The application of tertiary biofilters -- submersed aerated upflow filmreactors -- can be classified as follows:

1. treatment of biologically treated wastewater with BOD below 25 mg/l;

2. treatment of biologically treated wastewater with BOD between 20 and 50 mg/l;

3. treatment of biologically treated wastewater with BOD below 30 mg/l followed by partial oxidation with ozone, resulting in BOD of up to 80 mg/l.

These three types have to be discussed seperately. Conditions in these plants are quite different from the well-known secondary biofilters following chemical-mechanical pretreatment. The differences have to be regarded in design and operation of the plants. Several questions still have to be evaluated. Based on operational experience about details of microbiota, biofilm growth and attachment, treatment efficiency and stability of elimination, virtual independence of COD elimination from BOD elimination, retention of suspended solids, criteria for design and operation of these plants are discussed. Biofilters following the ozonation plants after biological treatment are quite different to operate. Special points of view to be taken into account at this are: no entry of microorganisms, possible ozone influences, inert solids produced in ozonation (mainly oxalate). These items are discussed based on experimental results.

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