Towards the integration of oxidative and reductive activities: application to nitrogen removal by co-immobilized microorganisms

Background

Complete degradation of many pollutants requires sequenced anaerobic-aerobic biotreatment steps. Many compounds that are difficult to degrade aerobically are readily biotransformed anaerobically. The products of anaerobic biotransformation, however, will frequently resist to further mineralization; yet, they will be good substrates for aerobic biodegradation. Examples of this are the sequential biodegradation of highly chlorinated aromatics and aliphatics, azo-dyes, TNT, inorganic nitrogen compounds (NH ₄⁺ , NO ₂^- and NO ₃^- ) and pesticides such as DDT, HCH's or methoxychlor. In waste-and groundwater treatment, these sequenced biotransformations are commonly achieved either by using aerobic and anaerobic (anoxic) reactors in series or by alternating periods of aerobiosis and anaerobiosis in a treatment unit. Ideally, however, these biodegradation processes would take place in a single, compact continuous-reactor system under carefully controlled conditions. In many instances, the benefits of such an integrated system would be clearly greater than the mere sum of the advantages of each individual process.

Magic beads: an advanced engineering concept for process integration

This work addresses the possibilities of integrating oxidative and reductive complementary biodegradation processes in compact systems by using co-immobilized mixed-culture systems. The central idea throughout the book is that aerobic and anaerobic niches will eventually develop and coexist within a single biocatalytic particle so that oxidative and reductive activities (e.g nitrification and denitrification, respectively) can be accomplished simultaneously (Magic-beads). Therefore, multiple-step complementary biodegradation and biotransformation processes could be conducted as single staged (Figure 1). The rationale behind this idea relies on sound experimental evidence (e.g. time-dependent measurements of oxygen gradients across biocatalyst particles or biofilms) that shows that such niches do indeed establish under aerobic process conditions.

Figure 1 - Schematic representation of the "Magic-bead Concept".

Case study: Integrated nitrification and denitrification

The potential of the general concept outlined above for combining oxidative and reductive processes with relevance to the biodegadation of recalcitrant compounds is assessed in this work by studying in detail coupled nitrification and denitrification within (double-layered) gel beads for high-rate removal of nitrogen from wastewaters. In such beads, the nitrifying microorganisms (aerobes) immobilized in an outer layer would oxidize ammonium into nitrite that would then diffuse inwards, where immobilized denitrifiers (either facultative heterotrophs or obligate anaerobic ammonium oxidizers) would reduce this nitrite into the harmless gaseous nitrogen. The biocatalyst particle is used optimally because both the external layers and core are active. The beads are placed in a common airlift reactor through which the waste streams can flow at almost any rate, without the need of recirculation to or from any anoxic compartment or reactor.

Aims of the dissertation and outline

This research project aimed at a) the development and characterization of a coupled system for integrated nitrogen removal, b) understanding the mechanisms underlying the processes involved and; c) providing knowledge for the integration of oxidative and reductive activities in a single compact system. With these aims in mind, the stepwise strategy depicted in Figure 2 was developed. Every stage of the project was comprised by a series of self-contained studies addressing different aspects of the proposed system.

Figure 2 - Structured contents of this dissertation.

In the first stage (chapter 2) the scope of the problem was defined (apparent incompatibility of oxidative and reductive activities of environmental relevance), the needs were addressed (urge to integrate processes and reduce reactor size) and the state-of- the-art of the field (conventional systems and emerging technologies) was presented. In the following phase (chapters 3 to 6), a compact process was proposed and the procedures for biocatalyst production, and its characterization and mechanical stability were assessed. In the next stage (chapter 7 to 9), achievement of in-depth insight into the system's behavior was pursued by means of mathematical modeling and concomitant experimental validation using specific microelectrodes. The knowledge gathered up to this point was subsequently used successfully for the design of a fully autotrophic system for nitrogen removal (chapters 10 and 11). Finally, the possibilities of integrating other oxidative and reductive complementary biodegradation processes in compact systems by using co-immobilized mixed-culture systems were discussed in Chapter 12.