In Nepal, both horizontal bed and vertical bed subsurface flow constructed wetlands have been used for wastewater treatment. However, these units were designed based on the empirical findings from other countries. The rational design criteria developed so far has some limitations as the performance of the units is sensitive to the behaviour of the microorganisms, climatic conditions and other attributes pertaining to the local contexts. Secondly, only limited numbers of studies have been carried out to assess the performance of these systems leading to the development of rationale design criteria. Considering these facts, the major objectives of the study were set to: (1) evaluate the performance of the subsurface flow reed bed system in terms of organic matter and ammonia removal; (2) estimate the reaction rate constant and effective specific surface area; (3) assess the relationship between performance and age of the system; and (4) investigate the dynamic behaviour of the reaction rate constant. The study was carried on three full-scale domestic wastewater treatment units and one pilot-scale ranging the age of horizontal beds from 1 to 5 years.