Removal of nutrient and heavy metal loads from sewage effluent using vetiver grass, <i>Chrysopogon zizanioides</i> (L.) Roberty

Abstract

Urban cities in many developing nations face the challenge of relieving the pressure exerted on overloaded sewage treatment works. Apart from limited financial capacity, complementary treatment methods like phyto-extraction of pollutants from the effluent have not been fully exploited, particularly in southern Africa. A study was conducted in Harare, Zimbabwe, to assess the potential of vetiver grass, Chrysopogon zizanioides, in removing N, P, Zn, Mn and Ni loads in sewage effluent from primary clarification, under hydroponic conditions. Vetiver grass was grown from tillers on floating trays suspended in effluent, and the total concentrations of selected parameters were monitored in both effluent and vetiver grass at 0, 7, 14 and 21 days of retention in effluent. Higher pollutant reduction (62–100%) in effluent, with respect to all parameters, under vetiver grass compared to no vetiver treatment (9–27%), was recorded by Day 21. Effluent pH remained stable at 7.44–7.64. Smaller changes were found for N (9.8 from 27.5 mg∙ℓ^–1) and P (2.0 from 5.3 mg∙ℓ^–1) than for the heavy metals. Vetiver grass dry biomass accumulated at 3.8–4.7 g∙tiller^–1∙week^–1, while heavy metal extraction (up to 6.2 mg Zn, 3.3 mg Mn and 0.06 mg Ni tiller^–1) by root uptake increased with time at an exponential rate (R², 0.73–0.83). The study established that, while root uptake was a significant feature of the clean-up process, the corresponding high reduction of heavy metals in effluent suggests multiple removal mechanisms, including bio-sorption by microorganisms producing an immobilised microbial biomass on the container wall. A deliberate lowering of effluent pH may increase root uptake thereby possibly reducing the clean-up time and improving effluent quality. There is potential for application of this technology in cities struggling with the cost of conventional sewage treatment.

Keywords: bio-accumulation, bio-sorption, heavy metals, hydroponic, nitrogen, phosphorus