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Redox measurement of treatability by the activated sludge process
Water Research Pergamon Press 1969. Vol. 3, pp. 955-961. Printed in Great Britain.
REDOX MEASUREMENT OF TREATABILITY BY THE ACTIVATED SLUDGE PROCESS D. DICKINSON Institute for Industrial Research and Standards, Dublin, 9, Ireland (Received 10 July 1969) Abstract--The change in redox potential with time following the addition of a substtate to an aerated activated sludge suspension follows a well-defined pattern chaxacteristie of the substrate. The shape of the trace indicates the relative ease or difficulty with which the sludge can purify a substrate or waste material. Successiveadditions causing a sludge to adapt result in a change in the shape of the trace. R~ma~--Le changement avec le temps darts le potentiel du redox, suivant l'addition d'tme touche inf6rieure ~t tree suspension de d6p6t activ6, a6r6, suit une caract6ristique bien d6tiule de la couche inf6rieure. Le contour du trac6 indique la r~lative aisance ou difficult6 avec laquelle le d6p6t peut purifier une couche inf6rieure ou un materiel de d~het. Des additions successives provoquent un d~l~t pour adapter les r6sultats ~tun changement darts la forme du trac~. Zusammeafassung--Der Wechsel in einem Redox Potential mit nach einen Zeitabstand folgendem Zusatz eines Substrats zu einer lufthaltigen, aktivierten Schlammuspensionfolgt einem genau dellnierten Muster, welches ftir das Substrat charakteristisch ist. Die Form der Spur zeigt die relative Leiehtigkeit oder Schwierigkeit an, mit welcher Schlamm ein Substrat oder Abflussmaterial reinigen kann. SukzesziveZustttze, welche Schlamm zur Anpassung bringen, haben eine Verttnderung in der Form der Spur zum Resultat. CHANGES in the redox potential of aerated bacterial cultures were investigated many years ago by HEwrrr (1930). Typically, the addition of nutrient results in a rapid fall in potential, and this is followed by a more or less exponential return to the original Eh level as the nutrient is oxidized and utilized by the organisms. On theoretical grounds, the present author showed that in an activated sludge suspension, the potential change should follow a similar pattern (DICKINSON,1940). Probably because of the practical difficulty of distinguishing between inter- or extra-cellular changes or of knowing precisely what is measured by a platinum electrode suspended in aerated activatedsludge, comparatively little attention has been given to the use of redox measurements other than to their possible application to aeration control (RUDD, 1961 ; O'RotmKE, 1963 ; BmGGS and KNOWL~S, 1968). In experiments carded out over a number of years it has been established that (1) the actual level of the potential of a bright platinum electrode suspended in aerated activated-sludge is affected by the "condition" of the sludge and by the concentrations of inorganic salts, particularly nitrate, present in the mixture; (2) changes in potential are characteristic of the nutrient added and reflect the ability of the sludge to oxidize it. For the purpose of assessing the treatability of an effluent by a particular sludge, or by any sludge, the actual level of redox potential is of no immediate significance. The 955
956
D. DICKINSON
change in potential induced by the efiluent immediately on addition, and the further changes which take place during prolonged aeration are, however, very informative. EXPERIMENTAL
The apparatus used throughout these experiments has been of the simplest kind. The sensor element was simply a bright platinum electrode, approximately 52 mm. A saturated calomel half-cell was used as reference. A glass tank contained about 22 1. water + activated-sludge and the electrodes were suspended in this. The mixture was aerated and suspended by compressed air introduced through diffusers (of the type used in aquaria) at the bottom of the tank. The potential was measured continuously by an EIL pH/mV meter, modified to read 140 mV at full scale, and this in turn was recorded on an Everet-Edgecombe strip-chart recorder. The reference half-cell was connected to the terminal normally occupied by the glass electrode cable, and it wss found necessary to screen the connector wire to earth; otherwise stray currents or static effects confused the record. No special precautions were needed with the platinum electrode. The EIL meter was operated on the ApH circuit as absolute measurements in mV were not required and this further increased the sensivitity to about 70 mV, full scale deflection. The activated-sludge was built up in the laboratory over a period of several months using a variety of nutrients--including molasses, milk, dried media etc. The initial inoculum was cultivated soil. The top-water was replaced by fresh tap water at intervals. The activated-sludge eventually achieved was in every way comparable to that in a sewage treatment plant. A suspended solids content in excess of 7000 ppm was maintained and the sludge rapidly oxidized ammonia to nitrate. Species ofciliates, flagellates and protozoa, including rotifers, were normally present, but no systematic counts of the higher organisms were undertaken. The ability of the sludge to oxidize solutions of molasses, of ammonium salts and urea, of "slops" (characteristically, tea and coffee residues), of slaughter house waste, and particularly of whey solids was traced and measured over quite long periods. RESULTS
AND
DISCUSSION
When an activated-sludge is able to purify a waste without difficulty, the shape of the redox/time trace is typically as shown in FIG. l(a). Such curves were obtained most readily with wastes from meat products factories. Similar wastes containing particles of fat, however, gave curves of the type l(b). Sludges actively oxidizing ammonia when fed with ammonium carbonate or chloride gave redox traces as shown in FIG. 2 (a, b, c different quantities). Tea slops again gave readily reproducible traces (FI~. 3). Other materials, much less easily oxidized such as molasses, gave traces illustrating a more gradual return to the base level (Fro. 4). When the sludge was capable of oxidizing a waste completely, or nearly so, the area enclosed by the redox/time curve and the base line (obtained by producing the final line backwards on the time scale) was proportional to the quantity of waste added. Such a relationship did not apply, however, in the case of ammonia. If, as in the case
Redox Measurement of Treatability by the Activated-Sludge Process
957
f
Eh
Time
=
FIG. 1. Diagram of redox curves given by (a) easily oxidized material. (b) effluent containing material resistant to oxidation.
=0 b Eh
NH3=Oc
..NH~4,2 '~ NH3=2,5
0
I
I
I
2
~
J
NH~I,2
NH~3
I
I
4 Time, hr 3
I
5
L
6
I
7
I
8
FIO. 2. Traces from chart records; ammonia oxidation (a) NH3/N added = 200 rng. (b) NH3/N added = 100 mg. (c) NH3/N added = 50 rag. The concentrations of NH3/N rag/l, in the mixed liquor sampled at various times are entered on the chart.
958
D. DICKINSON
2 mt
5 ml
Eh
--
I
l
]o
I
20
I
30
I
40
Time,
I0 ml
5O
min
F~G. 3. Traces f r o m chart records: additions o f 2, 5 and 10 rrd tea slops.
Eh
0
i
I
I
2
I
I
3 Time,
4
1
@
hr
F~o. 4. Diagram of redox curve: oxidation of molasses.
I
6
Redox Measurementof Treatabilityby the Activated-SludgeProcess
959
of a fat-free slaughter-house waste the proportionality was linear, there was dearly no difficulty in purifying the waste and no adaptation of the sludge was necessary. On the other hand, a eurvilinear relationship with the line inclining towards the Area axis, was interpreted as indicating that the sludge could accept only a limited quantity per hour of the waste in question. Adaptation of a sludge to deal with a particular waste was shown by a change in the nature of the redox/time curve. This has been followed in some detail for the treatment of whey. Whey was reconstituted from dry whey solids as a 6~o solution. A supply of material of constant composition was thus assured. When first added to the activatedsludge tank, the traces obtained were somewhat erratic, but after additions of whey had been continued over a period of several days, curves of the shape in FIG. 5 were regularly obtained. Furthermore, proportionality between curve area and quantity of whey was achieved. The peculiar upward shift of Eh on addition of the whey persisted, however.
{
y/
Eh
0
I I
I
2
I
1
3 Time,
4
1
$
hr
FIG. 5. Traces from chart records: purificationof whey. Successiveamounts of 40 ml 6~. solids solution. Most of the work undertaken so far has been of a qualitative nature, but quantitative possibilities are apparent. Several prerequisites for success may, however, be stated on the basis of the experience gained. It is first essential that the sludge solids be in a state of "starvation" with the enzyme systems mainly in the oxidized form, and with an excess of dissolved oxygen. Under such conditions the E h as measured by the system is virtually constant. A comparatively large volume (22 1.) of sludge at 7000 ppm suspended solids as used in these experiments has a good buffer capacity and comparatively small quantities of substrate serve to give satisfactory traces. Nevertheless, there must obviously be a minimum quantity of any substrate which will give
960
D; DICKINSON
a measurable change in Eh and this has to be determined. If the sludge is currently incapable of oxidizing a substrate, then its introduction may cause a positive or negative shift in Eh value followed by a very slow return to a steady state; sometimes as long as 24 hr is required. When whey solids were first introduced to an actively nitrifying sludge, 7.5 mg of whey solids per g of dry sludge had no detectable effect on the Eh value; 2 hr later, 15 mg/g produced a negative shift of 7 inV. Further additions were made at approximately 2 hr intervals throughout that day, each producing a negative shift, and on the following day 7.5 mg/g gave an immediate negative shift of 11 inV. The return after each addition followed the expected pattern. For quantitative work it is advisable to use as little substrate as is consistent with the production of a useful Eh trace; this minimizes the time required for a complete cycle and avoids any material change in the quantity of activated-sludge solids involved during this time. When following the oxidation of ammonia, quantities of 50-200 mg NH3/N were added to the tank--in terms of concentration in solution 2.5-10 mg/l., or 0.25-1.0 mg/g of dry sludge solids. Assessment of the activity of the sludge is possible in terms of a standard substance. Glucose has been used with some success, although it has the disadvantage that it may cause an initial increase in Eh and if the activity of the sludge is very high, this increase may not be followed by a clearly marked decrease. In such event, a substantially larger quantity of glucose is added, when a more normal curve is obtained. The activity of the sludge is then expressed in a single figure: Area enclosed by Redox curve × Suspended solids, rag/1. Wt. of glucose added per I. × 1,000. i.e. the area of Redox/time curve representing the oxidation of 1 g of glucose by one g of activated-sludge solids. The area may be regarded as a measure of the energy required for purification of the glucose solution and it is therefore an inverse measure of the activity of the sludge. The same argument may be applied to any other substrate so that the curve area may be used as a measure of the activity of the sludge with regard to a substrate; or the ratio Area enclosed by glucose Redox curve × wt. of substrate added Area enclosed by substrate Redox curve × wt. of glucose added measures the treatability of the substrate by comparison to glucose. Thus measured, whey solids gave a ratio of 0.5-0.65; molasses gave 0.4. Although this ratio is a useful figure, it has to be considered in relation to the shape of the curve, the dimensions of which are Eh and time. Thus with a highly active and acclimatized sludge, the shape of the whey solids curve was exactly similar to the shape of the glucose curve; but the molasses curve was different, being the result of a much smaller Eh displacement maintained over a much longer time. REFERENCES Btu~3s R. and JoNss K. (1968) The significance ofredox potential measurements in sewage treatment. In: E~ltuent and Water Treatment Manual, pp. 155-161. DxCg,.nCZOND. (1940) Changes in oxidation-reduction potential within the sludge floc. J. Soc. Chem. Ind., Lond. LIX, 257-258.
Redox Measurement of Treatability by the Activated-Sludge Process
961
HEwrrr L. F. (1935) Oxidation-Reduction Potentials in Bacteriology. London County Council Technical Publication. O ' R o u ~ J. J., TOMLn~SONH. D. and BtrRBA~r~ N. C. (1963) Variation in ORP in an activated sludge plant with industrial waste load. Ind. War. Wastes 8, 15-21. RtrDD D. A., ROBERTSF. W. and BROOKSD. E. (1961) Oxidation-reduction potential measurements in sewage purification. Instrum. Engng 3, 61-67.
REDOX MEASUREMENT OF TREATABILITY BY THE ACTIVATED SLUDGE PROCESS D. DICKINSON Institute for Industrial Research and Standards, Dublin, 9, Ireland (Received 10 July 1969) Abstract--The change in redox potential with time following the addition of a substtate to an aerated activated sludge suspension follows a well-defined pattern chaxacteristie of the substrate. The shape of the trace indicates the relative ease or difficulty with which the sludge can purify a substrate or waste material. Successiveadditions causing a sludge to adapt result in a change in the shape of the trace. R~ma~--Le changement avec le temps darts le potentiel du redox, suivant l'addition d'tme touche inf6rieure ~t tree suspension de d6p6t activ6, a6r6, suit une caract6ristique bien d6tiule de la couche inf6rieure. Le contour du trac6 indique la r~lative aisance ou difficult6 avec laquelle le d6p6t peut purifier une couche inf6rieure ou un materiel de d~het. Des additions successives provoquent un d~l~t pour adapter les r6sultats ~tun changement darts la forme du trac~. Zusammeafassung--Der Wechsel in einem Redox Potential mit nach einen Zeitabstand folgendem Zusatz eines Substrats zu einer lufthaltigen, aktivierten Schlammuspensionfolgt einem genau dellnierten Muster, welches ftir das Substrat charakteristisch ist. Die Form der Spur zeigt die relative Leiehtigkeit oder Schwierigkeit an, mit welcher Schlamm ein Substrat oder Abflussmaterial reinigen kann. SukzesziveZustttze, welche Schlamm zur Anpassung bringen, haben eine Verttnderung in der Form der Spur zum Resultat. CHANGES in the redox potential of aerated bacterial cultures were investigated many years ago by HEwrrr (1930). Typically, the addition of nutrient results in a rapid fall in potential, and this is followed by a more or less exponential return to the original Eh level as the nutrient is oxidized and utilized by the organisms. On theoretical grounds, the present author showed that in an activated sludge suspension, the potential change should follow a similar pattern (DICKINSON,1940). Probably because of the practical difficulty of distinguishing between inter- or extra-cellular changes or of knowing precisely what is measured by a platinum electrode suspended in aerated activatedsludge, comparatively little attention has been given to the use of redox measurements other than to their possible application to aeration control (RUDD, 1961 ; O'RotmKE, 1963 ; BmGGS and KNOWL~S, 1968). In experiments carded out over a number of years it has been established that (1) the actual level of the potential of a bright platinum electrode suspended in aerated activated-sludge is affected by the "condition" of the sludge and by the concentrations of inorganic salts, particularly nitrate, present in the mixture; (2) changes in potential are characteristic of the nutrient added and reflect the ability of the sludge to oxidize it. For the purpose of assessing the treatability of an effluent by a particular sludge, or by any sludge, the actual level of redox potential is of no immediate significance. The 955
956
D. DICKINSON
change in potential induced by the efiluent immediately on addition, and the further changes which take place during prolonged aeration are, however, very informative. EXPERIMENTAL
The apparatus used throughout these experiments has been of the simplest kind. The sensor element was simply a bright platinum electrode, approximately 52 mm. A saturated calomel half-cell was used as reference. A glass tank contained about 22 1. water + activated-sludge and the electrodes were suspended in this. The mixture was aerated and suspended by compressed air introduced through diffusers (of the type used in aquaria) at the bottom of the tank. The potential was measured continuously by an EIL pH/mV meter, modified to read 140 mV at full scale, and this in turn was recorded on an Everet-Edgecombe strip-chart recorder. The reference half-cell was connected to the terminal normally occupied by the glass electrode cable, and it wss found necessary to screen the connector wire to earth; otherwise stray currents or static effects confused the record. No special precautions were needed with the platinum electrode. The EIL meter was operated on the ApH circuit as absolute measurements in mV were not required and this further increased the sensivitity to about 70 mV, full scale deflection. The activated-sludge was built up in the laboratory over a period of several months using a variety of nutrients--including molasses, milk, dried media etc. The initial inoculum was cultivated soil. The top-water was replaced by fresh tap water at intervals. The activated-sludge eventually achieved was in every way comparable to that in a sewage treatment plant. A suspended solids content in excess of 7000 ppm was maintained and the sludge rapidly oxidized ammonia to nitrate. Species ofciliates, flagellates and protozoa, including rotifers, were normally present, but no systematic counts of the higher organisms were undertaken. The ability of the sludge to oxidize solutions of molasses, of ammonium salts and urea, of "slops" (characteristically, tea and coffee residues), of slaughter house waste, and particularly of whey solids was traced and measured over quite long periods. RESULTS
AND
DISCUSSION
When an activated-sludge is able to purify a waste without difficulty, the shape of the redox/time trace is typically as shown in FIG. l(a). Such curves were obtained most readily with wastes from meat products factories. Similar wastes containing particles of fat, however, gave curves of the type l(b). Sludges actively oxidizing ammonia when fed with ammonium carbonate or chloride gave redox traces as shown in FIG. 2 (a, b, c different quantities). Tea slops again gave readily reproducible traces (FI~. 3). Other materials, much less easily oxidized such as molasses, gave traces illustrating a more gradual return to the base level (Fro. 4). When the sludge was capable of oxidizing a waste completely, or nearly so, the area enclosed by the redox/time curve and the base line (obtained by producing the final line backwards on the time scale) was proportional to the quantity of waste added. Such a relationship did not apply, however, in the case of ammonia. If, as in the case
Redox Measurement of Treatability by the Activated-Sludge Process
957
f
Eh
Time
=
FIG. 1. Diagram of redox curves given by (a) easily oxidized material. (b) effluent containing material resistant to oxidation.
=0 b Eh
NH3=Oc
..NH~4,2 '~ NH3=2,5
0
I
I
I
2
~
J
NH~I,2
NH~3
I
I
4 Time, hr 3
I
5
L
6
I
7
I
8
FIO. 2. Traces from chart records; ammonia oxidation (a) NH3/N added = 200 rng. (b) NH3/N added = 100 mg. (c) NH3/N added = 50 rag. The concentrations of NH3/N rag/l, in the mixed liquor sampled at various times are entered on the chart.
958
D. DICKINSON
2 mt
5 ml
Eh
--
I
l
]o
I
20
I
30
I
40
Time,
I0 ml
5O
min
F~G. 3. Traces f r o m chart records: additions o f 2, 5 and 10 rrd tea slops.
Eh
0
i
I
I
2
I
I
3 Time,
4
1
@
hr
F~o. 4. Diagram of redox curve: oxidation of molasses.
I
6
Redox Measurementof Treatabilityby the Activated-SludgeProcess
959
of a fat-free slaughter-house waste the proportionality was linear, there was dearly no difficulty in purifying the waste and no adaptation of the sludge was necessary. On the other hand, a eurvilinear relationship with the line inclining towards the Area axis, was interpreted as indicating that the sludge could accept only a limited quantity per hour of the waste in question. Adaptation of a sludge to deal with a particular waste was shown by a change in the nature of the redox/time curve. This has been followed in some detail for the treatment of whey. Whey was reconstituted from dry whey solids as a 6~o solution. A supply of material of constant composition was thus assured. When first added to the activatedsludge tank, the traces obtained were somewhat erratic, but after additions of whey had been continued over a period of several days, curves of the shape in FIG. 5 were regularly obtained. Furthermore, proportionality between curve area and quantity of whey was achieved. The peculiar upward shift of Eh on addition of the whey persisted, however.
{
y/
Eh
0
I I
I
2
I
1
3 Time,
4
1
$
hr
FIG. 5. Traces from chart records: purificationof whey. Successiveamounts of 40 ml 6~. solids solution. Most of the work undertaken so far has been of a qualitative nature, but quantitative possibilities are apparent. Several prerequisites for success may, however, be stated on the basis of the experience gained. It is first essential that the sludge solids be in a state of "starvation" with the enzyme systems mainly in the oxidized form, and with an excess of dissolved oxygen. Under such conditions the E h as measured by the system is virtually constant. A comparatively large volume (22 1.) of sludge at 7000 ppm suspended solids as used in these experiments has a good buffer capacity and comparatively small quantities of substrate serve to give satisfactory traces. Nevertheless, there must obviously be a minimum quantity of any substrate which will give
960
D; DICKINSON
a measurable change in Eh and this has to be determined. If the sludge is currently incapable of oxidizing a substrate, then its introduction may cause a positive or negative shift in Eh value followed by a very slow return to a steady state; sometimes as long as 24 hr is required. When whey solids were first introduced to an actively nitrifying sludge, 7.5 mg of whey solids per g of dry sludge had no detectable effect on the Eh value; 2 hr later, 15 mg/g produced a negative shift of 7 inV. Further additions were made at approximately 2 hr intervals throughout that day, each producing a negative shift, and on the following day 7.5 mg/g gave an immediate negative shift of 11 inV. The return after each addition followed the expected pattern. For quantitative work it is advisable to use as little substrate as is consistent with the production of a useful Eh trace; this minimizes the time required for a complete cycle and avoids any material change in the quantity of activated-sludge solids involved during this time. When following the oxidation of ammonia, quantities of 50-200 mg NH3/N were added to the tank--in terms of concentration in solution 2.5-10 mg/l., or 0.25-1.0 mg/g of dry sludge solids. Assessment of the activity of the sludge is possible in terms of a standard substance. Glucose has been used with some success, although it has the disadvantage that it may cause an initial increase in Eh and if the activity of the sludge is very high, this increase may not be followed by a clearly marked decrease. In such event, a substantially larger quantity of glucose is added, when a more normal curve is obtained. The activity of the sludge is then expressed in a single figure: Area enclosed by Redox curve × Suspended solids, rag/1. Wt. of glucose added per I. × 1,000. i.e. the area of Redox/time curve representing the oxidation of 1 g of glucose by one g of activated-sludge solids. The area may be regarded as a measure of the energy required for purification of the glucose solution and it is therefore an inverse measure of the activity of the sludge. The same argument may be applied to any other substrate so that the curve area may be used as a measure of the activity of the sludge with regard to a substrate; or the ratio Area enclosed by glucose Redox curve × wt. of substrate added Area enclosed by substrate Redox curve × wt. of glucose added measures the treatability of the substrate by comparison to glucose. Thus measured, whey solids gave a ratio of 0.5-0.65; molasses gave 0.4. Although this ratio is a useful figure, it has to be considered in relation to the shape of the curve, the dimensions of which are Eh and time. Thus with a highly active and acclimatized sludge, the shape of the whey solids curve was exactly similar to the shape of the glucose curve; but the molasses curve was different, being the result of a much smaller Eh displacement maintained over a much longer time. REFERENCES Btu~3s R. and JoNss K. (1968) The significance ofredox potential measurements in sewage treatment. In: E~ltuent and Water Treatment Manual, pp. 155-161. DxCg,.nCZOND. (1940) Changes in oxidation-reduction potential within the sludge floc. J. Soc. Chem. Ind., Lond. LIX, 257-258.
Redox Measurement of Treatability by the Activated-Sludge Process
961
HEwrrr L. F. (1935) Oxidation-Reduction Potentials in Bacteriology. London County Council Technical Publication. O ' R o u ~ J. J., TOMLn~SONH. D. and BtrRBA~r~ N. C. (1963) Variation in ORP in an activated sludge plant with industrial waste load. Ind. War. Wastes 8, 15-21. RtrDD D. A., ROBERTSF. W. and BROOKSD. E. (1961) Oxidation-reduction potential measurements in sewage purification. Instrum. Engng 3, 61-67.