Abstract
Vacuum toilets have gained increasing attention in circular urban development projects, because of their marked water saving qualities compared to conventional flush toilets and the increased resource recovery potential for energy in the form of biogas and phosphorous as, e.g., struvite from the resulting concentrated wastewater. A further reduction of the flushing volume of vacuum toilets would also bring nitrogen recovery options in reach. In the framework of the EU Horizon 2020 project Run4Life, a novel dual-flush vacuum toilet was developed and tested at two sites and combined with an analysis of the flushing patterns and a qualitative user survey. The results show that a 25–50% lower flushing water consumption and accordingly 1.5–2 times higher nutrient concentrations are achievable with this novel type of vacuum toilet. The usage frequency of the dual flush feature was higher in residential homes than in an office building, which also had urinals installed at the men toilets. A notable fraction of toilet visits in which the toilet was flushed twice as well as user feedback on dissatisfactory cleaning effects suggest that the applied reduction in water use is most likely the upper limit of what can be achieved in this type of toilet.
Highlights
Introduction and Günter LangergraberVacuum toilets, mainly applied on ships, trains and airplanes, are gaining increased attention in green urban development projects due to their low flushing water consumption.The first installations are more than 20 years old, but, until recently, larger vacuum toilet installations in urban buildings have mainly be selected for the purpose of water saving in urban regions with water scarcity
A vacuum toilet is flushed with 0.8–1.5 L, resulting in a notable water saving effect compared to ordinary gravity toilets that have a flushing volume in the range of 3–9 L per flush [1]
On the other hand, only limited, small-scale and not very cost-efficient recovery processes are available with the present type of blackwater from vacuum toilets
Summary
Mainly applied on ships, trains and airplanes, are gaining increased attention in green urban development projects due to their low flushing water consumption. More efficient recovery methods for nitrogen (e.g., developed for separated urine), would require a concentration level that is notably beyond the capability of existing vacuum toilets. For this purpose, a novel type of vacuum toilet was developed within the European research project (H2020) Run4Life with the goal to halve the flushing water consumption. One key component of this concept is a toilet with an extremely low flushing volume, which produces blackwater of a high concentration with high resource recovery potential. This was achieved by implementing saving flush mode for urination. The results encompass quantitative data on the frequency of double flushing as well as the water-saving flush usage with qualitative user feedbacks
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