Citrus processing wastewater was treated in aerated pilot plants in order to evaluate the following: (a) energy efficiency under different air flow rates and times; and (b) limits of spontaneous microflora in adapting to essential oils. In comparison to permanent air flow, night aeration for 12 hours determined an increase of up to 12% of the monthly removal rate of chemical oxygen demand (COD) and a consequent reduction by 10% of energy consumptions per unit of COD removed from 0.63 to 0.57 kWh/kg_COD. Lowering night aeration from 14 to 7 l/m³/h reduced by only 10% the removal rate of COD; the energy consumption per unit of COD removed (0.32 kWh/kg_COD) was consequently reduced by more than 40%. Dissolved oxygen was maintained at very low level, rarely exceeding 0.2 ppm, with no bad smell. The consequent high oxygen deficit of 98–99% of saturation induced high oxygen transfer efficiency. The microbial population was characterized mainly by aerobic bacteria; only 5–8% of bacteria were strictly anaerobic. In the deep tank layer under the air diffuser a small amount of sludge settled (0.03–0.04 kg of dry matter per kg of COD removed), containing only 3% of total organic matter detected at the end of the depuration process. The fact that the concentration of essential oils could be progressively increased up to 1400 ppm without noticeably slowing down the biological processes demonstrated the remarkable microbial adaptation.

Orange peel (OP), the main residue of the citrus industry, is usually used for animal feeding and soil fertilisation if more advanced options are lacking. In areas with warm and dry climatic conditions, OP is land-spread for solar-drying on the fields, the leachate produced is a potential pollution factor for soil especially due to the release of organic matter; heavy rainfalls could even aggravate the hazard. Since literature does not report any quantitative evaluation of this risk, this study presents three OP drainage tests in lysimeters, where OP was left releasing leachate on a soil layer. A first test was carried out on raw OP naturally draining, while, in a second and a third test, a rainfall of 100 mm was applied on already drained and solar-dried OP, respectively. After drainage, raw OP reduced its initial volume by about 90% and the leachate production accounted only for about 20% of the initial volume. The simulated rainfall produced even lower volumes of leachate (2–3% of the initial biomass volume), in spite of the high rainfall volume and long drainage time after its application. The COD concentration in the leachate from the raw OP was significantly higher than those produced after simulated rainfall. However, the COD amount released to the soil was negligible. The lysimetric tests showed that the release of leachate occurs mainly during the first phase of drainage and that rainfall is absorbed and does not produce significant leaching. Overall, the risk of soil pollution due to the natural drainage of OP is negligible, due to both limited amounts of leachate and organic loading.

The uncontrolled disposal of olive oil mill wastewater (OMW) is hazardous for the health of water and soil, since this wastewater shows low pH and high contents of organic matter and polyphenols (PP). Lagooning is one of the most common treatment systems for agro-industrial wastewater (such as OMW), due to its low cost and easy management. Aeration allows reducing the low depuration time, which is a constraint for this system. Despite this potential feasibility, literature about OMW lagooning is not abundant. Moreover, the effects of the aeration rates, concentration of polyphenols and nitrogen shortage on depuration performance of lagoons treating OMW have not been properly explored. This study analyzes the removal rates of COD and PP, and variations of pH in OMW treated in aerated tanks at the laboratory scale simulating lagooning systems. Compared to the non-aerated tanks, aeration of OMW increased the removal rates from 61% to 90% (for COD) and from 52% to 64% (for PP). Permanent aeration was more advisable compared to intermittent flow rates. Increasing concentrations of PP noticeably reduced the COD removal rates, which were halved at a 4-fold PP concentration. In contrast, the PP removal rate was constant at every concentration experimented. Compared to the COD:N value suggested by literature for aerobic processes (100:5), a shortage in nitrogen availability reduced both COD (by about 20–25%) and PP removal rates (by 25%), the latter only when COD:N was higher 400:5. The pH was less influenced by the variations in aeration rates, PP concentration and COD:N ratio compared to COD and PP removal. This investigation provides indications about the most suitable operation parameters (airflow rates, inhibiting PP concentration, and optimal COD:N) in aerated lagooning of OMW towards environmentally sound treatments of highly polluting wastewater.