NOVELTY - Complete anaerobic digestion of polymer mixtures of composition comprising (a) a polyhydroxyalkanoate including poly-4-hydroxybutyrates, poly-3-hydroxybutyrates, poly(3-hydroxybutyrate-co-3-hydroxyvalerates), poly(3-hydroxybutyrate-co-3-hydroxyhexanoates) and poly(3-hydroxybutyrate-co-4-hydroxybutyrates), and (b) an aliphatic-aromatic polyester comprising 5-36C-dicarboxylic acid derivatives or 5-36C-dicarboxylic acids, (ii) a terephthalic acid derivative or a terephthalic acid, (iii) 2-8C-alkylenediol or 2-6C-oxyalkylenediol, and (iv) polyfunctional compound, is claimed.    USE - The process is useful for complete anaerobic digestion of polymer mixtures of composition and disposing of biomass in a biogas plant (all claimed) to produce films: such as agricultural films (mulch films), peelable closure film, transparent or opaque-weldable closure film, transparent or opaque, sausage skin, salad film, freshness retention film (stretch film) for fruit and vegetables, meat and fish, stretch film for wrapping of pallets, film for nets, packaging films for snacks, chocolate bars and muesli bars, and peelable lid films for dairy packaging (yoghurt and cream), semirigid packaging for smoked sausage and cheese and packaging of meat, poultry, meat products, processed meat, sausages, seafood, fish, crab meat, cheese, cheese products, desserts, pies, poultry products, tomato filling, pastes and bread spreads, bread, cakes, other bakery products, fruit, fruit juices, vegetables, tomato puree, salads, animal food, pharmaceutical products, coffee, coffeelike products, milk powder or cocoa powder, coffee whitener, baby food, dried foods, jams and jellies, bread spreads, chocolate cream and ready meals: for basal fold bags, lateral seam bags, carrier bags with a hole grip, shrink labels or vest-type carrier bags, inliners, heavy-duty sacks, freezer bags and composting bags; and for coating of paper cups and paper plates.    ADVANTAGE - The process produces mechanically durable films with good anaerobic biodegradability, good adhesion properties and good processability.    DETAILED DESCRIPTION - Complete anaerobic digestion of polymer mixtures of composition comprising (a) 25-95 wt.% of a polyhydroxyalkanoate including poly-4-hydroxybutyrates, poly-3-hydroxybutyrates, poly(3-hydroxybutyrate-co-3-hydroxyvalerates), poly(3-hydroxybutyrate-co-3-hydroxyhexanoates) and poly(3-hydroxybutyrate-co-4-hydroxybutyrates), and (b) 5-75 wt.% of an aliphatic-aromatic polyester comprising (i) 65-95 mol.% of one or more 5-36C-dicarboxylic acid derivatives or 5-36C-dicarboxylic acids, (ii) 5-35 mol.% of a terephthalic acid derivative or a terephthalic acid, (iii) 98-100 mol.% of an 2-8C-alkylenediol or 2-6C-oxyalkylenediol, and (iv) 0-2 wt.% of at least one polyfunctional compound comprising at least two isocyanate, isocyanurate, oxazoline or epoxide groups or at least three alcohol or carboxylic acid groups, is claimed. An INDEPENDENT CLAIM is included for disposing of biomass in a biogas plant comprising collecting or dispensing biomass in a package comprising polymer mixtures of the composition, collecting biomass in the package by a waste management company, and sending the biomass to anaerobic fermentation in a biogas plant.