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The Effect of Farm Scale Aerobic Treatment of Piggery Slurry on Odour Concentration, Intensity and Offensiveness
J. agric. Engng Res. (1998) 71, 203—211 Article No. ag980313
The Effect of Farm Scale Aerobic Treatment of Piggery Slurry on Odour Concentration, Intensity and Offensiveness C. H. Burton1; R. W. Sneath1; T. H. Misselbrook2; B. F. Pain2 1Silsoe Research Institute, Wrest Park, Silsoe, Bedford MK45 4HS, UK; 2Institute of Grassland and Environmental Research, North Wyke Research Station, Okehampton, Devon, EX20 2SB, UK (Received 3 December 1996; accepted in revised form 26 March 1998)
Piggery slurry was aerobically treated in a farm scale treatment plant with mean residence times of between 1)7 and 6)3 d. The treated slurry was analysed using standard laboratory methods including key parameters such as COD (chemical oxygen demand) and VFA (volatile fatty acids) concentrations. Further samples were taken and analysed either fresh from treatment or after a period of storage, for odour using olfactometric methods. These included the determination of odour concentration, by dynamic dilution, and offensiveness and intensity, by use of a panel score. Chemical analysis of the treated samples indicated a breakdown of organic material broadly in line with that expected, although, in the short treatment, insufficient aeration may have retarded the extent of the process. The olfactometry clearly demonstrated that reduction of odour in terms of concentration and offensiveness were achieved by aerobic treatment. The reduced level of odour was evident even after 28 d of subsequent anaerobic storage. Typically, the treatment reduced the concentration by 50—75% although this was insufficient to reduce the perceived intensity at source to below two, (equating to ‘‘faint odour’’) from the high valve of over five (equating to ‘‘very strong odour’’) measured for the untreated slurry. The effect of the duration of treatment on odour abatement was mixed. Reductions of odour concentration were broadly similar in all treatments. However, in terms of odour offensiveness, the best result was clearly achieved by the longest treatment of 6)3 d. In this case, the score of the odour quality of the freshly treated slurry was below two (equating to better than ‘‘faintly offensive odour’’), compared with a score of four for the untreated slurry (equating to ‘‘strongly offensive odour’’) ( 1998 Silsoe Research Institute
0021-8634/98/100203#09 $30.00/0
1. Introduction Odours from storage and land spreading of piggery slurry have been a major cause of nuisance reported to Environmental Health Officers1 throughout the past 10 years. Previous work reported by the authors and others2,3 has sought aerobic and anaerobic treatments that can reduce or remove the odours from the slurry, but few of the papers report direct objective measurements of the relative odour concentrations from the untreated and treated slurries. Some results were reported by Pain et al.4 where odours from two pilot scale aerobic treatments were compared with odours from untreated whole slurry and separated slurry. In all cases, the odour was measured following application of the slurry to grassland. The results indicated a reduction in the total odour emission of 55% for the aerated slurry compared with the untreated slurry. Some reduction in odour was also perceived by separation alone. In a separate study, Pain et al.5 carried out similar measurements on anaerobically reated slurry and reductions in odour of 70—80% were reported. However, evaluation of the treatment process is complicated by the interactions of the slurry with the soil system and the prevailing weather. In the verification of the performance of farm scale treatments reported here, odour measurements were made on slurry taken directly from the reactor or storage vessel.
2. Materials and methods 2.1. ¹he treatment plant and slurry The treatment plant (illustrated in Fig. 1) had been set up for use in earlier studies and a detailed description can be found elsewhere.2 The facility had been set up at a 2000 place pig unit which was run by the Institute of
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A total of three trials were carried out. The mean composition of the raw slurry used in each of the three trials is shown in Table 1. The characteristics (when compared as a percentage of total solids, TS) are similar to those of the slurry from the same piggery in previous studies2,6 and also to slurry used in laboratory and pilotscale experiments by Williams et al.3 and, to a lesser extent, with that used by Evans et al.7 The TS content of the slurry varied depending upon the activities within the piggery, which could not easily be controlled as part of the treatment experiment. In consequence, the concentrations of the slurry used in trials B and C were less than for A as a result of more dilution with wash water.
2.2 ¹rials carried out
Fig. 1. Farmscale treatment plant for pig slurry. A collection pit for raw slurry; B separator; C feed slurry tank; D aeration vessel (12 m3 working capacity); E recirculation pump and air injection; F treated slurry pit
Animal Health (Compton, Berkshire). The processing capacity varied depending on the treatment but nominal throughput was around 6 t/d of pig slurry. Although the plant approximated to a true continuous process, some parts of the operation were carried out on a regular cyclic pattern to enable easier control. Once a day, raw slurry was screened by a brush- roller separator. The screened slurry was itself fed to the main aeration vessel once an hour with the volume added determined by the pump run-time to achieve the required residence time. The level in the vessel was held constant by pumping out (treated) slurry. A recirculation loop provided both an entry point for air (supplied by compressor) and mixing. Aeration was controlled to maintain a redox potential in the aerated slurry within a chosen range. Excess foam was effectively controlled by a rotating mechanical disk set up at the top of the vessel just above the liquid surface. The plant was run automatically and included datalogging facilities for all the main parameters (temperatures, redox and pH valves and slurry throughput.)
Each of the three trials was carried out using a different mean hydraulic residence time of the main aeration vessel. The shorter treatments of 1)7 and 2)4 d in trials A and B, respectively, were not expected to allow nitrification. This was expected to be a factor in the final trial, which was run with a residence time of 6)3 d, following from the work of Williams et al.,3 where the presence of nitrates was shown to enhance odour control during subsequent storage (the nitrates providing an additional oxygen source). The target aeration level was a redox potential in the slurry in the range of !50 to !150 mV E where, for the probe used, E "E !241 mV; low #!#!) valves led to an air compressor switching in until the value was again in range. However, the limited capacity of the aerator led to aeration difficulties, especially for the shorter treatments and enough air could not always be supplied. The result was a mean value below the target range. Table 2 summarizes the conditions maintained for each of the trials. The trials were all carried out in the late Autumn as reflected by the low ambient temperatures. The reactor contents were well within the mesophilic temperature range in all cases; a slightly higher mean temperature recorded for trial B reflected a period of unseasonally warm weather that occurred during this part of the study.
Table 1 Average composition of feed slurries (standard deviation in parentheses) Definitions of the analyses are given in Section 2.3
¹rial A B C Mean value % w/w of TS
¹S, kg/m~3
»S % of ¹S
COD kg/m~3
Kj-N kg/m~3
Am-N kg/m~3
»FA kg/m~3
No. of measurements
32)3 (6)2) 24)3 (3)0) 19)8 (3)9)
76)0 (1)7) 72)8 (1)5) 70)5 (1)9)
48)8 (8)4) 35)2 (6)2) 29)8 (6)4)
2)95 (0)4) 2)80 (0)2) 2)43 (0)37)
1)85 (0)2) 1)92 (0)1) 1)74 (0)23)
3)87 (0)5) 4)44 (0)4) 3)64 (0)60)
7 11 19
100
73
149
11
7
16
—
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Table 2 Mean treatment conditions during the steady-state periods (values in parantheses are one standard deviation)
¹rial
Resid time, d
Monioring period*, d
A B C
1)7 2)4 6)3
7 11 19
pH
Redox potential Ecal
¹emperature, °C Feed
Reactor
Ambient
No. of measurements
7)2 (0)1) 8)2 (0)2) 7)9 (0)4)
!278 (112) !134 (44) !73 (84)
— 11)8 (1)6) 11)7 (2)9)
21)8 (1)5) 32)5 (3)0) 21)8 (2)2)
2)3 (1)2) 9)4 (3)5) 3)5 (2)5)
78 134 215
*Following a settling down period equal to three residence times.
2.3. Slurry sampling, storage and analysis Three types of slurry sample were taken during each trial. Daily samples were taken throughout the monitoring period of raw and treated slurry for biochemical analysis. Standard methods8 were used to analyse for TS (total solids content), VS (volatile solids proportion), COD (chemical oxygen demand), Kj-N Kjeldahl nitrogen; (total nitrogen excluding nitrates and nitrites), Am-N (ammoniacal nitrogen) and VFA (total volatile fatty acids). A second set of samples of treated slurry was taken and stored anaerobically at 10°C. These were analysed for VFA after storage of between 7 and 75 d of storage; the increase in VFA concentration has been shown by Williams9 to be linked to the offensive quality of the slurry odour. Samples for the olfactometric odour evaluation were taken at the end of each trial. For trials A and C, paired 15 l samples were taken of both raw and treated slurry. Odour measurements were carried out on all of these on the same day as sampling. For trial B, the sampling was done in triplicate. One pair of raw and treated slurry samples were analysed on the day of sampling (as for trials A and C). The remaining samples were stored anaerobically at 10°C with odour determinations being carried out after 14 and 28 d respectively.
2.4. Olfactometric evaluation of odour 2.4.1. Collection of the odour samples Collection of the sample of odorous air emitted from the slurries was done using equipment based on that developed by Lockyer to measure other emissions from soil and crops.10 The apparatus consisted of a portable semi-cylindrical wind tunnel 2)0 m long]0)5 m diameter. A fan was used to induce airflow (controlled at 1 m/s) over two stainless steel trays each 1)0 m long]0)5 m wide]0)02 m deep which contained 10 l of the slurry sample to be assessed. Air that had passed over the slurry was sampled using a stainless-steel bellows pump and captured in Teflon FEP odour sample bags with a nom-
inal volume of 60 l. This procedure continued for 5 min immediately after spreading the slurry in the trays. Three replicate tunnels were used to collect three replicate samples for each feed and treated slurry sample. 2.4.2. Measurement of odour concentration The odour concentration was measured using two dynamic dilution olfactometers with a forced choice method of sample presentation to an odour panel consisting of eight people. A typical panel would be drawn from a larger pool of 10—20 individuals who had previously been screened to rule out certain categories (e.g. those with a poor sense of smell, smokers, etc). Six dilutions of each sample, each differing by a factor of two, were presented to the panellists three times. Dilutions were made using odour-free air supplied by a compressor fitted with filters and an air dryer. Both olfactometers had two sniffing ports, one containing diluted sample air and the other contained odour-free air. For each presentation, panellists indicated via a keypad which port contained the odorous air. The mean threshold value (the dilution at which 50% of panel members could just perceive an odour) for each sample was calculated by probit analysis.11 The number of dilutions required to bring the sample down to threshold equates to the concentration of the sample. The measurement is given the units of odour units per m3 (OU m~3) 2.4.3. Odour intensity assessment Odour intensity measurements were made on the samples collected from the untreated slurry following the procedure given in detail by Pain et al.12 A range of dilutions differing by a factor of two each time were presented to each panellist a number of times through one known port of the olfactometer. The highest dilution was equivalent to the 75% odour threshold value. At each dilution, the panellists were required to indicate the subjective strength of the odour, according to the following scale derived by Paduch:13 0 No odour 1 Very faint odour
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2 Faint odour 3 Distinct odour 4 Strong odour 5 Very strong odour 6 Extremely strong odour The relationship between intensity and odour concentration was calculated by linear regression according to eqn (1) below. This follows the general form of the responses of human senses to stimuli (also known as Fechner’s law): I"k log C#k 1 10 2
(1)
where I is the perceived intensity, C the relative odour concentration (equal to the reciprocal of the number of dilutions applied, D) and k and k are constants 1 2 determined from the linear regression. This equation can be rearranged to enable calculation of how many times the odour would have to be diluted to achieve an odour intensity which would be scored as two or ‘‘faint odour’’ (which can be considered an acceptable level), thus giving an indication of the abatement required. 2.4.4. Odour offensiveness A quantity of slurry of 30 ml was put into each of six small pots (three replicates of each treatment) and one further pot contained brown odourless water. Lids were removed from the pots 5 min before presentation to panellists. Panellists were required to smell each pot and indicate the offensiveness of the odour according to the following category scale derived by Williams:9 0 1 2 3 4 5
Inoffensive odour Very faintly offensive odour Faintly offensive odour Definitely offensive odour Strongly offensive odour Very strongly offensive odour
The mean offensiveness score was calculated for each treatment.
3. Results and discussion 3.1. Biochemical performance of treatments The mean analyses of the slurries following each of the three treatments are summarized in Table 3; these can be compared directly with the composition of the raw slurries given in Table 1. In all cases, the reduction of the VFA concentration, an indicator of odour, approached 100%. The elimination of this component is nearest completion in the longer trials as was expected. The effect of the longer treatment was much more apparent in terms of COD reduction. Relative to the raw feed, these reductions were 39% for the 6)3 d treatment of trial C, 40% for trial B (2)4 d) but only 11% for the 1)7 d treatment of trial A. The figure for trial A was lower than expected from correlations for COD reduction developed by Evans et al.7 which predict a fall of around 25% in COD for a 1)7 d treatment (for whole slurry). The removal of some suspended matter by screening is unlikely to have affected the performance as this fraction is largely unaffected by the biological process anyway. The poor performance of trial A probably reflects the difficulty in maintaining the aeration level in this particular trial. This was also seen in terms of reduction in the organic-N component (i.e. the difference between Kjeldahl and ammoniacal N). The similar performance in the reduction of COD between trials B and C was a consequence of better than expected results from trial B. No explanation is offered but it is noted that in terms of N reduction, the longer trial C does give better results than trial B. The significance in the degradation of the organic matter in the slurry lies with its capacity to regenerate the compounds that give rise to offensive odour, of which VFA is an indicator. Thus although VFA was virtually removed in all treatments, its more rapid regeneration in slurry from the shorter treatments was expected on the basis that less degradation of organic material had been achieved. Concentrations of nitrates and nitrites were undetected in any of the samples taken. However, the unavoidable delay in presenting the samples to the laboratory (4—6 h) would have been enough for natural degradation of any such material present by denitrification. It is likely
Table 3 Average composition of treated slurries, with standard deviations in paratheses Trial
Res. time, d
TS, kg/m3
VS, kg/m3
COD, kg/m3
Kj-N, kg/m3
Am-N, kg/m3
Org-N, kg/m3
VFA, kg/m3
No. of measurements
A B C
1)7 2)4 6)3
35)4 (4.)8) 20)2 (1)8) 17)0 (0)12)
77)5 (1)6) 70)8 (1)4) 68)6 (1)7)
43)5 (7)2) 21)1 (2)9) 18)2 (2)8)
2)97 (0)35) 2)62 (0)19) 1)60 (0)23)
1)49 (0)15) 1)65 (0)09) 1)00 (0.23)
1)48 (—) 0)97 (—) 0)60 (—)
0)22 (0)13) 0)08 (0)01) 0)01 (0)01)
7 11 19
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Table 4 Odour concentration values for raw, treated and anaerobically stored slurries ¹rial/sample
Treatment time, d
Storage time, d
Concentration, OU/m3
95% confidence interval
A A B1 B1 B2 B2 B3 B3 C C
Untreated 1)7 Untreated 2)4 Untreated 2)4 Untreated 2)4 Untreated 6)3
0 0 0 0 14 14 28 28 0 0
111 24 177 70 239 65 163 88 57 55
60—330 18—40 110—360 40—170 190—400 24—150 110—280 14—170 27—100 40—190
that nitrates were present in the fresh samples taken from trial C as losses of nitrogen were most probably removed by the nitrification—denitrification route.
3.2. ¹reatment performance in terms of odour control 3.2.1. Odour concentration Table 4 shows mean threshold values for each treatment along with 95% confidence intervals as given by probit analysis.11 Despite the wide confidence interval, the general effect of treatment in reducing the odour concentration is clear. The raw slurry generate odours with a concentration value between 57 and 239 odour units (OU/m3). In most cases, aerobic treatment reduced this by between a half and a quarter of the original value. The exception was trial C where an unusually low value for the raw slurry left little scope for further reduction. Apart from the effect of aeration, no further discernment can be made from these results, i.e. either in terms of treatment times or storage times. The low value of 88 OU/m3 for the treated slurry from trial B, after 28 d of anaerobic storage was only half of the value for the raw slurry thus indicating some lasting effect of aerobic treatment. 3.2.2. Odour intensity Mean intensity scores from the panel for each of the samples at various dilutions are set out in Fig. 2. From this data the regression equations derived for the five untreated (raw) slurries were: log C#4)74, r2"0)87 (2) 10 log C#5)90, r2"0.98 (3) 10 log C#5)01, r2"0)98 (4) 10 log C#5)13, r2"0)97 (5) 10 log C#5)25, r2"0)95 (6) 10 Setting an acceptable value of I (odour intensity) as 2 (faint odour), the required number of dilutions, D, can be calculated by substituting I"2 in the above A B1 B2 B3 C
I"2)46 I"2)29 I"2)16 I"2)53 I"2)76
equations. The % abatement (i.e. reduction in odour concentration) required to reach I"2 is given by
A B
1 1! ]100% D
(7)
The % abatement achieved by the aerobic treatment and subsequent anaerobic storage is given by untreated threshold (OU/m3)!treated threshold (OU/m3) untreated threshold (OU/m3) ] 100%
(8)
using the odour concentration data given in Table 4. These values are compared in Table 5 for each of the five treatments (inclusive of subsequent anaerobic storage where indicated). Valves relating to the untreated slurry imply that an observer would need to stand some distance away before an acceptable ‘‘faint odour’’ was perceived. Complete abatement (achieved by the treatment process) would enable the observer to stand alongside the slurry store and only experience ‘‘faint odour’’. However, this may not always be required. The picture is thus slightly different from that of simply looking at odour concentration. The intensity parameter relates to the actual perception of odour strength by an individual. The number of dilutions required gives an indication of how far from the source the odour could still be a nuisance. From the values reported here, the raw slurry used in trial B is seen to have a strong intensity quality which is reflected in the corresponding measured concentration. However, the effect of storage is to achieve a fall in this quality suggesting that anaerobic storage alone has some benefit in odour control. None of the treatments achieved the complete odour abatement required, i.e. the intensity of odour close to treated slurry would be deemed to be greater than a score of two. However, substantial reduction in intensity was achieved in all except trial C. The apparent poor result in this longest of treatments is in contrast with the high breakdown of organic matter observed in the
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Fig. 2. Correlations of perceived odour intensity (mean value on a scale 1 to 6, as judged by a panel) to odour concentration (varied by dilution) for each of the five samples analysed
corresponding slurry samples analysed. The most likely explanation for this discrepancy probably lies will the odour sampling and analysis, especially as there was a relatively low odour with the slurry used in this trial. 3.2.3. Odour offensiveness and »FA concentrations In many respects, it is the offensive quality of the odour which is of greatest concern because few people will
complain of odours if they are deemed pleasant! Table 6 lists the scores awarded and compares these values to the VFA concentrations measured in the corresponding slurries. In previous work, Williams9 demonstrated that the VFA concentration is a good indicator of the odour offensiveness that would be perceived by a panel. High offensive scores coincide with high concentrations of VFA. A score of 2, equating to ‘‘faintly offensive odour’’,
T H E EF F ECT O F FA RM S CA LE A E RO B IC TR EAT ME NT OF PI G GE RY S LU R RY
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Table 5 Odour abatement by aerobic treatment. Slurry samples B2 and B3 were taken after 14 and 28 d of subsequent anaerobic storage Untreated slurries
¹rial/sample A B1 B2 B3 C
Dilutions required to reach I"2 for raw slurry 13 50 25 17 15
Corresponding % abatement required 92 98 96 94 93
¹reated slurries
% abatement achieved 78 60 73 46 4
occurs with a VFA concentration of 0)23 kg/m3; this has been used as a limit of tolerable odour2 . The correlation derived by Williams is reproduced in Fig. 3 with the data from Table 6 added. Higher scores have generally been given for the samples from this study. Hence, whilst the general trend is confirmed, there can be large variations which limit the value of VFA concentration as an indicator of odour offensiveness. In terms of odour offensiveness, the long treatment (trial C) produced a freshly treated slurry with a score of better than 2 (faintly offensive odour). The odour from slurry from trials A and B was definitely worse bordering on the unacceptable. Once again, the storage of samples from trial B led to a small reduction in odour with a fall in the offensiveness index. Figure 4 illustrates the increases in VFA concentrations in the treated slurries over an aerobic storage period up to 75 d. The various olfactometric tests were only carried out
Fig. 3. Values of odour offensiveness (mean value on a scale 1 to 6, as judged by a panel) for treated slurries related to the VFA concentration of the slurry sample. A straight line correlation linking these two parameters developed in earlier work by Williams9 is included for comparison
on slurry from trial B and up to 28 d. In that period the concentration of VFA remained below 1 kg/m3 which may explain the lack of observation of an increased odour nuisance following storage of aerobically treated slurry for 28 d. The VFA concentration is seen to rise further to 1)6 kg/m3 after 50 d of storage, still much less than the value for the untreated slurry. Concentrations of VFA for treated slurry from trial A are seen to rise faster, as might be expected from the shorter treatment. Likewise, the longer treatment of trial C is seen to produce a more stable slurry in terms of VFA regeneration.
Table 6 Odour offensiveness of untreated and aerobically treated pig slurries
¹rial/sample ¹reatment A B1 B2 B3 C
Untreated Aerated Untreated Aerated Untreated Aerated Untreated Aerated Untreated Aerated
Mean offensiveness score
»FA, kg/m3
4)0 2)8 4)7 3)7 3)6 3)0 ND ND 3)8 1)8
3)68 0)07 4)83 0)11 5)11 0)42 4)99 0)73 3)83 0)01
Fig. 4. Regeneration of VFA during anaerobic storage following aerobic treatment. j Trial A (1)7 d treatment); m Trial B (2)4 d treatment); d Trial C (6)3 d treatment)
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4. Discussion Assessment of the quality of odours is still in its infancy and perhaps it is not surprising that this study has produced some apparent contradictions. The inevitably subjective nature of odour is seen to produce a relatively wide confidence interval despite the objective approach of the olfactometric procedure. (The latest draft European standard for olfactometric methods for odour measurement14 addresses the problem and the methods proposed in it and improves greatly the repeatability of the procedures.) Despite the problems encountered, some clear trends have emerged from the study, together with verification that aerobic treatment can achieve a substantial reduction in the odours produced by pig slurry. To summarize such an abatement with a single figure can be very misleading both in terms of meeting absolute targets and in comparing one system with another. This indeed is the nub of the problem; that is, odour assessment requires several parameters to be specified to give the complete picture. This contrasts with many other treatment objectives, such as COD or nitrogen reduction, which can be determined easily and unambiguously as single measurements from laboratory analysis. Of all the odour parameters measured, it is the odour offensiveness which is perhaps the key quality as it is the unpleasantness of pig slurry smell that initiates most complaints. The beneficial effect of aerobic treatment is very clear in this respect from the results. Williams,9 in relating odour offensiveness to VFA concentration, provided a sound basis for the understanding and development of treatment processes. The removal of the reactive organic materials that give rise to offensive odour by aerobic treatment is both observed and expected. The removal of more complex organic material that can lead to the regeneration of the odorous compounds would also be expected to enable a period of storage without the return of offensive odours; some supporting evidence for this is also seen in this work. However, an offensive odour will only be a problem if it can be detected by people living nearby. This leads to the qualities of odour intensity and odour concentration. The latter gives an idea of the quantity of odour but this by itself may not equate to its perception when diluted with fresh air by a given amount. The concept of intensity gives a clear idea of how much dilution is required to reduce the smell to a tolerable level. The reduction in odour concentration achieved by a given treatment can then be compared with that required. If this reduction is insufficient, then the odour will be perceived close to the source and for some distance away. However, if there has been at least some reduction in odour concentration, then it would be necessary to be closer to the source before the odour becomes
noticeable. Hence, in some cases, where slurry stores are set well back from farm boundaries, this will still equate to adequate abatement. In the final analysis, satisfactory odour abatement will clearly be a combination of sufficient reduction in its offensiveness and its perceived strength in the areas around the farm where unpleasant smells can potentially cause annoyance. Aerobic treatment can achieve such abatement but the extent of treatment required will depend as much on the farm location as on any target quality of the treated slurry.
5. Conclusions (1) Farm scale aerobic treatment of pig slurry achieved a reduction in odour concentration of between a half and a quarter of the value of the raw slurry. This achieved between 46 and 78% of the abatement required to reduce odour intensity to that of faint odour in all except one case. Hence, the intensity of odour close to the slurries would still be rated above ‘‘faint odour’’ but with natural dilution it would not be detected so far away. (2) Aerobic treatment of pig slurry achieved a reduction in the offensiveness rating of the slurry odour in all cases. For the longest treatment (6)3 d), the value for the freshly treated slurry was below 2 ‘‘faintly offensively odour’’. The concentration of VFA correlates to the offensiveness score but there are differences with the results reported by other workers. Such indicators of odour should thus be used with caution. (3) The aerobic breakdown of organic matter in the pig slurry followed general expectations with a fall in COD of 39% for a 6)3 d treatment and just 11% for a short 1)7 d treatment. The effect of duration of treatment on the odour was evident from the reduction of the offensiveness score, the long treatment gave the best result. However, in terms of the reduction of odour concentration, no differences could be perceived between the treatments. (4) Anaerobic storage of pig slurry treated for 2)4 d led to a rise in the VFA concentration from 0)1 to 1)0 kg/m3 after 25 d and to 1)6 kg/m3 after 50 d. However, no regeneration of odour was discerned over the first 28 d. Raw slurry stored over the same period produced an odour of falling intensity with time indicating some benefit from storage alone. (5) The use of olfactometric methods has verified the value of aerobic treatment in achieving odour abatement. However, the establishment of specific targets remains difficult due to the combination of factors that describe an odour nuisance and the wide
T H E EF F ECT O F FA RM S CA LE A E RO B IC TR EAT ME NT OF PI G GE RY S LU R RY
confidence interval that can occur with some of the methods used.
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References 8 1
2
3
4
5
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IEHO Annual Report 87/88 London: The Institution of Environmental Health Officers Sneath R W; Burton C H; Williams A G Continuous aerobic treatment for piggery slurry for odour control scaled up to a farm-size unit. Journal of Agricultural Engineering Research, 1992, 53, 81—92 Williams A G; Shaw M; Selviah C; Cumby R J The oxygen requirements for deodorizing and stabilising pig slurry by aerobic treatment. Journal of Agricultural Engineering Research, 1989, 43, 291—311 Pain B F; Phillips V R; Clarkson C R; Misselbrook T H; Rees Y J; Farrent J W Odour and ammonia emissions following the spreading of aerobically-treated pig slurry on grassland. Biological Wastes, 1990, 34, 149—160 Pain B F; Misselbrook T H; Clarkson C R; Rees Y J Odour and ammonia emissions following the spreading of anaerobically-digested pig slurry on grassland. Biological Wastes, 1990, 34, 259—267 Burton C H; Sneath R W Continuous farm-scale aeration plant for reducing offensive odours from piggery slurry:
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control and optimisation of the process. Journal of Agricultural Engineering Research, 1995, 60, 271—279 Evans M R; Deans E A; Hissett R; Smith M P W; Svoboda I F; Thacker F E The effect of temperature and residence time on aerobic treatment of piggery slurry—degradation of carbonaceous compounds. Agricultural Wastes, 1983, 5, 25—36 APHA, AWWA, WPCF Standard Methods for the Examination of ¼ater and ¼astewater, 16th edn, Washington, DC, 1985 Williams A G Indicators of piggery slurry odour offensiveness. Agricultural ¼astes, 1984, 10(1), 15—36 Lockyer D R A system for the measurement in the field of losses of ammonia through volatilisation. Journal of the Science of Food and Agriculture, 1984, 35, 837—848 Finney D J Probit Analysis. 3rd edn. Cambridge: Cambridge ºniversity Press, 1971 Pain, BF; Phillips VR; Clarkson CR; Misselbrook TU; Rees YJ; Farrent JW Odour and ammonia emissions following the spreading of aerobically-treated pig slurry on grassland. Biological Wastes, 1990, 34, 149—160 Paduch M Present state of VDI guidelines on odour assessment. In »olatile Emissions from ¸ivestock Farming and Sewage Operations. (Nielsen V C, Voorburg J H; L’Hermite P, eds), pp. 38—53. London: Elsevier Applied Science, 1988 CEN TC 264 WG 1 ODOURS Draft European standard. Odour concentration measurement by dynamic olfactometry, 1995
The Effect of Farm Scale Aerobic Treatment of Piggery Slurry on Odour Concentration, Intensity and Offensiveness C. H. Burton1; R. W. Sneath1; T. H. Misselbrook2; B. F. Pain2 1Silsoe Research Institute, Wrest Park, Silsoe, Bedford MK45 4HS, UK; 2Institute of Grassland and Environmental Research, North Wyke Research Station, Okehampton, Devon, EX20 2SB, UK (Received 3 December 1996; accepted in revised form 26 March 1998)
Piggery slurry was aerobically treated in a farm scale treatment plant with mean residence times of between 1)7 and 6)3 d. The treated slurry was analysed using standard laboratory methods including key parameters such as COD (chemical oxygen demand) and VFA (volatile fatty acids) concentrations. Further samples were taken and analysed either fresh from treatment or after a period of storage, for odour using olfactometric methods. These included the determination of odour concentration, by dynamic dilution, and offensiveness and intensity, by use of a panel score. Chemical analysis of the treated samples indicated a breakdown of organic material broadly in line with that expected, although, in the short treatment, insufficient aeration may have retarded the extent of the process. The olfactometry clearly demonstrated that reduction of odour in terms of concentration and offensiveness were achieved by aerobic treatment. The reduced level of odour was evident even after 28 d of subsequent anaerobic storage. Typically, the treatment reduced the concentration by 50—75% although this was insufficient to reduce the perceived intensity at source to below two, (equating to ‘‘faint odour’’) from the high valve of over five (equating to ‘‘very strong odour’’) measured for the untreated slurry. The effect of the duration of treatment on odour abatement was mixed. Reductions of odour concentration were broadly similar in all treatments. However, in terms of odour offensiveness, the best result was clearly achieved by the longest treatment of 6)3 d. In this case, the score of the odour quality of the freshly treated slurry was below two (equating to better than ‘‘faintly offensive odour’’), compared with a score of four for the untreated slurry (equating to ‘‘strongly offensive odour’’) ( 1998 Silsoe Research Institute
0021-8634/98/100203#09 $30.00/0
1. Introduction Odours from storage and land spreading of piggery slurry have been a major cause of nuisance reported to Environmental Health Officers1 throughout the past 10 years. Previous work reported by the authors and others2,3 has sought aerobic and anaerobic treatments that can reduce or remove the odours from the slurry, but few of the papers report direct objective measurements of the relative odour concentrations from the untreated and treated slurries. Some results were reported by Pain et al.4 where odours from two pilot scale aerobic treatments were compared with odours from untreated whole slurry and separated slurry. In all cases, the odour was measured following application of the slurry to grassland. The results indicated a reduction in the total odour emission of 55% for the aerated slurry compared with the untreated slurry. Some reduction in odour was also perceived by separation alone. In a separate study, Pain et al.5 carried out similar measurements on anaerobically reated slurry and reductions in odour of 70—80% were reported. However, evaluation of the treatment process is complicated by the interactions of the slurry with the soil system and the prevailing weather. In the verification of the performance of farm scale treatments reported here, odour measurements were made on slurry taken directly from the reactor or storage vessel.
2. Materials and methods 2.1. ¹he treatment plant and slurry The treatment plant (illustrated in Fig. 1) had been set up for use in earlier studies and a detailed description can be found elsewhere.2 The facility had been set up at a 2000 place pig unit which was run by the Institute of
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A total of three trials were carried out. The mean composition of the raw slurry used in each of the three trials is shown in Table 1. The characteristics (when compared as a percentage of total solids, TS) are similar to those of the slurry from the same piggery in previous studies2,6 and also to slurry used in laboratory and pilotscale experiments by Williams et al.3 and, to a lesser extent, with that used by Evans et al.7 The TS content of the slurry varied depending upon the activities within the piggery, which could not easily be controlled as part of the treatment experiment. In consequence, the concentrations of the slurry used in trials B and C were less than for A as a result of more dilution with wash water.
2.2 ¹rials carried out
Fig. 1. Farmscale treatment plant for pig slurry. A collection pit for raw slurry; B separator; C feed slurry tank; D aeration vessel (12 m3 working capacity); E recirculation pump and air injection; F treated slurry pit
Animal Health (Compton, Berkshire). The processing capacity varied depending on the treatment but nominal throughput was around 6 t/d of pig slurry. Although the plant approximated to a true continuous process, some parts of the operation were carried out on a regular cyclic pattern to enable easier control. Once a day, raw slurry was screened by a brush- roller separator. The screened slurry was itself fed to the main aeration vessel once an hour with the volume added determined by the pump run-time to achieve the required residence time. The level in the vessel was held constant by pumping out (treated) slurry. A recirculation loop provided both an entry point for air (supplied by compressor) and mixing. Aeration was controlled to maintain a redox potential in the aerated slurry within a chosen range. Excess foam was effectively controlled by a rotating mechanical disk set up at the top of the vessel just above the liquid surface. The plant was run automatically and included datalogging facilities for all the main parameters (temperatures, redox and pH valves and slurry throughput.)
Each of the three trials was carried out using a different mean hydraulic residence time of the main aeration vessel. The shorter treatments of 1)7 and 2)4 d in trials A and B, respectively, were not expected to allow nitrification. This was expected to be a factor in the final trial, which was run with a residence time of 6)3 d, following from the work of Williams et al.,3 where the presence of nitrates was shown to enhance odour control during subsequent storage (the nitrates providing an additional oxygen source). The target aeration level was a redox potential in the slurry in the range of !50 to !150 mV E where, for the probe used, E "E !241 mV; low #!#!) valves led to an air compressor switching in until the value was again in range. However, the limited capacity of the aerator led to aeration difficulties, especially for the shorter treatments and enough air could not always be supplied. The result was a mean value below the target range. Table 2 summarizes the conditions maintained for each of the trials. The trials were all carried out in the late Autumn as reflected by the low ambient temperatures. The reactor contents were well within the mesophilic temperature range in all cases; a slightly higher mean temperature recorded for trial B reflected a period of unseasonally warm weather that occurred during this part of the study.
Table 1 Average composition of feed slurries (standard deviation in parentheses) Definitions of the analyses are given in Section 2.3
¹rial A B C Mean value % w/w of TS
¹S, kg/m~3
»S % of ¹S
COD kg/m~3
Kj-N kg/m~3
Am-N kg/m~3
»FA kg/m~3
No. of measurements
32)3 (6)2) 24)3 (3)0) 19)8 (3)9)
76)0 (1)7) 72)8 (1)5) 70)5 (1)9)
48)8 (8)4) 35)2 (6)2) 29)8 (6)4)
2)95 (0)4) 2)80 (0)2) 2)43 (0)37)
1)85 (0)2) 1)92 (0)1) 1)74 (0)23)
3)87 (0)5) 4)44 (0)4) 3)64 (0)60)
7 11 19
100
73
149
11
7
16
—
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Table 2 Mean treatment conditions during the steady-state periods (values in parantheses are one standard deviation)
¹rial
Resid time, d
Monioring period*, d
A B C
1)7 2)4 6)3
7 11 19
pH
Redox potential Ecal
¹emperature, °C Feed
Reactor
Ambient
No. of measurements
7)2 (0)1) 8)2 (0)2) 7)9 (0)4)
!278 (112) !134 (44) !73 (84)
— 11)8 (1)6) 11)7 (2)9)
21)8 (1)5) 32)5 (3)0) 21)8 (2)2)
2)3 (1)2) 9)4 (3)5) 3)5 (2)5)
78 134 215
*Following a settling down period equal to three residence times.
2.3. Slurry sampling, storage and analysis Three types of slurry sample were taken during each trial. Daily samples were taken throughout the monitoring period of raw and treated slurry for biochemical analysis. Standard methods8 were used to analyse for TS (total solids content), VS (volatile solids proportion), COD (chemical oxygen demand), Kj-N Kjeldahl nitrogen; (total nitrogen excluding nitrates and nitrites), Am-N (ammoniacal nitrogen) and VFA (total volatile fatty acids). A second set of samples of treated slurry was taken and stored anaerobically at 10°C. These were analysed for VFA after storage of between 7 and 75 d of storage; the increase in VFA concentration has been shown by Williams9 to be linked to the offensive quality of the slurry odour. Samples for the olfactometric odour evaluation were taken at the end of each trial. For trials A and C, paired 15 l samples were taken of both raw and treated slurry. Odour measurements were carried out on all of these on the same day as sampling. For trial B, the sampling was done in triplicate. One pair of raw and treated slurry samples were analysed on the day of sampling (as for trials A and C). The remaining samples were stored anaerobically at 10°C with odour determinations being carried out after 14 and 28 d respectively.
2.4. Olfactometric evaluation of odour 2.4.1. Collection of the odour samples Collection of the sample of odorous air emitted from the slurries was done using equipment based on that developed by Lockyer to measure other emissions from soil and crops.10 The apparatus consisted of a portable semi-cylindrical wind tunnel 2)0 m long]0)5 m diameter. A fan was used to induce airflow (controlled at 1 m/s) over two stainless steel trays each 1)0 m long]0)5 m wide]0)02 m deep which contained 10 l of the slurry sample to be assessed. Air that had passed over the slurry was sampled using a stainless-steel bellows pump and captured in Teflon FEP odour sample bags with a nom-
inal volume of 60 l. This procedure continued for 5 min immediately after spreading the slurry in the trays. Three replicate tunnels were used to collect three replicate samples for each feed and treated slurry sample. 2.4.2. Measurement of odour concentration The odour concentration was measured using two dynamic dilution olfactometers with a forced choice method of sample presentation to an odour panel consisting of eight people. A typical panel would be drawn from a larger pool of 10—20 individuals who had previously been screened to rule out certain categories (e.g. those with a poor sense of smell, smokers, etc). Six dilutions of each sample, each differing by a factor of two, were presented to the panellists three times. Dilutions were made using odour-free air supplied by a compressor fitted with filters and an air dryer. Both olfactometers had two sniffing ports, one containing diluted sample air and the other contained odour-free air. For each presentation, panellists indicated via a keypad which port contained the odorous air. The mean threshold value (the dilution at which 50% of panel members could just perceive an odour) for each sample was calculated by probit analysis.11 The number of dilutions required to bring the sample down to threshold equates to the concentration of the sample. The measurement is given the units of odour units per m3 (OU m~3) 2.4.3. Odour intensity assessment Odour intensity measurements were made on the samples collected from the untreated slurry following the procedure given in detail by Pain et al.12 A range of dilutions differing by a factor of two each time were presented to each panellist a number of times through one known port of the olfactometer. The highest dilution was equivalent to the 75% odour threshold value. At each dilution, the panellists were required to indicate the subjective strength of the odour, according to the following scale derived by Paduch:13 0 No odour 1 Very faint odour
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2 Faint odour 3 Distinct odour 4 Strong odour 5 Very strong odour 6 Extremely strong odour The relationship between intensity and odour concentration was calculated by linear regression according to eqn (1) below. This follows the general form of the responses of human senses to stimuli (also known as Fechner’s law): I"k log C#k 1 10 2
(1)
where I is the perceived intensity, C the relative odour concentration (equal to the reciprocal of the number of dilutions applied, D) and k and k are constants 1 2 determined from the linear regression. This equation can be rearranged to enable calculation of how many times the odour would have to be diluted to achieve an odour intensity which would be scored as two or ‘‘faint odour’’ (which can be considered an acceptable level), thus giving an indication of the abatement required. 2.4.4. Odour offensiveness A quantity of slurry of 30 ml was put into each of six small pots (three replicates of each treatment) and one further pot contained brown odourless water. Lids were removed from the pots 5 min before presentation to panellists. Panellists were required to smell each pot and indicate the offensiveness of the odour according to the following category scale derived by Williams:9 0 1 2 3 4 5
Inoffensive odour Very faintly offensive odour Faintly offensive odour Definitely offensive odour Strongly offensive odour Very strongly offensive odour
The mean offensiveness score was calculated for each treatment.
3. Results and discussion 3.1. Biochemical performance of treatments The mean analyses of the slurries following each of the three treatments are summarized in Table 3; these can be compared directly with the composition of the raw slurries given in Table 1. In all cases, the reduction of the VFA concentration, an indicator of odour, approached 100%. The elimination of this component is nearest completion in the longer trials as was expected. The effect of the longer treatment was much more apparent in terms of COD reduction. Relative to the raw feed, these reductions were 39% for the 6)3 d treatment of trial C, 40% for trial B (2)4 d) but only 11% for the 1)7 d treatment of trial A. The figure for trial A was lower than expected from correlations for COD reduction developed by Evans et al.7 which predict a fall of around 25% in COD for a 1)7 d treatment (for whole slurry). The removal of some suspended matter by screening is unlikely to have affected the performance as this fraction is largely unaffected by the biological process anyway. The poor performance of trial A probably reflects the difficulty in maintaining the aeration level in this particular trial. This was also seen in terms of reduction in the organic-N component (i.e. the difference between Kjeldahl and ammoniacal N). The similar performance in the reduction of COD between trials B and C was a consequence of better than expected results from trial B. No explanation is offered but it is noted that in terms of N reduction, the longer trial C does give better results than trial B. The significance in the degradation of the organic matter in the slurry lies with its capacity to regenerate the compounds that give rise to offensive odour, of which VFA is an indicator. Thus although VFA was virtually removed in all treatments, its more rapid regeneration in slurry from the shorter treatments was expected on the basis that less degradation of organic material had been achieved. Concentrations of nitrates and nitrites were undetected in any of the samples taken. However, the unavoidable delay in presenting the samples to the laboratory (4—6 h) would have been enough for natural degradation of any such material present by denitrification. It is likely
Table 3 Average composition of treated slurries, with standard deviations in paratheses Trial
Res. time, d
TS, kg/m3
VS, kg/m3
COD, kg/m3
Kj-N, kg/m3
Am-N, kg/m3
Org-N, kg/m3
VFA, kg/m3
No. of measurements
A B C
1)7 2)4 6)3
35)4 (4.)8) 20)2 (1)8) 17)0 (0)12)
77)5 (1)6) 70)8 (1)4) 68)6 (1)7)
43)5 (7)2) 21)1 (2)9) 18)2 (2)8)
2)97 (0)35) 2)62 (0)19) 1)60 (0)23)
1)49 (0)15) 1)65 (0)09) 1)00 (0.23)
1)48 (—) 0)97 (—) 0)60 (—)
0)22 (0)13) 0)08 (0)01) 0)01 (0)01)
7 11 19
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Table 4 Odour concentration values for raw, treated and anaerobically stored slurries ¹rial/sample
Treatment time, d
Storage time, d
Concentration, OU/m3
95% confidence interval
A A B1 B1 B2 B2 B3 B3 C C
Untreated 1)7 Untreated 2)4 Untreated 2)4 Untreated 2)4 Untreated 6)3
0 0 0 0 14 14 28 28 0 0
111 24 177 70 239 65 163 88 57 55
60—330 18—40 110—360 40—170 190—400 24—150 110—280 14—170 27—100 40—190
that nitrates were present in the fresh samples taken from trial C as losses of nitrogen were most probably removed by the nitrification—denitrification route.
3.2. ¹reatment performance in terms of odour control 3.2.1. Odour concentration Table 4 shows mean threshold values for each treatment along with 95% confidence intervals as given by probit analysis.11 Despite the wide confidence interval, the general effect of treatment in reducing the odour concentration is clear. The raw slurry generate odours with a concentration value between 57 and 239 odour units (OU/m3). In most cases, aerobic treatment reduced this by between a half and a quarter of the original value. The exception was trial C where an unusually low value for the raw slurry left little scope for further reduction. Apart from the effect of aeration, no further discernment can be made from these results, i.e. either in terms of treatment times or storage times. The low value of 88 OU/m3 for the treated slurry from trial B, after 28 d of anaerobic storage was only half of the value for the raw slurry thus indicating some lasting effect of aerobic treatment. 3.2.2. Odour intensity Mean intensity scores from the panel for each of the samples at various dilutions are set out in Fig. 2. From this data the regression equations derived for the five untreated (raw) slurries were: log C#4)74, r2"0)87 (2) 10 log C#5)90, r2"0.98 (3) 10 log C#5)01, r2"0)98 (4) 10 log C#5)13, r2"0)97 (5) 10 log C#5)25, r2"0)95 (6) 10 Setting an acceptable value of I (odour intensity) as 2 (faint odour), the required number of dilutions, D, can be calculated by substituting I"2 in the above A B1 B2 B3 C
I"2)46 I"2)29 I"2)16 I"2)53 I"2)76
equations. The % abatement (i.e. reduction in odour concentration) required to reach I"2 is given by
A B
1 1! ]100% D
(7)
The % abatement achieved by the aerobic treatment and subsequent anaerobic storage is given by untreated threshold (OU/m3)!treated threshold (OU/m3) untreated threshold (OU/m3) ] 100%
(8)
using the odour concentration data given in Table 4. These values are compared in Table 5 for each of the five treatments (inclusive of subsequent anaerobic storage where indicated). Valves relating to the untreated slurry imply that an observer would need to stand some distance away before an acceptable ‘‘faint odour’’ was perceived. Complete abatement (achieved by the treatment process) would enable the observer to stand alongside the slurry store and only experience ‘‘faint odour’’. However, this may not always be required. The picture is thus slightly different from that of simply looking at odour concentration. The intensity parameter relates to the actual perception of odour strength by an individual. The number of dilutions required gives an indication of how far from the source the odour could still be a nuisance. From the values reported here, the raw slurry used in trial B is seen to have a strong intensity quality which is reflected in the corresponding measured concentration. However, the effect of storage is to achieve a fall in this quality suggesting that anaerobic storage alone has some benefit in odour control. None of the treatments achieved the complete odour abatement required, i.e. the intensity of odour close to treated slurry would be deemed to be greater than a score of two. However, substantial reduction in intensity was achieved in all except trial C. The apparent poor result in this longest of treatments is in contrast with the high breakdown of organic matter observed in the
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Fig. 2. Correlations of perceived odour intensity (mean value on a scale 1 to 6, as judged by a panel) to odour concentration (varied by dilution) for each of the five samples analysed
corresponding slurry samples analysed. The most likely explanation for this discrepancy probably lies will the odour sampling and analysis, especially as there was a relatively low odour with the slurry used in this trial. 3.2.3. Odour offensiveness and »FA concentrations In many respects, it is the offensive quality of the odour which is of greatest concern because few people will
complain of odours if they are deemed pleasant! Table 6 lists the scores awarded and compares these values to the VFA concentrations measured in the corresponding slurries. In previous work, Williams9 demonstrated that the VFA concentration is a good indicator of the odour offensiveness that would be perceived by a panel. High offensive scores coincide with high concentrations of VFA. A score of 2, equating to ‘‘faintly offensive odour’’,
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209
Table 5 Odour abatement by aerobic treatment. Slurry samples B2 and B3 were taken after 14 and 28 d of subsequent anaerobic storage Untreated slurries
¹rial/sample A B1 B2 B3 C
Dilutions required to reach I"2 for raw slurry 13 50 25 17 15
Corresponding % abatement required 92 98 96 94 93
¹reated slurries
% abatement achieved 78 60 73 46 4
occurs with a VFA concentration of 0)23 kg/m3; this has been used as a limit of tolerable odour2 . The correlation derived by Williams is reproduced in Fig. 3 with the data from Table 6 added. Higher scores have generally been given for the samples from this study. Hence, whilst the general trend is confirmed, there can be large variations which limit the value of VFA concentration as an indicator of odour offensiveness. In terms of odour offensiveness, the long treatment (trial C) produced a freshly treated slurry with a score of better than 2 (faintly offensive odour). The odour from slurry from trials A and B was definitely worse bordering on the unacceptable. Once again, the storage of samples from trial B led to a small reduction in odour with a fall in the offensiveness index. Figure 4 illustrates the increases in VFA concentrations in the treated slurries over an aerobic storage period up to 75 d. The various olfactometric tests were only carried out
Fig. 3. Values of odour offensiveness (mean value on a scale 1 to 6, as judged by a panel) for treated slurries related to the VFA concentration of the slurry sample. A straight line correlation linking these two parameters developed in earlier work by Williams9 is included for comparison
on slurry from trial B and up to 28 d. In that period the concentration of VFA remained below 1 kg/m3 which may explain the lack of observation of an increased odour nuisance following storage of aerobically treated slurry for 28 d. The VFA concentration is seen to rise further to 1)6 kg/m3 after 50 d of storage, still much less than the value for the untreated slurry. Concentrations of VFA for treated slurry from trial A are seen to rise faster, as might be expected from the shorter treatment. Likewise, the longer treatment of trial C is seen to produce a more stable slurry in terms of VFA regeneration.
Table 6 Odour offensiveness of untreated and aerobically treated pig slurries
¹rial/sample ¹reatment A B1 B2 B3 C
Untreated Aerated Untreated Aerated Untreated Aerated Untreated Aerated Untreated Aerated
Mean offensiveness score
»FA, kg/m3
4)0 2)8 4)7 3)7 3)6 3)0 ND ND 3)8 1)8
3)68 0)07 4)83 0)11 5)11 0)42 4)99 0)73 3)83 0)01
Fig. 4. Regeneration of VFA during anaerobic storage following aerobic treatment. j Trial A (1)7 d treatment); m Trial B (2)4 d treatment); d Trial C (6)3 d treatment)
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4. Discussion Assessment of the quality of odours is still in its infancy and perhaps it is not surprising that this study has produced some apparent contradictions. The inevitably subjective nature of odour is seen to produce a relatively wide confidence interval despite the objective approach of the olfactometric procedure. (The latest draft European standard for olfactometric methods for odour measurement14 addresses the problem and the methods proposed in it and improves greatly the repeatability of the procedures.) Despite the problems encountered, some clear trends have emerged from the study, together with verification that aerobic treatment can achieve a substantial reduction in the odours produced by pig slurry. To summarize such an abatement with a single figure can be very misleading both in terms of meeting absolute targets and in comparing one system with another. This indeed is the nub of the problem; that is, odour assessment requires several parameters to be specified to give the complete picture. This contrasts with many other treatment objectives, such as COD or nitrogen reduction, which can be determined easily and unambiguously as single measurements from laboratory analysis. Of all the odour parameters measured, it is the odour offensiveness which is perhaps the key quality as it is the unpleasantness of pig slurry smell that initiates most complaints. The beneficial effect of aerobic treatment is very clear in this respect from the results. Williams,9 in relating odour offensiveness to VFA concentration, provided a sound basis for the understanding and development of treatment processes. The removal of the reactive organic materials that give rise to offensive odour by aerobic treatment is both observed and expected. The removal of more complex organic material that can lead to the regeneration of the odorous compounds would also be expected to enable a period of storage without the return of offensive odours; some supporting evidence for this is also seen in this work. However, an offensive odour will only be a problem if it can be detected by people living nearby. This leads to the qualities of odour intensity and odour concentration. The latter gives an idea of the quantity of odour but this by itself may not equate to its perception when diluted with fresh air by a given amount. The concept of intensity gives a clear idea of how much dilution is required to reduce the smell to a tolerable level. The reduction in odour concentration achieved by a given treatment can then be compared with that required. If this reduction is insufficient, then the odour will be perceived close to the source and for some distance away. However, if there has been at least some reduction in odour concentration, then it would be necessary to be closer to the source before the odour becomes
noticeable. Hence, in some cases, where slurry stores are set well back from farm boundaries, this will still equate to adequate abatement. In the final analysis, satisfactory odour abatement will clearly be a combination of sufficient reduction in its offensiveness and its perceived strength in the areas around the farm where unpleasant smells can potentially cause annoyance. Aerobic treatment can achieve such abatement but the extent of treatment required will depend as much on the farm location as on any target quality of the treated slurry.
5. Conclusions (1) Farm scale aerobic treatment of pig slurry achieved a reduction in odour concentration of between a half and a quarter of the value of the raw slurry. This achieved between 46 and 78% of the abatement required to reduce odour intensity to that of faint odour in all except one case. Hence, the intensity of odour close to the slurries would still be rated above ‘‘faint odour’’ but with natural dilution it would not be detected so far away. (2) Aerobic treatment of pig slurry achieved a reduction in the offensiveness rating of the slurry odour in all cases. For the longest treatment (6)3 d), the value for the freshly treated slurry was below 2 ‘‘faintly offensively odour’’. The concentration of VFA correlates to the offensiveness score but there are differences with the results reported by other workers. Such indicators of odour should thus be used with caution. (3) The aerobic breakdown of organic matter in the pig slurry followed general expectations with a fall in COD of 39% for a 6)3 d treatment and just 11% for a short 1)7 d treatment. The effect of duration of treatment on the odour was evident from the reduction of the offensiveness score, the long treatment gave the best result. However, in terms of the reduction of odour concentration, no differences could be perceived between the treatments. (4) Anaerobic storage of pig slurry treated for 2)4 d led to a rise in the VFA concentration from 0)1 to 1)0 kg/m3 after 25 d and to 1)6 kg/m3 after 50 d. However, no regeneration of odour was discerned over the first 28 d. Raw slurry stored over the same period produced an odour of falling intensity with time indicating some benefit from storage alone. (5) The use of olfactometric methods has verified the value of aerobic treatment in achieving odour abatement. However, the establishment of specific targets remains difficult due to the combination of factors that describe an odour nuisance and the wide
T H E EF F ECT O F FA RM S CA LE A E RO B IC TR EAT ME NT OF PI G GE RY S LU R RY
confidence interval that can occur with some of the methods used.
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References 8 1
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5
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IEHO Annual Report 87/88 London: The Institution of Environmental Health Officers Sneath R W; Burton C H; Williams A G Continuous aerobic treatment for piggery slurry for odour control scaled up to a farm-size unit. Journal of Agricultural Engineering Research, 1992, 53, 81—92 Williams A G; Shaw M; Selviah C; Cumby R J The oxygen requirements for deodorizing and stabilising pig slurry by aerobic treatment. Journal of Agricultural Engineering Research, 1989, 43, 291—311 Pain B F; Phillips V R; Clarkson C R; Misselbrook T H; Rees Y J; Farrent J W Odour and ammonia emissions following the spreading of aerobically-treated pig slurry on grassland. Biological Wastes, 1990, 34, 149—160 Pain B F; Misselbrook T H; Clarkson C R; Rees Y J Odour and ammonia emissions following the spreading of anaerobically-digested pig slurry on grassland. Biological Wastes, 1990, 34, 259—267 Burton C H; Sneath R W Continuous farm-scale aeration plant for reducing offensive odours from piggery slurry:
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