Sewage treatment plant that can cover a big part of its energy requirements by altering the processing techniques and by using sewage gas to produce electricity
Local municipalities need ca. 20% of their electricity demand for the operation of their sewage treatment plants, which often makes them the largest energy consumer, and energy amounts to ca. 30 percent of the annual operating costs. Sewage treatment plants that produce biogas in their sewage sludge treatment process with anaerobic digestion can produce enough energy to meet most of its energy needs.
The Verbandsgemeinde Weilberbach, Germany, 13,790 inhabitants, treating 1.6 million m3/y of wastewater consumed in the past 480,000 kWh/y of electric energy and 3,800 litres of liquid gas for heating. The processing technique was changed from aerobic digestion to anaerobic digestion with the production of biogas in the sewage sludge treatment process, and adding 2 CHPs (combined heat and power plants) for electricity and heat generation.
The energy demand was reduced by altering the processing technique and through other efficiency measures. The blower system usually needs up to 50% of the electricity. The agitators are usually the third biggest consumer in sewage plants. The following measures have been undertaken to save energy: the installation of a plate aerator/ membrane plate diffuser, optimized agitators/stirrers in both aeration tanks and the renewal of the blower.
Total costs: 4,000,000 €
For maintenance: € 1,200,000 €, for energy efficiency measures: € 2,800,000
Origin funds: own funds (credit) € 3,420,000; € 580,000, Federal Ministry for the Environment
Savings: ca. € 263,000/y, Amortization: ca. 17 years (interest 2.5%, increase energy costs 4%/y)
Evidence of success
During monitoring, approx. 294,560 kWh/a of electricity were produced using cogeneration (CHP). Additionally power was generated from PV: 1,814 kWh/y. Consumption 447.982,2 kWh/y ( - 32,000 kWh/y despite additional consumers) . 72.2% of energy demand covered with own production. Heat: 100%, no external energy source.
Reduction of dry sludge to be deposited: 35.8%
CO2 savings: 181,4 tons/y
During monitoring (09/2015 - 08/2016) minus 20% of waste water arriving produced sub-optimal values in the production of biogas. The energy self-sufficiency target since has been improved through reduction in aerating energy and sludge age/increase of energy content in the excess sludge.
Potential for learning or transfer
High potential for savings (emissions, energy, costs). Sewage collection and treatment is usually a municipal task and sewage treatment plants are major energy consumers. Potential saving of energy demand of municipal sewage treatment plants: 30%. In Rhineland-Palatinate a few municipalities already invested in energy efficiency measures in sewage plants and are able to show that is economically efficient, e.g. Pirmasens (40,000 inhabitants, cost savings/year: 162,000 €) and Wörrstadt (7900 inhabitants, cost savings/year 89,000 €). In Germany one out of eight sewage plants installed a CHP to use sewage gas for their own energy supply.
Sewage plants can be found in municipalities all over Europe. If energy efficiency measure are combined with making use of sewage gas the emission savings effects are even higher. The municipality of Weilerbach did both and this is why this practice has great potential for transfer and learning.
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