The choice of activating agent for the thermochemical production of high-grade activated carbon (AC) from agricultural residues and wastes, such as feedstock, requires innovative methods. Overcoming energy losses, and using the best techniques to minimise secondary contamination and improve adsorptivity, are critical. Here, we review the importance and influence of activating agents on agricultural waste: how they react and compare conventional and microwave processes. In particular, adsorbent pore characteristics, surface chemistry interactions and production modes were compared with traditional methods. It was concluded that there are no best activating agents; rather, each agent reacts uniquely with a precursor, and the optimum choice depends on the target adsorbent. Natural chemicals can also be as effective as inorganic activating agents, and offer the advantages that they are usually safe, and readily available. The use of a microwave, as an innovative pyrolysis approach, can enhance the activation process within a duration of 1–4 h and temperature of 500–1200 °C, after which the yield and efficiency decline rapidly due to molecular breakdown. This study also examines the biomass milling process requirements; the influence of the dielectric properties, along with the effect of washing; and experimental setup challenges. The microwave setup system, biomass feed rate, product delivery, inert gas flow rate, reactor design and recovery lines are all important factors in the microwave activation process, and contribute to the overall efficiency of AC preparation. However, a major issue is a lack of large-scale industrial demonstration units for microwave technology.

Summary Some intensive agricultural practices result in soil degradation through loss of soil organic matter. Organic farming may mitigate this problem, if managed properly, but may result in a yield penalty compared with conventional systems. Biochar addition to soil could influence both agricultural systems, but previous studies are not definitive about its impact on soil processes. Sandy soils are more susceptible to the effects of reduced soil organic matter on soil hydrology and nutrient dynamics. Nitrogen (N) is important for crop growth and soil water content can influence its transformation and cycling. This study explored the effect of biochar amendment on soil water retention and nitrification processes in soils under organic and conventional management. Carbon dioxide evolution was used as an indicator of related microbial activity. A water release curve study and a 60‐day incubation experiment were set up to consider the effect of biochar application on organically and conventionally managed sandy loam soils. The results showed that addition of biochar increased water retention for both soils and this is attributed to its porous structure. On incubation of an organically managed soil, with green‐waste compost, initial ammonium level was small, reflecting microbial demand for N. The large cation exchange capacity of the organically managed soil retained ammonium, reducing availability for nitrification. Carbon dioxide evolution increased with continuing small contents of ammonium and nitrate when biochar was added to the organically managed soil. Biochar enhanced nitrification without increased respiration during incubation of a conventionally managed soil with added mineral N; a possible explanation for this enhancement is the increase in pH resulting from the biochar addition.

Organomineral fertilisers (OMFs) were produced by coating biosolids granules with urea and potash. Two OMF formulations with N : P2O5 : K2O compositions: 10 : 4 : 4 (OMF10) and 15 : 4 : 4 (OMF15) were developed for application in grassland and arable crops. Routine fertiliser analyses were conducted on four batches of OMF and biosolids granules and compared with a sample of urea to determine key physical and chemical properties of the materials which affect handling and spreading, soil behaviour, and fertiliser value. Bulk and particle densities were in the range of 608 to 618 kg m−3, and 1297 to 1357 kg m−3, respectively. Compression tests showed that OMF particles undergo deformation followed by multiple failures without disintegration of the granules when vertical load was applied. Static particle strength was between 1.18 and 4.33 N mm−2depending on the particle diameter. The use of a model for fertiliser particle distribution studies showed that OMF granules should be between 1.10 and 5.50 mm in diameter with about 80% of the particles in the range of 2.25 to 4.40 mm to enable application at 18 m tramline spacing. This research utilises novel technology to improve the fertiliser value of biosolids, reduce disposal costs, and deliver a range of environmental benefits associated with recycling.

Biochar produced by pyrolysis of organic residues is increasingly used for soil amendment and many other applications. However, analytical methods for its physical and chemical characterization are yet far from being specifically adapted, optimized, and standardized. Therefore, COST Action TD1107 conducted an interlaboratory comparison in which 22 laboratories from 12 countries analyzed three different types of biochar for 38 physical-chemical parameters (macro- and microelements, heavy metals, polycyclic aromatic hydrocarbons, pH, electrical conductivity, and specific surface area) with their preferential methods. The data were evaluated in detail using professional interlaboratory testing software. Whereas intralaboratory repeatability was generally good or at least acceptable, interlaboratory reproducibility was mostly not (20% < mean reproducibility standard deviation < 460%). This paper contributes to better comparability of biochar data published already and provides recommendations to improve and harmonize specific methods for biochar analysis in the future.

Sewage sludge, a waste material commonly known as biosolids, has good potential as a valuable agricultural resource, providing that its nutrient imbalances could be overcome. Sewage sludge is rich in phosphorus but low in nitrogen and potassium. Technology exists to supplement sewage sludge with mineral fertilizers, such as urea and muriate of potash as sources of nitrogen and potassium, respectively, to produce an organo-mineral fertilizer with balanced crop nutrient requirements. Here, an experimental plot trial set up in 2008 was established at Broxton, Cheshire, UK, to compare crop yield response for typical crop varieties. Crops included wheat, oilseed rape, barley, beans and forage maize, treated with conventional fertilizer and organo-mineral fertilizer. The organomineral fertilizer is a nutrient-balanced sludge-based product produced by drying digested sewage sludge cake at 80°C in a tumbling evaporator, which produces sludge granules of 3-6 mm in diameter. Analysis was carried out on soil NPK and crop yield. N use efficiency was measured to assess N uptake. Results show that there is no significant difference in crop yield between treatments over the three trial years, with the exception of one crop. This finding demonstrates that the new organo-fertilizer is as efficient as conventional fertilizers. Moreover, levels of heavy metal in soil did not exceed permissible levels. The novelty of this research lies in the fact that it is the first field scale trial of a modified sewage sludge product that has the potential to transform a hitherto waste product into a practical fertilizer product. We conclude that the organo-mineral fertilizer is a promising alternative product for sustainable agriculture.

Peat is used as a high quality substrate for growing media in horticulture. However, unsustainable peat extraction damages peatland ecosystems, which disappeared to a large extent in Central and South Europe. Furthermore, disturbed peatlands are becoming a source of greenhouse gases due to drainage and excavation. This study is the result of a workshop within the EU COST Action TD1107 (Biochar as option for sustainable resource management), held in Tartu (Estonia) in 2015. The view of stakeholders were consulted on new biochar-based growing media and to what extent peat may be replaced in growing media by new compounds like carbonaceous materials from thermochemical conversion. First positive results from laboratory and greenhouse experiments have been reported with biochar content in growing media ranging up to 50%. Various companies have already started to use biochar as an additive in their growing media formulations. Biochar might play a more important role in replacing peat in growing media, when biochar is available, meets the quality requirements, and their use is economically feasible.

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Water scarcity severely affects drylands threatening their food security, whereas, the oil and gas industry produces significant and increasing volumes of produced water that could be partly reused for agricultural irrigation in these regions. In this review, we summarise recent research and provide a broad overview of the potential for oil and gas produced water to irrigate food crops in drylands. There is potentially sufficient water to irrigate about 130 000 ha/year of cropland in arid and semi-arid areas. The quality of produced water is often a limiting factor for the reuse in irrigation as it can lead to soil salinisation and sodification. Although the inappropriate use of produced water in irrigation could be damaging for the soil, the agricultural sector in dry areas is often prone to challenges in soil salinity. There is a lack of knowledge about the main environmental and economic conditions that could encourage or limit the development of irrigation with oil and gas effluents at the scale of drylands in the world. Cheaper treatment technologies in combination with farm-based salinity management techniques could make the reuse of produced water relevant to irrigate high value-crops in hyper-arid areas. This review paper approaches an aspect of the energy