Efficient and stable nitritation and denitritation of ammonium-rich sludge dewatering liquor using an SBR with continuous loading

Separate treatment of dewatering liquor from anaerobic sludge digestion significantly reduces the nitrogen load of the main stream and improves overall nitrogen elimination. Such ammonium-rich wastewater is particularly suited to be treated by high rate processes which achieve a rapid elimination of nitrogen with a minimal COD requirement. Processes whereby ammonium is oxidised to nitrite only (nitritation) followed by denitritation with carbon addition can achieve this. Nitrogen removal by nitritation/denitritation was optimised using a novel SBR operation with continuous dewatering liquor addition. Efficient and robust nitrogen elimination was obtained at a total hydraulic retention time of 1 day via the nitrite pathway. Around 85-90% nitrogen removal was achieved at an ammonium loading rate of 1.2 g NH4+-N m(-3) d(-1). Ethanol was used as electron donor for denitritation at a ratio of 2.2gCODg(-1) N removed. Conventional nitritation/denitritation with rapid addition of the dewatering liquor at the beginning of the cycle often resulted in considerable nitric oxide (NO) accumulation during the anoxic phase possibly leading to unstable denitritation. Some NO production was still observed in the novel continuous mode, but denitritation was never seriously affected. Thus, process stability can be increased and the high specific reaction rates as well as the continuous feeding result in decreased reactor size for full-scale operation. (c) 2006 Elsevier Ltd. All rights reserved.

Nitrogen removal from sludge digester liquids by nitrification/denitrification or partial nitritation/anammox: environmental and economical considerations

In wastewater treatment plants with anaerobic sludge digestion, 15-20% of the nitrogen load is recirculated to the main stream with the return liquors from dewatering. Separate treatment of this ammonium-rich digester supernatant significantly reduces the nitrogen load of the activated sludge system. Two biological applications are considered for nitrogen elimination: (i) classical autotrophic nitrification/heterotrophic denitrification and (ii) partial nitritation/autotrophic anaerobic ammonium oxidation (anammox). With both applications 85-90% nitrogen removal can be achieved, but there are considerable differences in terms of sustainability and costs. The final gaseous products for heterotrophic denitrification are generally not measured and are assumed to be nitrogen gas (N-2). However, significant nitrous oxide (N2O) production can occur at elevated nitrite concentrations in the reactor. Denitrification via nitrite instead of nitrate has been promoted in recent years in order to reduce the oxygen and the organic carbon requirements. Obviously this achievement turns out to be rather disadvantageous from an overall environmental point of view. On the other hand no unfavorable intermediates are emitted during anaerobic ammonium oxidation. A cost estimate for both applications demonstrates that partial nitritation/anammox is also more economical than classical nitrification/denitrification. Therefore autotrophic nitrogen elimination should be used in future to treat ammonium-rich sludge liquors.

Two-stage thermophilic-mesophilic anaerobic digestion of waste activated sludge-from a biological nutrient removal plant

A two-stage thermophilic-mesophilic anaerobic digestion pilot-plant was operated solely on waste activated sludge (WAS) from a biological nutrient removal (BNR) plant. The first-stage thermophilic reactor (HRT 2 days) was operated at 47, 54 and 60 degrees C. The second-stage mesophilic digester (HRT 15 days) was held at a constant temperature of 36-37 degrees C. For comparison with a single-stage mesophilic process, the mesophilic digester was also operated separately with an HRT of 17 days and temperature of 36-37 degrees C. The results showed a truly thermophilic stage (60 degrees C) was essential to achieve good WAS degradation. The lower thermophilic temperatures examined did not offer advantages over single-stage mesophilic treatment in terms of COD and VS removal. At a thermophilic temperature of 60 degrees C, the plant achieved 35% VS reduction, representing a 46% increase compared to the single-stage mesophilic digester. This is a significant level of degradation which could make such a process viable in situations where there is no primary sludge generated. The fate of the biologically stored phosphorus in this BNR sludge was also investigated. Over 80% of the incoming phosphorus remained bound up with the solids and was not released into solution during the WAS digestion. Therefore only a small fraction of phosphorus would be recycled to the main treatment plant with the dewatering stream.