High-rate anaerobic digestion of screened dairy manure

J. agric. Engng Res. (1985) 32, 349-358

High-rate

Anaerobic

Digestion of Screened Dairy Manure K. V. Lo; P. H.

LIAO*

Laboratory-scale fixed-film reactors were used for the high rate production of methane from screened dairy manure under mesophilic temperatures. The active biomass, growing on the support structures, enabled the reactors to withstand high organic loadings as well as very short hydraulic retention times down to 1 h. Screened dairy manure (3.25% VS) was added intermittently at loading rates ranging from 2.25 to 778 gram volatile solids per like per day (g VS l- 1 d- ‘) for fixed-film reactors. The maximum methane production rate was 6.20 like methane per litre reactor per day (6.20 1CH, l- ’ d- ‘) when operated at a loading rate of 259 g VS 1-l d- ’with a 3 h hydraulic retention time. The fixed-film reactor was capable of sustaining a loading rate of 778 g VS I- ’ d- ’ (1 h hydraulic retention time). The data illustrated the advantages of fixed-film reactors. The results obtained with reactors of varying sizes, and different fixed-film support materials are discussed.

1. Introduction In recent years, biogas production from food processing, animal and industrial organic wastes through anaerobic fermentation has received increased attention. The major obstacle to the adoption of the biogas technology has been the high capital investment that is required for the construction of large-volume, conventional (completely-mixed) anaerobic digesters’ ‘* Considerable technical improvements and cost reduction measures in anaerobic digestion are being pursued in many laboratories. The concept of biological solids recycle, which led to the introduction of the anaerobic contact or anaerobic activated sludge process, permitted a longer residence time for the active flora within the reactor and resulted in significantly higher gas volume efficiencies.” Retention of the active biomass, independently of the waste flow, reduced the hydraulic retention time (HRT) necessary for effective treatment, thereby resulting in a smaller volume reactor unit. The retention of the active biomass within the reactor has been achieved in the advanced reactors such as the upflow anaerobic sludge blanket reactor, 4 the anaerobic filter,5 the fluidizedbed reactor6 and the fixed-film reactor.’ By maintaining a high concentration of active biomass within the reactor for long periods, these new reactor designs allow the efficient digestion of soluble wastes at relatively short hydraulic retention times and also ensure that both high and low strength soluble wastes can be satisfactorily treated at mesophilic temperatures under more economically viable conditions. The search for alternative energy sources has led to the application of advanced reactor designs to biogas production from agricultural wastes. * The purpose of this study was to develop means of improving methane production from dairy cattle manure. The aim of the work presented in this paper was to determine the effects on methane production of liquid-solids separation when used in conjunction with fixed-film reactors.

2. Materials and methods 2.1. Reactor feedstock Manure from the confined Holstein dairy herd of the University of British Columbia Dairy Barn 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 incorporated 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) ‘Department of Bto-Resource Received

19 March

Engineering,

1984: accepted

Universay

of British Columbia,

in revised fom~ 24 September

Vancouver,

B.C. V6T lW5, Canada

1984.

349 0021-8634;85/080349+

10 $03,00/O

0

1985 The British Society

for Research

m Agncultural

Engmeering

350

ANAEROBIC

DIGESTION

OF DAIRY

MANURE

and 6% volatile solids (VS) content. A portion of the slurry was then passed through a vibrating screen liquid-solid separator with a 2.0 mm (No. 10 mesh) screen opening (Prater VSl-13- 1H eccentro set) to yield a liquid filtrate which was then transported to the laboratory and stored at 4°C prior to use. 2.2. Reactor design and operation All cylindrical fixed-film reactors were constructed of acrylic plastic tubing (152 mm i.d.). A fixed-film support structure built of rigid polyvinyl chloride “biopods” used commonly in sewage treatment plants for aerobic trickling filters were installed in two duplicate reactors having a working volume of 211 each. To enhance the entrapment of bacteria, the surface of the biopods was roughened by sandblasting. The bipod structure was placed about 10 cm above the reactor floor and 3 cm below the operating fluid level. The fluid in the reactor and the headspace above the liquid level were kept at 130 cm and 15 cm in height respectively. A fixed-film support structure made from 3 mm thick acrylic panels was installed in a reactor having a working volume of 4 1. Two sets of five parallel panels were slotted together at right angles to form an open-ended “honeycomb” matrix having vertical four-sided square channels each being 2.4 cm square by 17 cm deep. The structure was positioned about 6 cm above the reactor bottom and 3 cm below the operating fluid level. Fixed-film materials made from woven fibreglass cloth with waxed resin, sand-blast plexiglass, nylon cloth with thread reinforcements being 0.6 cm square, fibreglass door screen, woven fibreglass and nylon mesh were set up in a spiral configuration with 1.27 cm spacing and 27.94 cm deep. The structures were positioned about 6 cm above the reactor bottom and 3 cm below the operating fluid level. These reactors had working volume of 5 1. The configurations of the fixed-film reactors are shown in Fig. 2. The surface:colume ratios of the reactors are listed in Table 1.

(b)

(0)

(c)

Fig. 1. Cross sectional view of the$xed-film support structure: (a) poiyvinyl chloride ‘biopod”; conjiguration with variousjibreglass or nylon materials

(b) acrylic

(c) spiral

Substrate was fed in at the top, and effluent was withdrawn from the bottom. This ensured that suspended solids did not accumulate in the reactor. Mixing was provided by biogas recirculation using a peristaltic pump operated by an automatic timer for 15 min every hour. Reactors were maintained at required temperatures using a thermostatically controlled heating pad and insulation. At start-up, each reactor was fed a mixture of effluent from other laboratory scale anaerobic reactors and fresh screened dairy manure. After a one week period, daily feeding was then commenced. Prior to feeding each reactor was mixed thoroughly, then a predetermined volume of effluent was withdrawn and an equal volume of feed material added. 2.3.

Measurements

Biogas production was monitored daily and the gas was analysed for methane content at least twice during each HRT cycle. Gas samples collected in gastight sampling tubes were analysed with a Fisher-Hamilton Gas Partitioner equipped with a Chromosorb W column. The values

351

K. V. LO; P. H. LIAO TABLE 1

Film supportmaterials for the fixed-Clmreactors

Supporting

material

I Acrylic panel Biopods Woven fibreglass with resin Sand-blasted plexiglass Nylon cloth with thread reinforcements Fibreglass door screen Woven fibreglass (no resin) Nylon Mesh

I

I 139.0 99.4 112.0 119.0 110.0 110.0

110.0 110.0

4

35

21

22

5 5 5 5 5 5

35 35 35 35 35 35

presented in the tables or figures are the means of the results obtained from a minimum of four replicate runs at any given HRT. Chemical analyses were performed on all feeds immediately after preparation and on all effluents once per HRT cycle. Determination of chemical oxygen demand (COD), total solids (TS), volatile solids (VS), and pH were carried out according to standard methods.9 Total Kjeldahl nitrogen (TKN) and ammonia-nitrogen (NH,-N) were determined using a block digester and a Technicon Auto Analyser II according to the method of Schumann et al.‘O 3. Results and discussion Lo et al.“,” have shown the effects of solids separation pretreatment on biogas production from dairy manure over a range of HRTs. Screening out coarse solids prior to feeding virtually eliminates the material handling problems. Due to the reduced feed volume, a reduction in reactor volume can be achieved with no corresponding drop in gas production. The successful anaerobic digestion of screened dairy manure using a conventional reactor could be obtained at very short HRTs. The low gas production at shorter HRTs proved to result from the loss of bacterial biomass. In order to achieve high rates of methane production at shorter HRTs, the fixed-film reactor design concept was adopted in this study. Tables 2,3 and 4 illustrate the increase in methane production rate, methane yield and loading capability made possible through bacterial biomass retention. In order to demonstrate the performance of fixed-film reactors, the methane production rates of completely-mixed reactors receiving screened dairy manure operated at various temperatures are also included in Table 2. The fixed-film reactors maintained at 22°C could be operated successfully over a range of HRTs from 15 to one day. Biogas methane contents ranged from 69.3 to 62.1%. At this temperature, a maximum methane production rate of 1.OO1Cl-& 1 - 1 d-i was achieved at a two day HRT and at a loading rate of 16.8g VS 1-i d-i. The highest methane yield, 0.1041 CH,/g VS added, occurred at 10 d- ’ HRT and 3.40 g VS 1- 1 d- ’ loading rate. It is interesting to note that methane production rates of the fixed-film reactors operated at 22°C were similar to those of the conventional reactor operated at 55°C over HRTs below 10 d. The results indicate that operating a fixed-film reactor at the lower mesophilic temperature, may yield more net energy than a conventional reactor operated at 55°C. At 35°C the operational HRT of the fixed-film reactors could be reduced further to 1 h without loss of gas production. The methane production rate remained constant over eight cycles. Although this indicates the very high

ANAEROBIC

352

DIGESTION

OF DAIRY

MANURE

TABLE 2

Methane production

T

Merhaneproduction

HRT d-’

Loading rate, g VSI-‘d-l

Methane composition

22°C Conventional

16 15 12 10 10 8 6 4 3 2

2.1 1.9 2.9 2.8 2.9 4.7 6.3 7.8 10.9 15.6

63.5 64.6 63.5 64.6 62.7 53.6 48.2 45.8 49.2 53.3

0.16 0.18 0.20 0.19 0.22 0.13 0.08 0.07 0.05 0.03

0.079 0.096 0.071 0.066 0.072 0.027 0.012 0.010 OX@4 0.002

35°C Conventional

16 12 10 8 6 5 5 4 4 3 2

2.3 2.5 3.1 3.9 5.5 6.9 3.8 8.9 7.5 11.7 21.0

59.8 61.4 62.5 63.8 60.7 53.4 57.4 44.0 44.0 46.8 43.4

0.32 0.38 0.47 0.49 0.63 0.53 0.28 0.39 0.30 0.32 0.42

0.138 0.152 0.145 0.127 0.116 0.078 0.066 0.044 0.040 0.030 0.021

55°C Conventional

15.0 10.0 8.0 6.0 4.0 3.0 2.0 1.5

2.1 3.1 3.9 5.7 9.2 11.5 17.7 24.5

65.2 63.6 59.9 46.6 59.8 56.7 58.1 51.3

0.31 0.39 0.42 0.53 0.77 0.92 1.03 0.83

0.147 0.125 0.107 0.092 0.084 0.080 0.058 0.014

35°C Acrylic panel (fixed-film)

16.0 12.0 8.0 6.0 4.0 3.0 1.0 0.25 0.21 0.17 0.13 0.08 0.04

2.3 2.4 4.3 5.6 8.6 12.3 32.8 130.0 156.0 194.0 259.0 389.0 778.0

67.7 67.8 67.0 64.0 64.8 64.0 65.8 65.8 65.8 65.8 65.8 65.8 65.8

064 0.76 0.88 0.89 1.42 1.50 3.56 5.62 5.70 5.90 6.20 6.14 5.78

0.286 0.313 0.203 0.158 0.133 0.121 0,109 0.044 0.037 0.030 0.024 0.016 OX)07

22°C Biopod (fixed-film)

15 10 10 6 4 3 2 1

1.9 2.8 3.4 5.2 7.8 11.2 16.8 31.0

64.6 64.6 62.1 65.1 62.8 63.4 65.3 69.3

0.18 0.19 0.35 0.45 0.61 0.84 10l 0.93

0.096 0.066 0.104 0.087 0.078 0.075 0.059 0.030

Reactor

ICH, I-’ d-’

1 CH,/g

VS added

353

K. V. LO; P. H. LIAO TABLE3 Digestion of screened dairy manare using laboratory-scale

fixed-film reactors daring tbe steady-state

conditions Loading rate, g VSI-’ d-’

~~ Nylon cloth with thread enforcement

tCH,I-Id-’

1 CH,/g

VS

p~;i~~

No&: Feed: Screened dairy manure: VS: 3.4%; COD: 50.6 g/l (range 44.5-55.5 g/l). Temperature: 35°C. HRT: one day.

the bacteria film for producing methane, it remains to be seen whether the film the high gas production rate for long periods of time. The maximum methane production rate at this temperature was 6.20 1CH, 1-l d-’ at 3 h HRT or a loading rate of 259 g VS 1 - l d- ‘. The highest yield obtained was 0.3131 CH,/g VS added at 12 d HRT or 2.3 g VS l- 1 d - ’ loading rate. All support materials used in this study are readily available and inexpensive. They were also very effective for the development of the active biomass films. The bacterial film displaced approximately 1100 ml of fluid by the end of this experiment (14 months) reducing the working volumes of reactors to 3.9 1. During the course of this study, the working volumes of reactors were checked biweekly and the data were calculated accordingly. A comparison of the steady-state reactor performance at one day HRT for the same configuration with different support materials, showed that the films developed on the fibreglass door screen, woven fibreglass and nylon mesh produced more methane than films developed on the other support materials. At HRTs longer than one day, the differences in methane production were quite small (Table 4). The reactor with acrylic panels support was shown to be the most efficient in methane production among the reactors operated at 35°C and one day HRT. The fixed-film reactors regardless of operating temperature consistently produced more methane than the conventional reactors operated at 55°C. The average values of the chemical composition of the feed materials and effluents for each experimental run are presented in Table 5. Differences in TKN between input and output showed negligible change in all reactors, regardless of temperature. NH,-N increased through treatment, as would be expected from biodegradation of nitrogenous organic compounds in the manure-urine mixture. The pH in the fixed-film reactors remained stable within the range of 6.9-7.2 at all temperatures studied. However, the conventional reactors demonstrated a tendency toward acidification when operated at 35°C and two days HRT, and especially at 55”C, and 1.5 d HRT when alkalinity adjustment with NaOH became necessary. This may have been due to the accummulation of acetate resulting from the loss of acid-consuming, slow-growing methanogenic bacteria. The results indicate that activation of the bacterial film in the tied-film reactor requires nutrient loading rates which are higher than the loading capability of the conventional digester. It is believed that the fixed-film design is an important concept in animal waste management and biogas production. The potential for reducing biogas plant costs by decreasing the required digester volume appears significant.

capabilities of would maintain

3.1. Net energy output In order to make a direct comparison of net energy yields between reactors, it can be assumed

354

ANAEROBIC

DIGESTION

OF DAIRY

MANURE

TABLE 4

Methane production from fixed-l&n reactors with different support materials

Fixed-film reactor

Woven fibreglass

with resin

HRT, days

T

Methane production lCH,I-‘d-’

I CH,/g

VS added

1 2 3 4 8 16

1.92 1.25 1.23 0.11 0.57 0.27

0,057 0.103 0.138 0.120 0.161 0.165

1 2 3 4 8 16

1.61 1.29 1.31 0.75 0.54 0.31

0,050 0.099 0.146 0.118 1.161 0,184

Nylon cloth with thread reinforcements

1 2 3 4 8 16

1.95 1.41 1.32 0.75 0.62 0.29

0.058 0,108 0.148 0.118 0.186 0,180

Fibreglass

1 2 3 4 8 16

2.26 1.42 1.32 0.80 0.61 0.34

0.069 0.109 0.148 0.125 0.183 0.205

1 2 3 4 8 16

2.26 1.35 1.3-l 0.85 0.61 0.33

0.068 0,103 0.154 0.132 0.184 0,200

1 2 3 4 8 16

2.24 1.35 1.35 0.78 0.61 0.32

0,067 0.103 0.151 0,121 0.184 0.195

Sand-blasted

acrylic panel

door screen

Woven fibreglass

Nylon mesh

(no resin)

that the reactors are insulated such that the energy required for temperature maintenance is the same for all reactors. Then a conservative net energy output estimation can be made by subtracting the heat energy required to raise the feed slurry from a mean ambient temperature to the desired digestion temperature. The calculations of net energy ouput were based on the following equation. ‘I3 Q=CPM@,-t,),

where Q is the heat energy required, Cp is the specific heat of the slurry, A4is the TS feeding rate,

1z.z 96.1 1I.Z L6.1 0O.Z 66.1 98.Z f8.Z z1.z 60.2 60.2 1s.z 0I.Z 1z.z 1z.z ss.z 89.2 S6.Z

81.2 68.1 9I.Z 91.Z z1.z z1.z 1L.Z 99.z LE.Z VI 9.L z31 O.fZ 9.PZ VLZ LSZ Z.PZ Z.91

1.L L.6 b.01 O.ZI O.PI 8.15 f,IZ f4JI 8.fI Z.E 05 8.5 6.5 9.z s.z 9.z L.Z 9.z

E.E 8.Z 0.f 0.f L.Z s.z L,Z tr.Z 5.z Z,f Z.E f.f L.E VE FE S.E 6.2 1.f

9.E 1.f P.f vi 1.f 1.f bf 8.Z 6.2

81.0 81.0

If.0 9z.o s1.z 81.Z sz.z 6Z.Z PZ.Z fZ.Z LVZ zs.z 96.2

EP.0

sz.0 sz.0 Zf.0 ZE.0

PE.0 SE.0 zs.0 ss.0 E9.0 E9.0 16.0 89.0 6P.O 69.0 69.0 9f.O oz.0 IS.0 6Z.0 Pp.0 WO ILa.0

UI

ZL.0 LL.0 PL.0 09.0 IS.0 99.0 PI.1 91.1 ZI.1 Y

UI

Iis 'N-NXl

% ‘S/l

I"0 l/s ‘N-‘HN

z.1 f.9 f.91 I.fI 9.61 o.oz 8.LI Z.fI Z.ZI

L.9 S.L 1.6 1.6 I.01 f.91 VLI Z.fI VII

Ino

f.P 0.P 9.f 03 9.f 9.f L.E f.f 9.f

f.P L.E 0.P 0.P L.E VE 9.f EJi f.f

UI

E.P E.P f.P 947 S.P S.P S,P 8.f 1.P

9.P 0.P VP P.P 13 1.P VP 8.f L5

% XL

9.1 z.z O.LI P.61 f.91 L.61 L.61 Z.fI Z.ZI

8.L 2.6 5.6 E.6 I.01 9.11 Z,LI L.6 S.LI

Z.6P 6.8P 1.9P P.8P f.IP I.OP L.IP 8.Sf 8.6E

L.SS VOP LW 9437 P.8E 8.Lf 6.ZP E.LE VLE

UI

0.0s 0.0s s.ss 0.09 6.6P 6.6P z.zs Z.IP f.SP

s.09 SW Z.6P Z.6P L.ZP L.ZP 8.15 f.IP L.Sb

bO.0 sz.0 0.1 0.f 0.P 0.9 0.8 o.zr 0.91

I I Z f P 9 01 01 SI

P ‘A?H

I"0

IIS ‘a03

WJ-PFr Podw 30zz

&SE

w-Paxy Iauedqhv

356

ANAEROBIC

DIGESTION

OF DAIRY

MANURE

The value of Cp used in

t, is the ambient temperature and t, the desired digestion temperature. our calculations was calculated on the basis of the equation:

Cp = 4.19 - O.O0275[TS],

provided by Chen“’ for beef manure, where TS is the concentration of total solids in the feed material, expressed in kg/m3, and Cp is expressed in kJ/kg “C. For the purposes of this study, a mean ambient temperature of 9°C for Vancouver, B.C., Canada was used.ls Figure 2 compares the net energy output of the fixed-film reactor operated

L

I

I

I

I

I

I

I

0

2

4

6

0

IO

12

14

Hydraulic

retention

time,

I

16

I6

d

Fig. 2. Net energy outpul from reactors operated at different temperatures and hydraulic retention times; A-A, jixed-film reactor with ‘biopodr”; O-O, 35°C conventional reactor; O--O, 55°C conventional reactor

22°C

at 22°C with that of conventional reactors operated at 35°C and 55”C,12 based on published and unpublished data generated in our laboratory. The curves for all three temperatures show an increase in net energy output with increasing HRT, as would be expected from the decreasing daily volume of feed being preheated. At one day and two day HRT the highest net energy output is obtained at 22°C. At three day HRT the net outputs at 22°C (fixed-film) and 55°C (conventional) are almost equal, with the latter being slightly higher at this and greater HRTs. The operation of a conventional system at 35°C shows the highest net energy output at HRTs

357

K. V. LO; P. H. LIAO

between six and about 14 days, being surpassed at longer HRTs by the 55°C operation. Interestingly, the 35°C operation at HRTs of less than six days produced less net energy than either 22°C or 55°C. The net energy outputs of fixed-film reactors at 22°C (biopod) and 35°C (acrylic panel) are listed in Table 6. The net energy output of the fixed-film reactor at 35°C (acrylic panel) is much higher than that of fixed-film reactor at 22°C (biopod). The difference is too great to be explained by temperature effects alone. This may have been due to the larger surface:volume ratio of the fixed-film reactors (acrylic panel). These data also indicate that there is a kinetic advantage in fermenting screened manure at 35°C. TABLE6 Net energy output (MJ (kg TS)-’ d-l) HRT, d 16 15 12 10 8 6 4 3 1

22°C Biopod

35°C Acrylic panel 9.16

2.67 8.97 2.91 2.40 2.17 2.08 0.81

5.60 4.29 460 3.55 2.98

Chenq4 in a kinetic analysis comparing 35°C and 55°C conventional reactor digestions of beef cattle wastes, found that the net energy output at 55°C would be higher than at 35°C if the HRT was shorter than 12 days. Our findings with dairy manure show a similar pattern, wherein the critical HRT is about six days. Of particular interest in this case, is the fact that the use of a fixedfilm reactor operated at a low mesophilic temperature of 22°C could, at HRTs of less than three days, produce more net energy than conventional reactors operated at either 35°C or 55°C. Given that maximizing methane production per unit cost is dependent upon the reduction of reactor volume,16 then reactor designs capable of operating at very low retention times should be sought. The results of the present study suggest that fixed-film reactors operated at the lower mesophilic temperature could potentially be cost competitive with conventional systems operated at standard mesophilic (35°C) or thermophilic (55’C) temperatures. These findings could also have considerable implications in the economics of biogas technology in tropical developing countries where heated digesters are often inappropriate due to financial constraints. 4. Conclusions The fixed-film reactors fed with screened slurry performed better than the conventional reactors receiving the same feed material. The fixed-film reactor could be operated at HRTs as short as 1 h with high gas production whereas the conventional design could only maintain low gas output at hydraulic retention times of less than eight, six or two days at temperatures of 22°C 35°C or 55”C, respectively. A potential significant volume reduction is possible by using the fixed-film design, together with screening pretreatment of the slurry.

358

ANAEROBIC

DIGESTION

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MANURE

A comparison of net energy outputs between conventional reactors operating at 22”C, 35°C and 55”C, and a fixed-film reactor operating at 22°C showed that the latter can have a higher net energy output when operated at an HRT of less than three days. The results support the concept that a combination of macro-solids removal from the digester feed and the use of a fixed-film reactor design can contribute significantly to an increase in the gas production per unit reactor volume.

Acknowledgement The authors acknowledge Research Council of Canada.

the financial support provided by the Natural

Sciences and Engineering

REFERENCES

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Lo, K. V.; Bulley, N. R.; Liao, P. H.; Whitehead, A. J. The effect of solids-separation pretreatment on biogasproduction from dairy manure. Agric. Wastes, 1983 S(3) 155 Lo, K. V.; Liao, P. H.; Whitehead, A. J.; Bulley, N. R. Mesophilic anaerobic digestion of screened and unscreened dairy manure. Agric. Wastes, 1984 ll(4) 269 Persson, S. P. E.; Bartlett, H. D,; Branding, A. E.; Regan, R. W. Agricultural Anaerobic Digesters. Design and Operation. Bulletin 827. Pennsylvania State University, College of Agricultural Experiment Station, University Park, Pennsylvania, 1979 Chen, Y. R. Kinetic analysis of methane production from cattle wastes and its design implications. Paper No. 82-3593 presented at the 1982 Winter Meeting of the ASAE, Palmer House, Chicago, Illinois. December 12-14,1982 Ministry of Environment. Climates of British Columbia. Air Studies Branch, Ministry of Environment, Province of British Colombia, 1978 Hashimoto, A. G.; Chen, Y. R. Economic optimization of anaerobic fermenter designs. Paper presented at the Fourth International Symposium on Livestock Wastes, Amarillo, Texas, April 15-17,198O