The high power consumption associated with a sewage treatment works (STW) has a significant impact on its operating costs and, no less importantly, on its sustainability measured in terms of its carbon footprint. Reducing power consumption is therefore a priority. This article describes the successful measures taken to reduce energy consumption at the Baix Llobregat STW using the advanced control system platform, atl, from SISLtech. A reduction of 22.5% was achieved in the study year compared with the previous year, without affecting the quality of the effluent, while continuing to comply with current legislation on water discharges and reclamation and ensuring the treatment process equipment functioned correctly. The energy savings in the aeration operations alone amounted to around 35.5%. The measures described were implemented with the prior approval of the Catalan Energy Institute (ICAEN), which granted a subsidy covering 40% of the total cost of the investment.
Keywords: STW, water treatment, water reclamation, energy efficiency, carbon footprint, bioreactor, aeration, control system
The Baix Llobregat STW (Figure 1) is located to the south of Barcelona and serves a population equivalent of 1,700,000 with a treatment flow rate of 315,000 m3/day. It also has a water reclamation plant with water quality criteria that meet the requirements of Royal Decree 1620/2007 (Spanish legislation for water reclamation) and a thermal sludge drying installation connected to a CHP plant with natural gas and biogas engines and installed capacity of 10MW.
The STW discharges into the Mediterranean Sea via a 3.5 km marine outfall. The treated wastewater is expelled at a depth of 60 metres through a diffuser system with duck-bill valves, compliant with the limits set out in Royal Decree 509/1996 implementing Directive 91/271/EEC. STW effluent also feeds the reclamation plant for water to be used in agricultural irrigation and for environmental, urban and industrial purposes. For water destined for environmental uses, which will be used in sensitive areas as defined by the EC Directive, it is necessary to reduce the nitrogen and phosphorus content. This process is carried out during the biological treatment stage at the STW prior to feeding the water through to the water reclamation plant.
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