SUSTAINABILITY /

These measurements are still considerably lower than the Irish EPA willow system size of 187.5m2/person. Even though a larger system will mop up more nutrients, any tree will mop up nitrates and phosphates better than a lawn, the standard percolation area cover plant. Whether the willow system is large or small, any nutrients locked into willow biomass are nutrients not showing up as pollution in nearby soils and waterways.

What I would love to see for Ireland is a general reliance on plants – and willows in particular – in the normal run of events for new and existing percolation areas and sewage discharges alike.

EXPANDING WHAT’S ALREADY HERE

We’re already getting biomass electricity from Edenderry Power Station in Co Offaly, which is co-fired with willows. That would be easy to expand upon, ramping up willow biomass growing, and ensuring that all nutrient inputs for willow crops are met from sewage rather than artificial fertilisers.

The proposed sewage outfall at Portmarnock in Co Dublin would dump c. 550T of nitrogen and almost 70T of phosphorus into Dublin Bay each year – enough to fertilise 240km2 of willow plantation as a biomass crop!

Resource-use can be minimised for willow-planted percolation areas by relying on infiltration and evapotranspiration combined, and omitting the plastic liner. On free-draining soils, willow trees capture the nitrates that otherwise migrate into the groundwater. On poorly-draining soils, the trees capture the phosphates that otherwise flow overground into rivers and streams.

A new set of guidelines on willow-planted percolation areas will be needed – specifically modifications to the piping set-up to avoid willow roots blocking the standard 4” percolation pipe runs. While pumping of effluent is the Danish standard distribution method to ensure even feeding of the full run of willows, gravity distribution systems are possible using modern splitter units or flush box applications.

These measures further improve the carbon balance; not only mopping up atmospheric carbon dioxide in the growing trees, but minimising or designing out electricity inputs in the operation of the system. Standard EPA percolation area depths to bedrock or water table should still apply where possible, but the beauty of these planted systems is that where the code requirements cannot be met for some reason, the trees still help to create a safer ecological solution by compensating for the limitations of the site.

At a time when climate and biodiversity are both at crisis levels, plants are so often the answer, providing a suite of solutions: less pollution, more renewable electricity generation, improved habitat (willows are second only to oak trees in the number of species they support), and carbon sequestration.

To quote Bill Mollison, grandfather of permaculture: “We are surrounded by insurmountable opportunities.” Let's grasp them and move into a greener, plant-rich future! ✽

FÉIDHLIM HARTY is director of FH Wetland Systems and a writer. FHWS offers specialised courses and seminars on ecosystem repair and eco-friendly sewage treatment, and consultancy on habitat creation and regeneration. Harty is the author of Septic Tank Options and Alternatives, Permaculture Guide to Reed Beds, and other books on practical environmental solutions. www.wetlandsystems.ie

Bord Bia promoted fresh produce at Bord Bia Bloom and the Body & Soul Festival. WORDS: Lorcan Bourke