Water sector is a major energy consumer. According to the International Energy Agency, in 2014 four per cent of global electricity consumption was used to extract, distribute and treat water. Projections indicate that this percentage could double by 2040, due to increase in the demand of desalination and more efficient wastewater treatment plants. Therefore, there is an urgent need to develop and implement more environmentally sustainable wastewater treatment processes.

Most current urban wastewater treatment plants are based on a conventional activated sludge treatment (CAS), with clarifiers, biological reactors where the organic matter is oxidised, and aeration and recirculation systems. Many facilities also integrate the anaerobic digestion of sludge that allows biogas production and, consequently, the recovery of part of the consumed energy. In the past few decades, researchers have investigated the optimisation of this process by the use of a high rate biological process instead of the primary clarifier. It is the so-called «high-rate activated sludge (HRAS) process», an “accelerated” sanitation process able to remove more organic matter and to recover more energy. Nevertheless, we still need to get to know HRAs in depth to ensure its stability, and implement it widespread at full scale.

Joan Canals, with a long professional record within the water sector, was aware of this gap when he initiated his doctoral thesis. Because of this, he decided to study the different parameters involved in a HRAS process stability and that would allow optimising it. The experimental work was developed in Montornès del Vallès wastewater treatment plant, in the province of Barcelona. An HRAS pilot plant with two biological reactors and two clarifiers (operated alternatively) was designed and constructed to treat 35m³ of water per day. The researcher monitored the operation of the plant with different flow rates during 497 days by means of a digital system and studied the following parameters: dissolved oxygen, oxidation-reduction potential and the influent, effluent and recirculation flow rates. Moreover, he determined the suspended solids contents in continuous mode in the influent, the biological reactor, the effluent and the recirculation flow. Finally, he carried out simulations to assess the process robustness and feasibility.

Results obtained demonstrated that HRAS could be an efficient and stable process, with a high capacity to remove pollutants and a low energy content. Thus, HRAS achieves higher removal of several pollutants (nitrogen and phosphorous compounds) as compared with the primary clarifier, with removal rates correlated with influent nutrient concentration. A remarkable finding is that nitrogen and phosphorous removal show positive correlations with total and soluble chemical oxygen demand (COD), highlighting the importance of adsorption and entrapment processes. HRAS also shows low specific oxygen consumption (SOC) for several parameters such as total and soluble COD, and biological oxygen demand in 5 days (BOD5), indicating high-energy efficiency. Long-term process analysis proves that the best pathway to remove nitrogen in the following treatment step is nitrification/denitrification with anaerobic ammonium oxidation (anammox) in the side stream. Additionally, HRAS exhibits heightened energy efficiency at elevated influent concentrations, with specific oxygen consumption influenced by influent concentration and biodegradability.

The potential impact of these findings is very high. If an optimized HRAS process were applied altogether with an anammox process in the side stream, we would achieve 40% reduction of electricity consumption in a wastewater treatment plant and 34% of reactor volume reduction. This would only require increasing by 11% the anaerobic digestion reactor volume (due to the increase of biogas production). Such significant numbers challenge us to continue working to upscale and implement the process at full scale.

This doctoral thesis was directed by Dr Hèctor Monclús, Dr Maria Martín and Dr Alba Cabrera from the research group “Laboratory of Chemical and Environmental Engineering” (LEQUIA) of the University of Girona (UdG). The experimental work was carried out within the framework of an industrial RDI project at company GS INIMA Environment co-funded by CDTI. The defence, which is open to the public, will take place next 21 May at Aula Magna of UdG Faculty of Sciences.