Effect of chemical and biological surfactants on activated sludge of MBR system: Microscopic analysis and foam test
Graphical abstract
Introduction
Foaming phenomenon in wastewater treatment plants (WWTP) is an issue that adversely affects the management of the process; for this reason several important scientific efforts have been focused to the investigation of its occurrence (see among others Jenkins et al., 1986, Jenkins et al., 2004). Studies on chemical and biological foaming in conventional activated sludge (CAS) plants moved by the end of ’60: particularly, stability and persistence of foam in aeration and settling units were accurately investigated among others by Torregrossa et al. (2005).
Foaming appears on aeration tank surfaces as stable air bubbles enclosed in liquid film. Several factors contribute to the arising of such phenomenon but, generally, two are the most relevant: presence of synthetic surfactants (white scum/foam) and co-presence of hydrophobic substances secreted by particular filamentous microorganisms (brown scum/foam) (Davenport et al., 2000, Davenport et al., 2008, De los Reyes et al., 2002).
Raw wastewater features, in terms of: Carbon-to-Nitrogen ratio (C:N); surfactants presence; and toxic substances (which can damage cellular membrane with a consequent production of Extra Cellular Polymers or EPSs) (Jenkins et al., 2004), represent the main causes of foam occurrence. In particular, C:N ratio significantly influence foam production when is value deviates from a correct balance (BOD:N:P = 100:5:1). Moreover, a strong imbalance between organic matter and nutrients can facilitate the proliferation of filamentous bacteria, so called “foam-former”, that contribute to the arising of foaming. At the same time, organic load shock can likewise cause temporary foaming phenomenon, e.g. during a start-up phase (Tandoi et al., 1998).
Although foaming originally was associated with CAS operation, recently this phenomenon has been observed also in advanced treatment processes such as membrane bioreactors (MBR) (Nakajima and Mishima, 2005, Di Bella et al., 2011, Di Bella and Torregrossa, 2013).
It is worth noticing that in such systems foaming can occur even when synthetic surfactants or filamentous microorganisms are absent or negligible; indeed, MBR lay-out affects formation and dimension of activated sludge flocs (Durante et al., 2006). In this context, biological deflocculation coupled with organic load shock, temperature variation and high concentration of dissolved oxygen (DO) contribute to increase the EPSs concentration (Judd, 2011, Mannina and Di Bella, 2012).
EPSs produced or assimilated by bacteria, making up the structure of activated sludge floc, are composed by several organic substances, in the majority carbohydrates and proteins (Judd, 2011). It has been demonstrated that high concentration of EPSs produced by bacteria present in mixed liquor, even not foam-formers, can worsen than “conventional” phenomenon of foaming: therefore, it is possible to provide a particular development in foam formation if the surfactants from synthetic and biological act together, especially in the MBR systems that is a “foam’s traps” (Nakajima and Mishima, 2005, Di Bella et al., 2011).
Bearing in mind what discussed above, with the aim to increase knowledge on causes and features of foam in MBR systems, the present paper reports on the results of MBR foaming development in presence and absence of synthetics surfactants in the bioreactor. Furthermore, a new analytic method, based on “ink test”, has been applied and proposed, in order to make the EPSs estimation in the mixed liquor faster.
Section snippets
MBR system description
A submerged MBR pilot plant has been realized a fed with synthetic wastewater made up mainly of sodium acetate (CH3COONa) and ammonium chloride (NH4Cl) and of a secondary elements blend. The oxidation unit, continuously aerated (DO concentration > 3 mg L−1), was a perspex tank (15 L) provided with a level sensor which controlled the feeding pump. Filtration was operated by a GE hollow fiber membrane module (ZeeWeed™01; porosity 0.04 μm; specific surface 0.093 m2). A Programmable Logical Controller
General features of the mixed liquor
After the start-up, during phases I and II, the mixed liquor suspended solid (MLSS) concentration, depicted in Fig. 1a, remained quite constant and close to inoculum value (5 g TSS L−1), probably due to biomass acclimatization to synthetic wastewater. In the meanwhile VSS/TSS ratio remained close to 70%. From phase III (C:N = 100:1) MLSS concentration gradually rose up to about 8 g TSS L−1. The biomass increase (in terms of MLSS) occurred also in phase IV, when the ratio COD/N was set at 200:1.
Finally,
Conclusions
Synthetics surfactants were adequately removed by the biological system. It was noticed a contribution of residual surfactants not biologically removed in increasing the mixed liquor aptitude to foaming. Biological foam sampled was characterized by a not negligible presence of filamentous foam-former bacteria. The acquired experience suggests that surfactants do play an important role in foaming phenomena in MBR system, moreover with regard at the achieving of optimal condition to arise the
Acknowledgements
This research was funded by the National Research Project PRIN 2009 (Project Title: Experimental analysis on MBR systems to study and to realize a model of fouling including the physical and microbiological features of sludge and the operational conditions that influence this phenomenon); Italian Ministry of Education, University and Research. Authors are very grateful to Ph.D. Fulvio Capodici for his support in image processing.
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