Enhanced ammonia nitrogen removal using consistent biological regeneration and ammonium exchange of zeolite in modified SBR process
Introduction
High concentration of organics and ammonia nitrogen is commonly present in industrial wastewaters such as tannery, textile, landfill leachate, and fertilizer wastewater. Biological processes for nitrogen removal is very important for wastewater treatment because discharge of nitrogen into surface water results in oxygen depletion and algae bloom. In order to enhance the nitrogen removal from ammonium-rich wastewater, several studies on pretreatment such as air stripping and chemical precipitation with magnesium ammonium phosphate have been conducted. These processes, however, require complicated configuration and have difficulties in maintenance due to the scale formation [1], [2], [3].
Zeolite is a well-known material for its ability to preferentially remove ammonium ions from wastewater. Unlike synthetic ion exchange resins, zeolite is known to possess a higher selective ion-exchange capability for ammonium ion than Ca2+ and Mg2+, even when the concentration of the latter is higher than the former [4]. Natural zeolite was mainly used to remove ammonium ions from secondary effluent by selective ion exchange [5], but it was rarely tested for the wastewater of high ammonium nitrogen level, due to the chemical regeneration cost of the used zeolite [6].
Several researchers have developed hybrid biological-ion exchange systems, using the zeolite as ion exchange material [7], [8]. Green et al. [9], [10] recently presented a dual mode process consisting of ion exchange and bioregeneration mode in a single reactor using zeolites for ammonium removal, followed by bioregeneration. In addition to the ammonium removal step with bioregeneration, however, a denitrification step should be provided for complete nitrogen removal from nitrogen stream generated from bioregeneration.
Jung et al. [11], [12] conducted studies on the bioregeneration and ammonium exchange capacity of the bio-flocculated zeolite, having powdered zeolite added to a sequencing batch reactor. However, they could not enhance overall nitrogen removal efficiency in a zeolite-added SBR. Therefore, modified zeo-SBR is recommended for nitrogen removal process that has a special function of consistent ammonium exchange capacity and bioregeneration of the bio-flocculated zeolite as described in Jung et al. [11], [12].
The objectives of this study were to test whether a modified zeo-SBR process could enhance the nitrogen removal more than control and zeo-SBR reactors could, by using the role of powdered zeolite in a sequencing batch reactor.
Section snippets
Principle of bioregeneration
The equilibrium exchange reaction between the NH4+ ion in the solution and the Na+ ion attached to the zeolite can be expressed as stoichiometric reaction:where Z=zeolite.
The selectivity of various ions is dependent on the size, electron, and electric structure of cations. Once the zeolite is saturated with ammonium, chemical regeneration is possible when the large amount of sodium chloride is added into the solution. As soon as the ammonium-saturated zeolite is placed in
Experimental apparatus
Three sets of sequencing batch reactors, control, zeo-SBR, and modified zeo-SBR were tested to assess nitrogen removal efficiency. 240 mg zeolite/L was daily added to zeo-SBRs. The powdered zeolite passed through a No. 100 sieve (0.15 mm). As shown in Table 1, true density and specific surface area of the powdered zeolite were 1.73 g/mL and 49.6 m2/g, respectively. The working volume of each reactor was 2.5 L. The anoxic condition was maintained with a mechanical mixer, while mixing and aeration
Mechanism and roles of the zeolite in the SBR
The roles of powdered zeolite in a sequencing batch reactor are summarized in Fig. 3 as described in our previous work [11], [12]. As the powdered zeolite was added to the SBR, microorganisms that were attached on the surface of zeolite or were entrapped the powdered zeolite particles formed zeolite-floc. These zeolite-flocs enhanced the settleability of microbial floc because of the higher specific gravity of zeolite. This zeolite-floc adsorbed ammonium nitrogen during the anoxic-fill phase.
Conclusions
Zeolite- floc in the sequencing batch reactor had the ammonium adsorption capacity of 6.0–7.4 mg NH4+-N/g FSS.
Control reactor with the sequential denitrification and nitrification during the anoxic-fill and aeration-mixing phases, respectively, showed a 70% of T-N removal at 33% decanting volume for one cycle (influent ammonia nitrogen=300 mg/L).
Zeo-SBR reactor represented the same nitrogen removal as the control reactor, but zeolite-folc in the reactor showed the ammonium adsorption and
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