A laboratory scale external nitrification (EN) biological nutrient removal, ativated sludge (BNRAS) system and a VCT, BNRAS system with similar design and operating parameters receivng the same influent wastewater were operated in parallel for 234 d to compare their N and P removal performance and to establish the advantages and disadvantages of the ENBNRAS configuration. For both systems, the COD mass balances, COD removals and filtered and unfiltered effluent COD concentrations were virtually identical, i.e. 78%, 93%, 40 and 50 mgCOD/iota respectively. However, the oxygen demand in the ENBNRAS system was only 23% of that in the UCT system. The N mass-balances and TKN removals also were virtually the same, i.e. 87% band 94% respectively. The filtered and unfiltered effluent TKN concentrations were somewhat higher for the ENBNRAS system (4.0 and 4.8 mgN/iota) than the UCT system (2.6 and 4.2 mgN/iota) due to a slightly higher effluent free and saline ammonia (FSA) concentration. (3.5 vs 1.8 mgN/iota),because nitrification of FSA from (i) the EN part of the system(ii) that, released in the anoxic reactor; and (iii) the internal settling tank underflow, was not complete in the aerobic reactor. The effluent nitrate concentration was significantly lower for the ENBNRAS system (4.0 mgN/iota) than the UCT system (12.3. mgN/iota) due to (i) the larger anoxic mass fraction and (ii) nitrification preceding denitrification so that 100% denitrification is possible without nitrate recycling. Consequently, the overall N removal performance of the ENBNRAS system was significantly better [88%, effluent total N (TN= TKN + NOx) = 9.3 mgN/iota] than the UCT system (78%, effluent TN = 16.7 mgN/iota). The biological excess P removal (BEPR) in the ENBNRAS system was associated with 55-65% anoxic P uptake throughout the investigation and was around 9.7 mgP/iota. In the UCT system; the BEPR was predominantly aerobic (>95%) for the first half of the investigation and the P removal was 3.6 mgP/iota higher, (13.5 mgP/iota) than in the ENBNRAS system (9.9 mgP/iota). When anoxic, P uptake (similar to20%) BEPR was induced in the UCT system by dosing FSA to the influent, the P removal declined and was 1.7 mgP/iota lower (8.3 mgP/iota) than the ENBNRAS system (9.9 mgP/iota). Dosing FSA to both systems caused the nitrate concentration in the outflow, from the main anoxic reactors to increase above 2 mgN/iota. This stimulated a deterioration in sludge settleability in the UCT system (DSVI from 120 to 200 miota/g), while that in the ENBNRAS system remained very good (80 to 100 miota/g). Anoxic/aerobic P uptake BEPR appears to be stimulated in BNRAS systems with (i) small aerobic and large anoxic, mass fractions and (ii) anoxic reactor nitrate loads greater than the denitrification, potentials. However, associated with this is a decrease P removal compared with predominantly aerobic (>90%) P uptake BEPR. While anoxic P uptake BEPR often occurs in ENBNRAS systems because conditions (i) and (ii) are usually met, the system can be designed for aerobic P uptake BEPR by countering conditions (i) and (ii) above. However, the more conditions conducive for aerobic P uptake BEPR are created in the ENBNRAS system, the more sensitive its sludge settleability becomes to the nitrate concentration at the anoxic-aerobic transition like in conventional (internal nitrification) BNR systems.
