Source-separated, nitrified, and decontaminated human urine constitutes a promising plant fertilizer that contains a large share of the nitrogen and phosphorus in household wastewater, and other plant nutrients. However, human urine contains high levels of sodium and chloride that can affect salt-sensitive greenhouse crops. Replacing mineral fertilizer with nitrified urine fertilizer could reduce the environmental impact of lettuce production in hydroponic systems, if marketable yield, appearance, and produce quality are not affected. In the present study, a treatment combination of a nitrified urine fertilizer and mineral fertilizers was used to grow lettuce through the nutrient film technique. This was compared to a conventionally fertilized control treatment. No significant differences were observed regarding yield, phenotype, and contents of nitrate, heavy metals, phenolic acids, and chlorophyll in leaf tissue. Calcium content was significantly reduced and sodium was elevated in nitrified urine treatment. For the elements nitrogen, phosphorous, and potassium, a saving of 48%, 13%, and 15% was calculated, respectively. The calculated carbon footprint from the total fertilizer production was reduced by 20.52%, caused by the nitrified urine treatment. Based on these results, a nutrient solution composed of nitrified urine fertilizer combined with mineral fertilizer may be a promising alternative for growers to produce lettuce with a reduced environmental impact without loss of plant quantity and quality.

Controlled environment agriculture is a promising alternative to conventional production methods, as it is less affected by climate change and is often more sustainable, especially in circular and recycling frameworks such as aquaponics. A major cost factor in such facilities, however, is the need for skilled labor. Depending on available resources, there are endless possibilities on how to choose ingredients to realize a desired nutrient solution. At the same time, the composition of the desired solution is subject to fluctuations in fish water quality, fertilizer availability, weather, and plant development. In high-evaporation scenarios, e.g., summer, nutrient solutions might be mixed multiple times per day. This results in a complex, multi-variable task that is time-consuming to solve manually, yet requires frequent resolution. This work aims to help solve this challenge by providing methods to automate the nutrient mixing procedure. A simple mass-balance-based model of a nutrient mixing tank with connections to different water sources, drains, and fertilizers is provided. Using methods of static optimization, a program was developed which, in consideration of various process constraints and optimization variables, is able to calculate the necessary steps to mix the desired solution. The program code is provided in an open-source repository. The flexibility of the method is demonstrated in simulation scenarios. The program is easy to use and to adapt, and all necessary steps are explained in this paper. This work contributes to a higher automation level in CEA.

Artificial lighting accounts for a substantial share of electricity use in indoor farming. At constant photosynthetic photon flux density (PPFD), shortening the photoperiod reduces both daily light integral (DLI) and lighting electricity consumption. However, most lettuce studies have investigated comparatively large DLI differences, leaving marginal reductions poorly characterized. This study tested whether a 30-min reduction in photoperiod could lower lighting electricity use without compromising crop performance. Red oak leaf lettuce (Lactuca sativa L. ‘Saturdai RZ’) was grown at 163 µmol m⁻² s⁻¹ PPFD under 15.0 h (Control) and 14.5 h (DLIreduced), corresponding to 8.80 and 8.51 mol m⁻² d⁻¹. The 3.33% DLI reduction did not significantly affect final shoot fresh or dry weight, dry matter content, leaf area, leaf number, chlorophyll a or b, effective quantum yield of photosystem II, apparent electron transport rate, total phenolic acids, or the quantified flavonoid. Caffeoylmalic acid increased by approximately 36%, while the overall phenolic acid pool remained unchanged. Incident light-use efficiency and lighting energy-use efficiency were also unaffected. LED electricity consumption decreased by 3.33%, equivalent to 316.79 MWh ha⁻¹ yr⁻¹ and approximately €50,686 ha⁻¹ yr⁻¹ for one cultivation layer at €0.16 kWh⁻¹. In a theoretical five-layer system, annual savings reached 1,583.94 MWh and €253,431 per hectare of ground footprint. Break-even analysis indicated that a hypothetical fresh-weight reduction of 5.9-14.7%, depending on product value, could be tolerated before the electricity-cost advantage was eliminated. Marginal DLI reduction therefore provided energy and economic savings without detectable losses in crop performance under the tested conditions.