Characterization of rice husk and cork as waste materials and its integration in a WETWALL system

Abstract

Wastewater effluents from soilless crop cultivation contain high loads of nutrients and are poor in organic carbon, what makes its purification with conventional treatment methods challenging and expensive. Constructed Wetlands (CWs) have been proven to be a promising technology for the treatment of hydroponic wastewaters, as nutrients can be removed mainly by adsorption, precipitation and biological denitrification/nitrification processes. However, the provision of sufficient organic carbon to maintain a continuous nutrient removal by microorganisms is still a key challenge. Moreover, there is a lack of research on the potential of organic substrates that provide appropriate adsorption capacities and can serve simultaneously as a source of carbon matter for denitrification. The aim of the study is the investigation of untreated rice husk as possible substrate in CWs. In particular, the provision of organic carbon and its removal potential for nutrients out of synthetic hydroponic wastewater is going to be examined in greater detail. Therefore, an initial characterization of rice husk is carried out, performing a Fourier-transform-infrared-spectroscopy (FTIR) and determining the point of zero charge (pzc) by immersion technique. Subsequently, batch studies were performed to examine the influence of pH and adsorbent dosage (Ad) on the release of organic carbon and the removal of nutrients. The study shows that the rice husk surface contains mainly silica groups. In addition, it is a promising material in the liberation of organic matter. An Ad of 100 g/L and a pH of 10 are found to be the optimum conditions to release 508 mg/L of organic carbon after 24 hours. Moreover, batch studies reveal its capability in the removal of nitrates. Under pH 4 and with an Ad of 100 g/L, a nitrate removal of 55% is observed.