Survey of salinity effect on performance of denitrification beds resulted of soil and sugarcane bagasse for nitrate -- oval

Nitrate is a common ion in nitrogen cycle. Nitrate can reduced to nitrite, nitrogen and other forms through microbial activities. In recent decades, high nitrate concentrations in drinking water have been became a serious concern in the world. Nitrate distribution in the groundwater is controlled through hydraulic parameters, concentration of dissolved oxygen and availability of carbon source as electron donors. There are some methods for nitrogen removal such as biological processes, chemical processes, and Biological denitrification. Biological denitrification is the process by which nitrate is reduced to nitrogen gas, through a sequence of enzymatic reactions. Changing methods of use or increase the absorption of nitrate nitrogen in agricultural systems has been known as the best way to reduce pollution of groundwater nitrate to prevent the transmission of groundwater in the first place. Denitrification beds have been known as the most common method for nitrate removal with high efficiency. In this study, salinity effect on performance of denitrification beds, constructed of mix of soil and sugarcane bagasse, was surveyed. This study was done in the research laboratory of Shahid Chamran University, Ahvaz, Iran. A physical model consists of 9 polyethylene columns with 350 mm length and 76.2 mm diameter was constructed. A 5-cm layer of fine sand was placed at the top and bottom of the soil columns to ensure that the inlets and outlets of the columns did not clog. Three replications were made for each treatment. The columns were fed via an upward flow by falling head. An upward flow was used to ensure the entire pore volumes of the columns were filled. Three treatments with different salinity levels, 2, 5 and 8 ds m-1, were considered. Water samples were collected from the effluent of the columns in clean plastic containers during the 14-week experiment, and were then subjected to nitrate, ammonium, and pH measurements. Sampling of influent and effluent was done in the beginning of experiments, daily for one week. After one week, sampling was limited to once in a week and finally once in every two weeks. The nitrate and ammonium concentrations were analyzed immediately in the samples by using spectrophotometer (DR5000, Hach, USA) at wave lengths of 275 and 425 nm, respectively. For all samples, the outflow discharge rates were conducted through collecting of a given volume of water in a specific time. Duration of experiments was 98 days. Entrance constant head of 102 cm were used. Fourier transform infrared spectroscopy (FTIR) (Bruker model, Germany) was employed according to the potassium bromide (KBr). The changes in functional groups of sugarcane bagasse, before and after use in biofilters, were examined by FTIR spectra. Also, the external morphology of the biomass was studied by scanning electron microscopy (SEM, 1455VP, LEO, Germany). All statistical analyses were performed using SPSS 21 software. Correlation analysis was done for determining correlation kind (positive or negative) and the values of significant. All significant testing was at the 95 % confidence level. Reported error values were ±1 standard deviation. The results of this research were shown that nitrate removal efficiency reduced with increasing of salinity. The removal efficiency was 83.87, 78.72 and 63 %, respectively in the salinity levels of 2, 5 and 8 ds m-1. But, there was a threshold for salinity level. Moreover, in the constant hydraulic loading rate, the lowest nitrate removal rate was belonged to the highest salinity level. Also, it was be observed that the values of C:N reduced with increasing of salinity. The results of correlation analyzes between the effective parameters on performance of beds indicated which hydraulic loading rate, hydraulic retention time and salinity has significant effect on removal efficiency. Also the results showed that, hydraulic factors such as hydraulic retention time and hydraulic loading rate have positive and negative impacts on performance of bioreactor beds, respectively. The SEM images showed considerable differences between sugarcane bagasse images before and after experiment. The results of FTIR and SEM analyze approved degradation of sugarcane bagasse during the experiments through activities of bacteria available in beds. In the higher salinity level, carbon source was consumed faster and more than other lower salinity levels. The FTIR and SEM results confirmed reduction of C:N ratio in the highest salinity level than other levels.