From grass to gas and beyond: Anaerobic digestion as a key enabling technology for a residual grass biorefinery

Roadside grass clippings hold potential as a sustainable source of bioenergy as they do not compete with crops for land use, and are only partially utilized for low-value applications. In this study, we proposed using roadside grass as a sole feedstock for anaerobic digestion (AD) in three different settings, and assessed the potential of producing biomaterials and fertilizers from grass-based digestate. Wet continuous digestion at pilot scale and dry batch digestion at pilot and large scales resulted in biogas yields up to 700 Nm3.t-1 DOM with a methane content of 49-55 %. Despite promising results, wet AD had operational problems such as clogging and poor mixing; once upscaled, the dry digestion initially also presented an operational problem with acidification, which was overcome by the second trial. Digested grass fibers from the pilot dry AD were processed into biomaterials and performed similarly or better than the undigested fibers, while around 20 % performance reduction was observed when compared to reference wood fibers. A mass balance indicated reduced fiber recovery when higher biogas production was obtained. The liquid fraction from the pilot dry AD was characterized for its nutrient content and used as a biofertilizer in another study. In contrast, the leachate collected from the large-scale dry AD had a low nitrogen content and high chloride content that could hinder its further use. Finally, a regional market analysis was conducted showing that the biocomposites produced with the available grass fibers could substitute at least half of the current European market based on our results.

Simultaneous detection of pesticides and pharmaceuticals in three types of bio-based fertilizers by an improved QuEChERS method coupled with UHPLC-q-ToF-MS/MS

Bio-based fertilizers (BBFs) have the potential to contain both pesticides and pharmaceutical residues, which may pose a threat to soils, crops, and human health. However, no analytical screening method is available currently to simultaneously analyze a wide range of contaminants in the complex origin-dependent matrices of BBFs. To fill this gap, our study tested and improved an original QuEChERS method (OQM) for simultaneously analyzing 78 pesticides and 18 pharmaceuticals in BBFs of animal, plant, and ashed sewage sludge origin. In spiked recovery experiments, 34-58 pharmaceuticals and pesticides were well recovered (recovery of 70-120%) via OQM at spiking concentrations levels of 10 ng/g and 50 ng/g in these three different types of BBFs. To improve the extraction efficiency further, ultrasonication and end-over-end rotation were added based on OQM, resulting in the improved QuEChERS method (IQM) that could recover 57-79 pesticides and pharmaceuticals, in the range of 70-120%. The detection limits of this method were of 0.16-4.32/0.48-12.97 ng/g, 0.03-11.02/0.10-33.06 ng/g, and 0.06-5.18/0.18-15.54 ng/g for animal, plant, and ash-based BBF, respectively. Finally, the IQM was employed to screen 15 BBF samples of various origins. 15 BBFs contained at least one pesticide or pharmaceutical with ibuprofen being frequently detected in at concentration levels of 4.1-181 ng/g. No compounds were detected in ash-based BBFs.

A comparative study of thermally and chemically treated dairy waste: Impacts on soil phosphorus turnover and availability using 33P isotope dilution

Dairy processing sludge (DPS) and DPS-derived secondary products such as struvite, biochar, hydrochar and ash (collectively known as SRUBIAS) are emerging as alternatives to fertilizers produced from mined rock phosphate. However, little is known about how these products affect soil P availability and daily P turnover rates.. A lack of such information prevents precision nutrient management planning using these products out on farms. This study used a novel isotope dilution technique (IPD) with ³³P as a tracer to compare P turnover in soils amended with chemically (alum-treated DPS and struvite) and thermally (biochar, hydrochar, ash) treated DPS. Results showed that thermally treated products exhibited poor agronomic performance as P fertilizers, potentially inhibiting P availability when applied to soils. For example, a P deficient soil amended with hydrochar treatment at the highest application rates did not record a build-up of available P to agronomic target values. In ash and biochar treated P deficient soils, available P increased but only with very high application rates of 150 and 80 mg P kg ^-1. The application of these products as fertilizers could have negative implications for both environmental and agronomic goals. Conversely, chemically treated fertilisers demonstrated better agronomic performance. The same agronomic target value was reached with application rates of only 20 mg P kg ^-1 soil for DPS and 50 mg P kg ^-1 soil for struvite. However, the techniques deployed revealed that these products exhibited slower rates of available and exchangeable P build-up when compared with chemical fertilisers. This suggests that these bio-based alternatives require higher application rates or earlier application times compared to conventional chemical fertilizers. Regulations providing advice on P use in agricultural soils need to account for slower P turnover in soils receiving recycled fertilizers. The IPD technique is transferrable to all wastes to examine their performance as fertilizers.

Antibiotic residues in substrates and output materials from biogas plants - Implications for agriculture

Bio-based fertilizers including farmyard manure, sewage sludge, meat and bone meal, composts, digestates and derived products are nutrient-rich fertilizers. They deliver organic matter but may pose the risk to contaminate soils by pollutants such as pharmaceuticals, heavy metals, resistance genes or pathogens. Manure and sewage sludge are increasingly used in biogas plants for energy production with the residue being used as fertilizer. It is therefore important to understand the fate of pharmaceuticals during anaerobic digestion. In the present study, 29 biogas plants from three countries were studied. The different input materials and output after digestion were analyzed for selected examples of antibiotics from three different classes, namely tetracyclines (TCs), sulfonamides (SAs) and fluoroquinolones (FQs). These classes are frequently found in animal manures and sewage sludge and display differing mobility and persistence. The results revealed that antibiotics could be detected in 81% of the substrates derived from animal manures and sewage sludge and in 83% of the digestates. Antibiotics were determined with the highest frequency of 100% in sewage sludge where especially ciprofloxacin and tetracycline were found. Highest concentrations were analyzed in poultry dung with in maximum of 8.6 and 8.2 mg/kg DW of enrofloxacin and tetracycline, respectively. After digestion, slightly lower concentrations of antibiotics were determined for most substrates. However, in one biogas plant using poultry dung as an input material a maximum concentration of 15.2 mg/kg DW of tetracycline was determined in the digestate, which after separation accounted for 29.8 mg/kg DW of tetracycline in the liquid phase.

A systematic comparison of commercially produced struvite: Quantities, qualities and soil-maize phosphorus availability

Production of struvite (MgNH₄PO₄·6H₂O) from waste streams is increasingly implemented to recover phosphorus (P), which is listed as a critical raw material in the European Union (EU). To facilitate EU-wide trade of P-containing secondary raw materials such as struvite, the EU issued a revised fertilizer regulation in 2019. A comprehensive overview of the supply of struvite and its quality is presently missing. This study aimed: i) to determine the current EU struvite production volumes, ii) to evaluate all legislated physicochemical characteristics and pathogen content of European struvite against newly set regulatory limits, and iii) to compare not-regulated struvite characteristics. It is estimated that in 2020, between 990 and 1250 ton P are recovered as struvite in the EU. Struvite from 24 European production plants, accounting for 30% of the 80 struvite installations worldwide was sampled. Three samples failed the physicochemical legal limits; one had a P content of <7% and three exceeded the organic carbon content of 3% dry weight (DW). Mineralogical analysis revealed that six samples had a struvite content of 80-90% DW, and 13 samples a content of >90% DW. All samples showed a heavy metal content below the legal limits. Microbiological analyses indicated that struvite may exceed certain legal limits. Differences in morphology and particle size distribution were observed for struvite sourced from digestate (rod shaped; transparent; 82 mass% < 1 mm), dewatering liquor (spherical; opaque; 65 mass% 1-2 mm) and effluent from upflow anaerobic sludge blanket reactor processing potato wastewater (spherical; opaque; 51 mass% < 1 mm and 34 mass% > 2 mm). A uniform soil-plant P-availability pattern of 3.5-6.5 mg P/L soil/d over a 28 days sampling period was observed. No differences for plant biomass yield were observed. In conclusion, the results highlight the suitability of most struvite to enter the EU fertilizer market.

Nutrient recovery from digested waste: Towards a generic roadmap for setting up an optimal treatment train

This paper aims to develop a generic roadmap for setting up strategies for nutrient recovery from digested waste (digestate). First, a guideline-based decision-tree is presented for setting up an optimal bio-based fertilization strategy as function of local agronomic and regulatory criteria. Next, guidelines and evaluation criteria are provided to determine the feasibility of bio-based fertilizer production as function of the input digestate characteristics. Finally, a conceptual decision making algorithm is developed aiming at the configuration and optimization of nutrient recovery treatment trains. Important input digestate characteristics to measure, and essential factors for monitoring and control are identified. As such, this paper provides a useful decision-support guide for wastewater and residuals processing utilities aiming to implement nutrient recovery strategies. This, in turn, may stimulate and hasten the global transition from wastewater treatment plants to water resource recovery facilities. On top of that, the proposed roadmap may help adjusting the choice of nutrient recovery strategies to local fertilizer markets, thereby speeding up the transition from a fossil-reserve based to a bio-based circular nutrient economy.

Fertilizer performance of liquid fraction of digestate as synthetic nitrogen substitute in silage maize cultivation for three consecutive years

Following changes over recent years in fertilizer legislative framework throughout Europe, phosphorus (P) is taking over the role of being the limiting factor in fertilizer application rate of animal manure. This results in less placement area for spreading animal manure. As a consequence, more expensive and energy demanding synthetic fertilizers are required to meet crop nutrient requirements despite existing manure surpluses. Anaerobic digestion followed by mechanical separation of raw digestate, results in liquid fraction (LF) of digestate, a product poor in P but rich in nitrogen (N) and potassium (K). A 3-year field experiment was conducted to evaluate the impact of using the LF of digestate as a (partial) substitute for synthetic N fertilizer. Two different fertilization strategies, the LF of digestate in combination with respectively animal manure and digestate, were compared to the conventional fertilization regime of raw animal manure with synthetic fertilizers. Results from the 3-year trial indicate that the LF of digestate may substitute synthetic N fertilizers without crop yield losses. Through fertilizer use efficiency assessment it was observed that under-fertilization of soils with a high P status could reduce P availability and consequently the potential for P leaching. Under conditions of lower K application, more sodium was taken up by the crop. In arid regions, this effect might reduce the potential risk of salt accumulation that is associated with organic fertilizer application. Finally, economic and ecological benefits were found to be higher when LF of digestate was used as a synthetic N substitute. Future perspectives indicate that nutrient variability in bio-based fertilizers will be one of the greatest challenges to address in the future utilization of these products.