Humic-like substances extracted from different digestates: First trials of lettuce biostimulation in hydroponic culture.

Digestate valorization is a key challenge for the feasibility of Anaerobic Digestion plants. In this study, humic-like substances (HLS) extracted from two digestates (sewage sludge and manure) were used for the biostimulation of hydroponic cultures of Lactuca sativa (lettuce) at different doses and compared to a commercial reference of leonardite HLS. Aerial biomass increase averages ranged from 7 to 30 %, but the results presented a high coefficient of variation (around 20 %). The commercial reference did not present statistically significant biomass yield improvement. The application of manure digestate extract at the higher fulvic-like acids dose (4.6 mg/L of dissolved organic carbon) presented the best and most significant results compared to the blank (Hoagland's solution only). However, this result cannot be strictly dissociated from the supplementary amounts of nutrients brought by the extracts. Additionally, all the products presented low heavy metal content compared to the recent EU regulation for biostimulants (2019/1009). This preliminary study confirmed the interest of extracting HLS from two digestates for application as biostimulants, shedding light on a new perspective for digestate valorization.

First fertilizing-value typology of digestates: A decision-making tool for regulation.

Defined as the residue from anaerobic digestion (AD), digestate refers to a set of materials with varied biochemical compositions. The objective of this study was to establish a digestate typology according to its fertilizing-value with data from literature and internal unpublished databases. To establish a relatively big database allowing the application of advanced statistics, usual fertilizing-value parameters were used: dry matter, volatile solids, C/N, C/Organic-N, total N (TN), total ammoniacal nitrogen (TAN), TAN/TN, total P and total K. Statistical analysis was performed on a dataset of 91 raw digestates, 34 solid fractions and 25 liquid fractions after separation. The resulting typology outlined that fertilizing-values are linked to AD feedstock and process. As case study regulations, no digestate (without any post-treatment) fulfilled French standards and the latest European Union regulation proposal on fertilizers. Options to reach regulations' product categories were discussed according to the typology. For the first time, a digestate typology was established based on fertilizing value, which can be a useful tool enhancing digestate management and policy making.

Digestate mechanical separation: Efficiency profiles based on anaerobic digestion feedstock and equipment choice.

Digestate mechanical separation is present in numerous anaerobic digestion plants. In this study, data from literature and from unpublished analysis were gathered to evaluate digestate separation efficiency for different mechanical separators. For the first time, efficiency indicators allowed the definition of two mass distribution profiles. The low-performance profile was characterized by each component being mainly destined to the liquid fraction, excluding P, Mg and Ca in a few cases. Screw presses represented 68% of these separators and 78% of digestates came from mainly fibrous inputs such as cow manure and silage. In the high-performance profile, digestate compounds were effectively concentrated in the solid fraction, except nitrogen. The great majority of separators were decanting centrifuges, and the anaerobic digestion inputs were principally non-fibrous such as pig slurry, sludge and agro-industrial waste. This study represents a source for benchmarking digestate separation and opens a possibility to forecast more realistically digestate separation performance.

Treatment of the biodegradable fraction of used disposable diapers by co-digestion with waste activated sludge.

The results presented in this paper are part of a project aimed at designing an original solution for the treatment of used disposable diapers permitting the recycling of materials and the recovery of energy. Diapers must be collected separately at source and transported to an industrial facility to undergo special treatment which makes it possible to separate plastics and to recover a biodegradable fraction (BFD) made up mainly of cellulose. The methane yield of BFD was measured and found to be 280 ml CH4/g VSfed on average. 150 kg of dry BFD can be retrieved from the treatment of one ton of used disposable diapers, representing an energy potential of about 400 kW h of total energy or 130 kW h of electricity. As the treatment process for used diapers requires very high volumes of water, the setting up of the diaper treatment facility at a wastewater treatment plant already equipped with an anaerobic digester offers the advantages of optimizing water use as well as its further treatment and, also, the anaerobic digestion of BFD. The lab-scale experiments in a SBR showed that BFD co-digestion with sewage sludge (38% BFD and 62% waste activated sludge on volatile solids basis) was feasible. However, special attention should be paid to problems that might arise from the addition of BFD to a digester treating WAS such as insufficient mixing or floating particles leading to the accumulation of untreated solids in the digester.

Assessment of the effectiveness of an industrial unit of mechanical-biological treatment of municipal solid waste.

An assessment of the French municipal solid waste (MSW) mechanical-biological treatment (MBT) unit of Mende was performed in terms of mass reduction, biogas emissions reduction and biostability of the biologically treated waste. The MBT unit consists of mechanical sorting operations, an aerobic rotating bioreactor, forced-aeration process in open-air tunnels (stabilization), ripening platforms and a sanitary landfill site for waste disposal in separated cells. On the overall plant, results showed a dry matter reduction of 18.9% and an oxidative organic matter reduction of 39.0%. A 46.2% biogas production decrease could also be observed. Concerning the biotreatment steps, high reductions were observed: 88.1% decrease of biogas potential and 57.7% decrease of oxidative organic matter content. Nevertheless, the usually considered stabilization indices (biogas potential, respirometric index) remained higher than recommended by the German or Austrian regulation for landfilling. Mass balance performed on each step of the treatment line showed that several stages needed improvement (especially mechanical sorting operations) as several waste fractions containing potentially biodegradable matter were landfilled with very few or no biological treatment.

Effect of biological pretreatment of coarse MSW on landfill behaviour: laboratory study.

Mechanical and biological pre-treatment (MBT) of residual Municipal Solid Waste (MSW) is considered as a promising technical option prior to landfilling. The aim of MBT is to control the biological landfill activity to minimize biogas and leachate production. Laboratory-scale bioreactors were set up to study the behaviour of untreated and pre-treated residues. The bioreactors were designed to simulate the anaerobic condition of sanitary landfill. Initial water addition has been performed to ensure optimal condition of biological degradation. The incubation time was 400 days to achieve the biodegradation. Experiments have been carried out with untreated or treated waste collected from a mechanical-aerobic biological treatment plant located in middle south of France. Chemical and biological analyses have been performed to characterise the waste samples before and after the incubation. Results showed that a residual anaerobic activity does exist for the pre-treated waste when incubated in optimal moisture condition: biogas production does still exist even after a long period of aerobic hot fermentation and maturation.

Mass balance to assess the efficiency of a mechanical-biological treatment.

Using mechanical-biological treatment of residual municipal solid waste, it is possible to significantly lower landfill volume and gas and leachate emissions. Moreover, the landfill characteristics are improved. The performance of the Mende (France) mechanical-biological treatment plant is assessed via mass balances coupled with manual sorting according to the MODECOM methodology and biochemical methane potential after 90 days of incubation. The site includes mechanical sorting operations, a rotary sequential bioreactor, controlled aerobic stabilisation corridors, maturation platforms, and a sanitary landfill site for waste disposal in separated cells. Results showed that several steps could be improved: after a first sieving step, about 12% of the potentially biodegradable matter is landfilled directly without any treatment; mechanical disintegration of papers and cardboards in the rotary sequential bioreactor is insufficient and leads to a high proportion of papers and cardboards being landfilled without further treatment. Two fine fractions go through stabilisation and maturation steps. At the end of the maturation step, about 54% of the potentially biodegradable matter is degraded. The biochemical methane potential after 90 days of incubation is reduced by 81% for one of the two fine fractions and reduced by 88% for the other one. Considering the whole plant, there is a reduction of nearly 20% DM of the entering residual municipal solid waste.