Complex sentence: Most bioplastics (eg. PH) only biodegrade quickly in the tightly controlled conditions of industrial composting units; they do not biodegrade quickly in compost piles or in the soil/water.
Split into simpler sentences: Most bioplastics (eg. PH) only biodegrade quickly in industrial composting units. These materials do not biodegrade quickly in ordinary compost piles or in the soil/water.