Vergara, L.1, Nickel, K.2 and Neis, U.2,3
Wastewater treatment plants (WWTPs) are typically designed under conservative design guidelines and are operated based on historic practices. Such facilities often have considerable additional capacity that can be realized through optimisation. Improvements in wastewater and sludge treatment units mean an important saving in operational costs which take a special importance due to the unmanageable quantities of sludge produced by the rapid urbanization growth and difficult economic situations nowadays. Ultrawaves ultrasonic technology has been mainly designed to improve WWTPs biological processes as anaerobic stabilisation and biological reactor. The first full-scale applications of Ultrawaves technology were installed more than ten years ago in Germany for the pre-treatment of waste activated sludge (WAS) prior to anaerobic digestion (AD). Today more than 150 ultrasound reactors worldwide maximise anaerobic digesters as well as biological reactors in many WWTPs, providing confirmation of the success of this technology. Research is still carried out to better understand how ultrasound manifestations can impact specifically on aerobic processes since activated sludge contains numerous microorganisms which are imbedded in a slime-matrix and it is not easy to interpret the impact of sonication on all aerobic biomass. This article reports on fundamental research as well as full scale Ultrawaves experience for the following applications:
Ultrasound, optimisation, aerobic process, anaerobic digestion, excess sludge, nitrogen removal, filamentous microorganisms.
Ultrasound technology in environmental engineering started in the mid-nineties last century and was mainly driven by work in Germany. From the beginning it was stated to be a very effective mechanical pre-treatment method to enhance the AD of the sewage sludge produced during the biological wastewater treatment process. The AD process had been in use for more than half a century with no major improvement and no serious attempt to overcome its technical limitations. The Ultrawaves idea was to overcome the rate limiting hydrolysis step of the sludge AD by ultrasound, which disintegrates sludge cells and subsequently intensifies the anaerobic degradation process, eventually resulting in more biogas and less residual sludge. The first full-scale installation appeared in Germany after the turn of the millennium and today WWTPs equipped with Ultrawaves technology can be found around the world in different countries like Spain, Ireland, Brazil, Switzerland, the Netherlands, Denmark, Poland, Hungary, Australia, and others.
It has been demonstrated that low frequency ultrasound waves generate the cavitation necessary to produce mechanical shear forces associated with sludge disintegration. Combined with high intensity ultrasound, the cell aggregates as well as single cells are destroyed and enzymatic and intracellular material is released into the medium resulting in a higher degree of substrate bio-availability for the remaining living microorganisms. In effect, the enzymatic biological hydrolysis, which is the initial and rate limiting of the biological food chain, is substituted and catalysed by this mechanical disintegration of the sludge (Tiehm et al., 2001).
Continuous research and development has created Ultrawaves ultrasound technology new and revolutionary applications in aerobic processes. There has been a big effort to understand the best way to optimise a biological reactor with ultrasound which has now been reached through different applications. It is now possible the complete elimination of associated problems with bulking and foaming through a selective sonication of a very tiny amount of Returned Activated Sludge (RAS). In addition it is also possible to enhance a dramatic excess sludge reduction when a partial WAS flow is disintegrated and returned back to the biological reactor whenever a cell lysis and cryptic growth process is induced. Sonication causes cell lysis with the consequent solubilisation of cellular constituents which become substrate available for further biodegradation which in turn results in an overall reduction of the excess sludge production (Hamer and Mason 1987; Canales et al., 1994). The most recent application has been carried out in nitrogen removal biologically, where sonicated WAS represents a very good readily biodegradable autochthonous carbon source to denitrify. When WAS is sonicated and recycled back to the anoxic zone of the biological reactor, then is used as carbon source to support denitrification and the facility transforms a waste into a resource with the subsequent saving in carbon source purchase and sludge disposal cost.