It is highly suggested always to use a fresh batch of growing medium each time you sow new seeds as the old medium contains the leftover roots & stems, which may interfere with the growth of new seeds. You can add your leftover medium to your compost bin or use it as a mulch for your plants. Still, if you want to reuse it to grow another batch, make sure to properly wash, dry and filter it before use so that it's properly clean.