Study On Coupled Attenuation Of Nitrogen And Phosphorous Removal Using Dissimilatory Reduction In Activated Sludge

Microbial dissimilatory Fe（Ⅲ） reduction is an important biogeochemical process innonsulfidogenic anoxic environments, yet iron-reducing bacteria plays principal roles in thisprocsses. Iron-reducing bacteria might also be the principal microbial groups whichwidespread existed in sedimentary and subsurface environments. It is now known that theircommon feature is the ability to take advantage of Fe（Ⅲ） as the terminal electron acceptor,capabling of reducing Fe（Ⅲ） while fermenting electron donor and obataining energy forgrowth. Microbial dissimilatory Fe（Ⅲ） reduction has several siginficant environmentalimpacts, especially coupled to the oxidation of organic refractory organics （such aschlorinated organic compounds, azo dyes, petrochemical organic compounds, etc.） andmineralization of toxic heavy metal ions （uranium, chromium, arsenic, etc.）, as well as itsinfluences on the participation in the nitrogen and phosphorus cycle in nature. Therefore, ithas attracted widespread concern to environmental protection workers at home and abroad inrecent years. In addition, iron-reducing bacteria are the normal members of the microbialcommunity in activated sludge and that the numbers accouted for3%of the total amount ofactivated sludge microorganisms. Hence, this study showed significance in strengthening themicrobial dissimilatory Fe（Ⅲ）-reducing ability, improving the degradation efficiency ofrefractory organics and perfecting the traditional treatment process of nitrogen andphosphorus removal.In the present study, we have investigated the dissimilatory Fe（Ⅲ） reduction process bydosing different Fe（Ⅲ） sources at constant temperature under facultative/strictly anaerobicconditions in enrichment cultures obtained from microcosms previously inoculated withactivated sludge, and the nitrogen and phosphorus removal efficiency were also elucidated.Specifically, the influencing factors and mechanism of dissimilatory Fe（Ⅲ） red uction coupledto denitrification processes from the perspective of thermodynamics and kinetics analysiswere focused. And the Fe（Ⅲ）-reducting efficiency of Fe⁰passive film was also studied,which provided the theoretical basis for regeneration in situ of carrier in the the biologicalsponge iron system.The mian results showed as follows:（1） Fe（Ⅲ）-reducing ability of different Fe（Ⅲ） forms showed obvious diversity, whichmore easier to take advantage of non-crystalline and soluble iron with large specific surfacearea （Fe（OH）₃）, and the amorphous iron oxide（such as iron oxide scale） was usually easier tobe utilited rather than the stable crystal structure of iron oxide（such as green ore, red ore）. Itwas noteworthy that the dissimilatory Fe（Ⅲ） reduction process coupled to obviousphenomenon of nitrogen and phosphorus removal, and the nitrogen and phosphorus removalefficiency was positively correlated to Fe（Ⅲ）-reducing ability.（2） The ability of dissimilatory Fe（Ⅲ） reduction and the phosphorous removal efficiencyunder different conditions were compared. The results showed there was consistency in the Fe（Ⅲ） reduction ability of different conditions, followed by Fe（OH）₃＞iron oxide scale＞green ore＞red ore, but strictly anaerobic showed better. While the phosphorus removalefficiency was positively related to produced Fe（Ⅱ） concentration. At the7d of enrichmentculture, the phosphate concentration of Fe（OH）₃and iron oxide scale systems decreasedbelow2mg/L, and ultimately to0.5mg/L. Bonding mechanism of phosphorous removal usingdissimilatory Fe（Ⅲ） reduction, its main role of the precipitation removal by iron-reducingbacteria rather than surface adsorption of Fe（Ⅲ） source was proved.（3） The results showed there was great consistency between ability of Fe（Ⅲ） reductionand effects of nitrogen and phosphorous removal, yet facultative anaerobic conditions showedbetter. However, NH₄⁺-N and TN showed a removal hysteresis. The denitrification capacityunder different influent loading was positive correlated with Fe（Ⅱ） production concentrationwithing no initial nitrite additon. The accumulated Fe（Ⅱ） concentration showed successivelyincreased with the improvement of influent loads, and the maximum nitrogen removal ratewas also increases gradually, especially the system of high ammonia concentration, whichwas the highest level in all systems, that the Fe（Ⅱ） production rate was28.63mg/（Lï¹'d） at theperiod time of30d. At the same time, NH₄⁺-N and TN removal rate were reached up to90.61%and90.15%respectively. In addition, nitrite and nitrate producted transientaccumulation, which might be elucidated that NH₄⁺oxidation was coupled to Fe（Ⅲ）reduction under anaerobic conditons.（4） The NH₄⁺oxidation coupled to Fe（Ⅲ） reduction, anaerobic ammonia oxidation andnitrate-dependent Fe（Ⅱ） oxidation under iron reducing anaerobic conditions arethermodynamically feasible and the Eh/pH of vast majority systems were located in Fe（Ⅱ）region of Pourbaix diagram. The effluent concentration of NH₄⁺-N and TN in differentreaction systems, with the relationship between the logarithmic values and reaction timesfitted by the first-order decay kinetics mode with a correlation coefficient, Adj.R-Square, inrange of0.744to0.971. The study also indicated that the whole interaction mechanismsinvolved in activated sludge can be divided into three sub-processes, there are microbialreduction of Fe（Ⅲ） solution coupling to NH₄⁺oxidation, anaerobic ammonium oxidationprocess and nritrate-dependent Fe（Ⅱ） oxidation. Furthermore, microbial redox processesbased on iron-reducing bacteria is potentially a critical component of the N cycle in activatedsludge.（5） Fe⁰passivation film can be well utilited by iron-reducing bacteria. Fe（Ⅱ） productionrate under facultative and strict anaerobic conditions were reached up to88.67mg/（Lï¹'d） and98.69mg/（Lï¹'d） respectively, even were the441%and487%of iron oxide scale system. Thephosphate removal efficiency eventually reached94.4%and97.1%, nitrogen removal showeda removal hysteresis, NH₄⁺-N removal rate were78.1%and86.6%, while TN removal efficiency reached68.0%and76.1%respectively. The results investigated that the Fe⁰passivation film in the biological sponge iron system was easily illustrated by thedissimilatory Fe（Ⅲ）-reducing process.Cheap, easy to get and environment-friendly materials, such as iron oxide scales wasadopted as the sole electron acccptor of iron-reducing bacteria. The effects and mechanism ofactivated sludge iron reduction coupled nitrogen and phosphorus removal were alsoinvestigated, which might provide new ideas for traditional treatment process of nitrogen andphosphorus removal.