The primary settling tank is a preliminary step to the biological wastewater treatment to remove settleable organic particles and reduce the load for the following treatment. However the settleable particles removed can also provide the necessary carbon source for the biological nutrient removal process. Therefore a balance between preserving the carbon source and reducing the burden for aeration tank needs to be reached. This demands a clear understanding of the particle settleability and carbon source availability. In this study, the granulometric and respirometric analysis have been carried out on the raw, 15, 30, 45, 60 and 90 min settled wastewater to study the effect of primary settlement on the wastewater carbon source availability. Granulometric analysis indicated that the majority of particle number and volume reduction were achieved in the initial 15 and 30 min settling. The settling of submicron particles was enhanced by the interactions between the submicron particles and larger particles. Respirometric analysis showed that the slowly biodegradable chemical oxygen demand could be correlated to the sum of each particle volume divided by the square or cubic of particle diameter. The readily biodegradable chemical oxygen demand was found to be correlated to the particle number in wastewater. (C) 2012 Elsevier B.V. All rights reserved.