Trickling Filters

A trickling filter consists of a layer of solid particles or bundles of synthetic material inside a cylindrical (Fig. 7.2) or prismoid container. Wastewater must be distributed uniformly at the top of the fixed bed - frequently by a rotating system of two or four horizontal tubes equipped with many nozzles.

To compensate for the fact that the area of a circular section of the reactor increases with distance from the center, the distance between nozzles must decrease the further they are away from the center in order to have an even distribution of water over the surface. Furthermore, the changes in available pressure in the rotating tubes must be considered as a function of the flow rate. Uniform distribution of the wastewater and uniform packing of the reactor with solid substances are of high importance for a high loading and removal rate. It is critical to ensure that two conditions are met:

Fig. 7.2 Trickling filter, BIO-NET, Norddeutsche Seekabelwerke, Germany.

• The downward flowing liquid films must be in direct contact with the biofilm (i.e. the biofilm has to be trickled over all places and at all times) and must be in contact with the upward or downward flowing air (i.e. the trickling filter should not be flooded at any location or time).

• The wastewater must be practically free of solids. It is absolutely necessary that the wastewater passes a primary settler under controlled conditions which is never overloaded.

We distinguish between:

• Natural aeration as a result of density differences between the air saturated with moisture inside the trickling filter and the air outside the trickling filter, and

• Forced aeration by a ventilator at the top of the trickling filter. In this case, the reactor may have a height of up to 12 m and is filled with packages of synthetic supporting material.

Effective natural aeration can be expected in winter and summer. In winter, the density of cold air is greater than that of the warmer air inside the trickling filter and the air flows upwards. Accordingly, the air flows downwards in summer. In every year, there are some critical situations when water and air temperatures are nearly the same. During those times, the efficiency of trickling filters is reduced.

The composition of the biofilm of a trickling filter varies with the season. In summer, the activity of the organisms is very high. Small filter flies (Psychoda) and other insects, such as water springtails (Podura) and midges (Chironomus) as well as various protozoa lead to a thicker and more porous biofilm (Fair et al. 1986). During the autumn, this third phase of the biofilm is nearly completely sheared off and a thinner film of higher density remains. In every season of the year, smaller parts of the biofilm are rinsed away, but if the ecosystem is changed completely, the biofilm is changed in a characteristic way.

Table 7.1 presents some data for trickling filters. They can be subdivided into four types according to their hydraulic loading, their loading of substrate and their difference from low-rate to super-high-rate trickling filters.

Using these data, one can perform the initial scale concept design for the treatment of municipal wastewater (see Problem 7.1).

Table 7.1 Design and operation data for trickling filters (NazarofT and Alvarez-Cohen 2001).

Parameter Unit Type of trickling filter

Parameter Unit Type of trickling filter

Table 7.1 Design and operation data for trickling filters (NazarofT and Alvarez-Cohen 2001).

Low rate

Intermediate rate

High rate

Super-high rate

Support material

-

Rock, slag

Rock, slag

Rock

Plastic

Specific surface area

m2 m-3

40-70

40-70

40-70

80-200

Porosity

m3 m-3

0.4-0.6

0.4-0.6

0.4-0.6

0.90-0.97

Density of support

kg m-3

800-1500

800-1500

800-1500

30-100

materiala)

Hydraulic loadingb)

m3 m-2 d-1

0.5-3.0

3-10

8-40

10-70

Loading per volume

g m-3 d-1 BOD5

100-400

200-500

500-1000

500-1000

Height

m

1.0-2.5

1.0-2.5

1.0-2.5

3.0-1.02

Recirculation ratio

-

0

0-1

1-2

1-2

Removal efficiency

% bod5

80-90

50-70

65-85

65-80

a) Volume includes voids.

b) Includes recycled flow (Metcalf and Eddy 1991; Tschobanoglous and Schroeder 1985).

a) Volume includes voids.

b) Includes recycled flow (Metcalf and Eddy 1991; Tschobanoglous and Schroeder 1985).

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