Table 4.

Main advantages and limitations related to the valorization of OMWs and their derivative by-products 

Valorization method	Advantages	Limitations
Energy generation	-Replaces fossil fuel as an energy source -Contributes to the optimization of natural resources’ use -Contributes to minimize GHG emission -Provides an extra income to farmers and olive-mills` operators -Creates new source/ opportunities of employment and enhances the social fabric of rural and agricultural areas -Contributes to the national GDP (gross domestic product)	-Cost requirement to collect and transport OMWs from mills or farms to the utilization facilities -High operation and overhead cost that could impact the economically benefits -Toxicity of OMWs causing process stability problems during biological conversion methods: Required pre-treatment is not cost-effective and could affect negatively on the efficiency of energy recovery process -Lack of resources (technological, financial, and innovation challenges) -Environmental impacts of energy recovery process (digestate, HTC liquid effluent) in case of unrestricted long-term management plan
Agriculture applications	-Great potential of OMWs to improve soil fertility and soil organic matter: high content in valuable plant nutrients (nitrogen, phosphate, potassium, iron, and magnesium) -Slowing down soil-erosion processes specially in hilly areas -Improve the structure of soil aggregates and consequently increase soil porosity and water retention capacity -Supports the sustainability of the agricultural sector -Increase land yield -Strengthen and extend sorption of insecticides and herbicides, thus reducing their biodegradation, slow down their leaching, which reduces groundwater pollution risk	-Some chemical components in OMWs might result in soil and water pollution, in addition to the risk of phytotoxicity -Raw OMWs application to soil could deteriorate the oxygen uptake efficiency -Constraints related to the availability of OMWs, investment costs and the industrial or agronomic environment -Local regulations and laws could restrict the optimum benefits of OMWs applications -Lack of long-term sustainability strategies
Extraction of bioactive compounds	-Support green chemistry principles -Minimize environmental source depletion -Contributes to reducing chemicals demand and minimize environmental impacts related to chemicals ingredients production cycle	-Generates solvent waste -The application of bioactive compounds extracted from OMWs are subjected to restricted regulations and requirements, which could be considered as a challenge -Limited market demand -Technical limitations that include but not limited to substrate seasonal storage at ideal conditions and bioactive compounds extraction -Stability of extracted bioactive compounds during the utilization cycle
Construction applications	-Minimizes the pressure on freshwater sources -Reduces the overall process costs by replacing conventional water with unconventional sources with a lower cost per litter -Minimizes carbon footprint associated to construction industry -Lightweights’ construction material that leads to minimizing economic and environmental impacts -Good insulating characteristics that reflect on the energy consumption at purpose of heating/cooling -Reduces the heat required during the ceramic production process, resulting in lower greenhouse gas emissions from production line	-Cost requirement to collect and transport OMWs from mills or farms toward the utilization facilities