Biomass 8 (1985) 185-194

Two-phase Thermophilic Anaerobic Digestion of Screened Dairy Manure

P.H. Liao and K.V. Lo

Bio-Resource Engineering Department, University of British Columbia, Vancouver, B.C. V6T lW5, Canada

(Received: 1 December, 1984)

ABSTRACT

Thermophilic anaerobic digestion (55°C} was studied using screened dairy manure as feed substrate. The results indicated that satisfactory high-rate thermophilie digestions could be obtained at short hydraulic retention times for both one- and two-phase systems. There was no marked differ- ence in performance between mesophilic and thermophilic temperatures in the acid-phase reactor. There was no indication that a two-phase system wouM be superior to one-phase thermophilic digestion o f screened daio' manure,

Key words: Anaerobic digestion, two-phase, thermophilic, screened dairy manure.

INTRODUCTION

Considerable work has been done in recent years to improve the biologi- cal process of methane production by anaerobic digestion. Two-phase digestion is one of those unconventional and innovative fermentation modes.

As the two groups of bacteria, the acid-forming and the methane- forming species, differ widely with respect to their physiology and nutritional requirements in an anaerobic digestion process, Pohland and Ghosh ~ proposed to separate the acid-forming from the methane- forming bacteria and to isolate them in two separate reactors. This

185 Biomass 0144-4565/85/$03.30-© Elsevier Applied Science Publishers Ltd, England, 1985. Printed in Great Britain

186 P. H. Liao, K. V. Lo

separation can be achieved by maintaining the hydraulic retention time (HRT) in the acid-phase reactor at a value that is greater than the spcific growth rate (SGR) for the acid-formers but less for the methane- forming bacteria, which are subsequently washed out. The effluent from the acid-forming phase is then used as the substrate for the methane reactor. Ghosh e t al. 2 suggested that for treating sewage sludge, the two-phase process was superior in performance to the conventional one-phase.

Lo e t al. 3 have shown, for dairy manure, that the performance of a conventional completely mixed digestion system can be improved by screening out a portion of the coarse, poorly biodegradable material from the diluted whole manure substrate. This resulted in a mesophilic (35°C) methane production rate which was about twice that obtained from the use of unscreened manure, based on the same volatile solids (VS) loading rate. The effects of increased maximum biogas produc- tion rates and increased reactor stability at higher loading rates with the use of screened manure were even more pronounced at 55°C (Lo e t al.,

unpublished results). The purpose of this study was to investigate the performance of two-

phase digestion at thermophilic temperature (55°C)using screened dairy manure. The specific objectives were: (1) to compare the biogas production and process stability at thermophilic temperature of the one- and two-phase digestion processes; and (2) to examine tile chemi- cal characteristics of the acid phase that related to the two-phase system.

MATERIALS AND METHODS

Feed material

Manure from the confined Holstein dairy herd of the University of British Columbia Dairy Unit was used in this study. The cows were fed a ration consisting of four parts alfalfa cube, three parts grain pellet (14% protein) and two parts beet bulbs. No antibiotics were incorpor- ated into the feed. Faeces and urine from the concrete-floored holding pens were collected weekly and diluted on site with an equal volume of tap water to give a slurry of about 7.5% total solids (TS) and 6% volatile solids (VS) content. A portion of the slurry was then passed

Two-phase thermophilic anaerobic digestion o f screened dairy manure 187

through a vibrating screen liquid-solids separator (Prater VSI-13-1H eccentro set) with a 2.0 mm screen opening (No. 10 mesh) to yield a liquid filtrate with approximately 3% VS. The filtrate was then trans- ported to the laboratory and stored at 4°C prior to use.

Anaerobic reactor systems

Two sets of the two-phase fermentation system were fabricated. In each system, the first unit (acid phase) was a 3 litre completely mixed reactor for the acidification reaction and the second unit (methanogenic phase) was a 3 litre completely mixed reactor for methanat ion reaction. The cylindrical reactors were constructed of 15.2 cm (6 in) ID acrylic plastic tubing. The reactors for the acid and methanogenic phases were identified as A1, A2 and B1, B2, respectively. The methanogenic reactors and one of two acid-phase reactors had been in use previously during one-phase digestion at 55°C. The reactors for the acid phase and the methanogenic phase are hence called acid reactors and methanogenic reactors, respectively. Biofilm development on the reactor wall was observed in these aged reactors. The surface area of the film (reactor internal wall and floor) was calculated to be 614 cm 2, giving a surface area to fluid volume ratio of 0.2 m2/m 3.

Internal mixing was achieved by recirculating biogas using a time- controlled peristaltic pump operating for 15 rain every hour. The temperature of each reactor was maintained at 55°C using a thermo- statically controlled heating pad. Feed was added through the top and effluent withdrawn from the bot tom. The product biogas displaced saline water from a calibrated collector to a reservoir. Daily gas produc- tion was recorded by noting the volumetric gas production along with the headspace pressure as indicated using a standard mercury manometer .

Reactor operation

The study was carried out from March to August, 1984. The acid reactors A1 and A2 were started using a 50/50 mixture of fresh dairy manure filtrate and effluent from operational laboratory anaerobic reactors. Daily feeding of the acid reactors was begun at a HRT of 1-5 days. The effluent from the acid phase was used as the substrate for the methanogenic phase. Once a day a fixed volume of manure was with- drawn after thorough mixing, and an equal amount of feed material was added.

188 P. H. Liao, K. V. Lo

Acid-phase reactor A1 was tested at HRTs of 1.5, 1.2 and ! .1 days. Acid-phase reactor A2 was tested at a HRT of 1.1 days. The methane- phase reactors, B1 and B2, were tested at a HRT of 2 days. This HRT was selected on the basis of results from previous studies which indi- cated that maximum gas production could be obtained at a HRT of 2 days. The high-rate operation of the two-phase process is outlined in Table 1.

Analyses

Biogas product ion rates were monitored daily and calculated as litres CH4/litre/day and litres C H J g VS added. Gas samples, collected in gas- tight glass sampling tubes, were assayed on a Fisher-Hamilton Gas Partitioner equipped with a Chromosorb W column. No correction for water vapour was made as it was assumed to be negligible.

Values for total solids (TS), volatile solids (VS), chemical oxygen demand (COD), volatile fat ty acids (VFA) and pH were determined in both feeds and effluents, once for each HRT cycle according to the procedures in Ref. 4. Total Kjeldahl nitrogen (TKN) and ammonia- nitrogen (NH3-N) were determined by using a block digester and a Technicon Auto Analyser II as described by Schumann e t al. s

All values reported represent averaged results from experimental runs in the steady-state condition.

TABLE 1 Scheme of Process for Thermophilic Anaerobic Digestion

Acid H R T Loading Methane H R T Loading reactor {days) rate reactor (days) rate

{gVS/ (gVS/ litre/day ) litre/day )

System 1

System 2

A1 A1 A1

A2 (no

biofilm)

1.5 22.0 B1 2.0 16.5 1.2 25.9 B1 2.0 15.5 1.1 28.8 B1 2.0 15.7

1.1 29.6 B2 2-0 16.2

Two-phase thermophilic anaerobic digestion of screened dairy manure 189

RESULTS AND DISCUSSION

Performance of acid-phase reactors

High methane production and good stability characteristics for thermo- philic digestion of screened dairy manure using completely mixed reactors have been previously observed. Thermophilic digestion was capable of much higher rates of methane production than was the mesophilic, and at lower HRTs (e.g. 1.5 days). Our studies of the anaerobic digestion of screened dairy manure using a two-phase process at 35°C indicated that VFA were the major products of biochemical transformations in the acid-phase reactors. The acid-phase reactor, operated at a HRT of 1-5 days, gave rise to a 80-160% increase in VFA (as acetic) concentrations over those in the influent. Based on these findings, this research was undertaken to investigate the acidification reaction under thermophilic conditions (55°C).

Chemical characteristics of the influents are shown in Table 2. The feed composition ranged from 3-1 to 3-3% of VS. Steady-state per- formance data for acid reactors are presented in Table 3. Methane production rates and yields are expressed in terms of litres CH4/litre/

TABLE 2 Chemical Characteristics of Feed Materials in Two-Phase Anaerobic Digestion

System 1 Systern 2

3.5days 3.2days 3.1days 3.1da3's

Total solids (%) 4.0 3.8 3.8 3.9 Volatile solids (%) 3.3 3.1 3.1 3.2 Chemical oxygen demand (mg/litre) 49 400 48 700 43 200 53 900 Kjeldahl nitrogen (mg/litre) 1 530 1 500 1 640 1 890 Ammonia-nitrogen (mg/litre) 225 325 420 270 pH 7.0 6.9 6.9 6.9 Volatile fatty acids (mg/litre)

Acetic 828 1 000 651 804 Propionic 240 325 200 358 Butyric 116 130 68 124 Total as acetic 1 100 1 351 861 1 178

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Two-phase thermophiIic anaerobic digestion o f screened dairy manure 191

day and litres CHJg VS added, respectively. Acid reactor A1 was initiated at a HRT of 1.5 days, then decreased to 1.2 days and subse- quently to 1.1 days. The methane production rates and yields of acid reactor A1 increased with an increase in HRT (a decrease in loading rates). The highest methane production rate and yield occurred at a HRT of 1.5 days (0.89 litres CHjl i t re /day and 0.040 litres CHJg VS added, respectively).

At 1.1 day HRT, the effluent VFA concentration of reactors A1 and A2 increased to 1498 and 2070 mg/litre, respectively. The increases in the effluent VFA concentrations were very similar for both reactors, showing an increase of 70% over influent concentrations. Reactor AI produced 0.71 litres CH4/litre/day whereas reactor A2 yielded only 0-14 litres CH4/litre/day. Reactor A1 also had a higher pH and a higher biogas methane content than did A2. This might have been due to the contribution of immobilized bacteria in the matured reactor A1, which had been in use for 2½ years. The surface area to fluid volume ratio was 0.2 m2/m 3. The results indicated that the development of the fixed bacterial film in the matured reactor increases reactor efficiency drasti- cally. Lo e t al. 6 reported that for mesophilic anaerobic digesters operated at 1.5 day HRT with equal VS loadings, the fixed bacterial film (surface area to fluid volume ratio of 0.32 m2/m 3) contributed an additional 85% to the methane production rate, and 67% to the methane yield, over the filmless condition. The effect of immobilized bacterial film in thermophilic anaerobic digestion was more pronounced as shown in this study.

The increase in VFA concentrations for both the aged and the newly constructed reactors (A1 and A2) operated at a HRT of 1.l days sug- gested that they had similar biochemical conversion rates under the thermophilic condition, whereas a higher VFA conversion in the matured reactor under mesophilic (35°C) digestion was reported by Lo and Liao (unpublished results). They reported that the increases in the VFA concentrations were 91 and 164%, respectively, for matured and newly constructed acid reactors operated at a HRT of 1.5 days.

The results obtained in this study indicated that VFA production was not the major reaction in matured reactor A1 operated at HRTs of 1.5 and 1.2 days, and that the methanogenic reaction was still signifi- cant. The production of VFA was insignificant until the reactors operated at a HRT of 1.1 days. The increases in VFA concentrations were accompanied by decreases in methane production rates.

192 P. H. Liao, K. V. Lo

Comparing the results from this study with those obtained at lower temperatures (Lo and Liao, unpublished results) it appeared that there was no marked difference in performance for mesophilic and thermo- philic temperatures in the acid-phase reactors. The VFA concentrations in the acid reactors were 80% higher than those in the feed during the mesophilic digestion of screened dairy manure.

Performance of methane reactors

The methane production data of reactors B1 and B2 are presented in Table 3. The methane production rates of reactor B1 ranged from 0.82 to 0.98 litres CH4/litre/day, while the methane yields ranged from 0.049 to 0.063 litres CHJg VS added. The methane production rate and yield of 1.03 litres CHJlitre/day and 0-058 litres CH4/g VS added were obtained at HRT of 2 days during one-phase digestion of screened dairy manure using completely mixed reactors. It appeared that higher methane production rates and yields were obtained in a conventional completely mixed reactor than in the two-phase methane reactor, B1, operated under similar conditions.

For reactor B2, the methane production rate and yield of 1-21 litres CH4/litre/day and 0.075 litres CH4/g VS added were obtained at a HRT of 2 days. The highest methane production rates and yields were obtained at a HRT of 2 days, for both one- and two-phase digestions. The methane production rate and yield were only 0.85 litres CHJ litre/day and 0.054 litres CHJg VS added, respectively, for reactor B1 operated under similar conditions. These results show that the higher the methane production in the acid-phase reactor, the lower the methane production in the methane-phase reactor, for the digestion of screened dairy manure.

The methane production data for the one-phase digestion of screened dairy manure are presented in Table 4.

Comparison of one- and two-ohase operations

From the results obtained with one- and two-phase anaerobic digestions of screened dairy manure, the performance of the two two-phase systems in terms of the methane production rates (litres CH4/litre/day) and methane yields (litres CH4/g VS added) were very similar (Tables 3 and 4). Using screened dairy manure as the feed substrate there was no

Two-phase thermophilie anaerobic digestion of screened dairy manure

TABLE 4 Methane Production of One-Phase Digestion

193

HR T Loading Methane (days) rate composition

(g VS/litre/day) (% CH4)

Methane production

litres CH4/ litres CH4/g VS litres/day added

15 2.07 65.2 0.31 0.149 10 3.11 63.6 0.39 0.125 8 3.93 59.9 0.42 0.107 6 5.70 46.6 0.53 0.092 4 9-20 59.8 0.77 0.084 3 11-53 56.7 0.92 0.080 2 17.65 58.1 1-03 0.058 1.5 24-53 51.3 0-83 0.033 1.5 22-00 47.2 0.89 0.040 1-3 25.86 47.5 0.81 0.031 1.1 29-61 48.5 0.64 0.022

indication that a two-phase system would be superior to the one-phase system under thermophilic conditions. This contrasted to the results obtained by Ghosh et al. 2 which suggested that the two-phase process was superior to the conventional one-phase when used for treating sewage sludge.

It is also interesting to note that the performance of both two-phase systems, A1 and B1, and A2 and B2 operated at combined HRTs of 3.1 days were similar, regardless of the initial state of the reactors (matured or newly constructed) employed in the first phase.

CONCLUSIONS

This research leads to the following conclusions regarding one- and two- phase anaerobic thermophilic digestion (55°C) of screened dairy manure :

(1) Satisfactory high-rate thermophilic digestion has been attained in both one- and two-phase systems.

194 P. H. Liao, K. V. Lo

(2) The two-phase thermophilic digestion process is not recommended for general application to very strong wastes with a large insoluble fraction, such as the screened dairy manure.

(3) There is no marked difference in performance for the acid-phase reactors under mesophilic and thermophilic temperatures.

REFERENCES

1. Pohland, F. G. & Ghosh, S. (1971). Environ. Lett., 1,255. 2. Ghosh, S., Conrad, J. R. & Klass, P. L. (1975). Anaerobic acidogenesis of sew-

age sludge. J. Wat. Poll. Contr. Fed., 47, 30-45. 3. Lo, K. V., Liao, P. H., Bulley, N. R. & Chieng, S. T. (1984). A comparison of

biogas production from dairy manure filtrate using conventional and fixed-film reactors. Can. Agric. Eng., 26, 73-8.

4. American Public Health Association (1975). Standard Methods for the Examina- tion o f Waste and Wastewater. 14th edn, APHA, AWWA, WPCF, Washington, D.C.

5. Schumann, G. E., Stanley, M. A. & Knudsen, D. (1973). Automated total nitrogen analysis of soil and plant samples. Proc. Soil Sei. Soe. Amer., 37,480-1.

6. Lo, K. V., Liao, P. H., Whitehead, A. J. & Bulley, N. R. (1984). Mesophilic anaerobic digestion of screened and unscreened dairy manure. Agricultural Wastes, 11,269-83.