Production of sulphides in denitrifying woodchip bioreactors
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RESEARCH ARTICLE
Production of sulphides in denitrifying woodchip bioreactors Jitka Malá 1
&
Karel Hrich 1 & Kateřina Schrimpelová 1 & Zuzana Bílková 1
Received: 19 April 2020 / Accepted: 9 July 2020 # Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract Denitrifying woodchip bioreactors, natural treatment systems used for the reduction of nitrates in agricultural runoff or groundwater, may cause adverse side effects within receiving waters. One of the least studied but nonetheless still serious issues is the production of hydrogen sulphide, which occurs in bioreactors under operating conditions favourable to its creation. The aim of this paper is to elucidate the effect of process parameters on the production of sulphides and the proportion of hydrogen sulphide in a 1-year-long experimental study with four laboratory-scale denitrifying bioreactors. During the study, the strong dependence of sulphate reduction and the production of sulphides on the effluent oxidation-reduction potential (ORP) and nitrate-nitrogen (NO3-N) concentrations of bioreactors became evident. Sulphide formation occurred at concurrent effluent NO3-N concentrations below 3 mg/L and ORPs lower than − 100 mV. The tested hydraulic retention time of 1.7 days was sufficiently long to achieve these conditions. At an effluent pH of 7 or lower, the majority of the total sulphides present were in the form of hydrogen sulphide. It is suggested that in order to avoid the production of hydrogen sulphide, the production of total sulphides has to be minimised. Keywords Agricultural runoff . Denitrifying bioreactor . Groundwater . Hydrogen sulphide . Pollution swapping . Sulphate reduction
Introduction Denitrifying woodchip bioreactors are simple applications used for nitrate reduction in agricultural runoffs or groundwater. They take the form of a container or trench filled with woodchips. Nitrate-rich water flows through the organic material, which releases bioavailable organic carbon and so promotes heterotrophic denitrification, a process that converts nitrates into nitrogen gases (N2 and N2O), which are released into the atmosphere (Schipper et al. 2010; Christianson and Schipper 2016). The anoxic conditions prevailing in denitrifying bioreactors, together with a supply of sulphate, can lead to sulphide production (Martínez-Santos et al. 2018; Azhar et al. 2014), which may cause serious problems particularly due to the Responsible Editor: Ta Yeong Wu * Jitka Malá [email protected] 1
Brno University of Technology, Faculty of Civil Engineering, Institute of Chemistry, Žižkova 17, 602 00 Brno, Czech Republic
toxicity and odour of its undissociated form, i.e. hydrogen sulphide (Powell and Arp 1989). The ratio of hydrogen sulphide to the total amount of sulphides grows undesirably with decreasing pH and/or increasing temperature. At a temperature of 25 °C, hydrogen sulphide is created if pH drops below 9; if pH is below 7, hydrogen sulphide comprises a major part of total sulphides (Stumm and Morgan 1996). Sulphates are the predomina
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