Effects of free cyanide on microbial communities and biological carbon and nitrogen removal performance in the industrial activated sludge process

Abstract

The changes in process performance and microbial communities under free cyanide (CN⁻) were investigated in a lab-scale activated sludge process treating industrial wastewater. The performance of phenol degradation did not appear to be adversely affected by increases in CN⁻ concentrations. In contrast, CN⁻ was found to have an inhibitory effect on SCN⁻ biodegradation, resulting in the increase of TOC and COD concentrations. Nitratation also appeared to be inhibited at CN⁻ concentrations in excess of 1.0 mg/L, confirming that nitrite-oxidizing bacteria (NOB) is more sensitive to the CN⁻ toxicity than ammonia oxidizing bacteria (AOB). After CN⁻ loads were stopped, SCN⁻ removal, denitrification, and nitrification inhibited by CN⁻ were recovered to performance efficiency of more than 98%. The AOB and NOB communities in the aerobic reactor were analyzed by terminal restriction fragment length (T-RFLP) and quantitative real-time PCR (qPCR). Nitrosomonas europaea lineage was the predominant AOB at all samples during the operation, but an obvious change was observed in the diversity of AOB at the shock loading of 30 and 50 mg/L CN⁻, resulting in Nitrosospira sp. becoming dominant. We also observed coexisting Nitrospira and Nitrobacter genera for NOB. The increase of CN⁻ loading seemed to change the balance between Nitrospira and Nitrobacter, resulting in the high dominance of Nitrobacter over Nitrospira. Meanwhile, through using the qPCR, it was observed that the nitrite-reducing functional genes (i.e., nirS) were dominant in the activated sludge of the anoxic reactor, regardless of CN⁻ loads.

Introduction

Biological nitrification coupled to denitrification is commonly used in wastewater treatment plants to simultaneously remove carbon and nitrogen. Nitrification is achieved through the aerobic oxidation of ammonium (NH₄⁺) or ammonia (NH₃) into nitrite (NO₂⁻) by ammonia oxidizing bacteria (AOB), often Nitrosomonas spp., and followed by the oxidation of the nitrite (NO₂⁻) into nitrate (NO₃⁻) by nitrite-oxidizing bacteria (NOB), often Nitrobacter spp. The former is called nitritation and the latter nitratation. Denitrification consists of consecutive reaction steps in which nitrate is reduced to dinitrogen gas by denitrifying bacteria using the organic matter of wastewater under anoxic conditions: the reduction of nitrate via nitrite and nitric oxide to nitrous oxide or dinitrogen gas (Zumft, 1997).

Although the biological processes treating industrial wastewater are efficient and reliable, they may be susceptible to disturbances and toxic loadings (Juliastuti et al., 2003, Mertoglu et al., 2008, Kim et al., 2009). In particular, the presence of cyanide in industrial wastewaters from mining, coke or steel industries leads to severe problems for biological wastewater treatment (Wild et al., 1994). It is known that cyanide inhibits nitrification and denitrification in activated sludge systems (Kelly et al., 2004, Kim et al., 2008a), with only 1–2 mg/L of free cyanide in the form of HCN or CN⁻ being more toxic to biological processes than thiocyanate and metal–cyanide complexes (Park and Ely, 2008). Although the inhibitory effects of free cyanide on microbial reactions have been investigated by many researchers, most of them have focused on the threshold levels of free cyanide, at which pure or mixed cultures can tolerate without any inhibition (Kim et al., 2008b, Neufeld et al., 1986, Park and Ely, 2008). A few researchers have investigated the process performances on free cyanide (Lewandowski, 1984, Richards and Shieh, 1989). However, little is known about the microbial populations of nitrifiers and denitrifiers in an activated sludge system treating industrial wastewater containing free cyanide. Therefore, this study aimed to investigate bacterial populations relevant to nitrification and denitrification processes, particularly those in an activated sludge system treating wastewater from a coke plant under free cyanide shock loading. Diversity surveys, and the relationship of the bacterial abundance and activity to the overall processing conditions, may lead to an understanding of the basis of the process instability under free cyanide shock loading.