Treatment wetlands in Iran: A review

Abstract

This paper presents a comprehensive review and meta-analysis of treatment wetland technology trends and effectiveness in Iran since the 1990s. A web search identified 69 relevant studies since 1998, highlighting various types of treatment wetlands designed at pilot and full scales. Horizontal subsurface flow (HSSF) systems and hybrid wetlands were most investigated, while full-scale studies were relatively scarce. Treatment wetlands were evaluated under diverse climatic conditions, particularly the Csa climate of the Köppen classification, with a primary focus on domestic (66 %) and industrial (14 %) wastewater treatment. Applications also extended to agriculture, leachate, aquaculture, greywater, livestock, and pharmaceuticals. The meta-analysis revealed the prevalent use of sand and gravel (15 ± 10.09 mm) as substrates, although contemporary materials like zeolite are emerging. Among 31 identified plant species, Phragmites australis was most frequently utilized. Key design criteria, such as hydraulic retention time (HRT), hydraulic loading rate (HLR), and the width-to-length ratio of reed beds, were estimated. HRT and HLR for HSSF wetlands were 3.35 ± 2.16 days and 10 ± 2 cm.day^-1, respectively, while hybrid wetlands were 4.15 ± 1.80 days and 59 ± 25 cm.day^-1. Effective operation and maintenance, including periodic vegetation harvest and even influent distribution, were crucial for optimal performance. HSSF and hybrid systems showed significant efficiency in removing various pollutants, with hybrid systems excelling in nitrogen removal due to oxygen variability in vertical and horizontal reed beds. This study emphasizes the potential of treatment wetlands as a sustainable alternative to conventional systems.