# Digestate Management and Processing Practices: A Review
## 5 Digestate Processing
### Treatments of the Solid Fraction
#### 5.2.1 Composting
Recent efforts have been applied to improve composting performance, such as optimizing process parameters (aeration rate, feedstock properties, and matrix structure), adding useful additives (bulking agents), and integrating hybrid systems [102]. Aeration plays an important role in oxygen supply, temperature regime, and moisture control of composting [94]. Optimum aeration rates have been systematically studied for various organic solids and it was concluded that increasing the aeration rate can promote the activity of aerobic microorganisms for the decomposition of the OM. However, increased aeration may increase greenhouse gas and ammonia emissions, and facilitates the release of generated heat and water which can inhibit microbial growth and activity. Therefore, optimal aeration intensities should be determined for the specific composting process [102]. The addition of bulking agent, which helps to construct the composting pile and assures a good distribution of the airflow and hence the free-air space inside the piles [94], improves the physio-chemical properties of organic solid wastes, particularly those with high moisture and density, and consequently advance their composting [102]. Furthermore, the bulking agent can have positive effects such as increasing nutrient concentration, decreasing electrical conductivity, decreasing N concentration in terms of increasing C/N ratio, and diluting HM content in compost [98]. Special attention should be paid to the ratio of bulking agent and DG as it affects the initiation of composting [102]. Commonly used bulking agents are mostly derived from plant material [99] e.g., lignocellulosic waste such as corn stalks, straw, mushroom residues, sawdust, and green wastes. Numerous researchers also suggest materials with high porosity and a large specific surface area such as biochar, ceramsite, and zeolite [104]. Vermicomposting, which uses earthworms to stabilize DG and eliminate pathogens, is also a sustainable approach to DG management that, like composting, is an easy-to-use, eco-friendly and cost-effective technology [91]. Besides all the advantages, composting DG also has some disadvantages [94]. During AD, organic N is transformed through a series of reactions like ammonization, anaerobic ammonia oxidation, and denitrification. Most of these N substances result in high NH\({}_{3}\)-N content in DG which can be easily volatilized in the form of smelly and toxic NH\({}_{3}\) in the thermophilic period of composting [104]. Once released into the atmosphere, NH\({}_{3}\) emissions may lead to acid rain and eutrophication of local systems. Furthermore, the nitrifying microorganisms can develop during composting and oxidize NH\({}_{3}\)-N to nitrites (NO\({}_{2}\)\({}^{-}\)) and nitrates (NO\({}_{3}\)\({}^{-}\)) and can further reduce NOx\({}_{x}\)\({}^{-}\) to N\({}_{2}\). Both nitrification and denitrification can emit N\({}_{2}\)O which can be known for its high global-warming potential and for causing ozone depletion. Other disadvantages include high moisture content, which affects very low composting efficiency, low C/N ratio, which requires a long composting process, and low DG degradability, which results in a low-temperature rise during composting that cannot ensure the required disinfection of the compost [94]. In literature as well as in industry, there is insufficient information to optimize the operational parameters of DG composing [94]. Table 5 summarizes studies related to DG processing via composting. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Substrate** & **Process Conditions** & **The Aim of the Study** & **Results** & **Ref. ** \\ \hline DG (household wastes) & Mixing ratios WC:DG = 1:1, 2:1, 3:1, 4:1, 5:1 in volume; aeration rate: 7, 15 and 30 1h\({}^{-1}\) kg OM\({}_{0}\) & Influence of different size of BA and different mixing ratios of DG and BA on compost quality. & Higher mixing ratios of substrates increased O\({}_{2}\) utilization and self-heating potential, and reduced gas emissions.