Heavy Metal Phytoextraction in Sewage Sludge using Sunflower

Abstract

Agricultural application of sewage sludge is an effective disposal method as it is beneficial to agricultural productivity. However, there is need to regularly monitor levels of heavy metals in sludge. Such monitoring is currently lacking in our sewage treatment plants-thereby, leading to informal use of sewage sludge in agriculture and lack of quality control. Furthermore, there is an absence of locally available technologies for heavy metal removal. Conventional processes for heavy metal removal such as chemical precipitation and membrane filtration are too expensive, require technologically advanced systems, are difficult to maintain, require a lot of expertise and are therefore not locally accessible. Heavy metals have adverse effects on human life when consumed. There is therefore need to come up with affordable, innovative technologies that can be locally used to remove heavy metals from sewage sludge used in agriculture. This study used phytoextraction, a phytoremediation process in which certain plants have the ability to absorb toxic contaminants from a soil matrix, to remove heavy metals from sewage sludge. The objective was to investigate the potential of heavy metal phytoextraction in sewage sludge using sunflower. The sewage sludge was obtained from Dandora and Kariobangi Wastewater Treatment Plants. The experimental set up was in three sets. The first containing soil and Kariobangi sewage sludge mix in the ratio 1:1. The second containing soil and Dandora sewage sludge mix in the ratio 1:1. The third set contained 100% soil which served as the control experiment. Sunflowers were grown in each of the sets and heavy metal levels were monitored in the plant roots, shoots and soil sludge mixes for a period of four months using atomic absorption spectroscopy. After the four months, cadmium levels in the sewage sludge were reduced by 84%, manganese by 91%, copper by 85%, lead by 89% and zinc by 84%. The stated heavy metals were all brought down to levels acceptable for garden soil. This proved that sunflower phytoextraction is a technology that can be assimilated in wastewater treatment plants to ensure safe agricultural use of sewage sludge. However, this should be done with keen analysis so as to avoid depleting the sludge of useful nutrients as the heavy metals are absorbed. Hence, further study should be done to determine the appropriate density of sunflower plants that should be planted on a given quantity of sewage sludge during the phytoextraction process.