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The large proportion of samples of feeding stuffs which was found to contain
deleterious substances was due to the attention concentrated during the year by
the inspectorate on the quality of linseed cake supplied, and to the fact that, having
found that a sample from bulk contained evidence of the presence of castor seed,
further samples from different portions of the same consignment were examined.
It is evident that a considerable amount of contaminated linseed cake is still being
put on the market, and in a number of cases the amount of sand and very coarse
grit was also so great as to constitute a danger to young stock. A sample of ground
nut-cake was reported upon adversely for the latter reason. An interesting case which
occurred was that of a cargo of grass nuts imported from Egypt, part of which
had been contaminated with green paint from the sacking in which it had been
packed. One portion of the grass nuts free from any sacking contained arsenic
equivalent to 50 parts per million, and a piece of the sacking itself heavily impregnated
with paint contained 5.8 per cent. of arsenic. A sample of white fish meal was
reported against as having an admixture of at least 1.55 per cent. of crustacean
shell. Three samples of alleged bone meal were found to contain amounts of acid
insoluble matter ranging up to 28.7 per cent., and these were reported as not complying
with the definition given.
Drinking
water
supply.
A regular chemical examination has been made of the drinking water at those
institutions of the Council which derive their supplies from private wells, and water
from other sources has been tested when required. During the year 191 samples
have been examined from 33 institutions.
Sewage
treatment.
The use of ferric chloride added to the sewage has been continued with satisfactory
results.
Chlorination in the sewers has been found to improve the condition of the
sewage and is being continued and extended.
The activated sludge plant at the Northern outfall has continued to treat,
with satisfactory results, ten million gallons per day of sedimented sewage. The
construction of five activated units similar to the one in use is proceeding, and it
is hoped that one unit will be at work in the early summer of 1937.
The dry matter in individual cargoes of sewage sludge sent to sea from the
Northern outfall varied from 2 per cent. to 15 per cent., the average being 7.9 per
cent. In the case of the cargoes from the Southern outfall, the variations were
from 2 to 13 per cent., with an average of 6.9 per cent., similar to that of last year.
The dry solids in the greater number of cargoes from both outfalls did not diverge
much from the average.
The condition of the river, which had been affected by the prolonged dry
weather, has improved steadily since October, 1935, when the increase in the rainfall
made itself felt. The steps taken by the Council, described above, also contributed
materially to the improved conditions. The new units at the Northern outfall as
they come into being will tend to restore the balance between oxygen absorption and
re-absorption at the critical times of low flow of fresh water and high temperature.
Vehicular
tunnels.
The systematic investigations, commenced in 1928, as to the condition of
the air in Blackwall and Rotherhithe tunnels, were continued during 1936, 103
samples of air from each tunnel having been examined for the carbon monoxide
content and 26 from each for black suspended matter. The remarkable increase
in motor traffic using the tunnels has continued, and at Blackwall some changes
have been made in the ventilation scheme in an attempt to cope with the large
amount of polluting exhaust gas emitted by these vehicles.