Septage management is a challenging task across India as more than 44% of the population depends on septic tanks for human waste management. Septage collected from Chennai city, India, was found to be rich in nutrients even though the total solids content (<2%) and C/N are low. The current practice of septage disposal in Chennai city is co-treatment in existing sewage treatment plants. The main drawback of this method is that treatment efficiency could deteriorate if the organic load increased much beyond the design load of the treatment plant and would result in poor treated-water quality. In this context, the present study focused on the potential of in-vessel co-composting as a sustainable treatment option. In order to enhance the co-composting process, suitable co-substrates were identified for making the process efficient and cost-effective. The area under the temperature profile during composting was used as an assessment tool for the identification of the proper mix. Addition of mixed organic waste and coir pith waste (bulking agent) to the dewatered septage gave the highest area among the different combinations. Different waste combinations were investigated in order to assess the suitability for field scale application. © IWA Publishing 2018.