Trickling filter: use and construction

A trickling filter (percolating or biological filter) is a secondary treatment unit in which settled sewage is sprayed over a bed of coarse media; a biological film (slime) of micro-organisms grows on the media and oxidises the organic matter as the sewage trickles down in contact with air.

Why it is used

• Primary settling removes only 30-35% of BOD; dissolved and colloidal organics remain.
• A trickling filter removes 65-90% of the BOD with simple operation, low power need and low skill, and with good tolerance to shock loads.
• Suitable for small and medium towns where land and a little head are available.

Construction

        rotating distributor arm (sprinklers)
   inlet pipe  |
        +------+-------+------------------+
 wall   | o o o o o o o o o o o o o o o o | <- filter media
 (brick/| o o o o  stones 40-80 mm  o o o |    1.8 - 3 m deep
 conc.) |_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o|
        | underdrain tiles (slope 1:100)  | -> collecting
        +---- air ventilation channels ---+    channel -> to
                                               secondary clarifier

1. Circular (or rectangular) tank of brick masonry or concrete, with walls about 0.3 m above the media; diameter up to 60 m.
2. Filter media: hard, durable, rough stones, crushed rock, slag or plastic modules of 40-80 mm size (25-40 mm at the top in some designs), depth 1.8-3 m (standard rate), up to 12 m for plastic media.
3. Distribution system: a rotating arm distributor with nozzles driven by the jet reaction of the sewage (or fixed nozzles in rectangular beds) that spreads the effluent evenly.
4. Underdrainage system: perforated or half-round tile channels on the floor with a slope of about 1 in 100 to collect the effluent and also admit air.
5. Ventilation: natural draft through the underdrains and side openings, so the film stays aerobic.
6. Dosing and recirculation arrangement: a dosing tank or siphon for constant head and a pump for recirculation in high-rate filters.
7. A secondary settling tank follows the filter to remove the sloughed film (humus).