The first vermicomposting flush toilet system to be installed by the Câmara Municipal of Arganil was commissioned in July 2016. It was installed to replace a failing septic tank for a 2-person household in the village of Pai das Donas in the freguesia of Benfeita.
The system was closely modelled on the original system at Quinta do Vale which is close by. The container is built from concrete blockwork. As a retrofitted system, this one also has to deal with greywater, so the drainage pipework was installed with an option to divert water to additional ‘greenfilter’ soakaways if they were found to be needed.
As with the site at Quinta do Vale, the nature of this mountainous region with its steep slopes (averaging 30-45°) and thin soils makes it a particularly challenging environment for conventional septic tanks which rely on soil biota to treat tank effluent. Slopes are too steep and soils too thin to fall within recommended ranges for successful treatment. On top of that, the absence of any significant rainfall during the summer months combined with soils lacking in organic matter (and hence a dearth of soil life) frequently make for dysfunctional leachfields.
Not surprisingly then, the nearest stream evidenced septic tank pollution. The freguesia suspect the principal cause of the pollution in this stream is due to the failing septic tank of this household. It will be interesting to see how measures of bacterial contamination change over time once the vermicomposting ecosystem in the treatment tank has had a chance to fully develop. The water will be retested at this point. The initial readings were …
After 4 months, there was clearly a problem with the quality of the organic material. Most of what was initially added was hay, straw and leaves and this didn’t really provide enough substance to make a suitable substrate for the ecosystem to start to develop, or to act as a proper filter. When we inspected the tank at the end of November, we could see it wasn’t functioning properly. The hay round the edges was still fresh and hadn’t been incorporated with the material in the centre, and we couldn’t see much evidence of faecal material or worms. The tank was refilled, using a good proportion of partially decayed wood chippings plus autumn leaves, and more worms were added.
The good news was that the tank enclosure had finally acquired its roof.
We inspected the tank again after another 4 months. This time all was as it should be and functioning well. There was no hint of odour and faecal material was clearly being consumed at a satisfactory rate by the worms. No additional organic material was required at this stage.
We noticed the grass at the base of the wall on the terrace below the greenfilter was growing much more vigorously than the grass further out on the terrace. We suspect this was at least in part because the tank wasn’t functioning properly to begin with and fertility in the form of nitrates was finding its way past the greenfilter. To some extent, this is to be expected initially since the vermicomposting ecosystems, both in the tank and in the greenfilter, take time to develop. But this is something to be monitored, because this tank processes grey water as well as black water and it’s possible the one greenfilter alone isn’t sufficient. It does show how easy it is to spot when something isn’t working quite right though. If we notice a continuing lushness in the vegetation at this spot, a second greenfilter will be dug here to process what is escaping the first.