An eco-friendly solution to Dublin sewage

 

Sir, – The proposed new Greater Dublin Drainage Project, with a discharge point 1km north of Ireland’s Eye, presents not only a threat to the quality of the bay, but a great opportunity for taking a different approach. As we are reminded so succinctly by the generation coming after us, we are in the middle of a climate and biodiversity emergency. We need to do things differently – and a consultation process between reed bed system designers and local community groups in Portmarnock has yielded just such an opportunity, (see solutionnotpollution.com).

At present we discharge our sewage effluents – and the nutrients, biomass, toxins and residual pharmaceuticals within them – into our rivers and seas on an ongoing basis, treated to a greater or lesser degree with considerable electricity inputs beforehand. For the proposed Portmarnock outfall, we would be dumping circa 550 tonnes of nitrogen and nearly 70 tonnes of phosphorus each year – even after treatment. Nitrogen fertilisers have a very high carbon footprint and phosphorus has only limited global supplies remaining.

Instead of dumping effluent into the sea, at a direct cost to marine life, we could pump it to large willow plantations further inland. We have space in cut-over bogs. We have personnel in the form of Bord na Móna, already harvesting willows for biomass burning and available to move into sustainable employment as we move away from peat harvesting. We have an abundance of nutrients in the sewage to fertilise the willows on nutrient-poor peatland soils. Not only would we achieve a cleaner Dublin Bay, but the trees will also take up atmospheric carbon as they grow, providing a carbon-neutral fuel for electricity generation or carbon sequestration in the form of compost, biochar or maturing woodland.

The discharge from Portmarnock is projected to initially serve 500,000 persons equivalent, which would provide enough fertiliser to fuel a willow woodland of circa 240km2 (24km x 10km). The overall area of cutaway bogland is well over this size, so there is ample space for an extensive willow plantation. Using other natural technologies, a constructed wetland system or a zero discharge willow system would be able to deal with the same volumes of effluent on a much smaller footprint area (15km2 and 100km2 respectively), but remember that we urgently need more tree cover in Ireland to sequester carbon, and if we are to continue operating our power stations, then we’ll need a source of biomass willow to replace peat, so the larger willow plantation has clear advantages.

Instead of building the Portmarnock sewage treatment system, all sewage for Dublin could be routed through the Ringsend site for initial (partial) treatment and then pumped to a suitably sized willow plantation (circa 500km2) for multiple benefits: a clean Dublin Bay, continued employment for Bord na Móna for biomass electricity generation, carbon sequestration and recycling of nutrients.

As we face into the current climate and biodiversity emergency we need to find solutions within the problems that confront us. This is such an opportunity. This is time to shift our focus and do things differently. – Yours, etc,

FÉIDHLIM HARTY,

FH Wetland Systems Ltd

environmental consultancy,

Ennis, Co Clare &

OLLAN HERR,

Herr Ltd reed bed designs

and source separation

technologies,

Dundalk, Co Louth.