on water quality and possible sources of pollution. The direct transfer of agricultural runoff to accumulation zones allows us to imagine the places suitable for the restoration or construction of wetlands. It also makes it possible to imagine the modification of land use to reduce the input of nutrients and toxins if the environment is too small to treat this flux. It is then necessary to test the land use modifications as potential levers in pollution control or to insert CWs in areas with potential for accumulation along the transfer axes.
1.17 Conclusion
Due to population growth and increasing urbanization, water managers are faced with several major health and environmental challenges to which the implementation of NBSs can provide economic solutions without increasing the pressure and artificialization of aquatic environments. NBSs offer efficient and sustainable low-tech water management and sanitation opportunities, easily implemented at relatively low costs and with minimal maintenance.
Any bioremediation system needs continuous monitoring and maintenance. Indeed, performance monitoring and maintenance of the bioremediation system are also key conditions for the proper operation and thus the achievement of the expected performance. They must therefore be considered from the design stage and conducted according to a schedule adapted to the seasonal operating cycle of the bioremediation system. Where NBS bioremediation techniques are based on nature, this does not mean that one can abandon maintenance on the pretext that nature does not need maintenance. As we have seen, these systems evolve naturally over time, and minimal maintenance is necessary to maintain their maximum purification capacities.
Export of biomass should be planned in advance: plants will often fix metals which can cause a safety issue when further processed by composting or anaerobic digestion.
GHG emissions are a course of great concern in terms of climate change, but this concern is typical to any type of wetlands. Careful explanation to the public that a good balance of biodiversity will maintain low mosquitoes populations will be helpful, in order to convince the general public of the global usefulness of NBSs.
The overall effectiveness of NBSs is highly dependent on the context and options chosen at the facility design stage. The projects must be tailor-made, integrating the local geochemical specificities, the load and type of pollution to be treated and the objectives to be achieved.
A major issue for bioremediation implementation is the time it takes to get started. Indeed, biological processes are slow compared to excavation and hauling the polluted sediments to a landfill for hazardous waste or to an incinerator. The start-up of bioremediation systems based on nature requires between four and five years (the time when plant colonization is completed and the ecosystem has reached a sort of functional peak), validated by monitoring the activity of the expected processes. It will then be a matter of maintaining at a sufficient stage of regeneration to ensure maximum purification efficiency. There is a need to continue to develop global models which can help to design the best available NBSs, to evaluate their performance and to maintain them.
CWs provide a more natural alternative to chemical-based methods in water treatment and are also more feasible in terms of cost. Based on available literature on wetlands and case studies conducted, CWs have great potential in terms of passive water treatment. However, it must be noted that the outcomes in CWs vary greatly and cannot always be predicted due to the large number of factors that are associated with wetland design and upkeep. Management of remediation approaches and a desired effect target facilitate effective wetland design and execution. In projection of wetland remediation treatments, the time factor plays a vital role as funds for upkeep or replenishment of substrates are important in maintaining the success of a wetland.
The natural buffering capacity is an asset for adaptation to climate change, the effect of which will change the temporality of the flows of water and associated substances. This characteristic of natural systems needs to be well identified for each NBS in order to ensure management of the water resource and its extremes. Similarly, the choice of locations for NBS must take advantage of the existence of metabolic hot spots. Applied research is now investing in these different issues through international and national demonstration projects.
The social acceptance is another concern which needs to be considered when implementing NBS. The sustainability of the system depends to a great extent on the acceptance of stakeholders. They must be convinced by the impact and the benefits of implementing the NBS compared to conventional methods. They also need to be aware and trained on how to maintain and improve it if necessary.
Acknowledgement
The authors thank the French Research Agency (projects ANR-10-ECOT-007 EPEC and ANR-14-OHRI-0016 El Hamico), Campus-France (PHC Imhotep Mareoti 3785QA), the AZHUREV Project (Ministry for the Ecological Transition (France), Seine-Normandy Water Agency, Grand-Reims), the European Union’s Horizon 2020 Research and Innovation Programme (agreement no. 689162: AfriAlliance Action Group, SoWat project; LIFE 18 IPC-FR-000007-LIFE IP ARTISAN project and Water JPI Joint Call/ANR-18-WTW7-0005-04 ATENAS project), the LTSER-France network and the EcoHydrology group from UNESCO-IHP programme (http://ecohydrology-ihp.org/demosites/), and the Rhône-Méditerranée-Corse Water Agency, RIZHU project (AAP 722 2017 014 0SB), for their scientific and financial support.
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