id	keyword
american_scientific_journal-3563	biogas
american_scientific_journal-3563	digestion
american_scientific_journal-3563	pre
american_scientific_journal-3563	production
american_scientific_journal-3563	substrates
american_scientific_journal-3563	anaerobic
american_scientific_journal-3563	treatments
american_scientific_journal-3563	technology
american_scientific_journal-3563	solids
american_scientific_journal-3563	engineering