Wastewater treatment plants (WWTPs) play a crucial role in safeguarding public health by treating wastewater to return it to the natural environment in the best conditions. However, they are high-energy intensive.  According to some estimations, wastewater treatment accounts for about 3% of the total electricity consumption worldwide, with significant associated greenhouse gas emissions and operational costs.

In this context, GS INIMA, a leading global company within the water sector, is currently working in a new water treatment configuration in WWTPs to reduce energy consumption (due to a lower need of aeration) and optimize biogas production and recovery from sludge anaerobic digestion (through a lower organic matter consumption in the denitrification process).

Patented and registered under the trade name PROGRAMMOX®, this configuration is based on a first high rate activated sludge (HRAS) reactor where organic matter is redirected to the sludge line to boost biogas production. This is followed by the autotrophic removal of nitrogen in two stages: i) a partial nitritation (PN)-aerobic granular sludge (PN-AGS) reactor, and ii) an anaerobic ammonium oxidation (anammox) reactor. The goal is to save energy and make the wastewater treatment process more environmentally and economically sustainable.

To achieve this goal, GS INIMA has collaborated with the Laboratory of Chemical and Environmental Engineeering of the University of Girona (LEQUIA), one of the pioneer groups in Spain in studying PN-AGS and anammox treatments. Oriol Carbó’s thesis, focused on both processes, had two scientific objectives: 1) to optimize these processes to ensure its robustness and replicability, and 2) quantify the energy consumption and the potential of energy recovery in WWTPs. The experimental work was initiated at LEQUIA laboratories with an AGS reactor. The researcher then continued working in a pilot plant specially constructed to study the PROGRAMOX® configuration (HRAS + PN-AGS + anammox) and able to treat 1.5m³ per day. The plant was installed in La Garriga WWTP (Barcelona) and was operated with wastewater from this facility.

Results obtained allowed the research group and the enterprise to acquire new knowledge and expertise on the processes investigated. In fact, GS INIMA is currently working on the construction of a new demonstration plant to validate the PROGRAMOX® configuration at full-scale. In the AGS reactor, a selective wasting from the top of the settled sludge bed was found to be a successful strategy to effectively trigger the formation and establishment of aerobic granules in the system. However, full granulation was not seen as crucial to the proper performance of the AGS reactor. The maintenance of a low solids volumetric index (SVI < 100 mL/g) with a mixture of granules and flocs was helpful enough for an improved operation with high organic matter removal and nitratation repression. Stable PN was achieved under mainstream conditions by treating real HRAS effluent. Nitrite oxidizing bacteria (NOB) activity was successfully repressed by the free nitrous acid (FNA) produced in the reactor. In the anammox reactor, an effluent with a suitable quality (i.e., total nitrogen, TN, < 8 mg/L) could be achieved in the reactor by controlling the previous PN conversion at NO₂^–/NH₄⁺ ratios lower (0.80 g N/g N) than the theoretical 1.32 g N/g N, which implies a lower oxygen requirement. Finally, the energy balance indicates that energy savings (kWh/m3) of 44 % could be reached, which could have a significant impact on the sustainability of these facilities.

This doctoral thesis was carried out within the framework of an industrial doctoral project cofounded by the Spanish Ministry of Science and had two directors from academia (Dr Jesús Colprim and Dr Albert Magrí) and one from the business sector (Dr Belén Gutiérrez, RD Area of GS INIMA).

Publication: Oriol Carbó, Jaume Teixidó, Joan Canals, Antonio Ordóñez, Albert Magrí, Mercè Baldi, Belén Gutiérrez, Jesús Colprim, Achieving mainstream partial nitritation with aerobic granular sludge treating high-rate activated sludge effluent, Journal of Water Process Engineering, Volume 60, 2024, 105165, ISSN 2214-7144, https://doi.org/10.1016/j.jwpe.2024.105165.