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Aquatic invasive weeds affect hydrological, ecological, and socio-economic activities on freshwater ecosystems. On the Lower Volta River (LVR) of Ghana, invasive aquatic weeds have been known to be nuisance to fishing, navigation, aquaculture, hydropower production and other agricultural practices in the area. While information on the spatial and temporal distribution of aquatic weeds would be beneficial in improving weed management and control measures on the river, such information is very scanty. Also, these aquatic weeds are also biomass resources, that can be transformed to bioenergy. Thus, this study evaluated the spatial and temporal variations of aquatic weeds on the Lower Volta River, and assessed their potential biomass for bioenergy production.Random Forest (RF) algorithm and Landsat images were used to map the distribution of the weeds in 1975, 2003, and 2020, respectively. Accuracy assessment results showed mean Overall Accuracy (OA) of 83.44% and mean User Accuracy (UA) of 79.24%. The results indicated that as of 1975, aquatic weeds covered only 1495 ha and appeared in some specific locations such as Kpong and Ada. However, by 2003, the weeds had spread to most parts of the river covering 5600 ha, which was an increase of approximately 4-fold within a period of 28 years. The area covered by the weeds, however declined by 1505 ha between 2003 and 2020. Thus, in 2020, water hyacinth covered about 36% of the aquatic weeds relative to 28% in 2003. The results showed that, the quantity of the water hyacinth biomass per unit area was 21.5 kg/m 2 . This result can also be used as the basis for resource assessment as well as determination of its viability for bioenergy production and strategies for its modern utilisation. The conversion of water hyacinth into bioenergy remains one of the best aquatic weed management strategies that must be adopted in LVR.

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Generally, full‐scale biogas plants require trace elements supplementation to sustain a stable process. The aim was to apply total reflection X‐ray fluorescence (TXRF) to monitor the dissolved, active elements, for the first time, at μg/L levels in biogas plants. The digester liquid is comparable to sludge with high solids content and high salt concentrations. The ions are usually measured in the dried fermenter slurry after acid digestion in its inactive bound form, although microorganisms can take up only dissolved ions as active agent. In this study, a procedure was developed to monitor dissolved ions in such environmental processes to prevent an overdose or underdose. The analytical figures of merit considered were the lowest possible limit of detection, the recovery of spiked ions and linearity. Samples were spiked with nickel, selenium and tungsten in the range of 10–200 μg/L. Highest recovery rates of >93% for concentrations of 10 to <100 μg/L were obtained after the samples were centrifugated twice, followed by application of microwave‐assisted acid digestion. The calculated lowest limit of detection was 2–4 μg/L. The developed method was first tested in laboratory‐scale biogas digesters with wheat straw as substrate. Successful application of TXRF was also achieved in a full‐scale biogas plant to estimate the dynamics of “active” trace element ions in the range of 2–1,000 μg/L for several months. Therefore, cost‐effective biogas plant management is possible through the application of TXRF spectrometry to monitor trace elements.

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