Nitrification and sludge characteristics in a submerged membrane bioreactor on synthetic inorganic wastewater

A submerged membrane bioreactor (SMBR) treating ammonia-bearing synthetic inorganic wastewater without sludge purge was studied in respect to nitrification and microbial parameters over a period of 210 days. The reactor + was operated at a hydraulic retention time (HRT) of 24 h, and the NH4 -N volumetric loading rate increased from 0.18 + !3 !1 + to 1.30 kg NH4 -N m d by increasing influent NH4 -N from 180 mg/l!1 to 1300 mg/l!1. With the exception of a short + period after the failure of pH control, the NH4 -N+ removal rate was constantly above 99%. Due to the influent characteristics and the interception of the membrane module, the mixed liquor suspended solids (MLSS) varied from 3000 to 5000 mg/l!1 and the nitrifying bacteria were dominant in the SMBR. The numbers of ammonia oxidizers and nitrite oxidizers in the mixed liquor increased from 0.9×108 ml!1 and 1.0×108 ml!1 to 1.6×109 ml!1and 9×108 ml!1, + respectively, and the specific nitrification rate from 0.27 to 0.56 g NH4 -N g!1 SS!1 d!1. An increasing occurrence of extracellular polymeric substances (EPS) around microbial clusters with operation time was clearly observed on scanning electron micrographs (SEM). The pressure difference of the membrane module was not over 0.01 mPa over the whole operation period. Present results show that SMBR can be operated efficiently and stably as a high-rate nitrifying technology.

Satisfactory nitrification performance in a SMBR was achieved to treat synthetic inorganic + wastewater under NH4 -N concentration varying from 180 mg/l!1 to 1300 mg/l!1. Almost all of the ammonia-nitrogen was transformed into nitrate, but nitrite accumulation was not observed. Due to the influent characteristics and the interception of the membrane module, ammoniaoxidizing bacteria and nitrite-oxidizing bacteria were dominant in the SMBR. Ammonia-oxidizing bacteria and nitrite-oxidizing bacteria were gradually covered by EPS, which mainly consisted of polysaccharides and proteins with the extension of the operational period; however, the nitrifying sludge had an excellent specific nitrification rate + (up to 0.56 g NH4 -N g!1 MLSS!1 d!1). High free ammonia concentrations or temporary accumulations of nitrite cannot affect the stable operation of SMBR. On the other hand, EPS and soluble organic substances accumulated in the reactor before day 140. However, the membrane filtration was not affected by the accumulation of EPS. The present results show that SMBR can be operated efficiently and with stability as a highrate nitrifying technology that can be applied in the nitrification of wastewater containing a high concentration of ammonia nitrogen.