Several methods for wastewater treatment have been introduced but how it should be undertaken perfectly is unclear. Recently, biological methods are being focused to avoid secondary pollution. Denitrification is a biological method for wastewater treatment which needs carbon source. This is an environmental friendly and cost effective approach because the external carbon source can be obtained from solid waste. In this way, the waste can be managed and the carbon source will be utilized for water treatment. Though, several studies have identified different types of carbon source, research focusing on obtaining more new carbon sources from different solid waste is needed. Therefore, this special issue welcomes the papers concerning the external carbon source for denitirification.