Biostream Floating Treatment Wetland

Design Approaches

FTW design responds primarily to the water treatment requirement for the site, the timeline that standards must meet, and the project budget. Installations can be installed to extend infrastructure, increase the performance of existing treatment solutions, or provide a new treatment function. Size is a common factor in all FTW designs. What issue is being addressed is the next consideration.

Size

New Zealand floating treatment wetlands, that have been installed by Waterclean, range from approximately 32 square metres to 2000 square metres. Size very much reflects the purpose of the installation, the available budget, and the characteristics of the water and its location.

Currently, there is no international standard that correlates floating wetland surface area with specific performance outcomes. Drawing on New Zealand experience, a baseline of 15% pontoon cover, relative to water area, is used as a starting point for general water quality treatment and then developed if other factors are relevant. These may include the time the water is within the treatment area and its flow rate (refer FAQ ‘What Affects Performance’).

Floating Treatment Wetland (FTW) Designs

FTWs operate effectively below the waterline, regardless of pontoon configuration, provided factors such as water retention time, oxygen availability, and sediment levels are within tolerances. This adaptability allows for a wide range of application, and addressing water quality while contributing to visual amenity and broader environmental and design goals. Examples include extending wastewater treatment to remove final nutrients and trace elements before discharge to a local stream, enhancing high-profile public spaces with integrated water treatment and landscape design, and improving golf courses by managing sediment and nutrients while supporting new riparian planting themes.

The download below offers an overview of these examples and others, detailing their function and delivery.

How Do Floating Treatment Wetlands Work?

Floating Treatment Wetlands (FTWs) naturally filter water by capturing sediment, dissolved chemicals, trace elements, and heavy metals, and by converting nitrates and nitrites into nitrogen gas. Root structures positioned under FTWs capture and remove sediment efficiently and continue to do so for up to 20 years following installation.

Biological Mechanism

In open water, microorganisms break down organic matter through an aerobic (oxygen-requiring) process. When these microorganisms attach to FTW root structures, they form multicellular colonies and produce a sticky biofilm, shifting to an anaerobic (low oxygen) process. This biofilm captures and processes dissolved chemicals and nutrients, initiating denitrification—a nitrogen removal process that only occurs in anoxic (no oxygen) environments.

Sediment, Trace Elements, and Heavy Metals

As water flows around the FTW roots, biofilm captures sediment, trace elements, and heavy metals, which often form nodules and eventually detach, settling on the pond base (benthic layer). Elements like phosphorus and potassium are absorbed into the plants or settle in this benthic layer, which requires periodic removal, typically every 20 years. Nitrates and nitrites are absorbed within the biofilm and converted to nitrogen gas, a process known as denitrification.

Nitrogen Conversion Process

FTWs facilitate denitrification through these stages:

1. Nutrient Uptake: Plants absorb some nutrients for growth and provide sucrose to bacteria.

2. Biofilm Formation: Microorganisms form biofilm on roots, creating an anaerobic zone that maintains processing conditions. This zone stays cooler than surrounding water.

3. Anoxic Processing: Within the biofilm, bacteria convert nitrate (NO₃⁻) to nitrogen gas, releasing it gradually into the atmosphere, with a minor amount escaping via plant foliage.

Are There Other Benefits?

Floating Treatment Wetlands are not dependent on a connection to the bottom and continue to operate with water fluctuation and do not sludge up or are subject to water tracking. Pontoon coverage provides an area of calm water and shade that enhances sediment settlement. Water temperature under FTW’s is approximately 3 degrees centigrade lower than the surrounding water column which reduces evaporation in low rainfall/high temperature periods and helps retain oxygen.

Application Sites

Floating Wetlands are implemented worldwide, with their applications influenced by economic activity, population density, land availability, environmental priorities, and cost. Since the early 2000s, interest has grown in low-technology, passive water treatment systems with increased installation of floating wetlands at commercial scale being part of this trend.

Urban and Municipal

• Stormwater Management: Installed in-line within drainage systems.

• Residential Subdivisions: Collective stormwater pond to optimise cluster housing.

• Wastewater Treatment: Primary or supplementary support to existing plants.

• Parks and Public Spaces: Sediment and nutrient control, water clarity and amenity value, part of city-wide water management and conservation, e.g. Singapore.

Animal and Aquaculture

• Livestock Wastewater: Runoff from indoor livestock production, e.g. pig farming.

• Effluent Treatment: Fish farming and breeding.

Industry

• Stormwater Treatment: Yard runoff.

• Process Water Treatment: Food processing plants, e.g. plant based dairy replacements.

• Landfill Leachate: Processing for sediment, nutrient and heavy metal leachate run off.

Existing New Zealand Applications

• Wastewater and Stormwater Treatment: Primary treatment in small rural communities and water polishing, odour control and environmental enhancement at Municipal plants.

• Recreation: Nitrogen runoff from adjacent irrigated land.

• Public Venues: Stormwater management tool and amenity value.

• Residential Subdivisions: In-line service in wider stormwater management.

• Environmental Restoration: Lake water treatment.

• Coastal sediment control: Installation near the mouth of small freshwater estuary.

Site Suitability

Floating treatment wetlands (FTWs) are highly effective at filtering sediment and contaminants when site conditions include sufficient water flow, depth, dissolved oxygen, and adequate contact time for bioremediation. While most sites are suitable, these criteria are considered in each case.

Key Requirements:

• Inflow Rate: Provides a hydraulic retention time of at least 3 days with an ideal time of 7 days for effective treatment.

• Root Mass: Adequate to handle the water volume passing it (hydraulic loading rate).

• Dissolved Oxygen: Sufficient to support bacterial activity in open water areas before the FTW.

• Depth: A minimum of 1.2 meters to prevent root contact with the pond bottom.

• Temperature: A minimum annual temperature of 5 degrees Celsius for sediment removal and a minimum annual temperature of 10 degrees Celsius for full bioremediation to occur are the parameters for Waterclean FTW’s.

Constraints and Mitigation:

• Public Safety: Conduct a risk assessment; FTWs have a strong safety record with no incidents reported over 20 years of operation and high buoyancy reducing risks.

• High Sediment Levels: Pre-treatment with a settlement pond may be required in areas with high erosion or runoff from industrial or agricultural sites, to reduce the BOD5 levels that result.

• Low or No Inflow: Design for a depth of 1.5 meters and include low-pressure aeration to mitigate algae blooms and oxygen depletion in static flow periods, e.g. standing stormwater ponds.

• High Flows: Secure anchoring systems can accommodate peak water levels and prevent displacement and design to avoid exposure, including site selection.

• Elevated Ammonia: FTW “blankets” over pre-treatment ponds can effectively reduce ammonia concentrations (refer Marton WWTP project example), either by direct inflow from source or by using a bi-pass method of transfer of waste water from a larger WWTP unit to the FTW pond and then returning to the larger treatment processing plant and cycle.

This guideline outlines key factors for site suitability. Detailed technical responses are provided for each site as part of the scoping process. Contact us for more information.

Performance Factors for Floating Treatment Wetlands

The bioremediation performance of Floating Treatment Wetlands (FTWs) depends on water quality, contact time with root structures, and environmental conditions, including local geology and temperature. Optimising performance involves chose of site, or modifying for ideal conditions, adjusting FTW size and employing targeted management strategies.

Key factors include water conditions, such as Total Suspended Solids (TSS) and Biochemical Oxygen Demand (BOD₅). High TSS can overload root filtration and reduce the efficiency of both the wider water body and the FTW, potentially requiring pre-treatment through a settlement pond. Elevated BOD₅, frequently indicates significant organic content, which can deplete oxygen levels. Aerobic bacterial water treatment is highly effective and not precluded by the use of a FTW. Low-pressure aeration may be needed to support aerobic activity and to prevent the pond from becoming anaerobic.

Hydraulic Retention Time (HRT) and Hydraulic Loading Rate (HLR) provide measures for ensuring contaminant removal. Optimal HRT is 7+ days, though a minimum of 3 days is effective for water polishing when primary treatment has been undertaken. HLR depends on water volume and depth, with adjustments to the size of the surface cover of the FTW, and underwater root mass, made to match contaminant levels and water standard requirements. Baffle systems are placed on the down-stream size of the FTW pontoons to maximise HLR where more demanding water treatment is required.

This guideline outlines key performance factors. Detailed technical responses are provided for each site as part of the scoping process. Contact us for more information.

Warranty and Maintenance

A one-year warranty covering the integrity and operation of all FTW components, including pontoons, baffles, plant material, anchoring systems, and any additional items specified in the contract is provided. Most issues, if they arise, occur within the first few months of operation. The warranty excludes damage caused by chemical inflow, vandalism, or natural disasters such as earthquakes and significant flooding.

Maintenance Within the first three months after installation, a follow-up visit ensures anchoring systems, pontoons, and plant material are performing as expected. Any dead plants are replaced at this stage. If Pūkeko are present, bird guards will be installed to protect young plants and removed once vegetation is well-established.

12-Month Maintenance A final check at 12 months includes trimming foliage, replacing any dead plants, and inspecting overall system performance. This is typically handled by a two-person crew using scrub cutters. The high buoyancy of the FTW mostly avoids the need for boats. All cut foliage is removed to prevent organic matter from re-entering the water. This prevents further nutrients entering the water and removes phosphorus and potassium that may be sequestered in the foliage. Regular trimming promotes fresh root growth, enhancing bioremediation capacity.

Post-Warranty Maintenance Options After the first year, clients can continue with a maintenance contract or manage the FTW installation themselves. Training is offered for client representatives who chose in-house maintenance. Experience indicates that long-term maintenance depends on the continuity of trained staff passing on their knowledge of the project. We are available to help update information or advise where required.

The cost of a Floating Treatment Wetland (FTW’s) will reflect the area of water cover and supporting items that may be required to meet consent conditions or other design requirements. A standard FTW will include pontoons, anchoring wires, and plant material, all in place and ready for operation.

Additional elements can be added to improve performance or address the site layout. These may include plastic baffles and buoys, to control and direct water flow along the pontoons. The plastic baffles can also be used to screen off an area of open water to create a treatment bay for water circulation in and out of the FTW. Other preparation may include a de-sludge operation of the pond base to remove historical buildup of ammonia and other contaminants. This requirement is shared by several water treatment processes.

Floating treatment wetland installations cost approximately 50% less than comparable land-based mechanical filtration system for the same task. Both types of installations may require preparatory work but the lower product and infrastructure cost of installing a FTW system makes a significant difference. In addition, FTW’s can operate completely off-grid as they are effectively ‘solar powered’ and may not require connection to reticulation if the water quality standards that apply are met.