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WA T E R R E S E A R C H 4 0 ( 2 0 0 6 ) 1 6 2 – 1 7 4

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A bench-scale aeration study using batch reactorson swine manure stabilization to control odour inpost treatment storage

ZhiJian Zhanga, Jun Zhub,�, Keum J. Parkc

aDepartment of Environmental Engineering, Zhejiang University, Hangzhou 310029, PR ChinabDepartment of Biosystems and Agricultural Engineering, Southern Research and Outreach Center, University of Minnesota,

35838 120th Street, Waseca, MN 56093, USAcDepartment of Biomachinery Engineering, Sunchon National University, South Korea

a r t i c l e i n f o

Article history:

Received 22 September 2003

Received in revised form

24 January 2005

Accepted 8 November 2005

Available online 19 December 2005

Keywords:

Manure

Stabilization

Aeration

Odour generation potential

nt matter & 2005 Elsevie.2005.11.004

uthor. Tel.: +1 507 837 562zjzhangox@yahoo.com (Z

A B S T R A C T

A bench-scale study on swine manure stabilization for odour control was conducted using

batch aeration reactors. In trial 1, two aeration lengths, i.e., 0.5 and 4.0 day, were used

under uncontrolled ambient temperature that increased gradually over the experimental

period. While in trial 2, a 16.0-day aeration scheme was employed under constant 17 1C. An

airflow rate of 1.2 L/s/m3 was used for both trials to aerate batch reactors containing

finishing pig manure with initial total solids (TS) levels ranging from 0.5 to 4.0%. Manure

stabilization during the 90-day post-treatment storage was evaluated by the changes in

organic materials, nitrogen and volatile fatty acids (VFA). The odour generation potential in

the treated manure was determined by the changes in VFA. Up to 827 mL of liquid was lost

due to aeration related foaming. The reductions in total volatile solids (TVS), 5-day

biochemical oxygen demand (BOD5), total Kjeldahl nitrogen (TKN) and VFA during storage

were improved when aeration length increased. Low solids levels offered a more

advantageous circumstance for manure stabilization and odour control. Biodegradation

of organic matter, removal of nitrogen, and breakdown of VFA would increase with

increasing ambient temperature. VFA removals in manure under 16.0-day aeration were

higher than those under 0.5- and 4.0-day aeration; however, VFA regeneration started to

exceed its consumption on day 20 (4 days after the aeration treatment). BOD5 was the best

estimate of VFA concentration in the aerated manure during storage. The 4.0-day aeration

scheme was sufficient to stabilize manure to effectively assuage odour generation potential

during the 90-day storage under increasing ambient temperature conditions.

& 2005 Elsevier Ltd. All rights reserved.

1. Introduction

Presently, cost-effective engineering technologies that can be

used on a massive scale for animal manure treatment are still

in the process of optimization for energy and cost savings. As

such, manure stabilization becomes one of the popular

r Ltd. All rights reserved.

6; fax: +1 507 835 3622.. Zhang), zhuxx034@um

options for treating manure before disposal; however, the

associated odour issue is considered as a ‘bottle-neck’ in the

application of this treatment.

Aerobic treatment is regarded as any process that attempts

to improve the supply of oxygen to aerobic microorganisms

responsible for the conversion of waste into a relatively

n.edu (J. Zhu), pkj@sunchon.ac.kr (K.J. Park).

ARTICLE IN PRESS

Insoluble organics

Soluble organics

Hydrolysis

Enzymes Assimilation

Decay

Microorganism

+ Soluble organics

Nitrogenous

component

Sulfuric

component

VFA and

carbohydrate

Aeration

Aerobic reactor

CO2, H2O, SO4, NH4/NH3 (or NO3) and other oxides

Biodegrad

ation

Aeration process storage

Anaerobic reactor

Biodegrad

ation CO2, H2O, CH4, H2S, NH4, N2, sulfides, and other reductants

Post-aeration/open storage

Assimilation

Fig. 1 – Possible manure stabilization for odor control in storage (based on references from Burton, 1992; Zhang et al., 1997;

Grady et al., 1999; Westerman and Bicudo, 2002).

WAT ER R ES E A R C H 40 (2006) 162– 174 163

biologically stable end product (Robinson, 1974). An overview

of the biological processes involved in manure stabilization in

storage is pictured in Fig. 1. The aeration process prevents or

reduces the activity of anaerobic microorganisms that convert

the waste into incomplete end products, many of which are

noxious, odourous, or toxic (Robinson, 1974; Burton, 1992;

Zhu, 2000). Therefore, aeration process may offer a pathway

for biologically stabilizing manure for odour control.

Odour releases appear to be related to manure total solids

(TS) content, e.g., 2.7 OUE s�1 m�2 for 0.45% TS, and

1.9 OUE s�1 m�2 for 0.34% TS in swine lagoon water (Lim et

al., 2003). Odour control should be adequate if O2 is supplied

to satisfy 50% of the chemical oxygen demand (Westerman

and Zhang, 1997). Generally, total volatile solids (TVS) would

have more contribution to odour generation than the coarse

fraction of TS during decomposition of manure by aerobic

treatment process (Burton, 1992; Zhang and Westerman,

1997). Research found that release of ammonia and the

reduced malodourous gases to which odour concentrations

proportionally responded was directly proportional to the

volatile solids loading rate (Lim et al., 2003). Thus, biodegra-

dation of TVS by aeration should be beneficial to manure

stabilization.

Odourous nitrogen (N) compounds, such as volatile amines,

and part of volatile fatty acids (VFA) may accumulate under

prevailing anaerobic conditions (Zhu, 2000). While under

aerobic conditions, the nitrogen compounds (proteins, pep-

tides, and amino acids) could be converted to ammonium and

then oxidized to nitrate eventually (Westerman and Zhang,

1997). Ammonia stripping and nitrogen gas emission (includ-

ing nitrous oxide) by the nitrification–denitrification process

are commonly considered as the two most important path-

ways available for nitrogen losses associated with various

aeration technologies (Hashimoto, 1974; Loynachan et al.,

1976; Munch et al., 1996; Beline et al., 1999; Bernet et al., 2000;

Ni et al., 2000). Therefore, it can be inferred that aeration

process could enhance the stability of aerobically treated

manure by removing nitrogen, thereby reducing odour

emission.

Odour may be assessed by olfactometry evaluation (scored

by a group of panelists) (Lim et al., 2003; Zhang et al., 1997;

Burton et al., 1998). However, when the odour offensiveness

test is accepted as a measurement of manure odour, its

frequent use for routine monitoring of the performance of

treatment systems is normally too expensive, because of the

panelists’ time involved. Although a variety of compounds in

swine manure are responsible for producing malodour (such

as alcohols, carbonyl compounds, sulfur-containing com-

pounds, fatty acids, aromatic carboxylic acids, phenols and

indoles etc.), VFA has been found to be a suitable indicator of

manure odours (Williams, 1984; Zhu et al., 1999). Uses of VFA

were numerously reported to evaluate the effectiveness of

techniques for swine manure odour reduction. Some of the

typical studies include continuous aerobic treatment at pilot-

scale (Sneath et al., 1992) or farm scale aeration plants

(Burton and Sneath, 1995), aerated ponds (Westerman and

Bicudo, 2002), a laboratory study of surface aeration for

anaerobic lagoons (Zhang et al, 1997), and anaerobic sequen-

cing batch reactors (Zhang et al., 2000). Research showed that

VFA was formed mainly from the solids in the smallest

particle size in manure by microbial degradation (Yasuhara et

al., 1984; Zhu et al., 2001). Also, the biochemical oxygen

demand (BOD5) concentrations correlated well with VFA in

the manure (Zhu et al., 2001). Since aeration treatment

enhances the decomposition of organic compounds in

manure, evaluating the effectiveness of odour control by

specially designed aeration treatments based on the changes

of solids and BOD5 in the treated manure should be

considered appropriate.

The aerobic–thermophilic treatment through semi-contin-

uous feeding with a 6-day hydraulic retention time was

reported for stabilization of liquid manure (Burton, 1997).

Mean residence times of between 1.7 and 6.3 days under the

target aeration level, Eh of +91 to +191 mV, could maintain 28

days of subsequent anaerobic storage without offensive

odour in the treated pig slurry (TS ranging from 1.9% to

3.2%) (Burton et al., 1998). However, high operation cost due

mainly to energy consumption could impair farmers’ en-

thusiasm. Post-aeration storage is a process in which micro-

organisms are used under anaerobic conditions to convert

biodegradable organic materials to odourless gases, such as

methane and carbon dioxide, and nonbiodegradable solids

(Zhu, 2000; Williams et al., 1984). It thus implies that

the manure that has been biodegraded to some extent by

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WA T E R R E S E A R C H 4 0 ( 2 0 0 6 ) 1 6 2 – 1 7 4164

short-term aeration could continue its decomposition during

the post-aeration storage. Little information is available in

relation to the length of short-term aeration that affects

manure stabilization in the subsequent storage.

In this paper, a bench-scale study was conducted to reveal

the effect of short-term aeration treatment on manure

stabilization and reduction in odour generation potential

based on the analysis of characteristics of organic materials,

nitrogen, and VFA during the 90-day post-treatment storage.

The experiments consisted of 0.5-, 4.0-, and 16.0-day aeration

schemes using batch reactors filled with finishing pig manure

at initial TS levels ranging from 0.5% to 4.0%. The manure

stabilization was evaluated by the changes in TVS, BOD5, total

Kjeldahl nitrogen (TKN) and VFA. Besides, VFA was also used

to evaluate the odour generation potential.

pH meter

DO meter

Stirrer

15.3 cm

Air flow meter

Air pump

Tygon tube

Air bubble stone

91.6 cm

Fig. 2 – Schematic of the aeration apparatus used in the

experiment.

2. Materials and methods

2.1. Manure sources

Two sources of fresh finishing manure were available for the

study in southern Minnesota. One source was located in

Waseca for trial 1, while the other in New Richland for trial 2.

Pigs at both sources were fed on a regular corn (71%) soybean

(25%) meal with other ingredients including tallow, mineral

and vitamin. The finishing facilities at both locations featured

a pull–plug manure handling system. Prior to the experiment,

the collected manure was screened using a sieve with 2 mm

openings to remove coarse materials. The characteristic of

the sieved liquid manure is shown in Table 1. In general, the

manure compositions for the two trials did not differ much

from each other. However, the pH in the manure for trial 2

was 0.5 unit higher than that for trial 1 while VFA in the

manure for trial 2 was half that in the manure for trial 1.

2.2. Experimental setup

The screened manure for both trials was categorized into four

solids levels, i.e., 0.5, 1.0, 2.0, and 4.0% by diluting the manure

with tap water. Relatively speaking, the nutrients and organic

matter concentrations (data not shown) in tap water were

considered negligible. Therefore, the TKN, VFA, and BOD5

concentrations in the diluted manure at the four TS levels

before experiment were assumed to be linearly proportional

to the initial concentrations in the screened manure.

The aeration apparatus simulating a batch reactor for both

trials was shown in Fig. 2. The columns were made of PVC

materials, 91.6 cm in height and 15.3 cm in internal diameter.

Table 1 – Characteristics of the screened finishing manure for

Manure pH TS TSS TVS TVS/TS

TKN NH4–N

(%) (%) (%) (mgN/L)

(mg NL)

Trial 1 7.26 4.06 3.28 2.60 0.64 6429 5012

Trial 2 7.74 4.50 3.45 2.84 0.63 5325 4426

Manure in each column was 76.3 cm deep with the top

15.3 cm as a headspace. For trial 1, two aeration lengths were

used to treat the manure, i.e., 0.5 and 4.0 days. The

experiment was thus composed of a 4�2 factorial design,

and a total of 16 reactors for the batch were built to

accommodate duplication. Trial 1 started from May 23 to

August 21. The columns in the experiment were housed

under ambient temperatures ranging from 15 1C to 32 1C

during both aeration and post-aeration storage. For trial 2,

the aeration length was set at 16 days. Totally four columns

were used for four solids levels without duplication under a

constant room temperature of 17 1C. Aeration for both trials

was realized by pumping air through columns from the

bottom using an air pump (Catalog No. 13-875-220, Fisher

Scientific) at an airflow rate of 1.2 L/s/m3. For trial 2, the

dissolved oxygen concentration in the manure was measured

during aeration using an oxygen meter (Extech, Model

407510). After the treatment, the manure was left in the

original columns up to 90 days to simulate the long-term

post-aeration storage before land application.

2.3. Sampling and analysis

Two well-mixed samples from the sieved manure were

collected one day before the experiment, representing the

the two trials

BOD5 VFA VFA/TKN

VFA/BOD5

COD TP

/ (mg/L) (mg/L) (mg/L) (mg P/L)

20608 18397 2.86 0.89 48392 1581

19200 9570 1.80 0.50 32400 1603

ARTICLE IN PRESS

WAT ER R ES E A R C H 40 (2006) 162– 174 165

initial manure characteristics for the two individual trials.

During aeration and post-aeration storage, liquid manure

samples from each column were collected on day 5, 10, 20, 40,

60, and 90. All liquid samples, including the two initial

samples, were drawn from a location approximately half way

down from the liquid surface during mixing using a

motorized paddle-stirrer (Tline Laboratory Stirrer, Model

102). At the same time, the manure pH was measured by a

DIGI-SENSE digital pH/temperature/mV/ORP meter (catalog

number: 5938-10, Coleparmer Company, Vernon Hills, Illinois,

USA). The samples were stored at �20 1C immediately after

collection, and only thawed and allowed to reach room

temperature prior to analysis.

American Public and Health Association (APHA) Standard

methods were used to run the liquid manure sample analyses

for TS, TVS, BOD5, and TKN (APHA et al., 1998). According to

Hach (Hach Company, 1993), the VFA measurement is based on

esterification of the carboxylic acids present in the sample

followed by colorimetric determination of the esters produced

by the ferric hydroxamate reaction. The concentration of VFA

was measured at a wavelength of 495 nm by a DR/3000

spectrophotometer. All volatile acids are reported as their

equivalent mg/L acetic acid. The concentration of chemical

oxygen demand (COD) was measured according to the dichro-

mate reflux method presented by Hach (Hach Company, 1993).

The removal efficiency (R) for solids, BOD5, TKN and VFA

was determined with a presumption that the chemical and

biochemical compositions of the initial screened manure

(Table 1) remained unchanged throughout the duration of

experiment. Therefore, the percent removal efficiency was

calculated by dividing the difference between the initial and

measured concentrations of a concerned parameter during

storage by the initial concentration, and then multiplied by

100. A formula for this calculation is presented below:

Rð%Þ ¼Cin � Ct

Cin� 100%,

0

2

4

6

8

10

12

14

0 4

Aeratio

DO

(m

g/L

)

0.50% 1.00% 2.0

Valley for 2.0%

Vall

Valley for 1.0%

Valley for 0.5%

Fig. 3 – DO variations in the manure

where R is the removal efficiency (%), Cin is the initial

concentration, Ct is the concentration at time t during

storage.

3. Results and discussion

3.1. Foaming observation

Foaming normally results when airflow rates exceed 2.8 L/s/

m3 for manure aeration treatment (Burton, 1997). In this

study, two measures were adopted to prevent foaming. First,

the airflow rate was controlled at 1.2 L/s/m3, which was less

than half of that prescribed by Burton (1997). Second, small

doses of vegetable oil were periodically sprayed (totally 1.2 g/

column) onto the liquid surface to reduce surface tension, a

cause for producing foaming. However, despite these pre-

ventive techniques, liquid loss in some columns due to

foaming (also might be caused by water evaporation) was

still observed during the aeration process. The maximum

liquid losses from a single column, which were recorded

immediately after aeration, in the three aeration lengths, i.e.,

0.5, 4.0, and 16.0 day, were 110, 386, and 827 mL, respectively,

accounting for 0.78%, 2.76%, and 5.91% of the initial liquid

volume. Losses of liquid volume at these levels, although not

estimated, may not be considered substantial in affecting

data analysis for the collected samples.

3.2. Changes of dissolved oxygen and pH

The dissolved oxygen (DO) concentrations in all columns with

TS levels ranging from 0.5% to 4.0% in trial 2 were shown in

Fig. 3. Within a few hours of aeration, the DO levels in all

columns were sharply increased to around 8–11 mg/L (as O2),

approaching or exceeding the saturation concentration. Dur-

ing the 16-day aeration, four individual DO valleys occurred

approximately on day 1.3 (8.6 mg/L), day 2 (4.6 mg/L), day 3

8 12 16

n time (d)

0% 4.00%

ey for 4.0%

treated with 16.0-day aeration.

ARTICLE IN PRESS

WA T E R R E S E A R C H 4 0 ( 2 0 0 6 ) 1 6 2 – 1 7 4166

(0.2 mg/L), and between day 6 (1.2 mg/L) and 12 (0.6 mg/L) for

manure with initial TS content of 0.5%, 1.0%, 2.0%, and 4.0%,

respectively. It appeared that the higher the solids content,

the longer the time was needed to reach DO valleys featuring

lower DO values. This could be due to the abundance of

nutrients in manure of high solids content with which both

aerobic population and activity were enhanced by aeration,

leading to rapid depletion of oxygen available in the solution

(Loynachan et al., 1976). Also, it was reported that higher

organic solids content in the wastewater generally would

impede oxygen transfer (Westerman and Zhang, 1997),

leading to low DO in the liquid. These two factors may

explain the observed low values of DO valley in the manure of

high solids content.

Also in trial 2, aeration sharply increased the manure pH by

more than 1.0 unit, from about 7.7–9.1, within the first day

(Fig. 4). In the next 7 days, it appeared that the level of pH

increase was somewhat related to manure solids content,

with high pH increase corresponding to high TS content.

According to past research, the substantial increase in pH

during aeration is mainly due to CO2 stripping, in which the

bicarbonate equilibrium is shifted towards the right side of

the equation (HCO3�+H+2H2CO32H2O+CO2) (Gernaey et al.,

2002; Pratt et al., 2003). Since any molecule of CO2 stripped

from liquid consumes a proton, a quick rise in pH will occur

as a result. During post-aeration storage, the manure pH was

basically maintained at a range from 8.6 to 9.2, except for day

90. And the pH values were clearly in a descending order with

respect to the manure TS levels of 4.0%, 2.0%41.0% 40.5%.

For trial 1, aeration schemes of 0.5 and 4.0 days remarkably

increased pH in manure in post-aeration storage (Table 1 and

Fig. 5), during which longer aeration time and higher initial

solids content appeared to result in higher pH values.

Researchers have found that odour can be reduced in varying

degrees when manure pH is raised to a range from 8 to 11

7.0

7.5

8.0

8.5

9.0

9.5

0 25

pH

7.5

8.0

8.5

9.0

9.5

0 4 8

pH

Aeration

15

Aeration tim

Storag

Fig. 4 – pH variations during manure storage

(Vincini et al., 1994; Bundy and Greene, 1995). One major

reason that the raised pH may reduce odour is that it inhibits

the growth of odour-causing bacteria indigenous to swine

manure (Zhu, 2000). Obviously, the effect of increasing pH

caused by the three aeration schemes studied should be

beneficial to odour reduction.

3.3. Characteristics of organic material

3.3.1. Change of solids compositionVariations of TVS levels in the manure treated by the three

aeration schemes are shown in Fig. 6. Compared to the levels

in the initial manure, only slight decreases in TVS were seen

for the four different TS levels under the 0.5-day aeration

scheme during storage. In trial 1, the columns were placed

indoors with uncontrolled ambient temperature, which

increased from May to August (Fig. 5). Since rising tempera-

ture is usually conducive to the enhancement of microbial

metabolic processes in decomposing organic materials (Zhu,

2000), the nearly unchanged TVS levels in all columns thus

imply that the 0.5-day aeration at the current intensity is

insufficient for manure stabilization in term of TVS biode-

gradation. For the 4.0-day aeration, a slight reduction in TVS

from 4% to 28% was observed shortly after aeration was

stopped and the biodegradation continued during storage for

manure with up to 2% solids content. In trial 2 with constant

temperature of 17 1C and 16.0-day aeration, the TVS in the

columns with initially different TS concentrations were

gradually broken down and a reduction from 22% to 42%

was achieved on day 20, but the trend leveled off thereafter

and approached a ‘steady-state’ during post-aeration storage.

There exists a common feature for both trials (Fig. 6) that

TVS reductions for manure with low TS content appear to be

more significant than those of high TS content. Therefore,

given the identical aeration intensity, diluted manure (or

50 75 100

0.50%

1.00%

2.00%

4.00%

12 16

e (d)

e time (d)

under the treatment of 16.0-day aeration.

ARTICLE IN PRESS

7.5

8.0

8.5

9.0

0 20 40 60 80 100

pH

0

5

10

15

20

25

30

35

0.50% 1.00%

2.00% 4.00%

7.5

8.0

8.5

9.0

9.5

0 20 40 60 80 100

pH

0

5

10

15

20

25

30

35

0.50% 1.00%

2.00% 4.00%

Storage time (d)

Storage time (d)

Tem

pera

ture

(°C

)T

empe

ratu

re (

°C)

(a)

(b)

Fig. 5 – Variations of ambient temperature and pH during manure storage under the treatments of (a) 0.5-day aeration and (b)

4.0-day aeration.

WAT ER R ES E A R C H 40 (2006) 162– 174 167

solids removed manure) offers an advantageous condition for

TVS biodegradation.

3.3.2. Change of BOD5

For the 0.5-day aeration, the BOD5 in manure with TS from

0.5% to 2.0% decreased rapidly after day 10 and became

leveled after day 40 (Fig. 7(a)). This could be due to the

increasing temperature since day 20 (Fig. 5), leading to

increase in activities of microorganisms that caused reduc-

tions in BOD5 (Burton, 1992). For 4.0% TS, the reduction in

BOD5 during the storage period was also observed, but at a

much slower pace. For the 4.0-day aeration, reductions of

BOD5 from 17% to 54% were observed on day 5 for columns

with various solids levels (Fig. 7(b)). However, in the rest of the

storage period, only a minor decrease of BOD5 over time was

found for the columns with 4.0% TS, as compared to the

others showing progressive reductions. Interestingly, the

trend of BOD5 reduction during post-aeration storage for the

0.5-day aeration was more apparent than that for the 4.0-day

aeration. This infers that natural biodegradation may con-

tribute more to BOD5 reduction than aeration in the case of

0.5-day aeration. At the end of 90-day storage in trial 1, BOD5

removals from 20% to 63% and 27% to 77% were achieved for

the 0.5- and 4.0-day aeration, respectively. In trial 2 with 16-

day aeration, the BOD5 concentrations in manure decreased

markedly for all solids levels before day 20 and became nearly

unchanged thereafter (Fig. 7(c)). Approximately 75–90% of

BOD5 was biodegraded at the end of 90-day storage. Similar to

removals of TVS, for both trials, high levels of initial TS were

found to be associated with low BOD5 removals during

storage.

The TVS in manure generally represents total organic

matter while BOD5 is referred to the biological portion of

organics that is utilized by microbial production and meta-

bolism. As described in Fig. 1, the biodegradable soluble

organic matter is converted into carbon dioxide, water and

active biomass through the action of heterotrophic bacteria,

meanwhile the active biomass undergoes decay resulting in

the generation of additional carbon dioxide and water along

with inactive biomass (Grady et al, 1999). Therefore, the

effectiveness of aeration to reduce both TVS and BOD5 is

expected to be enhanced when aeration length is increased.

ARTICLE IN PRESS

(c)

(b)

(a)

Storage time (d)

Storage time (d)

Storage time (d)

4.0%2.0%1.0%0.5%

900 600 40 25 1

3.0

2.5

2.0

1.5

1.0

0.5

0.0

00 90 60 40 25 1

3.0

2.5

2.0

1.5

1.0

0.5

0.0

TV

S le

vels

(%

)T

VS

leve

ls (

%)

TV

S le

vels

(%

)

00 90 60 40 25 1

3.0

2.5

2.0

1.5

1.0

0.5

0.0

Fig. 6 – Variations of TVS levels during manure storage under the aeration schemes: (a) 0.5-day aeration, (b) 4.0-day aeration,

and (c) 16.0-day aeration. Bars are standard errors.

WA T E R R E S E A R C H 4 0 ( 2 0 0 6 ) 1 6 2 – 1 7 4168

However, the data from this study shows that the reductions

in manure BOD5 (Fig. 7) by the aeration process are more

pronounced than those of TVS (Fig. 6). In addition, the

increasing ambient temperature appears to be able to

improve organic matter biodegradation even with short-term

aeration (e.g., the 0.5-day aeration used in this study). For the

aerobically treated pig manure, the removal of organic

materiel that can lead to the regeneration of odourous

compounds will enable a period of storage without the return

of offensive odour (Burton et al., 1998). Williams (1984) also

pointed out that BOD5 reduction would be helpful to reduce

odour offensiveness from manure. Therefore, the removal of

organics, particularly BOD5 in the aerated manure in this

study, can mitigate the potential of odour generation and

maintain manure stabilization during post-aeration storage.

Nonetheless, it was found that high solids content would

militate against oxygen transfer (Fig. 3), which could impair

the microbial performance on breakdown of organics. Based

on the data from the two trials, manure with low solids

content would produce advantageous conditions for biode-

gradation of TVS and BOD5 during the aeration process, and

the post-aeration storage as well.

ARTICLE IN PRESS

0

5000

10000

15000

20000

25000

5 10 20 40 60 90

BO

D5

leve

ls (

mg/

L)

BO

D5

leve

ls (

mg/

L)

BO

D5

leve

ls (

mg/

L)

4000

8000

12000

16000

20000

24000

5 10 20 40 60 90

0

3000

6000

9000

12000

5 10 20 40 60 90

Storage time (d)

0.5% 1.0% 2.0% 4.0%

0

Storage time (d)

Storage time (d)(a)

(b)

(c)

Fig. 7 – Variations of BOD5 levels during manure storage under the aeration schemes: (a) 0.5-day aeration, (b) 4.0-day aeration,

and (c) 16.0-day aeration. Bars are standard errors.

WAT ER R ES E A R C H 40 (2006) 162– 174 169

3.3.3. Characteristics of TKNFigure 8 shows the results of TKN changes due to the aeration

treatments for manure with different solids levels and the

effects of time on TKN changes during the storage. It was

obvious that the TKN levels in manure treated with three

different aeration schemes decreased with the storage time.

The TKN removals in the manure treated with 16.0-day

aeration were commonly higher than those in manure treated

with 4.0-day aeration. At the end of the experiment, the

removals of TKN reached from 17% to 22%, 19% to 33%, and

26% to 38% for aeration lengths of 0.5-, 4.0-, and 16.0-day,

respectively. Since land disposal of manure contributes

excessive nitrogen to soil that could be easily washed out

into water (Stone et al., 1998; Yang and Wang, 1999), the

aeration treatments reported herein, especially the 16.0-day

aeration, can reduce the N loading to land, thereby potentially

minimizing water pollution due to N losses from manure

applied to soil.

The removal of TKN can be attributed to three gaseous N

emissions. First, continuous batch aeration shows that the

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0

1000

2000

3000

4000

5000

6000

7000

5 10 20 40 60 90

0

1000

2000

3000

4000

5000

6000

7000

5 10 20 40 60 90

0

1000

2000

3000

4000

5000

5 10 20 40 60 90

0.5% 1.0% 2.0% 4.0%

Storage time (d)

Storage time (d)

Storage time (d)

TK

N le

vels

(m

g/L

)T

KN

leve

ls (

mg/

L)

TK

N le

vels

(m

g/L

)

(a)

(b)

(c)

Fig. 8 – Variations of TKN levels and TKN removal percentages during manure storage under the aeration schemes: (a) 0.5-day

aeration, (b) 4.0-day aeration, and (c) 16.0-day aeration. Bars are standard errors.

WA T E R R E S E A R C H 4 0 ( 2 0 0 6 ) 1 6 2 – 1 7 4170

percentage of ammonia emission to total N could reach

16–26% by 12 weeks aeration (Loynachan et al., 1976), and

6–10% by 16 days aeration (Dewes, 1999). Over 112 days of

storage period, ammonia emissions may increase linearly by

39.3 mg/m2.d from aging swine manure (3.7% TS, 15–20 1C)

(Hobbs et al., 1999). Moreover, the rate of ammonia desorption

from solution increases as pH rises (Hashimoto, 1974).

Although no data were collected to quantity the ammonia

stripping in this study, ammonia emission could be one of the

essential factors for the reduction of TKN over time during

the period of aeration as well as the post-aeration storage.

This postulate is supported by the ratio of initial concentra-

tions of ammonium to TKN (over 78% of TKN) and the

increased pH in the manure (Figs. 3 and 4), along with the VFA

disappearance during aeration (Fig. 9). Increasing aeration

length apparently resulted in increased removal of TKN in the

manure, as shown by the data from the three aeration

schemes studied. In addition, past research indicated that

ammonia release rates from pig manure could increase

significantly when ambient temperature was increased (Ni

et al., 2000; Dewes, 1999; Harper et al., 2000). Therefore, it is

reasonable to assume that another factor for the decrease of

TKN concentrations over time for the two aeration schemes

in trial 1 is due to the increase of ambient temperature from

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8000

12000

16000

20000

5 10 20 40 60 90

VFA

s le

vels

(m

g/L

)V

FAs

leve

ls (

mg/

L)

VFA

s le

vels

(m

g/L

)

0

4000

8000

12000

16000

20000

5 10 20 40 60 90

0

1000

2000

3000

4000

5000

5 10 20 40 60 90

0.5% 1.0% 2.0% 4.0%

Storage time (d)

Storage time (d)

Storage time (d)

(a)

(b)

(c)

Fig. 9 – Variations of VFA levels during manure storage under the aeration schemes: (a) 0.5-day aeration, (b) 4.0-day aeration,

and (c) 16.0-day aeration. Bars are standard errors.

WAT ER R ES E A R C H 40 (2006) 162– 174 171

spring to summer during the experiment. Second, Beline et al.

(1999) reported that 7–31% of the total N of the raw swine

manure could be lost as nitrous oxide (N2O) in the headspace

gas during 4–6 days of aeration, which implied that some of

the TKN in the manure could be released into atmosphere as

N2O in this experiment. Finally, nitrogen gas loss from

wastewater by denitrification normally occurs in the

anoxic stage after nitrification by aeration (Munch et al.,

1996; Bernet et al., 2000; Westerman and Bicudo, 2002). The

total nitrogen losses in the aerated manure sampled on day 5

in trial 1 and on day 20 in trial 2 should therefore include a

portion of nitrogen gas emission caused by nitrification–de-

nitrification.

3.4. The odour generation potential (VFA)

The changes in VFA concentrations in manure over the 90-

day storage are shown in Fig. 9. Under 0.5-day aeration, a

slightly progressive breakdown on VFA was found in manure

with initial TS content less than 4.0%; however, under 4.0-day

aeration, the VFA levels in the same solids categories

fluctuated (Fig. 9(b)). Huge fluctuations of VFA in manure

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WA T E R R E S E A R C H 4 0 ( 2 0 0 6 ) 1 6 2 – 1 7 4172

with 4.0% TS were found under both aeration schemes in trial

1, particularly a surge on day 60. Under 16.0-day aeration with

fixed temperature, the VFA levels in all columns, regardless of

solids content, decreased rapidly until day 20, and started to

increase thereafter due possibly to VFA regeneration. VFA

reductions from 40% to 86%, 30% to 90%, and 45% to 92% were

found at the end of 90-day storage under aeration schemes of

0.5, 4.0, and 16.0 days, respectively. Again, the VFA removal

rates are related to the TS level in the manure in the order

from high to low of 0.5%41.0%42.0%44.0% under all

aeration schemes.

y = 4431.2x - 1r = 0.8416

0

5000

10000

15000

20000

0.0 0.5 1.0

y = 0.6167x - 305.31r = 0.9005

0

5000

10000

15000

20000

0 5000 10000

y = 2.1714x - 1794.8r = 0.8504

0

5000

10000

15000

20000

0 1000 2000 3000

VF

As

leve

ls (m

g/L

)V

FA

s le

vels

(mg/

L)

VF

As

leve

ls (m

g/L

)

BOD

TVS

TKN l

Fig. 10 – The overall linear relationships for VFA concentrations

lumped data from all three aeration schemes.

The volatile fatty acids are the intermediate products

generated during the microbial decomposition of manure

(Zhang et al., 1997). The key to preventing odour generation is

that the production of acids by the indigenous bacteria and

the consumption of acids by the methanogens to produce

methane and carbon dioxide have to be in equilibrium (Zhu,

2000). Since there are abundant substrates in manure with

4.0% TS, such equilibrium is probably difficult to be main-

tained under increasing ambient temperature in trial 1. From

day 5–40, the biodegradation of VFA might exceed the VFA

regeneration, which was normally a result of the enhanced

436.7

1.5 2.0 2.5 3.0

15000 20000 25000

4000 5000 6000 7000

5 levels (%)

levels (%)

evels (mg/L)

versus TVS, TKN, and BOD5 during 90 days of storage with

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WAT ER R ES E A R C H 40 (2006) 162– 174 173

activity of microorganisms due to gradual increase in

temperature. But that balance appeared to be reversed when

the ambient temperature continued to increase, leading to a

net accumulation of VFA around day 60. For the 16.0-day

aeration treatment in trial 2, 85–97% of VFA had already been

biodegraded on day 10. However, since day 20 (shortly after

aeration was completed), the VFA regeneration through

biodegradation of organic matter started to exceed the VFA

consumption under the constant temperature of 17 1C, which

simultaneously resulted in a pH decrease (Fig. 4).

In comparison, the VFA removals in manure under 16.0-day

aeration were generally higher than the other two aeration

schemes (0.5- and 4.0-day). As discussed early, a trend of

growing removal efficiencies on TVS, BOD5, TKN, and VFA

over the entire storage was found when aeration length was

increased. However, the relatively small increases in these

removals may not be worth the additional cost of operating

aeration for 16 days. According to this study, the 4.0-day

aeration scheme might be sufficient to stabilize manure to

effectively reign over the odour generation potential during

post-aeration storage.

3.5. Odour generation potential evaluated by TVS, BOD5

and TKN

Regression analyses between VFA and TVS, BOD5, and TKN

will be helpful to elucidate the stabilization of aerated

manure during storage as well as the strategy for odour

control. The correlation coefficients for VFA with TVS, BOD5,

and TKN throughout the 90 days of storage are presented

in Fig. 10.

Linear regression equations for VFA versus TVS, BOD5, and

TKN in manure treated by three different aeration schemes

proved highly significant with correlation coefficients, r, of

0.8416, 0.9005, and 0.8504, respectively. Thus, TVS, BOD5 and

TKN could be used to predict the odour generation potential

for the aerated manure in storage. Comparing the correlation

coefficients indicates that BOD5 is the best estimate of VFA

concentration in manure. Pervious studies demonstrated

similar linear correlations between BOD5 and VFA for swine

manure with no aeration treatment (Zhu et al., 2001), and for

manure subject to aerobic treatment (Williams, 1984). Re-

search also showed that manure VFA generation was relevant

to the portion of solids mainly in the form of TVS by microbial

degradation (Yasuhara et al., 1984; Zhu et al., 2001). Therefore,

although different characteristics on the changes of TVS,

BOD5, TKN and VFA over time are observed in manure

stabilization, the odour generation potential in the aerated

manure may still be linearly evaluated by the concentrations

of TVS, BOD5 or TKN in the post-aeration storage, with BOD5

being the best estimate.

4. Conclusions

Swine manure stabilization was experimentally investigated

in bench-scale bioreactors using three batch aeration

schemes in two trials. The decomposition of organics,

nitrogen and volatile fatty acids increased with increasing

aeration time. Low solids level in manure offered a more

advantageous environment for manure stabilization, and

thus odour control. Maximum liquid losses due to foaming

were 110, 386, and 827 mL for the 0.5-, 4.0-, and 16.0-day

aeration schemes at an airflow rate of 1.2 L/s/m3, respectively,

accounting for 0.78%, 2.76%, and 5.91% of the initial liquid

volume. Increasing ambient temperature would enhance

biodegradation of organics, removal of nitrogen and break-

down of VFA. Although the VFA removals in manure under

16.0-day aeration were generally higher than those of 0.5- and

4.0-day aeration, the VFA regeneration started to exceed its

consumption on day 20. The 4.0-day aeration scheme was

sufficient to stabilize manure to effectively assuage odour

generation potential during the 90-day storage under increas-

ing ambient temperature. Among parameters of TVS, BOD5

and TKN, BOD5 was the best estimate of VFA concentration in

the aerated manure.

Acknowledgements

We express our appreciation to the Minnesota Legislature

Rapid Agricultural Response Fund for funding this project.

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