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CE 421/521- Environmental Biotechnology

 Introduction of external nitrification in biological nutrient removal activated sludge systems

 Cheng Li

 ABSTRACT

The ENBNRAS system was proposed to intensify the treatment capacity of biological nutrient removal activated sludge (BNRAS) systems by addressing two difficulties often encountered in practice: (a) the long sludge age for nitrification requirement; and (b) sludge bulking. In the ENBNRAS system, nitrification is transferred from the suspended growth activated sludge in the aerobic reactor to a fixed film nitrification system, i.e. the ENBNRAS system is a combined fixed film and suspended growth system. Thus, the sludge age of suspended growth activated sludge can be reduced from 20 –25 d to 8-10 d, resulting in the decrease in the reactor volume per ML wastewater treated by about 30%. Furthermore, the aerobic mass fraction also can be reduced from 50-60% to less than 30% (alternatively the anoxic mass fraction is increased from 25-35% to large than 55% if the anaerobic mass fraction is 15%), and thus complete denitrification in anoxic reactors become possible. Research indicates that both the short sludge age and complete denitrification could ameliorate the AA filamentous bulking (Casey et al., 1994) and hence can reduce the surface area of secondary settling tanks or increase the treatment capacity of existing systems. The lab-scale experimental investigations indicated that the ENBNRAS system can obtain (i) very good COD removal, even with as low as a 20% aerobic mass fraction; (ii) high nitrogen removal, even for the wastewater with the high TKN/COD ratio up to 0.14, and that (iii) good settling sludge (DSVI less than 100 mL/g) and (iv) significant reduction in oxygen demand have been consistently observed (Hu, et al., 2000, 2002a, 2002b, 2002c).

 KEYWORDS

Activated sludge, biological nutrient removal (BNR), external nitrification (EN)

 INTRODUCTION

 The biological nutrient (N + P) removal activated sludge (BNRAS) system has become an established technology to control eutrophication in wastewater treatment practice, and this has been facilitated by an improvement in understanding of nitrification, denitrification and biological excess phosphorus removal (BEPR) processes. However, implementation of the BNRAS system has brought with it a new set of difficulties (Ekama and Wentzel, 1999a), the main ones being: (1) long sludge age required for nitrification; (2) Filamentous organism bulking; (3) treatment/disposal of liquors / supernatants generated from sludge treatment. The proposed ENBNRAS system focuses on the first two, namely the long sludge age for nitrification requirement and filamentous organism bulking.

 Long sludge age requirement for nitrification

 Typically, a BNR process has anaerobic, anoxic and aerobic reactors; the activated sludge is repeatedly exposed to these conditions. In the anaerobic reactor, the activated sludge releases phosphorus, which is stored as polyphosphate in the aerobic reactor, and accumulates polyhydroxylalkanoates (PHA) when the carbon substrate is more abundant. Subsequently, the released phosphorus is uptake in excess by phosphate-accumulating organisms (PAOs) in aerobic conditions; therefore, phosphorus can be removed from wastewater. In the anoxic reactor, nitrate decreases due to denitrification. Moreover, in the aerobic reactor, nitrification, organic substrate oxidation, and phosphorus uptake occur at the same time. Consequently, organic substrate, nitrogen and phosphorus can be removed simultaneously in a BNR process.