Nitrogen removal in saline wastewater by an activated sludge process with eross-flow filtration

Çarpraz akışlı filtrasyon kullanan aktif çamur sistemleri ile tuzlu atıksulardan azot giderimi

Abstract

Natural gas production is generally based on the purification of methane from hydrocarbons (ethane, propane, butane, pentane), H2S, CO2, helium and nitrogen after drilling from the gas well reservoirs and during the purification process of raw natural gas, wastewater containing high concentration of ammonium nitrogen and salinity is generated. The objective of this study was to investigate the effect of salinity on the nitrification and denitrification processes. Within this context the optimum conditions were determined for complete nitrogen removal in saline wastewater by an activated sludge process with cross-flow filtration . In recent years, membrane bioreactors have been widely used for nutrient removal. Biomass seperation membrane bioreactors can be operated at low hydraulic retention times and long sludge ages without the problem of biomass washout, which is very common in activated sludge systems. They enable complete solids removal from effluent, have a capability of high loading, produce low/zero sludge, effectively can be operated without sludge bulking problem. In this study, a bench scale continuous feeding activated sludge process with cross-flow filtration was used. Polysulfon ulta-filter with nominal molecular weight cut-off value of 200000 Da and filtration area of 35 cm2 was used for biomass seperation and nitrification-denitrification was performed with intermittent aeration in the reactor vessel. The gas well wastewater containing 3% NaCl similar to marine water and 200 mg/l NH4-N was seeded with activated sludge provided from a fish processing plant. The operational conditions of the reactor was set up as 6 hours intermittent cycle, 3 hours of anoxic and 3 hours of aeration with 10 minutes of feeding at the beginning of the anoxic period and the effluent was withdrawn during the aeration period and the system was run with NH4-N loading rate of 0.1 g/l-day, and methanol loading rate of 0.19 g/l-day in the starting phase. Nitrification and denitrification performance in highly saline wastewater was determined by carrying out batch tests. Nitrification rate was gradually increased from 94 mg N/l.day to 370 mg N/l.day and stable nitrification was obtained up to 70th day of the study. The performance of the reactor on the 50th day was studied and it was observed that nitrate concentration during nitrification period increased to a stable value within 90 minutes therefore the intermittent cycle was decreased to 4 hours; 2 hours anoxic and 2 hours aeration, which lead to an increase in flow and in nitrogen loading rate to 750 ml/day and 0.15 g/l.day respectively. In the first phase of this study, methanol was added in theoretical value of stoichiometric equation of denitrification but it was observed that this value was not enough for complete denitrification. Thus addition of methanol was increased to 1.5 times of its theoretical value. The batch denitrification tests showed that methanol as an external carbon source worked efficiently whereas denitrification performance of the original activated sludge was not good with methanol. Nitrogen removal efficiency increased from 53% to 90% and denitrification rate increased from 22 mg/l.day to 254 mg/l.day on the 37th day and reached to 414 mg/l.day on the 57th day of the study due to the addition of methanol. So it was concluded that halophilic (salt-loving) microorganisms became dominant within the acclimation to saline environment. Batch nitrification tests also proved the acclimation of the activated sludge to highly saline wastewater, as the nitrification rates were better than that of the activated sludge in low salinity wastewater. In literature, it was emphasized that denitrifiers possessed a higher tolerance capability and better adaptation to high salinity when it is compared with the nitrifiers which was also observed in this study that nitrification process was more sensitive to salinity than denitrification. In the last phase of this study, the nitrogen load was increased from 0.1 to 0.15 g/ l.day and the sludge was removed daily with a sludge age of 40 days. It was observed that in this period the mixed liquor concentrations became stable with a higher ratio of VSS/SS and the removal efficiency of total nitrogen reached to 95%. Consequently, it was demonstrated that the complete nitrogen removal in highly saline wastewater can be accomplished with cross-flow filtration due to the capability of operating at high nitrogen loading and longer sludge ages.

Özet

Gaz rezervlerinden sondajlama işlemi ile çıkarılan ham doğal gaz, çeşitli hidrokarbonlar (etan, propan, butan, pentan), su buharı, H2S, CO2, helyum, azot gibi bileşenleri içermektedir. Ham doğal gazın bu bileşenlerden arındırılarak saf metanın elde edilmesi işlemi sonucunda yüksek konsantrasyonda amonyum azotu ve tuzluluk içeren atıksu oluşturmaktadır. Çalışma kapsamında, doğal gazın üretimi sonucunda oluşan atıksuda, tuzluluğun nitrifikasyon ve denitrifikasyon proseslerine etkisinin araştırılması hedeflenmiştir. Bu amaçla, çalışmada, laboratuvar ölçekli sürekli beslenen çapraz akış filtrasyonlu aktif çamur prosesi kullanılmıştır. Özellikle son yıllarda, membran biyoreaktörlerin nutrient gideriminde kullanılması ile ilgili çalışmalar ve uygulamalar gün geçtikçe artmaktadır. Membran biyoreaktörler, düşük hidrolik bekletme sürelerinde ve uzun çamur yaşlarında, aktif çamur sistemlerinde sıkça rastlanan biyokütle yıkanması problemine neden olmadan işletilebilmektedir. Yüksek yükleme potansiyeline sahip olması, sistemden atılacak atık çamurun minimum olması, kabarma problemi olmadan yüksek verimde işletilebilmesi başlıca avantajlarıdır. Bu çalışmada aşırı tuzlu atıksudan, çapraz akışlı filtrasyon kullanan aktif çamur sistemleri ile toplam azotun yüksek verimde giderilebileceği gösterilmiştir. Membran biyoreaktörlerin uzun çamur yaşlarında ve yüksek biyokütle konsantrasyonlarında işletilebilmesi sayesinde aktif çamur, aşırı tuzlu atıksuya alıştırılarak halofilik (tuz seven) mikroorganizmalar ortamda baskın hale gelebilmiştir. Bunun sonucunda da tuzluluğun azot giderme verimi üzerine olumsuz etkisi gözlemlenmemiştir. Çalışma kapsamında, toplam azot giderimi için optimum koşullar, çevrim içi analizler ve kesikli deneylerle belirlenerek toplam azot giderimi % 95 verimle sağlanmıştır.