Onsite individual wastewater treatment systems can provide a financially attractive alternative to a sewer connection in locations far from the existing sewer network. These systems are, however, relatively new, and practical experiences, especially long-term field studies, are lacking. Therefore, a thorough study of two compact biofilm-based, aerobic onsite systems, both of five population equivalents, was started in 2001. The assessment of the treatment performance of these systems, as well as the maintenance requirements and the characterisation of the feed are of great importance for the better understanding of the systems in order to optimise their design and performance. This paper presents an evaluation and discussion of the start-up and a starvation period of the two studied systems, followed by a characterisation of the incoming wastewater using activated sludge respirometry experiments in the context of the assessment and improvement of the denitrification process.

Individual wastewater treatment systems are characterised by a rather long start-up period of 70–120 days. An important characteristic during the start-up is the nitrite peak, which indicates the initiation of the nitrification process. The respirometric experiments reveal that the failing denitrification is probably caused by an insufficient amount of readily biodegradable COD in the influent.

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