in storage psychrophilic anaerobic digestion of swine slurry

In storage psychrophilic anaerobic digestion ofswine slurry

J. Abou Nohra a, S. Barrington a,*, J.C. Frigon b, S.R. Guiot b

a Department of Biosystems Engineering, Macdonald Campus of McGill University, 21, 111 Lakeshore, St.

Anne de Bellevue, Quebec, QC, Canada H9X 3V9b Environmental Bioengineering Group, Biotechnology Research Institute, National Research Council, 6100

Royal Mount, Montreal, QC, Canada H4P 2R2

Received 20 July 2001; accepted 9 July 2002

Abstract

Anaerobic digestion of manures induces deodorization and conserves nitrogen. Livestock

producers could transform their long-term (250�/300 day) manure storages into psychrophilic

anaerobic digesters, thus greatly reducing the investment, operational and technical cost

associated with the treatment. But, sufficient inoculation is required and the feeding regime

differs from that of a fixed volume digester, as the fed manure volume to digester content ratio

is ever decreasing over time and the retention time is as long as the storage period. The project

was designed to establish, in the laboratory, the inoculation conditions and feeding regime

leading to the efficient establishment and operation of anaerobic digestion in conventional

storages. In the laboratory using the biochemical methane potential test (BMP), methane

production was monitored from swine manure inoculated with one of the three inocula (cattle

manure*/B; municipal wastewater anaerobic sludge*/R, and; food processing wastewater

anaerobic sludge*/C) at four ratios (0, 10, 30 and 50%). Also, six prototype digesters were

used to simulate VSS digestion in conventional storages, under two feeding regimes (once

every week and every 2 weeks) and using three inocula. With the BMP and for all three

inocula, a ratio of 10% provided the fastest initiation and the highest methane production.

Both R and C initiated methane production as of the 1st day because of their active bacterial

population, while B, obtained from a facultative system, required 24 days. After 100 days, all

three inocula had produced the same amount of methane, for the same inoculation ratio. In

the prototype digesters, both R and C inocula produced methane as of the first day, while the

B inoculum produced methane after day 45 and 78 days, for the 1/week and 1/2week feeding

* Corresponding author. Tel.: �/1-514-398-7776; fax: �/1-514-398-8387.

E-mail addresses: [email protected] (S. Barrington), [email protected] (S.R. Guiot).

Resources, Conservation and Recycling 38 (2003) 23�/37

www.elsevier.com/locate/resconrec

0921-3449/02/$ - see front matter # 2002 Published by Elsevier Science B.V.

PII: S 0 9 2 1 - 3 4 4 9 ( 0 2 ) 0 0 0 9 2 - 7

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regime, respectively. For both feeding regimes and after 148 days, all three inocula produced

the same cumulative amount of methane. Thus, 10% B can initiate anaerobic conditions in

swine manure during storage, as long as the storage period exceeds 140 days and a 45 day

initiation period at a temperature is 35 8C.

# 2002 Published by Elsevier Science B.V.

Keywords: Anaerobic digestion; Swine slurry; Inocula; Inoculation ratio

1. Introduction

Anaerobic digestion can partially deodorize swine manure. For livestock opera-

tions, adding an anaerobic digester increases the investment, handling and technical

costs of manure management. These costs could be substantially reduced if

anaerobic conditions could be introduced in the long-term (250�/300 day) storage

using an airtight cover. Anaerobic conditions within the manure storage would differ

from that of a digester with a constant hydraulic retention time. As the storage is

filled by adding manure on a weekly to monthly basis, the fresh manure would

represent an ever-decreasing fraction of the total storage content. Thus, the

microbial population would have to adjust to: a food source diluted with time as

the storage fills up; high levels of ammonia contained in the manure; and low winter

temperatures of 10�/15 8C which develop in storages during the winter, in Canada,

and the Northern states of the USA.

Psychrophilic anaerobic digestion (15�/20 8C) is the type of digestion most likely

to operate within a long-term manure storage, for Northern regions North. As

compared to mesophilic anaerobic organisms, those operating in the psychrophilic

range are more tolerant of high ammonium levels (Masse, 1995) and can be fed more

intermittently. Nevertheless, mesophilic temperatures can initiate anaerobic diges-

tion faster than psychrophilic temperatures (Chynoweth et al., 1999) and produce

more gas. The slower initiation period required by the psychrophilic organisms is not

an obstacle, because the storage would offer a relatively long digestion period of

250�/300 days. Also, the lower level of gas production is not vital, since the main

objective is to deodorize the manure, conserve nitrogen, and decrease its degree of

bacterial contamination.

The only condition requiring attention is the temperature of the manure. If the

livestock operation empties its storage in late summer, and starts to fill it regularly

there after, anaerobic initiation would occur under temperatures of 30�/35 8C. This

will permit favourable conditions for start-up as the micro-organisms will be allowed

to acclimate faster to pscychrophilic anaerobic digestion conditions (Zeeman et al.,

1988). By the time winter arrives, the anaerobic microbial population would be

established. Nevertheless, the manure temperatures would be kept at 15�/20 8C by

insulating the sealed storage cover.

The main objective of the project was therefore to test a simple management

scheme, which could lead to the establishment of psychrophilic anaerobic conditions

in long term swine manure storages. The project therefore tested, in the laboratory,

J.A. Nohra et al. / Resources, Conservation and Recycling 38 (2003) 23�/3724

the effect of inoculum type and inoculum to substrate ratio on the start-up

operation. Start-up success was measured by monitoring VSS, VFA, and methane

gas production and COD degradation. Swine manure was used as the substrate while

the inocula were municipal and industrial anaerobic sludge (R and C), and cattle

manure (B). The project also simulated, in the laboratory, the feeding of

conventional storages managed as anaerobic digesters: prototype digester start up

and operation was monitored using two feeding regimes (1 every week and every 2weeks) for each of the three inocula tested earlier. All prototype digesters were fed

for 80 days, and then observed for an additional 60 days, without emptying their

content. All tests were initiated at a temperature of 35 8C to simulate end of the

summer start up conditions for storages in North America.

2. Literature review

Rieradevall et al. (1983) applied anaerobic digestion of swine manure atpsychrophilic temperature, in 13 Spanish manure storage pits. The pits were

transformed into anaerobic digesters by installing a gas circuit, including a

membrane retaining the biogas produced, and installing a pig manure circuit. These

adapted digesters had an HRT of 100 days, and were operated according to seasonal

temperature. The results showed that the low cost digesters running at psychrophilic

temperatures produced between 0.03 and 0.09 m3 of biogas per cubic meter of

digester.

Likewise, methane digesters are used in different places around Europe as well,but the conventional mesophilic systems do not perform well on livestock farms:

treatment volumes are limited, resulting in high digester construction cost, low

energy output and high energy requirement to maintain the system in operation. In

storage psychrophilic digestion is proposed as a solution (Wellington and Kaufman,

1982). It requires feeding through siphons for air tightness and retaining 20% of the

volume, when emptied, to inoculate the next incoming manure. For a storage volume

of 140 m3 at 20 8C over 2 months, 34 m3/day of biogas was produced with methane

content of 60%.Covers floating over lagoons have successfully collected biogas from manure in

anaerobic lagoons kept at temperatures below 20 8C. Chandler et al. (1983) reported

collecting 0.66�/0.92 m3/m2/day of biogas (70% methane) from a lagoon in California

where the manure temperature ranged between 10 and 11 8C and the hydraulic

retention time was 50 days. Safley and Westerman (1988, 1989, 1990) successfully

collected biogas using floating covers over lagoons. Biogas production was initiated

between 3 and 9 8C, but the initiation temperature drops with the age of the manure

in the lagoon. A biogas production rate of 0.20 m3/m2/day (70% methane) wasmeasured from a dairy lagoon in Raleigh, North Carolina.

Conducting anaerobic digestion in storage implies exposing the micro-organisms

to low temperatures, which has a major impact on the rate of biological processes. It

can affect the microbial population in two ways: by immediate change in activity

rate, and by a change, over longer periods of time, in the composition of the

J.A. Nohra et al. / Resources, Conservation and Recycling 38 (2003) 23�/37 25

population itself. According to recent research conducted by Van Lier et al. (1997),

temperature is not a limiting factor in implementing anaerobic treatment, but the

suitable design should be provided. Namely, micro-organisms can adapt to

psychrophilic anaerobic conditions if the hydraulic retention time is twice that of

a mesophilic treatment and the micro-organisms are well acclimated at start-up

process.

Masse (1995) applied psychrophilic anaerobic digestion to swine slurry at 20 8Cusing a sequential batch reactor. For Canadian farms, psychrophilic anaerobic

digestion is more feasible than mesophilic digestion which is relatively unstable, and

requires heating and high technical expertise under extended cold winter conditions.

Masse (1995) developed a sequential batch reactor for anaerobic digestion because

such digesters can be fed in series with mixing only at feeding, can be fed

intermittently and the fed manure can have a high total solids and ammonium

content. Manure deodorization also happens.

The factors involved in the start-up of a digester are the quantity and quality of

inoculum, the rate of adaptation of these micro-organisms to the waste, the rate of

growth of these micro-organisms, and the hydraulic characteristics of the digester

(Chynoweth et al., 1999). The amount of dead space and flow short-circuits in a

digester decrease the start-up speed (Kennedy and van der Berg, 1987). The

inoculum must have a high methanogenic activity otherwise the quantity of

inoculum must be increased (Rebac et al., 1998). Under inoculating a digester

results in inadequate performance due to the more rapid growth of acid formers

leading to accumulation of organic acids and consequent pH reduction (Chynoweth

et al., 1999).

3. Method

3.1. Experimental material

Three different inocula were used to inoculate swine manure: a municipal

anaerobic sludge (R) from the Repentigny wastewater treatment plant operated at

a temperature of 35 8C; an industrial anaerobic sludge (C) from a food processing

factory in Cornwall, Ontario, also operated at a temperature of 35 8C, and: cattle

manure (B) from the dairy manure barn of the Macdonald Campus farm of McGill

University at ambient temperature (20�/25 8C). Large solid particles were removed

from the cattle manure using a 1.7 mm sieve. The substrate was swine manure

collected from the farrowing to finishing piggery of the Macdonald Campus farm of

McGill University. All inocula and the swine manure were analyzed for TS (total

solids), SS (suspended solids), VSS (volatile suspended solids) and soluble COD

(chemical oxygen demand).


3.2. Prototype digesters

Six prototype digesters were built to simulate the feeding of a conventional storage

converted into an anaerobic digester by means of an airtight cover. The digesters

were built of polyethylene vinyl chloride (PVC) tubing, 200 mm in inside diameter

and 750 mm in height (Fig. 1). Oil based gas meters, in the shape of a U with a

bubble counter, were installed at the top of each digester to measure gas production.

Each digester had four ports, the first located at 30 mm from the bottom and all

other located at 230 mm from each other.

3.3. Specific activity experiments

Activity tests were performed to characterize the three inocula and establish their

rate of consumption of glucose, acetate, propionate, and H2. Each inoculum was

diluted separately to 5 g/l of VSS with a propane buffer, and placed in separate

triplicated serum bottles maintained under anaerobic conditions by flushing with N2/

CO2. Each bottle was then injected with 0.05 ml of substrate per 10 ml of diluted

sludge. All bottles were incubated in a New Brunswick shaker (35 8C, 100 rpm) for a

period of 48 h. Liquid samples were collected after 1, 3, 7, 24, 32 and 48 h and

analysed for the specific substrates. Suspended solids were measured at the end of the

experiment to express the results as gram of substrate consumed per gram of

biomass. The gas produced was sampled and analysed for methane content. For the

hydrogen activity test, a pressurized mixture of gas containing 80% H2 and 20% CO2

was injected in the bottle. The specific activity was estimated from the depletion rate

of all substrates (Guiot et al., 1986).

Fig. 1. The prototype digesters.


3.4. Biochemical methane potential

To establish the optimum ratio of inoculum required to start-up anaerobic

digestion in storages, a laboratory-scale experiment was conducted using the

biochemical methane potential (BMP) test measuring the biodegradability of

wastewater through the cumulative methane production.

The BMP experiment was performed in 160 ml serum bottles. The substrate, swineslurry, was mixed with the three different inocula, at four different ratios (0, 10, 30,

and 50%). Bottles containing inoculum but no substrate were also incubated to have

the endogenous methane production (self-digestion of the inoculum) and to

differentiate in the tests, methane from the substrate itself. The swine manure and

inoculum mixture were diluted to a soluble COD level of 4.8 g/l. A typical bottle

contained 8 ml of sludge, 10 ml of defined media with nutrient supplementation, 2 ml

buffering solution, and 0.5 ml reducing solution were added to the inoculum and

substrate mixture for a total volume in each bottle of 40.5 ml. All the bottles wereprepared under an oxygen free environment and incubated for 96 days, at 35 8C in a

New Brunswick shaker agitated at 100 rpm. Gas samples were collected at constant

time intervals from day 1. The volume of gas produced was measured using a

volume-displaced method and its methane content was determined by gas

chromatography. Every 16 days, a serum bottle from each treatment was opened

and sacrificed to analyse for VSS, COD and VFA. All results were calculated based

on their COD equivalence: 1.4 g COD/g of biomass-VSS (Loehr, 1974); 355 ml of

methane/g of COD; 1.07, 1.51 and 1.82 g COD/g of acetate, propionate andbutyrate, respectively.

3.5. Feeding simulation of storages

To simulate the feeding of conventional storages, each one of the six 22.5l

prototype digesters received 7.5l of inoculum at a VSS of 40 g/l, for an inoculum to

substrate ratio of 30%. A 30% ratio provided sufficient initial volume for mixing and

sampling. Each inoculum was diluted using tap water, to a VSS concentration of 40

g/l, to start each digester with the same biomass. Likewise, all digesters received atotal volume of 14.6 l of swine manure over a period of 80 days, but were observed

over a period of 140 days. Thus, three digesters received 1.125 l, once a week and

three others received 2.25 l every 2 weeks. None of the digesters were emptied

through out the experiment.

At every feeding, the content of each digester was mixed and sampled from the

second port located at 260 mm from the bottom, and analysed for VSS and soluble

COD. Oil based gas flow meters were used to measure gas emissions, but these could

not measure the large volumes of gas released during the mixing operation. Thus,methane production was calculated from a COD equivalent mass balance, by

equating 1.4 g of VSS (Loehr, 1974) and 355 ml of methane to 1 g of COD. All

digesters were started at a temperature of 35 8C for 60 days, to simulate the start up

of storages at the end of the summer. The temperature was then gradually decreased

to 15 8C from day 60 to 80 to imitate field conditions in the fall. All digesters were


operated without feeding and at 15 8C, from day 80 to 148. To obtain the desired

temperature, the digesters were placed in a temperature controlled room kept at

within 1.0 8C of that specified.

3.6. Analytical procedure

All inocula and the swine manure were analyzed using standard methods (APHA,

1995). All solids and volatile solids were determined, respectively, by drying at

103 8C for 24 h and by burning at 600 8C for 1 h. TKN was determined by

digesting the material with sulphuric acid at 500 8C and measuring the NH3�/Ncontent at a sample pH of 13, using a NH3 sensitive electrode. NH3�/N and NO3�/N

were determined using NH3 and NO3 sensitive electrodes, respectively, connected to

a pH meter. COD was measured by digesting the samples at 150 8C with chromium

permanganate and measuring the permanganate remaining after digestion using a

colorimetric method. All volatile fatty acids were determined by gas chromatography

(GC, HP 69 800) and all gases were quantified by GC.

3.7. Statistical analysis

For the specific activity and BMT tests, all treatments were triplicated. ANOVAwas used to determine whether or not the treatments were significantly different, and

the Least-Square Method was used to identify the significantly different treatment.

For the prototype digester test, none of the treatments were repeated as the inocula

produced similar results, as compared to the BMT test.

4. Results and discussion

4.1. Characterizing the swine manure and the inocula

Swine manure, at 7.35% TS, was used as substrate (Table 1). This swine manure

offered a COD equivalent content of 80 g/l, considering its VSS and soluble COD

level of 47 and 14 g/l. Inoculum R offered SS and VSS levels twice as high as that ofboth inoculum B and the swine manure (Table 2). The R and C inoculums exhibited

low levels of soluble COD, as compared to the B inoculum.

Table 1

Characteristics of the swine manure

Unit Value

SS g/l 57.0

VSS g/l 47.0

Soluble COD g/l 14.0

pH 7.4

Total nitrogen g/l 4.6


Each inoculum demonstrated a different specific activity (Table 3). Inoculum B

showed a high specific activity for glucose and H2, and a moderate activity for

acetate. Inoculum C showed a high specific activity for acetate and H2. The specific

activity of inoculum R was moderate for all substrates and as compared to the other

two inoculums.

As compared to inoculum B, inoculums C and R demonstrated similar and low

methane percentages in the gas produced. This results from the higher soluble CODlevel found in inoculum B, producing more methane under the activity test. This also

demonstrates that soluble COD gives a better indication of methane production

potential, as compared to VSS which may be relatively nonbiodegradable. Also,

inoculums C and R had been exposed to anaerobic digestion earlier, implying that a

greater portion of the biodegradable VSS was used, as compared to inoculum B

which was relatively fresh and had been exposed to limited microbial digestion.

The structure of the biomass differed among the three inocula. Inoculum B’s

biomass did not exhibit a distinct structure; rather it was all one-colloidal mass,while the R and C biomass manifested a good settleable quality. Inoculum C’s

biomass developed macroscopic bacterial granules, which settled easily, while the R

biomass demonstrated a filamentous structure.

4.2. Biochemical methane potential

The extent of anaerobic digestion can be estimated from methane production (Fig.

2a�/c). Both inoculums C and R started to produce methane as off day 0, while the

inoculum B started to produce methane as off day 24. The control (swine manure

with no inoculum) also started to produce methane as off day 24. The later start up

Table 2

Characteristics of three inocula

Units R C B

SS g/l 39.3 76.1 45.4

VSS g/l 35.4 66.7 33.9

Soluble COD g/l 24.5 12 44.9

VFA COD Eq g/l 1.49 7.8 3.7

Note: SS, suspended solids; VSS, volatile suspended solids; VFA, volatile fatty acids.

Table 3

Average specific substrate activity of the three inoculums

Units C R B

Glucose mg/g VSS/day 235 148 537

Propionate mg/g VSS/day 134 33 104

Acetate mg/g VSS/day 655 124 220

Hydrogen mg/g VSS/day 1087 166 2664

Methane produced % 18 20 30


time obtained with inoculum B and the control was expected, as both were obtained

from an facultative system, while inoculums C and R were obtained from an

anaerobic system. Nevertheless, anaerobic conditions were initiated in swine manure

without inoculum, given enough timed. Anaerobic sludge can initiate faster

anaerobic conditions at an inoculation ratio as low as 10%.

For both inoculums C and R, the 10 and 30% inoculation ratio produced more

methane, as compared to the 50%. For inoculum B, the 10 and 30% inoculum ratios

produced, respectively, 25 and 22% more methane than the 50% inoculum ratio.

Fig. 2. Methane production for all three inoculums at ratios of 0, 10, 30 and 50%; the end. Curve is the

methane produced by the inoculum alone.


Thus, at inoculation ratios above 30%, inoculum B produced less gas because of its

higher fibre content and lower biodegradability, as compared to swine manure

composed of more starchy feed residues. The 0% ratio produced the same methane

as the 50% ratio, indicating a lower methanogen population despite the higher starch

content.

Volatile suspended solids, VFA and methane are illustrated in equivalent COD

levels, in Fig. 3a�/c, for the three inoculums at 10%. The total COD equivalent mass

is not constant from day 1 to day 40�/60, indicating that the VFAs measured (i.e.

Fig. 3. Performance of all three inoculums with time at a ratio of 10%.


acetate, propionate, butyrate) did not represent all soluble COD. Inoculum R

produced the least VFA, followed by inoculums C and B. Furthermore, inoculum B

produced high VFA levels over a longer period, because its methanogen population

was not as well developed as that of inoculums R and C.

Volatile fatty acid levels were much higher with inoculums B and C, reaching

levels of 20 g/l as compared to that of inoculum R reaching levels of 8 g/l. For all

three inoculums, the 50% inoculation ratio showed 5 and 10% higher peaks in VFAlevels, as compared to the inoculation ratios of 30 and 10%, respectively. Thus,

higher inoculation ratios produced temporary conditions of methanogen inhibition.

In all cases, propionic and acetic acids were predominant with no measurable butyric

acid.

The BMP tests therefore suggest that a 10% inoculum ratio suffices in initiating

anaerobic digestion as of day 1. Nevertheless, anaerobic conditions will appear in

swine manure alone, given at least 24 days. Adding 10�/30% B inoculum, to swine

manure, does not change the start up time, as compared to swine manure alone, butinduces more methane production.

4.3. Prototype digesters

The performance of each digester was evaluated based on soluble COD and VSS

variations over time and as sampled from the 2nd port from the bottom of the

digester. While VSS differentiates between the feed, effluent solids and gas, soluble

COD estimates the oxidation�/reduction balance between the feed and the product.Methane yield is preferred over gas yield because pH variations cause interference

with the liquid release or uptake of CO2. The methane content in the gas is also an

indicator of stability. Hence, the reduction in the VSS and COD and the production

of biogas indicate a successful performance of the digester (Chynoweth et al., 1999).

In general and for all digesters, the VSS and COD sampled at the second port

increased with every feeding and decreased in between feedings (Figs. 4 and 5). The

VSS concentrations generally dropped with time for all six digesters while soluble

COD was more irregular because soluble COD was continuously produced fromVSS and transformed into methane, while VSS was degraded into soluble COD.

Also, the digesters were able to liquefy the VSS added with each feeding. The erratic

concentrations of COD and VSS especially for the first 40 days resulted from the

substrates and inoculums having a high total solids content and being hard to mix.

Thus, sampling from the second port produced very high VSS and soluble COD,

especially at feeding, because of the settling process occurring with the swine manure.

Lowering of the digester temperature from day 60 on ward, to reach a final 15 8Ctemperature on day 80, resulted in a sharp increase in soluble COD for the swinemanure fed every 2 week, and inoculated with cattle manure. For all other

treatments, COD concentrations remained stable. The drop in temperature produced

no marked effect in VSS for the swine manure inoculated with both inoculums R and

the B, but did cause an increase with inoculum C. Since this increase only lasted 2

weeks, inoculum C was able to readjust despite its higher sensitivity to lower


temperatures. Thus, the drop in digester temperature did produce some changes in

COD and VSS concentrations, but these changes were short lived.

Inoculums C and R started to produce methane as early as day 14 while inoculum

B started after day 45 and 78 days, for the 1/week and 1/2week feeding regime,

respectively (Fig. 6). But, the feeding regime has no effect on methane production for

the swine manure inoculated with inoculums C and R. Nevertheless, all digesters

produced 55 g of methane (COD equivalent)/l of swine manure fed after 148 days of

observation. These results were similar to those obtained with the BMP test where

again, the swine manure inoculated with B produced methane after 24 days while

that inoculated with either the C and R produced methane from day 1.

Fig. 4. Prototype digester soluble COD levels with time under both feeding regimes and an inoculum ratio

of 30%.


5. Conclusion

Sludge from an active anaerobic digester can initiate anaerobic conditions in swine

manure within a day, as compared to cattle manure inoculum or swine manure alone

requiring 24 days. Based on the amount of methane produced after 100 days, an

inoculation ratio of 10�/30% is adequate to initiate anaerobic conditions and to

optimize methane production. Higher inoculation ratios lower the production of

methane because the inoculum has a lower biodegradability, as compared to the

substrate. The 10 and 30% inoculation ratio also lead to lower VFA levels,

demonstrating a more balanced microbial population, between that of the acid

producers and the methanogens.

For the prototype digester test, feeding regime had no effect on cumulative

methane production for the swine manure inoculated with both inoculums R and C.

Fig. 5. Prototype digester VSS levels with time under both feeding regimes and an inoculum ratio of 30%.


Under the weekly feeding scheme, the swine manure inoculated with B produced

methane 23 days sooner than that fed 1/2 weeks. But after 148 days, all inoculums

and feeding rates produced the same cumulative amount of methane. Thus, cattle

manure can initiate anaerobic conditions for swine manure in storage as long as the

storage period exceeds 140 days and the initiation temperature is 35 8C for at least

45 days. Nevertheless, the BMP test showed that even alone, swine manure could

produce anaerobic conditions given enough time.

It can therefore be concluded that anaerobic conditions can be initiated in a swine

manure storage facility by preferably adding, as inoculum, 10% cattle manure. This

10% is calculated based on the total volumetric capacity of the storage. If no

inoculum is used, anaerobic digestion will still proceed, but less gas will be produced

Fig. 6. Prototype digester methane production with time under both feeding regimes and an inoculum

ratio of 30%.


since lower methanogen population will develop. The start up should be conducted

at the end of the summer, as the higher prevailing temperatures lead to a faster

initiation. More research is needed to verify the prototype start up period of swine

manure alone and a feeding of once every 3 and even 4 weeks.

Acknowledgements

This project was supported by the Natural Science and Engineering Research

Council of Canada and the Biotechnology Research Institute of the National

Research Council of Canada.

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in storage psychrophilic anaerobic digestion of swine slurry

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