# Biochemical Methane Potential of Cork Boiling Wastewater at Different Inoculum to Substrate Ratios
## 1 Introduction  
The cork oak (_Quercus suber_ L.) is a native forest species protected by Portuguese legislation; it is distributed throughout the western part of the Mediterranean region. Historically, this primitive species has had greater importance in the Portuguese forest due to its wide distribution throughout the country and in the south. In terms of the manufacturing industry, Portugal's position is even more relevant since 60% of the raw material collected is processed by national industries, which export 90% of the processed cork [1, 2, 3]. Cork is mainly composed of suberin (between 40% and 50%), lignin (between 20% and 30%), polysaccharides (between 10% and 20%), and extracts (ceroids and tannins are between 10% and 15%). However, its composition depends on many variables such as climatic conditions, soil conditions, origin, dimensions, and age of trees [3, 4]. The extracts present in cork are nonstructural organic components of the cellular wall. They include families of compounds such as n-alkanes, n-alkanols, waxes, fatty acids, triterpenes, glycerides, sterols, phenols, and polyphenols. Phenolic compounds represent about 90% of the contaminants present in CBW [5]. Among the acids present, ellagic acid was identified as the main phenolic compound [3]. The process consists of immersing the cork boards in hot water for up to 2 h to allow the material to acquire elasticity for manufacturing and also to clean and disinfect the boards [6]. It is estimated that in Portugal, around 34% of its total area is currently exploited, which corresponds to an average annual production of 150 thousand tons. The cork transformation process involves the use of large amounts of water and, consequently, high production of wastewater. It is estimated that 400 L of water is consumed per ton of cork processed in the raw material purification stage, known as cooking, where the raw material is purified by immersion in hot water (temperature 95 to 105 \({}^{\circ}\)C) for one hour. The 150 thousand tons of cork creates about 6.0 \(\times\) 10\({}^{7}\) L of CBW. During the process of cooking cork boards, a wide range of organic compounds is transferred to the boiling water, thus resulting in a generation of wastewater with a high organic load and little biodegradability [7; 8]. The compounds accumulated during the cooking process include phenolic acids, namely, gallic, vanillic, syringic, ferulic, and ellagic acids, in addition to tannins, 4,6-trichloroanisole, and pentachlorophenol. Carboxylic acids of polyphenols contribute to the acidification of the medium, which characterizes the effluent with a pH between 4 and 5 [9]. Although the cork boiling wastewater has a low biodegradability (low BOD\({}_{5}\)/COD ratio, between 0.3 and 0.45) due to the presence of polyphenols and tannins, currently, no specific treatments are applied to these waters before discharge into wastewater treatment plants (WWTPs) [10]. The environmental impact of this activity is worsened by the fact that the treatment of this effluent is difficult to achieve through conventional purification processes, namely, those based on biological treatment, due to the high concentration of organic charge, with a recalcitrant character normally present [8]. This problem is potentially even more serious because there are still many small industrial factories with low and discontinuous wastewater flows that are sent to the receiving WWTPs without any prior treatment [11]. In this sense, these effluents from the cork industry can constitute a serious environmental problem, namely, the contamination of soils and groundwater. The most technologically advanced treatments for the effluent from the cork transformation process go through early removal of the organic matter through chemical coagulation and flocculation processes, followed by a biological process. In addition to these methods, membrane purification processes and chemical oxidation processes have also been used, with reagents such as Fenton or ozonation (advanced oxidation processes--POAs).