Report: TR 2004/15

Author: James Sukias, Chris Tanner (NIWA)

Abstract

Constructed wetlands are an established technology for secondary or tertiary treatment of wastewaters, with over 80 systems in operation in New Zealand (Tanner, Sukias et al. 2000, appended to this report). In addition to their treatment role, they appeal to communities because of their ‘naturalness’ and function as a refuge to wetland plants and birdlife, and also the intimate contact of the wastewater with the wetland substrate, which helps meet Maori cultural requirements for wastes to be cleansed by passage through the earth.

There are a wide range of constructed wetland designs in use, which reflect the diversity of engineering approaches and treatment objectives at different sites. Factors such as the quality of construction, success of plant establishment, and level of operational maintenance can all affect treatment performance. Performance is also influenced by the age of the system. Constructed wetlands include a number of ‘treatment compartments’ (e.g., plant biomass, bacterial biomass, soil matrix) which change in their ability to process nutrients as they mature. For instance, a gaseous removal mechanism for phosphorus is generally absent, thus phosphorus removal is limited to plant uptake and storage within the soil matrix. Once initial soil adsorption and plant biomass compartments are filled, phosphorus removal occurs only as a result of accretion of sediments and organic matter. Thus performance of constructed wetlands, like all stages of waste treatment systems, requires regular monitoring and assessment.

Waikato Regional Council requested NIWA to undertake an assessment of the performance of constructed wetlands treating domestic wastewater in the Waikato region. Data for the assessment was provided by Waikato Regional Council, supplemented by data supplied by treatment plant operators.

Evaluation of the Performance of Constructed Wetlands Treating Domestic Wastewater in the Waikato Region [PDF, 206 KB]