# Digestate Management and Processing Practices: A Review
## 5 Digestate Processing
### Treatments of the Liquid Fraction  
After solid-liquid separation, some nutrients and suspended solids remain in the liquid fraction. Due to regulatory requirements, it cannot be discharged directly to a receiving water body [98] but can be sent to a wastewater treatment plant. There are several technologies recommended for the extraction and valorization of nutrients from the liquid fraction, the application of which depends on economic viability and environmental impact [91]. Besides further processing, the liquid fraction can be reused as a nutrient-rich medium for algae cultivation [146], or again in AD [120]. The recirculation of the liquid fraction of the DG in the biogas digester is an interesting option to reduce CH4 emissions and produce more biogas from feedstock [147]. Several studies have confirmed the residual CH4 potential of recirculated DG, e.g., [148] conducted mesophilic AD in continuously stirred tank reactors where they reused the liquid fraction of DG in AD of wheat straw and compared it to monodigestion of wheat straw. The methane yields achieved with reused liquid fraction showed an improvement of 21% compared to the process without a liquid fraction of DG. Furthermore, [149] investigated the potential benefits of recirculation of liquid fraction of DG to biogas digesters by determining the BMP of the DG and comparing it to the BMP of untreated cattle slurry. The results indicated much higher BMP from DG (up to about 45%) than from untreated cattle slurry and highlighted the potential of recirculating DG to reduce the hydraulic retention time in the biogas digester while maintaining a long solid retention time to ensure process efficiency and stability. A comprehensive study by [150] investigated the effect of the liquid fraction of DG recirculation under different ratios of corn straw and pig manure on semi-continuous AD. Additionally, bacterial and Archaeal community structure was investigated while quantifying the benefits of the liquid fraction of DG recirculation. The authors concluded that liquid DG recirculation could be successfully used in most biogas plants by appropriately adjusting feedstocks and ferentation parameters to reduce waste, recycle materials, water, and energy, and make biomethane production cleaner and more efficient. Furthermore, the liquid fraction can also be used to moisturize compost heaps or as a source of microorganisms to facilitate the composting process [98].  
When the nutrients in the liquid fraction are not utilized, it is discharged to wastewater treatment plants to remove N and P before the aqueous effluent is discharged to water bodies [151]. Liquid DG contains high contents of nutrients in the available form with the potential to promote plant growth and increase the tolerance to biotic and abiotic stress. Therefore, it can be applied to soil as a liquid fertilizer via seed soaking, foliar spraying, soil amendment, and fertigation [152]. However, the liquid fraction of DG can cause environmental problems such as N leaching and infiltration into the groundwater, polluting nearby waters, ammonia, and greenhouse gas emissions. To overcome such problems, recovery and concentration of nutrients from the liquid fraction may be a desirable option [153]. In the literature, nutrient recovery technologies for the liquid fraction of DG have been the subject of numerous studies with a particular interest in N and P [152].