Career & Education shares with you this week a related article to Dr Barry Wades’ piece headlined ‘A wetland wastewater solution’, carried in last Sunday’s publication — in celebration of World Wetlands Day. The day is celebrated internationally on February 2 each year to raise awareness about the value of wetlands.

CONSTRUCTED wetlands are known to be very effective elements in wastewater treatment systems.

In the system designed by Foreman, Chung and Sykes — consulting engineers for the Gore Developments Limited housing development at Florence Hall near Falmouth in Trelawny — three reed beds making up the constructed wetland are placed in series after anaerobic (septic) tanks to reduce the loads of dissolved organics, phosphates and nitrates (plant nutrients), and fecal coliforms in the effluent to be discharged to the natural mangrove wetland adjacent to the development.

Constructed wetlands can either have a free water surface or a submerged water surface. For wastewater treatment, either can be used. However, local health authorities are more accepting of those with a submerged water surface when treating sewage waste.

The constructed wetlands at Florence Hall as well as at New Harbour in St Catherine both have submerged water surfaces. Submerged surface wetlands are ponds filled with stones that allow the water to flow through them slowly. When treating wastewater in a constructed wetland, it is required to be lined by an impermeable liner so that contaminants cannot enter the soil and continue to the ground water layer.

In designing a submerged surface wetland, the most important items to consider are the volume and flow rate of water to be treated, the location, ambient temperature, rainfall, depths, the available wetland media (size, shape, hardness) and the root depth for the type of vegetation available. When all of these are chosen, it is an application of first order kinetic equations to determine how big the wetland should be in order to remove the organics (biological and chemical oxygen demand).

Then there is an application of basic physics to ensure the wastewater can flow in and out of the constructed wetland in the required timeframe and stay below the wetland media at the required level. The required plant root depth and nutrient removal ability are estimated as most empirical equations are not temperature specific and they have mostly been developed for temperate areas.

Local engineers have learnt to use rules of thumb to estimate removal for several plant species, such as the bulrush, Typha, which has been used at Florence Hall. However, with further research and experience, more precise nutrient removal equations that are temperature dependent can be developed for this species as well as for other tropical wetland plants.

Once the surface area and the depth have been determined, it is time to fit the constructed wetland in nature. Both Gore Developments Limited constructed wetland projects are adjacent to natural wetlands — one is adjacent to a coastal wetland in a mature limestone area; the other is in a riverine floodplain with fine sands and silty clays.

The construction challenges for those locations are quite different. A synthetic liner was used under both wetlands. The construction challenges at the coastal limestone location was getting the excavated area smooth enough to reduce the depth of bedding material (fine sand) under the liner so that we could be confident that it would not puncture when filled with the rock media. In the riverine location, the wetlands had to be raised to avoid the flood plain.

The construction challenge was to achieve the required compaction to form the sides of the wetland so it would keep its shape and elevation when filled with rock media. There are many liners of varying thickness and material on the market. We have used 60 mil High Density Polyethylene with success; perhaps a good choice in some locations and not in others. The manufacturer installation instructions are key to saving time and reducing waste.

Prior to commissioning the wetland, the vegetation has to be established. The plants will not grow without adequate water or nutrients. Controlled amounts of wastewater need to be introduced; enough that the root zone is wet, but not more than the emergent vegetation can manage.

The operators have to keep the media saturated, but ensure that it does not rise above the surface. The outlet has to be configured so that the water levels can be manipulated as necessary. The wetland should be able to drain free or remain full in order to perform its functions and facilitate periodic maintenance.

The process design for wetlands is the simplest part. Configuring the cells so that they can be constructed when needed and put into operation without much disruption is more challenging. Ensuring that there is adequate screening so that solids are not introduced is also important.

If the wetland is placed after a closed chamber, such as a septic tank, the inlet is configured differently than if it is placed after an open chamber, such as a pond. Planning how to deal with sediment flushing from large volumes of stormwater and screening of algal blooms without blockage is important.

Constant onsite maintenance is not necessary for constructed wetlands; however, there are times when maintenance will be needed. Regular inspections are necessary to ensure the integrity of the berms and the health of the vegetation so that the wetland will perform as designed.

Some of the outstanding advantages of wetland treatment systems are that the energy demand is only a fraction of that for mechanical plants, and the need for repairs, replacement parts and daily monitoring is virtually nil.

The results of one year’s sampling at Florence Hall, as reported by Environmental Solutions Ltd last week, show that the level of treatment is very high and consistent. We can expect that more housing and other developments in Jamaica will use this method of wastewater treatment in the future.

— Lise Walter is a professional engineer and the Jamaican representative for the Caribbean Water & Waste Water Association. She can be contacted at lmw@fcsconsultants.com.