Improving sludge digestion sustainability

Posted: Monday 17th May 2010

Bruno Bigot, Technical Director, Veolia Water Solutions & Technologies discusses the cost benefits achievable by using thermal hydrolysis as a pre-treatment before anaerobic digestion.

Anaerobic digestion of sewage sludge is an established process which generates biogas from the sludge and, at the same time, reduces the volume of sludge for ultimate disposal; but can it be made more efficient? This was the question posed almost a decade ago to Engineers at Veolia Water Solutions & Technologies. Their answer was BIOTHELYS, a thermal hydrolysis process that pre-treats thickened or dewatered sludges prior to anaerobic digestion. The process hydrolyses organic solids to solubilise them and make them more readily biodegradable. Hydrolysis disrupts cellular material, floc particles and organic macromolecules, so the pre-treated sludge is less viscous, easier to pump and allows a higher sludge solid concentration to be fed to the downstream anaerobic digester which is therefore smaller and lower capital cost. In addition, the greater biodegradability of hydrolysed sludge improves removal of volatile suspended solids which increases the biogas yield and reduces the quantity of sludge for final disposal.

The Veolia Water Solutions & Technologies BIOTHELYS plant at Saumur in the Loire Valley treats sludge from an extended aeration plant serving a population equivalent of 60,000. Since the thermal hydrolysis process operates at a temperature of 160C, an important sustainability criterion is minimising the energy needed for heating. This is achieved by dewatering the raw sludge to about -15 %ds using a centrifuge to reduce the volume and, hence, the heat input.

The dewatered sludge is pumped to two hydrolysis reactors working in parallel and out of phase with each other. Each reactor in turn goes through a two hour multi-step cycle. First the reactor is filled with raw sludge and this is pre-heated with recycled flash steam from the other reactor. Heating to hydrolysis temperature is completed by injecting live steam into the sludge from a steam generator fired with biogas. There are no mechanical rotating parts in the thermal hydrolysis reactors, sufficient mixing of raw sludge being achieved by steam injection.

Once hydrolysis temperature has been reached it is maintained for a period of 30 minutes before the pressure is reduced generating flash steam, which is recovered for pre-heating the other reactor. Finally the reactor is emptied using the residual pressure in the reactor to aid discharge to the buffer tank where the hydrolysed sludge is stored prior to being cooled to about 40C and pumped continuously to the anaerobic digester. The digester is mechanically mixed by a top-entry agitator, and there is no recirculation or heating system. The digester temperature is regulated by controlling the temperature of the hydrolysed sludge feed by means of a heat exchanger at the buffer tank outlet. The digested sludge is dewatered and stored in covered cells for nine months after which it is spread on agricultural land.

At Saumur, the thermal hydrolysis process is self sufficient in energy, using some of the biogas produced by the anaerobic digestion. Excess biogas is fed to two 20kW CHP plants which generate power for the Saumur works and also preheat the softened water feed to the steam generator.

The BIOTHELYS process followed by mesophilic anaerobic digestion at Saumur is currently achieving about 45-50% removal of volatile suspended solids compared to 30-35% removal typically achieved by a single stage mesophilic digester treating similar extended aeration sludge. Also for the final dewatering, a 30 % ds cake is achieved compared to 22 -23 % ds cake typically achieved on digested sludge from extended aeration treatment. That means a reduction in sludge for disposal of about 50%. The biogas production is increased in the same proportion, and the Saumur biogas is 66% methane and only about 34% carbon dioxide.

The success of the Saumur plant clearly demonstrates the cost benefits achievable by using thermal hydrolysis as a pre-treatment before anaerobic digestion. BIOTHELYS has been applied to various types of sludge including primary, biological and blends of primary and biological sludges and significantly enhances volatile solids removal and sludge dewaterability and gives a considerable reduction in the quantity of sludge for final disposal. The process also optimises energy recovery in the form of biogas from biological sludge digestion whilst producing a high quality sludge free of pathogens, stabilised, odourless and with good fertilising characteristics making final disposal to agriculture a practicable option.

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