Energetic autarky waste water treatment at Rheda-Wiedenbrück WWTP, Germany
During the waste water treatment process sewage sludge is produced. This sludge is containing energy and nutrients as valuable components. The state of art for the sludge treatment is the anaerobic digestion, which results in a stabilization of the sludge (= no smell in the following treatment steps) and a production of biogas. The biogas can be used for the production of electricity and heat, and with those electricity around 2/3 of the electrical energy required for the sludge treatment can be covered. The anaerobic digester is a fully mixed reactor, in which the sludge is heated up to 37 °C with a retention time of around 20 days.
The energy costs are one of the main parts of the operation costs of a wastewater treatment plant. It would be great if the complete energy demand could be covered by the plant itself. But how to get this energetic autarky? One approved solution is to add further organic residues to the digester to increase the gas production. At the same time a relevant problem of the treatment of those organic residues is solved. Typical suitable substrates for the digesters are residues form slaughterhouses, fruit juice production, diaries, agricultural residues or the organic fraction of the municipal waste.
This addition of organic residues to a municipal sewage sludge digester is called Co-Fermentation. The substrates have from case to case to be pre-treated (like the municipal organic waste fraction), to avoid operation problems in the digesters. Also it has to be checked whether in inhibition of the biological process in the digester might happen (like through the polyphenolic components of the residues from the olive oil processing).
Components installed in the solution
The main components on the waste water treatment plant Rheda-Wiedenbrück within the frame-work of the project on co-fermentation and energetic autarky are the following:
- Pre-treatment plant for the slaughterhouse waste water treatment (sieve 1 mm, chemical-physical flotation, biological pre-treatment);
- Digester 6,500 m3 volume;
- Agitator / stirrer;
- Heating system;
- Gas flare;
- Gas drying;
- Activated carbon unit;
- CHP (2 x 1.4 MW) including station building;
- Gas storage (2,000 m³);
- Post sludge thickener;
- Sludge liquor treatment plant (PANDA System).
In Rheda-Wiedenbrück the sewage of a slaughterhouse (approx. 650,000 PE) is pretreated on the site of the municipal sewage treatment plant and then commonly treated together with municipal wastewater (76,000 PE). The slaughterhouse wastewater is transported pneumatically via a 2.3 km long pressure pipe to the site of the pretreatment plant next to the municipal treatment facility. The compressed air cushions are expanded by means of a hydro cyclone and the waste water is fed to a sieve. With a chemico-physical flotation plant, approx. 80 to 85% of the COD load and approx. 60% of the N-load are removed from the slaughterhouse wastewater, fed into the digester as a co-substrate and anaerobically treated together with the primary and excess sludge of the municipal stage and the excess sludge from the biological slaughterhouse treatment. The location of the pretreatment plant in the close vicinity of the municipal treatment plant offers considerable advantages due to the short transport paths for the flotate sludge to the digester.
The common biological main stage has a capacity of approx. 100,000 PE and treats the municipal waste water and the slaughterhouse water (treated biologically after the flotation) by means of biological phosphate elimination, nitrification and denitrification. The biogas produced is used for the power generation by CHP units (total installed capacity approx. 4 MW el) in order to cover the electricity demand of the sewage treatment plant (approx. 1.2 MW el). The excess energy is fed into the power grid.
By the anaerobic digestion of the flotate sludge considerable amounts of nitrogen get transferred to the liquid phase. In order to reduce the energy demand for the N elimination in the sludge water and the required external carbon dosage, the nitrogen elimination can alternatively be conducted via nitrite instead of nitrate. For this reason the PANDA system was implemented.