The effect of ultrasonic sludge pre-treatment on anaerobic digestion

Abstract

As sewage sludge dumping at sea is banned from 31st December 1998, water companies are having to find alternative outlets for large volumes of sludge. This project looks at a method of improving the conventional sludge treatment process of anaerobic sludge digestion, by pre-treating sludge with ultrasound. Ultrasound produces cavitation, which breaks up sludge solids, making it easier for bacteria to utilise them. In theory, this leads to a greater reduction in volatile solids and an increase in the volume of methane produced. Sludge was thickened in a drum thickener and passed through a Nearfield Acoustical Processor (NAP-3606-HP-TC) ultrasound generator. Two different intensities were used, 5 amps and 16 amps. Sludge was then placed into the respective holding tanks of three 100 litre (1) capacity pilot scale digesters. Sludge treated at 5 amps was supplied to the first digester, sludge treated at 16 amps was supplied to the second digester, and the third digester received unsonicated sludge. Batch digestion tests, soluble chemical oxygen demand (sCOD) tests and capillary suction time (CST) tests were also carried out. No major differences in gas production or volatile solids reduction were found between the three pilot scale digesters, possibly because the effects of sonication were negated during the sludge storage stage, due to shearing by the mixing device. However, the batch tests, soluble COD tests and CST tests carried out all showed that sonication had a marked effect. The CST tests showed that sonication made sludge harder to dewater, before and after subsequent digestion. Future work should include a more detailed investigation into the effects of sonication on sludge dewaterability. This is because a pre-treatment process that produces a sludge that cannot be dewatered may not be economically viable, despite any advantages of the pre-treatment process.