110 
THOMPSON YATES LABORATORIES REPORT 
1. CONTACT BEDS 
These are only of experimental size, and, so far as I know, the system has not 
been tried on a large scale like in several English towns. The results are in 
agreement, for the most part, with those obtained in Great Britain ; but the amount of 
purification attained seems to have been less, and the technique of the investigation less 
complete. Experiments have been made with various filtering materials. The two 
places, Hamburg and Charlottenburg, also form a valuable contrast and comparison, 
since the former supplies only domestic, and the latter both domestic and trade 
sewage to the beds. 
Hamburg 
The experimental installation of contact beds (under cover) for the treating 
of the sewage of the General Hospital (2,000 persons) has been very thoroughly 
investigated by Dunbar and Zirn*, chiefly from the chemical aspect, since no 
bacteriological analyses are given in their published results. The sewage was purely 
domestic and was screened. It comes only a short distance, consequently the solids 
do not get much broken up. Yet with double filtration the best purification was not 
more than 86 p.c. diminution of oxygen absorbed, and the effluents which I saw 
myself were far from clear. (At Leeds, with a stronger sewage, purification up to 
93 p.c. has been obtained). The authors, however, consider their results favourable 
to the system and draw the same conclusions as English observers, viz. : (1) clogging 
of the beds will occur sooner or later ; (2) the diminution in capacity, due to clogging 
by inorganic sludge, will not be much improved by rest ; (3) the diminution due to 
clogging with organic sludge will be relatively more improved by rest, but such sludge 
should not be allowed on to the beds in properly arranged works. The experiments 
with various materials all pointed to coke of appropriate size being the best. Both 
from a theoretical as well as from a chemical point of view, these critical and well 
written papers deserve careful study from those interested in the question, and it is 
much to be regretted that more complete chemical and some bacteriological analyses 
were not made. 
r 
Charlottenburg 
The experimental beds had a gross capacity of 6 cbm.f and a net capacity of 
5 cbm. ; they took about two hours to fill and to empty. Experiments were made with 
(1) coke and sand ; (2) granite, gravel, and sand ; (3) sand and brick breeze. The 
coarser solids were screened off. 
*Vierttljahrschr.f,gerichtl. Medic, 1900 : Suppl. pp. 178 et seq. 
f 1 cbm. = 1,000 litres = 220 gallons. 
