Unit operation, process; need of trickling filter

Unit operation and unit process

• A unit operation is a treatment step in which pollutants are removed by physical forces only. Examples: screening, grit removal, sedimentation, flotation, filtration, flow equalisation and mixing.
• A unit process is a treatment step in which pollutants are removed or changed by chemical or biological reactions. Examples: activated sludge (biological oxidation), trickling filter, anaerobic digestion, coagulation, chlorination (disinfection), nitrification.

In a plant the operations and processes are combined (preliminary, primary, secondary, tertiary stages) into a treatment train.

Need of a trickling filter

Primary treatment removes only about 30-35% of the BOD, and sewage still holds dissolved and colloidal organic matter. A trickling filter is a secondary treatment unit that:

• gives 80-90% BOD removal (high rate 65-85%) with simple operation and no continuous power for aeration (only for dosing);
• needs less skill and cost than activated sludge, and handles shock loads better;
• is suitable for small and medium towns that have a suitable gravity head.

Pollutant removal mechanism

Sewage is sprayed over a bed of stones (40-80 mm) or plastic media. A slimy biological film (zoogloeal film) of bacteria, fungi, algae and protozoa grows on the media. As the sewage trickles down in a thin layer, air moves up through the voids. Organic matter is adsorbed on the film and oxidised aerobically by bacteria in the outer layer, which converts it to CO₂, water and new cells; the inner layer is anaerobic. Old film sloughs off and is removed in the secondary clarifier.

Comparison with intermittent sand filter and contact bed

Because of higher loading rate, continuous operation, less area and less clogging, the trickling filter has replaced sand filters and contact beds.