Methodology for Optimizing Sewage Sludge Treatment, Valorization and Disposal Methods. Application to the Marrakech (Morocco) Wastewater Treatment Plant

Abstract

Wastewater treatment leads to a significant production of about 150 to 200 kg of sludge per population equivalent (P.E) per year, with a humidity level higher than 90%. The treatment and discharge of this sludge involves high costs for wastewater treatment plants (WWTPs), which can amount to up to 60% of their total operating cost. The environmental and public health aspect should also be carefully considered during evacuation as sludge may contain harmful elements. Currently, several treatment, valorization and disposal methods are possible for sludge. In order to respect the various constraints on the possibilities of discharging sludge and to minimize its cost, a reasonable approach must be followed to establish an optimal management path according to the WWTP situation. The objective of this work is to develop a methodology for optimizing sludge treatment, valorization and disposal methods for a given WWTP. This method is based on operational research technique. The methodology follows the following six steps: description of the existing situation, identification of possible treatment, valorization and disposal methods for the WWTP and development of a decision tree specific to the WWTP, mathematical formulation and assumptions, mathematical resolution and sensitivity analysis, validation of the solution and application of the solution. After the application of this methodology, a decision support tool is developed to define the optimal method(s) for sludge management in the WWTP. According to the example of the application of this methodology on sludge management of the WWTP of Marrakech (Morocco), the optimal solution, with respect to the data used and keeping the installations in place, is the application of agricultural spreading after the sludge treatments which are: thickening, anaerobic digestion, mechanical dehydration and solar drying. On a 20-year operation, an average gain of about €10 (110 DHs) per ton of dry solid (DS) incoming is generated.