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Effects of Duration and Intensity of Aeration on Solids Decomposition in Pig Slurry for Odour Control
ARTICLE IN PRESS Biosystems Engineering (2004) 89 (4), 445–456 doi:10.1016/j.biosystemseng.2004.08.009 SE—Structures and Environment
Effects of Duration and Intensity of Aeration on Solids Decomposition in Pig Slurry for Odour Control ZhiJian Zhang1; Jun Zhu1; Keum J. Park2 1
Southern Research and Outreach Center, University of Minnesota, 35838 120th Street, Waseca, MN 56093-4521, USA; e-mail of corresponding author: [email protected] 2 Department of Biomachinery Engineering, Sunchon National University, Sunchon, Republic of Korea (Received 9 January 2004; accepted in revised form 9 August 2004; published online 22 October 2004)
A 30 day laboratory scale experiment was carried out using three different aeration rates, i.e. +35 mV oxidation–reduction potential (ORP: standard hydrogen electrode), 10, and 30 mg[O2] l1, to achieve solids decomposition and odour control in pig slurry with total solids (TS) levels from 05 to 40%. Changes during the aeration process were characterised by TS, total volatile solids (TVS), total suspended solids (TSS), and total volatile suspended solids (TVSS), all expressed as w/w. The measurement of volatile fatty acids (VFAs), expressed as w/v, was used to evaluate the potential odour generation in the aerated slurry. The TS removal efficiencies from 60 to 449, 186 to 503, and 423 to 564% were observed for the three different aeration levels, with reductions from 66 to 489, 260 to 611, and 573 to 699% for the slurry TVS. The ratios of TVS/ TS, TSS/TS, and TVSS/TS in slurry over the experimental duration were found to increase with increased TS levels in the 30 day aeration under the three aeration intensities. Reductions in the 5 day biological oxygen demand (BOD5) reached 785–920, 794–960, and 912–970%, while reductions of VFAs reached 127–990, 717–990%, and 878–993%. The biodegradation of solids, BOD5, and VFAs was effectively enhanced when aeration time and intensity were increased. A low level of solids in slurry promoted aerobic decompositions of solids, BOD5, and VFAs. Changes of state in the solids being aerated and changes in the BOD5 levels can be used to distinctly characterise the potential of odour generation from the slurry. Batch aeration of 5–10 days under intensities of 10 to 30 mg[O2] l1 is recommended for odour control at farm level. r 2004 Silsoe Research Institute. All rights reserved Published by Elsevier Ltd
1. Introduction Public complaints due to odour emission are a major concern of pig producers. Open slurry storages, such as lagoons and earthen basins, produce odorous gases and compounds due to anaerobic microbial decomposition of pig slurry. In an observational study, Gay et al. (2003) found that odour emission in odour units (OU) from pig slurry storage units reached anywhere from 10 to 114 OU s1 m2. It is well known that solids and organic materials in pig slurry are the first and foremost odour-producing matter under anaerobic microbial activity (Burton, 1992; Westerman & Zhang, 1997; Zhu, 2000). Zhang and Westerman (1997) reviewed the characteristic of the distribution of solids in pig slurry, and found 1537-5110/$30.00
that the total solids (TS) content could be partitioned between suspended solids (SS) and dissolved solids (DS). Each fraction (TS, SS, or DS) can be further divided between volatile solids (VS) and fixed solids (FS). The corresponding volatile fractions of TS, SS, and DS are total volatile solids (TVS), volatile suspended solids (VSS), and volatile dissolved solids (VDS). The VS fraction generally represents the organic component of the total solids, which is more biodegradable than FS, which is mainly associated with the coarse material in the slurry and effectively inert in the typical aerobic treatment process (Burton, 1992). Understanding of distribution of slurry solids presents important information for design and selection of proper solid–liquid equipment for odour control (Zhang & Westerman, 1997). However, the separated liquid is still dark 445
r 2004 Silsoe Research Institute. All rights reserved Published by Elsevier Ltd
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in colour and odour generation depends largely on the amount of odour-producing organic substances remaining in the liquid, which may require additional treatment to ensure an odour-free storage (Chastain et al., 2001). Generally, insoluble organic substances or organic particulates in various wastewaters can be converted into soluble organic matter through hydrolysis, which depends on extracellular enzymes secreted by endogenic microbes (Grady et al., 1999). Some of the soluble organic matter is consumed by microbes to support their own metabolism, while the death, and the subsequent decay of microorganisms, increases insoluble organic materials. The convertible organic matter is transformed into methane, carbon dioxide, water, and other odourless products under the restrictive and controlled anaerobic digestion system (Chynoweth et al., 1999; Magbanua et al., 2001; Kataoka et al., 2002). A variety of odorous compounds, however, such as volatile fatty acids (VFAs), aldehydes, alcohols, volatile amines, mercaptans, indoles, and skatols, classified as the intermediate products of anaerobic metabolism, are produced abundantly under the uncontrolled anaerobic condition (Westerman & Zhang, 1997; Hobbs et al., 1998; Zhu, 2000). When the oxygen supply to wastewaters is maintained, organic decomposition is achieved by the activity of aerobic microorganisms, with no apparent accumulation of odorous compounds (Zhang et al., 1997; Yang & Wang, 1999; Zhu, 2000). Moreover, both the growth in the population of aerobic microorganisms and the subsequent biodegradation rate of organic matter are accelerated rapidly in the aerobic system compared to non-controlled anaerobic systems (Burton, 1992). Whilst previous studies have found that the concentration of solids in slurry can be used as an indicator of organic particulates hydrolysis, microorganism metabolism, aerobic decomposition of soluble organic substance, and biomass production when aeration technique is adopted, few have reported on the role of solids in odour control during the aeration process, particularly in relation to different solids fractions. As a matter of fact, the total amount of solids and fractions, such as VS, SS and DS, changes due to particulate hydrolysis, microorganism activity (assimilation and decay), and breakdown of soluble organic matter during the aerobic process. Aeration offers an effective way of treating animal slurry to achieve solids decomposition and odour control, but the relatively high cost of the required energy has hampered the broad application of this method. Ample research has been conducted in both improving the efficiency of aeration (Cumby, 1987; Yang & Wang, 1999) and reducing the duration and intensity of aeration (Phillips & Bulley, 1980; Zhang
et al., 1997). A level of 1–2 mg[O2] l1 maintained in the wastewater ensures adequate oxygen transfer into the biomass solids and provides mixing, so that the aerobic environment can be kept consistent throughout the treatment vessel (Wang & Pereira, 1986). A study by Williams et al. (1989) showed that odour offensiveness could be controlled if the oxidation– reduction potential (ORP) was maintained at above +13 mV (expressed as the value of standard hydrogen electrode, Eh) in slurry. If the ORP was controlled at +133 mV, the odour intensity ranged between ‘very faint’ and ‘faint’ (Burton et al., 1998). Another study showed that once the ORP in the liquid exceeded +100 mV (Eh), the liquid environment was said to be aerobic with detectable dissolved oxygen present and prevalent aerobic respiration (Charpentier et al., 1989). In a previous study of bacterial counts corresponding to the ORP, it was suggested that, for odour control, the slurry ORP should be maintained at +35 mV (Eh) or higher to effectively support aerobic growth and simultaneously hinder anaerobic activity (Zhu et al., 2002). To date, the characteristics of solids decomposition under different aeration durations and intensities for slurry odour control using batch operation is not fully understood. Since complete stabilisation of livestock slurry by aerobic treatment is normally not economically justifiable, low levels of aeration have been recommended for partial odour control (Westerman & Zhang, 1997). In this study, a 30 day, laboratory scale batch experiment with three different aeration levels, i.e. +35 mV[ORP] (Eh), 10 mg[O2] l1, and 30 mg[O2] l1, was designed to treat pig slurry with TS concentrations from 05 to 40% in order to shed light on odour control and aerobic decomposition of solids. The changing characteristics of solids during the aeration process were evaluated by TS, TVS, total suspended solids (TSS), and total volatile suspended solids (TVSS). The values of the 5 day biochemical oxygen demand (BOD5) in the slurries were measured to show the extent of organic matter decomposition and to indicate the efficiency of the various aeration regimes. Although the measurement and quantification of odour have been widely debated over the development of strategies toward odour issues, VFAs are considered appropriate in evaluating the potential odour generation in slurry (Williams, 1984; Burton, 1992). The level of VFAs was often reported to quantify pig slurry odour reduction (Burton et al., 1998; Zhang et al., 1997, 2000; Zhu et al., 2001). The potential odour generation during the course of aerobic decomposition is also investigated by using VFAs in this study in order to develop strategies for odour control.
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Table 1 Characteristics of the screened raw finishing pig slurry for experiment start-up
2. Materials and methods 2.1. Slurry collection and experimental procedure
Items
Raw slurry for this study was collected from a reception pit of a finishing barn at the University of Minnesota Southern Research and Outreach Center, where fresh slurry in a shallow pit inside the barn was flushed out weekly. The pigs at the sampling site were fed with corn and soyabean. Prior to loading the batch reactors (Fig. 1), the collected slurry was passed through a sieve with 2 mm openings. Four sub-samples of the screened slurry were then diluted with tap water to achieve respective TS concentrations of 05, 10, 20, and 40%. The characteristics of the sieved liquid slurry before dilution are shown in Table 1. The prepared liquid slurry was concurrently placed in 12 columns, made of PVC materials, 916 mm in height and 153 mm in internal diameter, simulating batch reactors (Fig. 1). The slurry level in each column was 763 mm, giving a 153 mm headspace. Air was pumped through a bubble stone located at the bottom of each column. Four aeration durations (5, 10, 20, and 30 day) and three aeration levels (+35 mV[ORP], 10, and 30 mg[O2] l1) were tested. The aeration intensities were maintained by controlling the airflow rates according to the ORP and dissolved oxygen (DO) readings taken three times a day. The ORP was converted to standard hydrogen electrode Eh in mV by a formula E h ¼ E cal þ 241
(1)
pH Total solids (TS), % Total volatile solids (TVS), % Total suspended solids (TSS), % Total volatile suspended solids (TVSS), % Volatile fatty acids (VFAs), mg l1 Biochemical oxygen demand in 5 days (BOD5), mg l1 TVS/TS TSS/TS TVSS/TS
Initial For tests (based on content of TS, %) 05
10
20
40
62 425 341
61 051 041
62 100 080
61 201 162
62 402 322
348
042
082
164
329
071
141
281
3200
6413
297
035
6812
800
1600
21 800
2558
5119 10 249 20 508
0802 0819 0699
0807 0802 0806 0801 0822 0822 0818 0818 0699 0699 0699 0699
where Ecal in mV (calomel reference electrode) was measured by an ORP meter (DIGI-SENSE, Model 5938-10). The DO in the liquid was measured using an oxygen meter (Extech, Model 407510). The actual ORP or DO in the aerated slurries had fluctuated around the set values, which were recorded as +35725 mV[ORP], 10712, and 30721 mg[O2] l1. All columns were housed under constant room temperature at 17705 1C.
2.2. Sampling and analyses
Fig. 1. Schematic of the aeration apparatus used in the experiment: ORP, oxidation–reduction potential (expressed as standard hydrogen electrode, Eh in mV); DO, dissolved oxygen
After sieving, one mixed slurry sample was collected for analysis of the initial characteristics. During aeration, liquid slurry samples from all columns were collected on days 5, 10, 20, and 30. All slurry samples, including the initial sample, were drawn from the wellmixed slurry at approximately the mid-depth of each column (or container) during agitation using a motorised paddle-stirrer (Tline laboratory stirrer, Model 102). All the samples were stored at 20 1C immediately after collection, and thawed and allowed to reach room temperature only prior to analysis. American Public and Health Association Standard methods were used to run the liquid slurry sample analysis for TS, TVS, TSS, TVSS, and BOD5 (APHA et al., 1998). According to Hach (1993), the VFA measurement is based on esterification of the carboxylic acids present in the sample. Calorimetric determination of the esters follows a ferric hydroxamate reaction. The
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concentration of VFAs was measured at a wavelength of 495 nm using a DR/3000 spectrophotometer. All volatile acids are reported as their equivalent concentration of acetic acid. The removal efficiency, in terms of solids in different forms, BOD5, and VFAs, is calculated by dividing the difference between the treatment and the concurrent control by the concurrent control, and then multiplying by 100.
3. Results and discussion 3.1. Changes of total solids and total volatile solids Figure 2 presents the changes of TS levels in slurry treated by the 30 day aeration process. For the treatment under ORP of +35 mV, there is some decrease in TS levels for all slurry columns when aeration time increases from 5 to 20 days, compared to the control. However, the TS levels apparently decrease when the aeration process lasts 30 days, particularly for slurry with initial TS levels lower than 40%. With few exceptions, the TS levels in non-aerated columns decrease slightly (Fig. 2). Analysis of the TS removal efficiency reveals that 16–111% is removed by day 5 for columns initially filled with slurry with TS levels of 05–40%. As the aeration treatment approaches 30 days, the removal efficiencies reach 60–449%. When the aeration intensity is raised to 30 mg[O2] l1, the trend of decrease in TS levels and increase in TS removal efficiencies become pronounced as the aeration time increases (Fig. 2). TS removal efficiencies are enhanced from 84 to 305% by day 5 and between 423 and 564% by day 30. The effectiveness of TS removal under the aeration intensity of 10 mg[O2] l1 is found to be 17 to 42% by day 5 and 186–503% by day 30. The changes of TVS levels in slurry under the three aeration intensities are shown in Fig. 3. Similar to TS, the TVS levels in slurry columns are decreasing during the process of aeration. The overall TVS removal efficiencies under the aeration intensity of +35 mV[ORP] achieve 08–172% by day 5 and 66–489% by day 30. The TVS removal under 10 mg[O2] l1 increases from 29 to 218% by day 5 to 260–611% by day 30, while from 182 to 357% by day 5 to 573–699% by day 30 under 30 mg[O2] l1. 3.2. Changes of total suspended solids and total volatile suspended solids Figure 4 demonstrates the primary features of TSS change in the slurry during the aeration process under
Fig. 2. Variation of total solids (TS) in slurry treated by 30-day aeration under: (a) +35 mV oxidation–reduction potential (ORP); (b) 10 mg[O2] l1; (c) 30 mg[O2] l1. , Control, slurry with TS of 05%; ’, control, slurry with TS of 10%; ~, control, slurry with TS of 20%; m, control, slurry with TS of 40%; , aeration, slurry with TS of 05%; , aeration, slurry with TS of 10%; , aeration, slurry with TS of 20%; aeration, slurry with TS of 40%
the three different intensities. For example, for slurry with low solids content, such as TS at 05 and 10%, aerated at the intensity of 10 mg[O2] l1, the TSS levels in the briefly aerated slurry (5 day) are similar to those
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Fig. 3. Variation of total volatile solids (TVS) in slurry treated by 30-day aeration under: (a) +35 mV oxidation–reduction potential (ORP); (b) 10 mg[O2] l1; (c) 30 mg[O2] l1. , Control, slurry with TS of 05%; ’, control, slurry with TS of 10%; ~, control, slurry with TS of 20%; m, control, slurry with TS of 40%; , aeration, slurry with TS of 05%; , aeration, slurry with TS of 10%; , aeration, slurry with TS of 20%; , aeration, slurry with TS of 4.0%
of non-aerated slurry. After 5 days, the TSS levels decrease sharply. For slurry with high solids content (20 and 40%), little change is observed in TSS levels during the first 20 days of aeration, but a significant reduction occurs by day 30. Under +35 mV[ORP], apparent
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Fig. 4. Variation of total suspended solids (TSS) in slurry treated by 30-day aeration under: (a) +35 mV oxidation–reduction potential (ORP); (b) 10 mg[O2] l1; (c) 30 mg[O2] l1. , Control, slurry with TS of 05%; ’, control, slurry with TS of 10%; ~, control, slurry with TS of 20%; m, control, slurry with TS of 40%; , aeration, slurry with TS of 05%; , aeration, slurry with TS of 10%; , aeration, slurry with TS of 20%; , aeration, slurry with TS of 40%
decreases in TSS levels in the slurry are found only by day 30, regardless of the initial TS levels. With an aeration intensity of 30 mg[O2] l1, similar to TS and TVS, the TSS levels in all slurry columns decrease gradually with respect to the time of aeration. For the
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with different TS contents for aeration intensities of +35 mV[ORP], 10, and 30 mg[O2] l1, respectively. The changes of TVSS levels over time (Fig. 5) in slurry under the three aeration intensity levels are quite similar to those of TSS. A special feature is that, at all TS levels, TVSS decreases during aeration for all three aeration intensities. In the same manner, the TVSS are reduced from 73 to 568%, 272 to 582%, and 565 to 688% when aeration is run continuously for 30 days under conditions of +35 mV[ORP], 10, and 30 mg[O2] l1, respectively.
3.3. Characteristics of solids composition
Fig. 5. Variation of total volatile suspended solids (TVSS) in slurry treated by 30-day aeration under: (a) +35 mV oxidation–reduction potential (ORP); (b) 10 mg[O2] l1; (c) 30 mg[O2] l1. , Control, slurry with TS of 05%; ’, control, slurry with TS of 10%; ~, control, slurry with TS of 20%; m, control, slurry with TS of 40%. , aeration, slurry with TS of 05%; , aeration, slurry with TS of 10%; , aeration, slurry with TS of 20%; , aeration, slurry with TS of 40%
controls, the reasons that contribute to the low levels of TSS and TVSS by day 5 and day 10, respectively, in the slurry with initial TS of 20% cannot be explained. The TSS levels are reduced from 44 to 458%, 180 to 519%, and 390 to 572% by day 30 among the columns
The solids composition in slurry treated by the aeration process is characterised by the ratios of TVS/ TS, TSS/TS, and TVSS/TS (based on the concurrent TS values in each of the aerated samples), and the changes of these ratios may reveal the relative decomposition rates of various labile substances. For the control, the ratio of TVS/TS is found to increase slightly over time in slurry with different TS levels and to increase with the increase in the levels of TS under different aeration durations, but there are no remarkable trends for the changes of ratios of TSS/TS and TVSS/TS (Table 2). The ratios of TVS/TS, TSS/TS, and TVSS/TS in slurry are each found to increase with respect to the solids levels under the three aeration schemes within the 30 day duration. This indicates that the decompositions of TVS, TSS, and TVSS would be accelerated in slurry with low solids levels. High organic solids generally impede the oxygen transfer rate in water (Westerman & Zhang, 1997); thus, oxygen utilization should be efficient in the low-solids slurry because microorganisms are able to effectively assimilate organic compounds to support enhanced respiration. There is a decreasing trend on TVS/TS ratio in slurry with different solids sampled from the three different aeration intensities when the aeration time increases from 5 days to 30 days (Table 2). In other words, the relative rate of TVS breakdown gradually increases as the time of aeration increases, compared to the breakdown of TS. Ndegwa et al. (2002) found that the ratio of TVS/TS kept nearly constant in the non-treated animal slurry. The decrease in TVS/TS during aeration should reduce odour offensiveness since odorous compounds are derived from volatile solids and organics (Burton, 1992; Zhang & Westerman, 1997; Zhu, 2000). Interestingly, the TVS/TS ratios by day 5 are consistently lower than by day 10 in the slurry with initial TS of 20%, a relatively low value compared to those with initial TS of 10% by day 5. It appears that there is an optimal substrate level at 2% TS for the blooming activity of
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Table 2 Variation in ratios of total volatile solids (TVS), total suspended solids (TSS) and total volatile suspended solids (TVSS) to total solids (TS) in slurry treated under three different levels of aeration intensities and four durations Total solids, %
Ratios for various aeration intensities and durations +35 mV[ORP*]
Control
10 mg[O2]l1
30 mg[O2]l1
5 day 10 day 20 day 30 day 5 day 10 day 20 day 30 day 5 day 10 day 20 day 30 day 5 day 10 day 20 day 30 day TVS/TS ratio 05 0729 0685 0623 0675 10 0727 0722 0672 0680 20 0750 0734 0709 0715 40 0777 0747 0730 0745
0561 0756 0590 0799
0539 0629 0750 0766
0538 0715 0727 0744
0519 0564 0580 0740
0555 0669 0596 0801
0485 0552 0758 0786
0538 0577 0737 0751
0540 0518 0560 0677
0568 0672 0582 0804
0464 0542 0660 0783
0545 0542 0591 0589
0511 0485 0493 0513
TSS/TS ratio 05 0497 0496 0521 0637 10 0799 0740 0748 0665 20 0480 0408 0711 0723 40 0826 0843 0967 0713
0490 0542 0765 0753
0249 0454 0797 0775
0504 0769 0796 0882
0628 0685 0707 0892
0822 0786 0702 0798
0223 0408 0730 0760
0528 0706 0783 0832
0509 0642 0795 0884
0525 0733 0658 0749
0453 0321 0582 0705
0672 0566 0984 0920
0485 0554 0738 0892
TVSS/TS ratio 05 0486 0429 0222 0498 10 0633 0626 0573 0523 20 0361 0344 0620 0583 40 0715 0704 0840 0585
0478 0526 0454 0701
0208 0358 0680 0657
0413 0610 0638 0689
0391 0426 0450 0704
0300 0399 0515 0649
0172 0292 0636 0661
0354 0480 0626 0755
0328 0393 0485 0638
0433 0324 0429 0641
0289 0238 0471 0622
0445 0389 0651 0647
0322 0337 0417 0518
*
ORP, oxidation–reduction potential.
aerobic microorganisms around day 5, by which time a considerable portion of volatile organics might be consumed; this deduction needs to be verified. For the ratio of TSS/TS, some distinct features different from that of TVS/TS are found. At the +35 mV[ORP], the TSS/TS tends to increase in all columns except for the initial TS of 20%. Under the aeration intensities of 10 and 30 mg[O2] l1, a decreasing tendency on the ratio of TSS/TS is found in the slurry with low TS levels (05 and 10%). However, an opposite trend occurs in slurry with high TS levels (20 and 40%). Notably, the lowest TSS/TS appears in all of the columns sampled by day 10 with aeration intensity of 30 mg[O2] l1, while this peculiarity is found only in the low-solids slurry (05 and 10%) for the other two aeration levels. TSS essentially consists of particulate solids and biomass (including live and decaying microbes), and the former could be decreased during aeration. Thus, the ratio of TSS/TS under +35 mV[ORP] might be explained by massive aerobe growth (also reported by Zhu et al., 2002). Due to abundant substrate, biomass accumulation could occur in slurry with high solids levels (Grady et al., 1999; Henze et al., 2003) when aeration intensity is 10 or 30 mg[O2] l1. Biomass should decrease in slurry with low solids content. As for the changes of TVSS/TS ratio, there is no apparent indication over the entire aeration period. But the lowest ratio is found in the slurry with low solids
content (05 and 10% TS) when the aeration time reaches 10 days, regardless of the aeration intensity. The TVSS/TS ratios in slurry with a TS level of 20% are elevated between day 10 and day 20 under the three aeration intensities.
3.4. Change of the 5 day biochemical oxygen demand For the controls, slight increases are found by day 10 and day 20 for BOD5 for slurries with different TS levels, while for the treatments, the values of BOD5 decrease after day 20 (Table 3). This phenomenon indicates that BOD5 in untreated slurry could increase during a relatively short period of storage. The effect of aeration duration and intensity on BOD5 concentrations in slurry is shown in Table 3. Firstly, BOD5 decreases rapidly when the duration of aeration increases under the three aeration intensities. Secondly, the higher the aeration intensity, the lower are the BOD5 levels achieved. Thirdly, BOD5 increases by days 10 and 20, particularly for the slurry with 40% TS, the main reason for which might be the increased rate of particulate hydrolysis due to the growing microbial population, leading to a release of more biodegradable materials that contribute to the temporary rise of BOD5 in the treated slurry. The BOD5 removal efficiencies ranges from 00 to 697%, 16 to 820%, and 250 to 864% by day 5 under the aeration intensities of
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Table 3 Variation in levels of 5 day biochemical oxygen demand (BOD5) and volatile fatty acids (VFAs) in the control over four durations BOD5, mg l1
Total solids, %
05 10 20 40
VFAs, mg l1
5 day
10 day
20 day
30 day
5 day
10 day
20 day
30 day
2475 4800 9300 19 200
2400 5325 10 800 21 900
2550 4875 11 100 21 900
1920 4320 10 500 20 400
417 888 3323 6391
656 1312 5027 9333
387 1071 2608 7937
303 1471 3425 7481
Table 4 Variations in levels of 5 day biochemical oxygen demand (BOD5) and removal efficiencies in slurry treated under three levels of aeration intensity and four durations Aeration intensity
Aeration duration, day
BOD5 levels, ml, l1
BOD5 removal efficiencies, %
Total solids, %
Total solids, %
05
10
20
40
05
10
20
40
+35 mV [ORP]*
5 10 20 30
750 750 300 156
4200 2175 2370 348
8850 9000 2400 975
19 200 22 200 21 600 4380
697 688 882 919
125 592 514 919
48 167 784 907
00 14 14 785
10 mg[O2] l1
5 10 20 30
450 375 330 90
675 975 870 186
9450 10 050 5400 930
19 500 21 600 20 700 4200
818 844 871 953
859 817 822 957
16 69 514 911
16 14 55 794
30 mg[O2] l1
5 10 20 30
337 225 360 72
1050 600 540 120
5550 1350 1950 315
14 400 10 200 3900 1800
864 906 859 963
781 887 889 972
403 875 824 970
250 534 822 912
*
ORP, oxidation–reduction potential.
+35 mV[ORP], 10, and 30 mg[O2] l1, respectively, while these ranges become 785 to 920%, 794 to 960%, and 912 to 970% by day 30. Obviously, the BOD5 removal efficiencies are enhanced by the aeration intensity. Continuous aeration for 4 days could reduce the BOD component by over 90% (Burton, 1992). Similarly, 45 days of aeration, under the ORP (Eh) range of +50 to +200 mV, would cause 46–85% of reduction in BOD5 (Williams et al., 1989). Based on the data collected from four different aeration durations in this study, the BOD5 removal efficiencies under the three respective aeration intensities decrease remarkably with respect to TS levels in the order of 05%410%420%440%. Yang and Wang (1999) found that 32 days intermittent aeration with a ratio of aeration to non-aeration of 3:3 achieved 98% of BOD5 removal efficiency for low substrate pig slurry (with 02% TSS). Obviously, a low level of solids in slurry offers an advantageous condition for BOD5 removal during the aeration process (Table 4).
3.5. Characteristics on potential of odour generation The changes of volatile fatty acids (VFAs) over time in the slurries with different TS levels under the control are quite similar to those of BOD5 (Table 3). However, the removal of VFAs by the aeration treatment at different treatment durations is much more significant than removals of solids or BOD5 (Table 5). Under the three aeration intensities, the VFA levels for those columns with a TS content of 05% are greatly reduced by day 5 and become nearly undetectable thereafter. The VFA levels in the slurry with initial 10% TS gradually decrease over the length of aeration under +35 mV[ORP], but are quickly reduced to undetectable levels after 5 days under aeration intensities of both 10 and 30 mg[O2] l1. For slurry with TS levels of 20 and 40%, good removal efficiencies are also obtained for all aeration intensities, with the exception of +35 mV[ORP] for slurry with 40% TS. Meanwhile, some increases in VFAs are found by day 10 (e.g. 40%
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Table 5 Variations of volatile fatty acids (VFAs) levels and removal efficiencies in slurry treated under three levels of aeration intensity and four durations Aeration intensity
Aeration duration, day
VFAs, mg l1
VFAs removal efficiencies, %
Total solids, %
Total solids, %
05
10
20
40
05
10
20
40
+35 mV[ORP]*
5 10 20 30
27 2 7 3
615 367 118 21
2100 1948 1083 404
6112 8043 8681 6535
93 100 98 99
31 72 89 99
37 61 59 88
4 14 9 13
10 mg[O2] l1
5 10 20 30
15 4 3 5
79 16 35 14
2332 772 808 347
5247 7389 6409 2115
96 99 99 98
91 99 97 99
30 85 69 90
18 21 19 72
30 mg[O2] l1
5 10 20 30
40 5 4 6
61 5 18 10
1948 187 101 303
1400 933 533 916
91 99 99 98
93 100 98 99
41 96 96 91
78 90 93 88
*
ORP, oxidation–reduction potential.
TS under +35 mV[ORP]) and by day 20 (e.g. 20% TS under 10 mg[O2] l1), even by day 30 (e.g. TS of 20 and 40% under 30 mg[O2] l1), probably because VFAs are the intermediate products generated during the microbial decomposition of slurry (Zhang et al., 1997; Zhu, 2000). The ORP level of +35 mV is shown to be insufficient for decomposition of odorous materials in the slurry with 40% TS because the concentrations of VFAs tend to increase over the duration of the process (Table 4). Although the VFA removal efficiencies increase from 44 to 934%, 179 to 964%, and 781 to 932% by day 5 for the three aeration rates, respectively, and these values are raised from 127 to 990%, 717 to 990%, and 878 to 993% by day 30 (Table 4), more than half of the VFAs in the slurries with TS less than 40% are removed when aeration duration is longer than 5 days, regardless of aeration intensities. Moreover, the VFA removal efficiencies are usually enhanced by the increase of aeration intensities for a given aeration duration and TS level in slurry. Burton et al. (1998) found that 85–99% of VFAs in slurry with initial TS around 25–30% were reduced by 17 to 63 days of aeration under an ORP (Ecal) level from +91 to +191 mV. According to this study, the ‘favourable conditions’ for reducing potential odour generation can be achieved by using aeration intensities equal to or higher than 10 mg[O2] l1 for slurry with TS less than 20%. And the effectiveness of aeration in odour control depends upon the changes of solids decomposition and BOD5 reduction discussed above.
Generally, a total VFAs level below 230 mg l1 indicates a slurry without offensive odour (Burton, 1992). By this standard, 5 days of aeration will suffice to treat slurry with TS content equal to or less than 05%, provided that the aeration level is at least +35 mV[ORP] (Table 4). For slurry with a TS content equal to or less than 10%, more than 10 days of aeration will be needed at +35 mV[ORP] (5 days at 10 and 30 mg[O2] l1). For slurry with TS equal to or greater than 20%, at least 5 days of aeration is required at a level of 30 mg[O2] l1 (Table 4). Continuous aeration of 5 days at 10 mg[O2] l1 in batch treatment is necessary for slurry with a solids level around 10% to ensure an economically efficient treatment. Since pig slurry after mechanical separation normally contains TS ranging from 10 to 25%, duration of 5–10 days with aeration intensities of 10–30 mg[O2] l1 is thus recommended for odour control at farm level according to this study.
3.6. Odour generation potential evaluated by solids and biochemical oxygen demand Regression analysis between VFAs and solids and BOD5 is conducted to develop the relationships for odour estimation. The linear relationships for VFAs with solids in different forms and BOD5 throughout the 30 days of aeration are presented in Figs 6 and 7. Linear regression equations for VFAs versus TS, TVS, TSS, and TVSS prove strongly significant with correlation
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Fig. 6. Linear regression relationships between solids and volatile fatty acids (VFAs) in the slurry treated by aeration, with intensity ranging from oxidation–reduction potential (ORP) of +35 mV, to 30 mg[O2] l1; R2, coefficient of determination
Fig. 7. Linear regression relationship between 5-day biochemical oxygen demand in (B0D5) and volatile fatty acids (VFAs) in the slurry treated by aeration, with intensity ranging from oxidation–reduction potential (ORP) of +35 mV, to 30 mg[O2] l1; R2, coefficient of determination
coefficients R of 08147, 08224, 08097, and 08200, respectively, while that for VFAs versus BOD5 reaches 08415. Previous studies demonstrated a similar linear correlation between BOD5 and VFAs in pig slurry with no aeration treatment (Zhu et al., 2001), and in slurry during aerobic treatment (Williams, 1984). Research also showed that the VFA generation was relevant to the portion of solids in the smallest particle size range (o0075 mm) in slurry by microbial degradation (Yasuhara et al., 1984; Zhu et al., 2001) and the odour
offensiveness was linearly related to the BOD in slurry storage (Williams, 1984; Ndegwa et al., 2002). Because aeration changes the composition of the solids and biological properties of the aerated slurry (as discussed above), the correlations in this study where odour generation potential is evaluated by the changes of solids in slurry under different aeration levels are different from the previous literature. Therefore, during the course of both decomposition of solids and organic materials and biodegradation of odorous compounds, the odour generation potential in slurry treated by aeration can be distinctly characterised by changes in TS, TVS, TSS and BOD5 concentrations.
4. Conclusions (1) Data from this study indicate that under a given intensity of aeration, solids decomposition, including total solids (TS), total volatile solids (TVS), total suspended solids (TSS), and total volatile solids (TVSS), 5 day biochemical oxygen demand (BOD5) reduction, and volatile fatty acids (VFAs) breakdown are improved by the duration of aeration for pig slurry with TS levels from 05 to 40%, which are enhanced with the increase of aeration intensities (i.e. +35 mV[oxidation–reduction potentials, ORP], 10, 30 mg[O2] l1). However, slurry containing a
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(2)
(3)
(4)
(5)
higher TS level would impair the efficiencies of aeration and aeration under +35 mV[ORP] is insufficient for odour control. Aeration intensities equal to or greater than 10 mg[O2] l1 for slurry with TS less than 20% would offer an advantageous condition for solids decomposition, BOD5 reduction, and VFA removal during the batch aeration treatment. Total solids (TS) removals with ranges from 60 to 449%, 186 to 503 and 423 to 564% are achieved by day 30 under aeration intensities of +35 mV[ORP], 10, 30 mg[O2] l1, respectively. The ratios of TVS/TS, TSS/TS, and TVSS/TS in slurry consistently increase with respect to the solids levels under the 30-day aeration process. The TVS/ TS ratio in slurry decreases, however, as aeration continues from day 5 to day 30. At the end of experiment, the BOD5 removal efficiencies range from 785 to 920%, 794 to 960% and 912 to 970%; the VFA removal efficiencies range from 127 to 990%, 717 to 990%, and 878 to 993%, under aeration intensities of +35 mV[ORP], 10, 30 mg[O2] l1, respectively. By the standard of VFA level (below 230 mg l1) for a slurry without offensive odour, duration of 5–10 days with aeration intensities of 10 to 30 mg[O2] l1 in batch treatment is recommended for farm practice for odour control. Linear regression reveals that the odour generation potential in slurry treated by aeration can be distinctly characterised by the change in the state of solids and also by the change in BOD5 levels.
Acknowledgement The authors wish to thank the Minnesota Legislature Rapid Agricultural Response Fund for funding this project. References APHA, AWWA, WEF (1998). In: Standard Methods for the Examination of Water and Wastewater (20th Edn.). (Clesceri L S; Greenberg A E; Eaton A D, eds). American Public Health Association, Washington, DC, USA Burton C H (1992). A review of the strategies in the aerobic treatment of pig slurry: purpose, theory and method. Journal of Agricultural Engineering Research, 53, 249–272 Burton C H; Sneath R W; Misselbrook T H; Pain B F (1998). The effect of farm scale aerobic treatment of piggery slurry on odour concentration, intensity and offensiveness. Journal of Agricultural Engineering Research, 71(4), 203–211 Charpentier J; Florentz M; David G (1989). Oxidation–reduction potential (ORP) regulation: a way to optimize pollution
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removal and energy savings in the low load activated sludge process. Water Science and Technology, 19(6), 645–655 Chastain J P; Lucas W D; Albrecht J E; Pardue J C; Adams III J; Moore K P (2001). Removal of solids and major plant nutrients from swine manure using a screw press separator. Applied Engineering in Agriculture, 17(3), 355–363 Chynoweth D P; Wilkie A C; Owens J M (1999). Anaerobic treatment of piggery slurry—review. Asian–Australasian Journal of Animal Sciences, 12(4), 607–628 Cumby T R (1987). A review of slurry aeration 3. Performance of aerators. Journal of Agricultural Engineering Research, 36, 175–206 Gay S W; Schmidt D R; Clanton C J; Janni K A; Jacobson L D; Weisberg S (2003). Odor, total reduced sulfur, and ammonia emissions from animal housing facilities and manure storage units in Minnesota. Applied Engineering in Agriculture, 19(3), 347–360 Grady C P L; Daigger G L; Lim H C (1999). Biological Wastewater Treatment (2nd Edn.), pp 48, 66–78, 637–638. Marcel Dekker, Inc. New York, USA Hach Company (1993). Procedure Manual, Loveland, CO Henze M; Harremoe¨s P; Jansen J L C; Arvin E (2003). Wastewater Treatment (3rd Edn.), pp 64–65, 120–122. Springer-Verlag, Berlin, Heidelberg, Germany Hobbs P J; Misselbrook T H; Pain B F (1998). Emission rates of odorous compounds from pig slurries. Journal of the Science of Food and Agriculture, 77(3), 341–348 Kataoka N; Suzuki T; Ishida K; Yamada N; Kurata N; Katayose M; Honda K (2002). Field test of methane fermentation system for treating swine wastes. Water Science and Technology, 45(12), 103–112 Magbanua B S; Adams T T; Johnston P (2001). Anaerobic codigestion of hog and poultry waste. Bioresource Technology, 76(2), 165–168 Ndegwa P M; Zhu J; Luo A (2002). Effects of solids separation and time on the production of odorous compounds in stored pig slurry. Biosystems Engineering, 81(1), 127–133 Phillips D; Bulley N R (1980). Odor reduction of swine manure by aeration stripping. Canadian Agricultural Engineering, 22(2), 113–116 Wang L K; Pereira N C (1986). Handbook of Environmental Engineering, Vol. 3. Biological Treatment Process. The Humana Press Inc., Clifton, NJ Westerman P W; Zhang R H (1997). Aeration of livestock manure slurry and lagoon liquid for odor control: a review. Applied Engineering in Agriculture, 13(2), 245–249 Williams A G (1984). Indicators of piggery slurry odor offensiveness. Agricultural Wastes, 10, 15–36 Williams A G; Shaw M; Selviah C M; Cumby R J (1989). The oxygen requirements for deodorizing and stabilizing pig slurry by aerobic treatment. Journal of Agricultural Engineering Research, 43(2), 291–311 Yang P Y; Wang Z Y (1999). Integrating an intermittent aerator in a swine wastewater treatment system for landlimited conditions. Bioresource Technology, 69(3), 191–198 Yasuhara A; Fuwa K; Jimby M (1984). Identification of odorous compounds in fresh and rotten swine manure. Agricultural and Biological Chemistry, 48, 3001–3010 Zhang R H; Dugba P N; Bundy D S (1997). Laboratory study of surface aeration of anaerobic lagoons for odor control of swine manure. Transactions of the ASAE, 40(1), 185–190 Zhang R H; Tao J; Dugba P N (2000). Evaluation of two-stage anaerobic sequencing batch reactor systems for animal
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wastewater treatment. Transactions of the ASAE, 43(6), 1795–1801 Zhang R H; Westerman P W (1997). Solid–liquid separation of animal manure for odor control and nutrient management. Applied Engineering in Agriculture, 13(3), 385–393 Zhu J (2000). A review of microbiology in swine manure odor control. Agriculture Ecosystems and Environment, 78, 93–106
Zhu J; Ndegwa P M; Luo A (2001). Effect of solid–liquid separation on BOD and VFA in swine manure. Environmental Technology, 22(10), 1237–1243 Zhu J; Ndegwa P M; Luo A (2002). Bacterial responses to temperature during aeration of pig slurry. Journal of Environmental Science and Health Part B—Pesticides Food Contaminants and Agricultural Wastes, 37(3), 265–275
Effects of Duration and Intensity of Aeration on Solids Decomposition in Pig Slurry for Odour Control ZhiJian Zhang1; Jun Zhu1; Keum J. Park2 1
Southern Research and Outreach Center, University of Minnesota, 35838 120th Street, Waseca, MN 56093-4521, USA; e-mail of corresponding author: [email protected] 2 Department of Biomachinery Engineering, Sunchon National University, Sunchon, Republic of Korea (Received 9 January 2004; accepted in revised form 9 August 2004; published online 22 October 2004)
A 30 day laboratory scale experiment was carried out using three different aeration rates, i.e. +35 mV oxidation–reduction potential (ORP: standard hydrogen electrode), 10, and 30 mg[O2] l1, to achieve solids decomposition and odour control in pig slurry with total solids (TS) levels from 05 to 40%. Changes during the aeration process were characterised by TS, total volatile solids (TVS), total suspended solids (TSS), and total volatile suspended solids (TVSS), all expressed as w/w. The measurement of volatile fatty acids (VFAs), expressed as w/v, was used to evaluate the potential odour generation in the aerated slurry. The TS removal efficiencies from 60 to 449, 186 to 503, and 423 to 564% were observed for the three different aeration levels, with reductions from 66 to 489, 260 to 611, and 573 to 699% for the slurry TVS. The ratios of TVS/ TS, TSS/TS, and TVSS/TS in slurry over the experimental duration were found to increase with increased TS levels in the 30 day aeration under the three aeration intensities. Reductions in the 5 day biological oxygen demand (BOD5) reached 785–920, 794–960, and 912–970%, while reductions of VFAs reached 127–990, 717–990%, and 878–993%. The biodegradation of solids, BOD5, and VFAs was effectively enhanced when aeration time and intensity were increased. A low level of solids in slurry promoted aerobic decompositions of solids, BOD5, and VFAs. Changes of state in the solids being aerated and changes in the BOD5 levels can be used to distinctly characterise the potential of odour generation from the slurry. Batch aeration of 5–10 days under intensities of 10 to 30 mg[O2] l1 is recommended for odour control at farm level. r 2004 Silsoe Research Institute. All rights reserved Published by Elsevier Ltd
1. Introduction Public complaints due to odour emission are a major concern of pig producers. Open slurry storages, such as lagoons and earthen basins, produce odorous gases and compounds due to anaerobic microbial decomposition of pig slurry. In an observational study, Gay et al. (2003) found that odour emission in odour units (OU) from pig slurry storage units reached anywhere from 10 to 114 OU s1 m2. It is well known that solids and organic materials in pig slurry are the first and foremost odour-producing matter under anaerobic microbial activity (Burton, 1992; Westerman & Zhang, 1997; Zhu, 2000). Zhang and Westerman (1997) reviewed the characteristic of the distribution of solids in pig slurry, and found 1537-5110/$30.00
that the total solids (TS) content could be partitioned between suspended solids (SS) and dissolved solids (DS). Each fraction (TS, SS, or DS) can be further divided between volatile solids (VS) and fixed solids (FS). The corresponding volatile fractions of TS, SS, and DS are total volatile solids (TVS), volatile suspended solids (VSS), and volatile dissolved solids (VDS). The VS fraction generally represents the organic component of the total solids, which is more biodegradable than FS, which is mainly associated with the coarse material in the slurry and effectively inert in the typical aerobic treatment process (Burton, 1992). Understanding of distribution of slurry solids presents important information for design and selection of proper solid–liquid equipment for odour control (Zhang & Westerman, 1997). However, the separated liquid is still dark 445
r 2004 Silsoe Research Institute. All rights reserved Published by Elsevier Ltd
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in colour and odour generation depends largely on the amount of odour-producing organic substances remaining in the liquid, which may require additional treatment to ensure an odour-free storage (Chastain et al., 2001). Generally, insoluble organic substances or organic particulates in various wastewaters can be converted into soluble organic matter through hydrolysis, which depends on extracellular enzymes secreted by endogenic microbes (Grady et al., 1999). Some of the soluble organic matter is consumed by microbes to support their own metabolism, while the death, and the subsequent decay of microorganisms, increases insoluble organic materials. The convertible organic matter is transformed into methane, carbon dioxide, water, and other odourless products under the restrictive and controlled anaerobic digestion system (Chynoweth et al., 1999; Magbanua et al., 2001; Kataoka et al., 2002). A variety of odorous compounds, however, such as volatile fatty acids (VFAs), aldehydes, alcohols, volatile amines, mercaptans, indoles, and skatols, classified as the intermediate products of anaerobic metabolism, are produced abundantly under the uncontrolled anaerobic condition (Westerman & Zhang, 1997; Hobbs et al., 1998; Zhu, 2000). When the oxygen supply to wastewaters is maintained, organic decomposition is achieved by the activity of aerobic microorganisms, with no apparent accumulation of odorous compounds (Zhang et al., 1997; Yang & Wang, 1999; Zhu, 2000). Moreover, both the growth in the population of aerobic microorganisms and the subsequent biodegradation rate of organic matter are accelerated rapidly in the aerobic system compared to non-controlled anaerobic systems (Burton, 1992). Whilst previous studies have found that the concentration of solids in slurry can be used as an indicator of organic particulates hydrolysis, microorganism metabolism, aerobic decomposition of soluble organic substance, and biomass production when aeration technique is adopted, few have reported on the role of solids in odour control during the aeration process, particularly in relation to different solids fractions. As a matter of fact, the total amount of solids and fractions, such as VS, SS and DS, changes due to particulate hydrolysis, microorganism activity (assimilation and decay), and breakdown of soluble organic matter during the aerobic process. Aeration offers an effective way of treating animal slurry to achieve solids decomposition and odour control, but the relatively high cost of the required energy has hampered the broad application of this method. Ample research has been conducted in both improving the efficiency of aeration (Cumby, 1987; Yang & Wang, 1999) and reducing the duration and intensity of aeration (Phillips & Bulley, 1980; Zhang
et al., 1997). A level of 1–2 mg[O2] l1 maintained in the wastewater ensures adequate oxygen transfer into the biomass solids and provides mixing, so that the aerobic environment can be kept consistent throughout the treatment vessel (Wang & Pereira, 1986). A study by Williams et al. (1989) showed that odour offensiveness could be controlled if the oxidation– reduction potential (ORP) was maintained at above +13 mV (expressed as the value of standard hydrogen electrode, Eh) in slurry. If the ORP was controlled at +133 mV, the odour intensity ranged between ‘very faint’ and ‘faint’ (Burton et al., 1998). Another study showed that once the ORP in the liquid exceeded +100 mV (Eh), the liquid environment was said to be aerobic with detectable dissolved oxygen present and prevalent aerobic respiration (Charpentier et al., 1989). In a previous study of bacterial counts corresponding to the ORP, it was suggested that, for odour control, the slurry ORP should be maintained at +35 mV (Eh) or higher to effectively support aerobic growth and simultaneously hinder anaerobic activity (Zhu et al., 2002). To date, the characteristics of solids decomposition under different aeration durations and intensities for slurry odour control using batch operation is not fully understood. Since complete stabilisation of livestock slurry by aerobic treatment is normally not economically justifiable, low levels of aeration have been recommended for partial odour control (Westerman & Zhang, 1997). In this study, a 30 day, laboratory scale batch experiment with three different aeration levels, i.e. +35 mV[ORP] (Eh), 10 mg[O2] l1, and 30 mg[O2] l1, was designed to treat pig slurry with TS concentrations from 05 to 40% in order to shed light on odour control and aerobic decomposition of solids. The changing characteristics of solids during the aeration process were evaluated by TS, TVS, total suspended solids (TSS), and total volatile suspended solids (TVSS). The values of the 5 day biochemical oxygen demand (BOD5) in the slurries were measured to show the extent of organic matter decomposition and to indicate the efficiency of the various aeration regimes. Although the measurement and quantification of odour have been widely debated over the development of strategies toward odour issues, VFAs are considered appropriate in evaluating the potential odour generation in slurry (Williams, 1984; Burton, 1992). The level of VFAs was often reported to quantify pig slurry odour reduction (Burton et al., 1998; Zhang et al., 1997, 2000; Zhu et al., 2001). The potential odour generation during the course of aerobic decomposition is also investigated by using VFAs in this study in order to develop strategies for odour control.
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Table 1 Characteristics of the screened raw finishing pig slurry for experiment start-up
2. Materials and methods 2.1. Slurry collection and experimental procedure
Items
Raw slurry for this study was collected from a reception pit of a finishing barn at the University of Minnesota Southern Research and Outreach Center, where fresh slurry in a shallow pit inside the barn was flushed out weekly. The pigs at the sampling site were fed with corn and soyabean. Prior to loading the batch reactors (Fig. 1), the collected slurry was passed through a sieve with 2 mm openings. Four sub-samples of the screened slurry were then diluted with tap water to achieve respective TS concentrations of 05, 10, 20, and 40%. The characteristics of the sieved liquid slurry before dilution are shown in Table 1. The prepared liquid slurry was concurrently placed in 12 columns, made of PVC materials, 916 mm in height and 153 mm in internal diameter, simulating batch reactors (Fig. 1). The slurry level in each column was 763 mm, giving a 153 mm headspace. Air was pumped through a bubble stone located at the bottom of each column. Four aeration durations (5, 10, 20, and 30 day) and three aeration levels (+35 mV[ORP], 10, and 30 mg[O2] l1) were tested. The aeration intensities were maintained by controlling the airflow rates according to the ORP and dissolved oxygen (DO) readings taken three times a day. The ORP was converted to standard hydrogen electrode Eh in mV by a formula E h ¼ E cal þ 241
(1)
pH Total solids (TS), % Total volatile solids (TVS), % Total suspended solids (TSS), % Total volatile suspended solids (TVSS), % Volatile fatty acids (VFAs), mg l1 Biochemical oxygen demand in 5 days (BOD5), mg l1 TVS/TS TSS/TS TVSS/TS
Initial For tests (based on content of TS, %) 05
10
20
40
62 425 341
61 051 041
62 100 080
61 201 162
62 402 322
348
042
082
164
329
071
141
281
3200
6413
297
035
6812
800
1600
21 800
2558
5119 10 249 20 508
0802 0819 0699
0807 0802 0806 0801 0822 0822 0818 0818 0699 0699 0699 0699
where Ecal in mV (calomel reference electrode) was measured by an ORP meter (DIGI-SENSE, Model 5938-10). The DO in the liquid was measured using an oxygen meter (Extech, Model 407510). The actual ORP or DO in the aerated slurries had fluctuated around the set values, which were recorded as +35725 mV[ORP], 10712, and 30721 mg[O2] l1. All columns were housed under constant room temperature at 17705 1C.
2.2. Sampling and analyses
Fig. 1. Schematic of the aeration apparatus used in the experiment: ORP, oxidation–reduction potential (expressed as standard hydrogen electrode, Eh in mV); DO, dissolved oxygen
After sieving, one mixed slurry sample was collected for analysis of the initial characteristics. During aeration, liquid slurry samples from all columns were collected on days 5, 10, 20, and 30. All slurry samples, including the initial sample, were drawn from the wellmixed slurry at approximately the mid-depth of each column (or container) during agitation using a motorised paddle-stirrer (Tline laboratory stirrer, Model 102). All the samples were stored at 20 1C immediately after collection, and thawed and allowed to reach room temperature only prior to analysis. American Public and Health Association Standard methods were used to run the liquid slurry sample analysis for TS, TVS, TSS, TVSS, and BOD5 (APHA et al., 1998). According to Hach (1993), the VFA measurement is based on esterification of the carboxylic acids present in the sample. Calorimetric determination of the esters follows a ferric hydroxamate reaction. The
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concentration of VFAs was measured at a wavelength of 495 nm using a DR/3000 spectrophotometer. All volatile acids are reported as their equivalent concentration of acetic acid. The removal efficiency, in terms of solids in different forms, BOD5, and VFAs, is calculated by dividing the difference between the treatment and the concurrent control by the concurrent control, and then multiplying by 100.
3. Results and discussion 3.1. Changes of total solids and total volatile solids Figure 2 presents the changes of TS levels in slurry treated by the 30 day aeration process. For the treatment under ORP of +35 mV, there is some decrease in TS levels for all slurry columns when aeration time increases from 5 to 20 days, compared to the control. However, the TS levels apparently decrease when the aeration process lasts 30 days, particularly for slurry with initial TS levels lower than 40%. With few exceptions, the TS levels in non-aerated columns decrease slightly (Fig. 2). Analysis of the TS removal efficiency reveals that 16–111% is removed by day 5 for columns initially filled with slurry with TS levels of 05–40%. As the aeration treatment approaches 30 days, the removal efficiencies reach 60–449%. When the aeration intensity is raised to 30 mg[O2] l1, the trend of decrease in TS levels and increase in TS removal efficiencies become pronounced as the aeration time increases (Fig. 2). TS removal efficiencies are enhanced from 84 to 305% by day 5 and between 423 and 564% by day 30. The effectiveness of TS removal under the aeration intensity of 10 mg[O2] l1 is found to be 17 to 42% by day 5 and 186–503% by day 30. The changes of TVS levels in slurry under the three aeration intensities are shown in Fig. 3. Similar to TS, the TVS levels in slurry columns are decreasing during the process of aeration. The overall TVS removal efficiencies under the aeration intensity of +35 mV[ORP] achieve 08–172% by day 5 and 66–489% by day 30. The TVS removal under 10 mg[O2] l1 increases from 29 to 218% by day 5 to 260–611% by day 30, while from 182 to 357% by day 5 to 573–699% by day 30 under 30 mg[O2] l1. 3.2. Changes of total suspended solids and total volatile suspended solids Figure 4 demonstrates the primary features of TSS change in the slurry during the aeration process under
Fig. 2. Variation of total solids (TS) in slurry treated by 30-day aeration under: (a) +35 mV oxidation–reduction potential (ORP); (b) 10 mg[O2] l1; (c) 30 mg[O2] l1. , Control, slurry with TS of 05%; ’, control, slurry with TS of 10%; ~, control, slurry with TS of 20%; m, control, slurry with TS of 40%; , aeration, slurry with TS of 05%; , aeration, slurry with TS of 10%; , aeration, slurry with TS of 20%; aeration, slurry with TS of 40%
the three different intensities. For example, for slurry with low solids content, such as TS at 05 and 10%, aerated at the intensity of 10 mg[O2] l1, the TSS levels in the briefly aerated slurry (5 day) are similar to those
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Fig. 3. Variation of total volatile solids (TVS) in slurry treated by 30-day aeration under: (a) +35 mV oxidation–reduction potential (ORP); (b) 10 mg[O2] l1; (c) 30 mg[O2] l1. , Control, slurry with TS of 05%; ’, control, slurry with TS of 10%; ~, control, slurry with TS of 20%; m, control, slurry with TS of 40%; , aeration, slurry with TS of 05%; , aeration, slurry with TS of 10%; , aeration, slurry with TS of 20%; , aeration, slurry with TS of 4.0%
of non-aerated slurry. After 5 days, the TSS levels decrease sharply. For slurry with high solids content (20 and 40%), little change is observed in TSS levels during the first 20 days of aeration, but a significant reduction occurs by day 30. Under +35 mV[ORP], apparent
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Fig. 4. Variation of total suspended solids (TSS) in slurry treated by 30-day aeration under: (a) +35 mV oxidation–reduction potential (ORP); (b) 10 mg[O2] l1; (c) 30 mg[O2] l1. , Control, slurry with TS of 05%; ’, control, slurry with TS of 10%; ~, control, slurry with TS of 20%; m, control, slurry with TS of 40%; , aeration, slurry with TS of 05%; , aeration, slurry with TS of 10%; , aeration, slurry with TS of 20%; , aeration, slurry with TS of 40%
decreases in TSS levels in the slurry are found only by day 30, regardless of the initial TS levels. With an aeration intensity of 30 mg[O2] l1, similar to TS and TVS, the TSS levels in all slurry columns decrease gradually with respect to the time of aeration. For the
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with different TS contents for aeration intensities of +35 mV[ORP], 10, and 30 mg[O2] l1, respectively. The changes of TVSS levels over time (Fig. 5) in slurry under the three aeration intensity levels are quite similar to those of TSS. A special feature is that, at all TS levels, TVSS decreases during aeration for all three aeration intensities. In the same manner, the TVSS are reduced from 73 to 568%, 272 to 582%, and 565 to 688% when aeration is run continuously for 30 days under conditions of +35 mV[ORP], 10, and 30 mg[O2] l1, respectively.
3.3. Characteristics of solids composition
Fig. 5. Variation of total volatile suspended solids (TVSS) in slurry treated by 30-day aeration under: (a) +35 mV oxidation–reduction potential (ORP); (b) 10 mg[O2] l1; (c) 30 mg[O2] l1. , Control, slurry with TS of 05%; ’, control, slurry with TS of 10%; ~, control, slurry with TS of 20%; m, control, slurry with TS of 40%. , aeration, slurry with TS of 05%; , aeration, slurry with TS of 10%; , aeration, slurry with TS of 20%; , aeration, slurry with TS of 40%
controls, the reasons that contribute to the low levels of TSS and TVSS by day 5 and day 10, respectively, in the slurry with initial TS of 20% cannot be explained. The TSS levels are reduced from 44 to 458%, 180 to 519%, and 390 to 572% by day 30 among the columns
The solids composition in slurry treated by the aeration process is characterised by the ratios of TVS/ TS, TSS/TS, and TVSS/TS (based on the concurrent TS values in each of the aerated samples), and the changes of these ratios may reveal the relative decomposition rates of various labile substances. For the control, the ratio of TVS/TS is found to increase slightly over time in slurry with different TS levels and to increase with the increase in the levels of TS under different aeration durations, but there are no remarkable trends for the changes of ratios of TSS/TS and TVSS/TS (Table 2). The ratios of TVS/TS, TSS/TS, and TVSS/TS in slurry are each found to increase with respect to the solids levels under the three aeration schemes within the 30 day duration. This indicates that the decompositions of TVS, TSS, and TVSS would be accelerated in slurry with low solids levels. High organic solids generally impede the oxygen transfer rate in water (Westerman & Zhang, 1997); thus, oxygen utilization should be efficient in the low-solids slurry because microorganisms are able to effectively assimilate organic compounds to support enhanced respiration. There is a decreasing trend on TVS/TS ratio in slurry with different solids sampled from the three different aeration intensities when the aeration time increases from 5 days to 30 days (Table 2). In other words, the relative rate of TVS breakdown gradually increases as the time of aeration increases, compared to the breakdown of TS. Ndegwa et al. (2002) found that the ratio of TVS/TS kept nearly constant in the non-treated animal slurry. The decrease in TVS/TS during aeration should reduce odour offensiveness since odorous compounds are derived from volatile solids and organics (Burton, 1992; Zhang & Westerman, 1997; Zhu, 2000). Interestingly, the TVS/TS ratios by day 5 are consistently lower than by day 10 in the slurry with initial TS of 20%, a relatively low value compared to those with initial TS of 10% by day 5. It appears that there is an optimal substrate level at 2% TS for the blooming activity of
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Table 2 Variation in ratios of total volatile solids (TVS), total suspended solids (TSS) and total volatile suspended solids (TVSS) to total solids (TS) in slurry treated under three different levels of aeration intensities and four durations Total solids, %
Ratios for various aeration intensities and durations +35 mV[ORP*]
Control
10 mg[O2]l1
30 mg[O2]l1
5 day 10 day 20 day 30 day 5 day 10 day 20 day 30 day 5 day 10 day 20 day 30 day 5 day 10 day 20 day 30 day TVS/TS ratio 05 0729 0685 0623 0675 10 0727 0722 0672 0680 20 0750 0734 0709 0715 40 0777 0747 0730 0745
0561 0756 0590 0799
0539 0629 0750 0766
0538 0715 0727 0744
0519 0564 0580 0740
0555 0669 0596 0801
0485 0552 0758 0786
0538 0577 0737 0751
0540 0518 0560 0677
0568 0672 0582 0804
0464 0542 0660 0783
0545 0542 0591 0589
0511 0485 0493 0513
TSS/TS ratio 05 0497 0496 0521 0637 10 0799 0740 0748 0665 20 0480 0408 0711 0723 40 0826 0843 0967 0713
0490 0542 0765 0753
0249 0454 0797 0775
0504 0769 0796 0882
0628 0685 0707 0892
0822 0786 0702 0798
0223 0408 0730 0760
0528 0706 0783 0832
0509 0642 0795 0884
0525 0733 0658 0749
0453 0321 0582 0705
0672 0566 0984 0920
0485 0554 0738 0892
TVSS/TS ratio 05 0486 0429 0222 0498 10 0633 0626 0573 0523 20 0361 0344 0620 0583 40 0715 0704 0840 0585
0478 0526 0454 0701
0208 0358 0680 0657
0413 0610 0638 0689
0391 0426 0450 0704
0300 0399 0515 0649
0172 0292 0636 0661
0354 0480 0626 0755
0328 0393 0485 0638
0433 0324 0429 0641
0289 0238 0471 0622
0445 0389 0651 0647
0322 0337 0417 0518
*
ORP, oxidation–reduction potential.
aerobic microorganisms around day 5, by which time a considerable portion of volatile organics might be consumed; this deduction needs to be verified. For the ratio of TSS/TS, some distinct features different from that of TVS/TS are found. At the +35 mV[ORP], the TSS/TS tends to increase in all columns except for the initial TS of 20%. Under the aeration intensities of 10 and 30 mg[O2] l1, a decreasing tendency on the ratio of TSS/TS is found in the slurry with low TS levels (05 and 10%). However, an opposite trend occurs in slurry with high TS levels (20 and 40%). Notably, the lowest TSS/TS appears in all of the columns sampled by day 10 with aeration intensity of 30 mg[O2] l1, while this peculiarity is found only in the low-solids slurry (05 and 10%) for the other two aeration levels. TSS essentially consists of particulate solids and biomass (including live and decaying microbes), and the former could be decreased during aeration. Thus, the ratio of TSS/TS under +35 mV[ORP] might be explained by massive aerobe growth (also reported by Zhu et al., 2002). Due to abundant substrate, biomass accumulation could occur in slurry with high solids levels (Grady et al., 1999; Henze et al., 2003) when aeration intensity is 10 or 30 mg[O2] l1. Biomass should decrease in slurry with low solids content. As for the changes of TVSS/TS ratio, there is no apparent indication over the entire aeration period. But the lowest ratio is found in the slurry with low solids
content (05 and 10% TS) when the aeration time reaches 10 days, regardless of the aeration intensity. The TVSS/TS ratios in slurry with a TS level of 20% are elevated between day 10 and day 20 under the three aeration intensities.
3.4. Change of the 5 day biochemical oxygen demand For the controls, slight increases are found by day 10 and day 20 for BOD5 for slurries with different TS levels, while for the treatments, the values of BOD5 decrease after day 20 (Table 3). This phenomenon indicates that BOD5 in untreated slurry could increase during a relatively short period of storage. The effect of aeration duration and intensity on BOD5 concentrations in slurry is shown in Table 3. Firstly, BOD5 decreases rapidly when the duration of aeration increases under the three aeration intensities. Secondly, the higher the aeration intensity, the lower are the BOD5 levels achieved. Thirdly, BOD5 increases by days 10 and 20, particularly for the slurry with 40% TS, the main reason for which might be the increased rate of particulate hydrolysis due to the growing microbial population, leading to a release of more biodegradable materials that contribute to the temporary rise of BOD5 in the treated slurry. The BOD5 removal efficiencies ranges from 00 to 697%, 16 to 820%, and 250 to 864% by day 5 under the aeration intensities of
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Table 3 Variation in levels of 5 day biochemical oxygen demand (BOD5) and volatile fatty acids (VFAs) in the control over four durations BOD5, mg l1
Total solids, %
05 10 20 40
VFAs, mg l1
5 day
10 day
20 day
30 day
5 day
10 day
20 day
30 day
2475 4800 9300 19 200
2400 5325 10 800 21 900
2550 4875 11 100 21 900
1920 4320 10 500 20 400
417 888 3323 6391
656 1312 5027 9333
387 1071 2608 7937
303 1471 3425 7481
Table 4 Variations in levels of 5 day biochemical oxygen demand (BOD5) and removal efficiencies in slurry treated under three levels of aeration intensity and four durations Aeration intensity
Aeration duration, day
BOD5 levels, ml, l1
BOD5 removal efficiencies, %
Total solids, %
Total solids, %
05
10
20
40
05
10
20
40
+35 mV [ORP]*
5 10 20 30
750 750 300 156
4200 2175 2370 348
8850 9000 2400 975
19 200 22 200 21 600 4380
697 688 882 919
125 592 514 919
48 167 784 907
00 14 14 785
10 mg[O2] l1
5 10 20 30
450 375 330 90
675 975 870 186
9450 10 050 5400 930
19 500 21 600 20 700 4200
818 844 871 953
859 817 822 957
16 69 514 911
16 14 55 794
30 mg[O2] l1
5 10 20 30
337 225 360 72
1050 600 540 120
5550 1350 1950 315
14 400 10 200 3900 1800
864 906 859 963
781 887 889 972
403 875 824 970
250 534 822 912
*
ORP, oxidation–reduction potential.
+35 mV[ORP], 10, and 30 mg[O2] l1, respectively, while these ranges become 785 to 920%, 794 to 960%, and 912 to 970% by day 30. Obviously, the BOD5 removal efficiencies are enhanced by the aeration intensity. Continuous aeration for 4 days could reduce the BOD component by over 90% (Burton, 1992). Similarly, 45 days of aeration, under the ORP (Eh) range of +50 to +200 mV, would cause 46–85% of reduction in BOD5 (Williams et al., 1989). Based on the data collected from four different aeration durations in this study, the BOD5 removal efficiencies under the three respective aeration intensities decrease remarkably with respect to TS levels in the order of 05%410%420%440%. Yang and Wang (1999) found that 32 days intermittent aeration with a ratio of aeration to non-aeration of 3:3 achieved 98% of BOD5 removal efficiency for low substrate pig slurry (with 02% TSS). Obviously, a low level of solids in slurry offers an advantageous condition for BOD5 removal during the aeration process (Table 4).
3.5. Characteristics on potential of odour generation The changes of volatile fatty acids (VFAs) over time in the slurries with different TS levels under the control are quite similar to those of BOD5 (Table 3). However, the removal of VFAs by the aeration treatment at different treatment durations is much more significant than removals of solids or BOD5 (Table 5). Under the three aeration intensities, the VFA levels for those columns with a TS content of 05% are greatly reduced by day 5 and become nearly undetectable thereafter. The VFA levels in the slurry with initial 10% TS gradually decrease over the length of aeration under +35 mV[ORP], but are quickly reduced to undetectable levels after 5 days under aeration intensities of both 10 and 30 mg[O2] l1. For slurry with TS levels of 20 and 40%, good removal efficiencies are also obtained for all aeration intensities, with the exception of +35 mV[ORP] for slurry with 40% TS. Meanwhile, some increases in VFAs are found by day 10 (e.g. 40%
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Table 5 Variations of volatile fatty acids (VFAs) levels and removal efficiencies in slurry treated under three levels of aeration intensity and four durations Aeration intensity
Aeration duration, day
VFAs, mg l1
VFAs removal efficiencies, %
Total solids, %
Total solids, %
05
10
20
40
05
10
20
40
+35 mV[ORP]*
5 10 20 30
27 2 7 3
615 367 118 21
2100 1948 1083 404
6112 8043 8681 6535
93 100 98 99
31 72 89 99
37 61 59 88
4 14 9 13
10 mg[O2] l1
5 10 20 30
15 4 3 5
79 16 35 14
2332 772 808 347
5247 7389 6409 2115
96 99 99 98
91 99 97 99
30 85 69 90
18 21 19 72
30 mg[O2] l1
5 10 20 30
40 5 4 6
61 5 18 10
1948 187 101 303
1400 933 533 916
91 99 99 98
93 100 98 99
41 96 96 91
78 90 93 88
*
ORP, oxidation–reduction potential.
TS under +35 mV[ORP]) and by day 20 (e.g. 20% TS under 10 mg[O2] l1), even by day 30 (e.g. TS of 20 and 40% under 30 mg[O2] l1), probably because VFAs are the intermediate products generated during the microbial decomposition of slurry (Zhang et al., 1997; Zhu, 2000). The ORP level of +35 mV is shown to be insufficient for decomposition of odorous materials in the slurry with 40% TS because the concentrations of VFAs tend to increase over the duration of the process (Table 4). Although the VFA removal efficiencies increase from 44 to 934%, 179 to 964%, and 781 to 932% by day 5 for the three aeration rates, respectively, and these values are raised from 127 to 990%, 717 to 990%, and 878 to 993% by day 30 (Table 4), more than half of the VFAs in the slurries with TS less than 40% are removed when aeration duration is longer than 5 days, regardless of aeration intensities. Moreover, the VFA removal efficiencies are usually enhanced by the increase of aeration intensities for a given aeration duration and TS level in slurry. Burton et al. (1998) found that 85–99% of VFAs in slurry with initial TS around 25–30% were reduced by 17 to 63 days of aeration under an ORP (Ecal) level from +91 to +191 mV. According to this study, the ‘favourable conditions’ for reducing potential odour generation can be achieved by using aeration intensities equal to or higher than 10 mg[O2] l1 for slurry with TS less than 20%. And the effectiveness of aeration in odour control depends upon the changes of solids decomposition and BOD5 reduction discussed above.
Generally, a total VFAs level below 230 mg l1 indicates a slurry without offensive odour (Burton, 1992). By this standard, 5 days of aeration will suffice to treat slurry with TS content equal to or less than 05%, provided that the aeration level is at least +35 mV[ORP] (Table 4). For slurry with a TS content equal to or less than 10%, more than 10 days of aeration will be needed at +35 mV[ORP] (5 days at 10 and 30 mg[O2] l1). For slurry with TS equal to or greater than 20%, at least 5 days of aeration is required at a level of 30 mg[O2] l1 (Table 4). Continuous aeration of 5 days at 10 mg[O2] l1 in batch treatment is necessary for slurry with a solids level around 10% to ensure an economically efficient treatment. Since pig slurry after mechanical separation normally contains TS ranging from 10 to 25%, duration of 5–10 days with aeration intensities of 10–30 mg[O2] l1 is thus recommended for odour control at farm level according to this study.
3.6. Odour generation potential evaluated by solids and biochemical oxygen demand Regression analysis between VFAs and solids and BOD5 is conducted to develop the relationships for odour estimation. The linear relationships for VFAs with solids in different forms and BOD5 throughout the 30 days of aeration are presented in Figs 6 and 7. Linear regression equations for VFAs versus TS, TVS, TSS, and TVSS prove strongly significant with correlation
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Fig. 6. Linear regression relationships between solids and volatile fatty acids (VFAs) in the slurry treated by aeration, with intensity ranging from oxidation–reduction potential (ORP) of +35 mV, to 30 mg[O2] l1; R2, coefficient of determination
Fig. 7. Linear regression relationship between 5-day biochemical oxygen demand in (B0D5) and volatile fatty acids (VFAs) in the slurry treated by aeration, with intensity ranging from oxidation–reduction potential (ORP) of +35 mV, to 30 mg[O2] l1; R2, coefficient of determination
coefficients R of 08147, 08224, 08097, and 08200, respectively, while that for VFAs versus BOD5 reaches 08415. Previous studies demonstrated a similar linear correlation between BOD5 and VFAs in pig slurry with no aeration treatment (Zhu et al., 2001), and in slurry during aerobic treatment (Williams, 1984). Research also showed that the VFA generation was relevant to the portion of solids in the smallest particle size range (o0075 mm) in slurry by microbial degradation (Yasuhara et al., 1984; Zhu et al., 2001) and the odour
offensiveness was linearly related to the BOD in slurry storage (Williams, 1984; Ndegwa et al., 2002). Because aeration changes the composition of the solids and biological properties of the aerated slurry (as discussed above), the correlations in this study where odour generation potential is evaluated by the changes of solids in slurry under different aeration levels are different from the previous literature. Therefore, during the course of both decomposition of solids and organic materials and biodegradation of odorous compounds, the odour generation potential in slurry treated by aeration can be distinctly characterised by changes in TS, TVS, TSS and BOD5 concentrations.
4. Conclusions (1) Data from this study indicate that under a given intensity of aeration, solids decomposition, including total solids (TS), total volatile solids (TVS), total suspended solids (TSS), and total volatile solids (TVSS), 5 day biochemical oxygen demand (BOD5) reduction, and volatile fatty acids (VFAs) breakdown are improved by the duration of aeration for pig slurry with TS levels from 05 to 40%, which are enhanced with the increase of aeration intensities (i.e. +35 mV[oxidation–reduction potentials, ORP], 10, 30 mg[O2] l1). However, slurry containing a
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(2)
(3)
(4)
(5)
higher TS level would impair the efficiencies of aeration and aeration under +35 mV[ORP] is insufficient for odour control. Aeration intensities equal to or greater than 10 mg[O2] l1 for slurry with TS less than 20% would offer an advantageous condition for solids decomposition, BOD5 reduction, and VFA removal during the batch aeration treatment. Total solids (TS) removals with ranges from 60 to 449%, 186 to 503 and 423 to 564% are achieved by day 30 under aeration intensities of +35 mV[ORP], 10, 30 mg[O2] l1, respectively. The ratios of TVS/TS, TSS/TS, and TVSS/TS in slurry consistently increase with respect to the solids levels under the 30-day aeration process. The TVS/ TS ratio in slurry decreases, however, as aeration continues from day 5 to day 30. At the end of experiment, the BOD5 removal efficiencies range from 785 to 920%, 794 to 960% and 912 to 970%; the VFA removal efficiencies range from 127 to 990%, 717 to 990%, and 878 to 993%, under aeration intensities of +35 mV[ORP], 10, 30 mg[O2] l1, respectively. By the standard of VFA level (below 230 mg l1) for a slurry without offensive odour, duration of 5–10 days with aeration intensities of 10 to 30 mg[O2] l1 in batch treatment is recommended for farm practice for odour control. Linear regression reveals that the odour generation potential in slurry treated by aeration can be distinctly characterised by the change in the state of solids and also by the change in BOD5 levels.
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