## 4 Discussion
### Wastes and By-Products: Generation and Characteristics
There have been a number of studies about the quantities and characterization of winery wastes and by-products [42] as well as the economic significance of their valorization against the cost of not doing it [43], both under the viewpoint of the extraction of compounds in grape wastes and as micro-scale energy valorization of the remains [35]. The process of making _orujo_, basically consists on: firstly, the dried grape marc is soaked and moistened, after which the mix of marc and water is fermented by the yeasts which appear naturally in the marc; after fermentation, the whole mix is distilled in still pots. After distillation, the remains in the still pot are just discharged, loaded in containers and disposed of. According to official data of the ODECA in 2019 (the official organism that certifies the production of food and beverages in Cantabria, working as a regulation council) [44], the production of _orujo_ that can be set under the official denomination of \"orujo de Liebana\" in 2013 was of around 20,300 L. To this amount can be added the 5337 L commercialized as _orujo_ without official denomination and to the distillation products from the distilleries used to produce other alcoholic beverages (based in _orujo_), like liquefus and cremes. Taking in account data gathered in a visit to a well-established distillery and extrapolated to the regional production, a liter of _orujo_ requires more than 6 kg of dry grape marc which turn into 10 kg of hydrated exhausted grape marc after distillation. This gives us a total number of 126,450 kg of grape marc used for production which become 154,269 kg of solid waste. The production has two distinct features: seasonality (the whole of the distillations take place in around 3-4 weeks in October-November) and geographic restriction (the Liebana region spreads over 570 square kilometers and encompasses seven municipalities, where grape production is restricted to the less mountainous areas). Waste management is performed by an authorized waste manager, who provides the containers for the storage (of 9 cubic meters and able to contain up to 15 tons), transport and disposal of the used marc. About beer brewery wastes, they have been subject to extensive research due to a number of facts: They are usually produced in large amounts and they have high pollution potential. In fact, since centuries ago, breweries have been detected as pollution sources, therefore being among the first industries that applied systems to reduce or recycle waste and treat liquid emissions. Some treatment alternatives used historically for the solid or semi-solid wastes, such as spent brewer's grains (SBG) and used yeasts, have been using them as livestock food or, in the case of spent yeast (trub), as human food after some lysis processes of yeast cells (Marmite). Other not-so-easily treatable wastes are brewery slurries, composed of materials used to filter beers (kieselguhr of diatomaceous earth) and the remains of filtration. Brewery wastewaters have traditionally undergone conventional aerobic sewage treatment, though they have proven to be an ideal subject for anaerobic treatment due to their characteristics. In recent times and due to the increasing costs of energy, solid or semi-solid brewery wastes have been subject of research as a potential source of biofuels that might be used to reduce the costs associated to energy consumption (mainly in the shape of heat) in the brewing processes [45]. Anaerobic treatment of wastes with the production of biogas provides a source of energy as well as a way of treatment for wastes and by-products which fits particularly well in brewery production [46], as biogas is storable and can be used for discrete processes requiring energy, like the boiling of words or the pasteurization of bottled beer for example.