Production and removal of soluble organic nitrogen by nitrifying biofilm

Highlights

• Soluble organic nitrogen was produced in a nitrifying moving bed biofilm reactor.

• Highest soluble organic nitrogen degradation was observed at the lowest C/N tested.

• Soluble organic nitrogen (sON) cycled via production and ammonification.

• Bioavailability of organic C and ammonia level regulated sON activities.

Abstract

This study investigated soluble organic nitrogen (sON) activity in batch reactors mimicking nitrifying moving bed biofilm reactors (MBBRs). This work was divided into two objectives focusing on the production and degradation of sON, respectively. For the first objective, a synthetic feed that did not contribute any organic nitrogen was used and results indicated that irrespective of the presence of influent organic carbon (0 versus 400 mg COD/L) in the reactors, sON was contributed by the biofilm during nitrification. Although net production of sON was observed, both production and ammonification coexisted which regulated the sON concentration. For the second objective, actual wastewater was fed to the reactors to investigate sON degradation under different carbon to nitrogen (C/N) ratios. A higher concentration of sON was biodegraded in the reactor when fed with influent containing a lower C/N ratio. Overall results suggested that organic carbon bioavailability and/or ammonia concentration influenced the production and ammonification of sON. This study is the first to explore the sON activity by MBBR biofilm and findings from this work could extend the knowledge on the fixed film process with respect to sON activity to regulate and optimize reactor operation in meeting stringent total nitrogen discharge limits.

Introduction

Recent guidelines for discharging total nitrogen (TN) are approaching ≤ 5 mg TN/L for several parts of the United States. These guidelines aim to curb the hypoxic conditions and eutrophication issues in vulnerable receiving water bodies. With advancements in science and technology, water resource recovery facilities (WRRFs) are capable of removing > 95% of inorganic nitrogen resulting in soluble organic nitrogen (sON) being a major nitrogen fraction (> 50%) of the effluent TN [1]. Several studies have described that about 60–70% of the total influent sON is removed by activated sludge process (ASP) [2], [3], [4] while Simsek et al. [5] found that 37–50% of the influent sON is biodegraded by a trickling filter system. The majority of the research work related to sON degradation has focused on conventional ASP [6], [7], [8], [9] while few studies have touched on the fixed film processes, mainly on trickling filter and post-denitrification filters (DNF) [5], [10], [11].

At WRRFs, moving bed biofilm reactors (MBBRs) are employed usually as a separate stage nitrification process (to nitrify wastewater with a lower carbon to nitrogen (C/N) ratio). Considering the consequences of elevated fraction of sON in the effluent (complication with permit compliance and impairment of receiving water quality), it will be reasonable to identify the available strategies in an MBBR process to control the concentration of sON while avoiding the need for (additional) advanced removal technologies. Simsek et al. [10] investigated the fate of biodegradable sON (bsON) and bioavailable sON (AbsON) in a full-scale WRRF consisting of both ASP and MBBR. The biodegradable fraction of sON or bsON can be biochemically oxidized by bacteria to produce ammonia N [12] whereas the bioavailable fraction of sON or AbsON can be uptaken by algae or other aquatic plant species for growth [13], [14], [15]. Nitrogen cycling can be influenced by the form of sON in the effluent. For instance, dissolved free amino acids can be directly uptaken by (bioavailable to) the algae; however, other forms of sON might have to be first hydrolyzed and/or mineralized (biodegraded) by bacteria making them bioavailable to the algae or other phytoplanktons in the receiving waters [10], [16]. Simsek et al. [10] concluded that ASP removed 29% of sON whereas MBBR removed only 4% of sON. The authors suggested that a low C/N ratio, solubilization of particulate organics from the biofilm, and/or release of soluble microbial products (SMPs) might have affected the sON removal in the MBBR process [10].

Hu, Liao, Geng et al. [11] investigated the effect of different C/N ratios (3, 4, 5 and 6) on the removal of sON and AbsON in DNFs. They fed secondary effluent to the filters and noticed the maximum effluent sON at C/N ratio of 3 (1.91 mg sON/L) and no impact on effluent sON for higher C/N ratios i.e., 4 (1.70 mg sON/L), 5 (1.70 mg sON/L) and 6 (1.69 mg sON/L). However, effluent AbsON decreased with increasing C/N ratio suggesting that sON produced by DNFs at higher C/N ratios will be less bioavailable, a scenario favorable for the receiving waters [11]. The studies of Simsek et al. [10] and Hu, Liao, Geng et al. [11] indicated that relatively less sON removal should be expected under a lower C/N ratio. However, no study has explicitly investigated the removal of sON in an MBBR process under different C/N ratios.

Effluent sON from biological treatment processes is primarily from influent- and process-derived sources. The influent-derived sON is the result of recalcitrant organic nitrogen, which is not biodegraded or removed during wastewater treatment [16]. Process-derived sON is released by metabolic activities associated with biological processes (e.g., SMPs and extracellular polymeric substances) [7], [14], [17], [18]. Since process-derived sON is contributed by the growth and decay of microorganisms during the biological treatment processes, process-derived sON is unavoidable and more closely related to operational parameters than influent-derived sON [15]. Approximately 33% of the effluent sON are process-derived while the rest of it is from the influent [19], [20]. However, the extent of the biological production of sON varies from one biological system to another [6]. Therefore, reducing the formation of process-derived sON in biological treatment processes will be beneficial in achieving the low TN discharge limits and eventually safeguard the water bodies receiving treated wastewater. Parkin and McCarty [21] investigated the influence of organic loading (glucose, acetate, glucose-acetate mixture) on sON production and found that an increase in organic loading increased sON production in ASP [21]. Although there have been several studies investigating the effect of organic loading (measured as chemical oxygen demand (COD)) on nitrification in an MBBR process [22], [23], [24], [25], no study has investigated the effect of organic loading on the production of sON by biofilm particularly those in an MBBR.

Simsek et al. [10] reported sON removal of 29% by ASP and 4% by MBBR (for nitrification) in a full-scale WRRF. Their study suggested that in an MBBR, lower ammonification of sON occurred due to less ability of ammonifying bacteria to compete for oxygen compared to nitrifiers [10]. Ammonification is a major pathway for sON degradation and is considered to be achieved primarily by heterotrophs and phytoplanktons [26]. Ammonia produced from ammonification is transformed via nitrification and/or assimilated by biomass. Since nitrifiers are primarily autotrophs, they are not believed to be directly associated with sON degradation [10], [27]. Hence, sON degradation is largely considered to be a heterotrophic bacterial process.

Studies reported that while heterotrophic processes remove higher fraction of sON, reduction in sON concentration was also observed after nitrification stages at full-scale WRRFs highlighting the involvement of nitrifiers in sON biodegradation [5], [10]. Wadhawan et al. [28] reported 57% of sON removal through the nitrification process in secondary effluent and lesser removal through the heterotrophic process (38%). The nitrification process biodegraded higher concentration of sON compared to the heterotrophic process. The study also claimed that during the nitrification, ammonia oxidizing bacteria rather than nitrite oxidizing bacteria were responsible for sON degradation and it is the first study that reported the involvement of nitrification in sON degradation. Based on the results from these previous studies [5], [10], [28], this study aimed at exploring the production of sON and the effect of C/N ratios on sON degradation during nitrification in a MBBR.

The objective of this study was to identify the influence of organic loading and different C/N ratios on sON activity (production and removal) in bench-scale reactors that mimic the nitrification process of MBBRs. Specifically, this study examined the effect of readily biodegradable COD on the production of sON by feeding synthetic wastewater with no organic nitrogen. The study also investigated the effect of different C/N ratios on sON degradation for which the reactors received real wastewater samples representing different C/N ratios. Results from the work could extend our knowledge on the fixed film process with respect to sON activity to regulate and optimize reactor operation in order to achieve low TN discharge limits.