Paclobutrazol removal from irrigation water using a commercial-scale granular activated carbon system

A commercial-scale granular activated carbon (GAC) system was evaluated for removal of the plant growth regulator paclobutrazol [(2RS,3RS)-1-(4-chlorophenyl)-4,4-dimethyl-2-(1H-1,2,4-triazol-1-yl)pentan-3-ol] from recaptured irrigation water in an ornamental greenhouse operation. This greenhouse collected and re-used irrigation drain water from 8.1 ha of subirrigated greenhouse production space, resulting in approximately 5.4 million L of recycled irrigation water filtered annually by GAC. Recaptured water was pre-filtered with polyester bag filters and then passed through three 1000 L (454 kg) GAC vessels in series before being recycled for irrigation purposes. Each GAC filter contained 8 × 30 US mesh (595-2380 μm) bituminous coal-based GAC. In Experiment 1, paclobutrazol concentration was analyzed in pre- and post-GAC filtration samples approximately weekly with a total of 55 samples between March 2015 and August 2016. The highest concentration of paclobutrazol found in recycled irrigation water was 72.5 μg L−1, whereas residual concentration after GAC was below a target 5 μg L−1 in all samples. In Experiment 2, water samples collected pre- and post-GAC filtration four times over a ten-week period during a peak period of plant production and included chemical analysis and a broccoli bioassay. The GAC filtration in Experiment 2 was effective at paclobutrazol removal despite wide variation in flow rates and resulting contact times (11-31 min) through the filter. In Experiment 3, samples were taken in a random order from the outlet of each individual GAC filter with contact times of 0, 10, 21, or 31 min. After 31 min of GAC contact time, broccoli bioassay plants were 7% (0.22 cm) shorter than control plants that received 0 μg L−1 paclobutrazol. There was a 30% average reduction in paclobutrazol concentration with the addition of each GAC filter or approximately 10 additional minutes of contact time with each GAC filter, reduced from 16.3 μg L−1 for the control to 1.7 μg L−1 after 31 min contact time. A partial budget showed GAC media replacement to be the highest operating cost item associated with treating recaptured irrigation water, contributing approximately 30% of the total annual cost of $13,020. There was no indication of reduced effectiveness of GAC paclobutrazol removal after approximately one year of filtration without GAC replacement.