Pursuit of urine nitrifying granular sludge for decentralised nitrite production and sewer gas control

• Nitrifying SBRs operated for high rate urine oxidation with base addition.
• Granular sludge development explored through various operational parameters.
• Small, compact and strong granule-like aggregates developed.
• Granulation limited by pH/NH3 and opposing conditions for heterotrophs and nitrifiers.
• High-rate stable nitritation achieved at volumetric conversion rate of 1.1 kg-N/m3 d.

This study aims to develop a high-rate reactor for decentralised urine nitrification using granular sludge, which allows high biomass retention in a small footprint installation. The system incorporated alkalinity dosing to ensure full conversion of ammonia to nitrite for subsequent control of sewer gas production. The key operational parameters for granule formation such as feeding and settling duration were tested as well as environmental conditions including pH. Pulse feeding was found to be instrumental to both the treatment performance and development of well settling granule-like aggregates. These aggregates showed granular characteristics with a compact and well defined shape, excellent settleability and high strength. However, pulse feeding also led to increased pH and free ammonia concentrations which reduced heterotroph activity and aggregate growth. Furthermore, in order to prevent nitrification collapse and high pH and free ammonia associated with such an event the reactor nitrogen loading was limited. Due to the low organic to nitrogen ratio of urine this restrained the organics loading of the dominant heterotrophic community further limiting granule growth. Nevertheless, the development of compact well settling aggregates under pulse feed operation allowed the objective of high-rate nitrification to be achieved with sustained nitritation rates up to 1.1 kg-N/m3 d at a urine dilution of approximately 25%.

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