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Users of biochar in the field require this product to reliably meet its declared specifications.For the first time, this work investigated, whether these specifications could be reproducibly obtained as a sole function of the thermal history of the biomass feedstock during slow pyrolysis, irrespective of the type and scale of the production unit. Using volatile matter content as a proxy for a wider set of biochar quality parameters, biochar from units at scales from grams to hundreds of kilograms, representing three main types of slow pyrolysis units (fixed bed, screw reactor and rotary kiln) were investigated. For the first time we showed that comparable biochar could be produced by these very different pyrolysis units, with good reproducibility within individual as well as among separate production runs.

Phosphorus is a water pollutant of concern around the world as it limits the productivity of most freshwater systems which can undergo eutrophication under high phosphorus inputs. The importance of treating stormwater as part of an integrated phosphorus pollution management plan is now recognized. Bioretention systems are urban stormwater best management practices (BMPs) that rely on terrestrial ecosystem functions to retain storm flows and reduce pollutant loads. Bioretention has shown great potential for stormwater quantity and quality control. However, phosphorus removal has been inconsistent in bioretention systems, with phosphorus leaching observed in some systems. Numerical models can be used to predict the performance of bioretention systems under various conditions and loadings. The aim of this paper is to identify and characterize bioretention phosphorus cycling processes, with a particular focus on process modelling. Both soluble and particulate phosphorus forms are expected in significant proportions in bioretention system inflows. Sorption mechanisms are expected to dominate soluble phosphorus cycling, while particulate phosphorus transport occurs mainly through sedimentation. Vegetative uptake, mineralization, and immobilization are also known to play a role in the cycling of phosphorus; however, data is lacking to assess their importance. There is a need for simple mathematical equations to represent dissolution and precipitation reactions in bioretention systems. More research is also needed to characterize the rates of colloidal capture and mobilization within soils. Finally, approaches used to model phosphorus transport in systems similar to bioretention are not applicable to bioretention system modelling. This reinforces the need for the development of a bioretention phosphorus transport model.

Bioretention systems are designed to treat stormwater and provide attenuated drainage between storms. Bioretention has shown great potential at reducing the volume and improving the quality of stormwater. This study introduces the bioretention hydrologic model (BHM), a one-dimensional model that simulates the hydrologic response of a bioretention system over the duration of a storm event. BHM is based on the RECARGA model, but has been adapted for improved accuracy and integration of pollutant transport models. BHM contains four completely-mixed layers and accounts for evapotranspiration, overflow, exfiltration to native soils and underdrain discharge. Model results were evaluated against field data collected over 10 storm events. Simulated flows were particularly sensitive to antecedent water content and drainage parameters of bioretention soils, which were calibrated through an optimisation algorithm. Temporal disparity was observed between simulated and measured flows, which was attributed to preferential flow paths formed within the soil matrix of the field system. Modelling results suggest that soil water storage is the most important short-term hydrologic process in bioretention, with exfiltration having the potential to be significant in native soils with sufficient permeability.

No abstract

Bioretention is a relatively new stormwater management practice that relies on physical, chemical and biological processes within a terrestrial ecosystem to provide stormwater retention and treatment. Bioretention systems, also referred to as rain gardens, include a layer of high permeability soil filtration media, covered by an optional thin layer of mulch, and planted with woody and herbaceous plants. In areas with low permeability native soils, an underdrain structure is installed below the soil filtration media to prevent water from standing for excessive periods of time. An overflow structure is also incorporated into the system to drain excess water when the ponding capacity of the system is exceeded. Field monitoring and laboratory testing performed to date have demonstrated the ability of bioretention systems to significantly decrease runoff flows and to efficiently reduce a number of pollutant loads. However, large discrepancies in phosphorus removal efficiencies have been reported from the field monitoring of bioretention systems. Two bioretention cells on the University of Maryland campus, monitored by Davis (2007), achieved 79% and 77% total phosphorus mean mass removals, respectively, over 12 storm events. Conversely, Hunt et al. (2006) noted an increase of 240% in total phosphorus, on a mass basis, in the outflow of a bioretention cell in North Carolina over a 12-month monitoring period.