No abstract

This paper presents the characteristics of the operation of the system for recovery of water from exhaust air in moderate climates in the years 2012–2019. The proposed system for water recovery uses the phenomenon of condensation in a cross-flow heat exchanger operating as an element of the air conditioning system. The parameters of exhaust air behind the heat exchanger have been determined using a mathematical model of the so-called black box. The mathematical model considers the risk of the cross-freezing of the heat exchanger. The calculations carried out for variable parameters of external air during the analyzed period confirm that the system allows to cover the demand for water for lettuce irrigation during the cold and transitional period, which is a major part of the year. It has been noted that the effectiveness of the system is very high (av. 67.12% per year) due to the specific parameters of the internal air in which the lettuce must be grown and the need for continuous air exchange in such facilities. This means that air is a stable source of water recovery, where the recovery rate depends on the parameters of external air.

The aim of this study was to determine the suitability of a rainwater harvesting system to cover the water demand for indoor hydroponic lettuce cultivation located in Wrocław (Poland). The analysis was performed on the basis of the recorded rainfall in Wrocław in 2000–2019. The analyzed cultivation is located in a hall with an area of 300 m2, where the lettuce is grown vertically by the hydroponic method. The calculations of the rainwater harvesting (RWH) system were carried out considering the selection of the tank capacity for the collected water. The operation of the water storage is simulated using a yield after spillage (YAS) algorithm. It was evident that the proposed system might be an auxiliary system that relieves the water supply network or supports other water recovery systems (e.g., the water vapor condensation in a cross-flow heat exchanger operating as an element of the air conditioning system, proposed in Part 1 of this study). The harvesting system for the selected vertical farming indoor hall covers an average of 35.9% of water needs and allows a saving of 146,510 L of water annually for the cultivation. An average water demand coverage increases up to 90.4%, which allows a saving of 340,300 L per year when the RWH system is combined with water recovery from exhaust air from the hall.

The application of hydroponic cultivation fertilized with biologically nitrified synthetic urine can produce nitrate-rich fertilizer for lettuce (Lactuca sativa var. capitata L.). The mounting water crisis and depletion of natural resources makes nitrogen recovery from human urine a practical option. Nitrified urine can be used in indoor vertical hydroponic cultivation and is characterized by a high degree of element recovery. Because of its high ammonium content, hydrolyzed fresh urine may be toxic. A nitrification sequencing batch reactor with suspended activated sludge biomass ensured urine stabilization and biological conversion into nitrate-rich fertilizer. The diluted nitrate-rich fertilizer was then supplied for soilless cultivation. The results show that diluted nitrified urine is an excellent source of bioavailable nitrogen and phosphorus and, with proper enrichment with microelements, could replace commercial fertilizers in hydroponic systems. The yield and quality parameters of lettuce cultivated with enriched urine were comparable to those obtained with a commercial fertilizer. The mass balance calculation showed that industry-scale lettuce production can be based on urine fertilizer collected from a few hundred people for a single unit.