Recirculating Vertical Hydroponic Systems: Effect of Light and Nutrient Solution Composition on Nitrification Activity

Nitrogen-rich effluents from anaerobic processes present nutrient resource recovery opportunities for fertilizer applications in hydroponic systems, thus facilitating agricultural production in less conventional contexts such as urbanized areas. However, the high ammonia and soluble chemical oxygen demand, which is common in anaerobic digestate, can inhibit crop development in a hydroponic system, requiring conditioning to enable optimal performance of the system. This study examines the use of three nutrient sources to support the growth of lettuce (Lactuca sativa) in vertical hydroponic systems: (i) synthetic permeate (SP) solution, (ii) desorption solution (DS) from an anaerobic membrane bioreactor (AnMBR), and (iii) DS modified with acetic acid addition. Two light conditions were used to observe the effect of photon flux (from 150–200 to 10–15 μmol/[m2·s]) on lettuce crop development and nitrification efficiency of the treated AnMBR permeate. Fresh and dry mass of the harvested lettuce crops as well as chlorophyll content were measured as an indicator of crop quality after a 13-day development period. Crops grown under well-lit conditions in DS had harvested fresh weight (2929.0 ± 454.6 mg/plant) than SP-grown crops (2646.2 ± 908.8 mg/plant). The lighting conditions did not significantly impact the nitrification efficiency; thus nitrate, the preferred form of nitrogen for supporting lettuce crop development, was sufficiently available to support crop growth in the recirculating hydroponic systems.