Assessment and optimization of the operation of integrated membrane systems for wastewater reclamation

Referred to as integrated membrane systems (IMS), the combination of two membranes coupled in series is a standard technology to produce reclaimed water of high quality for potable or industrial applications. Despite the widespread experience gained utilizing IMS, there are still some aspects which require further investigation, such as the fate of compounds of emerging concern (CEC), the control of N-Nitrosodimethylamine (NDMA) formation, the energy demand and the total cost of producing the reclaimed water, and monitoring membrane integrity in reverse osmosis (RO). Julian Mamo’s doctoral thesis has investigated a component of these aspects in a MBR-RO/NF process, i.e. membrane bioreactor followed by reverse osmosis/nanofiltration. The work also discusses whether a decision support system (DSS) for the online monitoring and operation would improve the current state-of-the-art of IMS.

The fate of pharmaceuticals and their transformation products – Pharmaceuticals are often excreted from the human body at much higher concentrations than the corresponding parent chemical and can themselves be pharmacologically active. For this reason, the thesis has investigated the fate of a number of pharmaceuticals and, particularly, their main human metabolites through the IMS process. The results show that the two consecutive membrane processes, when seen as a whole, become a highly efficient process to remove all the studied compounds. Additionally, when comparing the removal efficiencies of the RO and NF membranes, as expected, the RO membrane showed near complete removal rates (>99%), whereas the NF membrane resulted also in high removal efficiencies (> 90%).

The control of N-Nitrosodimethylamine (NDMA) formation – Removal of nitrosamines by RO and NF membranes is quite poor. On the other hand, using UV as a last step in a treatment train to remove these compounds is costly in terms of energy demand. This thesis focused on the removal of NDMA formation potential and individual precursors under nitrifying and non-nitrifying conditions (achieved by changing the aeration conditions of the bioreactor). The work also looked at the removal of NDMA formation potential and individual precursors by using an NF membrane to understand whether an NF membrane would provide a high enough rejection of NDMA to achieve potable reclaimed water quality standards. The results showed that during normal aerobic operation, implying a fully nitrifying system, the MBR pilot plant was able to reduce NDMA formation potential above 94%; however, this removal percentage decreased to values as low as 72% when changing the conditions to avoid nitrification. These results suggest that a fully nitrifying MBR system will support better removal of NDMA precursors during wastewater reclamation.

The reduction of operational costs – Unlike seawater RO systems, where systems operate with rather constant salinity and temperature, wastewater shows diurnal variations, which are catchment dependant. Julian Mamo explored whether these diurnal variations in wastewater quality, in terms of inorganic constituents and temperature, would justify the modification of RO process conditions (in terms of system recovery and pre-treatment dosing) to minimise the operational cost while considering the control of membrane fouling. The results showed that although there are limitations to the use of electrical conductivity (EC) as a main parameter to deduce the individual ionic constituents in wastewater, given the right assumptions, it is possible to obtain a useful profile for a particular EC value which could be used in an online / real-time optimisation system. Through the presentation of a case study considering the cost of energy and pre-treatment chemicals, the thesis showed that there is room for online optimisation tool in terms of associating a cost to the current operating process conditions and then questioning whether there is a more cost-effective way to ‘set’ the system.

Better monitoring and membrane characterisation – The way EC sensors, which are found in any RO/NF system, are currently utilised does not provide much information to the operator in terms of membrane integrity monitoring. This thesis explored the limits of detection of membrane integrity methods using EC sensors and provides a number of strategies improve the  characterisation of  membrane integrity.

Decision Support Systems – All previous results were brought into a discussion aimed at providing a joint framework with an initial rule-base for a knowledge-based decision support system for the real-time control of integrated membrane systems for wastewater reclamation.

To sum up, the doctoral thesis, entitled “Assessment and optimization of the operation of integrated membrane systems for wastewater reclamation” is a significant step forward to better understand the physical and chemical mechanisms of wastewater treatment in integrated membrane systems MBR-RO/NF and optimise its operation. Directed by Drs Joaquim Comas, Ignasi Rodriguez-Roda and Hèctor Monclús, it boosts LEQUIA research lines on membrane technologies and decision support systems for water treatment.