Bioresource Technology 44 (1993) 71-76

BATCH ANAEROBIC DIGESTION OF WATER HYACINTH: EFFECTS OF PARTICLE SIZE, PLANT NITROGEN CONTENT,

A N D INOCULUM VOLUME*

K. K. Moorheada:l: & R. A. Nordstedt b

"Department of Soil Science, b Department of Agricultural Engineering, Institute of Food and Agricultural Sciences, University of Florida, Gainesville, FL 32611, USA

(Received 2 March 1992; revised version received 20 August 1992; accepted 25 August 1992)

Abstract Batch anaerobic digestion (at 35°C) of water hyacinth plant material was evaluated for differences in particle size and nitrogen content of plants, and for differences in inoculum volume. Particle sizes ofl.6, 6"4 and 12.7 mm and a nitrogen content of lO and 34 mg N kg - j dry plant material were evaluated as well as inoculum volumes of 2"5, 5 and 10 liters. Differences in cumulative biogas production were maximum at 15 days. Cumulative biogas production was highest for a plant particle size of 6.4 mm. Cumulative biogas production at 15 days increased with increasing inoculum volume for plants with a high N content but not for plants with a low N content. Total biogas and methane yields at 60 days were similar for plant material regardless of particle size, N content, or inoculum volume. Total biogas yields ranged from 0"20 to 0"28 liters g- 1 volatile solids. Mineralization of organic 15N to 15NH4-N accounted for 72% of added 15N for plant material with high N content and 35% of added JSN for plant material with low N content.

Key words." Anaerobic digestion, water hyacinth, inocu- lure volume, particle size.

INTRODUCTION

The potential productivity of water hyacinth (Eichhor- nia crassipe [Mart] solms) in nutrient-enriched waters has led to its selection as a biomass feedstock for methane generation while providing a means for waste- water treatment. Methane yields during anaerobic digestion depend on characteristics of feedstock (Wolverton & McDonald, 1981; Stack et al., 1982) as well as digester operating conditions (Hashimoto et al., 1980). Inoculum from operating anaerobic digesters is

~Present address: Environmental Studies, University of North Carolina at AsheviUe, Asheville, NC. *Approved as Florida Agricultural Experiment Stations Journal Series No. R-02345.

Bioresource Technology 0960-8524/93/S06.00 © 1993 Elsevier Science Publishers Ltd, England. Printed in Great Britain

71

commonly added as a microbial seed to initiate anaer- obic digestion in new digesters (Sievers & Brune, 1978; Field et al., t984). Information on the effect of inocu- lum volume on gas production is limited (Nordstedt & Thomas, 1985; Hashimoto, 1989).

Methane yields have been reported for water hya- cinth feedstock using a variety of digesters. Hanisak et al. (1980) found average methane yields of 0"24 liters g 1 volatile solids (VS) of shredded water hyacinths in 162-liter digesters at loading rates of 1"10-1"38 g VS liter -~ day -1 and residence times of 30-38 days. Chynoweth et al. (1983) reported methane yields of 0-19 and 0"28 liters g- ~ VS of water hyacinth and a 3 : 1 water hyacinth/primary sewage sludge blend, respect- ively, in 5-liter daily-fed digesters with a loading rate of 1-6 g VS liter -~ day -1. Shiralipour & Smith (1984) reported average methane yields of 0"32 and 0"17 liters g-I VS water hyacinth shoot and root samples, respectively, in a bioassay test of 100-ml culture volume. They also concluded that the addition of N in growth media for water hyacinth production increased methane yields of shoot and root samples.

The objectives of this study were to: (1) determine the effect of particle size, plant N content, and inocu- lum volume on biogas production during anaerobic digestion of water hyacinth; (2) determine N mineral- ization from 15N labeled water hyacinth during anaer- obic digestion; and (3) evaluate digester effluent (solids and liquid) composition based on inoculum volume.

METHODS

Batch anaerobic digestion tests with water hyacinth were conducted in temperature-controlled (35°C) incu- bation facilities (Table 1 ). Each series consisted of four 55-liter batch digesters, one control (inoculum and no hyacinth plant material) and threc inoculum and hya- cinth treatments. Each digester received 210 g NaHCO 3 to buffer pH at 7"0. The size of the digesters and the use of 15N-labeled plant material prohibited replication of treatments.

The first series was designed to determine the effect of particle size of water hyacinth on gas production.

72 K. K. Moorhead, R. A. Nordstedt

The water hyacinths were collected from a finishing pond of the University of Florida sewage treatment plant. They were frozen and chopped to a length of 1.6 mm in a Hobart T 215 food processor, and to lengths of 6-4 and 12.7 mm in a Dito Dean TR 21 food pro- cessor. Each digester received 4.7 kg fresh weight of water hyacinths and 5 liters of inoculum from an anaer- obic stirred tank digester receiving swine waste as feed- stock (Table 2). The moisture content of the fresh plant material was 93% and the water hyacinth loading was approximately 5.1 g VS liter -~. A control digester received 5 liters of inoculum and no plant material. Tap water was used to bring the weight of each batch di- gester to 54.7 kg. The contents of each digester were thoroughly mixed before digestion.

The second and third series were designed to deter- mine the effect of inoculum volume on gas production using water hyacinths with high and low tissue N con- tents. Water hyacinths with a high N content (34 g kg- 1 dry weight plant tissue) were obtained from the waste- water treatment plant of the Reedy Creek Utility Company, Inc., at Wait Disney World near Orlando, Florida. Water hyacinths with a low N content (10 g kg- 1 dry weight plant tissue) were grown in tap water at Sanford, Florida. Both types of hyacinths were grown in 15N labeled (NH4)2SO 4 for 2 weeks, frozen and chopped to 1-6 mm length using a Hobart T 215 food processor. Although maximum gas production was observed for a 6.4 mm particle size, the 1-6 mm length was chosen for the ~SN labeled plant material due to the relative ease of processing plant material through the Hobart T 215 food processor.

The digesters in the second and third series received 4.7 kg fresh weight of the 15N labeled water hyacinths and an inoculum volume of 2-5, 5 or 10 liters. A control digester received 10 liters of inoculum and no plant material. Tap water was used to bring each batch digester to 54"7 kg which resulted in a water hyacinth loading of 4-9 g VS liter- 1 for the high N plants and 4-7 g VS liter-~ for the low N plants. The inoculum used for plants with high N content was obtained from a continuous-fed upflow digester receiving a feedstock of water hyacinth and domestic sewage sludge in a blend ratio of 3:1 (Table 2). The inoculum used for plants with low N content was obtained from a continuous-

fed tank digester receiving water hyacinth as feedstock. The continuous-fed tank digester was not operating at the time of inoculum collection.

Gas production was monitored in each digestion series for 60 days by a floating gas collector. Biogas production for each digester was corrected by sub- tracting the gas produced in the control digesters. At the end of the digestion period, the digesters containing I SN labeled effluent were thoroughly mixed and the contents were passed through a 1.00-mm fiberglass screen into a 60-liter tub to separate most of the 15N labeled digested sludge from the effluent. The plants used for the particle size digestion (Series 1) were not labeled with ~5N and were not processed to separate sludge from effluent.

Samples of the screened digester effluents were analyzed for total solids (TS), VS, Kjeldahl nitrogen (TKN) (Nelson & Sommers, 1975), and NHn-N and NO3-N by steam distillation (Keeney & Nelson, 1982). The digester effluents were also filtered through a 0.2- /~m membrane filter and analyzed for Ca, K, Na and Mg by atomic absorption and P by an autoanalyzer (APHA, 1980). The pH and electrical conductivity (EC) were measured for the effluent after sludge sepa- ration.

The fresh plant material and digested sludge were freeze-dried (Thermovac-T) and analyzed for the following: TKN (Nelson & Sommers, 1973), total carbon (TC) (LECO Induction Furnace 523-300), lignin, cellulose and hemicellulose (Goering & Van Soest, 1970), and ashed mineral constituents (Gaines & Mitchell, 1979).

RESULTS AND DISCUSSION

Biogas production

A summary of biogas production, corrected to 0°C and 0"1 MPa, for each digestion series is given in Table 3. Biogas production was corrected for gas produced in the control digesters. Gas production essentially ceased after 60 days of digestion (data not shown). Biogas production was highest for plants chopped to 604 mm length, followed by a 1.6 and 12.7 mm length. Analysis of gas production data indicated that cumulative biogas

Table 1. Batch anaerobic digestion tests with water hyacinth

Series Inoculum source and volume Plant particle size, N content, TS and VS added

1 Swine waste digester 1-6, 6.4 and 12.7 mm 5 liters N content unknown

329 gTS, 282 gVS 2 1"6 mm

34 g N kg- ~ dry weight 320 gTS, 269 gVS

3 1.6 mm 10 g N kg- 1 dry weight

306 gTS, 257 gVS

Hyacinth/sewage sludge digester 2.5, 5 and 10 liters

Hyacinth digester 2.5, 5 and 10 liters

Batch anaerobic digestion of water hyacinth 73

production at 60 days was 12% higher for the 1"6 mm particle size compared to 12.7 mm. However, cumula- tive biogas production at 15 days was 25% higher for the 1"6 mm particle size compared to 12.7 mm. Thus, the additional biogas production would have to be weighed against the additional chopping energy input and the ease of handling or pumping of hyacinths which have a smaller particle size.

Table 2. Characteristics of the inocula used in thebatch digesters

Inocula characteristics

NH4-N NOa-N TKN COD pH TS FS VS (mg liter- l) (g liter- l)

Series 1 -- Swine digester 1190 33 4270 7700 8.2 67.7 14.6 53.1

Series 2 -- Hyacinth/sewage sludge digester 1072 48 1533 14200 6.3 17-5 5.9 11.6

Series 3 -- Hyacinth digester 535 21 562 784 7"7 2.9 2"4 0"5

Increasing inoculum volume resulted in increasing cumulative gas production for digesters with high N plants. However, biogas production for the low N plants was similar throughout 60 days of digestion regardless of inoculum volume. In addition, biogas production began earlier for high N plants compared to low N plants (data not shown). Cumulative biogas production at 60 days for hyacinths with a high N content was approximately 21% less with 2"5 fiters of inoculum compared to 10 liters. However, cumulative biogas production at 15 days was doubled for the digester receiving the 10 liters of inoculum. The amount of inoculum did not appreciably affect cumula- tive biogas production during digestion of plants with a low N content.

Cumulative biogas production at 60 days was simi- lar for plants with high or low N content. At 15 days, the biogas production was generally greater for high N plants. It appeared that N was not a limiting factor for cumulative or total gas production in either digestion test. Sievers & Brune (1978) reported higher methane yields as the carbon/nitrogen (C/N) ratio increased for

Table 3. Gas production during anaerobic digestion of water hyacinth

Inoculum Particle volume size (liters) (mm)

Cumulative biogas production" Total gas yields

15 days 30 days 60 days Biogas Methane (liters) (liters g- ~ VS)

Series 1 -- Particle size plant material 5 1-6 77.3 103.0 134.4 0.24 0.16 5 6.4 78.7 112.5 138.9 0.28 0.18 5 12.7 61.8 95.8 120.3 0.22 0-14

Series 2 -- High N plant material 2.5 1.6 16"4 40-8 60"3 0"21 0.14 5 1.6 28.1 53"5 73"0 0-23 0-15

10 1"6 34.5 59.1 75.4 0"20 0.13 Series 3 -- Low N plant material

2.5 1.6 16.9 45.5 67.9 0.25 0.16 5 1.6 20.0 51.6 72.6 0.27 0-17

10 1.6 14.7 52.2 67-4 0.25 0.16

~Biogas production was corrected by subtracting gas produced in the control digesters.

Table 4. Characteristics of fresh plant material and digested sludge (based on 1 kg of freeze-dried material)

Inoculum TC TKN C/N Lig a Cell Hemc Ca K Mg volume (g kg- ~) (g kg- 1) (liters)

Na

Series 2 -- High N plant material Fresh 385 34.0 12 43 167 182 17.6 23.5 3-2

2.5 449 37.2 12 136 159 NA b 17.6 2.2 1.8 5 446 39.3 12 130 207 180 18.4 3.8 2.2

10 441 30.8 14 111 168 NA 17.8 3.6 2.0 Series 3 -- Low N plant material

Fresh 373 10.6 35 83 266 247 21.0 22.0 6.7 2.5 441 27.4 16 149 177 NA 22.1 2.8 2.8 5 425 26-6 16 145 181 242 24.2 2.8 2-2

10 433 25.2 17 163 163 NA 26-8 4-6 2.7

8.0 15.3 26-0 20.8

10.9 14.8 17.4 19.5

aLig, Cell, Hemc = Lignin, cellulose and hemicellulose, respectively. bNA= not available.

74 K. K. Moorhead, R. A. Nordstedt

swine waste feedstock. They concluded that the opti- mum C/N range for maximum methane production was 15.5/1-19/1. The initial C/N ratio of the high N plant material was 12 compared to 35 for the low N plant material (Table 4).

Cumulative 60-day biogas production converted to total gas yields (liters of biogas or methane g-1 VS added) is also presented in Table 3. Total VS added were calculated by adding VS from water hyacinths and inoculum. Methane yields were similar for all three digestion series and ranged from 0" 13 to 0" 18 liters g- 1 VS. The similar gas yields were due to differences in VS of the various inocula used for digestion (Table 2). The inoculum from the swine waste digester (Series 1) contained 53"1 g VS liter-1. The inoculum from the water hyacinth/sewage sludge digester (Series 2) con- tained 11"6 g VS liter-1 and the inoculum from the water hyacinth digester (Series 3) contained 0"5 g VS liter- 1. Gas production expressed as liters per total VS added as inoculum and plant material suggested that inoculum volume did not appreciably affect total bio- gas or methane yields. The average methane content of the biogas was 65"6 + 5"1% based on 30 samples (12 samples from Series 1, 3 from Series 2, and 15 from Series 3).

Shiralipour & Smith (1984) reported that methane production for water hyacinth roots was lower than for shoots and that increasing N in growth media for water hyacinths resulted in increased methane yields. Water hyacinths typically have greater root masses as water fertility declines. Shoot/root dry weight ratios of water hyacinth were higher when nutrients were not limiting and decreased significantly when plants grew in nutrient-poor waters (Reddy, 1984). The shoot/root dry weight ratio of the high N plants was 4-2 compared to 1"6 for low N plants. It was anticipated in this study that gas production, both cumulative and yields, would be greater for the high N plants. However, differences in inoculum characteristics and lack of replication make comparisons of gas data difficult.

Nitrogen mineralization The digested sludge was separated from effluent for high and low N plant digestion series in which the initial plant material was labeled with 15N. Nitrogen and 15N were determined for effluent and sludge frac- tions to evaluate N mineralization during anaerobic digestion. A mass balance of 15N is presented in Table 5. The 15N recovered as organic N decreased after anaerobic digestion for each treatment. The majority of the 15N was recovered in the effluent as NH4-N.

Total 15N recovered as NH4-N in the screened efflu- ent was 72 + 4% for high N plant material compared to 35 + 9% for low N plant material (Table 5). Approxi- mately 11 and 20% of the added 15N was recovered as organic N in the screened effluent for digested high and low N plants, respectively. Most of the N that is placed in a digester should be recovered in the effluent, and the proportion of NH4-N to total N tends to increase (Sievers & Brune, 1978). Organic 15N recovered in digested sludge accounted for 20 + 5% of the added 15 N from fresh plant material regardless of N content.

The low total recovery of 15N for low plant material after anaerobic digestion is difficult to explain. Nitrogen cycling during anaerobic digestion was pri- marily mineralization of organic N to NH4-N. Volatili- zation of NH4-N may occur but the potential increases as NH4-N concentrations increase or at higher pH values (Freney et al., 1983). Each digester received the same amount of buffer and the pH after digestion was similar for all digester effluents (Table 6). The NH4-N concentrations after digestion were much higher for the effluents of high N plant material.

Characteristics of digester effluents and digested slurry The digester effluents from high N plant material had higher concentrations of N and P but lower electrical conductivity (EC) than the effluents from low N plant material (Table 6). As the inoculum volume increased, the EC and the NH4-N, TKN and K concentrations increased. Increasing inoculum volume did not alter

Table 5. Nitrogen-t5 balance for thebatch digesters

Inoculum volume

High N plant material (g) Low N plant material (g)

2.5 5 10 2.5 5 10

~5N Added Water hyacinth

Organic N 15 N Recovered Screened effluent

Organic N Inorganic N

Digested sludge Organic N

Total

% Recovered

10"40

1"18 7"06

1'67

9'91

95

10"40 10"40 3"24 3"24 3"24

1"48 0"93 0"84 0"65 7"78 7"68 0"79 1"25

2"53 2"21 0"87 0"57

11"79 10"82 2"50 2"46

113 104 77 76

0"34 1"34

0"52

2"20

68

Batch anaerobic digestion of water hyacinth

Table 6. Characteristics ofdigester effluent after sludge separation

75

Inoculum pH EC a volume (dS m- l) (liters)

NH4-N TKN Ca Mg K Na P TS VS (mg liter- ~) (g liter- L)

Series 2 -- High N plant material 2.5 7.6 4.3 161 188 32 5 7-6 4.7 212 256 30

10 7.6 5"3 289 326 23 Series 3 -- Low N plant material

2"5 7.5 5"6 22 45 147 5 7.4 5"9 48 72 53

10 7-5 6.7 91 111 61

13 82 750 12"8 3"67 1-56 12 80 595 12"8 3"42 1"42 14 123 915 11"5 3"69 1"52

50 195 1220 3-1 3"67 1"11 53 245 1 235 4-6 4"08 1"24 63 325 1 200 4"5 4"25 1"19

aEC = electrical conductivity.

the concentrations of Na, P and Mg. The concentra- tions of Ca generally decreased as the inoculum volume increased. The effluents from low N plant material contained more Ca, Mg, K and Na than the effluents from the high N plant material.

Anaerobic digestion resulted in increases in sludge total carbon (when expressed as g kg- 1 plant material) and TKN (Table 4). The increases in sludge TKN after digestion of low N plant material caused a reduction of the C/N ratio from 35 to 16. The changes in TC or TKN of the digested high N plant material did not appreciably alter the C/N ratio. The digested sludge had a higher lignin content compared to fresh plant material. The increase in lignin was due to the loss of readily-decomposable C during anaerobic digestion. Lignin appears to be practically inert to anaerobic digestion (Hashimoto et al., 1980).

The cellulose concentration of low N plant material decreased after digestion. Cellulose may have been a primary C source during anaerobic digestion of low N plant material. The high N plant material had less C as cell wall constituents and probably more C that was readily-decomposable, such as sugars or starch. The 15-day cumulative biogas differences may be attri- buted to the composition of C constituents of the initial plant material or differences in inoculum charac- teristics.

Anaerobic digestion resulted in losses of K and Mg from fresh plant material, but increased the concentra- tion of sludge Na and Ca. The Na increase probably due to the addition of NaHCO3 as a buffer during anaerobic digestion. The volume of inoculum did not appreciably alter the mineral concentrations of digested sludge.

The size of the batch digesters and the use of 15N labeled plant material limited the replications for this study. Although statistical information is lacking for this research, the general trends of biogas production and N mineralization and the composition of digester effluents and digested sludge may be of practical value for initiating anaerobic digestion of plant material. Optimizing biogas production includes rapid initiation of anaerobic digestion. The data from this study suggest that anaerobic digestion can be enhanced

initially by decreasing particle size of plant material and increasing inoculum volume.

ACKNOWLEDGEMENTS

This paper reports results from a cooperative program between the Institute of Food and Agricultural Sciences (IFAS) of the University of Florida and the Gas Research Institute (GRI) entitled 'Methane from Biomass and Waste'. The technical assistance of Mr Terry Slean, Mr Bill Pothier, and Ms Veronica Camp- bell is gratefully acknowledged. Manuscript prepara- tion was partially supported by DOE Contract DE-AC09-76SROO-819 between the US Department of Energy and the University of Georgia's Savannah River Ecology Laboratory.

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