Can constructed wetlands be more land efficient than centralized wastewater treatment systems? A case study based on direct and indirect land use

Compared with centralized wastewater treatment systems, constructed wetlands are generally regarded as not suitable for wide deployment due to the comparatively larger direct land area. Much of the traditional thinking is based on an onsite perspective, while the offsite information is left out. By a comparative case study with systems accounting of both onsite and offsite land use, this study questioned the traditional picture and found that constructed wetlands can be more land use efficient than centralized wastewater treatment systems. On a unit of wastewater treated basis, the land use induced by a typical constructed wetland in China is revealed to be less than half of that by the case of a centralized wastewater treatment plant or a hybrid system. On a unit removal basis for biological oxygen demand (BOD₅), chemical oxygen demand (COD), total suspended solid (TSS) and ammonia‑nitrogen (NH₃−N), the land use induced by a constructed wetland is only around 61%, 67%, 73% and 64% of that by a centralized wastewater treatment system, respectively. Meanwhile, the indirect effect is demonstrated to be significant for these three systems: this magnitude amounts to three times the direct land occupation for a constructed wetland, and one order of magnitude higher of that for the a centralized wastewater treatment system. By a scenario analysis for China in 2017, it is preliminarily estimated that over two billion square meters of land use could be reduced if all the centralized wastewater treatment systems are replaced by constructed wetlands. The outcome may serve a benchmark and offers a new way of thinking for management of wastewater treatment systems.