Design of Trickling Filters & Common Operational Issues

Types of Trickling Filters & Common Operational Issues

by Pankaj Bansal

What is a trickling filter??

Trickling filter is a type of waste water treatment process, which is an attached growth process. In this process the microorganisms responsible for digestion are attached to an inert filter material. This Packing material can be rock, gravel, sand and a wide range of plastic and other synthetic materials. In other words the removal of pollutants from waste-water  involves both absorption & adsorption of organic materials by the layer of microbial bio-film. The packing media is typically chosen to provide a very high surface area to volume. It is Also known as  trickling bio-filter, trickle filter, bio-filter, biological filter.

These systems have also called as roughing filters, intermittent filters, packed media bed filters, alternative septic systems, percolating filters, attached growth processes, and fixed film processes.

What can this process do?

1. Remove Nutrient
2. Remove dissolved organic solids
3. Remove suspended organic solids
4. Remove suspended solids

Process flow:

Tank is filled with solid media like Rocks or Plastic, bacteria grows on surface of media. Wastewater is trickled over media, at top of tank. As water trickles through media, bacteria degrade BOD, The thickness of the aerobic layer is limited by the depth of penetration of oxygen into the microbial layer. Bacteria eventually die, fall off of media surface, known as sloughing. The sloughed off film and treated wastewater are collected by an underdrainage which also allows circulation of air through filter. Finally liquid is collected and passed to a settling tank used for separation of solid- liquid. Part of water is recycled back to the filter in order to maintain moist condition.

Design consideration

1. In-fluent waste water characteristics
2. Degree of treatment anticipated (BOD & TSS removal)
3. Temperature range of applied waste-water
4. Pretreatment processes
5. Type of filter media
6. Recirculation rate
7. Hydraulic & organic loadings on the filter
8. Under drainage and ventilation systems

Design consideration – Filter media

The ideal filter packing is material that has a high surface area per unit of volume, is low in cost. It has a high durability. It has a high enough porosity so that clogging is minimized. It provides good air circulation

Types of Trickling Filters

I. According to the technological parameters

• Low rate trickling filters
• High rate trickling filters
• Rotating disks

II. According to the carrying material

• Natural (gravel)
• Plastics
• Grains
• Sheets

III. According to the necessity of waste-water recirculation

• With purified waste-water recirculation
• Without purified waste-water recirculation

IV. According to the the kind of biological processes

• Aerobic
• With natural circulation of air
• With ventilators or blowers
• Anaerobic

V. According to the technological stages

• One stage (single)
• Two stages

VI. On the basis of filter media

Crushed rock

• Durable & insoluble
• Locally available
• But, reduce the void spaces for passage of air
• Less surface area per volume for biological growth

Plastic media

• lighter – can get deeper beds (up to 12 m)
• reduced space requirement
• larger surface area for growth
• greater void ratios (better air flow)
• less susceptible to plugging by accumulating slime

Purpose of Under drain System

Is to to carry the filtered wastewater and the biomass lump (sloughed solids) from the filter to the final clarification process. It provides ventilation for filter to maintain aerobic conditions. The under drain system is generally designed to flow one-third to one-half full to permit ventilation of the system.

Ventilation systems 

In TF system, Air is supplied by natural draft or forced draft fan. Forced draft fans is applied in order to provide the adequate oxygen.

Common operating problems

1. Filter Ponding

If the voids in the media get plugged, flow can collect on the surface in ponds. Excessive sloughing, excessive organic loading, non-uniformity in size of media and improper functioning of primary treatment units are its chief causes.Remedies

• Wash the filter surface with a stream of water under high pressure.
• Dose the filter with heavy applications of chlorine.
• Take the filter out of service for a period of one day or longer to allow it to dry out.

2. Filter Flies

primary nuisance insect- tiny, gnat-size filter fly, or Psychoda. Filter flies develop most frequently in an alternately wet and dry environment. remedies:

• Dose filter continuously, not intermittently.
• keep orifice openings clear
• apply insecticides to filter walls
• dose filter with chlorine
• keep weeds and tall grass cut around filter

3. Odours

The presence of “rotten egg” odour is an indication of anaerobic condition.Remedies

• Maintain aerobic conditions in all units, including settling tanks and waste water system.
• Recirculate to filters.

4. Icing for Filter Surface

Cold weather not only reduces the efficiency of trickling filters by decreasing the activity of the microorganisms, but in severe cases actually can cause the wastewater to freeze on the medium surface.Remedies

• decrease recirculation to the filter (influent is usually warmer than recycled flows)
• construct wind screens
• operate two-stage filters in parallel rather than in series

 

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