Nature-based solutions for septic waste

Monday April 16, 2018 Written by Published in Environment
Nature-based solutions for septic waste

This weekly column is supplied by Te Ipukarea Society. It deals with environmental and conservation matters of concern to the Cook Islands.

 

There have been signs of significant impacts from septic waste in our lagoon in Rarotonga and Aitutaki, as the coastal populations have increased, mainly as a result of a corresponding increase in tourism. 

The most obvious indicator of an increase in nutrients entering the lagoon has been the seaweed blooms at the popular Muri Beach lagoon area.

These algal blooms are a result of an influx in nutrients which can come from various land based sources including agriculture and storm runoff.  However, grey water (kitchen sink, washing machine, shower etc), from commercial and domestic buildings, along with the septic waste from toilets, has significantly increased in coastal areas as our tourism numbers have escalated, and must be a significant cause of these seaweed blooms.

With tourism continuing to increase, the situation can only get worse, unless something is done very soon.

Plans to cater to the increase in septic waste have been drafted and include the possibility of construction of an over the reef pipeline to discharge septic waste. This solution poses many concerns, particularly with the poorly-understood nature of currents on the foreshore which may result in surges washing the septic waste back onto our reef. 

There is always the impending threat of a major cyclone event and the impact it could have on this sort of development. It is not a matter of if, but when we experience a catastrophic cyclone of a higher scale than those of the past 40 years. Remember Cyclone Sally which crippled the Cook Islands economy in 1987 was only a category 3!

Alternative septic waste management development options need to be considered and explored, particularly nature-based solutions, which have been proven to work.

Constructed wetlands are a potential option that could work relatively well in the Cook Islands environment. Constructive wetlands are an artificial wetland constructed through the use of natural functions from vegetation, soil and organisms to treat wastewater.

A taro swamp is a good example of a constructed wetland.  Similar to natural wetlands, constructed wetland can also act as a bio filter and/or can remove a range of pollutants such as organic matter, nutrients, pathogen and even heavy metals from the water.

Reed bed treatment systems are one possible type of constructive wetland that could be suitable on Rarotonga. Reed bed systems are aquatic plant based systems which allow bacteria, fungi and algae to digest the sewage and clean the water.

Reed bed systems operate aerobically (with oxygen) to break down pollutants, including turning toxic ammonia into nitrates.

A reed bed treatment system needs to be constructed above the water table. It will require a basin lined area with a waterproof based covering that is then filled with gravel and planted with reeds.  Rarotonga currently has two species of reed, the giant reed Arundo donax and the Fernland reed Miscanthus floridulus (kaka’o), both of which could be used for this type of treatment system.

Reeds play a crucial role in the natural treatment of septic waste, as they would feed air into the waste water creating an aerobic pockets in an otherwise anaerobic environment. This creates an ideal environment for micro-organisms to grow on the surface of the gravel and roots of the plant. These micro-organisms are largely responsible for the pollutant removal that occurs in the reed bed.

All in all, a reed bed treatment system would provide a much more environmentally friendly alternative solution to how Rarotonga goes about managing their septic waste. These types of treatment systems are also beneficial in terms of having low maintenance costs along with low capital costs.

The only challenge would be having to find a large area of land suitable enough to establish the constructive wetland treatment site.  If we assume a population of around 5000 people for the Muri/Titikaveka area, including tourists, and that 5sqm of wetland is required per person, we would need about 2.5 hectares of six acres of constructed wetland.  However, resorts and other high density accommodation businesses could make their own smaller constructed wetland within property, minimising their input of pollutants into the environment.

It is crucial for the sake of our natural environment and for the livelihoods of our future generations that we start to take development steps in the direction of nature based solutions. Living on a tropical island, our ecosystem operates differently to other urban environments, and therefore requires an alternative approach as to how we go about achieving sustainable development.

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