Problems with meeting nitrogen standards at the ‘s-Hertogenbosch WWTP in The Netherlands led to an examination of a number of technologies to improve the performance. A new side stream process, called BABE, was selected after an feasibility study. In September 2005, the BABE process went into operation. The first-ever application of this innovative process has proved successful. The augmentation of nitrifying bacteria within the activated sludge process increased the nitrification capacity at the plant and thus improves nitrogen removal.
The completion of Phase A of the extension of the ‘s-Hertogenbosch WWTP early 2000 was the first piece of an effort to upgrade three local WWTPs. The goal of these upgrades was to comply with European and Dutch effluent standards regarding nitrogen and phosphorus removal. Three phases were defined beforehand: Phase A comprised the introduction of anaerobic and anoxic zones in the existing aeration volume and various improvements in sludge treatment and dewatering. Phase B consisted of additional measures, in the form of sludge water treatment, to further improve nitrogen removal. Phase C (conventional extension of the secondary treatment) would come into force as a result of increased wastewater loads or new effluent standards, in which case Phase B would be skipped.
Since the first phase of the Dutch ‘s-Hertogenbosch wastewater treatment plant was commissioned, nitrogen removal had proved disappointing. As a result the yearly average effluent standard of 12 mg total N per litre was regularly exceeded. Various small measures to improve nitrogen removal have had little success. A risk analysis showed that once in every five years the effluent standards would be exceeded.
An up-front choice had to be made between implementing either sludge water treatment or a conventional extension of the activated sludge system. The outcome largely depended on the expected increase of wastewater loads and future effluent standards. Until 2018, an increase of 27,000 p.e. is expected, of which 80% would be designated to industrial wastewater. More stringent effluent standards are not expected until then and present hydraulic capacity of the WWTP will be sufficient to handle dry weather flow increase. Stormwater flow will remain the same.
SLUDGE WATER TREATMENT
An extensive analysis of the WWTP’s performance showed that the disappointing nitrogen removal was a consequence of the critical minimum aerobic sludge age, causing a decreased nitrification capacity at low temperatures. Of the various possibilities to resolve the problem, sludge water treatment seemed most feasible, because it has the potential to improve nitrogen removal in a compact and cost efficient way. A wide range of innovative and proven technologies is available, which calls for a system evaluation and selection process. Due to its poor cost effectiveness, conventional extension of the secondary treatment was not considered.
The effect of sludge water treatment on the effluent quality had been calculated prior to system selection [modelling by Delft University of Technology], distinguishing between techniques with and without augmentation of nitrifiers to the activated sludge system. Calculations were made for both present and future loads. The results show that augmentation of nitrifiers has far more effect on the effluent quality than just the removal of nitrogen from the sludge water, as shown in Table 1. This is remarkable, especially since the effect of augmentation on the nitrification capacity of the activated sludge was assessed rather conservatively.