## 1 Introduction  
Agro-industries are major contributors to the worldwide industrial pollution problem, since they produce large quantities of wastewater that are very often left untreated and eventually end up in the environment. Among these agro-industries, wineries are important pillars of the local economy in Mediterranean countries, including Greece [1]. The amount and complexity of wastes generated by wineries and the management of these wastes is very problematic [2; 3]. Additionally, most of these wineries are usually small-sized, scattered throughout the country, and produce byproducts and wastes seasonally. Financial resources for proper waste disposal are not usually available, and therefore, in order for these industries to survive and remain competitive in the market, in many cases legislative standards for wastes and wastewaters are not adhered to. The wine making process comprises two main periods: harvest and post-harvest, and waste production is considerably higher in the harvest season [4]. Solid waste and sludge may be produced during several procedures, such as destemming, pressing, decanting, clarifying, etc., and these wastes include solid grape residues (grape stalks, etc.) and lees (winery sludge, or dregs). In addition, wine production generates large quantities of wastewater that originate from grape processing and washing operations such as grape crushing and pressing, and the rinsing of fermentation tanks, barrels and other equipment and surfaces [5]. Winery wastewater (WW) is characterized by fluctuations in both quality and quantity during the year [2; 3; 6; 7], causing environmental concern in wine-producing countries. WW is characterized by low pH values (in the range of 3-6 (acidic), with the exception of some winemaking stages such as bottling, where pH values increase), low nutrient content [8; 9; 10], and high organic content, which according the literature is readily biodegradable [2; 4; 11; 12; 13; 14]. Although several technologies have been proposed for winery wastewater treatment in Greece and other countries, [2; 10; 13; 15; 16; 17; 18; 19] (e.g., chemical precipitation, coagulation/flocculation processes, aerobic and anaerobic biological treatment, constructed wetlands (CWs), advanced oxidation processes, etc. ), unpretreated WW is usually discharged directly into the septic system or irrigated onto agricultural or other land [20]. A literature review reveals that most research concentrates on WW treatment using a single treatment method, while only recent research works have demonstrated a hybrid approach combining two or more treatment methods. [PERSON] et al. [21] showed that mild Fenton's treatment with biofililtration by Asian clams as a secondary treatment step can completely remove the Chemical Oxygen Demand (COD) of WWs, the concentration of which however was rather low (about \(1267\pm 101\) mg/L). A similar treatment system was also examined by [PERSON] et al. [22], who achieved COD removal near 100% from a WW with a relatively low initial COD concentration (up to 2280 mg/L). The use of solar Fenton as a pretreatment step before an immobilized biological reactor or activated sludge was studied by [PERSON] et al. [23] and [PERSON] et al. [24], respectively. They both achieved up to 96% COD removal with initial COD concentrations of up to 3300 mg/L. [PERSON] et al. [25; 26; 27] examined the ozonation of an aerobically pretreated WW (with an initial COD concentration of raw WW 27,000-29,000 mg/L) and found that the combination of these two processes leads to a COD removal efficiency of up to 67%. An aerobic biological process followed by a chemical oxidation process using Fenton's reagent (as a final polishing step) was examined by [PERSON] et al. [28], [PERSON] et al. [29] and [PERSON] et al. [30] using WW with a high COD concentration (up to 20,000 mg/L), and achieving a COD removal efficiency of 80%-99%.