Determination of Rate Constants of Organic Matter Degradation of an Intensified Constructed Wetland Treating Synthetic High Strength Wastewater

Abstract

Constructed wetlands (CWs) are identified as nature-based technology for treatment of wastewater (WW). This study focuses on a novel intensified horizontal sub-surface flow constructed wetland (HSSFCW) by integrating adsorption removal mechanisms into to purify high-strength industrial effluent. Coal bottom ash was used as the adsorbent in the intensified bed for chemical oxygen demand (COD) removal and gravel was used in the control bed. The aims of this paper were to determine and compare the COD rate constant using the Kickuth equation, the K-C model and the Convolution integral method and to assess the performance of the intensified bed and the control. Two HSSFCWs of dimensions of 3m x 0.5m x 0.5m were built and were started up at low COD concentration in the range of 107-130 mg/l. Thereafter, the systems were run using medium strength WW having COD concentration of 912-1563 mg/l for 18 weeks and then at higher COD concentration of 4801-5021mg/l over a period of 12 weeks. Samples from the inlet, within the bed and outlet were taken and analyzed for COD. An average COD removal efficiency of 89% was achieved in the intensified bed and 46% was obtained for the control bed. The COD rate constant was estimated by (1) fitting the observed progression of COD along the beds, (2) applying the Kickuth equation using the inlet & outlet COD values and (3) using the Convolution Integral Method integrating the residence time distribution (RTD) from a tracer test. The convolution integral was found to be most accurate method to determine the COD rate constant as non-ideal flow behaviour and degradation of COD along the beds were considered. Therefore, the rate constant obtained from the Convolution Integral method can be earnestly use for future design of the similar intensified HSSFCWs treating medium to high strength industrial wastewater.

Keywords: constructed wetland, adsorption, industrial effluent, rate constant, convolution integral