Risk Assessment and Risk Management in Managed Aquifer Recharge and Recycled Water Reuse: The Case of Sabadell

Abstract

The increasing practice of water recycling, including or not managed aquifer recharge (MAR), requires to thoroughly assess the risks posed by it in real systems, not only in laboratory and column studies.

The present work has been developed in a case study in Sabadell, Spain. For this site, the treated effluent of the Ripoll River WWTP is discharged into the Ripoll River, thus enhancing the natural infiltration to the alluvial aquifer. Pumping of the groundwater induces a riverbed filtration process (RBF), which is one of types of MAR. The recovered water undergoes further post-treatments, including UV, chlorination and sand filtration. After the post-treatments, the water is used for park irrigation and street cleaning. This site was part of the RECLAIM WATER project, supported by the European Commission and devoted to studying MAR and the use of reclaimed water for it in different locations in Europe, as well as in other countries outside Europe. For Sabadell case study, named “RISMAR” in the present work, a risk assessment and a risk management have been developed. In addition, a quantitative microbial risk assessment (QMRA) has been developed too. QMRA is not as usually applied to recycled water schemes as it is to drinking water ones, and even less to MAR.

In order to develop the risk assessment, it was necessary to gather data on the site. Most of the data used to develop the present work were generated in the framework of the RECLAIM WATER project, and it included not only basic wastewater and surface water regular parameters and microbiological indicators, but also trace compounds, pathogens and antibiotic resistance genes. Other data were available from public institutions and previous works.

The main results of the risk assessment indicate that for the uses considered for the final treated water the risk is low and in some cases medium, with the exception of using the final treated water as drinking water. Currently, this use it is not in place at the site, and it is not expected to be in the near future. The QMRA results additionally indicate that cross-connection, swimming and the immunocompromised population would be in the limit of the acceptable level of risk. Thus, the immunocompromised population should be considered in risk assessments, as the risk for them might be much higher than for the rest of the population. The residual risks that needed to be managed and considered were posed by inorganic compounds, organic compounds, salinity and mobilization of inorganic compounds from the sediments.

Another important result of the work is that the RBF and subsurface treatment proved to be very useful in reducing the risks posed by pathogens, nutrients, organic compounds and particulates. In contraposition, other risks appeared, like the mobilization of inorganic compounds from the aquifer. Then, these positive results support the request by many authors of treating MAR as an additional treatment.

Finally, a risk management plan has been developed, integrating the results of the risk assessment. For this risk management plan, not only the critical control points are identified, as it is typical for risk managements, but the twelve elements of the framework issued by the Australian Government (NRMMC-EPHC–NHMRC, 2009) have been assessed and developed, thus supporting a robust risk management plan. Emphasis is put in corrective and preventive actions for the system, as well as in defining the critical limits, monitoring program and sampling points. Besides, validation is given the importance it has in order to ensure a proper functioning of the system.

La regeneració d’aigües és una pràctica cada cop més generalitzada, que pot incloure o no recàrrega artificial d’aqüífers (MAR: Managed Aquifer Recharge), i que requereix una avaluació dels riscs en sistemes reals en ús.

L’estudi actual es desenvolupà a Sabadell, Espanya. En aquest cas de MAR la recàrrega de l’aqüífer es realitza a través del llit del riu Ripoll i s’utilitza l’efluent secundari d’una depuradora. L’aigua que posteriorment s’extreu de l’aqüífer passa per un tractament ultraviolat, cloració i filtre de sorra, i s’utilitza per al reg de parcs i neteja de carrers. Aquest sistema formà part del projecte europeu RECLAIM WATER. Al present treball s’ha desenvolupat una avaluació i gestió del risc. A més a més, s’ha fet un estudi del risc probabilístic, cosa habitual en aigües potables però no en aigües regenerades o en MAR.

Les dades utilitzades per a l’avaluació del risc es van generar en el marc del projecte RECLAIM WATER. Altres dades es van obtenir d’institucions públiques i altres estudis.

L’avaluació del risc per als usos considerats de l’aigua recuperada i tractada indica que aquest és baix i en alguns casos moderat, amb l’excepció de l’ús com a aigua potable, que no es preveu fer a Sabadell. Els riscs residuals que cal considerar i gestionar tenen com a protagonistes els compostos inorgànics, els compostos orgànics i la salinitat.

Un altre resultat important a tenir en compte és que la recàrrega a través del llit del riu és un tractament efectiu per a reduir els riscs derivats de patògens, nutrients, compostos orgànics i partícules. Aquest resultat dóna suport a la demanda de molts autors de considerar el MAR com a un tractament més.

Finalment, s’ha desenvolupat un pla de gestió del risc, integrant els resultats de l’avaluació del risc. En aquest pla no només s’han identificat els punts de control crític sinó que també s’han avaluat els dotze elements de les Guies Australianes per a la gestió del risc en MAR, fent molt més robust l’estudi. L’èmfasi ha estat posat en les accions correctives i preventives, la definició dels punts de control crític, la monitorització del sistema i els punts de mostreig.