CEE Oct-Dec 2012

Energy Efficiency &Conservation SET TURBO ON YOUR ENERGY SAVINGS Save more than 50% of energy by going over from surface aeration to bottom aeration Martin Carlsen Energy savings can be achieved by a fine tuning or improvement of existing equipment, the use of modern control systems based on the real time measurements of process data or even better, by the replacement of ageing technologies with modern and more efficient technologies. In Denmark alone, the annual electricity consumption of the public owned wastewater treatment plants is about 350 Gwh, of which, the country's 60 largest plants account for 70 per cent of the energy consumption. By rebuilding the four largest surface aerated plants in Denmark there is an energy-saving potential of at least 20 Gwh annually. By far the greatest amount of energy consumed by conventional wastewater treatment plants is the energy to the aeration process, which typically accounts to 40- 60 per cent of the overall plant energy consumption. In Denmark the potential savings from just the aeration process are particularly great, based on the many large plants, which are equipped with simple surface aerators or other types of mechanical aeration equipment. Aeration of wastewater Typical for the aeration process on activated sludge plants is that air (oxygen) is transferred to the water by means of mechanical aerators, which force air into the water or by purging air from the aeration tank bottom by use of compressed air that is purged th rough fine or coarse bubble diffusers. The amount of air is typically controlled by keeping a constant amount of Dissolved Oxygen Level (DOL) in the water or by controlling the DOL according to the amount of ammonia in the liquid. Several full scale tests have confirmed the theory that bottom aeration demands considerably less energy than surface aeration, and typically an average energy saving of more than 50 per cent may be achieved , at the same plant loading, by applying bottom aeration. See for example the report from the large Dradenau municipal sewage treatment plant in Hamburg (1.6 Mia PE), ISBN-3- 930400-81-2. The inability to achieve higher savi ngs is mainly attributed to the fact that surface aeration tanks are often not particularly deep, which makes them less suitable for bottom aeration. In addition, surface aeration has some other disadvantages, such as the fact that it is ~---------------------, difficult to regulate the amount of oxygen 250,~ 200,~ no.~ 100,~ so.~ 0,00% 3 Relative Compression Work 3 metre= 100% I 4 5 6 7 rt~tm• 1: Rel<~til(~ sorr .11 1 mvc = IOO% a supplied, that the water is cooled, which inhibits bacterial growth and oxygen diffusion, and that this type of aeration creates aerosols, which can spread the bacterial around to local residential areas and it can generate foul odours. Bottom aeration process In the case of bottom aeration there are two important, but conflicting factors. The first is that oxygen diffusion expressed as Standard Oxygen Transfer Efficiency (SOTE) will become better the deeper the aeration membrane is located in the tank, due to the increased oxygen partial pressure, and the longer route of the bubbles through the liquid (see Figure 2). 16