Blum, Jan-Michael

Spildevandsrensningsanlæg er designet til at fjerne patogener og forurenende stoffer fra spildevand for at sikre hygiejne og beskytte vandmiljøet mod eutrofiering. Reaktivt kvælstof, hovedsageligt i form af ammonium, er en af bestanddelene i spildevand, der omdannes til frit kvælstof. I processen med delvis nitrifikation og efterfølgende anammox (PNA på engelsk) udnyttes kapaciteten af autotrofe aerobe og anaerobe ammonium oxiderende bakterier (AOB og AnAOB) til behandlinge af spildevandstrømme med høje ammonium koncentrationer og lave koncentrationer af organisk kulstof. I forhold til konventionel kvælstoffjernelse er denne proces fordelagtig pga. et lavere energibehov til beluftning, intet behov for tilsætning af organisk kulstof og reduceret slamproduktion. Emissioner af drivhusgassen lattergas (N2O), der ofte dannes under procesdrift, kan have stor betydning for CO2-fodaftrykket under PNA og dermed ophæve de miljømæssige fordele. Der er derfor et behov for forskellige strategier til at reducere N2O emissioner fra PNA. I denne Ph.D. afhandling undersøges et laboratorie-baseret PNA system for at opnå en PNA proces drift med høj aktivitet og samtidig lave N2O emissioner. Lattergas emissioner samt produktion og forbrug af næringssalte blev kvantificeret for at identificere sammenhænge mellem N2O og procesparametre. Endvidere blev en metode til at konstruere det mikrobielle samfund vha. luftningsmønstre testet, samt blev et tiltag til nedbringelse af N2O emission baseret på pH værdier vurderet. Konceptet ”Microbial Resource Management” (MRM) blev brugt til at styre fraktionen af AOB og AnAOB i biogranulater og til at manipulere aktiviteten af enzymer, der er relevante for N2O omsætningen. Både mikroorganismer og enzymer blev betragtet som mikrobielle ressourcer. Karakteriseringen af PNA systemet viste at netto N2O produktion i beluftede faser udgjorde det meste af den samlede N2O emission (80-100%). Det resterende N2O blev dannet i de faser, hvor der ingen beluftning var. Sidstnævnte førte til en stigning i N2O emissionen lige efter beluftningen blev startet igen. Der var en positiv sammenhæng imellem netto N2O produktionsrater og specifikke ammoniakfjernelsesrater under beluftning, mens netto N2O-produktionsrater var positivt korreleret med nitritkoncentrationen (NO2-) i de faser, hvor beluftningen var slukket. Styring af PNA proces ved reducerede specifikke ammoniakfjernelsesrater er derfor en mulig strategi til at reducere frigivelsen af N2O. Hvis høje ammoniumfjernelsesrater skal opretholdes har en forøget NO2- reduktionskapacitet, igennem en relativt større brøkdel af AnAOB, potentialet til at reducere N2O produktionen. Ændringer af det intermitterende beluftningsregime blev anvendt til at konstruere det mikrobielle samfund. Faktisk var det med længere beluftningsfaser muligt at forøge størrelsen af bio-granulaterne med ca. en faktor 2, hvilket resulterede i en stigning i forholdet imellem AnAOB og AOB fra 0,4 op til 1,4. En effekt af median granulestørrelse og granulstørrelsesfordeling på procesydelse var imidlertid ikke påviselig, da periodiske faser uden beluftning havde en stærkere regulerende virkning på NO2--akkumulering end granulstørrelsesfordelingen. Således var granulat-størrelsesfordelingen mindre relevant for stabil procesydeevne i PNA systemer med intermitterende beluftning end det er i systemer med konstant beluftning Da netto N2O produktion er forskellen mellem N2O produktion og reduktion, vil højere N2O reduktionsrater i forhold til N2O produktion reducere netto N2O produktionen. Efter en omfattende opsummering af relevante publikationer, der sammenfatter den nuværende viden om effekten af pH på enzymer involveret i omdannelser af kvælstofforbindelser blev endnu et tiltag til reduktion af N2O emission udarbejdet. Under en test af den udarbejdede teori viste det sig at pH havde en stærk virkning på N2O emissioner, idet emissionsfaktoren (%N2O-Nproduceret/NH4+-Nfjernet) steg fra 2,5% ved pH 6,5 til 3,3% ved pH 7,0 og 7,5, før den igen faldt væsentligt til 1,6% ved pH 8,0. Årsagen til de lave N2O produktionsrater ved basisk pH kan skyldes de høje N2O-reduktionsrater ved hjælp af enzymet N2O-reduktase, som bliver relevant ved pH &gt; 7,5. Baseret på de opnåede resultater i ph.d. afhandlingen, foreslås en styring med forholdsvis lange beluftningsfaser, lave specifikke ammoniakfjernelsesrater og en pH på 8,0 som effektive handlingsplaner til at nedbringe frigivelsen af N2O fra PNA processen.Urban wastewater treatment plants are designed to remove pathogens and pollutants from wastewater in order to provide sanitation and to protect receiving water bodies from eutrophication. Reactive nitrogen, mainly in the form of ammonium, is one of the components in wastewater that is converted to dinitrogen gas during treatment. The Partial Nitritation-Anammox process (PNA) uses the capacity of autotrophic aerobic and anaerobic ammonia oxidizing bacteria (AOB and AnAOB) to perform this task. The process is mainly applied to treat ammonium-rich wastewater streams with low concentrations of organic carbon. Its advantages over conventional nitrogen removal technology include lower energy needs for aeration, no addition of organic carbon and reduced sludge production. However, emissions of the greenhouse gas nitrous oxide (N2O), that are commonly measured during process operation, have the potential to determine the CO2 footprint of PNA, thus off-setting its environmental benefits. Consequently, strategies to mitigate N2O emissions from PNA are needed. In the presented Ph.D. study a laboratory scale PNA model system was investigated to infer beneficial modes of process operation for high rate PNA with simultaneously low N2O emissions. Dynamics of N2O emission, production and consumption reactions were quantified to identify correlations between N2O and process parameters, a method to engineer the microbial community by aeration patterns was tested and a N2O mitigation strategy based on pH set-points was evaluated. The concept of Microbial Resource Management (MRM) was adopted to steer the fraction of AOB and AnAOB in bio-granules and to manipulate the activity of enzymes relevant for N2O metabolism. Both, microorganisms and enzymes were considered as microbial resources. The characterization of the system revealed that net N2O production in aerated phases primarily contributed to overall N2O emissions (80-100%). The remaining N2O was produced during non-aerated phases, leading to distinct N2O emission peaks at the onset of aerated phases. Net N2O production rates in aerated phases were positively related with the specific ammonia removal rate, while during non-aerated phases net N2O production rates were positively correlated with the nitrite concentration (NO2-). Operation of PNA at reduced specific ammonia removal rates is, therefore, a feasible strategy to mitigate N2O emissions. However, when high ammonium removal rates shall be maintained, an increased NO2- sink capacity by a relatively larger fraction of AnAOB has the potential to reduce N2O production. Changes of the intermittent aeration regime were applied to engineer the microbial community. Indeed, longer aerated phases were feasible to increase the size of bio-granules by approx. two-fold, which resulted in an increase of the AnAOB/AOB ratio from 0.4 up to 1.4. However, an effect of median granule size and granule size distribution on process performance was not detectable, since periodic non-aerated phases in intermittently aerated PNA had stronger regulating effects on NO2- accumulation, than the granule size distribution had. Consequently, the granule size distribution was less relevant for stable process performance in intermittently aerated PNA systems, than for continuously aerated systems. Net N2O production is the difference between N2O production and consumption. Therefore, increased N2O consumption rates reduce net N2O production, also when production is not altered. A comprehensive review, synthesizing current knowledge on the effect of pH on enzymes involved in nitrogen conversions, suggested a N2O mitigation strategy based on pH set-points. Indeed, pH exerted a strong effect on N2O emissions, when emission factors (% N2O-Nproduced/NH4+-Nremoved) increased with pH from 2.5% at pH 6.5 to 3.3% at pH 7.0 and 7.5, before substantially decreasing to 1.6% at pH 8.0. The cause for low net N2O production rates at alkaline pH was assigned to high N2O reduction rates by the enzyme nitrous oxide reductase, which became relevant at pH &gt; 7.5. Based on the results obtained in the Ph.D. study, operation of Partial Nitritation-Anammox at relatively long aerated phases, low specific ammonia removal rates and pH 8.0 is suggested to reduce N2O emissions from the process

Online Research Database In Technology

Microbial resource management for the mitigation of nitrous oxide emissions from the Partial Nitritation- Anammox process

Microbial resource management for the mitigation of nitrous oxide emissions from the Partial NitritationAnammox process

http://orbit.dtu.dk/ws/files/149384078/Thesis_online_version_Jan_M._Blum.pdf

Microbial resource management for the mitigation of nitrous oxide emissions from the Partial Nitritation- Anammox process

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Online Research Database In Technology

Online Research Database In Technology