Bioaugmentation of cyanide-degrading microorganisms in a full-scale cokes wastewater treatment facility

Abstract

To enhance biological removal efficiency of total cyanides, bioaugmentation was applied to a full-scale cokes wastewaters treatment process. After a laboratorial-scale cultivation (up to 1.2 m³) of a cyanide-degrading yeast (Cryptococcus humicolus) and unidentified cyanide-degrading microorganisms, the microbial consortium was inoculated into a fluidized-bed type process (1280 m³), and then enriched for two months with a huge supply of glucose, KCN and other nutrients. Target wastewater was effluent of a biological pre-denitrification process for treating cokes wastewater, and contained about 14 mg/L of total cyanides in the form of ferric cyanide. This may be a first or rare report on the full-scale bioaugmentation of specialized-microorganisms. However, continuous operation of the full-scale cyanides-degrading bioprocess showed poor removal efficiency than expected owing to poor settling performance of microbial flocs, slow biodegradation rate of ferric cyanide and lack of organic carbon sources within the wastewater. Therefore, there is a need for further studies on how to solve these operating problems in full-scale bioaugmentation approach.

Introduction

Since cokes wastewater generated from coals coking process of steel industries contains various toxic compounds such as ammonia, thiocyanate, phenols and cyanides in high concentration range, it has been considered as the most toxic one to be treated before being discharged into environments (Li et al., 2003, Vázquez et al., 2006). Among various processes capable of treating cokes wastewater (Lee and Park, 1998, Ning et al., 2005, Yun et al., 1998), a biological pre-denitrification process has been preferred in Korea because of its simplicity and economic benefits (Kim et al., 2007).

The pre-denitrification process is a single-sludge system with recycle of nitrified effluent and consists of two distinct microbial reactions under anoxic followed by oxic conditions (Fig. 1). In anoxic condition, heterotrophic denitrifying bacteria convert nitrite and nitrate into nitrogen gas using organic carbon sources, thus most of organic pollutants are removed in this step. Additionally, very toxic free cyanide can be removed to some degree by anaerobes (Lewandowski, 1984). In oxic condition, autotrophic nitrifying bacteria convert ammonia into nitrite and nitrate, meanwhile autotrophic thiocyanate-degrading bacteria convert thiocyanate into ammonia, sulfate and bicarbonate (Paruchuri et al., 1990). These successive microbial reactions have completely removed most of toxic compounds within the cokes wastewater (Table 1). However, effluent from the pre-denitrification process has contained considerable amount of cyanide compounds (∼14 mg/L), which must be legally removed below 1 mg/L in Korea (Note that a post chemical treatment process has satisfactorily removed the residual cyanide compounds below the regulation level, but numerous costs has been paid to purchase chemical agents).

Bioaugmentation of specialized-microorganisms has been known to be a powerful tool to enhance the removal efficiency of recalcitrant and/or toxic compounds (Van Limbergen et al., 1998). The specialized-microorganisms include indigenous or allochthonous wild-types or genetically modified organisms. In spite of several successes of bioaugmentation in laboratorial-scale below 10 L (Head and Oleszkiewicz, 2004, Jianlong et al., 2002, Saravanane et al., 2001), its application to a full-scale wastewater treatment process has rarely been tried due to the risk of irrecoverable process failure by unexpected operating problems. For this reason, there are few or no reports on full-scale bioaugmentation of specialized-microorganisms (>1000 m³).

In this study, a cyanide-degrading yeast, Cryptococcus humicolus MCN2 (Kwon et al., 2002), and unidentified cyanide-degrading microorganisms were augmented in a full-scale cokes wastewater treatment facility to enhance removal efficiency of cyanide compounds.