Resources, Conservation and Recycling, 1 (1938) 145-151 Elsevier Science Publishers B.V./Pergamon Press plc - - Printed in The Netherlands

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Short Communication

A Comparative Study of Anaerobic Digestion Systems to Treat Piggery Waste

J. MATA-ALVAREZ and P. LLABRI~S

Department of Chemical Engineering, University of Barcelona, E-08028 Barcelona (Spain) (Received December 7, 1987; accepted in revised form February 6, 1988)

INTRODUCTION

There is an increasing interest in using anaerobic fermentation processes for stabilizing high and medium strength effluents such as livestock residues. Anaerobic treatment is generally described as a three-step process. In the first, complex organic compounds are converted to soluble compounds by enzymatic hydrolysis. In the second step, these products are converted to simple organic, mainly volatile fatty acids (VFA), yielding ultimately a mixture of methane and carbon dioxide cal]ed "biogas". Thus, the anaerobic digestion of animal wastes offers, together with its energy potential for heating and production of energy, an important reduction of the pollution load on the environment, the removal of odor problems and a digested product, which can be used as a fertilizer.

Although advantages of anaerobic fermentation are well recognized, the eco- nomics for the installation of a digestion system on a modern livestock farm in temperate climates are critical. Methane preduction from the fermentation of farm wastes would be more attractive if the high initial capital investment cost, and the level of operating maintenance, could be reduced.

The work presented here concentrates on the use of a high rate Down-flow Stationary Fixed Film (DSFF) digester to treat piggery waste. This reactor was developed in Canada [1,2] and the system could contribute significantly to improved economics of farm scale systems.

MATERIALS AND METHODS

Digesters design and operation

Three DSFF digesters have been used; (reactors A and B) at laboratory scale and a third (reactor C) at bench-scale. The digesters were packed with a film support consisting of straight vertical channels (see a schematic of a digester cross section in Fig. 1 ). This support was made of potter's clay. Table I sum° marizes the main characteristics of the digesters. Details of the design and operation of DSFF reactors have been described in [ 1,2 ].

0921-3449/88/$03.50 © 1988 Elsevier Science Publishers B.V./Pergamon Pre~ plc

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H)

@ (B)

(G)

Fig. l. Experimental device used in the anaerobic digestion of piggery waste: (A) down-flow sta- tionary fixed film digester; (B) digester cross-section; (C) substrate reservoir; (D) peristaltic pump; (E) effluent outlet; (F) recirculation, centritugal pump; (G) heat exchanger; (H) biogas outlet.

TABLE 1

Characteristics of the Down-flow Stationary Fixed Film digesters used in the study of the anaer- obic digestion of pig manure

Characteristic Reactors A and B Reactor C

Initial void volume (m :~ ) 0,004~ Support material Clay Support specific surface 56,4 (support area/void volume, m~/m ~) Reactor Height/Diameter (m/m) 1,~/0,009

0,135 Clay 46,2

0,9/0,5

Feed was pumped in at the top of the reactor intermittently, i.e. once every 60 minutes. Effluent was removed from the bottom through a syphon. To en- sure adequate mixing, the digester was mixed for 2 rain every 30 rain, by pump- ing the fermenter liquor from the bottom to the top of the reactor with a centrifugal pump. The bench-scale digester C was maintained at 35 ° C by means of a heat exchanger operating at the recirculation line. Digesters A and B were located ia a temperature controlled room. Gas measurement was by a test gas meter (reactor C) and with a simple displacement device for reactor A, B and D[3] ,

A fourth CSTR digester of 0.002 m ~ volume was operated as a control, with procedures similar to those described above. The reactors were inoculated with the effluent from a full scale CSTR digester treating pig manure. The start-uo

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procedure takes about 90 days. Start-up of digester A was accelerated by using aqueous methanol, in which methanogenic bacteria grow faster. Reactors were operated for at least five hydraulic retention times before the data presented here were taken.

Substrate

Manure from a farm near Barcelona was used as substrate in the experi- ments. The piggery waste consisted of urine and fecal material from growing and finishing pigs fed a ration free from antibiotics. To minimize pumping and line-blockage problems, the waste was separated using a screen (1.5 mm width of opening). The separated liquid fraction was diluted further with tap water in order to have more flexibility in controlling the loading rate while operating the digesters. A manure-water dilution ratio 1:3 was used. The composition of the filtered waste was determined (Table 2) and then the waste was stored at - 4 ° C until used.

Analytical methods

The basic process parameter, biogas production, was monitored daily. Meth- ane contents and effluent VFA were determined once a week by gas-chromat. ographic methods [4 ]. Total solids (TS), volatile solids (VS) and total and soluble oxygen demand (TCOD and SCOD) of the feed and effluent were also monitored once a week, according to standard methods [5 ]. Biogas flow-rate was corrected to standard temperature and pressure.

RESULTS AND DISCUSSION

Five hydraulic retention times (HRT) were tested in reactors A and B: from 7.5 to 1.5 days with a step of 1.5 days. Bench-~cale reactor C was operated at

TABLE 2

Composition of the diluted (1: 3) piggery waste used in digesters

Substance g/L

Total solids 15.0 Total volatile solids 10.0 Total chemical oxygen demand (TCOD, O~) 18.2 Soluble chemical oxygen demand (TCOD, O~) 6.4 Ammonia-nitrogen 800 Acetic acid 1,05 Proprionic acid 0.3 ! Butyric acid 0,05

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7.5 and 3 days, whereas the CSTR-type digester D was tested at 18, 14 and 12 days HRT. The feed composition was kept constant dining all the experimen- tation period and is presented in Table 2.

The methane production rate observed for the different reactors rreferred to the volatile solids fed (specific methane production rate, STP m s CHJkg YS added) is presented in Fig. 2 as a function of the organic loading rate. As can be seen, reactors A, B and C present similar behaviors, that is, the specific methane production decreases as the organic loading rate increases. At lower loading rates, reactor B presents lower yields as compared with reactor A, and this may be due to a inefficient start-up (7.5 days was the first HRT studied, then it was lowered step-wise until reaching 1.5 days). The operation of the pilot scale reactor C also results in a lower yield as compared with reactor A. In this case it may be due to the lower specific surface of the support. The different diameter-height ratio might also be a reason, because of a poorer mass transfer and the augmented possibility of dead support zones. However, this last factor was minimized by creating a thin (5 cm) intermedia~ zone without support. The results of reactor D are much lower as compar~ with reactors A, B and C. This is considered normal ~ a u s e of the type of digester. CSTR type digesters do not allow biomass retention and, as a result, the active biomass present to degrade a given amount of substrate, is several times smaller. As a consequence, the achieved specific COD reductions must be well under those achieved by an attached biomass digester, like DSFF reactors. However, the same trend is observed, that is, less biogas is produced per unit of biode- gradable substrate, as the organic loading rate increases.

Figure 3 presents the volumetric yields expressed as STP m '~ CH4/m ~ diges-

0,!

0 q f~

Fig, 2, P ~ r y waste anaerobic digestion results: specific methane production rate, as a function of the organic loading rate: (V) Digester A; ( ~ ) Digester B; (Q) Digester C; (V) Digester D.

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1.6

lm

_.R ,.,= 1 .2-

¢~

~.'~r 0,8-

~ 0.~

J

ORGANIC LO^I~ING R^t~ ~I¢~ VS/.~,t~)

Fig. 3. Piggery waste anaerobic digestion results: volumetric methane production rate a~ a function of the organic loading rate: (V) Digester A; ([]) Digester B; (®) Digester C; (V) Digester D.

ter/day, for the studied reactors. In this case, the yield increases as the loading rate does. This is because as more the substrate enters the digester, more biogas is produced. Of course, the substrate leaving the digester has a higher organic content as the HRT decreases, but in that range, the net result is an increase in the production of biogas. On the contrary, the inverse effect is observed in reactor D. Again, the reactor configuration dictates this different behaviour: the higher the HRT, the lower the biomass concentration in the reactor (bac- teria leaves the reactor together with the effluent). As a consequence, even with more substrate having entered the reactor, the effects of a lower contact time, and that explained by a minor bacteria content, results in a lower volu- metric yield when the organic loading rate increases.

The volatile fatty acid (VFA) level was always below 600 ppm in all the experimented HRT's even when working at the most severe conditions, i.e. at the lower HRT's. Because of the observed low specific degradation yields at these high loading rates (Fig. 2 ), the results lead to a consideration other than that the methanization step is limiting.

To determine more precisely the limiting step in the digestion of the sub- strate, the completeness of each of the three steps, liquefaction, acid formation and methanization, was calculated according to van Velsen [6 ]. Taking the total manure COD as a basis, the conversion to soluble material, VFA and CH4 was evaluated. The results for digester A are represented in Fig. 4 as a function of the HRT. As can be seen, the three steps were similarly affected by the organic loading rate. Moreover, the liquefaction step appeared as th~ limiting one in the degradation process. This conclusion agrees with previous results [41.

150 i00

~ 80.

~ " 6o

N ~, ~,~0

j qF "*'* ~ " ~ w w

, j o , " 0

HYDR,~ULIC RETENTION TIME (oArs)

Fig, 4, Piggery waste anaerobic d(gestion results (reactor A): percentages of the total chemical oxygen demand converted to ,~oluble nmteria| ((D), volatile, fatty acids ( V ) and methane ( 0 ), as a function of the hydraulic retention time.

It is important to point out that the reactors could be changed from one HRT to another without a reactor upset occurring or a long adaptation period.

CONCLUSIONS

Comparison of the results of the DSFF-type digester with the CSTR-type showed that the former was capable of handling much larger loading rates and hed higher rates of methane production than the latter. Results also indicated that microbial liquefaction of organic snlids was the rate limiting step of the process in terms of conversion efficiency.

Comparison of the results of the pilot and laboratory scale DSFF showed that the scale factor is not unfavourable, taking into account the large differ- ence in diameter-height ratio and also the less important difference in specific support surface.

ACKNOWLEDGEMENT

The work reported in this paper was supported by the CAIXA d'Estalvis de Barcelona.

N OMENCLATURE

B Specific methane production (m 3 CH4/kg VS)

CSTR DSFF HRT SCOD TS TCOD VS VFA

Cont inuous stirred t ank reactor Down-flow stat ionary fixed film Hydraul ic retention t ime (days) Soluble chemical oxygen demand ( g / L ) Total solids ( g / L ) Total chemical oxygen demand ( g / L ) Volatile solids ( g / L ) Volatile fat ty acids ( rag/din 3)

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REFERENCES

I vm~ den Berg, L+ and Lentz, C.P., 1979. ComparismL between up+ and down+flow anaerobic fixed film reactors of varying surface to volume ratios for the treatment of bean blanching waste. Proe. 34th Purdue Industrial Waste Conf., Lafayette, IN, pp. 319+325.

2 van den Berg, L+, Lentz, C.P. and Armstrong, D,W,, 1980. Anaerobic waste treatment effi° ciency comparisons between fixed film reactors, contact digesters and fully mixed continuously fed digesters. Proe. 35th Purdue Industrial Waste Conf., Lafayette, IN, pp. 788+79,3.

3 Mata, J., Martinez, A. and Torres, R., 1986. A simple device to measure biogas production in laboratory scale digesters. Biotechnol. Letters, 10: 719-720.

4 Kennedy, K.J. and van den Berg, L., 1982. Anaerobic digestion of piggery waste using a sta- tionary fixed film reactor. Agricultural Wastes. 4:151-182.

5 Standard Methods, 1975. Standard Methods for the Examination of Water and Waste+water. 14th edn., Amer. Public Health Assoc., Amvr. Water Work Assoc., Water Pollution Control. Federation.

6 van Velsen, A.F.M., 1977. Anaerobic digestion of piggery waste. I. The influence of detention time and manure concentration. Netherlands J. Agric. Sci., 25: 151-169.