AGNES – Aerobic Granular sludge – Nutrient removal and recovery Efficiency in Sweden

More stringent discharge limits and increased load due to population growth and centralisation create needs of upgrading and enlarging the existing municipal wastewater treatment plants (WWTPs). Also expansion of housing areas close to the treatment plants increase the demands of retrofits and upgrades that are space efficient. At the same time we want the treatment methods to be robust, energy, carbon and chemical efficient, have low carbon footprint and facilitate recovery of plant nutrients.

The aerobic granular sludge (AGS) technology is an innovative, compact and energy efficient wastewater treatment process that is mature for full-scale implementation around the world. During the years of 2017-2022 Sweden Water Research initiated two projects under the name AGNES, which is an acronym for Aerobic Granular sludge – Nutrient removal and recovery Efficiency in Sweden. In AGNES we cooperate with many stakeholders such as universities, companies and utilities that were interested in research and development and implementation of the AGS technology.

AGNES I – knowledge about AGS technology

Our first project Aerobic granules, a new technology for municipal wastewater treatment – the state-of-the-art (AGNES I) was a project co-financed by the Swedish Water & Wastewater Association (project no. 2017-19) which was performed to increase the knowledge about the AGS technology in Sweden.

Aerobic granules are distinguished from activated sludge flocs through their larger size and more compact and spherical structure. Due to these properties, granules settle rapidly and can therefore contribute to compact treatment processes through high sludge concentrations and short settling times. The most important factors to promote granulation include exposing the biomass to relatively high concentrations of contaminants in sequencing batch reactors (SBRs), promoting slow-growing microorganisms and applying a relatively short settling time. Enhanced biological phosphorus removal is preferably integrated with AGS and the large size of the granules makes simultaneous nitrification (at the surface of the granules) and denitrification (at the inner, anoxic parts) feasible. The AGS technology is energy efficient due to its lack of need of mixers and return pumping of sludge and nitrate, and its operation at low dissolved oxygen concentrations.

AGNES II – a full-scale study

At Österröd WWTP in the municipality of Strömstad the first AGS plant in the Nordic countries was started during the summer of 2018. A new project, also co-financed by the Swedish Water & Wastewater Association, called Implementation of aerobic granular sludge in Sweden – a full-scale study (AGNES II), evaluated th start-up and the operation of the first 3.5 years. A stakeholder group, consisting of the municipalities of Strömstad and Tanum, Chalmers University of Technology, Sweden Water Research, H2OLAND, Uppsala Vatten och Avfall, Gryaab, the Käppala Association, VA SYD, Västervik Miljö & Energi, TU Delft and Royal HaskoningDHV, contributed with knowledge and cash money in the project.

One of the two parallel AGS reactors was started with granules and one with flocculent activated sludge. In the reactor started with flocculent sludge, granules were formed and increased in proportion and grew in size over time. Hence, a start-up from activated sludge flocs seemed possible, but it took considerably longer than start-up with granules and various technical problems prolonged the start-up period.

The AGS plant achieved its guaranteed performance during a one-year warranty period by reaching effluent concentrations below 8 mg BOD7/L, 10 mg N/L and 1 mg P/L, as annual averages and averages over the period May to August. The results were obtained at nitrogen and phosphorus loadings that were approximately 40 % below the design loads and flows that were on average 17 % lower than the design flow. High flow rates, especially in combination with low temperatures, was observed to be the limiting factor for the performance of the plant.

The results from Österröd indicate a difference between AGS and activated sludge in terms of how well incoming particles are utilized for nitrogen and phosphorus removal. The low-loaded presettler removed more suspended organic matter than desirable with respect to sufficient nutrient removal in the AGS line. Hydrolysis of primary sludge in the presettler and bypass of the presettler were required to achieve sufficiently low effluent N and P concentrations.

The volume and surface requirements per volume of treated water were calculated to be 31 % and 48 % lower, respectively, for the AGS line than for the activated sludge line. The electricity usage was 0.22 kWh/m3 for the AGS line and 0.26 kWh/m3 for the activated sludge line, i.e. 15 % lower for the AGS line. Neither of the two processes had been subject to energy optimization and there was ample room to reduce electricity usage in both lines. The results confirmed that an AGS process can be compact without significantly increasing the electricity usage. However, in order to obtain an energy-efficient facility as a whole, it is critical to set the right requirements during procurement, especially regarding aeration systems, blowers and mixers.

The need for supervision by operating personnel was higher for the AGS line than for the activated sludge line and higher than expected. The control of the AGS process is largely based on sensors that require a considerable amount of work to function reliably. However, the difference has decreased and is expected to decrease further over time. The project group felt that the work with commissioning and tuning of the AGS process was quite extensive and could be more suitable for a somewhat larger water utility than Strömstad municipality.

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