Co-Digestion of Extended Aeration Sewage Sludge with Whey, Grease and Septage: Experimental and Modeling Determination

Gérard Merlin,Hervé Boileau,Jonathan Outin

doi:10.3390/su13169199

Abstract

The potential of co-digestion mixing thickened secondary sludge (TS) from extended aeration wastewater treatment plant and locally available substrates (whey, grease and septage) has been studied in this work, using three steps. The first step was a batch test to determine the biological methane potential (BMP) of different mixtures of the three co-substrates with TS. The second step was carried out with lab-scale reactors (20 L), simulating anaerobic continuous stirred tank reactors, fed by three mixtures of co-substrates that were determined according to the previous step results. Modeling was applied in the third step, using ADM1 as a mechanistic model to help understand the co-digestion process. According to the BMP step, septage used as a co-substrate has a negative effect on performance, and the addition of 10–30% grease or whey would lead to a gain of around 60–70% in the production of methane. The results from the reactor tests did not validate the positive effects observed with the BMP assay but confirmed good biodegradation efficiency (> 85%). The main purpose of co-digestion in this scenario is to recover energy from waste and effluents that would require even more energy for their treatment. The protein and lipid percentages of particulate biodegradable COD are important variables for digester stability and methane production, as predicted by modeling. The results of simulations with the ADM1 model, adapted to co-digestion, confirmed that this model is a powerful tool to optimize the process of biogas production.

Highlights

100% grease and became less and less visible, depending on the decrease in the proportion of grease in the mixture (Figure 1A). This behavior has been observed in biological methane potential (BMP) tests carried out with the solid fat waste, with a comparable substrate-over-inoculum ratio (S/I) ratio [30], or with grease trap sludge mixed with sewage sludge [31]
The numbers represent the mixture ratio in its respective percentages. These results show the possibility of using grease and whey as substrates for the anaerobic co-digestion of thickened wastewater treatment plants (WWTP) sludge
The results obtained confirm that it is possible to add local organic waste to the anaerobic digesters of an extended aeration wastewater treatment plant to treat the thickened sludge. This addition is even positive, as a way to valorize these effluents into biogas

Summary

Introduction

The activated sludge process is the one most widely applied to treat domestic wastewater, due to its well-known facility design and operation parameters. Among diverse activated sludge processes, the extended aeration activated sludge system (EAASS) is the most widely employed technique for the treatment of domestic wastewater in France, with more than 60% of domestic wastewater treatment plants (WWTP) and a capacity greater than the 2000 person-equivalent (PE) volume [1]. The extended aeration process is often better at handling organic loading and flow fluctuations and has a relatively low sludge production due to a long sludge age, as there is a greater detention time for the substrate to be assimilated by microorganisms [2]. Anaerobic digestion is an efficient method to reduce and valorize waste activated sludge [3]. In France, 97 WWTP were equipped with anaerobic digestion units in 2018, out of the nearly 20,000 WWTP in France at the time [4]

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Conclusion