The following conclusions can be made on the basis of a comparison between the nitrifying attached and suspended growth processes.

1. The nitrification rate for the attached-growth processes is higher than for the suspended-growth processes.

2. The attached-growth processes are generally used in small sewage works (less than 20 000 Person Equivalent (P.E.)), while the suspended-growth processes are used in large treatment works. Today much effort is being put into the development of large attached-growth sewage works. The future will, therefore, without any doubt show more and more use of the biofilm technology for even larger treatment plants.

3. Attached-growth processes normally carry more suspended solids in the effluent than the suspended-growth processes.

4. Activated sludge processes usually require more skilled operators and more frequent mainteinance than the attached-growth process.

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