Biomedical Engineering Reference
In-Depth Information
1. Introduction
EBPR (Enhanced Biological Phosphorus Removal) technology is promising for
phosphorus removal since it is more environment friendly and more cost-effective than
chemical phosphorus removal processes (Robert et al., 2003). The reliability of EBPR
processes, however, is notorious and prevents wide spreading of the technology (Csiti, 1991;
Robert et al., 2003). Failure or deteriorated performance of EBPR processes are inherently
related to PAO deactivation, which are triggered by improper application of easily adjustable
operational parameters such as sludge retention time (SRT) (Akar et al., 2006), hydraulic
retention time (HRT) (Csiti, 1991) , internal/external return ratio (Ochmen et al., 2005;
Beatons et al., 1999; Csiti 1991), and costly adjustable parameters such as temperature (T)
(Beatons et al, 1999), pH (Carsson et al., 1997; Kuba et al., 1997; Ochmen et al., 2005; Roske
and Schonborn, 1994; Smolders et al., 1994;) , and wastewater characteristics (Cech and
Hartman, 1993; Meinhold et al., 1998; Ahn et al., 2002; Maurer et al., 1997) . Numerous
studies have been done regarding to each single factor listed above, but mainly with synthetic
wastewater in laboratory scales (Csiti. 1991; Meinhold et al., 2005; Ochmen et al., 1998;
Smolders et al.,1994).
The parallel AN/AO process was first proposed by the authors to efficiently use
denitrifying phosphorus removing bacteria (DPB) (Xia and Liu, 2006), a kind of facultative
bacteria performing phosphorus and nitrogen removal simultaneously (Meinhold et al., 1999;
Kerrn et al., 1994; Kuba 1997; Kuba 1996; Chuang et al., 1998; ). The process performed
well with average COD, TN, TP removal efficiency 74%, 65%, and 84% respectively before
occurrence of PAO deactivation. However, in some operations of the parallel AN/AO
process, PAO deactivation occurred and decreased phosphorus removal efficiency of the
process to 55%; PAO deactivation occurred also in some operations of three anaerobic-anoxic
SBR reactors using seed sludge from the parallel AN/AO process.
Aim of this study was to investigate factors that trigger PAO deactivation in both
configurations, determine interaction effects of different factors and find solutions to
rejuvenate PAO from deactivated EBPR systems.
2 Materials and Methods
2.1. The Parallel AN/AO Process
Schematic diagram of the parallel AN/AO process was demonstrated in figure 1 together
with mass balance of main pollutants for a reoported operation (Xia and Liu, 2006).. Main
body of the process consists of an AN unit and a parallel AO unit. The AO unit consists of an
anaerobic compartment and two aerobic compartments (abrreviated as Ana1, Aer1 and Aer2
respectively in the schematic diagram) and the AN unit consists of an anaerobic compartment
and two anoxic compartments (abrreviated as Ana2, Ano1 and Ano2 respectively in the
schematic diagram). The 6 compartments are uniform in size and each has a 20L working
volume. Oxygen was provided by air diffusors in Aer1 and Aer2; nitrate needed in anoxic
compartments was provided by internal return stream from Aer2 to Ano1. Mixers were
installed in Ana1 , Ana2 , Ano1, and Ano2 to stir and suspend biomass. Extenal sludge return
ratios were identical in both AO and AN units, where mixed liquid solids (MLSS) in the AN
