Powerstep

During the POWERSTEP project, a full-scale treatment of this sidestream is realised using the process of membrane ammonia stripping. In this process, nitrogen is transferred in NH3 form from the sludge water via a gas-permeable membrane into sulfuric acid, which can then be sold as nitrogen fertiliser (diammonium sulfate). The membrane stripping process requires a multi-stage pre-treatment of the sludge water to remove any particles and adjust proper conditions for an efficient stripping (high pH and temperature).

In the POWERSTEP project, a power-to-gas (P2G) plant was operated and optimised to convert residual CO₂ in the biogas into biomethane while using renewable electricity from the grid during low prices (e.g. excess supply of wind and PV power). The P2G plant uses an electrolyser to produce H2 gas, which is fed into a biological reactor where special bacteria (archaea) convert H2 and CO₂ into CH4. The process can either work directly with biogas (mixture of CH4 and CO₂), or on pure CO₂ separated from biogas during gas upgrading (e.g. in a scrubber). The produced biomethane is further cleaned prior to injection of the renewable gas into the local gas grid. By-products of the P2G plant are pure oxygen and heat, which can both be utilized on-site at the WWTP.

In the POWERSTEP project, two measures for improved energy management were tested at WWTP Braunschweig. The first approach encompassed a desktop study to estimate potential cost savings by applying different strategies for energy management and integration into a “smart grid” (e.g. market price for electricity, optimised operation of CHP, storage of biogas, load shifting, power-to-gas). The second approach tested a “heat-to-power” process in pilot scale where excess heat can be converted into additional electricity, drawing on the available exhaust heat of the CHP units with a thermo-electric generator.

In the POWERSTEP project, a separate treatment step for the centrate from sludge dewatering is tested in full-scale to improve the energy balance of the plant. This sludge dewatering effluent is highly loaded with nitrogen, which is normally recycled to the inlet of the WWTP and increases the load and energy demand of the main line. The biological process of nitritation (conversion of ammonia to nitrite) is established for the sidestream, and the formed nitrite is recycled back to the high-load activated sludge tank where the chemically bound oxygen can be utilised by bacteria. Due to the better efficiency of oxygen transfer in the sidestream, this concept saves electricity for aeration and also improves COD extraction into sludge, increasing biogas production.

In the POWERSTEP project, a pilot plant for testing of mainstream N treatment via nitritation and anammox was operated after enhanced carbon extraction. The mainstream process was tested in two pilot reactors (50 m³) based on a two-stage configuration (MBBR for nitritation followed by MBBR for anammox) and a one-stage configuration with simultaneous nitritation and anammox, operated in integrated fixed-film activated sludge (IFAS) configuration. Influent for the mainstream anammox was provided either by high-load activated sludge (full-scale) or microscreen with upstream chemical dosing (pilot discfilter with xx µm mesh). Process control with STAR system was used to control the mainstream anammox process.

During the POWERSTEP project, the site was equipped with a microscreen (drum filter, 40 µm mesh) as primary treatment in 2016 to enable carbon extraction in form of primary sludge. This pre-treatment reduces influent COD load to the biological process and generates additional sludge for increased biogas production in sludge treatment. Depending on the target for carbon extraction, chemicals such as coagulant (metal salt) and flocculant (polymer) can be dosed upstream of the microscreen to increase COD elimination up to 70%. The WWTP has a discharge limit of 18 mg/L total nitrogen (TN), and an advanced control strategy is installed to provide sufficient COD into the SBRs so that TN levels in the effluent are safely below the limit.

Another process to remove TN without carbon source is tested on-site with a pilot reactor growing duckweed on the effluent of the microscreen. This process should remove nutrients N and P by uptake into duckweed plants floating on the water surface. These plants can then be harvested and used for biogas production.