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On farm aerobic treatment of piggery waste. The effect of residence time and storage on effluent quality
Water Research Vol. 14, pp. 805 to 808 O Pergamon Press Ltd 1980. Printed in Great Britain
0043-1354/80/0701-0805502.00/0
ON FARM AEROBIC'TREATMENT OF PIGGERY WASTE. THE EFFECT OF RESIDENCE TIME AND STORAGE ON EFFLUENT QUALITY D. R. FENLO~ and P. J. MILLS Bacteriology & Engineering Divisions, North of Scotland College of Agriculture, Aberdeen AB9 1UD, Scotland (Received August 1979) Almtraet--The effect of reducing the residence time of screened pig slurry in an on farm pilot scale oxidation ditch is described. The soluble organic fraction of the slurry was greatly reduced, either by mineralisation at long residence times, or, as residence times decreased, by conversion to suspended solids and organic nitrogen. Losses of inorganic nitrogen during treatment were high due to desorption and denitrification. Controlled loading of a laboratory system with the same waste showed that the amount of oxygen demand lost per day was constant, and that residence time determined whether mineralisation or solids production occured. A small increase in soluble COD took place during overwinter storage of aerobically treated waste, but it remained far more acceptable for land disposal than untreated material.
NOMENCLATURE
TER R CODT CODD CODs TS DS SS NH~-N NH3-N org-N NO~-N NO~-N YN
Treatment efficiency ratio, the ratio of the output to the input. Residence time in days. Total chemical oxygen demands. Dissolvedchemical oxygen demand. Suspended chemical oxygen demand. Total solids. Dissolvedsolids. Suspended solids. Ammoniacal nitrogen. Free ammonia nitrogen. Organic nitrogen. Nitrite nitrogen. Nitrate nitrogen. Total nitrogen.
Where possible, land spreading of untreated waste is the most practicable solution to the farm waste problem. However difficulties can arise from odour and from water pollution caused by soluble fractions of the waste leaching to watercourses. The main effort in research into aerobic treatment of agricultural wastes has therefore been concentrated on odour control and transformation of the soluble biodegradable components of waste to a form that will be retained by the soil and minimise the risk of pollution (ARC, 1976). Slurry treatment systems have to operate within the strict economic constraints imposed upon the livestock producer. It is therefore desirable that an aerobic plant treats the maximum quantity of waste in the shortest possible time so that capital outlay on equipment is minimised. Mathematical models to describe soluble COD removal have been produced by Woods & O'Callagban (1974) based on work published by Owens et al. (1973) which have demonstrated that, when laboratory treatment plants are operated as chemostats, variation in residence time can be used as a control of
effluent quality. Work on relatively uncontrolled farm-scale systems with long residence times (Robinson, 1975) has produced data in agreement with these authors, but most field scale systems in use fail to meet the necessary criteria for analysis on chemostat principles, e.g. insufficient run in time to achieve steady state, variability in slurries, operating and sampiing difficulties (ARC, 1976). Aerobic treatment does not eliminate the necessity to dispose of waste to land. Consequently storage of treated waste is essential until land application can be effected. Efficient treatment and reduced capital outlay must be accompanied by a reduction in the skilled management requirement if the process is to be cost effective in an on-farm situation. The object of the work described here was, firstly to examine the use and biological efficiency of a farm site system operated on or as near to chemostat principles as possible, in order to study the effect of residence time on the various components of the waste, secondly to compare the effects of long term winter storage on the stability of untreated and aerobically stabilised waste and thirdly to compare the results of the farm site system with those from a laboratory system with automated loading of a similar waste.
805
METHODS
The farm site system consisted of a plywood oxidation ditch of 3.405 m 3 capacity and a working depth of 0.25 m. The perforated disc aerator was capable of providing 117 mg 02 l- ~ h- ~ into clean water (K2o 12.8h- ~). Slurry was screened prior to treatment by passing it through a 1.0mm mesh rotary screen separator (Report, 1974) to remove coarse solids, which have shown a tendency to build up in oxidation ditches, causing settling and oxygen transfer problems (Robertson, 1972). A persistaltic pump (Watson and Mariow Model HRSV, Falmouth, Cornwall)
806
D R. FENLONand P. J. MILLS
loaded slurry into the ditch at half-hourly intervals. Alteration in residence time was achieved by changing the length of the dosing period. In order to achieve steady state the ditch was operated for a period of 3 x residence time before the results were used. Although loading was kept as consistent as possible on a volumetric basis no attempt was made to control either the waste concentration or temperature. Storage of treated and untreated waste was in 375 I. covered containers housed on the same site. After four months overwinter storage the contents were mixed and sampled. The laboratory treatment system, using pH controlled loading of screened waste was like that described by Robinson & Fenlon (1977) for lagoon supernatant. Loading was actuated by the pH falling below a preset value and ceased when the pH returned to that level The aeration rate was equivalent to 75 mg 02 1- z h - ~ into clean water (Kzo 8.2 h-Z). The methods of analysis have been described previously (Fenlon & Robinson 1977). Organic nitrogen was obtained by subtraction of ammoniacal nitrogen from total Kjeldhal nitrogen determined as in Standard Methods (1965).
z ,.r Z
z
O
RESULTS
O
The composition of the slurry treated over the trial period is shown in Table 1, and illustrates the wide variation in oxygen demand (OD) applied to a system when loading is on a volumetric basis. Treatment efficiency ratios (TER) were calculated for the various components by comparing effluent with influent concentrations, with adjustments for changes in the ML concentrations over the analysis period. The effect of changes in residence time on the treatment system, as measured by changes in TER, are shown in Table 2, together with the other physical and chemical parameters of the mixed liquor (ML). The dissolved oxygen (DO) concentration varied with the loading cycle, and only neared limitation. with a minimum of 4 ~ after loading, when R = 4 days. A further reduction in residence time to 2 days, at a temperature of 9-15°C, resulied in a DO ranging from 2 to 15~ but the system was unstable and began to fail probably due to a combination of limiting DO and R < microbial growth rate, leading to a slow washout of the treatment microflora. The ML pH was a reflection of the effect of temperature and dilution rate on the growth of the nitrifying microflora. The high surface area to volume ratio of the
o=
e-
(D
L~ e~
e~
eq 8" t¢3 m ~
~
=,O ¢~- ~ O
.q -
Table 1. The composition of screened raw waste (g 1- ~1
CODT CODD CODs TS DS SS org-N NHa-N
High
Low
Mean
SD
Var
95.3 18.9 76.4 54.5 12.1 42.4 2.1 2.3
35.0 9.7 25.3 20.8 7.0 13.8 0.7 1.4
56.9 16.3
13.3 2.8
174.2 7.6
34.8 9.5 25.3 1.14 1.83
6.9 2.6 6.1 0.25 0.24
46.1 6.9 36.8 0.06 0.06
g
Aerobic treatment of piggery waste Table 3. The effect of storage on the levels of constituents (g I - ~) in raw and treated waste
Kj N NH4-N org-N CODT CODD TS DS
Raw waste Before After
Treated waste Before After
2.8 1.7 i .0 45.8 13.7 28.0 8.5
1.8 0.6 1.2 21.1 2.1 19.2 4.5
2.1 (75) 1.4 (82) 0.7 (70) 26.0(57) 8.4 (61) 19.0 (68) 7.4 (87)
807
COD being in the range 1.0-3.0 g 1-m, depending on the concentration of the ingoing waste. The fate of oxidisable material tended towards mineralisation at longer residence times and biomass production as residence time decreased. Table 3 shows the effect of storage on treated and untreated slurry. No nitrification had occurred during treatment of the material used for storage due to the short residence time. Therefore the losses in available nitrogen were confined to desorption of ammonia, which were less than might have been expected due to replacement by degradation of the organic nitrogen. The small increase in the dissolved COD of treated waste on storage demonstrated that it is reasonably stable over a winter period and has a much lower pollution potential and odour than its stored untreated counterpart. The fermenter, with automatic loading linked to pH, maintained a DO between 3- 12To with a periodicity of about 20 min. Due to the more intimate mixing and refined temperature control more accurate analysis was possible. The results (Table 4) show that while the proportion of the waste OD lost by mineralisation decreases as R decreases, the amount of OD lost per day from the system remains relatively constant when oxygen is near limitation. The nitrogen components were affected as in the oxidation ditch trial, the lower the pH the greater the residence time and low N H ~ - N TElL high pH low residence and high org-N TER. The major losses of nitrogen took place at long residence times, and greatest accumulations of org-N occurred when R was short.
1.5 (83)* 0.6 (100) 0.9 (75) 17,6(83) 2.7 (129) 15.0 (78) 3.8 (84)
* ( )% of original remaining. ditch resulted in its temperature being near ambient, with wide diurnal and seasonal variations. Inhibition of nitrification at low temperatures (R = 18days) gave a high pH and 50% of the ammoniacal nitrogen (27% N) was lost by desorption. As the temperature increased (R --17.5 days) nitrification commenced, but the combination of high pH and ammonia resulted in levels of free NH3-N inhibitory to Nitrobacter sp. and accumulation of nitrite nitrogen to a peak of 640 mg 1-1. Losses of 26% of the total nitrogen occurred probably as a result of a combination of desorption and denitrification (Murray et ai., 1975). When R = 8.7 days, the higher temperatures resulted in almost complete conversion of ammonia to nitrate (max 740mg 1-1 N O i - N ) and mineralisation of organic nitrogen was also increased (TER org-N 0.63). Losses were greatest during this run amounting to 5390 of the total nitrogen. The shortest residence time of 4 days resulted in the greatest utilization of ammonia (TER 0.01) but at the high pH N O 2 - N accumulated (up to 410 mg 1-1) which at low levels of dissolved oxygen was readily lost by denitrification. Nitrogen losses were limited to 37To mainly due to the increase in organic nitrogen (TER 1.47). The results for the two longer runs at R -- 18 and 17.5 days show that treatment is relatively unaffected by temperature at longer residence times. Suspended material increased as residence time decreased and its higher level of organic nitrogen suggests that the increase is due to the production of microbial solids. The solids and COD ratios in this trial are lower than those reported by Evans et al. (1979) due to the screen removing a large proportion of the nonbiodegradable suspended matter. The efficiency of treatment, as measured by the TER for dissolved COD was consistently high, 0.1-0.17, the effluent
DISCUSSION
Cost is the principle constraint on treatment. In a comparison of aerobic and anaerobic treatment Mills (1977) has shown aerobic systems to have a lower capital cost but higher running costs, due to the need to provide constant aeration. The laboratory trial illustrated that, with oxygen limitation, DO was used efficiently and, depending on loading rate, it could be used for solids production or mineralisation. Both laboratory and field scale systems demonstrated that the main effect of altering residence times was on suspended solids, and the nitrogen components of the effluent, and that the major cause of pollution, the soluble COD of the treated waste was little affected. In order to control pollution the treatment system needs only
Table 4. The effect of residence time on the treatment efiiciency and oxygen demand (OD) removal (g day- 1) in the pH controlled system pH
R
6.0 7.0 7.5
15.0 6.3 3.0
Days Period 27 29 27
TER org-N
NH2-N
22N
CODT
CODD
CODss
g OD lost day-
0.94 1.17 1.63
0.07 0.24 0.31
0.58 0.74 0.80
0.50 0.62 0.81
0.10 0.11 0.13
0.74 0.93 1.27
18.7 19.3 20.4
808
D.R. FENLONand P. J. MILLS
a short residence time, since mineralisation is unnecessary when it can occur on land without odour or pollution hazard. Short residence times result in high rates of metabolism by treatment microflora which generate heat, Rieman (1972) has shown that by providing a suitable treatment vessel with low surface area and insulation heat losses can be minimised giving less seasonal fluctuation in treatment efficiency in contrast to the oxidation ditch. Losses in nitrogen have to be accepted as an inevitable effect of aerobic treatment. These were especially high when mineralisation occurred and would increase even more as dentrification took place on storage. Low residence times resulting in organic nitrogen accumulation gave optimum conservation and in a form relatively stable on storage. It is obvious that treatment is most efficient at low residence times, with a continuous loading of a consistent waste to reduce fluctuations in DO levels. Unfortunately the treatment system must be run to suit the piggery, not vice versa. Pigs tend to excrete after periods of feeding and prolonged rest, resulting in an uneven distribution of loading over the day. Pork and bacon pigs are produced in batches, therefore sizes and numbers in a given unit can fluctuate markedly. Fortunately the resultant sudden changes in waste production are downwards, increases are gradual as pigs grow, but the result of these variations is to produce difl~ulties in calibrating continuous loading systems. The management requirement for treating slurry at optimum residence time is obviously unrealistic in practice, but the information provided by the trial suggests methods of treatment and likely results. Waste tends to vary in consistency even when screened (Table 1). In a recent trial in an experimental piggery on the same site the average whole slurry production was 7.21. pig- ~ d a y - 1 at 5.5~ TS (Fenlon & Mills, 1980), and whole slurry from the pig unit during this trial averaged 4.3~o TS (Mills, 1977). Loading pumps and balancing tanks are expensive items, and would require frequent recalibration to account for changes in pig size and numbers, therefore gravity flow or flushing with mixed liquor would be most likely with the acceptance of uneven loadin& The necessary requirements for treatment would therefore be met by a vessel, insulated against heat loss, with a 4 day retention time based on the maximum number of pigs producing 8 1. of slurry per day, and an aeration rate of 75-117 mg 02 1. ML - t h - t . This would give a small provision for excess loading, as the trial indicated only a slow washout occurred at 2 day residence time. On this basis suspended solids would accumulate due to biomass production when the piggery population is at a maximum, but as pig numbers decrease residence time would increase, leading to a lowering in microbial SS as mineralisation occurred.
The laboratory data confirmed that loading rate ie residence time, pH and nitrification are linke~l, and the fixing of any one of these parameters tends to control the others (Owens et al., 1973; Robinson & Fenlon, 1977). It also showed that while the amount of oxygen demand removed was dependent on the aeration rate, the pH setting determined the residence time and therefore the effluent quality. As the provision of aeration is the major cost in aerobic treatment further work is needed on determining whether pH control of aeration rather than loading is capable of reducing costs on the treatment of wastes of different quantity and quality. Winter storage of aerobically treated slurry showed a reduction in the concentration of most components. Stability, as measured by increase in dissolved COD was reasonable compared to untreated material. Nitrogen losses were confined to the org-N fraction, presumably by its breaking down to N H ~ - N and replacing that available nitrogen being lost by desorption. REFERENCES
ARC Bulletin (1976) Studies in Farm Livestock Wastes. Agricultural Research Council, London. Evans M. R., Hisset R., Smith M. P. W., Ellam D. F. & Baines S. (1979) Aerobic treatment of piggery waste. Agric. Wastes 1, 67-85. Fenlon D. R. & Mills P. J. (1980) The stabilisation of pig slurry with lime. Agric. Wastes 2, 13-22. Fenlon D. R. & Robinson K. (1977) Dentrification of aerobically treated pig waste. Water Res. 11, 269-273. Mills P. J. (1977) A comparison of an anaerobic digester and an aeration system treating piggery waste from the same source. Proc. 1977 Cornell Aoric. Waste Management Conf. Ann Arbour Science, Michigan. Murray I., Parsons J. W. & Robinson K. (1975) Interrelationships between nitrogen balance, pH and dissolved oxygen in an oxidation ditch treatin8 farm animal waste. Water Res. 9, 25-30. Owens J. D., Evans M. R., Thacker F. E., Hisset R. & Baines S. (1973) Aerobic treatment of piggery waste. Water Res. 7, 1745-1766. Report (1974) Treatment of Piggery Wastes (Edited by Littlejohn L.) North of Scotland College of Agriculture. Rieman U. (1972) Aerobic treatment of swine waste by aerator-agitators ("Fuchs"). Proc. 1972 Cornell Agric. Waste Manooement Conf. pp, 537-543. Robertson A. M. (1972) Treatment of livestock wastes. Process Biochem. 7, 21-24. Robinson K. (1975) Waste management systems in relation to land disposal/utilization. Energy, Agriculture and Waste Manooement (Ed. Jewell W.) Ann Arbor Science Publishers Inc. Michigan. Robinson K. & Fenlon D. R. (1977) A comparison of pH controlled and dissolved oxgen-controlled nutrient addition for the maintenance of steady state in a mixed culture. J. Appl, Bact. 42, 393-403. Standard Methods for the Examination of Water and Waste Water (1965) 12th Ed. Health Assoc Inc., New York.
Woods J. L. & O'Callaghan J. R. (1974) Mathematical modelling of animal waste treatment. J. agric. Engtu3 Res. 19, 245-258.
0043-1354/80/0701-0805502.00/0
ON FARM AEROBIC'TREATMENT OF PIGGERY WASTE. THE EFFECT OF RESIDENCE TIME AND STORAGE ON EFFLUENT QUALITY D. R. FENLO~ and P. J. MILLS Bacteriology & Engineering Divisions, North of Scotland College of Agriculture, Aberdeen AB9 1UD, Scotland (Received August 1979) Almtraet--The effect of reducing the residence time of screened pig slurry in an on farm pilot scale oxidation ditch is described. The soluble organic fraction of the slurry was greatly reduced, either by mineralisation at long residence times, or, as residence times decreased, by conversion to suspended solids and organic nitrogen. Losses of inorganic nitrogen during treatment were high due to desorption and denitrification. Controlled loading of a laboratory system with the same waste showed that the amount of oxygen demand lost per day was constant, and that residence time determined whether mineralisation or solids production occured. A small increase in soluble COD took place during overwinter storage of aerobically treated waste, but it remained far more acceptable for land disposal than untreated material.
NOMENCLATURE
TER R CODT CODD CODs TS DS SS NH~-N NH3-N org-N NO~-N NO~-N YN
Treatment efficiency ratio, the ratio of the output to the input. Residence time in days. Total chemical oxygen demands. Dissolvedchemical oxygen demand. Suspended chemical oxygen demand. Total solids. Dissolvedsolids. Suspended solids. Ammoniacal nitrogen. Free ammonia nitrogen. Organic nitrogen. Nitrite nitrogen. Nitrate nitrogen. Total nitrogen.
Where possible, land spreading of untreated waste is the most practicable solution to the farm waste problem. However difficulties can arise from odour and from water pollution caused by soluble fractions of the waste leaching to watercourses. The main effort in research into aerobic treatment of agricultural wastes has therefore been concentrated on odour control and transformation of the soluble biodegradable components of waste to a form that will be retained by the soil and minimise the risk of pollution (ARC, 1976). Slurry treatment systems have to operate within the strict economic constraints imposed upon the livestock producer. It is therefore desirable that an aerobic plant treats the maximum quantity of waste in the shortest possible time so that capital outlay on equipment is minimised. Mathematical models to describe soluble COD removal have been produced by Woods & O'Callagban (1974) based on work published by Owens et al. (1973) which have demonstrated that, when laboratory treatment plants are operated as chemostats, variation in residence time can be used as a control of
effluent quality. Work on relatively uncontrolled farm-scale systems with long residence times (Robinson, 1975) has produced data in agreement with these authors, but most field scale systems in use fail to meet the necessary criteria for analysis on chemostat principles, e.g. insufficient run in time to achieve steady state, variability in slurries, operating and sampiing difficulties (ARC, 1976). Aerobic treatment does not eliminate the necessity to dispose of waste to land. Consequently storage of treated waste is essential until land application can be effected. Efficient treatment and reduced capital outlay must be accompanied by a reduction in the skilled management requirement if the process is to be cost effective in an on-farm situation. The object of the work described here was, firstly to examine the use and biological efficiency of a farm site system operated on or as near to chemostat principles as possible, in order to study the effect of residence time on the various components of the waste, secondly to compare the effects of long term winter storage on the stability of untreated and aerobically stabilised waste and thirdly to compare the results of the farm site system with those from a laboratory system with automated loading of a similar waste.
805
METHODS
The farm site system consisted of a plywood oxidation ditch of 3.405 m 3 capacity and a working depth of 0.25 m. The perforated disc aerator was capable of providing 117 mg 02 l- ~ h- ~ into clean water (K2o 12.8h- ~). Slurry was screened prior to treatment by passing it through a 1.0mm mesh rotary screen separator (Report, 1974) to remove coarse solids, which have shown a tendency to build up in oxidation ditches, causing settling and oxygen transfer problems (Robertson, 1972). A persistaltic pump (Watson and Mariow Model HRSV, Falmouth, Cornwall)
806
D R. FENLONand P. J. MILLS
loaded slurry into the ditch at half-hourly intervals. Alteration in residence time was achieved by changing the length of the dosing period. In order to achieve steady state the ditch was operated for a period of 3 x residence time before the results were used. Although loading was kept as consistent as possible on a volumetric basis no attempt was made to control either the waste concentration or temperature. Storage of treated and untreated waste was in 375 I. covered containers housed on the same site. After four months overwinter storage the contents were mixed and sampled. The laboratory treatment system, using pH controlled loading of screened waste was like that described by Robinson & Fenlon (1977) for lagoon supernatant. Loading was actuated by the pH falling below a preset value and ceased when the pH returned to that level The aeration rate was equivalent to 75 mg 02 1- z h - ~ into clean water (Kzo 8.2 h-Z). The methods of analysis have been described previously (Fenlon & Robinson 1977). Organic nitrogen was obtained by subtraction of ammoniacal nitrogen from total Kjeldhal nitrogen determined as in Standard Methods (1965).
z ,.r Z
z
O
RESULTS
O
The composition of the slurry treated over the trial period is shown in Table 1, and illustrates the wide variation in oxygen demand (OD) applied to a system when loading is on a volumetric basis. Treatment efficiency ratios (TER) were calculated for the various components by comparing effluent with influent concentrations, with adjustments for changes in the ML concentrations over the analysis period. The effect of changes in residence time on the treatment system, as measured by changes in TER, are shown in Table 2, together with the other physical and chemical parameters of the mixed liquor (ML). The dissolved oxygen (DO) concentration varied with the loading cycle, and only neared limitation. with a minimum of 4 ~ after loading, when R = 4 days. A further reduction in residence time to 2 days, at a temperature of 9-15°C, resulied in a DO ranging from 2 to 15~ but the system was unstable and began to fail probably due to a combination of limiting DO and R < microbial growth rate, leading to a slow washout of the treatment microflora. The ML pH was a reflection of the effect of temperature and dilution rate on the growth of the nitrifying microflora. The high surface area to volume ratio of the
o=
e-
(D
L~ e~
e~
eq 8" t¢3 m ~
~
=,O ¢~- ~ O
.q -
Table 1. The composition of screened raw waste (g 1- ~1
CODT CODD CODs TS DS SS org-N NHa-N
High
Low
Mean
SD
Var
95.3 18.9 76.4 54.5 12.1 42.4 2.1 2.3
35.0 9.7 25.3 20.8 7.0 13.8 0.7 1.4
56.9 16.3
13.3 2.8
174.2 7.6
34.8 9.5 25.3 1.14 1.83
6.9 2.6 6.1 0.25 0.24
46.1 6.9 36.8 0.06 0.06
g
Aerobic treatment of piggery waste Table 3. The effect of storage on the levels of constituents (g I - ~) in raw and treated waste
Kj N NH4-N org-N CODT CODD TS DS
Raw waste Before After
Treated waste Before After
2.8 1.7 i .0 45.8 13.7 28.0 8.5
1.8 0.6 1.2 21.1 2.1 19.2 4.5
2.1 (75) 1.4 (82) 0.7 (70) 26.0(57) 8.4 (61) 19.0 (68) 7.4 (87)
807
COD being in the range 1.0-3.0 g 1-m, depending on the concentration of the ingoing waste. The fate of oxidisable material tended towards mineralisation at longer residence times and biomass production as residence time decreased. Table 3 shows the effect of storage on treated and untreated slurry. No nitrification had occurred during treatment of the material used for storage due to the short residence time. Therefore the losses in available nitrogen were confined to desorption of ammonia, which were less than might have been expected due to replacement by degradation of the organic nitrogen. The small increase in the dissolved COD of treated waste on storage demonstrated that it is reasonably stable over a winter period and has a much lower pollution potential and odour than its stored untreated counterpart. The fermenter, with automatic loading linked to pH, maintained a DO between 3- 12To with a periodicity of about 20 min. Due to the more intimate mixing and refined temperature control more accurate analysis was possible. The results (Table 4) show that while the proportion of the waste OD lost by mineralisation decreases as R decreases, the amount of OD lost per day from the system remains relatively constant when oxygen is near limitation. The nitrogen components were affected as in the oxidation ditch trial, the lower the pH the greater the residence time and low N H ~ - N TElL high pH low residence and high org-N TER. The major losses of nitrogen took place at long residence times, and greatest accumulations of org-N occurred when R was short.
1.5 (83)* 0.6 (100) 0.9 (75) 17,6(83) 2.7 (129) 15.0 (78) 3.8 (84)
* ( )% of original remaining. ditch resulted in its temperature being near ambient, with wide diurnal and seasonal variations. Inhibition of nitrification at low temperatures (R = 18days) gave a high pH and 50% of the ammoniacal nitrogen (27% N) was lost by desorption. As the temperature increased (R --17.5 days) nitrification commenced, but the combination of high pH and ammonia resulted in levels of free NH3-N inhibitory to Nitrobacter sp. and accumulation of nitrite nitrogen to a peak of 640 mg 1-1. Losses of 26% of the total nitrogen occurred probably as a result of a combination of desorption and denitrification (Murray et ai., 1975). When R = 8.7 days, the higher temperatures resulted in almost complete conversion of ammonia to nitrate (max 740mg 1-1 N O i - N ) and mineralisation of organic nitrogen was also increased (TER org-N 0.63). Losses were greatest during this run amounting to 5390 of the total nitrogen. The shortest residence time of 4 days resulted in the greatest utilization of ammonia (TER 0.01) but at the high pH N O 2 - N accumulated (up to 410 mg 1-1) which at low levels of dissolved oxygen was readily lost by denitrification. Nitrogen losses were limited to 37To mainly due to the increase in organic nitrogen (TER 1.47). The results for the two longer runs at R -- 18 and 17.5 days show that treatment is relatively unaffected by temperature at longer residence times. Suspended material increased as residence time decreased and its higher level of organic nitrogen suggests that the increase is due to the production of microbial solids. The solids and COD ratios in this trial are lower than those reported by Evans et al. (1979) due to the screen removing a large proportion of the nonbiodegradable suspended matter. The efficiency of treatment, as measured by the TER for dissolved COD was consistently high, 0.1-0.17, the effluent
DISCUSSION
Cost is the principle constraint on treatment. In a comparison of aerobic and anaerobic treatment Mills (1977) has shown aerobic systems to have a lower capital cost but higher running costs, due to the need to provide constant aeration. The laboratory trial illustrated that, with oxygen limitation, DO was used efficiently and, depending on loading rate, it could be used for solids production or mineralisation. Both laboratory and field scale systems demonstrated that the main effect of altering residence times was on suspended solids, and the nitrogen components of the effluent, and that the major cause of pollution, the soluble COD of the treated waste was little affected. In order to control pollution the treatment system needs only
Table 4. The effect of residence time on the treatment efiiciency and oxygen demand (OD) removal (g day- 1) in the pH controlled system pH
R
6.0 7.0 7.5
15.0 6.3 3.0
Days Period 27 29 27
TER org-N
NH2-N
22N
CODT
CODD
CODss
g OD lost day-
0.94 1.17 1.63
0.07 0.24 0.31
0.58 0.74 0.80
0.50 0.62 0.81
0.10 0.11 0.13
0.74 0.93 1.27
18.7 19.3 20.4
808
D.R. FENLONand P. J. MILLS
a short residence time, since mineralisation is unnecessary when it can occur on land without odour or pollution hazard. Short residence times result in high rates of metabolism by treatment microflora which generate heat, Rieman (1972) has shown that by providing a suitable treatment vessel with low surface area and insulation heat losses can be minimised giving less seasonal fluctuation in treatment efficiency in contrast to the oxidation ditch. Losses in nitrogen have to be accepted as an inevitable effect of aerobic treatment. These were especially high when mineralisation occurred and would increase even more as dentrification took place on storage. Low residence times resulting in organic nitrogen accumulation gave optimum conservation and in a form relatively stable on storage. It is obvious that treatment is most efficient at low residence times, with a continuous loading of a consistent waste to reduce fluctuations in DO levels. Unfortunately the treatment system must be run to suit the piggery, not vice versa. Pigs tend to excrete after periods of feeding and prolonged rest, resulting in an uneven distribution of loading over the day. Pork and bacon pigs are produced in batches, therefore sizes and numbers in a given unit can fluctuate markedly. Fortunately the resultant sudden changes in waste production are downwards, increases are gradual as pigs grow, but the result of these variations is to produce difl~ulties in calibrating continuous loading systems. The management requirement for treating slurry at optimum residence time is obviously unrealistic in practice, but the information provided by the trial suggests methods of treatment and likely results. Waste tends to vary in consistency even when screened (Table 1). In a recent trial in an experimental piggery on the same site the average whole slurry production was 7.21. pig- ~ d a y - 1 at 5.5~ TS (Fenlon & Mills, 1980), and whole slurry from the pig unit during this trial averaged 4.3~o TS (Mills, 1977). Loading pumps and balancing tanks are expensive items, and would require frequent recalibration to account for changes in pig size and numbers, therefore gravity flow or flushing with mixed liquor would be most likely with the acceptance of uneven loadin& The necessary requirements for treatment would therefore be met by a vessel, insulated against heat loss, with a 4 day retention time based on the maximum number of pigs producing 8 1. of slurry per day, and an aeration rate of 75-117 mg 02 1. ML - t h - t . This would give a small provision for excess loading, as the trial indicated only a slow washout occurred at 2 day residence time. On this basis suspended solids would accumulate due to biomass production when the piggery population is at a maximum, but as pig numbers decrease residence time would increase, leading to a lowering in microbial SS as mineralisation occurred.
The laboratory data confirmed that loading rate ie residence time, pH and nitrification are linke~l, and the fixing of any one of these parameters tends to control the others (Owens et al., 1973; Robinson & Fenlon, 1977). It also showed that while the amount of oxygen demand removed was dependent on the aeration rate, the pH setting determined the residence time and therefore the effluent quality. As the provision of aeration is the major cost in aerobic treatment further work is needed on determining whether pH control of aeration rather than loading is capable of reducing costs on the treatment of wastes of different quantity and quality. Winter storage of aerobically treated slurry showed a reduction in the concentration of most components. Stability, as measured by increase in dissolved COD was reasonable compared to untreated material. Nitrogen losses were confined to the org-N fraction, presumably by its breaking down to N H ~ - N and replacing that available nitrogen being lost by desorption. REFERENCES
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