Harnessing Packed-bed Biofilm Reactors for High-rate Saline Nitrification: An Innovative Application for Intensive Recirculating Aquaculture Systems

Abstract

Saline effluents containing ammonia are generated by various human activities, including aquaculture, food processing, chemical industries, landfilling, etc. necessitating treatment to mitigate environmental harm. The growing demand for freshwater has led to increased stress and scarcity, prompting the exploration of alternative water sources like seawater. However, this also results in the generation of ammonia-rich saline effluents. Biological nitrification is a crucial process for nitrogen removal, but high salinity levels can inhibit nitrification, reducing treatment efficiency. Packed bed biofilm reactors (PBBRs) have demonstrated potential for high-rate nitrification in diverse wastewater streams. However, limited research has been conducted on the impact of salinity on nitrification performance in PBBRs, highlighting the need for further investigation to elucidate their behaviour in saline matrices. This thesis focuses on assessing and optimizing high-rate PBBRs for efficient saline wastewater nitrification. The study investigates the effects of different operating parameters (water up-flow velocity & temperature) and salinity-adaptation strategies on nitrification performance and microbial community dynamics, aiming to develop a robust and efficient saline nitrification process. Additionally, the research examines the potential for integration of high-rate nitrifying PBBRs into recirculating aquaculture system (RAS), providing a promising solution for a globally important sector.

Specifically, this thesis studied nitrification using two distinct configurations of PBBRs: -

1. Configuration I – standalone high-rate nitrifying PBBR systems Objective 1: To investigate the impact of water up-flow velocity and the effect of temperature on nitrification rate in PBBRs The relationship between up-flow water velocity and nitrification rates was investigated using five PBBR systems with varying up-flow velocities (1, 5, 10, 15, and 20 m/h) and incrementally increasing Ammonia Loading Rates (ALR) from 200 to 2000 g N/m³·d. Further, the impact of temperature (10 °C to 28 °C) on nitrification in PBBR at a fixed ALR of 1600 g N/m³·d was examined, recognizing the critical role of temperature in biological nitrification processes. Objective 2: To assess gradual salinity incrementation strategy for salt-acclimatisation in high-rate nitrifying PBBRs After understanding the effects of up-flow velocity and temperature, the study evaluated the impact of salinity on nitrification performance of freshwater biofilms, introducing stepwise increases in salinity up to seawater levels (35 ‰) at a fixed ALR of 1600 g N/m³·d. Objective 3: To evaluate salinity priming for facilitating robust & efficient high-rate saline nitrification in PBRRs Further, the work investigated the role of salinity priming for enabling saline nitrification in PBBRs, priming the biofilms at 10 ‰ and 20 ‰ and then assessing the impact on nitrification performance as salinity was gradually increased in steps up to 50 ‰, at a fixed ALR of 1600 g N/m³·d.

2. Configuration II – PBBRs in lab-scale simulated RAS This part of the thesis centres on RAS, which requires compact, robust and efficient nitrifying reactors. Objective 4: To investigate, optimize, & integrate PBBR for high-rate nitrification in RAS, using simulated freshwater recirculating aquaculture system. This objective involved a comprehensive investigation for process optimization through system integration, reactor sizing, reactor & system dynamics, and biofilm development. The study consisted of three distinct parts: A. Integrating PBBRs into RAS: preliminary study was conducted using a simulated lab-scale setup to evaluate the performance of PBBRs in RAS. The reactor was loaded with stepwise increases in ALR up to 800 g N/(m³·d). B. Investigating the effect of Exchange Rate on Nitrification Dynamics: the impact of exchange rate on nitrification dynamics in PBBRs was examined with stepwise increases in ALR from 0 to 1900 g N/m³·d. The experiment was conducted in two phases: Phase I with a maximum ALR of 800 g N/m³·d using a 3.98 L PBBR, and Phase II with a maximum ALR of 1900 g N/m³·d using a 1.91 L PBBR. C. Efficient Nitrification in RAS, Rates and Kinetics Analysis: the rates and kinetics of nitrification in PBBRs was analysed with varying biofilm enrichment periods and realistic ammonia generation rates. ALR was increased based on standard fish growth pattern over two 90-day cycles, with biocarriers enriched for different durations (21 days and 35 days). Objective 5: To investigate gradual salinity incrementation and salt-acclimated biofilms for high-rate and efficient nitrification in simulated seawater recirculating aquaculture system. This objective explored nitrification in saline environments for RAS in a lab-scale study conducted in two distinct phases: i) Phase I: ALR was increased based on fish growth cycle, with initial salinity at 5‰, until ALR reached a maximum of 1900 g N/m³·d. Subsequently, salinity was increased stepwise up to 35‰ (maintaining ALR of 1900 g N/m³·d) to simulate seawater conditions. ii) Phase II: Using a salt-acclimated PBBR, ALR was increased based on fish growth maintaining salinity at 35‰, simulating the conditions of a marine RAS. It was found that up-flow velocity had no significant impact on nitrification rates for ammonia loading rates up to 2000 g N∙m-3∙d-1, and PBBRs achieved complete nitrification across a temperature range of 15-28 °C. Both non-primed and primed systems demonstrated saline nitrification capabilities at salinities of 35 and 45 ‰, with nitrification rates of approximately 1600 g N∙m-3∙d-1. However, non-primed reactors exhibited transient nitrification inhibition and shifts in biofilm community composition between 20-35 ‰ salinity, whereas priming enhanced system robustness, maintaining stable effluent ammonia levels (<1 mg N/L) and microbial communities up to 45 ‰ salinity. Furthermore, the study successfully integrated PBBRs into simulated freshwater and seawater intensive lab-scale RAS with stocking densities up to 48 kg/m3, achieving nitrification rates of approximately 1900 g N∙m-3∙d-1 and maintaining acceptable ammonia and nitrite concentrations in the culture tank. By exploring the application of packed bed biofilm reactors for high-rate nitrification under different salinity conditions and in recirculating aquaculture systems, this study provides valuable knowledge on designing, operating, and optimizing these systems for efficient and sustainable saline wastewater nitrification. The findings have important implications for promoting sustainable aquaculture practices and reducing the environmental impacts of saline ammonia-rich effluents.