Granular Sludge Technologies for Treatment of Saline Wastewater

Published: 22-02-2019

Per year a total of around 70 million tonnes of saline (waste)water originating form olive oil industry is produced in the EU alone. When all of this wastewater remains untreated, environmental damages, such as depletion of oxygen and eutrophication in receiving water bodies, can occur upon discharge. Biological treatment of non-saline wastewater has been done for many years, but addition of salts can severely hinder performance of biological treatment systems.

Due to these negative effects of salinity on biological treatment systems, commonly physicochemical methods are used for treatment of saline (waste)water, but their high energy demands and operations costs can make it unfeasible for large scale use. Biological wastewater treatment is a good alternative, making use of the flexibility of microorganisms to adapt to saline water for removal of a wide variety of pollutants.

Biological treatment is commonly achieved by conventional activated sludge (CAS), but this technology requires large surface area and multiple process units for complete treatment. The largest surface in CAS systems is usually required for settlers which allow separation of treated water from biologically active microorganisms before discharge. Recent advances in granular sludge technologies have overcome these problems, by making use of densely aggregated biofilms. This allows for rapid separation of the liquid and solid phase after settling, and for multiple simultaneous reactions in a single process unit, thereby reducing both energy costs and surface area requirements.

The granular sludge technologies of the Water Nexus consortium are focusing on are anaerobic granular sludge at Wageningen University (WUR-ETE) and aerobic granular sludge at Delft University of Technology (TUD-EBT). Anaerobic granular sludge is used for high strength wastewater, often coming from industrial sources. The available organic carbon (COD) can be converted into biogas, which can subsequently be used for a range of processes such as energy production. The aerobic granular sludge research is focused more on domestic wastewater streams, in which tidal variations and seawater intrusion into sewage systems can lead to increased salt concentrations. This technology is capable of simultaneously removing COD, nitrogen, and phosphorus down to effluent requirements for direct discharge into natural environments. Furthermore, the aerobic granular sludge can be applied to further polish the effluents of anaerobic bioreactors.

Lab-scale reactors in both Wageningen and Delft have been operated under highly saline conditions, and stable operation has been achieved in both labs. A synthetic wastewater stream with salinity originating mainly from NaCl could successfully be treated in the anaerobic granular sludge reactor in Wageningen. Successful granulation was achieved, which was previously believed to be impossible due to displacement of divalent cations from the EPS matrix. In Delft, a seawater-substituted aerobic granular sludge reactor has been operated up to complete removal of COD and phosphate. The granules appeared to be physically and biologically resistant to highly saline conditions, even after several salinity shocks.

These two projects were integrated through a case study experiment. The effluent of the anaerobic process was connected to the influent of the aerobic process, by which initially high-strength saline wastewater should be cleaned down to low effluent concentrations. Through batch test experiments it could be concluded that phosphate levels decreased, and removal of acetate and propionate from the effluent was successful. This case study gives good basis for future integration of multiple process steps for the complete treatment of saline wastewater.

Aerobic Granular Sludge at TUD

Anaerobic Granular Sludge at WUR

 

Aerobic Granular Sludge Microscopy Image Anaerobic Granular Sludge Microscopy Image

Dainis (left) and Danny (right) working on integrative tests at TUD