In most cases the process is continuous. Every day the digester is fed with substrates like manure and silage. The overflow goes into a second stage tank.
The digestion produces mostly methane (CH4), so only C and H leave the digester. N, P and K are (almost) fully retained and the value as a fertilizer is great. Best thing is, the manure stink is gone!
Unfortunately, "free fertilizer" leads to over-fertilization and nitrate in ground water. The use of organic fertilizer has been limited to 170 kg per hectare of total N lately, together with other restrictions. Some biogas investors fight to get rid of the end product.