In the light of growing water demand, wastewater is gaining importance as a reliable alternative source of water and nutrients for plant cultivation, shifting the paradigm of wastewater management from treatment and disposal to reuse, recycle and resource recovery. The urgency of this shift is clearly expressed in the United Nations Agenda 2030 targets for the Sustainable Development Goals, specifically target 6.3: “By 2030, improve water quality by reducing pollution, eliminating dumping and minimizing release of hazardous chemicals and materials, halving the proportion of untreated wastewater and substantially increasing recycling and safe reuse globally”.

Between the different scopes of water reuse, we chose to focus on kitchen wastewater recycling to irrigate vegetables, since this practice has historically already been exploited due to the high content of nutrients coming from food residues. Moreover, in the framework of sustainable development, on-site food production is gaining popularity because of the environmental benefits it can provide concerning reducing the environmental impact of food transport and supporting healthier diets.

To cope with these issues, we conceived a project called Jetsons’ kitchen aimed to reuse and up-cycle the kitchen wastewater in a vertical garden.

The foremost move of this direction is the Zero Mile system, an innovative biotechnological system, designed to recycle dishwasher effluents both in subsequent dishwasher rinsing cycles, and in onsite plants cultivation.

The dishwasher wastewater reuse is advantageous because of the low levels of contaminants. Indeed, in contrast to wastewaters from other sources, toxic compounds such as heavy metals, pathogens or pharmaceutical traces are not present in dishwasher effluents that only contain tap water, food leftovers and detergents traces. Furthermore, residues from food and detergents, especially eco-labeled ones, can be mineralized through a biological treatment to be up taken as fertilizers by plants.

How can microorganism help us?

The core of the Zero Mile system is a biofilter constituted by a microbial consortium, based on the mutualistic relationship between autotrophic and heterotrophic microbial partners, that makes the biodegradative process sustainable over time.

We run several experiments to test the capability of this microbial consortium to work in conditions as increasingly similar to operating ones.

Tests were performed to evaluate the consortium characteristics while exposed to a periodic replenishment of dishwasher wastewater to evaluate the efficacy of the consortium treated dishwasher wastewater as plant fertilizer.

The results of these tests show that the microbial consortium thrives well in dishwasher wastewater over time. The blue line indicates, by measuring photosynthesis trough light absorbance, the growth of the consortium which guarantees the wastewater treatment. The consortium grows because the microbes “feed” on the nutrients, e.g. Nitrogen (N) and Phosphorus (P), present in wastewater as food and detergent leftover. Consequently, Nitrogen and Phosphorus decrease throughout the experiment, and biomass is produced that can be used as plant fertilizer. 

Plants growth was also evaluated comparing seedlings of lettuce planted in the Zero Mile demonstrator watered with tap water with others watered with treated dishwasher effluent. At the end of the growing cycle (52 days) plants watered with treated dishwasher effluent showed the best growth performance.

The treated dishwasher wastewater proved to be useful for plant fertilization, as demonstrated by pigment content and quality indices (total phenols and flavonoids, soluble solids) measured in the lettuce plants watered with consortium reclaimed dishwasher wastewater.

Why is it important so important to upcycle?

The upcycling of the dishwasher effluent through the microbial consortium, core of Zero Mile System, allows the reduction of both the amount of fresh water consumed and the amount of wastewater discharged. Furthermore, the fertilizing potential of the treated wastewater fosters the quality of the plant food produced, increasing the content of beneficial metabolites. In addition, also the biomass produced in the biofilter can be upcycled as fertilizer, a secondary resource of nutrients fitting the plant's needs also as regards added value properties, such as plant disease prevention.

Hence, besides the reduction of water use and disposal, further benefits of the Zero Mile System are the onsite production of vegetables, which allows the users to cultivate the desired vegetables and to freshly consume them, along with the gratification of ‘grow your own’ activities. Furthermore, the home-made production of edible vegetables may contribute to a sustainable diet, stimulating healthy eating behaviors, and, at the same time may reduce the environmental impact due to food production, transport and marketing by standard procedures. Finally, the Zero Mile system encourages a cultural change that, in the long run, can support the acquisition of a different perception about the real value of wastewater and the transition from an abstract concept of recycling to a real one. These changes may trigger a further positive feed-back in consumer lifestyle, pushing citizens to move from a linear economy to a regenerative circular one.