Anybody wanting to build or renovate their home in an eco-friendly way has lots of opportunities, challenges and decisions. We all want to do the right thing, but making the right decision for the environment can sometimes be a balancing act. How we treat our septic tank effluent is just one of many choices ahead. But it's an important one. It's surprisingly easy to cause water pollution — but fortunately it's quite straightforward to prevent it too.
By Féidhlim Harty
The national septic tank inspection plan has now been rolled out around the country. Existing homeowners and those facing into a new building project are all more aware of the importance of protecting the local environment, as well as meeting regulations. So what are the options available?
The most straightforward option is actually a well-functioning septic tank and percolation system. Where soil depths and site conditions are appropriate, this option provides good settlement in the tank and good biological filtration within the percolation trench. Disposal of treated effluent is to the groundwater beneath. However, where soils are thin, heavy or waterlogged, this option ceases to offer good biological filtration, and water pollution is likely.
So in such cases, what are our plan B options? If you have a small garden and want to minimise on space use, then a mechanical treatment unit may well be a good solution. These provide both the settlement and the biological treatment within a septic tank type chamber using pumps and blowers to diffuse air into the effluent.
If you have a little more space — about half an acre of garden will suffice — then you may want a more eco-friendly approach and use a gravel reed bed system. A septic tank provides the settlement, and the gravel reed bed provides the biological filtration. One amazing feature of wetland plants is that they draw oxygen down to the roots, where it can support the same aerobic bacteria as in mechanical treatment. Thus your system can have a lower carbon footprint, and no electricity bills.
For a more natural approach again, you could choose a soil-based constructed wetland system. Instead of treatment taking place in the gravel, this shallow marsh-type system provides treatment around the stems and leaf litter of the wetland plants. Wetland birds love this system. I’ve seen moorhens, mallard, herons and snipe all hanging out within or around treatment wetlands, adding life and beauty. On heavy clay subsoil you won’t even need the plastic liner that is necessary for gravel reed beds. Overall, this can be the most cost-effective approach for sites with heavy soil.
If minimising the carbon footprint of your building project is your main priority, then the obvious thing is to close a loop on your sewage treatment and use it as fertiliser for your firewood. In this way, your sewage treatment system can become a source of fuel to offset oil imports, and can actually reduce the overall carbon footprint for your household. In a climate conscious world, switching like this from net carbon outputs to net carbon uptake is one of the most practical steps we can take towards a healthier planet.
Willows love moist, nutrient rich soils. This makes them ideal for sewage treatment. I’ve used them for willow filters and zero discharge willow facilities. The willow filters (a row or two of willows, often on a boundary) should be down-gradient of percolation areas — far enough that they don’t clog the pipes and close enough that they take up nitrates and phosphates migrating through the subsoil. As the layout suggests, the cost can be close to nil.
Zero discharge willow facilities on the other hand cost quite a bit more than any of the systems already mentioned. These are large, lined, soil-filled basins, planted with quick growing willow cultivars. Their distinct advantage is that they either store or evaporate 100 per cent of the septic tank effluent pumped into them. Thus there is no risk of pollution to groundwater or surface water, and firewood growth can be exceptional.
Although willows are mentioned in the EPA Code of Practice, there are currently no national guidelines on willow filter or zero discharge willow facility construction. The EPA Code is the main government reference document on domestic treatment systems, and is free to download from the EPA website. Nonetheless, because willows do such an effective job at mopping up moisture and nutrients, they are an invaluable tool, particularly on existing problem sites where a practical solution is needed.
Dry toilets are another option that are high on the eco-index; saving water, nutrients and biomass and preventing water pollution. Scandinavian countries use hybrid systems that use modified flush toilets or in-pipe systems to separate out urine and faecal solids respectively into fertiliser and compost, leaving relatively clean effluent to be treated. Urine has just the right mix of nutrients for growing plants and was traditionally used as a clean and safe fertiliser. One to ponder next time you sit on a loo seat and think!
Féidhlim Harty is the author of Septic Tank Options and Alternatives – Your Guide to Conventional, Natural and Eco-friendly Methods and Technologies. See www.wetlandsystems.ie/watertips.html for more information on the above options and EPA Code of Practice download.