Comprehensive evaluation of substrates in vertical-flow constructed wetlands for domestic wastewater treatment

Substrates are the important component of constructed wetlands (CWs), which have an effect on construction cost, purification capability and stable operation, so that substrate optimization is the key part of CWs design. The comprehensive evaluation system, including four layers, eleven indicators and nine schemes, for substrates in vertical-flow CWs treating domestic wastewater was established based on analytic hierarchy process. Then combined with Delphi method and fuzzy synthetic evaluation approach, zeolite, anthracite, shale, vermiculite, ceramic filter material, gravel, steel slag, bio-ceramic and combination substrate (isopyknic layered anthracite, bio-ceramic and zeolite) were evaluated from the viewpoints of purification effect, practical performance and economic analysis. The results showed that phosphorus removal, nitrogen removal, chemical stability were the main factors of substrate selection. Combination substrate was the best scheme among nine substrates. Zeolite was ideal substrate for nitrogen removal and biocompatibility, while anthracite and steel slag were ideal substrates for phosphorus removal. The comprehensive evaluation system of substrates was beneficial to comprehensive compare all aspects of performance for different substrates, and could be improved according to the actual situation of engineering applications, so as to provide guidance of substrate selection for CWs design.