This closed system is called an anaerobic digester, biodigester or a bioreactor. Biological hydrogen can be produced in an algae bioreactor. Approximately 300 spheroids are usually cultured per bioreactor. With added and compacted wastes, the O content of the landfill bioreactor strata gradually decreases. Efforts have also been undertaken with genetically modified alga in a bioreactor. "Fouling" can harm the overall efficiency of the bioreactor, especially the heat exchangers. Mostly the feed solution is highly concentrated to avoid dilution of the bioreactor. An example of a continuous bioreactor is the chemostat. This yeast then makes the insulin, which is harvested from the bioreactor. A PBR is a bioreactor which incorporates a light source. In some cases, all the nutrients are fed into the bioreactor. This biohydrogen is produced in a bioreactor. To avoid it, the bioreactor must be easily cleaned.