The ærobic Contact Bed filters are merely open tanks filled with coal, coke, or large cinders, through which the liquid is allowed to percolate at intervals. The bacilli live and flourish in these, and do their work during the periods from 2 to 4 hours the tanks are charged.
The percolating continuous filters are those over which the effluent is continuously sprinkled and through which the fluid slowly percolates. Experiments have conclusively proved that good results can be obtained from either when a suitable sewage is applied.
CHAPTER X.
General Sanitation.
Trees and tall crops.
All the lower branches of trees in crowded areas should be pruned; these are useless or superfluous for shade, and only impede the free circulation of air. This should be done by sawing off any branch flush with the main trunk, or with its main branch. In no case should a ragged stump be left, and, if possible, the wound should be smeared with tar. Lopping must be done with a handsaw and in no case with an axe; the best season for doing this is at the end of the cold weather, but it can also be done at the end of the rains. The final form of a tree should be a straight stem up to 15 or 18 feet, without a bough, and above that height its natural shape whatever that may be. This will allow of free ventilation of the roads and streets. For the same reason no tall crops, such as Genera, or Indian corn, should be allowed near houses, and jungle should be kept cleared as far as possible.
Tanks.
Householders should not be allowed to excavate small tanks in their compounds to procure earth from for the plinths of their houses, as these become in time mere cesspools, where the inmates bathe, wash their clothes, and often drink the water. Further, these become suitable breeding-places for mosquitoes. Plots of ground should be set aside by Municipalities, where, for proper reasons, earth can be taken. These common excavations will in time become large tanks, which can be properly conserved and utilized. For the same reason earth for roads and railways should not be removed from isolated pits, but should be taken so that the resultant excavation forms one continuous channel running parallel with the road, or should be taken in shallow layers so as to avoid the formation of a pit or hollow, the existence of which is especially objectionable in the light of recent research as mosquitoes breed and generate in them, and so spread malarial fever. Where such hollows exist, the growth of fish should be encouraged, as they feed on the mosquito larvæ when present.
Cultivation within urban limits.
From the experience of troops on service, it is found that camping near recently turned-up soil is usually followed by fever. Ploughing up of land for agricultural purposes should, if possible, be prevented within municipal urban limits. In most municipalities there is a suburban area which is pretty well all cultivated, but cultivation in densely populated areas, as often occurs, should be discouraged.
Dhobies.