Bioresource Technology 43 (1993) 169-174

ANAEROBIC DIGESTION OF BAKER'S YEAST FACTORY EFFLUENT USING AN ANAEROBIC FILTER A N D A HYBRID

DIGESTER

M. Van Der Merwe & T. J. Britz*

Department of Microbiology, University of the Orange Free State, Bloemfomein 9300, South Africa

(Received 16 April 1992; revised version received 18 May 1992; accepted 25 May 1992)

Abstract A high-strength effluent from a baker's yeast factory was treated using a hybrid and an anaerobic filter digester under mesophilic conditions. The effluent COD com- postion varied considerably (11000--88000 mg liter -s) and contained high sulfate concentrations. The digesters were subjected to quantitative increases in organic loading rate (OLR; 1"8-100 kg COD m -3 day-9 at a set hydraulic retention time of 3"0 days. The data showed that the anaerobic digestion process is feasible for the treatment of, and methane generation from, baker's yeast factory effluent without pretreatment. The results obtained showed that, in general, the two digester designs reacted in a similar manner to increases in OLR. A COD removal efficiency and methane yield of 67% and t~207 m 3 kg-s CODrernove d for the anaerobic filter and 65% and 0"208 m 3 kg- t COD,e,,ove a for the hybrid, respectively, could be achieved at an OLR of 8,6 kg COD m-3 day-s. The data also showed that both diges- ters could be maintained at a hydraulic retention time of M9 day with an OLR of lifO kg COD m -3 day -1. Decreasing digester efficiency was characterized by accu- mulation of iso-butyric and propionic acids.

Key words: Baker's yeast wastewater, anaerobic diges- tion, anaerobic hybrid digester, anaerobic filter digester.

INTRODUCTION

Certain industries produce wastes that are difficult to manage and treat in order to meet standard water- quality requirements. Effluents from baker's yeast fermentation have high pollution loads, with Chemical Oxygen Demand (COD) values ranging between 10000 and 80000 mg liter-1. The effluent problem is

*To whom correspondence should be addressed.

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

aggravated by the high sulfate and variable phosphate concentrations. Treatment to reduce the pollution value is essential prior to disposal. Effluents of this type are reluctantly received into communal sewers by controlling authorities and the factory is thus faced with heavy trade-effluent charges. Land disposal options generate additional problems and possible groundwater pollution. Many local authorities are now insisting that industries undertake some form of efflu- ent treatment so as to protect the environment.

Anaerobic processes are well established for the treatment of high-strength industrial wastewaters. The development of new high-rate anaerobic bioreactor designs, with increased biomass retention and tole- rance to toxic and shock loadings, has led to the treat- ment of extremely recalcitrant industrial waste streams (Stronach et al., 1987). Among the newer designs is the hybrid anaerobic process instigated by Guiot & Van Den Berg (1985) and high removal rates have been achieved (Brim et al., 1990). The anaerobic filter process, first proposed by Young & McCarthy (1962), has also drawn attention and has been used success- fully in the treatment of high-strength wastewaters (Tesch et al., 1983; Silverio et al., 1986; Hilton & Archer, 1988).

The aim of the investigation was to evaluate anaer- obic digestion as a treatment option for a high-strength, sulfate-rich baker's yeast factory effluent. During the study, the performance of two digester types, a hybrid and anaerobic filter, were compared and evaluated at different organic loading rates.

169

METHODS

Digester design Two laboratory-scale upflow anaerobic digesters (hybrid and anaerobic filter design), each with a working volume of five liters, were constructed. The hybrid design (HYB) combined a f'Lxed-film of inert porous polyethylene foam (density=0.77 kg m -3) fitted to the inside wall and an upflow sludge blanket (Fig. la). The anaerobic filter digester (AF) was filled with 77 rings of the synthetic FLOCOR-RC material

170 M. Van Der Merwe, T. J. Britz

a umt~ng ~ n t

mlorial

hod boa

Fig. 1. Laboratory-scale anaerobic hybrid (a) and anaerobic filter (b) digesters.

(Fig. lb). The rings had a high specific surface area (230 m z m -3) and high void volume (95%). Each piece of the medium was a deep ring with a corrugated wall 25 mm in diameter and 34 mm in depth. The substrate in each case was introduced continuously via a hori- zontal inlet at the bottom of each digester. Biogas exited at the top via a gas-solids separator and gas production was determined by means of a brine- displacement system. The biogas volumes were cor- rected to standard temperature and pressure (STP). An operational temperature of 35"C was used (Meyer et aL, 1985).

Digester start-up Each digester was originally seeded using a mixture of municipal sewage-sludge and municipal digester efflu- ent, yeast factory effluent and sludge from an anaerobic laboratory-scale digester that had been fed with the yeast factory effluent for 3 weeks before the main digesters were seeded. At the start, raw effluent was diluted to give a COD of 3000 mg liter- 1 and then fed to the digesters. The substrate flow rate was set at a hydraulic retention time (HRT) of 3"0 days and main- tained until stable conditions persisted. The latter was assumed when, after five volume turnovers, parameters showed a variation of less than 10%. For the rest of the study the HRT was kept constant at 3"0 days with only the organic loading rate (OLR) being changed.

Balker's yeast effluent The effluent was obtained from a local baker's yeast factory and stored at 4*(] until required. The effluent

used as digester substrate during the start-up was supplemented with 20 mg liter-~ urea, 20 nag liter =I K2HPO4, 1000 nag liter-~ acetic acid and a sterile trace element solution (Nel et al., 1985), to stimulate the growth of the microbial consortium and to prevent any nutrient limitations during start-up. The pH of the substrate was adjusted to 6.5.

Analytical methods The following parameters were analyzed according to the APHA (1985): pH, COD, alkalinity, total Kjeldahl nitrogen (TKN), total solids (TS), total volatile solids (TVS), total non-volatile solids (TNVS), ortho- phosphate phosphorus (PO4-P) and sulfate (SO4).

The total volatile fatty acids (TVFA) were deter- mined using a Hewlett Packard (Avondale, PA) gas chromatograph, equipped with a flame ionization detector and a 30 m x 0.75 mm i.d. Nukol (Supelco, Inc., Avondale, PA) capillary column. The chromato- graph was programmed at an initial temperature of 120°C, then increased at a rate of 6°C per min to a final temperature of 1850C. The detector and the inlet tem- peratures were set at 2500C and 160°C, respectively, and nitrogen used as a carrier gas at a flow rate of 5 ml rain-'.

The biogas composition was determined on a Varian 3300 gas chromatograph (Varian Ass., Walnut Creek, CA) equipped with a thermal conductivity detector and column (2-0 m x 0.3 mm i.d.) packed with Porapak Q (Waters Association, Inc., Milford, M_A), 80-100 mesh. The oven temperature was set at 55°C and hydrogen was used as carrier gas at a flow rate of 40 ml man -1"

Digestion of yeast factory effluent 171

Experimental periods The study consisted of seven experimental periods. The OLR was increased stepwise to give, on average, 5000, 12000, 18000, 20000, 25000, 28000 and 30000 mg liter-1. Once stable-state conditions were obtained for each experimental period, the next step- wise increase was implemented. Stable-state conditions were assumed when, after five volume turnovers, para- meters showed a variation of less than 10%.

RESULTS AND DISCUSSION

Substrate The compositions of two baker's yeast effluents are given in Table 1. The concentrated effluent obtained from the first separation process showed less variation in composition than the general effluent obtained from the balancing tank. Because of the variations, different batches of the concentrated effluent were standardized to obtain the OLR required for each experimental period. The pH of the substrate was initially adjusted to 6.5 to optimize the environment for maximum microbial growth. As the digester effluent pH was found to be > 7.5 and high pH values are known to influence digester efficiency, the substrate pH was then adjusted to 6.0.

Substrate utilization A summary of the substrate composition and digester efficiency is given in Tables 2 and 3. During the study, seven OLR were used with stepwise increases from 1"8 to 10.0 kg COD m- 3 day- t, at a HRT of 3"0 day.

The percentage COD removal and COD removal rates (R), plotted as a function of the OLR, is presented in Fig. 2. The best percentage COD removal was found

at the lower OLR and decreased as the OLR was increased. However, the best removal rate (R-value) for both digesters was at an OLR of 8-6 kg COD m -3 day-1. It was also clear that, when compared with the HYB, the AF showed a lower COD percentage remo- val and R-value up to the best R-value, after which the data were very similar. One major advantage of both the hybrid and anaerobic filter designs is their ability to retain a high biomass holdup, which results in an increased substrate utilization (Fig. 2) greater than that of conventional designs (Anderson et al., 1990).

For both digesters, at the start of periods 1-3, it was found that extreme variations occurred in the COD removal efficiency and other parameters, although an increase in biogas production, alkalinity, TS, VS, TNVS, as well as phosphate and sulfate removal, occurred. This could be an indication of the selection of a specific microbial community as part of the stabili- zation in the digester. Indirectly, this could also be because of the wide variation in the raw baker's yeast effluent composition (Table 1) and, thus, in the digester substrate.

During the first few days of periods 4-7, the sudden change in OLR caused a progressive decrease in COD removal and pH and an increase in TVFA concentra- tion. In time these parameters stabilized. These results, after the increase in loading rate, are in agreement with data reported by other researchers (Asinari Di San Marzano et al., 1981; Dohhnyos et al., 1985; Silverio et al., 1986). Baker's yeast effluent contains high con- centrations of sulfate as well as other organics, which may cause unfavorable digester conditions and could be the reason for the reduction in digester performance at the higher OLR (Lo et al., 1990). High sulfate con- centrations in wastewaters are especially inhibitory to

Table 1. Compositions of two effluent types obtained from the baker's yeast factory

Parameters Concentrated effluent °

Minimum Maximum

General effluent b

Minimum Maximum

pH COD (mg liter- t) TVFA (mg liter- 1) Acetic acid (mg liter- l) Propionic acid (rag liter- l) iso-Butyric acid (mg liter- ~) n-Butyric acid (mg liter- ~) iso-Valeric acid (mg liter- l) n-Valeric acid (nag liter- ~) Caproic acid (mg liter- 1) TS (mg liter- l) VS (mg liter -~) TNVS (mg liter- l) PO4-P (rag liter- i) SO 4 (mg liter-l) TKN (mg liter- 1) Alkalinity (nag liter- ~ as CaCO3)

5'20 57728

2040 77O 109

0 0 0 0 0

73510 43830 28030

8 13696

874 1488

5"74 88 560

7 698 5 566 1400 1 502

516 0

486 87

111 100 70080 44690

289 18018

1 502 5 678

4"98 11671

343 285

52 0 0 0 0 0

15310 8620 6690

18 2221

319 1 193

6"10 54 940 12 943 5 347 4408

896 1870

238 1 185

133 64 570 44 780 19790

191 9375 2 180 2 980

aData obtained from 18 different batches. bData obtained from 10 different batches.

172 M. Van Der Merwe, T. J. Britz

Table 2. Average composition of the substrate used during the different experimental periods"

Parameters Periods

1 2 3 4 5 6 7

HRT (days) 3.0 3.0 3.0 3.0 3"0 3.0 3-0 pH 6.5 6.5 6.5 6.0 6-0 6.0 6.0 COD (mg liter- 1) 5 368 11838 18 630 20 960 26 224 28 520 29 990 Loading rate (kg COD m -3 day- 1) 1.8 3.9 6.1 6-9 8.6 9.4 10-0 TVFA (rag liter -~) 543 785 1 198 5 190 6344 7270 6232 Acetic acid (mg liter -t ) 438 617 907 4746 5877 4960 5102 Propionic acid (nag liter- i) 55 96 140 115 0 1348 608 iso-Butyric acid (mg liter- 1) 0 72 151 329 467 - 339 522 TS (mg liter -~ ) 10790 18410 22430 23630 29990 36920 37350 VS (mg liter -I ) 6810 12190 14810 15800 18970 25430 23090 TNVS (mg liter -I ) 3980 6220 7620 7830 11020 11490 14260 SO4 (mg liter -I ) ND 1485" 2120 2307 2907 4550 5892 PO4-P (mg liter-1) ND 31 43 46 75 74 33 TKN (mg liter- l) ND 219 311 359 424 567 586 Alkalinity (mg liter- t as CaCO3) ND 1 971 2 490 2 825 2 671 2 782 2 790

aData are means of four repetitions. ND, not determined.

Table 3. Efficiency of the digesters at each period of the study"

Parameters Periods

1 2 3 4 5 6 7

Hybrid digester

CH4 content (%) 62 57 58 62 59 47 38 CH4 yield (m 3 kg- l COD~mov~) 0"041 0"058 0"081 0"278 0"208 0"138 0"169 COD (% removal) 84 79 77 73 65 48 42 TVFA (% removal) 40 0 13 61 45 15 0 Acetic acid (% removal) 26 12 32 74 73 18 0 Propionic acid (% removal) 100 0 0 0 0 0 0 iso-Butyric acid (% removal) 100 100 100 100 100 0 0 TS (% removal) 28 38 43 33 38 36 45 VS (% removal) 35 49 55 49 56 48 6 TNVS (% removal) 15 16 19 2 5 9 9 SO4 (% removal) ND 27 57 44 32 54 56 PO4-P (% removal) ND 32 56 41 59 31 60 TKN (% removal) ND 16 23 24 18 6 8

Anaerobic filter

c n 4 content (%) 61 55 57 65 59 45 39 c n 4 yield (m 3 kg- l COD ..... ,~) 0"028 0"031 0"085 0-331 0-207 0"110 0"114 COD (% removal) 74 61 56 69 67 55 43 TVFA (% removal) 19 0 48 53 53 0 0 Acetic acid (% removal) 35 0 66 72 75 61 0 Propionic acid (% removal) 0 0 0 0 0 0 0 iso-Butyric acid (% removal) 100 51 100 100 100 0 0 TS (% removal) 28 38 44 36 39 35 47 VS (% removal) 31 42 50 52 58 47 63 TNVS (% removal) 22 30 33 4 6 10 9 SO 4 (% removal) ND 25 57 44 38 51 58 PO4-P (% removal) ND 32 55 42 59 26 35 TKN (% removal) ND 17 22 21 17 6 8

aData are means of four repetitions. ND, not determined.

methanogens and thus to methane generation during the anaerobic digestion process (Hilton & Archer, 1988; Ueki et al., 1989). Sulfate stimulates the activity of sulfate-reducing bacteria. These bacteria are in

direct competition with the methanogens for specific substrates. Furthermore, sulfate-reducing bacteria produce hydrogen sulfide which is known to be toxic to digester populations (Gadre, 1989).

Digestion of yeast factory effluent 173

1 oo 6

9 0

80 5

,-, 70

- - 6 0 4

E 50 Oo e

~: 40 3 0 Iz: o 30

20 2

10 i I I I I 1

2 4 6 8 10 OLR (kgC0D/m3.d)

Fig. 2. The effect of the increase in organic loading rate on the percentage COD removal (o hybrid; • anaerobic filter) and the COD removal rate (R) (v hybrid; • anaerobic

filter).

18000 10000

16000

14000 6000 --~"

12000 E 6000

10000 . 0 o 8000 G

4000 < @

= 6000

4000 2000

2000

. 0 2 4 6 8 10

0LR (kgCOD/m3.d)

Fig. 3. The effect of the increase in organic loading rate on the effluent COD (o), total volatile fatty acids (TVFA •), acetic acid (HAc v ) and the propionic acid (HPc • ) of the

anaerobic hybrid digester.

Effluent COD, pH and volatile acids The effects of changes in the OLR on the digester ef- fluent COD and the TVFA, acetate (HAc) and pro- pionate (HI~) remaining in the digester effluent for both digesters, axe shown in Figs 3 and 4. It was found for both digesters that the effluent COD increased with increases in OLR. This was concomitant with similar increases in the digester effluent TVFA. The best percentage TVFA, HAc and HPc removals for both digesters were obtained during periods 4 and 5 and this corresponded to the best COD removal rate during period 5. However, in this study volatile acid removal was based only on the influent value. Increases or decreases, resulting from the balance between genesis and utilization during the biomethanation processes, were not taken into consideration. Thus up to period 5, it appeared as if the effluent COD was largely com- posed of the unutilized volatile acids.

On changing the OLR it was usually found that the pH dropped, with a subsequent increase in the iso- butyric, butyric and propionic acid concentrations in the digester effluent. Within a few days the pH increased again to above 7.5.

Propionic acid was found to be the slowest para- meter to stabilize after an increased organic loading. Furthermore, as the OLR was increased, longer time- intervals were needed to reach stable conditions. The concentrations of both acetic and propionic acid in the digester effluent (Figs 3 and 4) were also found to increase drastically from period 5 onwards, suggesting that the digesters were being overloaded.

Gas production and composition The gas production trend as well as the composition was found to be very similar for both digesters (Fig. 5). The methane percentage for the HYB digester varied between 57 and 62% (average 59.6%) and for the A E 55 and 65% (average 59.4%), during the first five peri- ods. However, for both digesters, it decresed drasti-

18000

1 6 0 0 0

14000

~ 12000 E

a 10000 o u

8000

= 6000

4000

2000'

2 4 6 8 OLR (kgCOD/m3.d)

10000

8000

I E 6000 o

Z

4000

2000

I 0 10

Fig. 4. The effect of the increase in organic loading rate on the effluent COD (o), total volatile fatty acids (TVFA •), acetic acid (HAc v ) and the propionic acid (HPc • ) of the

anaerobic filter digester.

':i i 0.3. 80 1 0.30

7O 1 0.25 i

i 60 ~ 1 0.20

so 1 o16 -:

40 1 0.10 ~

: 10.+ 2 4 6 8 10

OLR (kgCOO/m3.d)

Fig. 5. The effect of the increase in organic loading rate on the percentage methane content (o hybrid; • anaerobic filter) and the methane yield (yield as m 3 methane per kg

CODremoved) ( V hybrid; • anaerobic filter).

174 M. Van Der Merwe, T J. Britz

cally at the higher OLR applied during periods 6 and 7, decreasing to below 40% during period 7.

The best observed methane production yield was obtained during period 4 at an OLR of 6.9 kg COD m -3 day -l. The AF gave the highest value of 0.331 and the HYB 0.278 m 3 kg -1 COD~mov~. During the whole study the methane production rates were much lower than the theoretical optimum of 0"395 m 3 kg- CODremo~ d at 35"C for glucose. The highest values represent, respectively, 83% and 70% recovery of energy value from the substrate used. Values from the other periods were all lower than 52% energy recovery. However, it must be taken into consideration that the effluent used in this study was the wastewater of a microbial fermentation and no easily metabolizable carbon sources remained for further microbial utiliza- tion.

CONCLUSIONS

The data from this study demonstrate that the anaer- obic digestion process is feasible for treatment, and methane generation from, a high-strength baker's yeast effluent. The results obtained encompass the effect of increasing OLR on the efficiency of two different anaerobic digester designs and show that, in general, the two designs react in a similar manner to the increase in OLR. However, small differences were also found. It was found that the A F required a longer period to stabilize after the start-up period. In comparison, the HYB, which has the advantage of a shorter start-up period, required a shorter period to obtain a relatively high digester performance efficiency. This digester also recovered more rapidly from increases in the loading rate (data not shown).

A COD removal efficiency and methane yield of 67% and 0.207 m 3 kg-~ CODremove d for the AF and 65% and 0-208 m 3 kg-~ CODremov, d for the HYB, respectively, can be achieved at an OLR of 8"6 kg COD m-3 day-J. The data also show that both digesters can be maintained at a HRT of 3-0 days with an OLR of 10.0 kg COD m -3 day -I, although the digesters are sensitive at this OLR and failures may result from rela- tively small variations in the operational conditions.

After continuous operation for 1 year, there was no obvious sign of clogging in either of the digesters. How- ever, a post-treatment will have to be undertaken in order to reduce further the level of organic matter to conform to local standards, before final disposal. In future studies the operation of the digesters at shorter HRT will have to be examined, as these could lead to the use of smaller digester sizes which are economically more viable.

ACKNOWLEDGEMENTS

The authors gratefully acknowledge the financial support of Welkom Yeast Ltd and National Chemical

Products -- a division of Sentrachem. The authors would also like to thank Mr C. Van Eck, of Welkom Yeast, for providing ready access to the baker's yeast effluents.

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