Deconvolution Methods and Applications of Auditory Evoked Response Using High Rate Stimulation
Yuanyuan Su, Zhenji Li, Tao Wang · InTech eBooks · 2010
IntroductionIt is widely known that many regions in the world have scarce water resources.In these areas the groundwater aquifers are also found to be in a critical condition as a result of overexploitation.That is why, in such regions, the reuse of wastewater is a common practice and the competent authorities undertake multiple courses of action to encourage its reuse.Legislation implementing the reclaimed wastewater reuse is likewise very demanding in terms of quality and health and safety, which has resulted in the application of new technologies for water treatment and purification.Among the new emerging technologies appears the use of micro and ultrafiltration membranes as highly efficient systems, which are economically feasible for obtaining high quality recycled water.Over the last two decades the technology of membrane bioreactors (MBRs) has reached a significant market share in wastewater treatment and it is expected to grow at a compound annual growth rate (CAGR) of 13.2%, higher than that of other advanced technologies and other membrane processes, increasing its market value from $ 337 million in 2010 to 627 million in 2015 (BCC, 2011).Aerobic MBRs represent an important technical option for wastewater reuse, being very compact and efficient systems for separating suspended and colloidal matter, which are able to achieve the highest effluent quality standards for disinfection and clarification.The main limitation for their widespread application is their high energy demand -between 0.45 and 0.65 kWh m -3 for the highest optimum operation from a demonstration plant, according to recent studies (Garcés et al., 2007;Tao et al., 2009).The advantages of this process over the conventional activated sludge process are widely known (Judd, 2010), among these one of the most cited is the reduction in sludge production which results from operation at high solid retention time (SRT).However, its consequences for the structure and metabolism of the microbial suspensions need to be studied in detail.Generally, we would expect that microorganisms subjected to severe substrate limitation should preferentially meet their maintenance energy requirements instead of producing additional biomass (Wei et al., 2003).This substrate limitation imposed on an MBR, by operating at low food-to-microorganism ratios (F/M), should modify the activity and characteristics of the sludge and could be the key factor for determining the process performance, particularly the membrane filtration (Trussell et al., 2006). www.intechopen.comBiomass -Detection, Production and Usage 266 Biokinetic models are widely used to design activated sludge process.Knowledge of biokinetics parameters allows modelling of the process including the substrate biodegradation rate and biomass growth.At low growth conditions, as is demanded in MBRs, other processes apart from microbial growth have to be taken into consideration.These have been recognized as the maintenance energy requirement, endogenous respiration and subsequent cryptic growth (Van Loosdrecht & Hence, 1999).Macroscopically they cannot be perceived, but, from a practical point of view, the global process can be described by Pirt´s equation (Pirt, 1965).Although there are several experiences with membrane bioreactors working without biomass purge (Rosenberger et al., 2002a;Pollice et al., 2004;Laera et al., 2005), none of these authors apply any kinetics models to describe process performance.Furthermore, these results were obtained in similar conditions, by treating raw municipal wastewater with a high substrate concentration, and it is interesting to compare this behaviour with an MBR treating wastewater with a low organic load.Additionally, not enough is known about the morphology and extracellular polymeric substance (EPS) production for total sludge retention and low F/M ratios.The aim of this chapter is to summarize the current status of membrane bioreactor technology for wastewater treatment (Section 2.1).The advantages against the conventional activated sludge process and technological challenges are assessed (Section 2.2).Some design and operation trends, based on full-scale experience, are reviewed (Section 2.3).To discuss both fundamental aspects, biotreatment and filtration, some experimental results are presented.Special attention was given to the microbial growth modelling (Section 4.1.1),biomass characterisation (Sections 4.1.2to 4.1.5)and membrane fouling mechanisms (Section 4.2).Some of these results have at the same time been compared with biomass from a conventional activated sludge process (CAS) operated in parallel. Membrane bioreactor (MBR) technology Current status and process descriptionThe current penetration in the wastewater treatment market of the membrane bioreactors gives an idea of the degree of maturity reached by this technology.The most cited market analysis report indicates an annual growth rate of 13.2 % and predicts a global market value of $ 627 million in 2015 (BCC, 2011).Actually MBRs have been implemented in more than 200 countries (Icon, 2008).Particularly striking is the case of China or some European countries with an implementation rate of over 50% and 20%, respectively.This technological maturity in urban wastewater market is also reflected in two main issues: the diversity of technology suppliers and the upward trend in plant size.Since 1990, the number of MBR membrane module products has grown exponentially until reaching over 50 different providers by the end of 2009 (Judd, 2010).However, globally, the market is dominated by three suppliers: Kubota, Mitsubishi Rayon and GE Zenon, which held about 85-90 % of the urban wastewater market (Pearce, 2008).In regard to the largest MBRs, there are 8 plants with a peak design capacity greater than 50 MLD (Table 1), all of them constructed before 2007 (Judd, 2010).MBR technology is based on the combination of conventional activated sludge treatment together with a process filtration through a membrane with a pore size between 10 nm and 0.4 microns (micro/ultrafiltration), which allows sludge separation.The membrane is a barrier that retains all particles, colloids, bacteria and viruses, providing a complete www.intechopen.com