# Digestate Management and Processing Practices: A Review
## 5 Digestate Processing
### Treatments of the Liquid Fraction
#### 5.3.1 Membrane Technology  
Membrane technology separates the liquid fraction of the DG into a solid rich fraction (retentate or concentrate) and an aqueous solution that passes through the membrane (permeate). Both fractions (retentate and permeate) contain valuable chemical building blocks that can be used for various purposes. The main disadvantages are mainly the significant upfront costs [154] and the tendency to membrane fouling and clogging which lead to loss of performance and significant operational costs [155]. Almost all types of membrane processes have been investigated for the treatment of anaerobic DG: microfiltration [154; 156], ultrafiltration [157; 158], nanofiltration [159; 160], reverse osmosis [159; 160], forward osmosis [155]. However, the introduction of membrane technology in farming environments has proven to be quite challenging. Among membrane process technologies, microfiltration and ultrafiltration are very broadly used. They concentrate molecules and particles above 0.01 and up to 1 \(\upmu\)m, e.g., pathogens (bacteria, viruses, etc.), organic macromolecules (proteins, carbohydrates, etc.), and minerals (clays, latex, etc.) can be well separated by microporous membranes. Their main advantage is the relatively low operating pressure (from 0.1 to 5 bar) and therefore limited energy costs. High-pressure membrane processes, such as reverse osmosis, retain the smallest organics (tannins, organic acids, polyphenols, pesticides, pharmaceuticals, etc.), HMs, and salts. In comparison with thermal processes, reverse osmosis proves to be less energy-intensive, requires fewer investment costs, and has a smaller footprint. As an alternative to microfiltration, ultrafiltration, and reverse osmosis, nanofiltration with intermediate properties (operating pressure, molecular weight cut-off), which can split compounds into divalent ions, may be a more suitable choice [161]. In addition, forward osmosis is considered to be very energy efficient and has a low fouling tendency, so purification is rarely required. In forward osmosis, a semipermeable membrane (ideally allowing only water molecules to pass) is placed between a feed (a solution to be treated whose osmotic pressure is low compared to the draw) and a draw solution (which has high osmotic pressure and in which pure water is extracted). The osmotic gradient arises from the difference in osmotic pressures of the feed and the draw solution [162] which means that the process does not require pressure vessels and expensive high-pressure pumps. Forward osmosis allows reaching high-concentrated [161] and high-quality products while the applied filters show an extended lifetime if compared to filters used in pressure-driven processes. However, forward osmosis has several limitations such as relatively low water flux, back salt diffusion, the necessity to recover the draw solution, the internal concentration polarization phenomenon, and fouling [162]. Despite significant advances in membrane design and operation, applications of osmotic processes remain mostly pilot scale to date [161].