Coordination

INRAE (Sylvie Gillot)

Active period

2023-2027

Funding

AERMC, INRAE, Suez

Partners

Suez

Abstract

The activated sludge process, developed in the early 20th century, is still the main process used to treat urban wastewater today, mainly because of its adaptability and the possibility of obtaining advanced treatment of nitrogen and phosphorus. Numerous improvements and variations of this process have been proposed over time, in line with changes in the discharge levels required to treat nitrogen and then phosphorus. However, the efficiency of an activated sludge system remains directly linked to the sludge ability to settle, and therefore to the efficiency of separating the biomass produced from the treated water.

Several activated sludge densification processes, such as aerobic granulation, have recently been proposed to intensify activated sludge systems by increasing the sludge settling capacity. Among these, the installation of hydrocyclones on the sludge extraction loop has demonstrated its ability to stabilize sludge volume indices at very low levels. This process involves applying a gravimetric selection pressure to retain the densest flocs and granules in the system. As shown in the figure below, the dense particles obtained from the underflow of the hydrocyclones are returned to the biological reactor, while the overflow containing the “light” sludge is directed to the sludge treatment line.

Installation of hydrocyclones on the sludge recirculation line

While the benefits of the system have been demonstrated, several questions remain unanswered. These concern the conditions under which the granules are formed (applied load, presence of a contact zone, etc.) and the operational limits of the system, particularly in terms of nitrification capacity. Energy issues (impact of changes in sludge properties on oxygen transfer parameters, pumping and mixing power, etc.) also deserve further study.

The HyBoues project aims at addressing these issues by monitoring several wastewater treatment plants equipped with hydrocyclones. The HyBoues project interfaces with the inDENSE project led by Suez in collaboration with INRAE. The purpose is to integrate hydrocyclone batteries into activated sludge systems and to determine the conditions for the efficient operation of the entire system. To achieve this objective, measurements will be taken in situ and ex situ at different plants equipped by Suez.

From a scientific point of view, the experimental work carried out in HyBoues will provide new insights into the functions (nitrification, dephosphatation, etc.) of the biomasses selected through the implementation of hydrocyclones. This knowledge will be integrated into an activated sludge model adapted to represent the coexistence of granular biomass and flocculated biomass in reactors (PhD of Mathilde LEPAGE).

The operational objective of the project is to redefine the rules for sizing and operating these densification systems (maximum sludge settling velocities, nitrifying sludge age, recirculation rates, etc.).