Primary sludge hydrolysis can enrich primary effluent with the soluble organics which in turn can be a valuable carbon source to subsequent nutrient removal processes. By controlling hydraulic retention time and temperature it is possible to confine the anaerobic digestion of the primary sludge to the acidogenic and acetogenic phase (hydrolysis/fermentation process), and take advantage of the soluble organics produced.

This paper presents the results of a research involving bench and pilot scale experiments related to primary sludge hydrolysis. The pilot scale sedimentation tank (4.10 m in diameter, 3.20 m in depth) operated over an expended period of 21 months as a conventional clarifier and following this as a fermentor unit employing sludge recirculation. Parallel to the pilot scale experiments, several batch and continuous flow bench scale experiments were conducted in order to determine the factors controlling the production of soluble organics and the effect of the latter on the denitrification process.

The conclusions drawn were that a) a soluble COD production of the order of 5-6% in terms of sludge TCOD can be expected in a batch fermentor operating with HRT≅2days at T≤ 20°C, b) in a continuous flow fermentor, combinations of T>20°C and SRT>2 should be applied in order to achieve a production of the order of 10%, c) significant soluble carbon production can be achieved in primary sedimentation tanks (over 30% in terms of influent SCOD) when relatively increased SRTs (4 to 5 days) in combination with sludge recirculation are employed, under T>22°C, and d) increased denitrification performance of the order of 9 mgNOx/g, can be achieved with hydrolysate as a carbon source.

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