Vertical Green Wall Systems for Rainwater and Sewage Treatment

Rainwater and sewage are important pollution sources for surface water bodies. Vertical greening systems (VGSs) are extensively employed for these wastewater treatments due to the green and sustainable characteristics, as well as their high-efficiency in pollutant (organic matter, nitrogen, and phosphorus) removal. At present, more and more VGSs are designed with green buildings, serving city ecosystems. This study provides an overview of different kinds of VGSs for rain and sewage treatment, emphasizing their types, design, mechanisms, selection of plants, and growth substrate. Plants play a crucial role in pollutant removal, and different plants usually obtain different efficiencies of water treatment. Climbing plants and ornamental plants with fast growth rates are priority selections for VGSs, including Canna lilies, Jasmine, Grape vine, Boston ivy, Pittosporum tobira, Pelargonium australe, Mentha aquatica, and Lythrum salicaria. The substrate is the most critical part of the VGS, which plays an important role in regulating water flow, supporting plant growth, promoting biofilm growth, filtering pollutants, and adsorbing nutrients. The single substrate either has a blockage problem or has a short holding time. Therefore, a number of studies have mixed the substrates and integrated the advantages of the substrates to form a complementary effect, thereby improving the overall purification efficiency and stability. Novel substrates (sand, spent coffee grounds, date seeds, coffee grinds, reed-based, etc.) are usually mixed with coco coir, light-weight expanded clay, growstone, or perlite at a certain ratio to obtain optimum treatment performance. Moreover, plants in clay show more significant growth advantages and health statuses than in zeolite or soil. Operating parameters are also significant influences on the treatment performance. This review provides theoretical and technical support for designing sustainable, environmentally friendly, and cost-effective VGSs in treating rainwater and sewage.