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Fast development of centralized agricultural biogas plants leads to high amounts of digestate production. The treatment and disposal of liquid fractions after on-site digestate solid–liquid separation remains problematic due to their high organic, nutrient and aromatic contents. This work aims to study the variability of the remaining compounds in the digestate liquid fractions in relation to substrate origin, process parameters and solid–liquid separation techniques. Twenty-nine digestates from full-scale codigestion biogas plants and one waste activated sludge (WAS) digestate were collected and characterized. This study highlighted the combined effect of the solid–liquid separation process and the anaerobic digestion feedstock on the characteristics of liquid fractions of digestates. Two major clusters were found: (1) liquid fractions from high efficiency separation process equipment (e.g., centrifuge and others with addition of coagulant, flocculent or polymer) and (2) liquid fractions from low efficiency separation processes (e.g., screw press, vibrating screen and rotary drum), in this latter case, the concentration of chemical oxygen demand (COD) was associated with the proportion of cow manure and energy crops at biogas plant input. Finally, SUVA254, an indicator for aromatic molecule content and the stabilization of organic matter, was associated with the hydraulic retention time (HRT).

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Phosphorus (P) recovery in wastewater treatment plants (WWTP) as pure crystals such as struvite (MgNH4PO4.6H2O), potassium struvite (KMgPO4.6H2O) and calcium phosphates (e.g. Ca3(PO4)2) is an already feasible technique that permits the production of green and marketable fertilizers and the reduction of operational costs. Commercial crystallizers can recovery more than 90% of soluble P. However, most of the P in WWTP sludge is unavailable for the processes (not dissolved). P solubilization and separation are thus the limiting steps in P-crystallization. With an innovative two-step sequencing acidification strategy, the current study has aimed to improve biological P solubilization on waste-activated sludge (WAS) from a full-scale plant. In the first step (P-release), low charges of organic waste were used as co-substrates of WAS pre-fermentation, seeking to produce volatile fatty acids to feed the P-release by Polyphosphate-accumulating organisms, while keeping its optimal metabolic pH (6-7). In this phase, milk serum, WWTP grease, urban organic waste and collective restaurant waste were individually applied as co-substrates. In the second step (P-dissolution), pH 4 was aimed at as it allows the dissolution of the most common precipitated species of P. Biological acidification was performed by white sugar addition, as a carbohydrate-rich organic waste model, which was compared to chemical acidification by HCl (12M) addition. With short retention times (48-96 h) and without inoculum application, all experiences succeeded on P solubilization (37-55% of soluble P), principally when carbohydrate-rich co-substrates were applied. Concentrations from 270 to 450 mg were achieved.

Séparation de phases et digestats : profils d’efficacité basés sur les intrants de digestion anaérobie et le choix du séparateur. Journées Recherche Innovation Biogaz Méthanisation - JRI 2018

International audience; In France, digestates from anaerobic digestion plants have to pass through homologation process or reach certain specific standards to be commercialized. In this study, valorisation processes regarding the production of added-value products from urban digestates are being explored. Humic Acids (HA) extracted from upcycled organic products such as composts have been demonstrated to improve soil fertility [1] and industrial applications have been proven more recently [2]. However, very few studies addressed HA-recovery from digestates [3]. Preliminary tests presented in this study were carried out in order to assess the feasibility of recovering HA from such urban digestates. The samples were a Food Waste/Cardboard digestate (FW/CB-D) from a pilot reactor and an Organic Fraction of Municipal Solid Waste digestate (OFMSW-D) from a full-scale plant. HA-extraction was tested on raw digestates and on its solid fractions after laboratory centrifugation. A practical approach for HA extraction was performed, adapted from [4]: NaOH 0.1 M was added until pH 12 was reached. The solution was stirred for 24h. After the alkaline extraction was performed, the digestate was centrifuged. The supernatant was filtered at 0.45 μm and acidified at different pH (1, 2, 3, 5 and 7). This solution was centrifuged to isolate the insoluble fraction containing HA. The recovered HA were redissolved for Fluorescence 3D analysis. Another acidification at pH 3 was performed where the precipitate was washed several times with distilled water and freeze-dried for Mid-Infrared analysis and calculation of recovering yields. Recovered HA were compared to HA isolated from a leonardite commercial solution and to references from the International Humic Substances Society (IHSS). Freeze-dried HA yields were low: from 3 to 4% (volatile solids:volatile solids) and 1 to 2%, for raw mass of FW/CB-D and OFMSW-D respectively. HA extracted from FW/CBD were observed to be similar to a river sediment reference from the International Humic Substances Society. The HA from OFMSW was closer to the HA isolated from the leonardite solution, but it presented more proteins in the 3D fluorescence spectra. When HAs were extracted from the solid fraction of digestates, HA extraction yields and spectra were almost unchanged. The pH for acidification played an important role in HA isolation. pH 1, 2 and 3 were observed to perform similarly (quality and quantity) but pH 5 and 7 resulted in less extraction of HA and poorer quality. Industrial feasibility of HA-like extraction from digestate would rely on extraction yields, product quality and residual matter management options. Experiments will be carried out in order to further evaluate the quality of the products regarding biodegradability and impact on root growth

International audience; Whether to enhance nutrients management for land spreading or as a previous-step for advanced treatments, digestate mechanical phase separation is present in many Anaerobic Digestion (AD) plants. A wide range of equipment is available including screw presses, centrifuges, belt presses and rotary drums. Up-to-date, few or maybe no study has critically regarded a database of digestate separation efficiency, mass balances and quality of outputs for different separators. To fill-up this gap, data from literature was gathered with an INRA’s database and operational data from SUEZ. From this database, two different analysis approaches were carried out. The first consisted to perform clustering analysis on complete fertilizing-value data of liquid fractions (LF, n = 38) and solid fractions (SF, n = 47) separately. The second approach consisted to calculate mass balances and separation efficiency indicators [1] when characterization of raw digestate and its associated LF and SF were available. The later consisted in digestate data from over 50 industrial scale digesters. The first approach resulted in a typology for LF (Figure 1A), and SF (Figure 2B). LF and SF were observed to be grouped according to AD feedstock (fibrous/non-fibrous) and separation equipment (low/high performance), with significant distinct fertilizing-values. The second analysis approach is still ongoing. Early results highlight the distinguishable performances of previously observed groups. A recurring lack of consistency on the mass balances of some elements such as nitrogen is observed and will be discussed.

International audience; Anaerobic digestion (AD) is a well-known technology for waste treatment and bioenergy production, but digestate management is still a key challenge. Defined simply as the digested residue of AD, digestate refers to a set of heterogeneous matters featuring different biochemical compositions and physical properties inherited from greatly diversified AD processes and feedstocks. This situation induces a blurry scenario for policy-makers, digestate producers, marketers and consumers regarding digestate treatment and valorization. In this context, the objective of this study was to establish a digestate typology with available data on literature and internal databases. Common fertilizing-value parameters were used in the analysis, which are also present in many policies: dry matter (DM), volatile solids (VS), C/N ratio, C/N org ratio, Total N (TN), Total Ammoniacal Nitrogen (TAN), P and K contents, TAN/TN ratio. Heavy metals contents were also separately assessed. In a first approach, a Principal Component Analysis was performed including raw digestates and mechanical separation fractions. With the selected variables, it was found no statistical difference between raw wet (Wet AD) and liquid fraction and between raw dry (Dry-AD) and solid fraction. Later, Hierarchical Clustering Analysis (HCA) was performed on raw digestates resulting in 8 fertilizing-value groups among a dataset of 91 raw digestates. The groups presented variable nutrients and organic matter contents that could be qualitatively linked to the AD feedstock and to the type of process in terms of moisture (Wet or Dry-AD). HCA was performed separately for liquid and solid fractions after mechanical separation and in both cases, digestates were found to be categorized into two groups depending simultaneously on the separation technique and AD feedstock. With regard to heavy metals content, a typology was found to be similarly grouped by AD feedstock.

Typologie des digestats de méthanisation à partir de paramètres usuels de valeur amendement/fertilisante. Journées Recherche et Industrie biogaz méthanisation - JRI 2017

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International audience

International audience; Phosphorus (P) recovery as struvite from wastewater activated sludge (WAS) is a developing technology usually applied to recycle dissolved P by adding magnesium (Mg) and increasing pH in the liquid effluent from the digestate dewatering step after anaerobic digestion (AD) of WAS. Consequently, the P recovery yield is depending on the dissolved P concentration in digestate which is usually only a small fraction of the P in WAS. The aim of this study was to increase the P recovery yield by favouring P dissolution by a biological way, not after but just before the AD of WAS as described in figure 1. Recovering P before AD is also a way to prevent struvite crystallization on pipes and equipment, reducing maintenance cost in AD plants. For that, acidogenesis was favoured by adding several different wastes as co-substrate in thickened WAS.

International audience; Phosphorus (P) recycling as mineral fertilizer from wastewater activated sludge (WAS) depends on the amount that can be dissolved and separated from the organic matter before the final crystallization step. The aim of the biological phosphorus dissolution potential (BPDP) test developed here was to assess the maximum amount of P that could be biologically released from WAS prior that the liquid phase enters the recovery process. It was first developed for sludge combining enhanced biological phosphorus removal and iron chloride. Because carbohydrates are known to induce acidification during the first stage of anaerobic digestion, sucrose was used as a co-substrate. Best results were obtained after 24-48 h, without inoculum, with a sugar/sludge ratio of 0.5 gCOD/gVS and under strict anaerobic conditions. Up to 75% of the total phosphorus in sludge from a wastewater treatment plant combining enhanced biological phosphorus removal and iron chloride phosphorus removal could be dissolved. Finally, the test was applied to assess BPDP from different sludge using alum compounds for P removal. No dissolution was observed when alum polychloride was used and less than 20% when alum sulphate was used. In all the cases, comparison to chemical acidification showed that the biological process was a major contributor to P dissolution. The possibility to crystallize struvite was discussed from the composition of the liquids obtained. The BPDP will be used not only to assess the potential for phosphorus recycling from sludge, but also to study the influence of the co-substrates available for anaerobic digestion of sludge.