Discussion by A.H. Potter
There are two major aspect of the nitrogen cycle in the freshwater portion of the environment
which have to be considered. One concerns waters which are not eutrophic and where nitrogen
removal from contributing waters may be needed. The other is where a potable supply is obtained
from that body of water and where a health hazard may occur from excessive nitrate and I assume it is
this aspect which the authors have in mind since the removal referred to in this paper is not a complete
one. My interest in this subject derives from my position in United Kingdom as Chief Chemist to the
Middle Lee Regional Drainage Scheme which is responsible for sewage purification at the Rye Meads
Works, situated in the middle reaches of the drainage basin of the River Lee, a tributary of the River
Thames. The effluent from this Works which has to be fully nitrified and reach a standard of 5 mg/1
B.O.D. and 5 mg/1 of suspended solids, has been discharged to the River Lee since 1956. Much interest
has been directed to the effect of the nitrate load on the water quality of the river, especially since the
Lee provides London with 20% of its drinking water. The nitrate content of the river at the point of
abstraction has risen from about 3 mg/1 in 1925 to almost 7 mg/1 in 1955 but has remained relatively
constant around this figure since that date despite the considerable amounts of nitrate discharged into
the river from Rye Meads effluent since 1956. Clearly there is no cause for complacency and interest
in denitrification and other methods of nitrogen removal is called for.
To return to the paper despite the fact that the authors refer to "low cost land" and climatic
conditions which are "adequate" I was not clear about the application of their proposals to a specific
project. They do refer to the Dan Region Renovation Project and I should be interested to know if
they consider the conditions there are favourable and precisely what they consider are adequate
climatic conditions. I should also like to know what treatment sewage will need before it can be
treated in holding ponds such as they describe. Some treatment is essential since raw sewage will need
a large amount of lime to bring it to a pH of 11 (about 500 mg/1 is needed in typical United Kingdom
sewage) and this results in about 3 - 4 times the normal amount of sludge being produced. A more
serious problem would be the presence of urea which would not be removed since at a pH of over 10.5
the enzymic conversion of urea to ammonia is inhibited. I did not notice any reference to the consequences on the receiving water of discharging an effluent of pH 11 with a possible ammonia content
of 10 mg/1.
I must also refer to the basic disadvantage of any method which merely transfers ammonia from
the water into the air - it will almost certainly find its way back into somebody else's water!
This disadvantage is now recognised in the latest designs of ammonia stripping towers - the air
from, them being passed through absorption towers containing sulphuric acid in order to remove the
ammonia.
There is yet another disadvantage which is not referred to in the paper, namely, losses due to
evaporation which would be at the maximum when maximum removal of ammonia occurred.
I must confess that I had a little difficulty with some of the mathematics. It would have helped me
if the authors had tabulated the symbols used and I was puzzled for some time by the anomaly of the
final equation No. 27 which in my copy did not contain a factor related to the surface area. I finally
discovered that there were two misprints on adjacent lines which caused confusion. You will realise
from this that I am not a mathematician.
The authors state on the last page that it should be possible to decrease the ammonia concentration
by 90%. This may well be true but since their incoming sewage contained 50-90 mg/1 of ammoniacal
nitrogen this would leave 5 - 9 mg/1 - a range not far short of the W.H.O. limit of 10 mg/1 of nitrogen
as nitrate. The description of the vessels in the wind tunnel experiments shows that rather small
containers were used. Are the authors really satisfied that they can make valid comparisons between
the results of such experiments and their calculations using a formula which takes into account such
complex factors as wind temperature stratification and convection currents? Perhaps the Authors
would state whether they consider that it is valid to compare such effects in small containers with a
large pond. Perhaps the authors would also clarify the references to the heights at which samples were
taken and how they relate these to the depth of vessels.
Over the whole of the United Kingdom the total rainfall is more than adequate for immediate and
prospective needs. In England however the available water per head of the population is amongst the
lowest in Europe.
But in the South-east of England where we have a very high population density, rainfall which is
only moderate and an evaporation rate which is equal to the rainfall the quantity of water obtained
from natural rivers and wells is insufficient for the needs of the population. Our solution to the
problem is to rely increasingly on the re-use of sewage effluent in admixture with river water. To
devote land use to a process using a 5 - day stabilisation pond plus a 15 day holding pond for partial
nitrogen removal would not be possible. However at Rye Meads we have demonstrated that lagoon
treatment of a highly purified and fully nitrified effluent before discharge to a river has a number of
517
There are two major aspect of the nitrogen cycle in the freshwater portion of the environment
which have to be considered. One concerns waters which are not eutrophic and where nitrogen
removal from contributing waters may be needed. The other is where a potable supply is obtained
from that body of water and where a health hazard may occur from excessive nitrate and I assume it is
this aspect which the authors have in mind since the removal referred to in this paper is not a complete
one. My interest in this subject derives from my position in United Kingdom as Chief Chemist to the
Middle Lee Regional Drainage Scheme which is responsible for sewage purification at the Rye Meads
Works, situated in the middle reaches of the drainage basin of the River Lee, a tributary of the River
Thames. The effluent from this Works which has to be fully nitrified and reach a standard of 5 mg/1
B.O.D. and 5 mg/1 of suspended solids, has been discharged to the River Lee since 1956. Much interest
has been directed to the effect of the nitrate load on the water quality of the river, especially since the
Lee provides London with 20% of its drinking water. The nitrate content of the river at the point of
abstraction has risen from about 3 mg/1 in 1925 to almost 7 mg/1 in 1955 but has remained relatively
constant around this figure since that date despite the considerable amounts of nitrate discharged into
the river from Rye Meads effluent since 1956. Clearly there is no cause for complacency and interest
in denitrification and other methods of nitrogen removal is called for.
To return to the paper despite the fact that the authors refer to "low cost land" and climatic
conditions which are "adequate" I was not clear about the application of their proposals to a specific
project. They do refer to the Dan Region Renovation Project and I should be interested to know if
they consider the conditions there are favourable and precisely what they consider are adequate
climatic conditions. I should also like to know what treatment sewage will need before it can be
treated in holding ponds such as they describe. Some treatment is essential since raw sewage will need
a large amount of lime to bring it to a pH of 11 (about 500 mg/1 is needed in typical United Kingdom
sewage) and this results in about 3 - 4 times the normal amount of sludge being produced. A more
serious problem would be the presence of urea which would not be removed since at a pH of over 10.5
the enzymic conversion of urea to ammonia is inhibited. I did not notice any reference to the consequences on the receiving water of discharging an effluent of pH 11 with a possible ammonia content
of 10 mg/1.
I must also refer to the basic disadvantage of any method which merely transfers ammonia from
the water into the air - it will almost certainly find its way back into somebody else's water!
This disadvantage is now recognised in the latest designs of ammonia stripping towers - the air
from, them being passed through absorption towers containing sulphuric acid in order to remove the
ammonia.
There is yet another disadvantage which is not referred to in the paper, namely, losses due to
evaporation which would be at the maximum when maximum removal of ammonia occurred.
I must confess that I had a little difficulty with some of the mathematics. It would have helped me
if the authors had tabulated the symbols used and I was puzzled for some time by the anomaly of the
final equation No. 27 which in my copy did not contain a factor related to the surface area. I finally
discovered that there were two misprints on adjacent lines which caused confusion. You will realise
from this that I am not a mathematician.
The authors state on the last page that it should be possible to decrease the ammonia concentration
by 90%. This may well be true but since their incoming sewage contained 50-90 mg/1 of ammoniacal
nitrogen this would leave 5 - 9 mg/1 - a range not far short of the W.H.O. limit of 10 mg/1 of nitrogen
as nitrate. The description of the vessels in the wind tunnel experiments shows that rather small
containers were used. Are the authors really satisfied that they can make valid comparisons between
the results of such experiments and their calculations using a formula which takes into account such
complex factors as wind temperature stratification and convection currents? Perhaps the Authors
would state whether they consider that it is valid to compare such effects in small containers with a
large pond. Perhaps the authors would also clarify the references to the heights at which samples were
taken and how they relate these to the depth of vessels.
Over the whole of the United Kingdom the total rainfall is more than adequate for immediate and
prospective needs. In England however the available water per head of the population is amongst the
lowest in Europe.
But in the South-east of England where we have a very high population density, rainfall which is
only moderate and an evaporation rate which is equal to the rainfall the quantity of water obtained
from natural rivers and wells is insufficient for the needs of the population. Our solution to the
problem is to rely increasingly on the re-use of sewage effluent in admixture with river water. To
devote land use to a process using a 5 - day stabilisation pond plus a 15 day holding pond for partial
nitrogen removal would not be possible. However at Rye Meads we have demonstrated that lagoon
treatment of a highly purified and fully nitrified effluent before discharge to a river has a number of
517