The aim of this research was to determine the influence of temperature, agitation, sludge concentration, and solids retention time (SRT) to obtain readily biodegradable organic matter on primary sludge (PS) fermentation, which would be used as substrate in a biological nutrient removal (BNR) process. Stirring and heating the sludge as well as increasing SRT improved the PS fermentation, producing a large amount of soluble chemical oxygen demand (SCOD). The influence of each operational parameter on PS hydrolysis was observed clearly. A great performance on SCOD production was obtained when the PS was stirred and heated for 3 days. However, PS concentration did not affect the fermentation. Sludge agitation is a simple process with minimal energy consumption. Warming the sludge is very interesting in those plants with anaerobic digestion, where heat energy is obtained from biogas. Therefore, PS fermentation can be improved with a minimum investment and leveraging existing resources in a wastewater treatment plant (WWTP). Fermenter volume can also be reduced if sludge is being heated and stirred during fermentation. 1. Introduction Optimization of wastewater treatment plants (WWTPs) equipped with BNR processes has been taking place in Spain, in recent years, to satisfy the requirements of phosphorus and nitrogen set by the Council Directive of 21 May 1991 concerning urban waste water treatment (91/271/EEC). These requirements are listed in Table 1. Table 1: Requirements for discharges from urban waste water treatment plants to sensitive areas which are subject to eutrophication. Council directive of 21 May 1991 concerning urban waste water treatment (91/271/EEC). The fraction of organic matter that is readily biodegradable, namely, the organic matter that could be metabolized directly by heterotrophic bacteria in anoxic and anaerobic zones is a key parameter on BNR processes. However, the influent of the vast majority of the WWTP contains low readily biodegradable organic matter, particularly in urban plants. Therefore, in many cases, it is impossible to achieve a total nitrogen lower than 10？mg/L in the effluent of WWTP with over 100,000 inhabitants. A supply of readily biodegradable organic matter is required. The supply of readily biodegradable organic matter can be external to the processes that occur in the WWTP. Many commercially available organic compounds, such as methanol or acetic acid, can serve effectively as a carbon source for a BNR process. However, the use of such external carbon sources results in an increase of the operational costs and
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