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The effects of removing solids from aerobically treated piggery slurry on the VFA levels during storage
Biological Wastes 26 (1988) 175-188
The Effects of Removing Solids from Aerobically Treated Piggery Slurry on the VFA Levels during Storage R. W. Sneath Waste Engineering Group, Buildings and Livestock Division, AFRC Engineering, Wrest Park, Silsoe, Bedford, MK45 4HS, UK (Received 28 November 1987; revised version received 10 March 1988; accepted 23 March 1988)
A BSTRA CT Piggery slurries treated in pilot-scale aeration vessels were sieved or centrifuged to remove a proportion of the solids. The resulting liquidfractions were stored at 10°C; at intervals samples were removed and analysed for volatile fatty acid ( VFA ) content. Removal of solids using fine sieves or the decanting centrifuge extended the storage times by one-third before the VFA level indicated that offensive odours had returned to the slurries. A laboratory centrifuge tripled the stable-storage period.
INTRODUCTION Aerobic treatment is an effective process for controlling slurry odours, but treatment times need to be minimised for economy. One problem with short treatment times is that the treated slurry will not necessarily remain stable during storage before land spreading. Previous work comparing the effects of the same period of aerobic treatment on slurries of 1.5-4-5% dry matter content (DM) showed that the slurry at 1-5% D M could, after treatment, be stored indefinitely without significant return of odour. In contrast, at 4.5% D M the period of odourfree storage was only a matter of days (Williams et al., 1984). Low dry matter contents can be achieved in two ways, either by dilution with water or by removing solids. Dilution with water, however, is likely to be impractical and expensive because of the cost of water, storage, transport and spreading. 175
Biological Wastes 0269-7483/88/$03.50 © 1988 Elsevier Science Publishers Ltd, England. Printed in Great Britain
176
R. W. Sneath
Removal of solids, on the other hand, will reduce the volume needed for storage and reduce the spreading and transport costs, but there will be an added cost for the process. Solids removal after biological treatment is arguably easier, since large bacterial flocs may have grown, thus increasing the numbers of large particles. In addition, degradation of soluble and colloidal compounds will have reduced the viscosity of the liquid. This makes centrifugation an appropriate process to consider. The solids removed ('fibre') should also be odour-free at the time of removal, unlike those removed before treatment, which require composting to become odour-free. The objectives of the work reported in this paper were to measure the effects of removing solids from slurries (using centrifuges, a vibrating screen separator and laboratory sieves) on the stability of aerobically-treated slurries during storage.
METHODS Treatments of slurries prior to solids removal
Raw piggery slurry was collected weekly from a fattening pig house containing 50-100 pigs. The slurry was separated with a brushed-screen roller-press separator (Hepherd, 1976) before aerobic treatment. All the solids removal techniques used were applied to slurries that had been aerobically treated in two ways. Oxygen was supplied to the two treatments at rates such that the redox potential was controlled between 0 and 100 mV Eh (corrected for the calomel-Pt potential) for the 'low O2', and for the 'high 02' the dissolved oxygen concentration was controlled at between 0.5 and 2.0mg/1. These treatments had nominal solids retention times (SRT) of 1 and 3 days respectively in a continuous-flow treatment vessel. The actual SRT and the corresponding solids removal techniques are described in Table 1. Temperatures in the treatment vessels were controlled at 30-35°C. (A fuller description of the treatment apparatus is given by Williams et al. (1986).) The effects of the aerobic treatments that the slurries had undergone are also shown in Table 1, where the chemical oxygen demand (COD), biochemical oxygen demand of the whole slurry (BODw) and the supernatant BOD (BODs) are given for the feed to the treatment plant and the treated slurry (ML) which was then physically treated. Since these aerobically treated slurries had had the coarse fibre removed with a brushed-screen and roller-press separator, any further removal of solids would need equipment capable of removing finer particles, e.g. fine sieves or a centrifuge.
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Solids removal equipment and techniques The laboratory sieves used for the first experiment had nominal hole sizes of 0.6, 0.3, 0.15 and 0-075mm and a diameter of 200mm. Samples (about 5 litres) of each treated slurry and each sieve were prepared and from each preparation 10 sub-samples of 250 ml were then taken for storage. The laboratory centrifuge was used at 1000g for 5 min and operated in a batch mode. The vibrating screen separator was a commercial slurry separator fitted with a 60 mesh (nominally 0.25 m m aperture) stainless steel screen. This being a full-scale machine, it was fed with a p u m p at about 1000 litres/h. The separator was used to separate solids from aerobically-treated slurry which had been collected over the previous 24-h period from each of the aeration vessels. Samples of the separator output were taken at intervals for storage, as described above. The decanting centrifuge was a Pennwalt-Sharples P600 Super-D-Canter operated at 5000 rpm and exerting 2200g at the bowl circumference. It was fed at 600 litres/h. The liquid level in the centrifuge bowl was set at 60.5 m m radius, this produced an acceptably-dry solid fraction. Liquid samples were taken for storage. Sieves and separator Further batches of treated slurry were separated using the vibrating screen separator and laboratory sieves to confirm and clarify the results obtained in the above experiments. The slurries from the same aerobic treatment processes as before were used (although the RT were slightly different), but this time solids were removed by using laboratory sieves with 0-3, 0-15 and 0.075 m m apertures. A large quantity of the same slurry was separated using the same vibrating screen separator with the same size mesh.
Storage Samples from each of the experiments were stored in a controlled temperature room at 10°C for up to 120 days. Ten samples, taken in triplicate, were stored in closed 250 ml containers, one sample was removed for analysis after approximately 0, 1, 3, 7, 14, 21, 28, 50 and 100 or 120 days.
Analyses Samples were taken of aerated slurry and of the liquids after solids removal; all were analysed for dry matter content (DM), and most were analysed for 5-day biochemical oxygen demand (BODs), soluble biochemical oxygen
Storage of pig slurry after solids removal
179
demand (BODs), and chemical oxygen demand (COD), using standard methods (APHA, 1975). The particle size distributions were determined on selected treated slurries and liquids by wet sieving through a 0.15 mm sieve and using a Coulter Counter (Coulter Counter Ltd) for the fraction passing the sieve. The volatile fatty acid (VFA) content of the liquid samples was determined after storage using a gas/liquid chromatograph (Williams et al., 1984). The VFA content of aerobically-treated piggery slurries had been found to be an indicator of the offensiveness of the odour of that slurry. Slurries containing up to 0.23 g/litre VFA were considered acceptable by an odour panel (Williams, 1984), while a threshold of unequivocal unacceptability was reached at 0-52 g/litre VFA. Thus, slurries stored until the VFA concentration reaches 0-23 g/litre should not cause odour problems, while those containing above 0.52 g/litre should release offensive odours.
RESULTS
The physical effects of removing solids The decanting centrifuge Using the decanting centrifuge on the aerated slurries from the two treatments (Table 1) reduced the dry matter content (Table 2) from 3"2 to 2-1% for the l.l-day SRT treatment and from 3"6 to 2"6% for the 3.6-day SRT treatment. During storage the dry matter content decreased further. The particle size analyses of the slurry treated for 1-1-day SRT show that 90% of the particles were less than 0" 15 mm diameter and 89% were less than 0"016 mm diameter (Fig. 1). The centrifuged slurry had 96% below 0-15 mm diameter and 94% less than 0.016 mm diameter. The vibrating screen The vibrating screen separator reduced the initial dry matter content of slurry from the 'low O2', 1-1-day SRT treatment from 3.4 to 2.9% and of slurry from the 'high O2', 3.6-day SRT treatment, from 4-0 to 3.2%. After 100 days of anaerobic storage the unseparated slurries were 3.1 and 3.5% DM respectively and the separated slurries were 2-6 and 3"0% DM. The second batches separated using the screen had dry matter contents reduced from 3-8 to 3-0% and 4.0 to 2.7% for the short and long SRT treatments respectively (Table 2). Particle size analysis of the 3.2-day SRT treatment screened and unscreened slurries (Fig. 2) showed all particles in the screened slurry to be under 0.25 mm. This is to be expected since the mesh in the separator was
R. W. Sneath
180
TABLE 2 Properties o f the Slurries before a n d after Storage
Retention Treatment time (days)
Dry matter contents ( % ) Initial
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centrifuge 2-6 2.2 2.3 2'1 2'1 1.8 1.1 1'0 2.2 2-0 2.3 1-9 1'9 1"8 1-2 1.1
Decanting centrifuge Whole Centrifuged Whole Centrifuged
3-2 3.2 3.6 3"6
b 2.1 -2-6
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31'4 27.8 27.2 21'5
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6-9 6"6 7"5 7"3
31'0 30"8 42.3 33"7
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6.2 5"8 5'1 5-9 1'2 5-2 4.9 4.7 6.1 3'3
51.2 36-0 30'2 43-6 32"4 52-4 35"0 36.2 34-1 28.1
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Blanks, not analysed. a Solids removal. b N o t applicable.
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Laboratory sieves Sieving slurry from the shorter SRT treatment using 0-30, 0-15, 0.075 m m sieves reduced the dry matter content from 3.8 to 2.9, 2.7 and 2.7% respectively (Table 2). The dry matter content of slurry from the longer SRT treatment was reduced from 4-0 to 3.0, 2.8 and 2.7% by the three sieves, respectively. The particle size analysis, Fig. 2, shows how the use of the 0.075 m m sieve changed the particle distribution. In the unsieved slurry 78% of the particles were smaller than 0.016 mm diameter and 60% smaller than 0.005 mm, in the sieved slurry 88% were smaller than 0.016 mm but only 34% were smaller than 0.005mm diameter. This suggests that many small particles were removed from the liquid, not because they were retained by the sieve but because they were attached to large particles which were retained by the sieve. This mode of separation has also been observed in a decanting centrifuge by Sneath et al. (1988). The effects of solids removal on slurry stability
The laboratory sieves and laboratory centrifuge The times to reach the threshold VFA level of 0.23 g/litre were similar for the liquids passing through the 0.15, 0"3"and the 0"6 mm sieves from the slurries
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aerated for 1 day and for 3 days (Fig. 3). The horizontal axis is marked off to show the treatment of the aerated slurry in order, from left to right, of effectiveness of solids removal. The laboratory centrifuge had the greatest effect on reducing the rate of increase of VFA content. Liquid aerobically treated for 1 day had similar VFA levels to that treated for 3 days. Both remained below the 'faintly offensive' level of 0.23 g/litre for about 35 days (Fig. 3). Neither reached a VFA level of 0-52 g/litre (Fig. 4), a level at which the odours might still not be rated as 'definitely offensive' according to a scale devised to correlate odour offensiveness of aerated piggery slurry with the total VFA content (Williams, 1984). The dry matter content of the slurries stored (Table 2) had a significant effect on the time taken to reach the threshold levels. There was a linear relationship between dry matter content at the beginning of storage and the time taken to reach the threshold levels. These relationships are for the 3-day SRT. 1 7 2 - 69DM~ at the 90% confidence level (c.1.) T.52 = 59 - 23DM~ at the 95% c.1. T.23 =
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where T.23 = time to reach the 0.23 g/litre threshold, T.52 = time to reach the 0"52 g/litre threshold, DM~ = dry matter content at the beginning of storage. The dry matter contents of the slurries decreased during storage (Table 2). This decrease is the result of solids in the slurry being converted by anaerobic micro-organisms to volatile materials. The decanting centrifuge Centrifuging reduced the initial VFA content of shorter-treatment slurry and this difference between the centrifuged and uncentrifuged slurries was maintained so that the centrifuged slurry remained below the threshold level of 0-23 g/litre for 5 days compared with 3 days for the uncentrifuged slurry (Fig. 5). The peak VFA level of 0.9 g/litre was reached on the 28th day for centrifuged slurry, while uncentrifuged slurry reached 2.6 g/litre on the 74th day. The times taken to reach the peak VFA levels are similar to those observed previously by Williams et al. (1984); i.e., lower dry matter slurries peak earlier. In that work 4-5% DM peaked at 90 days and 3% DM slurry peaked at about 60 days and 1.5% slurry peaked at 30 days, after 3 days of
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Fig. 3. The effect of solids removal methods on the time taken for the VFA to reach 0.23 g/litre. • = 1-day SRT; • = 3-day SRT. y-axis notation indicates solids removal methods: R = none removed; 0"6,0.3, 0,15, 0-075 refer to the laboratory sieveapertures in mm (top, first experiment; bottom, second experiment); C = decanting centrifuge; LC = laboratory centrifuge; S = vibrating screen. aerobic treatment in a batch reactor. More thoroughly-treated slurries tended to reach a peak later than less well-treated ones. Both centrifuged and uncentrifuged slurries from the 3.6-day S R T treatment had an initial V F A content of zero (Fig. 5), the centrifuged slurry remained below the 0-23 g/litre level for 35 days, 10 days longer than the uncentrifuged slurry and below the 0.52 g/litre V F A level for 44 days. The vibrating screen separator The aerobic treatment process from which the slurry for the first experiment using the commercial vibrating screen separator was taken, suffered a b r e a k d o w n 2 days prior to the experiment, therefore the slurry used here was less well-treated than had been expected and the initial V F A levels were higher than expected and higher than those in the previous experiments. Separation had no effect on the initial V F A level o f either slurry: it also had no effect on the rate of increase of V F A in slurries from the shorter retention time treatment. However, the effect on slurry treated for 3"6-day S R T was to increase the time to reach the peak V F A level from 70 days to more than 100 days. The peak V F A levels for the shorter S R T treatment occurred at a b o u t 90-100 days. This illustrates that solids removal is not a substitute for a poor aerobic treatment process, but can only improve a stable, well-designed regime.
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R. W. Sneath
In the second experiment using the vibrating screen separator the aerobic treatment process performed properly. The effects of using the vibrating screen separator and the laboratory sieves were compared. After 120 days of storage the dry matter contents had fallen further to 2.8 and 2.6% for the screened slurries (Table 2). The V F A levels in screened and unscreened slurry from the shorter SRT treatment remained about equal, passing the 0.23 g/litre threshold after about 5 days (Fig. 3). There was a more marked difference, in the time taken to reach the threshold level with slurries from the 3"2-day SRT treatment (Fig. 3). The screened samples took 45 days and the unscreened 32 days. Laboratory sieves, second experiment The initial V F A levels of the sieved slurries after 1.1-day SRT treatment were less than the unsieved and reached the 0.23 g/litre threshold after 6, 8 and 10 days compared with 5 days for unsieved slurry (Fig. 3). Sieved slurries from the 3-2-day SRT treatment had an initial VFA content of zero, as did the unsieved slurry. VFA levels for the sieved slurry began to rise at a similar rate to that when separated with the vibrating screen, however, the V F A level in slurry sieved through the 0.15 m m sieve reached the threshold value after 43 days (Fig. 3). The threshold value was reached in the slurry passing through the 0.075 and the 0.30 m m sieves after 46 and 48 days respectively, but they did not reach the next level of 0.52g/litre (Fig. 4). That sieved through the 0.075 mm sieve fell below 0"23 g/litre V F A after 107 days (i.e. the odour returned to being only faintly offensive.) The time in storage before the V F A levels reached the thresholds of 0.23 g/litre was correlated with the initial BOD s, COD and dry matter contents. There appears to be a trend of longer storage times resulting from lower initial BODs, COD and dry matter values, but in only two of the correlations were the regression coefficients significant, those for COD and initial dry matter content when aerated for 3-6-day SRT.
and
T.23 = 6 5 . 5 - 0.6 COD (sig. at 95% c.1.) T.z3 = 73 - 10-2 DMI (sig. at 95% c.1.)
DISCUSSION Within each batch of aerated slurry it appears that the more effective solids removal methods had greatest effect on increasing the length of the storage
Storage of pig slurry after solids removal
187
period (by more than one-third) before the odour returned, but with slurry from different batches the increases in the stable-storage periods were different. Some reasons for this are apparent. First, removal of solids from the slurry measured by the dry matter reduction may not be a satisfactory method of measuring the change in the slurry's ability to produce odorous compounds. For example, a sieving which removes only coarse particles from the slurry may reduce the dry matter content considerably but these particles would decompose relatively slowly and therefore have little effect on the stable storage period. Conversely, if the same dry matter reduction was achieved by removing the soluble and very fine fractions, those which decompose to odorous compounds quickly, the stable-storage time would increase greatly. The slurry would become odorous again only when the coarse particles decomposed. In order to show better how solids removal affects the length of the stablestorage period more information is required about the sizes of particles removed and the compounds produced when various sizes of particles decompose. The results of the laboratory sieving experiments indicate that a 0"075 mm hole-size sieve removed a similar quantity and size range of particles as the decanting centrifuge. Vibrating screen separators, with a 0-075 mm screen, therefore have the potential of reducing the cost of the aeration process to a similar extent to that which a decanting centrifuge does, but for a lower capital and running cost. (The main disadvantage of vibrating screens as slurry separators is the low moisture content of the solids removed.) The performance of a commercial machine fitted with the above mesh size would need to be measured to determine the possible running costs and capital requirements. The laboratory centrifuge increased the stable-storage period by about 200%. However, the degree of solids removal achieved in the laboratory could be achieved on a practical scale only by using flocculants prior to separation in a decanting centrifuge.
ACKNOWLEDGEMENTS Thanks are due to the Pig Breeders Supply Co. Ltd for kindly allowing me to use the Trobridge pig slurry separator installed on their piggery, to Paul Wood for doing the laboratory sievings, to the staff of the Central Laboratory for analysing all the samples and to Dr A. G. Williams for advice in preparing this report.
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REFERENCES APHA (1975). Standard Methods for the Examination of Water and Waste Water (14th edn). American Public Health Association, Washington, DC. Hepherd, R. Q. (1976). UK Patent specification No. 1447755, separating apparatus. Sneath, R. W., Shaw, M. & Williams, A. G, (1988). Centrifugation for separating piggery slurry. 1. Performance of a decanting centrifuge. J. agric. Engng Res., 39, 181-90. Williams, A. G. (1984). Indicators of piggery slurry odour offensiveness. Agric. Wastes, 10, 15-36. Williams, A. G., Shaw, M. & Adams, S. J. (1984). The biological stability of aerobically-treated pig slurry during storage. J. agric. Engng Res., 29, 231-9. Williams, A. G., Shaw, M., Selviah, C. M. & Cumby, R. J. (1986). Oxygen requirements for controlling odours from pig slurry by aeration. In Odour Prevention and Control of Organic Sludge and Livestock Farming Waste, ed. V. C. Nielsen, J. H. Voorburg & P. rHermite. Elsevier Applied Science Publishers, Barking, pp. 258-72.
The Effects of Removing Solids from Aerobically Treated Piggery Slurry on the VFA Levels during Storage R. W. Sneath Waste Engineering Group, Buildings and Livestock Division, AFRC Engineering, Wrest Park, Silsoe, Bedford, MK45 4HS, UK (Received 28 November 1987; revised version received 10 March 1988; accepted 23 March 1988)
A BSTRA CT Piggery slurries treated in pilot-scale aeration vessels were sieved or centrifuged to remove a proportion of the solids. The resulting liquidfractions were stored at 10°C; at intervals samples were removed and analysed for volatile fatty acid ( VFA ) content. Removal of solids using fine sieves or the decanting centrifuge extended the storage times by one-third before the VFA level indicated that offensive odours had returned to the slurries. A laboratory centrifuge tripled the stable-storage period.
INTRODUCTION Aerobic treatment is an effective process for controlling slurry odours, but treatment times need to be minimised for economy. One problem with short treatment times is that the treated slurry will not necessarily remain stable during storage before land spreading. Previous work comparing the effects of the same period of aerobic treatment on slurries of 1.5-4-5% dry matter content (DM) showed that the slurry at 1-5% D M could, after treatment, be stored indefinitely without significant return of odour. In contrast, at 4.5% D M the period of odourfree storage was only a matter of days (Williams et al., 1984). Low dry matter contents can be achieved in two ways, either by dilution with water or by removing solids. Dilution with water, however, is likely to be impractical and expensive because of the cost of water, storage, transport and spreading. 175
Biological Wastes 0269-7483/88/$03.50 © 1988 Elsevier Science Publishers Ltd, England. Printed in Great Britain
176
R. W. Sneath
Removal of solids, on the other hand, will reduce the volume needed for storage and reduce the spreading and transport costs, but there will be an added cost for the process. Solids removal after biological treatment is arguably easier, since large bacterial flocs may have grown, thus increasing the numbers of large particles. In addition, degradation of soluble and colloidal compounds will have reduced the viscosity of the liquid. This makes centrifugation an appropriate process to consider. The solids removed ('fibre') should also be odour-free at the time of removal, unlike those removed before treatment, which require composting to become odour-free. The objectives of the work reported in this paper were to measure the effects of removing solids from slurries (using centrifuges, a vibrating screen separator and laboratory sieves) on the stability of aerobically-treated slurries during storage.
METHODS Treatments of slurries prior to solids removal
Raw piggery slurry was collected weekly from a fattening pig house containing 50-100 pigs. The slurry was separated with a brushed-screen roller-press separator (Hepherd, 1976) before aerobic treatment. All the solids removal techniques used were applied to slurries that had been aerobically treated in two ways. Oxygen was supplied to the two treatments at rates such that the redox potential was controlled between 0 and 100 mV Eh (corrected for the calomel-Pt potential) for the 'low O2', and for the 'high 02' the dissolved oxygen concentration was controlled at between 0.5 and 2.0mg/1. These treatments had nominal solids retention times (SRT) of 1 and 3 days respectively in a continuous-flow treatment vessel. The actual SRT and the corresponding solids removal techniques are described in Table 1. Temperatures in the treatment vessels were controlled at 30-35°C. (A fuller description of the treatment apparatus is given by Williams et al. (1986).) The effects of the aerobic treatments that the slurries had undergone are also shown in Table 1, where the chemical oxygen demand (COD), biochemical oxygen demand of the whole slurry (BODw) and the supernatant BOD (BODs) are given for the feed to the treatment plant and the treated slurry (ML) which was then physically treated. Since these aerobically treated slurries had had the coarse fibre removed with a brushed-screen and roller-press separator, any further removal of solids would need equipment capable of removing finer particles, e.g. fine sieves or a centrifuge.
Storage of pig slurry after solids removal
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178
R. W. Sheath
Solids removal equipment and techniques The laboratory sieves used for the first experiment had nominal hole sizes of 0.6, 0.3, 0.15 and 0-075mm and a diameter of 200mm. Samples (about 5 litres) of each treated slurry and each sieve were prepared and from each preparation 10 sub-samples of 250 ml were then taken for storage. The laboratory centrifuge was used at 1000g for 5 min and operated in a batch mode. The vibrating screen separator was a commercial slurry separator fitted with a 60 mesh (nominally 0.25 m m aperture) stainless steel screen. This being a full-scale machine, it was fed with a p u m p at about 1000 litres/h. The separator was used to separate solids from aerobically-treated slurry which had been collected over the previous 24-h period from each of the aeration vessels. Samples of the separator output were taken at intervals for storage, as described above. The decanting centrifuge was a Pennwalt-Sharples P600 Super-D-Canter operated at 5000 rpm and exerting 2200g at the bowl circumference. It was fed at 600 litres/h. The liquid level in the centrifuge bowl was set at 60.5 m m radius, this produced an acceptably-dry solid fraction. Liquid samples were taken for storage. Sieves and separator Further batches of treated slurry were separated using the vibrating screen separator and laboratory sieves to confirm and clarify the results obtained in the above experiments. The slurries from the same aerobic treatment processes as before were used (although the RT were slightly different), but this time solids were removed by using laboratory sieves with 0-3, 0-15 and 0.075 m m apertures. A large quantity of the same slurry was separated using the same vibrating screen separator with the same size mesh.
Storage Samples from each of the experiments were stored in a controlled temperature room at 10°C for up to 120 days. Ten samples, taken in triplicate, were stored in closed 250 ml containers, one sample was removed for analysis after approximately 0, 1, 3, 7, 14, 21, 28, 50 and 100 or 120 days.
Analyses Samples were taken of aerated slurry and of the liquids after solids removal; all were analysed for dry matter content (DM), and most were analysed for 5-day biochemical oxygen demand (BODs), soluble biochemical oxygen
Storage of pig slurry after solids removal
179
demand (BODs), and chemical oxygen demand (COD), using standard methods (APHA, 1975). The particle size distributions were determined on selected treated slurries and liquids by wet sieving through a 0.15 mm sieve and using a Coulter Counter (Coulter Counter Ltd) for the fraction passing the sieve. The volatile fatty acid (VFA) content of the liquid samples was determined after storage using a gas/liquid chromatograph (Williams et al., 1984). The VFA content of aerobically-treated piggery slurries had been found to be an indicator of the offensiveness of the odour of that slurry. Slurries containing up to 0.23 g/litre VFA were considered acceptable by an odour panel (Williams, 1984), while a threshold of unequivocal unacceptability was reached at 0-52 g/litre VFA. Thus, slurries stored until the VFA concentration reaches 0-23 g/litre should not cause odour problems, while those containing above 0.52 g/litre should release offensive odours.
RESULTS
The physical effects of removing solids The decanting centrifuge Using the decanting centrifuge on the aerated slurries from the two treatments (Table 1) reduced the dry matter content (Table 2) from 3"2 to 2-1% for the l.l-day SRT treatment and from 3"6 to 2"6% for the 3.6-day SRT treatment. During storage the dry matter content decreased further. The particle size analyses of the slurry treated for 1-1-day SRT show that 90% of the particles were less than 0" 15 mm diameter and 89% were less than 0"016 mm diameter (Fig. 1). The centrifuged slurry had 96% below 0-15 mm diameter and 94% less than 0.016 mm diameter. The vibrating screen The vibrating screen separator reduced the initial dry matter content of slurry from the 'low O2', 1-1-day SRT treatment from 3.4 to 2.9% and of slurry from the 'high O2', 3.6-day SRT treatment, from 4-0 to 3.2%. After 100 days of anaerobic storage the unseparated slurries were 3.1 and 3.5% DM respectively and the separated slurries were 2-6 and 3"0% DM. The second batches separated using the screen had dry matter contents reduced from 3-8 to 3-0% and 4.0 to 2.7% for the short and long SRT treatments respectively (Table 2). Particle size analysis of the 3.2-day SRT treatment screened and unscreened slurries (Fig. 2) showed all particles in the screened slurry to be under 0.25 mm. This is to be expected since the mesh in the separator was
R. W. Sneath
180
TABLE 2 Properties o f the Slurries before a n d after Storage
Retention Treatment time (days)
Dry matter contents ( % ) Initial
1-0
3"0
1'1 3-6
1'1 3"6
0.7
3"2
After SR a
After storage
Oxygen demands before storage (g/litre ) BOD
COD
BOD s
0'3 0-3 0-3 0"5
L a b o r a t o r y sieves a n d l a b o r a t o r y 0.6 m m sieve 0.3ram sieve 0-15 m m sieve lab. centrifuge 0.6 m m sieve 0.3 m m sieve 0.15 m m sieve lab. centrifuge
centrifuge 2-6 2.2 2.3 2'1 2'1 1.8 1.1 1'0 2.2 2-0 2.3 1-9 1'9 1"8 1-2 1.1
Decanting centrifuge Whole Centrifuged Whole Centrifuged
3-2 3.2 3.6 3"6
b 2.1 -2-6
2"9 2.0 3-4 2-4
7"1 7'1 6"5 6"9
31'4 27.8 27.2 21'5
Vibrating screen Whole Screened Whole Screened
3"4 3-4 4.0 4-0
-2"9 -3-2
3-1 2'6 3'5 3-0
6-9 6"6 7"5 7"3
31'0 30"8 42.3 33"7
3.5 2"8 2'7 2.7 2'5 4-0 2'6 2.8 2-6 2'5
6.2 5"8 5'1 5-9 1'2 5-2 4.9 4.7 6.1 3'3
51.2 36-0 30'2 43-6 32"4 52-4 35"0 36.2 34-1 28.1
L a b o r a t o r y sieves a n d vibrating screen Whole 3-8 -Screened 3'8 3"0 0"3 m m sieve 3'8 2'9 0 . 1 5 m m sieve 3'8 2-7 0-075 m m sieve 3"8 2.7 Whole 4.0 -Screened 4"0 2'7 0'3 m m sieve 4.0 3.0 0 . 1 5 m m sieve 4.0 2-8 0.075 m m sieve 4.0 2.7
Blanks, not analysed. a Solids removal. b N o t applicable.
0.8 0"7 0.7
Storage of pig slurry after solids removal 100
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after using the vibrating screen, (c) after using the decanting centrifuge. nominally 0.25 mm, and 75% were under 0.020 mm. In the unscreened slurry 94% were less than 0.25 m m diameter and only 65% less than 0.020 mm.
Laboratory sieves Sieving slurry from the shorter SRT treatment using 0-30, 0-15, 0.075 m m sieves reduced the dry matter content from 3.8 to 2.9, 2.7 and 2.7% respectively (Table 2). The dry matter content of slurry from the longer SRT treatment was reduced from 4-0 to 3.0, 2.8 and 2.7% by the three sieves, respectively. The particle size analysis, Fig. 2, shows how the use of the 0.075 m m sieve changed the particle distribution. In the unsieved slurry 78% of the particles were smaller than 0.016 mm diameter and 60% smaller than 0.005 mm, in the sieved slurry 88% were smaller than 0.016 mm but only 34% were smaller than 0.005mm diameter. This suggests that many small particles were removed from the liquid, not because they were retained by the sieve but because they were attached to large particles which were retained by the sieve. This mode of separation has also been observed in a decanting centrifuge by Sneath et al. (1988). The effects of solids removal on slurry stability
The laboratory sieves and laboratory centrifuge The times to reach the threshold VFA level of 0.23 g/litre were similar for the liquids passing through the 0.15, 0"3"and the 0"6 mm sieves from the slurries
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Storage of pig slurry after solids removal
183
aerated for 1 day and for 3 days (Fig. 3). The horizontal axis is marked off to show the treatment of the aerated slurry in order, from left to right, of effectiveness of solids removal. The laboratory centrifuge had the greatest effect on reducing the rate of increase of VFA content. Liquid aerobically treated for 1 day had similar VFA levels to that treated for 3 days. Both remained below the 'faintly offensive' level of 0.23 g/litre for about 35 days (Fig. 3). Neither reached a VFA level of 0-52 g/litre (Fig. 4), a level at which the odours might still not be rated as 'definitely offensive' according to a scale devised to correlate odour offensiveness of aerated piggery slurry with the total VFA content (Williams, 1984). The dry matter content of the slurries stored (Table 2) had a significant effect on the time taken to reach the threshold levels. There was a linear relationship between dry matter content at the beginning of storage and the time taken to reach the threshold levels. These relationships are for the 3-day SRT. 1 7 2 - 69DM~ at the 90% confidence level (c.1.) T.52 = 59 - 23DM~ at the 95% c.1. T.23 =
and
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T.23 = 159 - 63DM! T.52 = 4 9 - 19DM I, both at the 95% c.1.
where T.23 = time to reach the 0.23 g/litre threshold, T.52 = time to reach the 0"52 g/litre threshold, DM~ = dry matter content at the beginning of storage. The dry matter contents of the slurries decreased during storage (Table 2). This decrease is the result of solids in the slurry being converted by anaerobic micro-organisms to volatile materials. The decanting centrifuge Centrifuging reduced the initial VFA content of shorter-treatment slurry and this difference between the centrifuged and uncentrifuged slurries was maintained so that the centrifuged slurry remained below the threshold level of 0-23 g/litre for 5 days compared with 3 days for the uncentrifuged slurry (Fig. 5). The peak VFA level of 0.9 g/litre was reached on the 28th day for centrifuged slurry, while uncentrifuged slurry reached 2.6 g/litre on the 74th day. The times taken to reach the peak VFA levels are similar to those observed previously by Williams et al. (1984); i.e., lower dry matter slurries peak earlier. In that work 4-5% DM peaked at 90 days and 3% DM slurry peaked at about 60 days and 1.5% slurry peaked at 30 days, after 3 days of
184
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Fig. 3. The effect of solids removal methods on the time taken for the VFA to reach 0.23 g/litre. • = 1-day SRT; • = 3-day SRT. y-axis notation indicates solids removal methods: R = none removed; 0"6,0.3, 0,15, 0-075 refer to the laboratory sieveapertures in mm (top, first experiment; bottom, second experiment); C = decanting centrifuge; LC = laboratory centrifuge; S = vibrating screen. aerobic treatment in a batch reactor. More thoroughly-treated slurries tended to reach a peak later than less well-treated ones. Both centrifuged and uncentrifuged slurries from the 3.6-day S R T treatment had an initial V F A content of zero (Fig. 5), the centrifuged slurry remained below the 0-23 g/litre level for 35 days, 10 days longer than the uncentrifuged slurry and below the 0.52 g/litre V F A level for 44 days. The vibrating screen separator The aerobic treatment process from which the slurry for the first experiment using the commercial vibrating screen separator was taken, suffered a b r e a k d o w n 2 days prior to the experiment, therefore the slurry used here was less well-treated than had been expected and the initial V F A levels were higher than expected and higher than those in the previous experiments. Separation had no effect on the initial V F A level o f either slurry: it also had no effect on the rate of increase of V F A in slurries from the shorter retention time treatment. However, the effect on slurry treated for 3"6-day S R T was to increase the time to reach the peak V F A level from 70 days to more than 100 days. The peak V F A levels for the shorter S R T treatment occurred at a b o u t 90-100 days. This illustrates that solids removal is not a substitute for a poor aerobic treatment process, but can only improve a stable, well-designed regime.
Laboratory sievesand laboratory centrifuge 120
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Fig. 4. The effect of solids removal methods on the time taken for the VFA to reach the threshold level of 0.52 g/litre. • = 1-day SRT; • = 3-day SRT. y-axis notation indicates solids removal methods: R = none removed; 0-6, 0"3, 0.15, 0-075 refer to the laboratory sieve apertures in mm (top, first experiment; bottom, second experiment); C = decanting centrifuge; LC = laboratory centrifuge; S = vibrating screen. 3.0
2.0
VFA g/I
(a)
1.0
~
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0
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20
40 60 Storage time, days
80
100
Fig. 5. Comparison of the changes in VFA level with storage time for aerated slurries and the liquid fractions from those slurries after passing through the decanting centrifuge: (a) uncentrifuged l.l-day SRT, (b) centrifuged l-l-day SRT, (c) uncentrifuged 3-6-day SRT, (d) centrifuged 3-6-day SRT.
186
R. W. Sneath
In the second experiment using the vibrating screen separator the aerobic treatment process performed properly. The effects of using the vibrating screen separator and the laboratory sieves were compared. After 120 days of storage the dry matter contents had fallen further to 2.8 and 2.6% for the screened slurries (Table 2). The V F A levels in screened and unscreened slurry from the shorter SRT treatment remained about equal, passing the 0.23 g/litre threshold after about 5 days (Fig. 3). There was a more marked difference, in the time taken to reach the threshold level with slurries from the 3"2-day SRT treatment (Fig. 3). The screened samples took 45 days and the unscreened 32 days. Laboratory sieves, second experiment The initial V F A levels of the sieved slurries after 1.1-day SRT treatment were less than the unsieved and reached the 0.23 g/litre threshold after 6, 8 and 10 days compared with 5 days for unsieved slurry (Fig. 3). Sieved slurries from the 3-2-day SRT treatment had an initial VFA content of zero, as did the unsieved slurry. VFA levels for the sieved slurry began to rise at a similar rate to that when separated with the vibrating screen, however, the V F A level in slurry sieved through the 0.15 m m sieve reached the threshold value after 43 days (Fig. 3). The threshold value was reached in the slurry passing through the 0.075 and the 0.30 m m sieves after 46 and 48 days respectively, but they did not reach the next level of 0.52g/litre (Fig. 4). That sieved through the 0.075 mm sieve fell below 0"23 g/litre V F A after 107 days (i.e. the odour returned to being only faintly offensive.) The time in storage before the V F A levels reached the thresholds of 0.23 g/litre was correlated with the initial BOD s, COD and dry matter contents. There appears to be a trend of longer storage times resulting from lower initial BODs, COD and dry matter values, but in only two of the correlations were the regression coefficients significant, those for COD and initial dry matter content when aerated for 3-6-day SRT.
and
T.23 = 6 5 . 5 - 0.6 COD (sig. at 95% c.1.) T.z3 = 73 - 10-2 DMI (sig. at 95% c.1.)
DISCUSSION Within each batch of aerated slurry it appears that the more effective solids removal methods had greatest effect on increasing the length of the storage
Storage of pig slurry after solids removal
187
period (by more than one-third) before the odour returned, but with slurry from different batches the increases in the stable-storage periods were different. Some reasons for this are apparent. First, removal of solids from the slurry measured by the dry matter reduction may not be a satisfactory method of measuring the change in the slurry's ability to produce odorous compounds. For example, a sieving which removes only coarse particles from the slurry may reduce the dry matter content considerably but these particles would decompose relatively slowly and therefore have little effect on the stable storage period. Conversely, if the same dry matter reduction was achieved by removing the soluble and very fine fractions, those which decompose to odorous compounds quickly, the stable-storage time would increase greatly. The slurry would become odorous again only when the coarse particles decomposed. In order to show better how solids removal affects the length of the stablestorage period more information is required about the sizes of particles removed and the compounds produced when various sizes of particles decompose. The results of the laboratory sieving experiments indicate that a 0"075 mm hole-size sieve removed a similar quantity and size range of particles as the decanting centrifuge. Vibrating screen separators, with a 0-075 mm screen, therefore have the potential of reducing the cost of the aeration process to a similar extent to that which a decanting centrifuge does, but for a lower capital and running cost. (The main disadvantage of vibrating screens as slurry separators is the low moisture content of the solids removed.) The performance of a commercial machine fitted with the above mesh size would need to be measured to determine the possible running costs and capital requirements. The laboratory centrifuge increased the stable-storage period by about 200%. However, the degree of solids removal achieved in the laboratory could be achieved on a practical scale only by using flocculants prior to separation in a decanting centrifuge.
ACKNOWLEDGEMENTS Thanks are due to the Pig Breeders Supply Co. Ltd for kindly allowing me to use the Trobridge pig slurry separator installed on their piggery, to Paul Wood for doing the laboratory sievings, to the staff of the Central Laboratory for analysing all the samples and to Dr A. G. Williams for advice in preparing this report.
188
R. W. Sneath
REFERENCES APHA (1975). Standard Methods for the Examination of Water and Waste Water (14th edn). American Public Health Association, Washington, DC. Hepherd, R. Q. (1976). UK Patent specification No. 1447755, separating apparatus. Sneath, R. W., Shaw, M. & Williams, A. G, (1988). Centrifugation for separating piggery slurry. 1. Performance of a decanting centrifuge. J. agric. Engng Res., 39, 181-90. Williams, A. G. (1984). Indicators of piggery slurry odour offensiveness. Agric. Wastes, 10, 15-36. Williams, A. G., Shaw, M. & Adams, S. J. (1984). The biological stability of aerobically-treated pig slurry during storage. J. agric. Engng Res., 29, 231-9. Williams, A. G., Shaw, M., Selviah, C. M. & Cumby, R. J. (1986). Oxygen requirements for controlling odours from pig slurry by aeration. In Odour Prevention and Control of Organic Sludge and Livestock Farming Waste, ed. V. C. Nielsen, J. H. Voorburg & P. rHermite. Elsevier Applied Science Publishers, Barking, pp. 258-72.