Characterization of Solid Digestates: Part 1, Review of Existing Indicators to Assess Solid Digestates Agricultural Use

Abstract

Anaerobic digestion of organic wastes is clearly encouraged by current regulations in Europe. In complement to the energy supply it represents, this biological treatment process also allows the recycling of organic matter and nutrients contained in biodegradable wastes. Indeed the digestion residue can be further promoted as soil improver or fertilizer. The sustainability of anaerobic digestion plants partly depends on the management of these digestion residues. Digestates present particular characteristics that can reduce their direct agricultural valorization and minimize by the way the benefit of such a biological treatment. Thus the first part of this two part paper aims at reviewing and discussing a wide range of biochemical, biological and physical indicators used to assess the agronomic quality of organic products and the feasibility of an aerobic treatment by composting. The definition of agronomic quality is very complex and no single parameter can be picked out to assess the quality of solid digestates coming from different sources. A relevant choice of these parameters will lead to state on digestates agricultural use, whether they can be directly used on soil after digestion or if they need a composting post-treatment before utilization. The second part of this two part paper will choose indicators to characterize several digestates in order to assess their future agricultural use.

Agronomic characteristics of five different urban waste digestates

“…The high TS and VS in FW1 and FW2 digestates could also be partly explained by the lower degradation during anaerobic digestion over 90% in FW3 and OFMSW) Overall, the results support the fact that the digestate TS concentration is dependent on the reactor configuration (e.g. wet/dry process) and process parameters (loading rate, retention time) (Teglia et al, 2011) despite the uniform characteristics of the feedstocks.…”

“…accounted for 100% of the P tot in 1/5 water extractions indicating good plant availability of P (Teglia et al, 2011) and was most likely due to the high P content of the wasteactivated sludge, as reported by Odlare et al (2008) and Zirkler et al (2014).…”

“…All digestates were neutral or slightly alkaline (pH 6.7-8.4), which is typical for food and green waste digestates (reviewed by Teglia et al, 2011). The neutral pH supports the use of digestates in agriculture, while the use of alkaline digestates could increase, e.g., NH 4 -N volatilization from soil during spreading depending on the temperature (Nkoa, 2014) and the acidic digestates can decrease soil pH and enhance the heavy metal mobilization in soils (Otabbong et al,.…”

“…heavy metals and pathogens, define the effect on soils and plants (Abubaker et al, 2012, Nkoa, 2014, Teglia et al, 2011, i.e. the agronomic value of the digestate.…”

Effect of Fertilizer and Storage Period on Oxidative Stability and Color of Rapeseed Oils

“…The pH, solids concentrations of the digestate, and feedstock are usually assessed to evaluate the effect of digestate on soil quality and plant growth. The use of alkaline digestate could increase NH 4 -N volatilization from soil during spreading, depending on the temperature [40], and the acidic digestates can decrease soil pH and enhance heavy metal mobilization in soils [41]. Subsequently, the effect of digestate pH on soil is dependent on soil characteristics [42].…”