Surface Flow (SF) Treatment Wetlands

Not all wetlands are the same. Although many people have an image of wetlands etched in their mind, of marshy wet landscapes where birds, frogs and insects coexist, ecological engineered treatment wetlands have very different features.

Surface Flow Treatment Wetlands are one type of ecological engineering wetland commonly used for treating wastewaters and stormwaters.  They comprise a shallow containment dam arrangement with dense coverage of plants.  Water flows through the stems of the plants about 200-300mm above the surface.  See diagram.

The stems, leaf matter, and detritus provided by the wetland plants afford a rich organic surface for healthy microbiological populations to grow and treat water, as it passes through the vegetation and biomass.  In process terms, surface flow wetlands can be said to operate as an ecological fixed film bioreactor.

Applications of Surface Flow Treatment Wetlands

Surface flow treatment wetlands are typically used for:

  • Algae removal and degradation
  • Sedimentation
  • Denitrification
  • BOD polishing
  • Hydrocarbon / recalcitrant organic compound decomposition
  • Flow balancing
  • pH stabilisation

SFW wetland diagram

7 Common Questions about Treatment Wetlands

Are they a bird habitat?

It is important to emphasise that Surface Flow Treatment wetlands are NOT designed to create habits that attract large water birds.  Large bird populations, while ubiquitous in natural wetland habitats, present a contamination risk in treatment wetlands.  It is therefore important to design systems that are not attractive to large water birds.

Treatment wetlands must avoid including large open water marshy areas within a design, because these attract water birds seeking a place to land or nest.  Birds will still coexist in a high density treatment wetland, but they are typically smaller species that do not impact water quality.

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Is harvesting required?

Surface flow wetlands owe much of their success to the ecological structures provided by vegetation (roots, stems, leaf litter etc.). For this reason, it is essential not to remove them from the system through regular harvesting.

During the life of the wetland, vegetation will have a cycle of growing and dying (for example some species die off in winter).  As vegetation dies it falls back into the wetland where it provides a carbon source for denitrification reactions, and contributes to the overall biomass structures that aid wastewater treatment.

Some specific types of wetland design (not discussed here) do rely on harvesting (e.g. floating wetlands, high rate algae harvesting systems) but these operate on absorption processes rather than creating a bioreactor environment.

Do engineered wetlands present a mosquito risk?

A well designed surface flow treatment wetland does NOT create a breeding ground for mosquito populations. They are not a concern for several reasons.

  • The average retention time of effluent in a surface flow treatment wetland is from 1-3 days; mosquitoes have a life cycle of >7 days.
  • Mosquitoes thrive in stagnant water.  Whilst areas of stagnant water occur in natural wetland marsh systems, an engineered wetland is designed to avoid stagnant water, and hence does not create a mosquito-friendly habitat.  Flow through the engineered wetland profile is designed to be even at a continuous rate of about 1-3mm/s.
  • Constructed Wetlands are designed to have a diverse and healthy plethora of macro and micro invertebrates that feed on mosquito larvae; keeping these populations in check and preventing them from increasing.
  • Finally, as a last resort safe guard, engineered wetlands are designed to be fully drained.  Should a mosquito event occur (which is unlikely and not in line with what has occurred at other operational sites), the wetlands can be fully drained to remove the risk.

Do treatment wetlands produce odours?

No. Surface flow treatment wetlands are not a significant source of odour.  Odours are generally associated with malodorous sulphide and volatile organic compounds that are largely the by-product of anaerobic reactions.   A well designed healthy surface flow treatment wetland is aerobic or anoxic (low oxygen/high nitrate) in nature.  The by-products of reactions in this environment are CO2, N2 or H2O instead of odour compounds seen in anaerobic operations.

Certain design features are necessary to ensure that aerobic/anoxic conditions are maintained and do not become anaerobic, such as:

  • ensuring that the wetland is not overloaded,
  • incorporating robust engineering hydraulic design, to provide an even distribution of flow, and avoid stagnation in pockets;  and
  • not exceeding the water level depth of 0.3m for more than a couple of weeks at a time, to avoid limiting oxygen transfer into the water profile.

Provided that good management and engineering design principles are incorporated into surface flow wetland projects, odour producing mechanisms will not exist.

Are ecological wetlands only suitable for small scale regional application?

Treatment wetlands are used around the world on many different scales.  For example, the world’s largest SF treatment wetland is the 700ha Nimr wetland in Oman.  This wetland treats 115ML/day of oil and gas wastewater.

The applicability of wetlands is not determined by whether a treatment plant is regional or metro, but rather whether there is sufficient area available – given that wetlands have larger footprints than mechanical process solutions, due to their lower energy.  There are many metro based plants, in Australia and overseas, that have large areas available for wetlands, and many regional facilities that are land constrained.  A regional or metro location should not determine where wetland application is appropriate.

Although the larger footprint of treatment wetlands can be a constraint of this technology, it is also a benefit. The large footprint contributes to attenuating flow variability more effectively than low footprint mechanical systems.

Oman wetlandINNOUT

What process design is used to size surface flow treatment wetlands?

The process design models that are used to size treatment wetlands are based on first order reaction kinetic process engineering models. These have been calibrated to the performance of hundreds of wetland operations around the world.

As a demonstration of a wetland design models’ accuracy, the Water and Carbon Group recently prepared a paper with Queensland Urban Utilities comparing the theoretical modelled outcomes for nitrogen, BOD and TSS with the actual results seen at the Helidon SF treatment wetland (designed and built by The Water and Carbon Group in Qld).  The paper was presented at the AWA regional conference (July 2014).  Click here for a full copy of this paper.

The analysis of this comparison is summarised in the figures below. The bars show the actual influent quality at Helidon, the theoretical effluent quality as predicted by the models, and what was actually measured in the final effluent.

The conclusion: the design models used to design and predict the wetland performance at Helidon accurately reflect the actual performance of the system.

Actual BODActual TSSActual TN

Can treated effluent from wetlands be used for high level reuse application?

Process trains that incorporate ecological treatment systems can be designed for high level reuse applications.  Whether the solution is a mechanical activated sludge based system, or a trickling filter/wetland arrangement, it is the disinfection barriers at the end of the process train, for example chlorine, UV, and sand filtration, that largely dictate reuse applications.

In fact, A+ class effluent can even be achieved using a trickling filter or surface flow treatment wetland, simply by adding a chlorine – UV module at the end of the process train (and possibly sand filtration if Helminths removal is needed). This removes the need for a membrane based system.

The benefit of lower energy, ecological primary or secondary treatment trains is that the outcomes can be achieved with a lower cost per kL compared to other mechanical solutions.  There is also greater flexibility to direct varied reuse streams through different disinfection systems that are “fit for purpose”. This avoids the expense of treating 100 per cent of effluent to A+ quality when a lower amount of A+ quality is needed. The remainder, sometimes as much as 80 per cent, can be treated to Class B effluent via lower cost chlorination methods.