Silvana Luz

The utilization of agro-industrial wastewaters (AIWWs), pretreated by immediate one-step lime precipitation + natural carbonation, as a nutritive solution for the hydroponic production of lettuce was evaluated. The AIWWs studied were olive mill wastewater (OMW), winery wastewater (WW), and cheese whey wastewater (CWW). Lettuces (Lactuca sativa L. var. crispa) were grown in a closed nutrient film technique hydroponic system, using the pretreated AIWWs (OMW-T, WW-T, and CWW-T) and a control nutrient solution (CNS). The growth and sensory analysis of lettuces and the environmental parameters of effluents after hydroponics were evaluated. The average number of lettuce leaves obtained with nutrient solutions prepared with AIWW-T was lower than that from CNS, but the highest lettuce chlorophyll content was attained with CWW-T, which also presented the best grow results. In general, sensory analysis did not show significant differences from the lettuces grown in the different pretreated AIWWs and CNS. As for the environmental parameters of the effluents from hydroponics, according to the Portuguese legislation, only the chemical oxygen demand of the OMW-T and WW-T presented slightly higher values than that of the environmental limit values for discharge in surface waters, showing the feasibility of using pretreated agro-industrial effluents in hydroponic lettuce cultivation, while obtaining a treated effluent, in a circular economy perspective.

A short review on the treatment of effluents from food processing industries by electrochemical oxidation (EO) was performed. Olive mill wastewater (OMW) and boron-doped diamond (BDD) are the most reported effluent and anode material, respectively. The addition of NaCl or Na₂SO₄ as supporting electrolytes is common in these studies, and their influence on the EO performance depends, among other things, on the anode material, since the electrolyte oxidation mechanism is different when active and non-active anode materials are utilized. A case-study on the application of a pilot plant, working in batch mode with recirculation, equipped with a BDD anode, to treat 4 L of OMW, slaughterhouse (SW) and winery (WW) wastewaters, with initial chemical oxygen demands (COD) of 20.5, 3.6 and 0.26 g L⁻¹, respectively, is presented and discussed. In 16 h assays, 94% COD removal was achieved for OMW, and for SW and WW the Portuguese COD legal discharge limit of 150 mg L⁻¹ was accomplished. Process efficiency decreased for lower organic load. NaCl addition increased COD removal in SW and WW, but presented an adverse effect for OMW COD removal, when compared to Na₂SO₄ addition. Nevertheless, lower specific energy consumptions were attained in chloride medium (48 Wh (g COD)⁻¹).

This study reports the first time that cheese whey wastewater has been treated by the hydrogen peroxide oxidation alone (uncatalytic reaction). This oxidation type was capable of eliminating the total phosphorus content in about 46–53%, regardless of the dose applied (1–7 g L⁻¹). For total phenols contents, the oxidation system was capable of reducing around 40% under the lowest concentration investigated (1 g L⁻¹). Similar results were obtained for calcium content with a reduction of 55%. Turbidity was reduced in about 90% when hydrogen peroxide was applied with concentrations of 2, 3 and 7 g L⁻¹. Characteristic absorbances were reduced in the range of 12–83% depending on the operating conditions. Chemical oxygen demand (COD) was recalcitrant to hydrogen peroxide oxidation alone (uncatalytic reaction), needing a post-treatment.