6. Conclusion
Overall, this paper revealed that the MEEs wastewater treatment technology was mostly used in developing country and various wastewaters have been successful treated by MEEs mainly at pilot scale. However, the literature review also illustrated that the treatment performance of MEEs would be affected by design parameters, operational conditions, and environmental factors. In order to realize long-term effective treatment and sustainable operation and meet increasingly stringent water quality standards, future studies should concentrate on the comprehensive assessment of earthworms and filter media under real life conditions, optimization of operating and design parameters, exploration of novel enhancement technologies.