Reclamation and Agricultural Reuse of Mixed Industrial and Domestic Wastewater Treated by Aerobic Biological Process and Ozonation

Pollution and increasing water demand, especially for agriculture, put severe stress on freshwater sources, and as a result, there is progressive deficit in the global water supply. In the face of growing water scarcity and droughts, integration of high water-volume and nutrient-rich industrial effluents, into the existing water management plans for agriculture, could be a viable option to mitigate water scarcity and support the agriculture sector. Suspended growth biological process (SGBP) and ozone (O3) were studied for the treatment of mixed industrial and domestic wastewater, and the effluents were used for the growth of lettuce and silverbeet in hydroponics.SGBP proved to be efficient in biodegradation of complex organic pollutants and performed optimally at 60/30-min on/off alternated aeration cycles, achieving up to 92.1, 90.6, 83.6 and 93.5% reductions in COD, BOD5, diesel oil and methylene blue (MB) dye compounds, respectively. Thanks to the aeration conditions and dissolved oxygen levels (3.21–0.32 mgDO/L) which allowed the growth and co-existence of diverse microbial strains with a wide range of metabolic pathways potential for hydrolysis and subsequent degradation of poorly biodegradable organic complexes. In addition, extended biomass acclimation (lasting up to 98-d) to the wastewater conditions allowed the proliferation and enrichment of bacterial consortia with hydrocarbon and organic complexes degrading traits. Nutrients were removed considerably, up to 88.3% TN and 83.8% PO4-P. Nitrification activity (64.1%) was uninhibited, even on transition to pulsed aeration cycles. Further treatment by ozone at a fixed doze of 250 mgO3/h and up to 60-120 min exposure time was accompanied by degradation and structural modification of complex organic compounds into lower molecular weight compounds, thus attaining up to 14.8, 45.9 and 31.8% reductions in COD, diesel oil and MB dye compounds, respectively. The reclaimed streams were analysed for metals using the ICP-MS instrument and the concentrations of all elements tested, exception of Cu, Mn and Mo, fall within the permissible range for crop irrigation.Regarding the hydroponic cultivation, compared to the control, plants that received effluents with external nutrient had a comparable aerial biomass and vegetative growth, higher photosynthetic pigments but exhibited some degree of root development impairments. This was probably due to high activity of H+ in irrigation streams, which probably promoted secondary toxicity of metals including Al3+, Mn2+, Cu2+ on plant roots. However, under pH-controlled irrigation, an improvement in root growth was observed. The biomass of plants watered with only the treated wastewater were more than 50% higher than the yield in tap water control and plants exhibited high degree of root foraging. The root concentrations of the studied metals, particularly Al, Mn, Cu and Zn were higher than the corresponding shoot concentrations, and the concentrations were considerably lower than the maximum allowable concentrations in leafy vegetables. Pearson correlation analysis showed a significant positive interaction (p