This is an excellent report on the liquid leaving a biogas digester - especially in farming areas.
"Anaerobic Digestion (AD) in a biogas plant is a well proven process in which organic matter breaks down naturally in the absence of oxygen to produce two valuable products - biogas and digestate. Biogas is an extremely useful source of renewable energy, whilst digestate is a highly valuable biofertiliser (Lukehurst 2010). AD can also offer a range of other benefits." IEA report 2010.
"Digestate is an easy product to handle and apply and can be used successfully as a substitute for mineral fertilisers. The fertiliser value of digestate depends on the nutrients present in the feedstock. However, digestate is the result of a living process and therefore has characteristics that are specific to each digester tank. These characteristics can vary between batches from the same digester and even within the same batch of digestate, following storage."