The HELCOM Baltic Sea Action Plan is an ambitious program to obtain good ecological status of the Baltic marine environment by 2021. Sweden has a preliminary commitment to reduce nitrogen by 21 000 ton, partly from the wastewater sector (3 000 ton nitrogen), and due to this many wastewater treatment plants need to improve their nitrogen removal. On top of that, the incoming load of nitrogen is increasing due to rising protein consumption. Co-digestion of external substrates (often rich in nitrogen) with wastewater sludge for increased biogas production further contributes to higher nitrogen loads. Increased nitrogen removal capacity often means more bioreactor volumes, which can be very expensive and sometimes impossible if space is limited. Therefore, there is a need for compact hybrid solutions that can increase capacity within existing volumes.

The Sundet WWTP in Växjö received a new environmental permit in 2010 for 95 000 pe. Emission criteria for treated water are 10 mg BOD7/l, 0.2 mg P/l, and 60% nitrification. During a five year evaluation period (2012- 2016) assessments to ensure 15 mg N/l in the effluent should be conducted. To increase the nitrogen removal capacity within existing volumes, two processes using MBBR technology with plastic carriers were chosen:

• Increased nitrification capacity in the existing activated sludge system through the Hybas™ combination process of MBBR and activated sludge (Integrated Fixed-Film Activated Sludge, IFAS).

• Separate biological treatment of sludge liquor with the AnitaMox™ process, using autotrophic Nremoval through anaerobic ammonium oxidation (anammox) in a one-stage process with carriers.

One of the six treatment trains was rebuilt to fit the integrated fixed-film activated sludge process for improved nitrification. The anoxic zone in this train was doubled for improved denitrification capacity. Data collected onsite from over a year (from October 2011 to present) are analyzed and compared with the performance of a conventional activated sludge train operated in parallel.

The Hybas trained nitrified more consistently than the reference train, with effluent concentrations <1 mg NH4- N/l for most of the time. To keep the nitrification capacity in the reference train during the winter, a higher MLSS concentration was required, as well as larger aerated volumes. This lead to inadequate denitrification, while the capacity was substantially higher in the Hybas train, with emission concentrations well under the objective of 15 mg N/l. No additional carbon was added to the system. Practical experiences concerning operational requirements and challenges of the Hybas process (air flow and hydraulic loading, carrier management, and nutrient limitations) are discussed in the paper.

The existing sludge liquor treatment (SBR, sequence batch reactor) was retrofitted in 2011 to an AnitaMox process. Thanks to a seeding start-up strategy, the process reached full capacity with more than 90% ammonia removal within two months from start-up. By applying a nitrogen loading strategy to the reactor that matches the capacity of the seeding carriers, more than 80% nitrogen removal could be obtained throughout the start-up period. Full-scale experiences from more than a year has proven AnitaMox to be an energy- and cost efficient nitrogen removal process compared to the previous SBR process, and at the same time robust and relatively simple to operate. However, an increased process control and more on-line instruments require resources and qualified personnel.

These two full-scale demonstration projects have been a successful learning experience in identifying and correcting both process and operational issues, which may not have arisen at pilot scale. The set objectives in terms of nitrogen removal were met for both processes and design modifications will improve the future operation at the Sundet WWTP.