In the past decade nutrient removal and recovery by treatment of source separated urine has received attention based on its high concentrations of nitrogen and phosphorus. In dense urban environments nutrient removal via struvite precipitation, urine nitrification and in-sewer denitrification allows phosphorus recovery, efficient use of sewer infrastructure and wastewater-carbon, and a possible shift to high rate carbon removal at centralized plants. Combination with seawater toilet flushing provides numerous additional benefits including freshwater savings, an economical magnesium source and additional autotrophic denitrification in sewer. To realize such a system a compact nitrification reactor such as an aerobic granular sludge reactor is required, which due to unfavourable c...[ Read more ]

In the past decade nutrient removal and recovery by treatment of source separated urine has received attention based on its high concentrations of nitrogen and phosphorus. In dense urban environments nutrient removal via struvite precipitation, urine nitrification and in-sewer denitrification allows phosphorus recovery, efficient use of sewer infrastructure and wastewater-carbon, and a possible shift to high rate carbon removal at centralized plants. Combination with seawater toilet flushing provides numerous additional benefits including freshwater savings, an economical magnesium source and additional autotrophic denitrification in sewer. To realize such a system a compact nitrification reactor such as an aerobic granular sludge reactor is required, which due to unfavourable chemical oxygen demand to nitrogen ratio, high pH and free ammonia is difficult to achieve.

In this study a novel micro-granular sludge with size between 100 - 180 μm was cultivated for nitrifying urine. Detailed characterization of the microgranules confirmed a compact structure with specific gravity of 1.02 - 1.10 eligible to be termed aerobic granules. Granule characteristics were improved when treating urine from seawater toilet flushing systems, although 4-5 mg/L of bioavailable phosphorus was required to maintain biomass growth and stable nitrification. Pulse feeding was the major operational parameter identified for granule formation. Pulse feeding created low substrate gradients and reduced growth rates through free ammonia inhibition. These two factors resulted in homogenous radial growth and dense microcolonies giving the sludge its properties of high density and small size.

Microbial community analysis between reactors identified a common core of key bacteria while activity measurements on various substrates indicated specific degradation niches. Nevertheless, seawater toilet flushing selected for a number of unique organisms including an ammonia oxidizer not typically found in wastewater systems, and potentially an unidentified nitrite oxidizer.

Microgranules, as developed in this study bring a number of benefits over conventional aerobic granules, possessing increased surface area, active biovolume and loading rates over conventional aerobic granules (typically 0.2 - 6 mm). As a result they combine the benefits of flocculent and granular sludge.