Standard Trickling Filters: Merits and Demerits | Waste Management

In this article we will discuss about:- 1. Constructional Features of Conventional Trickling Filters or Standard Trickling Filters 2. Performance and Efficiency of Conventional or Standard Trickling Filters 3. Merits and Demerits.

Constructional Features of Conventional Trickling Filters or Standard Trickling Filters:

In general a trickling filter consists of a watertight tank having a bed of filter media, sewage distribution system and underdrain system.

The various constructional features of a conventional trickling filter are discussed below:

1. Shape of Filter:

Trickling filters may have circular, rectangular or square shape, out of which the circular shape is the most common. For filters of circular shape rotary distribution system may be adopted for spreading or spraying sewage on the surface of the filter bed. The circular shape also has the advantage of structural economy.

2. Filter Walls:

Filter walls may be of either reinforced cement concrete, or fully plastered brick or stone masonry or hollow concrete blocks. For flooding operation reinforced cement concrete is preferred.

3. Filter Floor:

The filter floor is designed to support the underdrain system and the fully loaded superimposed filter media. The usual practice is to provide a nominally reinforced cement concrete slab 100 to 150 mm thick, over a proper levelling course. The floor should slope towards the main collecting channel. The slope of the floor is between 0.5 and 5%, with the commonly used value being 1%. Further in larger filters flatter slopes are used.

4. Underdrain System:

The underdrain system is intended to collect the trickling sewage and sloughed solids and to convey them to the main collecting channel and also to ventilate the filter media. The underdrains cover the entire floor of the filter to form a false bottom and consist of drains with semi-circular or equivalent inverts.

They are formed of precast vitrified clay or concrete blocks, complete with perforated cover, or they may be formed insitu with concrete or brick and covered with perforated precast concrete slabs.

The slope of the underdrains should be the same as that of the floor. The drains shall be so sized that the flow occupies less than 50% of the cross-sectional area with velocity of flow not less than 0.6 m/s at average design flow, and not less than 0.75 m/s at peak flow.

The cover over the drains shall be perforated to provide a total area of inlet openings into the drains not less than 15% of the surface area of the filter. Underdrains may be open at both ends, so that they may be inspected easily and flushed out if they become clogged.

5. Main Collecting Channel:

The main collecting channel is provided to carry away the flow from the underdrains and to admit air to the filter. In a circular filter, the main collecting channel may be located along a diameter, suitably curved around the central feed well or parallel to the diameter with a slight offset from the centre.

Alternatively the channel may be provided along the outer periphery of the filter. If inside the filter, the channel shall be provided with perforated covers to enable drainage and also the ventilation of the filter media above the channel. The channel should be extended outside the filter, both at the upper and lower ends with vented manholes to facilitate ventilation and access for cleaning.

The main collecting channel shall have semicircular or other rounded inverts. The velocity of flow in the channel shall not be less than 0.6 m/s for the average hydraulic loading, and not less than 0.75 m/s at peak hydraulic loading, and the flow shall be only half-depth particularly where recirculation is low. At the peak instantaneous hydraulic loading, the water level in the channel should not rise above the inverts of the underdrains at their junctions with the channel.

6. Ventilation:

Adequate natural ventilation can be ensured by proper design of the underdrains and effluent channels. For filters larger than 30 m diameter a peripheral head channel on the inside of the filter with vertical vents is desirable to improve the ventilation (see Fig. 14.5). 1 m² of open grating in ventilating manholes and vent stacks should be provided for 250 m² of filter area. The vertical vents can also be used for flushing the underdrains.

In case of extremely deep or heavily loaded filters forced ventilation may be provided with advantage if it is properly designed, installed and operated. It consists of forcing the air vertically upwards through the filter by the use of fans or other suitable mechanical equipment.

Such a design of forced ventilation should provide for an air flow of 1 m³/min/m² of filter area in either direction. It may be necessary during periods of extremely low air temperature to restrict the flow of air through the filter to keep from freezing. However, a minimum air flow of 0.1 m³/min/m² of filter area should be provided.

7. Filter Media:

The filter media used for trickling filters should have high specific surface area, high percent void space, resistance to abrasion or disintegration during placement, insolubility in sewage or other wastewater and resistance to spalling and flaking.

The particles of the filter media should be round or cubical in shape and free from thin, elongated and flat pieces. Not more than 5% of the media (by weight) should have the longest dimension greater than 3 times the smallest dimension. The other physical properties of the filter media are indicated in Table 14.3.

The most commonly used filter media is broken stone (trap rock, granite or limestone), slag or gravel graded to a uniform size within the range of 25 to 75 mm. The size of the filter media is of considerable significance as the specific surface area decreases with increase in media size but percent void space increases.

Stones less than 25 mm diameter do not provide sufficient pore space between them to permit free flow of sewage and sloughed solids and also lead to plugging of media and ponding of filters. Large size stones greater than 100 mm diameter avoid the plugging and ponding problem but have a relatively small surface area per unit volume, thus they cannot support as large a microbial population as the smaller size stones.

The filter material should be washed before it is placed in position. The filter material should be placed and packed by hand for at least a height of 300 mm above the underdrains to avoid damage to the underdrain system. The remainder of the material may be placed by means of wheel barrows or boxes or belt conveyors. They should not be dumped or tipped from lorries.

8. Filter Dosing:

In case of low rate filters, the minimum flow rate of sewage inflow may not be sufficient to rotate the distributor and discharge sewage from all nozzles. Hence when adequate head is available dosing tank is provided to collect the sewage and dose the filter through a siphon intermittently.

When head is inadequate, a collection well is provided to store the sewage and a suction level controlled pump, intermittently pumps the sewage to the filter. The dosing siphons are designed to dose the filters once in about 5 minutes under average flow conditions. In case of high rate filters, there is no need for the special dosing device since continuous dosing is possible.

9. Flow Distribution:

The influent sewage is distributed uniformly on the surface of the filter bed by spraying with the help of distributors. The distributors may be classified as fixed distributors and movable distributors.

The fixed distributors are in the form of spray nozzles which are fixed to a network of pipes at the surface of the filter bed and hence these are also known as fixed nozzle distributors. These nozzles which are pointing upward discharge the sewage into atmosphere and the sewage then falls under gravity on the surface of the filter bed in the form of fine drops.

The fixed nozzle distributors are employed only for rectangular or square trickling filters. However, fixed nozzle distributors are no longer used because of the elaborate piping requirements and the necessity of dosing tanks, siphons or motor operated valves to obtain variable dosing rates.

The movable distributors may be further classified as longitudinally travelling distributors (or rectilinear distributors) and rotary distributors.

The longitudinally travelling distributors are in the form of a series of nozzles which move back and forth from one end of the filter bed to the other end. They are supported on rails. The sewage is discharged through the moving nozzles on the surface of the filter bed. These distributors are suitable only for rectangular or square trickling filters.

However, the longitudinally travelling distributors are not common because of the long resting period associated with their time of travel from one end of the filter bed to the other, and the need for a reversing gear at each end of the filter bed to change the direction of motion.

The present practice is to employ mostly circular trickling filters for which only rotary distributors are used. The rotary distributor consists of a feed column at the centre of the filter, a turntable assembly at the top and two or more hollow radial distributor arms with orifices or nozzles.

The distributor arms are usually rotated by the dynamic reaction of the sewage discharging from the orifices or nozzles and hence these are also known as reaction type rotary distributors. The distributor arms are generally two in number, multiples of two also being adopted. When multiple arms (greater than two) are provided, low flows are distributed through two arms only and as flow increases, it is distributed by the additional arms.

This is achieved by overflows from weirs incorporated in the central column diverting the higher flows into the additional arms. The distributor arms are so designed that peak velocities do not exceed 1.2 m/s. The distributor arms are generally fabricated of steel and are liable to rapid corrosion.

As such they are fabricated and bolted together in such lengths as to facilitate dismantling for periodic repairing of their inside surface. The orifices in the distribution arms are composed with aluminium orifice plates. Spreader plates preferably of aluminium are provided below the orifices to spread out the discharge.

A clearance of 150 to 225 mm is provided between the bottom of the distributor arm and the top of the filter bed. This permits the sewage stream from the orifices or nozzles to spread out and cover the bed uniformly.

Distributor arms are provided with gates at the end for flushing them. Atleast one end plate has arrangement for a jet impinging on the side wall to flush out fly larvae. The distributor arms may be of constant cross-section for small units but in larger units they are tapered from the centre towards the end to maintain the minimum velocity required in the arms.

Further in order to ensure uniform distribution of sewage over the surface of the filter bed, the size and spacing of the orifices or nozzles is varied from the centre towards the periphery. The distributors are so designed that under average flow conditions, the rate of dosing per unit area at any point in the filter is within ± 10% of the calculated average dosing rate per unit area of the whole filter, and also it is ensured that the entire surface of the filter is wetted and no area is left dry.

Reaction type rotary distributors require adequate hydraulic head for operation. The head required is generally 1 to 1.5 m measured from the centre line of the distributor arms to the low water level in distribution well or the siphon dosing tank preceding the filter. When the available hydraulic head is inadequate to provide the reaction drive, the rotary distributor may be driven by electric motor.

The speed of rotation of the distributors is such that the intervals of successive dosings is between 15 and 20 seconds. The speed of rotation may vary from 2 rpm for small distributors to less than 1/3 to 1/2 rpm for large distributors.

Rotary distributors are commercially available in our country upto 60 m diameter. The piping to the distributor is generally taken below the filter floor and in rare cases through the filter media just above the underdrains. The pipe is designed for a velocity not greater than 2 m/s at peak flow and a velocity not less than 1 m/s at average flow.

10. Provision for Filter Flooding:

Provision for flooding the filters is useful for controlling filter flies and ponding. To enable flooding, the filter walls must be designed for the internal water pressure and the main collecting channel must be placed inside the filter and provided with gate valves. An overflow pipe leading from the filter to the main collecting channel downstream from the gate valve is also necessary.

Provision for filter flooding should always be made in the case of small filters, especially the low-rate filters. Such a provision in large filters would not only increase the cost but is also likely to cause hydraulic problems with the sudden discharge of large volumes of sewage when the flooded filter is drained. In such cases alternate methods may be required for controlling filter flies and ponding.

Performance and Efficiency of Conventional or Standard Trickling Filters:

Performance:

The conventional or standard trickling filters are suitable for the treatment of low to medium strength domestic wastewaters. These filters are highly efficient in the removal of BOD and other organic matter. The effluent obtained from a conventional or standard trickling filter plant is highly nitrified and stabilized.

A conventional or standard trickling filter plant may remove 75 to 90% BOD. The BOD left in the effluent obtained from such a trickling filter plant is generally less than 20 ppm. The sludge obtained in the secondary settling tank is thick (with moisture content about 92%) heavy and easily digestible.

Efficiency:

The efficiency of conventional or standard trickling filter can be expressed by the empirical equation developed by National Research Council (NRC) of Canada on the basis of operation results of trickling filters serving military installations in U.S.A.

The NRC equation for the efficiency of a conventional or standard trickling filter is as follows:

Merits and Demerits of Conventional or Standard Trickling Filters:

The following are the merits of conventional or standard trickling filters:

(i) The effluent obtained from these trickling filters is highly nitrified and stabilized, and hence it can be disposed of in smaller quantity of dilution water.

(ii) These trickling filters can withstand the application of variety of sewage having different concentration and composition.

(iii) They possess unique capacity to handle shock loads. Even if they are overloaded they can recoup after rest.

(iv) These trickling filter plants may remove 75 to 90% BOD.

(v) The rate of hydraulic loading for these trickling filters is relatively higher than that for intermittent sand filters and contact beds, and hence they require lesser land space.

(vi) The working of these trickling filters is simple and it does not require skilled supervision.

(vii) As these trickling filters contain less mechanical equipment, mechanical wear and tear is small.

(viii) Operation of these trickling filters requires less electrical power to run the mechanical equipment.

(ix) The moisture content of sludge obtained from these trickling filter plants is as high as 92%.

The following are the demerits of conventional or standard trickling filters:

(i) The head loss through these trickling filters may be 1.5 to 3 m, which may not permit gravity flow if the site is too flat, and may necessitate dosing through siphonic dosing tanks.

(ii) The cost of construction of these trickling filters is high.

(iii) They require large area in comparison to other biological treatment processes.

(iv) These trickling filters require primary treatment of sewage and hence cannot treat raw sewage as such.

(v) In these trickling filters there is a nuisance of bad odour and filter flies Psychoda which may breed in the filters and may be carried away into human habitation creating serious health problems.