21. In 2013, Harvest Power began operations of three anaerobic digesters in North America. 22. California has 242 sewage wastewater treatment plants, 74 of which have installed anaerobic digesters . 23. Neither approach has mixing inside the digester . 24. It also consists of the mineralised remains of the dead bacteria from within the digesters . 25. The cake can also be used as feed in digesters and gasifiers to produce biogas. 26. As of July 2013, his company has installed over 4, 500 Biofil Digesters . 27. It can be produced using anaerobic digesters ( air-tight tanks with different configurations ). 28. Anaerobic digesters can also be fed with purpose-grown energy crops, such as maize. 29. The primary styles of dry digesters are continuous vertical plug flow and batch tunnel horizontal digesters. 30. The primary styles of dry digesters are continuous vertical plug flow and batch tunnel horizontal digesters . 