs/liter); they correspond to the domestic pollution of a big city with around 1"5 million inhabitants. Although vinasse is not characterized as toxic, because it does not contain toxic metals or other chemicals dangerous to the natural ecosystem, its free disposal creates a serious problem.

Several treatment methods have been examined for the safe disposal of vinasses; among them, chemical or biological treatment (aerobic or anaerobic classical methods, trickling filters, lagoons, etc.), evapora- t ion-condensation with or without combustion, direct dispersion on soil as a fertilizer, etc. (Sheehan & Greenfield, 1980; Rao, 1972). A common feature of all these methods is their relatively high cost, and sometimes simultaneous creation of other hazardous byproducts/pollutants.

131

132 A. Vlissidis, A. I. Zouboulis

The aerobic biological treatment of high-organic- load wastes, like vinasses, is associated with operational difficulties of sludge bulking, inability of the system to treat high BOD or COD loads economically, relative high biomass production and high cost in terms of energy. On the other hand, with the diminishing supply of natural gas and other fossil fuels, bacterial conversion of liquid (or solid) wastes to methane and stabilized byproducts through anaerobic digestion would be beneficial (Fannin et aL, 1986, 1987; Good- win & Hickey, 1988; Hickey & Goodwin, 1989). These byproducts could subsequently serve as feed or fertilizer and generally could be disposed of without many problems (easier dewatering, smaller amounts). It is worth pointing out also, that the high temperatures and high-organic-load concentrations of the effluents to be treated, as well as the high energy requirements of the distillery process, are very suitable conditions for the application of anaerobic digestion.

Anaerobic treatment, though slow, presents a number of advantages; for example, a quite high degree of purification with high-organic-load feeds can be achieved, low nutrient requirements are necessary, small quantities of excess sludge are usually produced and finally, but equally important, a combustible biogas is generated. The production of biogas enables the process to generate or recover energy instead of just energy-saving; this can reduce operational costs by a big margin compared with the high-energy-consump- tive aerobic process (Lettinga et aL, 1979, 1981). On the other hand, the treated effluents have solubilized organic matter which is amenable to quick subsequent aerobic treatment. Thus, there is an argument for combining anaerobic and aerobic units for the treatment of these wastewaters (Atanasoff & Haberl, 1985; Shrihari & Tare, 1989).

The net sludge production resulting from anaerobic contact treatment followed by aerobic polishing was estimated to be only 20-30% of that which would result from the application of aerobic treatment alone. Pilot-study results also suggest that biomass solids production would be of the order of 8% of the mass of organic substrate removed (Farmer et al., 1989).

For these reasons, the anaerobic digestion process has been examined by a number of researchers in laboratory or pilot-scale digesters, but less often in full- scale mesophilic plants, as the initial (pretreatment) processing step for slops treatment (Shea et aL, 1974; Roth & Lentz, 1977; Braun & Huss, 1982; Szendrey, 1984; Wang et aL, 1986; Athanasopoulos, 1987; Carozzi & Pfeiffer, 1988; Montanari & Basilico, 1989). Several related reviews have been presented recently and many pilot-scale investigations have been reported, using different anaerobic reactor concepts (Chiesa & Manning, 1986, 1987; Manning & Chiesa, 1988).

The main application problem of this process to the distillery slops treatment seems to be its relative insta- bility. The most significant reason for this is the daily large changes of load, and the presence of high potas-

sium, ammonia and sulfate concentrations. The presence of sulfate creates the toxic and corrosive H2S, which also inhibits the methane fermentation.

Mainly for these reasons, very few data have been reported for full-scale applications of this method, and in these cases mainly mesophilic temperatures have been applied. In a novel anaerobic process for treating molasses waste, using an upflow packed-bed reactor, sulfate was reduced to H2S and then oxidized to ele- mental sulfur by photosynthetic bacteria (Maree & Strydom, 1987). Thermophilic digestion was also investigated for distillery slops using pilot-scale CSTR and UASB digesters, but the performances were more related to toxicity than loading (Chiesa & Manning, 1987).

Several types of anaerobic treatment systems have been examined in the literature for the treatment of liquid wastes (Bowker, 1983). The present method is similar to the anaerobic contact process, as biological solids are recycled from a solids separation unit to the anaerobic contact reactor; this results in a relatively high retention time of solids.

It has also been demonstrated that fermentations operating at thermophilic temperatures (around 50"C), result in more rapid degradation (accelerated stabilization) of the organic matter, leading to a lower hydraulic retention time and higher loading rate and BOD reduction. These systems also have more efficient biogas production with higher methane percentage and calorific value of the produced biogas than those operating in the mesophilic range, i.e. around 35"C (Pearson et a/., 1953; Lanting etal., 1989; Wohit etal., 1990).

The applied process is composed mainly of two stages (Fig. 1). Comparison between the one- and the two-stage anaerobic treatments has been presented in the literature, showing some advantages when the separation between acidogenesis and methanogenesis has been applied (Gieseke et al., 1988). In the case of the process described below, separation has been applied between different operating conditions of the same bacteria cultures; so it looks like a conventional two-stage process or a contact process, but it is not exactly the same, presenting certain peculiarities.

Specific advantages of this process include: influent suspended solids are accumulated or degraded in the reactor due to the high solids retention, and loading fluctuations can be readily accommodated since the hydraulic retention time of reactors of this type is typically 5-15 days, while the mean solids (and cells) retention time is greater (22-50 days).

The basic question is no longer whether this waste can be anaerobically degraded to methane, since most organics are amenable to anaerobic treatment, but at what rate it is degradable and to what degree, how the maximum yield of biogas can be obtained and also how the overall process can be made cost attractive. This paper attempts to evaluate the performance of a full- scale anaerobic thermophilic digestion plant treating distillery slops, as well as to discuss the results and to present, where this is possible, some generalized con-

Digestion of alcohol distillery wastes 133

(a)

q

' L I .E~VAl VoF ,~, SLUDGE EXCESS

biogas l a n e

flare

(b)

Vinassa

Bioreactor

B-I

Bioreector

G a s h o l d e r

biogas

£arobXc

/proposed ~ use of gas % , \

I !

electrical generator x /

tank

anaerobic lime

t, ~ ~ ammonia stripper. eo-composting

pile

primary settling tank

"~"1=~'e ' r~, to a e r o b i c

Fig. 1. (a) Schematic diagram of the applied thermophilic anaerobic digestion unit. (b) The actual plant.

clusions. To the authors' knowledge, this is the first paper of this kind to have appeared in the literature. For this reason a detailed description of the main parameters affecting the process has been attempted.

M E T H O D S

Distillery slops characteristics Several parameters were examined routinely: pH, temperature, influent and effluent COD, total volatile acids content, bicarbonate alkalinity, biogas production (expressed as m3/day or m3/kg COD removed). A number of other parameters: nitrate and phosphate content, total, dissolved and volatile solids, etc., were examined occasionally. In all cases standard methods were employed for the evaluation of the above mentioned parameters (APHA, 1971 ).

From these measurements, the following parameters were also calculated:

- - loading rate (influent and effluent): kg COD/ n~cto r day,

-- percentage COD removal: Re% COD = ( 1 - CODe/CODi)x 100, where COD e and CODi correspond to the effluent and influent values (g/liter), biogas specific productivity: 3 3 - - m g a s / m r e a c t o r day, and

- - oil-saving: estimated using the equation

kgoil = m~iogas x %CH4 x 0"8899

as the biogas calorific value is given by %CH4 x 8899 kcal/m 3 and the oil calorific value is estimated as 10 000 kcal/kg.

Some selected typical parametric values of the main types of Greek distillery slops are presented in Table 1. Vinasses produced from beet molasses have a dark brown or black color, due to the presence of several organic compounds, like melanoidrines, polyphenol- iron complexes, melanines, etc. As the influent nitrogen content was in excess and phosphates were found to be sufficient, additional nutrients were not supplied. On the other hand, the sulfates and potas- sium concentrations in the wastewater were at levels reported to be moderately inhibitory for the anaerobic digestion process (Basu, 1975).

Description of the thermophilic anaerobic digestion plant The alcohol distillery studied in this paper is located in an industrial zone. The simpfified overall flowsheet of the process is given in Fig. 2; the two stages will be described shortly in the following. It is worth noting

134 A. Viissidis, A. I. Zouboulis

Table 1. Composition (g/liter) of the distillery slops (vinasses) used in the present study (average values over a 3-year period)

Parameter Vinasses from Vinasses from Vinasses from Vinasses from beet molasses raisins wines figs

pf-f 4.3 f 05 3.2 f 05 4.2 f 2 3.6 f 0.5 BOD 27.5 f 9.5 30 f 2.2 16.3 f 1.5 20.4 f 5.5 COD 55.5 f 25 57.5 f 15 27.5 f 2.1 35.4 f 15 Total solids 95f25 73.2 f 22 36.6 f 0.69 48.5 f 18 Organic solids 55*15 44.5 f 15 32.2 f 6.2 32f 12 Volatile acids 1.95 f 0.185 3.2 f 0.35 1.1 f 0.6 Total-N 4.75 f 0.15 0.75f0.15 0.65 f0.12 OzY.18 Total-P ND 0.22 f 0.06 0*12%*055 017fQ05 Sulfates 3.5 * 1.05 0.48 fO.15 09f0.18 W 6.7 f 3.5 ND ND ND

ND: not determined.

Fig. 2. Flowsheet of the thermophilic anaerobic digestion plant treating distillery slops. (1) h&rent, (2) recirculation loop for B-l content, (3) removal of sludge excess, (4) sludge return from the second- to the first-stage reactor, (5) feed of the second reactor B-2 from the supematant of the first reactor B-l, (6) mixing of B-2 content with lime, (7) effluent

for subsequent treatment, (8) biogas.

that nowadays there are three full-scale plants in suc- cessful operation in Greece, treating similar wastewaters.

Vinasses are collected after alcohol distillation, cooled (in a cooling tower) from 80-90°C initial temperature to 60-70°C final and directed into a tem- porary storage tank before being fed into the bioreactors. These are made from 2 X lOOO-m3 vitri- fied concrete tanks for the first stage (B- 1) and 1 x 40- m3 carbon steel tank for the second stage (B-2). The daily feed volume lies in the range 160-200 m3, equal to 8-12 t of COD/day. Fluctuations in quantities of vinasses produced, due to different origins of raw materials or to different quantities of alcohol produced, can be easily handled by the manual pH control of the second-stage reactor (B-2). For example, when 10 kg COD/m3 day loading rate is applied, the pH value should be maintained around 1@5, while with 3 kg COD/m3 day loading rate it is 8. Thus, a daily COD loading measurement or prediction is necessary for process controlling purposes. For protection from hydraulic or mechanical failures, appropriate con-

trollers have been installed, such as safety valves for tanks and lines, CO* for biogas depressures, etc.

The first stage (biomethanogenesis) is an upflow sludge bed bioreactor (B-l) that operates under the following (optimum) conditions: pH 7 f O-3, temperature 51 f 3”C, volumetric load around 7 kg COD/ mzzactor day, alkalinity (due to HCO; ) lo-20 g/liter, hydraulic retention time about 11 days, solids retention time 22 days or more. The influent/feed of this reactor is continuous, but the exit of the effluent is discontinu- ous. When biogas production is low (start-up period, failure problems, etc.), the contents are agitated by a liquor recycling loop of the upper liquid level to the bottom (20 m3/h), while in normal operation the produced biogas provides the appropriate agitation energy and the liquid recirculation is used only from time to time for temperature measurements or sampling purposes. The height of the sludge bed must be more than 2.3 m and up to 2.8 m, while the blanket bed is from 5.9 to 6.8 m; at the same time the suspended solids concentrations are about 12% and 2*8%, respectively. The heating of the B-l content is achieved by the 70°C hot vinasse, which is fed to the bottom of the sludge bed.

The ‘second stage (B-2) is a batch-operated bioreactor-flocculator-precipitator with the following operation steps per cycle: (1) feeding of B-2 from the upper content of B-l, (2) mixing of the B-2 content with lime up to appropriate pH value (between 8 and 1@5), (3) coagulation-flocculation-precipitation of the mixture for 2.5 h, (4) directing the supematant for subsequent treatment to the central biological (aerobic) treatment plant of the industrial zone, (5) recirculating the biological/chemical settled sludge back to the B-l reactor. Using this operation, the following advantages can be observed:

(1)

(2)

Quantitative sludge precipitation because of lime addition/action. Adsorption of certain amounts of CO* from biogas because of high pH value and so enrich- ment of biogas in methane content and of liquid phase in bicarbonates.


(3) Possibility of easy subsequent ammonia-strip- ping/removal, because of existing high pH values and high temperatures.

(4) Entrapping of phosphates, because of high pH value and Ca 2 * presence.

(5) Fast bacterial growth and spore generation because of high pH stress. In these conditions, biosynthesis is much more favorable than bio- reaction for energy production (Vlissidis, 1987). It is important to mention also that in this case higher hydrocarbons than methane were observed in the biogas. The fast cell synthesis happens for a short time of no more than 3 h; after this time the death and hydrolysis of bacteria cells are the major phenomena occur- ring.

(6) Precipitation of sulfates as Ca804.

The good collaboration between the two stages increases the bicarbonates up to 60 g/liter, mainly because of CO2 absorption from the gas phase. For this reason the process performs well even under high volatile acids concentrations (up to 16 g/liter) or high COD loading rates (up to 12 m3/m 3 day). Also because of the high bicarbonate concentrations, methane fermentation overcomes the inhibition caused by the presence of high sulfate concentrations (up to 4 g/liter) (Thauer, 1977) and because of the high Ca 2+ concentrations ( > 1 g/liter), it overcomes the toxic inhibition caused by high NH~" concentrations (up to 5 g/liter)(Sprott, 1987). On the other hand, the presence of Ca 2. improves the dewatering properties of the sludge produced, so the sludge removed from the bottom of the B-1 reactor could have increased solid concentrations (up to 18%). The observed sludge production is low (about 0"12 kg/kg CODr) due to the long solids retention time.

The biogas produced is directed into a steam generator in this factory. Alternatively it could be used to drive an electric generator or a security torch. The daily excess sludge (1-2 m3/day), the characteristics of which are presented in Table 2, is directed to a nearby sludge dewatering bed.

RESULTS AND DISCUSSION

In general, three separate operation stages could be anticipated in an anaerobic digestion process: start-up, routine operation, reinitialization. In the present paper, results for the intermediate stage are presented, i.e. for a 7-month period of continuous operation covering winter and summer months.

pH value Although the principal parameter governing the stabi- lity of the digestion process is pH value, this is not considered as a sensitive indicator, since when the pH falls, bicarbonate alkalinity has already declined. In Fig. 3 pH values are presented against time for the whole period.

Table 2. Characteristics ofbinmus contained in the first- stage bioreactor

Component Com o~sition (%)

Inorganics 34"2 a Carbon 48"7 b Nitrogen 10"7 b Oxygen 22"8 b Sulfur 2.1 b Phosphorus 2.1 b Remainder 8.3 b

°Percentage of total solids. ~Percentage of organic solids.

p H

£3

, 6

6.6 ~ ~

40

Fig. 3.

[3

I I I I 80 '120 180 200

Da~ pH measurements against time.

The optimal pH range for methane production lies in the range 7.0-7.5 and digester pH should, if possible, be held within these limits. Below pH 6"0 and above pH 8"0 methane production declines rapidly. Only in 40% of the total measured values did pH fall into the optimal range. This indicates that when pH was lower than 7 (20% of the total values), feed organic loading was in excess of the desired levels, producing more volatile acids whilst the alkalinity present was not enough to buffer the system; at the same time biogas production was noticed to decrease. The pH control was accomplished with lime addition in the second- stage reactor and controlled manually according to the observed pH values.

In almost 40% of the measured values pH were higher than 7.5, thus indicating that an excess of lime was used for alkalinity control, while in about 80% of all cases pH was between 7.0-8.0.

Temperature Temperature is one of the principal factors affecting the metabolic activity of microorganisms and hence the permissible load rate. In Fig. 4 the measured temperature values are presented against time. Thermophilic temperatures often bring about more efficient digestion but in this case, the effective control of temperature seems to be the greater problem.

The temperature is controlled automatically, so when it was lowered during the winter months, addi-

136 A. Vlissidis, A. L Zouboulis

Temperature, (grad O) SO

61"

40

44 0

Y'7" I I I t

40 80 120 160 200 Days

Fig. 4. Temperature measurements against time.

ooo, (g~) 8o,

0 1 o : 0

Fig. 5.

,to so Days

120 1(I0 200

Influent and effluent COD (g/liter) (COD i and COD e respectively) against time.

tional heating was provided to the first-stage digester through a closed steam loop. The optimum temperature conditions for an anaerobic thermophilic range (51 ± 3°C) can be found in almost 60% of the measured values, while for 35% of the cases the measured temperatures were higher and only in 5% were lower. The higher temperatures were noticed during the summer months, when ambient temperatures around 40°C are quite common in Greece.

lnfluent-eflluent COD, influent loading rate and Re% COD In Fig. 5 the influent and effluent COD values (g/liter) and in Fig. 6 the influent loading rate (kg/m a day) and the Re% COD are presented against time. It can be seen that the influent load was highly variable, ranging for short periods of a few days from zero, due to main- tenance or other reasons, to more than 12000 kg COD/day. Typically, around 80% of the total influent COD was soluble. It was estimated that in most of the cases (90%) the influent load was between 7200 and 12000 kg COD/day, while in only 30% of the measurements the effluent COD was found to be below 20 g/liter. However, the majority (60%) of the calculated COD removals were above 60% and in some cases (20%) even more than 80% COD removal was found.

Since alcohol is sometimes produced seasonally, another important aspect studied here was the effect of shock loading and starvation on digester operation. The effect of shock loading and reloading after a short starvation period showed that even a 100% daily increase in loading rate did not greatly affect the sta- bility of the digester. The sudden higher loading rates produced a slight increase in the volatile acids content, but the digester continued to operate effectively. Starvation periods up to 10 days were accommodated without problems; loading shocks at a rate of 2 kg/m a day after a short starvation period did not have detri- mental effects on the operation of the digester, while the volatile acids content stabilized after a short period at a concentration similar to that before starvation was initiated. Longer starvation periods (in one case almost 2 months), resulted in a sharp rise in the volatile acids

1 0 0

SO

SO

4 0

2 0

0 0

v

)

d 0

I

40

0 I I

80 1"J0 Days

( t )

0

t I

'ms

L o a o i ~ ~ (~1 COO/inS.a)

e~

4

2 D

0 0

Fig. 6.

Co)

w o c

2 :i

. i - i / ,

Days

• l

(a) Re% COD and (b) influent loading rate (kg/ m~e,cto , day) against time.

content; in this case much more time was needed (about 2 months) for the system to settle and resume normal operations.

Total volatile acids OvA) and bicarbonate alkalinity (ALK) concentrations Although there is still much controversy as to the permissible volatile acid concentrations ([VA]) in an anaerobic digester, it can be assumed that a steady increase in [VA] will be an indication of possible failure of the process as a result of unbalanced conditions. A typical composition of the volatile acids in the digester content was: acetate 78%, propionate 6%, butyrate 12% and higher acids 4%.

In Fig. 7 the VA and alkalinity concentrations are presented against time. These data show an increase in


Concqmtrltton, (g/I)

S0"

4 0

2 0

0

--O

4 0 8 0 q ~ 0 1 6 0 2 0 O

Days

Fig. 7. Total volatile acids (VA, g/liter) and bicarbonate alkalinity (ALK, g/liter) concentrations against time.

RQ Index:VAIALK 0.8

0.S

0 . 4

0 . 2

rl

C) C31 0

* i t i i

40 80 120 160 200 Daya

Fig. 8. R0 index (:VA/ALK) against time.

the likelihood of digester failure when the unionized volatile acids reached concentrations above 10 g/liter (as CH3COOH). They also indicate that under the optimum conditions the volatile acids concentrations should be kept below 5 g/liter and alkalinity concentrations below 20 g/liter. Only in 10% of the cases was the first condition achieved, with the majority of the values (60%) having more than 10 g/liter volatile acids.

Alkali addition for pH control and for substrate buffer potential is particularly important for systems with a natural alkalinity below 1 g/liter as CaCO 3. In the present case this was accomplished at the same time as the pH control by adding lime to the second- stage reactor. It was noticed that after lowering the lime addition, the effluent pH decreased to 4.5 within a few days; this was accompanied by a rapid reduction in biogas production. Only in 20% of the measurements was alkalinity below 20 g/liter, while in most cases (60%) it was above 25 g/liter.

The need for a balance between alkalinity and volatile acids for normal digestion implies that variations in pH occur only after the volatile acids/alkalinity balance has been destroyed or seriously affected. When the ratio RQ :VA]ALK is higher than 0"8, the concentration of volatile acids is too high to be equalized by the alkalinity present and unbalanced conditions in the digester usually develop. In the examined process, as can be seen in Fig. 8, this condition was never noticed, i.e. RQ was always lower than 0.8. Half of the cases were even below 0"4, indicating use of excess lime.

Lime addition (Fig. 9), in the form of a Ca(OH)2 slurry of 30% solids content, was very variable (0"3-10 t/day), but this does not seem to have any correlation with the measured alkalinities, because of the high solids retention time of the system; in half of the cases lime addition was kept below 3 t/day.

Biogas The mean composition of the produced biogas was: CH4 76%, CO2 22%, H 2 1% and H2S 0"08%. In Fig. 10 results are presented for biogas production against time. As can be seen, it was quite variable, even from day to day, but in most cases (60%) the biogas produc-

Lime I i~ l l t ion ( tn ld)

10 D n

8

. I I ' -

0 '

0 4 0 8 0 1 2 0 ' l e 0 2 0 0

Days

Fig. 9. Lime addition (t/day) against time.

glO(pMI prodgotk)fl,(k M i d ) o

8

i D i

0 ~

0 4 0

g

o

8O 120 160 200 Day~

Fig. 10. Biogas production (m3/day) against time.

tion was above 2500 m3/day. The biogas produced was used in the factory for steam production. The corre- sponding oil-saving (average value 877 kg/day) follows a similar pattern.

A summary of the above mentioned observations is presented in Table 3.

Relationships between the different parameters of the anaerobic digestion system Previous results have indicated that no single factor (parameter) is sufficiently sensitive to permit reliable forecasting of incipient overloading or digester failure, particularly at high load rates. Also, no single factor


.$

r ~

~4 "-z I [..

.o

.=

v

¢,

r~

t ' 4

¢',1

e ~

6-

O'3

-i

O~ t/% 6-

¢¢3

O

tt'3

o< O~ O~ O~

d=

.2=

can be used as a control measure of the process of anaerobic digestion. None of the parameters presented above was sufficient alone to fully describe anaerobic digestion. Thus effective monitoring requires a combi- nation of physical, chemical and biological indices, notably interrelationships such as that between loading rate and biomass retention time (Thiel et al., 1968).

In Table 4, a first related attempt is made and the correlation coefficients for linear regression analysis between the examined parameters are presented.

Some general remarks can be made as follows:

(1) There is a small correlation between date and temperature. This is easily explained by climatic conditions.

(2) The highest, though still quite small, correlation between pH values and the other examined parameters is observed between pH and Re% COD, which explains the dependency of the process efficiency upon the pH value.

(3) Produced biogas shows a higher correlation with the influent COD load than with the Re% COD.

(4) Effluent COD is very well correlated with volatile acid concentrations of the bioreactor, and to a less extent with Re% COD and alkalinity.

(5) Re% COD is correlated to a great extent with volatile acids concentrations in the bioreactor. This is easily explained by the previous remark.

(6) There is a small correlation between volatile acid concentrations and bicarbonate alkalinity of the bioreactor.

(7) RQ index is well correlated with volatile acids concentration and, to a smaller extent, with effluent COD and Re% COD.

(8) Lime addition is not well correlated with any of the other examined parameters.

The correlations found between different characteristics of the ecosystem indicated that these were largely interdependent, i.e. a change in one of these would either cause changes in others, or perhaps be itself a direct or indirect result of a change in another characteristic.

CONCLUSIONS

Selected parameters of the anaerobically treated effluent (overflow from the second-stage reactor) are presented in Table 5 (average values). The efficiency of the process can be deduced through a comparison between some selected parameters in Table 5 in terms of percentage removal.

Comparison of the previous results with those reported in the literature (Bories et al., 1988) shows that the performances of the applied system are among the best for biogas production and loading rate reported so far. Unlike other anaerobic digestion processes, the described method can treat undiluted distillery waste. With regard to pollution control, the results confirm that COD breakdown is higher than 60%, which is the range usually obtained with anae-

Digestion of alcohol distillery wastes

Table 4. Correlation coefficients for linear regression analysis between the parameters examined

139

pH T * L O A D i COD, Re% COD VA ALK GAS RQ Lime

0.014 0-276 0.164 0.120 0.009 0.043 0-057 0-002 0-002 0.247 0.000 0.022 0.158 0.234 0.158 0.009 0.031 0.162 0.054

0.230 0.078 0.002 0.025 0.141 0.141 0-028 0-217 0.093 0-042 0.052 0.038 0-220 0 " 0 0 5 0-133

0.580 0.900 0.376 0.036 0.329 0-199 0"582 0.145 0.205 0-355 0-044

0.265 0-049 0.502 0.151 0.006 0.035 0-100

0"108 0.001 0.028

Date pH T LOADi COD~ ReCOD VA ALK GAS RQ Lime

aFor symbols see text.

Table 5. Average characteristics of the anaerobically treated effluent and percentage removal of some pollutants during the proposed thermophHic process

Parameter Value Removal (%)

pH 9.2 ± 0.3 S u s p e n d e d solids 450 ± 110 mg/liter Total solids 10 380 :t: 1 250 rag/liter BOD 4 200 + 580 rag/liter COD 19 000 ± 2 310 rag/liter Sulfates 820 :t: 130 rag/liter Phosphates 0 Temperature < 40"C Total-N 1 830 ± 150 rag/liter Chlorides 1 850 + 130 rag/liter Hydrogen sulphide 2 + 0.5 mg/liter Toxic metals (As, Cd, Mn, Hg, Ni, Pb, Cu, Zn) 0 Oils 0 Hydrocarbons 0

88+3 86 +4 71+5

65:1:9

84+2

robic treatment processes. Under optimum conditions the thermophilic digested waste can have a final COD below 10 g/liter.

It is also concluded that the low BOD/COD ratio of the treated effluent underlines the poorly biodegra- dable nature of the residual COD. Therefore, methane fermentation leads to an advanced stabilization of the distillery slops by an optional removal of the biode- gradable organic matter.

The thermophilic anaerobic digestion described here has been shown to be an efficient and versatile method for reducing significantly the organic load from an alcohol distillery wastewater, as well as a fractional source of the energy required for distillation. The most significant features demonstrated so far are:

(1) It can be used for high-organic-load effluents (up to 50 g/liter BOD5) with high organic loading rates (up to 7 kg COD/m 3 of bioreactor) and high sulfate concentrations (up to 4 g]liter), without either pretreatment or water dilution.

(2) Bioreactor volume is small because of the low hydraulic retention time, so the biogas production per cubic meter of bioreactor, as well as the volumetric load rate, are higher.

(3) It is a flexible, i.e. quite stable in load and temperature variations, and highly buffered system, i.e. wastewater even with pH value 4, can be fed without prior neutralization.

(4) Although comparably short residence times have been applied, satisfactory biogas yield and productivity could be obtained. The methane concentration of the biogas is high (76%), thus making it a valuable fuel, while H2S appeared only in traces. The methane produced can be burned in boilers to producesteam, drive a gas turbine to produce electricity, or fuel an internal combustion engine.

(5) Energy consumption for the bioreactor mixing is relatively low (in the range of 0-0128 kW/m 3 when applied) because of the small bioreactor volume and the high biogas production.

(6) The NH 3 contained in the treated effluent can be removed easily with a subsequent air-strip- ping step, because of the relatively high final pH value and temperature.

(7) The net production of biological sludge is smaller than in other biological methods (about 0.08 kg]kg CODr), making disposal easier.

(8) The net production of energy is positive, i.e. the system produces more energy than it consumes.


(9) Finally, as our research is continued with laboratory and pilot-scale experiments using other substrates, the proposed method is found to be applicable to many wastewater streams containing organic material, e.g. food, beverage, brewery and other related industries.

Nevertheless, it is concluded once more that anaerobic treatment alone cannot be a unique solution to the vinasse treatment problem. It is essential that further treatment of anaerobically treated distillery effluents should be considered.

ACKNOWLEDGEMENT

This study was supported in part by the EEC Project no. EE/152/82.

REFERENCES

APHA, AWWA, WPCF (1971). Standard Methods for the Examination of Water and Wastewater (13th edn). Washington, D.C.

Atanasoff, K. & Haberl, R. (1985). Beitrag zur anaerob- aeroben reinigung yon organisch hochbelastertem indus- trieabwasser am Beispiel der Melasseschlempen. Oesterreichische Wasserwirt., 37, 99-110.

Athanasopoulos, N. (1987). Anaerobic treatment of beet molasses alcoholic fermentation wastewater in a downfiow filter. Biol. Wastes, 15, 147-50.

Basu, A. K. (1975). Characteristics of distillery wastewaters. J. Water Pollut. Contr. Fedn, 47, 2184-90.

Bories, A., Raynal, J. & Bazile, E (1988). Anaerobic digestion of high-strength distillery wastewater (cane molasses stillage) in a fixed-fdm reactor. Biol. Wastes, 23, 251-67.

Bowker, P. G. (1983). New wastewater treatment for industrial applications. Environ. Progr., 2,235-42.

Braun, R. & Huss, S. (1982). Anaerobic filter treatment of molasses distillery slops. Water Res., 16, 1167-71.

Carozzi, A. & Pfeiffer, W. (1988). Untersuchungen zur anaeroben behandlung von Brennerischlempen mit hohem Anteil ungelosster Inhaltstoffe. Wasser-Abwasser, 129, 340-4.

Chiesa, S. C. & Manning, J. E Jr (1986, 1987). Fermentation industry. J. Water Pollut. Contr. Fedn, 59, 554-6; 58, 445-8.

Fannin, K. E, Conrad, J. R., Srivastava, V. J., Chynoweth, D. P. & Jerger, D..E. (1986, 1987). Anaerobic processes. J. Water Pollut. Contr. Fedn, 55,504-10; 59,403-10.

Farmer, J. K., Friedman, A. A. & Hazen, W. C. (1989). Anaerobic treatment of winery wastewater. In Proceedings of the 43rd Purdue Industrial Waste Conference. Lewis Publications, Michigan, USA, pp. 525-34.

Gieseke, U. E, Aivasidis, A. & Wandrey, C. (1988). Vergleich des ein- und zweistufigen anaeroben Abbaus von Bren- nerreischlempen. Biotech-Forum, 5, 76-86.

Goodwin, S. & Hickey, R. E (1988). Anaerobic processes. J. Water Pollut. Contr. Fedn, 60, 831-7.

Hickey, R. E & Goodwin, S. (1989). Anaerobic processes. J. Water Pollut. Contr. Fedn, 61,814-21.

Lanting, J., Jordan, J. A. Schone, M. T. & Kimey, B. L. (1989). Thermophilic anaerobic digestion of coffee wastewater. In Proceedings of the 43rd Purdue Industrial Waste

Conference, Lewis Publications, Michigan, USA, pp. 513-24.

Lettinga, G., Geest, A. v.d., Hobma, S. & Lama, J. v.d. (1979). Anaerobic treatment of methanolic wastes. Water Res., 13, 725-37.

Lettinga, G., Zeeuw, W. & Ouborg, E. (1981). Anaerobic treatment of wastes containing methanol and higher alco- hols. WaterRes., 15, 171-82.

Manning, J. E Jr. & Chiesa, S. C. (1988). Fermentation industry. J. WaterPollut. Contr. Fedn, 60, 872-4.

Maree, J. P. & Strydom, W. E (1987). Biological sulfate removal from industrial effluent in an upflow packed bed reactor. WaterRes., 21,141-8.

Montanari, T. & Basilico, C. (1989). Produzione di biogas da borlande di distillerie. Inquinamento, no. 5, 96-100.

Pearson, H. A., Weinberg, C. J. & Whitsell, W. W. (1953). Thermophilic digestion of monosodium-glutamate end liquor. In Proceedings of the 8th Purdue Industrial Waste Conference, Purdue University, USA, pp. 275-83.

Rao, B. S. (1972). A low cost waste treatment method for the disposal of distillery waste (spent wash). Water Res., 6, 1275-82.

Roth, L. A. & Lentz, C. P. (1977). Anaerobic digestion of rum stiUage. Can. Inst. Food Sci. Technol. J., 10, 105-8.

Shea, T. G., Ramos, E., Rodriguez, J. & Dorion, G. H. (1974). Investigation of rum distillery slops treatment by anaerobic contact process. In Proceedings of the 5th National Symposium on Food Processing Wastes, Monterey, USA, pp. 155-98.

Sheehan, G. T. & Greenfield, P. E (1980). Utilization, treatment and disposal of distillery wastewater. Water Res., 14, 257-77.

Shrihari, S. & Tare, V. (1989). Anaerobic-aerobic treatment of distillery wastes. Water, Air and Soil Pollut., 43, 95-108.

Sprott, G. D. (1987). Structure and function of the methano- gen cell surface. In Anaerobic Digestion: Results of Research and Demonstration Projects, eds M. P. Ferrant, G. L. Ferrero & P. L'Hermitte. Elsevier, Amsterdam, pp. 328-37.

Szendrey, L. M. (1984). Anaerobic treatment of fermentation wastewaters. Environ. Progr., 3,222-8.

Thauer, R. K. (1977). Energy conservation in chemotrophic anaerobic bacteria. BacterioL Rev., 100-80.

Thiel, P. G., Toerien, D. E, Hattingh, W. H. J. & Siebert, M. L. (1968). Interrelations between biological and chemical characteristics in anaerobic digestion. Water Res., 2, 393-408.

Vlissidis, A. G. (1986). Anaerobic digestion of liquid wastes of spirit factories. In Anaerobic Digestion: Results of Research and Demonstration Projects, eds M. P. Ferrant, G. L. Ferrero & P. L'Hermitte. Elsevier, Amsterdam, pp. 261-6.

Vlissidis, A. G. (1987). An optimised model of anaerobic digestion in thermophilic region. PhD thesis, Chemical Engineering Dept, N.T.U. Athens (in Greek).

Vlissidis, A. G., Zouboulis, A. I. & Hatzifotiou, A. (1988). Economical evaluation of anaerobic digestion as an energy production p rocess - The case study of vinassa treatmem. In Proceedings of the 3rd National Conference on Alternative Ways for Producing Energy. I.S.T., Thessa- ioniki, Greece, pp. 941-8 (in Greek).

Wang, B. Z., Sui, J., Liu, IL E, Yang, G. & Qi, P. S. (1986). Anaerobic reactors treating beet sugar effluents. Effluent Water TreatmentJ., no. 5, 150-62.

Wohlt, J. E., Frobish, R. A., Davis, C. L., Bryant, M. P. & Mackie, R. 1. (1990). Thermophilic methane production from dairy cattle waste. Biol. Wastes, 32, 193-207.

thermophilic anaerobic digestion of alcohol distillery wastewaters

Documents