Study of the treatment and the recirculation of water from soil-less horticulture
The management of water resources in soil-less cultures presents two difficulties. On one hand, the quality of these resources has to be good, that is to say without pesticides or pathogens. On the other hand, the effluents contain high concentrations of nutrients (NO3-, PO43-), damageable for the environment, and should be recycled. Thus, recycling has to include necessarily a disinfection step to satisfy the quality requirement. The main disinfection treatments used in soil-less cultures are slow sand filtration, ultraviolet treatment, heat treatment, nanofiltration, ozone or hydrogen peroxide oxidation, iodine or chlorine treatment. In order to control the chemical as well as the microbiological quality of the recycled nutrient solution, we suggest oxidation (O3) and advanced oxidation (O3/H2O2) processes, carried out in static mixers as chemical reactors instead of bubble columns. We have been studying this process in situ for the treatment of a 1-hectare greenhouse. The pilot plant unit can be configured under three setups (Figure 2) according to the aim to favor either the molecular action of ozone or the formation of very reactive radical species such as the hydroxyl radical. In this second case, the mechanism of ozone decomposition is given by Figure 1. The first step of the study was to measure the influence of the nutrient solution to be recycled on the efficiency of atrazine removal (Figures 3 and 4). In comparison with tap water, the percentage of pesticide removal is lower by about 10 to 20 %. Solutions with nutrients do not drastically change the process efficiency. The experiments were carried out with various ozone dosages and various ozone / hydrogen peroxide mass ratios, using the three configurations (Figures 5 and 6). With these results, the best operating conditions for micropollutant removal are a treatment rate of about 10 g O3 /m3 of treated solution, a H2O2/O3 ratio equal to 0.15 g/g and a contact time in the reactor in the range of 1 to 2 seconds. The influence of the configuration type is not really marked. The results show that, under these conditions, this technique leads to good pesticide removal efficiencies (about 90 % for atrazine). In a second step, experiments were carried out on real solutions containing microorganisms from the greenhouse, sometimes spiked with special bacteria (Clavibacter) or fungi (Fusarium). Some results are reported in Figures 7, 8 and 9. With the same oxidant dosage conditions, the role of the configuration is clearly demonstrated. The best results are obtained with a molecular action of ozone in the first static mixed reactor followed by a free-radical action within the second reactor. Thus, it is possible to prevent germ proliferation (aerobic mesophilic flora and fungi flora) and particularly pathogenic species. The abatement of Clavibacter michiganensis reaches 3.5 to 4 logarithmic units, 1 to 1.5 units for Pythium and 2 to 4 units for Fusarium. The treatment does not effect a complete sterilization, e.g., the beneficial bacterium Pseudomonas fluorescens survives. The global impact of the treatment on the nutritive quality of the treated solution is negligible. Nevertheless, we can note that the process induces a decrease of the ion concentrations of Fe (II) (- 5 to 30 %) and Mn (II) (-10 to 15 %) as a result of the oxidation of the EDTA chelate. In fact, this problem is observed with all oxidation and UV treatments. The residual oxidant (O3, H2O2) concentrations are low and do not induce obvious toxic effects on the cultures. Thus, the technique is consistent with a recycling of the treated effluents. The advantages of the process include very short contact times, compactness of the equipment, no need for pretreatment, reasonable investment and operating costs, an increase of the oxygen concentration in the treated effluent, and possible curative effects on the culture's germ contamination due to the residual concentration of hydrogen peroxide. The disinfection efficiency of this suggested process is similar to those obtained with more common techniques like UV irradiation. Moreover, the studied process can also reduce, for example, an eventual chemical pollution of the water resource. In conclusion, the O3, H2O2 process in static mixers appears to be a new solution for greenhouse farmers.