Effects of Hydraulic Retention Time of Aquaculture Effluent on Nutrient Film Technique Lettuce Productivity

Abstract

Nutrient film technique (NFT) is a popular, ergonomic, hydroponic system, but is not often used in commercial aquaponic systems due to lower efficiency in overall nutrient removal. Experiments were conducted to assess if NFT lettuce production could be improved by exchanging aquaculture effluent more frequently, and if so, determine the optimal water exchange rate. The AE was taken from a biofloc-based nile tiapia production system. Treatments consisted of increasing hydraulic retention time (HRT (d)) viz: four-, eight-, twelve-, or sixteen-day water exchanges arranged in a randomized complete block design with five blocks. In one trial (trial 1) where iron (Fe) was not supplemented, there was one replication. There were three replications for the second trial with iron supplementation. The analysis of lettuce plant size index and chlorophyll index (SPAD units) in trial 1 was statistically different among the HRTs beginning 14 days after planting, exhibiting negative linear trends with increasing HRT. However, most foliar micronutrients were borderline sufficient, and all treatments showed foliar Fe deficiency. After iron supplementation (trial 2), lettuce plant chlorophyll and size index exhibited quadratic trends with increasing HRT on 14 and 21 DAP, respectively. In trial 2, plant fresh mass decreased linearly from 162.1 g/head to 147.1 g/head, with increasing HRT. Furthermore, iron supplementation eliminated Fe deficiencies in the plants albeit only up to 14 DAP. Our findings suggest that shorter hydraulic retention times provide a solution to using the NFT system in aquaponics especially with iron supplementation.