The objective was to evaluate the effect of four sanitizers, including hydrogen peroxide plus peroxyacetic acid (H2O2), quaternary ammonium chloride (QAC), sodium hypochlorite (Chlorine), and d-limonene (Limonene) products on (A) biofilm control within mist irrigation lines, and (B) biofilm and plant growth in hydroponic lettuce cultivation. Two greenhouse experiments were run for objective (A) where sanitizers were dosed into a nutrient solution and delivered with each irrigation through a mist propagation system. In the first experiment, solutions included clear water, a nutrient solution control (100 mg L-1 N) or nutrient solution plus either H2O2 (54 mg L-1), QAC (5 mg L-1), Chlorine (2 mg L-1) or Limonene (350 mg L-1), with 16 replicated irrigation lines per treatment. Biofilm mass per unit of internal pipe area was 0.1 mg cm(-2) for tap water, increasing to 1.82 mg cm(-2) with the nutrient control. Limonene (0.43 mg cm(-2)) resulted in less biofilm than any treatment containing nutrient solution other than QAC (1.0 mg cm(-2)). Biofilm growth was greater in the second irrigation line experiment than in experiment 1, with 10.2 mg cm(-2) of biofilm in the nutrient control. Biofilm reduction compared with the nutrient solution was observed in QAC (an 8% reduction) and H2O2 (9%), but Limonene (at a reduced concentration of 32 mg L-1) and Chlorine did not reduce biofilm. In the lettuce hydroponics experiment, the four sanitizing treatments did not lower biofilm mass in hydroponics compared with the nutrient control. However, plant dry mass was reduced at the H2O2 high dose of 149 mg L-1 (8.2 g plant(-1)) compared with nutrient solution (14.3 g plant(-1)) and plant evapotranspiration was reduced at both levels of H2O2 (30 or 149 mg L-1). Overall, a low continuous dose of a residual sanitizer is unlikely to provide adequate biofilm control under conditions that strongly favor biofilm growth, and dosage in hydroponics is limited by phytotoxicity.
