Attached Growth Reactors

5.1 Introduction

In attached growth systems the waste water is in contact with a microbial film, attached to the surface of a solid material/medium. The surface area for growth of the biofilm is increased by the use of a porous medium in the reactor. The biological reactions take place in the biofilm, while suspended bacteria are washed out of the systems.

When randomly packed reactors, are used and the waste water flows by gravity as a free surface stream, the reactor is called a trickling filter. The use of rotating discs, covered with biofilm, partially submerged in waste water is called a rotating biological contactor (RBC) process, where the biofilm development is controlled by the rate of rotation.

Other attached-culture systems are the submerged filters with up-flow or down flow application, i.e. Up-flow Fixed Bed Reactors (UFBR) and the Fluidized Beds. Thay may both have applications under certain conditions such as high-nutrient-containing waste water. Figure 5.1 shows some of the nitrifying attached growth units in use.

In the trickling filter, the medium is stationary and the waste water is passed over the biofilm in intermittent doses. In the RBC, the medium moves the biofilm alternately through water and air. Using the UFBR, the waste water is pumped up-flow through a fixed medium. The fluidized bed consists of spherical particles coated with a biofilm fluidized by up-flowing water. A segregation generally occurs; the apparent density of the particles decreases, as the thickness of the biofilm increases.

Continuous control of the biofilm Is not possible with a stationary support medium. The filters therefore have to be backwashed in order to prevent clogging.

Experience has shown that many kinds of support material can be used, for example stones, gravel, sand, plastic, asbestos plates, wood, zeolites, and activated carbon particles.

In addition to the biological reactor, an attached growth system usually includes both primary and secondary clarification. Recirculation of sludge is normally not necessary in biofilm reactors, because the amount of biomass is huge compared with activated sludge systems.

The major work in an attached growth system is to establish a unit configuration, where oxygen can uniformly be supplied during the nitrifying process and where water at the same time can pass through the support media without any limitations.

Many experiments have been carried out, assuming that the nitrifying rate per unit surface of biofilm is the same from one reactor to the other, independent of influent characteristics. Serious errors have, therefore, been made, for example with respect to scaling up. Results from pilot scale experiments with submerged filters cannot be scaled up on the assumption that the rate of substrate removal per unit m3 is the same at full scale.

The processes for which the biofilm reactors have been used or proposed for use in waste water treatment are oxidation of organic matter, nitrification, denitrification or combinations of these.

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