Modelling of longterm simultaneous nitrification and denitrification performance of a pilot scale membrane bioreactor

Membran biyoreaktörlerin nütrient giderim kapasitesi son yıllarda bu sistemlerdeki karbon gideriminden daha çok dikkat çekmiştir. Membran biyoreaktörlerde toplam azot giderimi için konvansiyonel olarak bilinen proseslerin yanında eşzamanlı nitrifikasyon ve denitrifikasyon (SNdN) gelecekte en çok araştırma gerektiren prosestir. SNdN’in tüm etkisini ve esaslarını tanımlamak amacıyla pilot bir membran biyoreaktör teşkil edilmiştir. Bu çalışma, membran biyoreaktör sistemlerinin azot gideriminde etkili olan ana mekanizmalarının ve özellikle SNdN’in etkisinin değerlendirilmesi üzerine odaklanmıştır. Bu kapsamda yeni bir matematik model geliştirilerek model sonuçlarının ölçülen değerlerle uyumluluğu üzerinden kalibrasyon gerçekleştirilmiştir. Önerilen bu matematik modelde ilk olarak membran biyoreaktörler esas olarak biyokütlenin tamamının sistem içinde tutulduğu ve geri devrettirildiği ve bu sayede yüksek biyokütle  konsantrasyonlarının sağlandığı askıda çalışan bir aktif çamur prosesi olarak tanımlanmıştır. Bu kapsamda, AÇM1 (Aktif Çamur Modeli 1) baz olmak üzere içsel solunum fazı yaklaşımına dönüştürülmüş yeni bir aktif çamur modeli geliştirilerek ölçüm sonuçlarının dinamik olarak modellenmesi ve kalibrasyonu yapılmıştır. Yüksek biyokütle konsantrasyonlarında difüzyonun sistem performansı ve denitrifikasyon üzerine olan önemli etkisi ortamdaki oksijen konsantrasyonuna bağlı olarak nitrifikasyon ve denitrifikasyonu kontrol eden anahtar fonksiyonları sayesinde başarı ile açıklanabilmiştir. Geliştirilen model sonuçları, ölçülen sonuçlar ile oldukça yüksek bir uyumluluk göstermiştir. Bu uyum da modelin güvenilirliğini teyit etmiştir. Membran tank içinde oksijenin flok içine olan difüzyon limitasyonu, anahtar fonksiyonlarında yer alan yarı doygunluk sabitlerine spesifik değerlerin verilmesiyle açıklanabilmiştir. Söz konusu yarı doygunluk sabitleri KOH=1 mg/L, KOA=1.25mg/L, KNH=2mg/L ve KNO=2 mg/L olmak üzere önceki aktif çamur modellerine göre oldukça yüksek değerler olarak belirlenmiştir. Anahtar Kelimeler: Eşzamanlı nitrifikasyon ve denitrifikasyon, difüzyon limitasyonu, azot giderimi, çözünmüş oksijen, oksijen yarı doygunluk sabiti, membran biyoreaktör.Membrane bioreactors (MBR) are simply described as a membrane filtration process where the activated sludge flocs are filtered through a porous medium with a cutoff size of 0.1-0.4µm. Complete retention of sludge is maintained within the system whereas suspended solids are usually not detectable on the permeate side. Membranes are either submerged within the activated sludge reactor or configured externally in a separate tank. Membrane bioreactors provide superb treatment efficiency in terms of organic and nutrient removal. Although the organic removal efficiency of membrane bioreactors is very well known; nitrogen removal and especially simultaneous nitrification and denitrification capacity of membrane bioreactors still remains to be explored. Simultaneous nitrification and denitrification (SNdN) brings the advantage of high nitrogen removal efficiencies due to increased denitrification potential. A better understanding of the biological reactions occuring in a membrane bioreactor will provide an optimized design and energy consumption which in turn will promote the faster commercialization of the membrane bioreactors. This study focuses on two initiatives and objectives: i. to identify the true potential of membrane bioreactors for nitrogen removal with simultaneous nitrification and denitrification thereby deriving the denitrification potential of the system with respect to available carbon and dissolved oxygen (for longterm conditions) ii. to minimize and to avoid irreversible fouling to a maximum time extent by implementing a different operational mode from the ones conducted in previous studies. For the above mentioned objectives, a pilot scale membrane bioreactor based on microfiltration technology was setup at a domestic sewage treatment plant. The plant was fed with municipal wastewater. Flatsheet membranes with a cutoff size of 0.4mm were used. Total membrane area was 8 m2 and the operating flux was constant at 0.5 m3/m2-day. The membrane module was supplied from Kubota Inc., Japan. The system setup for nitrogen removal was based on predenitrification. Coarse bubble aeration was used to supply oxygen for the microorganisms to carry out the biological reactions where it also simultaneously provided the necessary scouring effect on the membrane surface. System was biologically modelled for all runs using BioWin 2.2 software from Envirosim Associates Ltd. whilst physical modelling was done by GPS-X from Hydromantis Inc. Physical-chemical methods were used to determine the influent readily biodegradable COD(SS) and the influent inert COD(SI). The modified model characterized the measured effluent COD efficiently. The effluent total COD only consisted of inert soluble fraction (SI) and inert soluble microbial products (SSMP). Under low dissolved oxygen (DO), the system yielded high and stable nitrogen removal performance in spite of oscillating COD/TKN ratio in the influent. Considering that the average influent TKN and ammonia concentrations were 45-60 mgN/L and 30-45 mgN/L respectively, the level of denitrification in the MBR tank which is oxic was observed to be approximately 20-30 mg/L. The level of denitrification is related to the denitrification potential, NDP within the MBR reactor which is solely dependant on available COD. However in membrane bioreactors, it can be stated that the level of dissolved oxygen inside the MBR also plays an important role in defining denitrification potential. The level of SNdN occuring in the membrane bioreactor suggests that this level of diffusion limitation is so high that it is even causing the anoxic fraction of biomass inside the floc to be dominant during high DO levels. It can be concluded that during high DO levels, this fraction of biomass shifts from being anoxic to aerobic decreasing the level of SNdN. The oxygen diffusion limitation from the bulk liquid into the flocs can be explained by assigning specific values to half saturation constants in the corresponding switching functions namely KOH=1 mg/L, KOA=1.25 mg/L , KNH=2 mg/L and KNO=2mg/L which are much higher than the values adopted to previous models. It can be concluded that the factors and parameters triggering SNdN in MBR can be listed as i.dissolved oxygen concentration, ii. floc size, iii. MLSS concentration of the bulk liquid which the latter two severely affects diffusion limitation of oxygen from the bulk liquid into the floc. The SNdN was also very sensitive to the hD factor which governed the degree of denitrification occuring during anoxic decay. Keywords: simultaneous nitrification and denitrification, diffusion limitation, nitrogen removal, dissolved oxygen, oxygen half saturation constant, membrane bioreactor