Trickling Filter

Trickling filters are conventional aerobic biological wastewater treatment units, such as active sludge systems or rotating biological contactors. The advantage of all these systems is that they are compact (i.e. applicable in densely populated urban settings) and that they efficiently reduce organic matter (JENSSEN et al. 2004). However, they are high-tech and generally require skilled staff for construction as well as for operation.

Trickling filters are a secondary treatment after a primary setting process (see also septic tanks or pre treatment).

The trickling filter consists of a cylindrical tank and is filled with a high specific surface area material, such as rocks, gravel, shredded PVC bottles, or special pre-formed plastic filter media. A high specific surface provides a large area for biofilm formation. Organisms that grow in the thin biofilm over the surface of the media oxidize the organic load in the wastewater to carbon dioxide and water, while generating new biomass. This happens mainly in the outer part of the slime layer, which is generally of 0.1 to 0.2 mm thickness (U.S.EPA 2000).

The incoming pre-treated wastewater is ‘trickled’ over the filter, e.g., with the use of a rotating sprinkler. In this way, the filter media goes through cycles of being dosed and exposed to air. However, oxygen is depleted within the biomass and the inner layers may be anoxic or anaerobic.

The word filter is somehow misleading, as physical straining of solids is only marginal. The removal of organic substances occurs by use of bacterial action (UNEP & MURDOCH UNIVERSITY 2004). Therefore trickling filters are also called bio-, or biological filters to emphasise that the filtration. Fixed film biological treatment are also used in other common treatment processes such rotating biological contactors of fixed film activated sludge systems.

The filter is usually 1 to 2.5 m deep, but filters packed with lighter plastic filling can be up to 12 m deep.

Oxygen is obtained by direct diffusion from air into the filter and the biological film (UNEP 2004) from the bottom through a spontaneous airflow due to temperature difference. Therefore, both ends of the filter should be ventilated (TILLEY et al. 2008), and sub-soil construction is not common (HEEB et al. 2008). However, in cold climates, and where energy for aeration and pumping is easily available, sub-soil construction can give protection against temperatures shocks.

The primary factors that must be considered in the design of trickling filters include (MOUNTAIN EMPIRE COMMUNITY COLLEGE n.y.):

• the type of filter media to be used
• the spraying system, and
• the configuration of the under-drain system

Filter media

The ideal filter material is low-cost and durable, has a high surface to volume ratio, is light, and allows air to circulate. Whenever it is available, crushed rock or gravel is the cheapest option. Specially manufactured plastic media, such as corrugated plastic sheets or hollow plastic cylinders, that optimise surface area for bacteria to attach free movement of air are also available (UNEP & MURDOCH UNIVERSITY 2004). The particles should be uniform and 95% of them should have a diameter between 7 and 10 cm. A material with a specific surface area between 45 and 60 m²/m³ for rocks and 90 to 150 m²/m³ for plastic packing is normally used. Larger pores (as in plastic packing) are less prone to clogging and provide for good air circulation. Primary treatment is also essential to prevent clogging and to ensure efficient treatment.

Spraying System

Adequate air flow is important to ensure sufficient treatment performance and prevent odours. To evenly distribute the water on the filter, a “rotary sprinkler/distributor” is most often used. The rotary distributor consists of a hollow vertical centre column carrying two or more radial pipes or arms some cm above the filter media (to spread out uniformly and prevent interfering with ice accumulation during winter season in colder climates), each of which contains a number of nozzles or orifices for discharging the wastewater onto the bed (MOUNTAIN EMPIRE COMMUNITY COLLEGE n.y.).

Underdrain System

The underdrains should provide a passageway for air at the maximum filling rate. A perforated slab supports the bottom of the filter, allowing the effluent and excess sludge to be collected. The trickling filter is usually designed with a recirculation pattern for the effluent to improve wetting and flushing of the filter material.

With time, the biomass will grow thick and the attached layer will be deprived of oxygen; it will enter an endogenous state, will lose its ability to stay attached and will slough off (U.S. EPA 2000) . High-rate loading conditions will also cause sloughing.

With time, growth and reproduction of the bacteria results in an increase of thickness of the biofilm layer, particularly at the top of the trickling filter. If the biofilm grows to thick, oxygen is prevented to enter and anaerobic organisms develop (U.S.EPA 2000; UNEP & MURDOCH UNIVERSITY 2004). The continuous growth, the metabolic waste products of the anaerobic bacteria, and the maintenance of a hydraulic load to the filter eventually lead to the fact that the microorganisms near the surface lose their ability to stick to the filter. When microorganisms fall off the medium and are carried with the effluent, this process is known as sloughing (U.S. EPA 2000). The under-drain system allows transporting these solid to a clarifier, where the solids settle and separate from the treated effluent (UNEP 2004).

To keep sloughing minimal, the organic and the hydraulic load to the filters should guarantee a balance between the growth of the biofilm and the amount of rinsed-out dead bio-film (HEEB et al. 2008). The collected effluent should be clarified in a settling tank to remove any biomass that may have dislodged from the filter. It can then be discharged to surface waters (see also surface water disposal and surface groundwater recharge), percolated to groundwater (see also sub surface and surface groundwater recharge) or used in irrigation (ECOSANRES 2008). The hydraulic and nutrient loading rate (i.e., how much wastewater can be applied to the filter) is determined based on the characteristics of the wastewater, the type of filter media, the ambient temperature, and the discharge requirements.

Trickling filters can be combined in decentralised wastewater treatment systems (e.g. following septic tanks or anaerobic baffled reactors) but are also often part of large centralised wastewater treatment plants (e.g. following activated sludge treatment).

Although trickling filters are more easily operated and consume less energy than activated sludge processes, they have a lower removal efficiency for solids and organic matter, they are more sensitive to low air temperatures, and can become infested with flies and mosquitoes (UNEP et al. 2004).

Trickling filters are designed primarily for BOD removal. Treatment performances depend on wastewater characteristics, hydraulic and organic loading, medium type, maintenance of optimal dissolved oxygen levels, and recirculation rates (UNEP 2004). A BOD reduction of 60 to 85 % can be expected with loading rates of 1 kg BOD/m3/day (SASSE & BORDA 1998; U.S.EPA 2000; UNEP 2004; WSP 2008;). Bacterial reductions have been reported to be 1 to 2 logs of faecal Coliforms (UNEP 2004), respectively 60 to 90 % of total Coliforms (WSP 2008). Physical adsorption of virus on the biofilm or elimination by predation are additional factors in pathogen elimination in trickling filters (STRAUSS n.y.). Total suspended solids (TSS) removal is expected to be very low (due to the down-flow regime) and pre-settling as well as removal of the solids from the effluent is recommended.

Because aerobic bacteria convert ammonia to nitrate (WSP 2008), some nitrification can also be achieved, depending on the organic loading rate to the filter, the temperature and the aeration. Total nitrogen removal varies from 0 to 35 % (UNEP 2004; WSP 2008), while phosphorus removal of 10 to 15 % might be expected (UNEP 2004). However, the capacity for nutrient removal of trickling filters depends strongly on the operation conditions, and while some sources indicate a high removal of ammonia (U.S. EPA 2000) other indicate no capacity of trickling filters for nutrients (UNEP et al. 2004).

Odour and fly problems require that the filter be built away from homes and businesses. Appropriate measures must be taken for pre- and primary treatment (settling), secondary treatment (eventually final clarifier) , effluent discharge and solids treatment, all of which can still pose health risks.

Capital costs are moderate to high depending on type of filter materials and feeder pumps used. Operation and maintenance costs are moderate or high depending on electricity consumption of feeder pumps (UNEP 2004). In any case expert design and skilled labour is required for construction and maintenance (e.g. prevent clogging, ensure adequate flushing, monitor hydraulic and organic loads, control filter flies, etc.). Another cost factor is energy consumption of pumps (e.g. to bring the water to the top of the filter) and for the sprinkler system. However, this requirements are low compared to actively aerated systems such as activated sludge processes.

A skilled operator is required to monitor the filter and repair the pump in case of problems. The sludge that accumulates on the filter must be periodically washed away once in five to seven years or more (WSP 2008) to prevent clogging and keep the biofilm thin and aerobic. High hydraulic loading rates (flushing doses) (> 0.8 m3/m3h, SASSE & BORDA 1998) and temporal collection of the effluent can be used to flush the filter. Optimum dosing rates and flushing frequency should be determined from the field operation.

The rotary distributor may also require regular cleaning or technical maintenance.

The packing must be kept moist. Constant hydraulic loading can be maintained through suction level controlled pumps or dosing siphons (HEEB et al. 2008). This may be problematic at night when the water flow is reduced or when there are power failures. Recirculation of effluent may also be required to avoid low flow conditions (WSP 2008), but a too strong flow overload would flush out the microbes.

Besides drying out, excessive odour can also arise when anaerobic conditions arise due to excessive organic loadings or insufficient aeration.

Snails grazing on the biofilm and filter flies are well known problems associated with trickling filters and must be handled by backwashing and periodic flooding.