Nutrient removal capacity of wood residues for the Agro-environmental safety of ground and surface waters

Abstract

The aim of this study was to determine the effectiveness of wood residues in the removal of nutrients (ammonium-N; NH4-N) from nutrient-rich(NH4-N) waters. The water holding capacity of the wood materials was also determined. Carried out at RothamstedResearch,North Wyke, UK, this controlled laboratory experiment tested two wood residues; in length, one being 1-2cm and the other from 150 μm (microns)to 9.5mm. Although awide range of studies have shown theeffectiveness and performance of various absorbent materials as animal beddings, such as straw (cereal straw), woodchip (sawdust, bark or wood shavings), bracken and rushes, only few have focusedon the NH4-N sorption/desorption capacity.The depuration capacity of wood residues from nutrient-rich effluents such as those fromcattle bedded on woodchip orstraw will be controlled by processes such as sorption (adsorption-absorption) and desorption of nutrients. Studies have reported the nitrogen removal capacity of woodchip materials and biochar from woodchip as well as removal of NH4+-N from domestic and municipal wastewater, farm dirty water, landfill and industry effluents. These studies have observed that the mechanism ofremoval of nitrogen isby either increasing NO3--N removal form leachate by enhancing N2O losses via denitrification (biochar as carbon source for denitrifiers) or by decreasing NH4+-N in leachate through adsorption to negatively charged sites. Results showed that although thecation exchange capacity (CEC) and surface area (SA) are both fundamental properties of adsorbent materials, no correlation was found with CEC and adsorption or desorption. Nor did changes in pHappear to be sufficiently important to cause changes in CEC.For this reason, osmotic pressure appeared to be a more predominant parameter controlling processes of adsorption and desorption of NH4+-N in both wood residues. Thus, wood residues high in NH4+-N should be avoided, as theycould have an opposite effect in the adsorption of nutrients from nutrient-rich effluents.The results also showed that somewood residues(G30) hadgreat capacity to adsorb NH4+-N to levels up to nearly 90% whilst demonstrating low desorption capacity of NH4+-N(less than 1%). These are ideal relevant features foran adsorbent material for the removal of nutrients (or heavy metals) from contaminated waters such us farmo industrial effluents, orfor the depuration of eutrophic watercourses. This could help reduce the concentration of farm effluents making them more manageable, subsequently contributing towards the compliance of new environmental regulations.