Discussion by A.L.H. Gameson
Water Pollution Research Laboratory of the Department of
the Environment, Stevenage, England
There are two main areas in which some clarification by the Authors would be appreciated:
Firstly in the reliability of the method itself and secondly in the reliability of the predictions that
can be made from the results obtained.
It is disquieting to read that the Authors consider that an instrumental accuracy of 0.01 mg/1 D.O.
is insufficient, since this would seem to imply that the concentrations being measured are meaningful
to this degree of accuracy; but Fig. 3 shows perturbations of the order of tenths of a mg/1 about the
smooth curves that can be drawn through the plotted points, and such variations may well be real ones
and would not be greatly reduced by improvement of the measuring instruments. The unconfined
probe appears to be, in fact, confined except in that it is open to the air. It is, in general, inadmissible
to measure reaeration in flowing water by confining the water so as to remove effects of turbulence
and wind. Furthermore, within each container there will be factors which inevitably affect the
measured D.O.; such factors may include the shadow cast by the probe in the clear-glass containers,
absorption of radiation in the glass, stratification of D.O. - particularly in the one where much of the
oxygen demand is exerted at the bottom and in the blackened one where in daylight there may well be
some thermal stratification. The assumption that P is equal to 0, 0.7 P max, or P max also seems to
introduce uncertainties. All things considered, how do the Authors justify their requirement of better
than 0.01 mg/1?
In Fig. 3 the D.O. values for the unconfined probe are generally below those for the bottom-free
probe - presumably because after the first hour or so the water was super-saturated with D.O. and
oxygen was escaping from the unconfined system - but after 6V&-7 h the position is reversed; does not
this imply that the overall value of K2 was negative?
In overcoming the limitations reported to be imposed by the 0.01-0.001 mg/1 D.O. criteria, the
Authors stated that L 0 was determined by the conventional laboratory method. Would they outline
this method, as although a 5-day BOD test is readily performed it is difficult to determine the relevant
value of L 0 . They refer to L 0 as the total organic BOD load, but presumably they are referring to
concentration rather than load. Did they establish that there was no nitrification occurring?
When it comes to the verification and application of the results, how is this done? Clearly to use
the results to 'predict' the D.O. curve for the unenclosed probe would be no verification, but
verification is needed before prediction. Suppose the polluting load were to be doubled, how would
the Authors set about predicting the effects on the D.O.? Doubling the load would be accompanied by
a change in flow, in composition, or in both. A change in flow may affect K2 and a change in
composition may materially alter Ki. Changes in either could affect algal growth (and hence R and P)
and may be expected to alter the rate of deposition and the nature of the deposits and hence to alter
B. If small changes are made in the polluting load, one can estimate their effects by relatively simple
methods, but if the changes are large it is difficult to see how one could reasonably assume that the
values found for K t , Κ2, R, B, and P could still apply.
Hermann H. Hahn, West Germany
Did the authors observe variations in the reported coefficient with respect to time and location?
The discussors' work on modelling the river Neckar showed such variations which have to be expected
if one considers how many different reactions are represented in these reactions coefficients.
Furthermore it appears that the attention focussed on determining the reported coefficient is not in
proportion to other elements of the quality model, such as the hydrological inputs; precision of the
used parameters should be comparable and increased efforts in detailed investigation of one or several
elements will most likely not improve the results obtained from calculations on this basis.
279
Water Pollution Research Laboratory of the Department of
the Environment, Stevenage, England
There are two main areas in which some clarification by the Authors would be appreciated:
Firstly in the reliability of the method itself and secondly in the reliability of the predictions that
can be made from the results obtained.
It is disquieting to read that the Authors consider that an instrumental accuracy of 0.01 mg/1 D.O.
is insufficient, since this would seem to imply that the concentrations being measured are meaningful
to this degree of accuracy; but Fig. 3 shows perturbations of the order of tenths of a mg/1 about the
smooth curves that can be drawn through the plotted points, and such variations may well be real ones
and would not be greatly reduced by improvement of the measuring instruments. The unconfined
probe appears to be, in fact, confined except in that it is open to the air. It is, in general, inadmissible
to measure reaeration in flowing water by confining the water so as to remove effects of turbulence
and wind. Furthermore, within each container there will be factors which inevitably affect the
measured D.O.; such factors may include the shadow cast by the probe in the clear-glass containers,
absorption of radiation in the glass, stratification of D.O. - particularly in the one where much of the
oxygen demand is exerted at the bottom and in the blackened one where in daylight there may well be
some thermal stratification. The assumption that P is equal to 0, 0.7 P max, or P max also seems to
introduce uncertainties. All things considered, how do the Authors justify their requirement of better
than 0.01 mg/1?
In Fig. 3 the D.O. values for the unconfined probe are generally below those for the bottom-free
probe - presumably because after the first hour or so the water was super-saturated with D.O. and
oxygen was escaping from the unconfined system - but after 6V&-7 h the position is reversed; does not
this imply that the overall value of K2 was negative?
In overcoming the limitations reported to be imposed by the 0.01-0.001 mg/1 D.O. criteria, the
Authors stated that L 0 was determined by the conventional laboratory method. Would they outline
this method, as although a 5-day BOD test is readily performed it is difficult to determine the relevant
value of L 0 . They refer to L 0 as the total organic BOD load, but presumably they are referring to
concentration rather than load. Did they establish that there was no nitrification occurring?
When it comes to the verification and application of the results, how is this done? Clearly to use
the results to 'predict' the D.O. curve for the unenclosed probe would be no verification, but
verification is needed before prediction. Suppose the polluting load were to be doubled, how would
the Authors set about predicting the effects on the D.O.? Doubling the load would be accompanied by
a change in flow, in composition, or in both. A change in flow may affect K2 and a change in
composition may materially alter Ki. Changes in either could affect algal growth (and hence R and P)
and may be expected to alter the rate of deposition and the nature of the deposits and hence to alter
B. If small changes are made in the polluting load, one can estimate their effects by relatively simple
methods, but if the changes are large it is difficult to see how one could reasonably assume that the
values found for K t , Κ2, R, B, and P could still apply.
Hermann H. Hahn, West Germany
Did the authors observe variations in the reported coefficient with respect to time and location?
The discussors' work on modelling the river Neckar showed such variations which have to be expected
if one considers how many different reactions are represented in these reactions coefficients.
Furthermore it appears that the attention focussed on determining the reported coefficient is not in
proportion to other elements of the quality model, such as the hydrological inputs; precision of the
used parameters should be comparable and increased efforts in detailed investigation of one or several
elements will most likely not improve the results obtained from calculations on this basis.
279