VPE 511: Environment and Environmental Hygiene (2+1)

LAND OR ARTIFICIAL BIOLOGICAL TREATMENT

Percolating or trickling filters

• Trickling filters are really aerating beds, not filters they consist essentially of circular beds of broken stone of various sizes on which sewage is distributed.
• The broken stone becomes coated after a short period with a gelatinous covering which in the presence of oxygen forms a nidus for aerobic bacteria which is then said to be ripened or activated.
• Distribution of the tank liquor is obtained by means of a revolving sprinkler driven by a constant head of water or by the intermittent discharge of a siphon.

Contact beds

• It comprise tanks filled with broken stone the object being to permit digestion of organic material by aerobic bacteria.
• The tank is filled and the liquor allowed to remain in contact with the material in the bed for a sufficient period to allow purification to take place, thereafter the tank is emptied, and in the process air is drawn into the interstices of the stone, thus providing the oxygen necessary for the bacteria.
• The cycle of operation occupies eight to twelve hours, the resting period being about half this to keep the beds in condition.

Humus tanks

• The effluent from percolating filters and contact beds often contain a good deal of fine organic matter termed humus.
• It is necessary to allow this to settle before discharging the final effluent into a stream. This may be done by holding the filter effluent in tanks.