Spatio-temporal variation of septage characteristics of a semi-arid metropolitan city in a developing country.

Septage is the solid sludge that accumulates in septic tanks over a period of time. Many of the developing countries in the world face the challenging task of septage management. Due to the high variability in the nature of septage, there is a need to study its physical and chemical characteristics in order to suggest a sustainable treatment methodology. The present study deals with the characterization of septage collected from different locations of Chennai city in India, in two different seasons (summer and winter). The characterization includes parameters such as solids, organics, inorganics, nutrients and heavy metals. Septage showed significant difference in concentrations of pollutants from one season to the other (p > 0.05). The average total solids concentration is found to be 1.6 times higher during winter season than compared to summer season. Average concentrations of solids were 2185 ± 1070 and 3555 ± 2935 mg/L during summer and winter, respectively. Liquid characterization of septage showed total chemical oxygen demands (COD) of 905 ± 603 mg/L (summer) and 1460 ± 1295 mg/L (winter). Similarly, average soluble biochemical oxygen demand (sBOD) was found to be 117 ± 54 mg/L during summer and 211 ± 220 mg/L during winter season. Studies also showed that septage was rich in nutrients such as total nitrogen (4-500 mg/L), ammoniacal nitrogen (2-129 mg/L), total phosphate (5-236 mg/L) and heavy metals such as copper, zinc, lead and manganese. Overall characterization indicates that the collected septage samples are highly variable in nature with respect to sources, season and locations of collection. As the septage is rich in organic matter and nutrients, it can be used as a resource like soil conditioner or as a substitute for chemical fertilizer in agriculture, after proper treatment, leading to a new evolution in sanitation.