Abstract
P-limitation in enhanced biological phosphorus removal (EBPR) systems fed with acetate, has generally been considered as a condition leading to enrichment of organisms of the genotype’ Candidatus Competibacter phosphatis’ expressing the glycogen-accumulating organisms (GAO) phenotype. Recent studies have demonstrated in short-term experiments that organisms of the genotype ‘Candidatus Accumulibacter phosphatis’ clade I and II, known to express the polyphosphate-accumulating organisms (PAO) phenotype can switch to the GAO phenotype when poly-P is absent, but are performing the HAc-uptake at lower kinetic rates, where clade I showed the lowest rates. The objective of this study was to verify whether organisms of the genotype ‘Candidatus Accumulibacter phosphatis’ can also be enriched under P-limiting conditions while expressing a GAO phenotype and more specifically to see which specific clade prevails. A sequencing batch reactor was inoculated with activated sludge to enrich an EBPR culture for a cultivation period of 128 days (16 times the solids retention time) under P-limiting conditions. A mixed culture was obtained comprising of 49 % ‘Candidatus Accumulibacter phosphatis’ clade II and 46 % ‘Candidatus Competibacter phosphatis’. The culture performed a full GAO metabolism for anaerobic HAc-uptake, but was still able to switch to a PAO metabolism, taking up excessive amounts of phosphate during the aerobic phase when it became available in the influent. These findings show that P-limitation, often used as strategy for enrichment of ‘Candidatus Competibacter phosphatis’, does not always lead to enrichment of only ‘Candidatus Competibacter phosphatis’. Furthermore, it demonstrates that ‘Candidatus Accumulibacter phosphatis’ are able to proliferate in activated sludge systems for periods of up to 128 days or longer when the influent phosphate concentrations are just enough for assimilation purposes and no poly-P is formed. The ‘Candidatus Accumulibacter phosphatis’ retain the ability to switch to the PAO phenotype, taking up phosphate from the influent as soon as it becomes available.Electronic supplementary materialThe online version of this article (doi:10.1186/s13568-016-0214-z) contains supplementary material, which is available to authorized users.
Highlights
Enhanced biological phosphorus removal (EBPR) is a microbial process for removal of excessive amounts of phosphorus from wastewater through storage of intracellular polyphosphate by polyphosphate-accumulating organisms (PAO) and excess sludge wasting.Due to its high efficiency and cost-effectiveness, the process is widely implemented in biological wastewater treatment systems
When activated sludge is cycled through alternating anaerobic and aerobic zones, organisms with the PAO phenotype are able to take up phosphate from the liquid phase and store it as intracellular polyphosphate, leading to P-removal from the bulk liquid via PAO cell removal through the wastage of activated sludge
To verify if the ‘Candidatus Accumulibacter phosphatis’, that seemed to be present, were able to take up phosphate, two consecutive cycle tests were conducted after approximately 14 solids retention time (SRT) (113 days), in which 0.65 P-mmol l−1 (20 mgP l−1) was added to the reactor at the end of each anaerobic phase (Fig. 3b)
Summary
Enhanced biological phosphorus removal (EBPR) is a microbial process for removal of excessive amounts of phosphorus from wastewater through storage of intracellular polyphosphate (poly-P) by polyphosphate-accumulating organisms (PAO) and excess sludge wasting.Due to its high efficiency and cost-effectiveness, the process is widely implemented in biological wastewater treatment systems. By linking microbial community composition with EBPR performance, “Candidatus Accumulibacter phosphatis” were identified as the organisms expressing the PAO phenotype in many laboratory EBPR systems (Bond et al 1995, 1999; Hesselman et al 1999; Crocetti et al 2000) as well as full-scale waste water treatment plants (Zilles et al 2002; Kong et al 2004; Gu et al 2005; He et al 2005; Wong et al 2005) Another group of organisms, ‘Candidatus Competibacter phosphatis’, are considered to compete with ‘Candidatus Accumulibacter phosphatis’ for acetate (HAc), expressing a so called glycogen-accumulating organisms (GAO) phenotype (Mino et al 1987; Nielsen et al 1999; Crocetti et al 2002). The competition between ‘Candidatus Accumulibacter phosphatis’ and ‘Candidatus Competibacter phosphatis’ communities in EBPR processes has been the subject of several laboratory studies, often using highly enriched EBPR cultures expressing the PAO and GAO phenotypes (Oehmen et al 2007)
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