That took me a bit to figure out as well. I have noticed that at the compost and board pallet factories that I cannot get a full fill of what it says that it will take. I rarely get more than about 5k liters at the board pallet, and around 25k at the compost factory. I've tried both tipping it in and using a bucket. There's a lot of pushing straw around.
Also, I think that at the compost factory the straw disappears (rots) at night or when it rains since I am using seasons. I could be wrong, but it seemed that I got very little compost - around 5k liters - for a lot of straw (25K) and water.
Found an interesting little bug at the desalination plant. Certainly not a game breaker:
Having said all that, I am enjoying this map immensely.