Reviving an Old Workhorse: Integration of Rock Media Trickling Filters for External Nitrification in BNR Activated Sludge

Abstract

In the external nitrification (EN) biological nutrient removal (BNR) activated sludge (AS) system, the nitrification process is removed from the main BNRAS system to a fixed media system external to the AS system. The ENBNRAS system provides considerable advantages over the conventional BNRAS system, e.g. reduced bioreactor volumes, secondary settling tank surface area and oxygen demand. Further, the ENBNRAS system provides opportunity for substantial system intensification. The performance and characterization of the ENBNRAS system has been successfully demonstrated at lab-scale, but has not yet been tested in full-scale implementation. In collaboration between the City of Tshwane Metropolitan Municipality (CTMM) and the University of Cape Town, ENBNR activated sludge is being implemented at full-scale at the Daspoort Wastewater Treatment Works (DWWTW) in Central Pretoria, South Africa. This paper describes the implementation and initial performance of the system.

REVIEW - Recent developments in biological nutrient removal

“…While certain problems with macro-fauna (snails, worms, larvae and f lies), which reduce nitrification rates, have been encountered, high removals of ammonia have been economically achieved in TNTFs (Parker et al 1989(Parker et al , 1995(Parker et al , 1996. This has also been found to be the case for rock media TFs (Muller et al, 2005). Therefore, implementing the external nitrification scheme (Fig.…”

“…2b) is entirely feasible. Despite the significant differences in technology, it is interesting that the specific surface nitrification rates in ventilated TNTF systems, viz., around 1.0 gFSA-N/(m 2 •d) for plastic media (Parker et al, 1995) and 0.86 gFSA-N/ (m 2 •d) for rock media (Muller et al, 2005), are of a similar magnitude to those in IFAS systems (see section on IFAS above). The significant reduction in nitrification rate with increasing organic load in IFAS systems is also observed in NTFs.…”

“…A significantly better system is obtained if the nitrification process is transferred to the TF and all of the WW flow discharged to the BNRAS system (Hu et al, 2000;Muller et al, 2005;Fig. 2b).…”

“…Therefore, nitrification in the aerobic reactor still takes place and the nitrate concentration in the aerobic reactor is governed by the ammonia concentration that enters it. Provided the TF nitrifies well (Muller et al, 2005), this nitrate concentration is mainly from the ammonia which bypasses the TF via the internal settling tank underflow, and therefore will be relatively low. If the TF does not nitrify well and the residual ammonia concentration from it is high, then, if sufficient nitrifiers are present in the aerobic reactor, the nitrate concentration will be high, with the result that a significant nitrate concentration will be present in the underflow from the final settling tank.…”

Improving Nitrification through Bioaugmentation

“…By placing the nitrification phase external to the process, the size of the biological stage could be materially reduced. A derivative of the process, termed the ENBNRAS process (External Nitrification Biological Nutrient removal Activated Sludge), has been tested at full scale in South Africa (Muller et al, 2005). …”