The effect of filamentous bacteria on foam production and stability

Abstract

Bacteria have been implicated in the formation of viscous brown foams that can appear suddenly on wastewater treatment plants. Three strains of the filamentous bacterium Gordonia amarae, isolated from wastewater treatment plants, were investigated to determine their effect on foam formation and stabilisation. During the exponential phase of the bacterial growth a biosurfactant was formed, causing a significant drop in the surface tension of the filtered medium and the formation of persistent foam. Foaming tests in the presence and absence of bacteria showed that bacteria increased foam persistence, most probably by reducing the drainage from the lamellae between bubbles. Experiments showed that ≥55% of the three bacterial strains partitioned into the foam produced by the biosurfactant, indicating that their surfaces were hydrophobic. The extent of partitioning was independent of the growth stage, suggesting that the cell surface hydrophobicity did not change with age, or with cell viability. This work shows that, although the G. amarae cells themselves do not cause foaming, they do produce biosurfactant, which aids foam formation, and they stabilise the foam by reducing the rate of drainage from the foam lamellae.

Introduction

The activated-sludge process is one of the most widely used treatments for domestic and industrial wastewater. One common problem, first noted in 1969 [1], is the formation of stable foam, which reduces oxygen transfer, decreases the quality of the effluent and therefore increases maintenance costs [2]. Foam also increases the potential release of airborne pathogenic bacteria [2]. Three types of foam in wastewater treatment plants have been described: those caused by excess surfactant in the influent, transient foams associated with the start-up of new plants and viscous brown foams that occur spontaneously during normal plant operation [3].

Foam is a collection of bubbles separated by thin liquid films or lamellae: its formation involves the dispersion of a gas in a liquid. The conditions necessary for the formation of foams have been well characterised: a surface-active agent must be present and there must be a source of gas bubbles [4]. Surface-active agents in the wastewater setting can include synthetic detergents, fats, oils, greases and biosurfactants. In fact any molecule that preferentially adsorbs at the liquid–air surface, and thereby decreases the interfacial free energy allowing the surface to expand, acts as a surface-active agent. Foams are stabilised when drainage from the lamellae is minimised by increasing the viscosity of the liquid phase [5], or when there are hydrophobic particles between the bubbles that prevent drainage [6]. The stability of foam can be influenced by the nature of the surfactants (e.g. anionic or non-ionic synthetic detergents or biosurfactants), and their adsorption to adjacent interfaces causing electrostatic and steric repulsion [5].

Wastewater treatment plants can potentially furnish all of the conditions required for stable foam formation. Surfactants enter with the influent and are produced by bacteria in the plant; gas bubbles are provided by aeration, which is necessary for aerobic digestion; hydrophobic particles can be delivered in the influent, and in addition hydrophobic bacteria are present in the plant within the mixed liquor [2], [7].

It has been proposed that bacteria are responsible for the formation of the viscous brown foams on wastewater treatment plants [8], [9]. Microscopic examination of such foams has identified a wide range of bacteria. Among the most common are the filamentous bacteria Gordonia amarae [2], [10] and Microthrix parvicella [2].

Several studies have investigated the role of bacteria in foaming. For example, Pagilla et al. [11] studied the variation in total suspended solids concentration and surface tension with growth phase for G. amarae grown with acetate, hexadecane or a mixture of the two as the carbon source. They found that more surfactant was produced in the presence of the hydrophobic substance, hexadecane. Despite the more recent data, the conclusion that the detailed mechanism by which bacteria may stabilise the foams has not been established [2], [12] is still valid.

In this work we studied pure cultures of three strains of the filamentous actinomycete G. amarae, isolated from the foam or mixed liquor of two wastewater treatment plants in Victoria, Australia. The aim was to identify how factors that may be associated with foam production and stability vary during the bacterial growth cycle. The study embraced a range of parameters: biomass, surface tension, respiratory status and foaming of both whole culture and filtered growth medium. Together this information helps us to understand the role of bacteria in foam production and stabilisation. We also sought to establish any differences between the behaviour of related, but different, bacterial strains.