Accepted Manuscript Title: Mechanisms of Foam Formation in Anaerobic Digesters Author: Bhargavi Subramanian Krishna R. Pagilla PII: DOI: Reference:
S0927-7765(14)00657-2 http://dx.doi.org/doi:10.1016/j.colsurfb.2014.11.032 COLSUB 6756
To appear in:
Colloids and Surfaces B: Biointerfaces
Received date: Revised date: Accepted date:
14-9-2014 29-10-2014 20-11-2014
Please cite this article as: B. Subramanian, K.R. Pagilla, Mechanisms of Foam Formation in Anaerobic Digesters, Colloids and Surfaces B: Biointerfaces (2014), http://dx.doi.org/10.1016/j.colsurfb.2014.11.032 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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*Graphical Abstract (for review)
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Highlights Interactions between uncontrolled and controlled factors contributing to AD foam are discussed.
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Nucleation type may be different for the two types of AD foam.
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Bubble nucleation plays a role in AD foaming though the type of nucleation is under question.
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Due to the presence of biogas, AD foaming contributors are required for the foaming problem to be evident.
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Mechanisms of Foam Formation in Anaerobic Digesters Bhargavi Subramanian1, and Krishna R. Pagilla1,* 1
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Department of Civil, Architectural, and Environmental Engineering, Illinois Institute of Technology, Chicago IL 60616, USA
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* Correspondence to:
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Prof. Krishna R. Pagilla, Ph.D., P.E., B.C.E.E. Department of Civil, Architectural, and Environmental Engineering 3201 S Dearborn Street Illinois Institute of Technology Chicago, IL 60616, USA Phone: +1 312 567 5717
[email protected] 2
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1 2
Abstract An anaerobic digester (AD) is the most essential step to generate energy in the form of biogas from waste. AD foaming is widespread and leads to deterioration of the AD
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process and operation. In extreme conditions, AD foaming poses a significant safety risk
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and considerable economic impacts. It is, therefore, necessary to understand the
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fundamentals of AD foaming to develop effective strategies that can help minimize the
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foaming impacts. Several aspects of AD foaming have attracted considerable research
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attention, however, the focused has been mainly on site specific causes and prevention.
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Here, the available three-phase foam literature is reviewed with an emphasis on the
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fundamental aspects of bubble formation in AD: similarities between AD foams and other
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“desirable” foams, surface rheology, physico-chemical aspects of carbon dioxide (CO2) in
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digesters, dynamics of the gas-phase, pH, alkalinity and certain relationships between these
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factors are discussed. All of the abovementioned fundamental aspects seem to be involved
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in AD foam formation. However, the detailed relationship between these uncontrolled and
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controlled factors, foam formation and its implications for process and operation of AD is
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still inconclusive.
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Keywords: Anaerobic digester foaming, three-phase foam, rapid volume expansion, bubble
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formation.
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Contents
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1
Introduction
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2
Background
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2.1
Three phase AD foam fundamentals
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2.2
Comparison between general three-phase foams and AD foams
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2.3
Possible types of bubble formation in AD
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Factors contributing to AD foam
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3.1
Role of controlled and uncontrolled factors in AD foaming
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3.1.1
Basic mathematical models for bubble growth
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3.2
Mechanisms of AD foam nucleation
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3.3
Contribution of the controlled and uncontrolled factors to AD foam formation
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3.3.1
Contribution of the gas phase to foaming - role of methane
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3.3.2
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3.3.3
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3.3.4
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3.3.5
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3.3.6
Interfacial phenomena between sludge solids, liquid and gas
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Discussion
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Acknowledgements
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Physicochemical characteristics of CO2 in AD Particle size distribution and surface characteristics Sludge rheology
Temperature and pressure effects
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References
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List of important abbreviations
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AD
Anaerobic Digester
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AS
Activated Sludge
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EPS
Extracellular Polymeric Substances
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OLR
Organic Loading Rate
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PS
Primary Sludge
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PS:WAS
Primary Sludge:Waste Activated Sludge (Ratio)
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SRT
Sludge Retention Time
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TS
Total Solids
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VFA
Volatile Fatty Acids
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VS
Volatile Solids
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WAS
Waste Activated Sludge
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WWTP
Wastewater Treatment Plant
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1. Introduction Anaerobic digestion (AD) is the primary sludge stabilization and energy production method from wastewater wherein organic material is anaerobically converted to biogas
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through a series of biochemical steps [1]. Consequently, it is the key to overall energy
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sustainability of wastewater treatment plants (WWTPs). One of the most common
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operating problems in AD process of WWTPs is foaming, leading to performance and
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operating problems. AD foaming is a complex three-phase phenomenon (liquid-solid-gas)
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contributed by surface active materials or surfactants (solids and soluble constituents) and
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water present in the sludge, and biogas produced within the digester. Such foam is mostly
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uncontrolled and bubble creation occurs when all of the three constituents are present at
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minimum threshold levels. A certain degree of foaming is always present in all AD
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systems, but becomes a problem when it begins to adversely affect the process, (popularly,
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termed as foaming problem or episode). In general, the main impacts caused by AD
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foaming episode are (i) reduced digester performance by removing active volume resulting
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in lowered gas production and volatile solids (VS) destruction; (ii) tank mechanical and
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structure failure due to persistent foaming episodes; (iii) significant maintenance required
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for cleaning biogas piping and foam overspills; (iv) potential short-circuiting of pathogens
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due to lower active volume in the digester. These impacts lead to economic losses of
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varying magnitude in the form of biogas loss, digester cleaning costs after a foam incident,
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digester downtime/active volume loss, repair costs, and personnel costs for cleanup and
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additional maintenance [2,3].
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In practice, severe AD foaming may be caused by a combination of several factors, initiated by one or more primary causes. The complexity of the AD system and the foaming
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phenomenon makes it difficult to correlate the type of foaming to a single cause directly.
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However, the several causes discussed in this review have been popularly accepted to
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cause/contribute to either type of foaming to various extents. Occurrence of biological
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foaming due to filamentous foaming bacteria (mainly nocardioforms (Gordona amarae))
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and/or Microthrix parvicella in the feed sludge originating from upstream biological
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wastewater treatment process such as activated sludge (AS) has been a key contributor or
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cause of AD foaming [2,5]. In addition to this biological cause, several others have been
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listed in the literature and have been popularly accepted to cause/contribute to AD foaming
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to various extent [2–11].
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Regardless of the cause/contributor, in spite of several decades of research,
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mechanisms of formation and stabilization of foam are not clear and general criteria to
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describe AD foam behaviour does not exist [12]. Therefore, the focus of this review is on
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various ways in which gas bubbles can be formed and stabilized leading to foam in ADs,
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which is an undesirable condition. The understanding of foam causing mechanisms is
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important for AD foaming for various reasons: (i) to determine how and to what extent the
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controlled and uncontrolled factors are related to each other and to foaming; (ii) to model
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the bubble growth that leads to AD foam; (iii) to predict if the foam in a digester is
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excessive and will become a problem; and (iv) to develop prevention and control methods.
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The ultimate objective of this review is to analyze mechanistic multidimensional
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knowledge on AD foaming. Such knowledge can relate AD foam characteristics with 7
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process and operational factors better, and in solving the problems associated with AD
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foaming.
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2
Background
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2.1
Three phase AD foam fundamentals
The various constituents required for three-phase foaming – liquid, surface active
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agents and solids, and gases (biogas) are present in all digesters at all times providing a
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conducive environment for foam formation. Though gas bubbles are the primary
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constituent in producing foams of any kind, surface properties of all the three phases
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influence foaming potential and foam stability in three-phase foams. Mainly, particulates
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stabilize foam by their attachment to the gas bubble surface, and by the flocculation of
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particles in the bulk solution at increasing particle concentrations [13–15]. Foamability
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increases with increasing particulate concentration until about 38% by weight in solution
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after which particles aggregate and fewer particles attach to the bubbles [16]. It is not
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certain whether this particle content threshold is applicable to all types of particles and all
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solutions since AD sludge usually contains less than 6% solids by weight in the bulk. The
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foamability of a three-phase system was observed to be directly proportional to particle
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concentration and inversely proportional to particle size [13]. Hence, there must be a
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particle concentration and particle size distribution where AD foams are more stable and
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vice versa. Figure 1 presents a schematic of three-phase foam in which the particles and the
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gas bubbles are relatively of the same size order. This visualization may be closer to AD
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foams than in systems where gas bubbles are much larger than the particles such as in AS
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foaming and detergent foams.
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2.2
Comparison between general three-phase foams and AD foams In addition to biological wastewater treatment and AD, foam generation also takes
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place in several industrial processes such as polymeric foam production [17], production of
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carbonated drinks [18,19], and processed food like whipped creams, cakes, bread, etc. [20].
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At the other end of the spectrum, bubble nucleation as foam also occurs in animals and
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humans (decompression sickness) [21], nuclear waste processing [16], and in nature as gas
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pressure driven eruption from volcanoes [22]. Thus far, published literature has only
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compared AD foams to AS foam [7], though some of the abovementioned foams are quite
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well understood. Though the specific mechanisms of bubble formation and stabilization are
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different in the various foam applications vs. AD, gas/air bubbles and liquids are the
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primary media in producing foams and surfactant molecules/solids are the stabilizing
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agents.
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Decades of research has resulted in fairly well developed systems of three-phase
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foams in various applications of “desirable foams” (whipped cream, cakes, carbonated
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drinks etc.) in food and other areas. Reviewing literature from these fields where foam
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formation is well developed has helped understand the behavior of three-phase foam in
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general, and to a certain extent, is the first step in adding to the understanding of the more
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complex digester foam. Each type of the foam structure in desirable foams is a result of
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proper blends of constituents, bubble/particle sizes, air entrainment/overrun (amount of air
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in the foam), temperature/pressure, blending/whipping time, etc. Most of the above-
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mentioned parameters that are important for formation of foam structures are mostly
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uncontrolled in digesters where the foam is “undesirable”, possibly explaining why
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different digester foams have been anecdotally referred to be similar to either
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detergent/soap lather, or gushing from a beer/carbonated beverage bottle, etc. Non-food foams such as detergent foams are similar to wine or beer foams because
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of high air content [23]. Aerated foods such as bread or whipped cream contain much lower
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air content than non-food foams. The constituent bubbles of such foams can have either one
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or two interfaces, based upon constituents and process parameters. Bubble surface may
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separate gas on the inside from the outside. These surfaces could be gaseous, liquid or solid
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up to the interface [24]. Soap bubbles have two interfaces, one on the inside, and one on the
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outside, with liquid in between. Bubbles in champagne or bread dough, have just one
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interface [24]. However, it seems that AD foam bubbles could be of either type as
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explained in this review; depending on whether it is a case of rapid volume expansion in the
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digester or conventional digester foam (discussed in Section 2.3).
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The particle-gas bubble system in AD is also different from most of the other three-
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phase foams. Almost all other foams involve smaller surface active particles/molecules
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stabilizing larger bubbles in a controlled manner. In AD foam, the gas phase is the biogas
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being produced or externally introduced into the digesters for gas mixing and contains
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soluble carbon dioxide and less soluble methane. AD foams involve stabilization of smaller
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bubbles by soluble surface active molecules and larger solids, in effect, behaving similar to
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a dissolved gas flotation system rather than dispersed gas flotation seen in traditional three-
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phase foams, including those found in foaming AS systems. Most probably, a combination
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of both dissolved flotation and dispersed flotation exists in ADs, depending on pH and
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solubility/pressure effects of the gas phase present in digesters. Since the biogas is forming
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throughout the volume of the digester, the bubble size could vary throughout the depth of
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the digester, with smaller bubbles at the bottom and larger ones at the top. Such a
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phenomenon of gas evolution is attributed to formation of many types of desirable foams
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[24].
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In an AD, the presence of sludge solids (with or without hydrophobic filamentous
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microorganisms) and soluble surface active materials act as stabilizers enhancing foam
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stability, quite similar to proteins and other stabilizers added to commercially available
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desirable foams.
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Possible types of bubble formation in AD
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In the case of uncontrolled foam structures, occurring in AD, the most common visual classification of type of AD foam by plant operators is the size of bubbles and their
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stability [26]. Foams with relatively large bubbles are short-lived when compared to foams
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with smaller bubbles which appear to be more stable. The properties of these foams and
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their mechanisms of their formation are not discussed in the literature. Consequently, it is
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also highly unlikely that a unified theory can explain all these kinds of digester foams. This
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discussion provides a brief insight into how different types of foam may be formed and
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stabilized in digesters. Based on the extensive review of three-phase foam literature, two
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types of foam bubbles could form in AD:
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1. Conventional foaming – Accumulates at the gas/liquid interface in the digester. Three-
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phase foam bubbles are formed in digesters with a coating of surface active material
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making them stable. CO2 and methane formed as biogas continuously diffuses into existing
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bubbles, entrapped during mixing [24] or getting attached to particles [19,27,28]. Bubbles
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incorporated in the sludge could serve as nucleation sites for the gas produced during AD.
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This type of bubble formation requires high concentrations of surfactants or solids with
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hydrophobic filaments; whose quantitative threshold estimates are case-specific and
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unknown. These bubbles contain much less gas and tend to have well separated spherical
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bubbles, mainly due to the thin films surrounding the bubbles in the foam structure.
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Following factors can potentially cause conventional foam events:
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Sludge feed characteristics: surface active agents in feed sludge; foam causing filaments
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Volatile fatty acid (VFA) production during digestion - Imbalances between the
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in feed sludge.
successive hydrolysis, acidogenesis and methanogenesis in AD process.
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2. Rapid expansion events – Such bubbles are formed by pressure difference and gas
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saturation in the digester. Solid particles and cell-wall fragments present in the digester may
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act as nucleation sites [29]. The main difference between this type of foam and
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conventional foaming is that gas holdup finally leads to the foam, in this case. It is
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postulated in this review that foaming in the conventional sense involves the stabilization
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step where surface active agents form a coating around the gas bubbles (more stable foam),
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whereas here, the foam is stabilized by interacting simultaneously with surface active
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material and particles (foam that is short lived). The digester contents appear to be rapidly
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expanding in volume at an alarming rate, causing digester overflows. The total volume of
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the digester can contract or expand according to the gas volume in the digester contents.
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Rapid volume expansion events could be caused by changes in biogas volumes present in
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the digester liquid rather than conventional three-phase foaming [30]. The source of bubble
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formation in the case of rapid expansion events is bubble nucleation, as discussed in
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subsequent sections of this review. The difference in the formation of these two types of
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foams is attributed to the heterogeneity of the species at the interfaces in AD and
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subsequent adsorption and interfacial interactions. The following controllable factors that
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exacerbate gas holdup can potentially cause rapid expansion events:
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Inconsistencies in organic loading rate (OLR) – shock loads, slug feeds, inconsistent
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feed rates.
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Inconsistencies in gas production leading to sudden gas release.
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Sudden pressure changes.
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Inadequate, intermittent, excessive or non-optimal high intensity mixing which results
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in dead zones and pockets of inhomogeneous material inside the digester.
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Both of these types of foams have been reported in full scale digesters and attributed
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to cause major foam episodes [30]. It is unknown if the two types of foaming events are
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mutually exclusive or one can lead to another; based on filament, particulate and surface
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active material thresholds. The bubble formation is mostly the same in both cases (gas
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surrounded by liquid and solids/surfactants), but in the case of rapid expansion, it is gas
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holdup (due to physical factors such as increase in gas pressure in the digester, etc.) that is
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finally released out of the sludge.
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Factors contributing to AD foam In most desirable foams, bubbles are produced by (i) agitating, whipping, shaking or
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beating (ii) by sparging or diffusing gas and (iii) bubble nucleation (depressurizing a gas
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saturated liquid); where the liquid becomes less supersaturated as a result of the pressure
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release. Then bubbles nucleate and grow to form foam. In AD, one or more of these sources
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of bubbles are known to occur. Mixing/whipping/mechanical agitation is the most widely 13
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used means of formation of foam structures in several applications, and the bubble size and
238
stability is maintained by careful process control [24]. In AD, there are several sources of
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mixing – natural and induced. The induced sources constitute the intended mixing system
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(installed mixers and equipment). Major sources of natural mixing include gas evolution
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during digestion as well as the inflow and outflow of sludge by pumping [31]. High
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intensity induced mixing sources are implicated to cause gas entrapment and release of
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bubbles, although the extent of their contribution is unclear. Bubble formation due to gas
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release is considered uncontrolled; though gas release is related to loading; gas quality and
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quantity are not directly controlled. The presence of CO2 in the digester at all times induces
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bubble nucleation depending upon pressure, pH, etc. and is considered to be uncontrolled.
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Therefore, such bubble creation through various means coupled with other inherent
248
characteristics could explain why only a fraction of the digesters experience foaming
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despite these conditions being present in all digesters. Such inherent or uncontrolled factors
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include:
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Surface characteristics, number of particles, and particle size distribution.
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Gas bubble size distribution, proportion of CO2/Methane, gas volume in liquid or foam.
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Natural mixing due to biogas production and convection.
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Nature and type of surfactants (in order to adhere to bubbles).
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Interfacial phenomena between sludge, liquid and gas.
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The above discussion leads to the understanding that foam formation and stability,
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may be a function of these uncontrollable factors or inherent qualities occurring in all
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digesters, while influenced by the controlled factors. 14
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On the other hand, controlled factors deal with the actual causes and contributors of
259
foaming, which influence the abovementioned uncontrolled factors in various ways to
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cause foaming. Table 1 presents a list of digester foaming causes from inlet to in-digester
262
conditions [32]. Such causes and contributors have been discussed in detail elsewhere and
263
will not be elaborated here. For the purposes of this review, we will consider how the
264
controlled and uncontrolled factors are related to each other and to AD foaming. The
265
underlying mechanisms of foaming cannot be completely explained without the potential
266
links between the groups of causes/contributors and uncontrolled factors. The following
267
sections will attempt to relate these groups of causes and mechanistically explain some of
268
these causes with respect to AD foam formation.
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3.1
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Role of controlled and uncontrolled factors in AD foaming Though the formation of foam is different from single bubble dynamics, the first
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step is bubble nucleation/formation. In general, the following non-independent steps are
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involved in the foam formation:
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Bubble formation/nucleation.
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Bubble growth.
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Bubble detachment/coarsening.
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Bubble stabilization.
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Figure 2 depicts the process of AD bubble formation and the role of controlled and
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uncontrollable factors. Bubble nucleation, subsequent growth and detachment of bubbles
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due to biogas production in digesters lead to undesirable foam episodes, mainly influenced
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by the controlled factors in various ways. In the following sections, we will attempt to
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explain the various ways in which these factors lead to AD foam.
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3.1.1
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Basic mathematical models for bubble growth Several published works have studied the generation and growth of bubbles in non-
284
Newtonian liquids containing dissolved gases and formulated basic equations for the steps
285
involved in bubble growth [33-37]. Single bubble growth dynamics are considered for most
286
models and use mass, momentum and diffusion equations.
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within finite radius boundaries. Bubble growth is considered to be affected by parameters
289
such as pressure difference (ΔP), difference of gas concentration between the phases (ΔC),
290
flow index (n), coefficient of consistency (mf or k), shear stress ( rr ) and surface tension
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( ). Flow behavior index (n) varies according to fluid type - n1
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(Dilatant) [12].
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The bubble growth is determined by solving the governing equations for
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the mass transfer and the momentum transfer that happen continuously between nucleated
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bubbles the surrounding digester contents [38,39]. The continuity equation
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is given by [40]:
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(Eqn 1)
Applying boundary conditions to the continuity equation, assuming that the inertial
299
forces are negligible and the pressure at the outer boundary is Psys(t), the momentum
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equation for the solution-gas surrounding the bubble is [39]:
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(Eqn 2)
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where Pbub(t, t’) is the pressure at time t of a bubble formed at time t’. All the τ
302
components are stress parameters in the r and θ directions and are determined by the
304
applicable rheological model of the system (discussed in Section 3.3.4 Rheology). During
305
the bubble growth process, the law of conservation of mass should be satisfied. The rate of
306
change of mass inside the gas bubble should be balanced by the mass of gas diffusing in or
307
out of bubble surface. Concentration of gas inside the bubble is increasing, while the
308
concentration in the bulk of the solution outside is decreasing [40]. It is assumed that the
309
gas is ideal. The concentration in the bulk around by a growing gas bubble is described by
310
the following diffusion equation [39, 40]:
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(For r ≥ Rbub) , where D is the gas diffusivity in the bulk solution. Appropriate initial and final boundary conditions that can describe the AD foaming
313
system are required to solve the equations [39].
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3.2
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(Eqn 3)
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Mechanisms of AD foam nucleation In general, bubble nucleation is of different types: homogeneous and heterogeneous
317
[41]. Homogeneous nucleation, which is spontaneous, typically occurs only if the
318
difference between the ambient and dissolved gas pressure is greater than 100 atm. [21].
319
Such high supersaturation values are unlikely in AD. Homogeneous nucleation involves
320
bubble formation in the bulk of a homogeneous solution. No gas nuclei should be present
321
prior to bubble formation [41]. Since gas bubbles form in several locations inside a digester
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simultaneously and are rising continuously to the surface, bubbles possibly do not form by
323
this nucleation mechanism. Heterogeneous nucleation usually requires supersaturation levels similar to
325
homogeneous nucleation [42]; however prior gas nuclei or cavities may be present. If the
326
AD is suddenly made supersaturated by pH variation (which influences CO2 solubility
327
dynamics) or pressure reduction, then it creates a bubble. It is assumed that the gas is
328
already not present in the digester and the formation of this first bubble is a heterogeneous
329
nucleation event. Once it is formed, the bubble then follows the steps outlined in Figure 2.
330
Then each bubble leads to a gas nucleus. Once this bubble forms, any subsequent bubble
331
production is due the presence of these nuclei or in other terms a cavity from which other
332
bubbles have nucleated. This type of nucleation that follows the initial heterogeneous
333
bubble formation is referred to as either type III or IV [43]. Sources in literature have
334
identified other sources of gas nuclei namely Type II (gas residues adsorbed on solid
335
“support”) and Type III (stabilized micro-bubbles) [44].
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In AD, heterogeneous bubble nucleation may be driven by: 1) Change in
337
temperature, pressure, or surface tension 2) New gas phase nuclei may form due to
338
impurities or particulates. This bubble nucleation seemingly requires lower gas
339
supersaturation than homogeneous nucleation due to the heterogeneous environment inside
340
the AD [45,46]. As hypothesized earlier in this paper, in conventional foam, gas bubbles
341
have a layer of surface active material. In case of rapid expansion, the surfactant layer is
342
absent. This will determine how the gas nuclei form. Increased agitation by mixing in AD
343
also influences type of nucleation [47]. Certain kinds of crevices and bends provide
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additional nucleation sites for bubbles indicating that digester shape and configuration
345
aspects may influence foaming [48]. It appears that gas nuclei formation is slow and requires a considerable number of
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nuclei to build up which could again be related to the presence of methane coupled with
348
CO2 in the digester gas. Bubbles in rapid expansion events seem to manifest faster as foam
349
(hence the name) when compared to conventional events. After a detailed review of the
350
nucleation mechanisms, the type of the nucleation mechanism is not obvious due to the
351
various controlled and uncontrolled phenomena occurring in AD. In fact, different
352
nucleation mechanisms may contribute to the two different types of foam events. But due
353
to the presence of CO2, possibility of supersaturation and bubble formation, it is evident
354
that nucleation is one of the methods of AD foam bubble formation.
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in the following section.
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3.3
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The roles of CO2 and methane in AD foam bubble formation are further discussed
355
Ac ce p
Contribution of controlled and uncontrolled factors to AD foam formation This section deals with how causes and contributors possibly influence uncontrolled
358 359
factors to aid in foam formation mechanisms.
360
3.3.1
361
Contribution of the gas phase to foaming - role of methane Gas phase in the digester mainly contains methane and CO2. Trace amounts of
362
hydrogen sulfide, ammonia, several non-methane volatile organic carbons (NMVOC),
363
volatile organic compounds (VOC), and siloxanes are also present [49]. The presence of the
364
former gases i.e., sparingly soluble methane along with CO2 affects bubble nucleation
365
mechanisms. The effects of the latter trace compounds on foaming are unknown and lacks
366
experimental data. Theoretically, in the presence of only CO2, the foaming phenomenon 19
Page 20 of 48
seems spontaneous, whereas due to the presence of methane, foam formation will be
368
slowed down due to lower rates of diffusion of methane gas [50]. Higher concentrations of
369
methane also result in smaller bubble sizes than if only CO2 was involved; thus increasing
370
the stability of the bubbles [51].
ip t
367
The solubility of methane in mesophilic conditions in the digester is one-twentieth
cr
371
of that of CO2 and has a coefficient of diffusion which is 4.5 times greater than that of CO2
373
[52,53]. The weight of methane is also much lesser than that of CO2 so it tends to rise-up
374
faster as bubbles in the digester. The concentration of dissolved CH4 will then be always
375
lesser than that of CO2 even during methanogenesis because it always forms bubbles [54].
376
Based on these properties, major portion of CH4 inside a digester will exist in the gas phase
377
as bubbles. CO2, on the other hand, based on the discussion in Section 3.3.2, exists in either
378
dissolved or gas form. Presence of methane in the digesters influences bubble nucleation
379
and stability because of low diffusion. The presence of two or more gases of very different
380
solubilities affects the bubble coarsening/detachment step of the bubble formation, in
381
particular [12].
Ac ce p
te
d
M
an
us
372
To sum up, the mechanism of bubble formation in digesters is similar to carbonated
382 383
liquids but occurs much slowly. Due to the presence of methane, bubble nucleation
384
processes are not spontaneous and require a mechanism in order to initiate or enhance the
385
foaming cause [55]. Such an additional mechanism is termed as a contributor and it favors
386
the foam persistence if the potential to foam or a fundamental cause already exists in a
387
digester.
388
3.3.2
Physicochemical characteristics of CO2 in AD 20
Page 21 of 48
In the realm of pH governing AD, CO2 though present in lesser quantity than
390
methane governs the chemistry in the digester (carbon dioxide-bicarbonate system). CO2 is
391
also a prerequisite to any effervescence as seen from carbonated beverages, beer and
392
champagne. In AD, CO2 gas most likely influences bubble nucleation and thus rapid
393
volume expansion as discussed in this section.
cr
394
ip t
389
During the various steps of AD, when gas is produced above the saturation levels in the digester, it is released as bubbles by nucleation. Under the conditions existing inside
396
digesters, the driving force for such bubble growth depends on pressure difference, surface
397
area, diffusion constants, and gas solubility [56,27,28,57,58]. Digester contents become
398
supersaturated when the total dissolved gas pressure exceeds the solution pressure and a gas
399
bubble can easily form in this condition. The total dissolved gas is the sum of the partial
400
pressure of each gas species dissolved in solution. Saturation conditions inside the digester
401
vary based on the pH. Due to this reason, in the case of carbonated beverages, CO2 is
402
usually injected and solubilized under pressure in beverage cans and bottles.
an
M
d
te
Ac ce p
403
us
395
For example, at pH 7.0 about 80 % of carbonic acid is dissociated as HCO3-
404
(bicarbonate) at 25°C [59]. These bicarbonates can form precipitates. If more CO2 is
405
dissolved, more H2CO3 is formed and will be ionized to HCO3-. Any increase in H+ results
406
in pH decrease due to acidity. When the pH is lesser than the pKa1, H2CO3 is finally
407
dissolved in CO2 [60]. Accumulation of this acidity may ultimately cause a pH drop once
408
the alkalinity is completely exhausted. Otherwise AD is assumed to be naturally buffered
409
systems. Both pH and alkalinity are not directly thought to cause foaming though they play
410
an important part in bubble nucleation by influencing supersaturation. In digesters the
21
Page 22 of 48
411
desired pH is between 6 and 8 and the alkalinity is typically between 2,000-2500 mg/L as
412
CaCO3. Based on the pH inside the digester, the CO2 formed in methanogenesis could
414
dissolve or be released into the gas phase. Since one of the major reasons for foaming is the
415
presence of gas bubbles, pH fluctuations directly influence bubble nucleation. Dissolved
416
CO2 in the digester and gaseous CO2 molecules in the headspace of the digester eventually
417
are in equilibrium according to Henry’s law which states that the partial pressure of a given
418
gas above a solution is proportional to the concentration of the gas dissolved into the
419
solution, as expressed by the following relation [19]:
420
C = kHPCO2
421
where C is the concentration of dissolved CO2 molecules, PCO2 is the partial pressure of
422
CO2 molecules in the gas phase, and kH is its Henry’s law constant. Inside the digester,
423
dissolved and gaseous CO2 are in equilibrium; mostly keeping the gaseous pressure at a
424
stable level. The quantities of these components vary with pH of the system. Free CO2 can
425
transfer out into the headspace of the digester from the liquid interface when CO2 is in
426
equilibrium between free CO2 and carbonic acid [47]. The excess of CO2 being generated
427
in the digester escape as bubbles based on nucleation rate and form foam. CO2 chemistry
428
within a digester is governed by both Henry’s law (for CO2-dissolved gas molecules) and
429
the ideal gas law (for the gaseous CO2 in the headspace) [28], quite similar to a bottle of
430
carbonated beverage.
431
Ac ce p
te
d
M
an
us
cr
ip t
413
Under any loss of pressure events or significant variations in gas holdup, the
432
gaseous volume of CO2 under pressure in the headspace suddenly expands and in several
433
cases overflows. After this loss of seal in the digester and gushing, the pressure of gaseous 22
Page 23 of 48
CO2 maintained inside the digester falls. The thermodynamic equilibrium is disturbed, and
435
the dissolved CO2 inside the digester escapes, to fulfill Henry’s law. If the excess of the
436
dissolved CO2 does not possess the necessary free energy to overcome the enthalpy of
437
formation of bubbles, then any source of external energy, e.g., high intensity AD mixing,
438
shaking, and/or particulates or surface active materials lowers the energy barrier. The size
439
of CO2 bubbles at this high pressure becomes critical. As a result, headspace volume
440
changes and leads to rapid rise just like a champagne or soda bottle when popped open after
441
shaking it and forms foam potentially becoming a rapid expansion foam event in the
442
digester [42].
cr
us
an
If the digester is undersaturated, free energy is not favorable for bubbles to be
M
443
ip t
434
thermodynamically stabilized. In case of saturation or supersaturation, the free energy has
445
to be supplied to achieve the critical radius of the bubble. Bubbles larger than this radius
446
will grow and those lesser will die. Rather than newer bubbles being generated in this case,
447
their mean size increases, because their nucleation is strongly dependent on supersaturation
448
[61]; as shown in Figure 3 [42]. The organic matter, hydrophobic components and solid
449
particles including inorganic bicarbonate precipitates formed in the digesters serve as
450
bubble nucleation sites and growth in the case of undersaturated digesters [62]. Bicarbonate
451
precipitates occur in areas of reduced partial pressure of CO2 which can result in
452
supersaturated conditions and release the CO2 to the gas phase similar to a rapid expansion
453
event [19, 47, 48].
454 455
Ac ce p
te
d
444
Mathematical formulations for critical radii for both types of bubble nucleation are expressed as [39]:
23
Page 24 of 48
(Eqn 4)
456
where Pbub is the pressure inside the bubble; Psys is the surrounding system pressure;
457
γlg is the interfacial energy at the liquid-gas interface. The free energy barrier for
459
homogeneous nucleation (Whom) is [63-66]:
cr
ip t
458
(Eqn 5)
us
460
From eqns 4 and 5, Rcr and Whom are functions of γlg and the degree of
461
supersaturation, which is defined as the difference between Pbub,cr and Psys.
an
462
For heterogeneous nucleation, the equation of Rcr is identical to that of
463
homogeneous nucleation but the maximum value of this critical radius is much lower than
465
the critical radius required for homogeneous nucleation. In case of the free energy barrier:
d
M
464
(Eqn 6)
te
466
where F is the energy reduction factor for heterogeneous nucleation, which depends
Ac ce p
467 468
on the geometry of the nucleation site [67]. These equations have been designed for
469
industrial applications that contain CO2 and non-Newtonian liquid systems similar to AD
470
sludge, where there are significantly lesser uncontrolled factors than AD foaming. In
471
developing equations for AD foaming, effect of more factors than just the geometry of the
472
nucleation site will have to be considered.
473
3.3.3
474 475
Particle size distribution and surface characteristics Even though the role of particle concentration and particle size has been explained
for other three-phase foams, similar effects in AD foaming have not been reported. In the 24
Page 25 of 48
presence of surface active substances and absence of particles, any foam developing in
477
digesters would rapidly collapse [68]. Yet whether this is the case remains unknown,
478
though digester foam collapses have been anecdotally reported by several WWTP operation
479
personnel, even in the presence of solid particles. The possibility of the sludge solids
480
aggregating in a digester leaving fewer particles for foam stabilization cannot be ruled out
481
and likely causes foam collapse. Smaller particle sizes also accelerate hydrolysis,
482
acidogenesis and the production of VFA resulting in organic overload to the digester.
483
Reduction of mean particle size of feed sludge cause increased VFA production and
484
reduced methane production [69]. Finer grit particles have an increased ability for gas
485
holdup compared to larger sized particles, in effect possibly aiding rapid volume expansion
486
[70].
cr
us
an
M
In AD, the particle size distribution of feed and that of the digester contents is likely
d
487
ip t
476
to be different [69]. In low sludge retention time (SRT) systems (highly loaded), the
489
particle size distribution in the digester might be strongly influenced by feed sludge
490
compared to that of high SRT systems. The digestibility of the sludge also plays a role in
491
the digester contents’ particle size distribution and concentration [71]. Several unpublished
492
reports state that digesters receiving feed with lower primary sludge: waste activated sludge
493
(PS:WAS) solids ratio tend to foam more compared to those with higher PS:WAS solids
494
ratio, which could be explained by particle size-concentration effects, but yet to be
495
experimentally determined due to the complexity of AD foam.
496
Ac ce p
te
488
The biogas production step (methanogenesis) itself is significant to foaming as the
497
concentration of gas bubbles, bubble size distribution as well as particle size distribution
498
has a role to play in foam formation. If the methanogenesis step is isolated from the other 25
Page 26 of 48
steps of AD, the physical absence of gas bubbles could reduce incidence of foaming, as in
500
standard two-phase or acid-phase digestion. Incidence of foam in the acid (first phase)
501
phase is potentially less due to: (i) less gas production; and (ii) lesser filament incidence
502
due to adverse conditions of low pH and higher VFAs [72]. Faster degradation rate of
503
Nocardia filaments in two-phase AD than in the single stage AD along with corresponding
504
lowered aeration foam potential has been reported [73]. However, if the reaction time in the
505
first phase is not sufficient to breakdown the foam-causing filaments to non-foaming state,
506
there could be excessive foaming in the acid phase stage of the two-phase digestion system
507
[2].
cr
us
an
Since the foamability of three-phase suspension is dependent on the particle
M
508
ip t
499
concentration (directly proportional) and size (inversely proportional) [13], the hydrolysis
510
of particulate organic matter and size distribution of both undigested and digested sludge
511
solids plays a significant role in AD foam formation. The initial steps in the AD process,
512
namely the hydrolysis and the acidogenesis are affected by physicochemical conditions
513
more than the biological factors. In acid-phase, the first stage consists of the hydrolysis and
514
the acidification step through careful operational control (mainly the SRT); which
515
influences particle size distribution (as discussed earlier in this section); in turn related to
516
foaming. While reduced foaming in such systems have been reported [74]; experiences of
517
plants with higher foaming in two-phase have also been reported citing operational issues
518
[75]. Limited full-scale data exists on two-stage treatment and its effects on the various
519
controlled and uncontrolled factors associated with foaming.
520 521
Ac ce p
te
d
509
Kinetics of foaming varies with the physicochemical properties of sludge due to the microbial adherence to sludges [76]. Recently, layering effects also have been linked to 26
Page 27 of 48
digestion performance [77]. Surface tension of sludge influences the microbial adhesion
523
phenomena. Methanogenic archaea rely on extracellular polymeric substance (EPS) to bind
524
to the sludge particles. The methanogens and acidogens form a layer on the surface of the
525
sludge, based on the surface tension of the liquid. If surface tension is low, the acidogens
526
are present in the outer layer and the hydrophobic methanogens are present in the inner
527
layers. If surface tension is high, methanogens are dominant on the surfaces, and biogas
528
bubbles produced adhere strongly to sludge, increasing sludge propensity to foam [76].
us
cr
ip t
522
Surface activity of sludge could be altered due to cationic polymers in feed leading
an
529
to foam [78] but this theory could not be corroborated in a full-scale study where the
531
polymer added for feed thickening did not have any effect of the foaming [79].
532
3.3.4
Sludge rheology
Rheology of sludge refers to flow of sludge with significant solids content. Digester
d
533
M
530
sludge has been characterized as either non-Newtonian fluid [80], Bingham plastic (high
535
solids up to 14% total solids (TS)), or viscoplastic non-Newtonian fluid (2.5-12% TS at
536
temperatures between 20-60°C) [81]. Rheological properties can affect foam stability due
537
to changes in yield stresses and apparent viscosity [82]. For instance, foams on low
538
viscosity liquids like beer are more short-lived than those on higher viscous ones such as
539
milk shakes or cappuccino. Higher viscosity sludge forms stable foams due to lesser
540
drainage of liquid from the bubble surfaces. Rheology changes during processes like
541
pumping or gas release/aeration, and making it difficult to measure real time inside
542
digesters. The pretreatment options used to enhance digestion also impact rheology due to
543
various forms of energy being applied to sludge. Many of these methods have been
544
reported to reduce foaming though mostly due to a decrease in the filamentous populations
Ac ce p
te
534
27
Page 28 of 48
545
[83-85]. No direct evidence is available to relate foaming to changes in rheology or other
546
uncontrolled factors. Irrespective of the type of foam bubbles formed surface tension, viscosity and
ip t
547
buoyancy forces impact bubble rise velocity. Sludge is viscous and needs more yield stress
549
to deform. Under these conditions, small bubbles are entrained in sludge; while larger
550
bubbles rise [86]. These entrained bubbles are similar to “sinking bubbles” in conjunction
551
with the presence of a sparingly soluble gas like methane in the case of AD [49,50,53].
552
Such type of bubbles could be evidence for the likes of “gas nuclei” [56], or
553
“microbubbles” [42,43] in AD.
554
3.3.5
us
an
Temperature and pressure effects
M
555
cr
548
Temperature could be a contributor to foaming events in digesters because of its impact on AD metabolism by hydrolysis and methanogenesis, and has a significant effect
557
on other factors such as gas transfer rates and settling characteristics of biosolids [87]. The
558
importance of temperature as an operational parameter (controlled) is more pertinent for
559
rapid volume expansion while its effects on filamentous bacteria or seasonal variations
560
(uncontrolled) are important with regard to conventional foam events. Temperature effects
561
on sludge properties such as viscosity or surface tension possibly impacts both types of
562
foam events.
te
Ac ce p
563
d
556
Bubble formation potential increases with temperature due to reduced Henry’s
564
constants and more rapid diffusion kinetics [88]. Bubble drag and bubble rise velocity are
565
impacted by temperature. Temperature produces changes in viscosity and the rheological
566
behavior, which in turn affects mixing, indirectly impacting gas holdup. Though
567
experimental evidence is lacking, inadequate mixing contributes lesser to foaming at higher 28
Page 29 of 48
temperatures than at lower ones [9]. High temperature of thermophilic digesters eliminates
569
foaming due to viscosity changes of fats, oils and grease at higher temperatures in foaming
570
digesters [9]. A few other studies attribute the diminished foaming to the effect of higher
571
temperatures on lowering viscosity of sludge hence increasing foam drainage and breakup
572
[89]. Temperature inside a digester may also be non-uniform due to improper mixing or
573
feed variations, which indicates that temperature indirectly is a cause of rapid volume
574
expansion, by different means.
cr
us
The solubility of CO2 in the digester contents depends on temperature (the lower the
an
575
ip t
568
temperature of the solution, the higher the gas solubility). Because the solubility of CO2
577
changes with temperature, the partial pressure of CO2 gas also strongly depends, in turn, on
578
the temperature. Digesters which feed infrequently with large volumes of colder feed
579
sludge may experience temporary temperature fluctuations that may aid gas dissolution into
580
the liquid phase. Depending on the duration of the intermittent feeding, there could be
581
changes in the gas phase pressure, particularly in fixed cover digesters.
d
te
Ac ce p
582
M
576
Digester pressure has not been implicated as a cause or contributing factor in
583
foaming in literature thus far. Gas phase pressure in the headspace of the digester and liquid
584
level impact bubble sizes and bubble rise velocity [90]. In digesters, pressure increases with
585
height of the liquid above and gas system pressures may be as high as 50 inches of water
586
column [30]. Slower bubble rise rates occur at higher pressure in the digester and slows
587
down coalescence leading to further bubble formation [90,91]. Such evidence from
588
literature point to lower bubble formation in the pressure ranges encountered in AD.
589
However, under these conditions, a significant volume expansion and subsequent gas
590
release in the form of foam from solution can occur when there is a sudden pressure drop. 29
Page 30 of 48
Presence of CO2 which is very soluble under higher pressure will be released under such
592
conditions. Therefore, it may be prudent to operate digesters at lower headspace gas
593
pressure, if possible, to minimize rapid expansion of sludge during withdraw-feed periods.
ip t
591
In summary, the role of digestion temperature in AD foaming seems to be mainly
595
due to indirect effects such as the gas production, biogas solubility, viscosity and surface
596
tension of the liquid phase in the digester. Pressure changes impact gas solubility and lead
597
to rapid volume expansion events.
598
3.3.6
us
cr
594
an
Interfacial phenomena between sludge solids, liquid and gas
The fundamental interfacial phenomena of AD foam are illustrated in Figure 4 [92].
599
These relationships can be identified from literature review but quantitative estimates for
601
AD foam are not available. The lack of such quantitative estimates is due to the
602
heterogeneity of the solid and liquid phase of wastewater.
d
M
600
Processes depicted in this schematic include: (i) the adsorption of different
te
603
constituents at the bubble interface, (ii) the interfaces between the solid, gas bubble, liquids
605
and various surface active components, (iii) processes occurring at the interface, (iv) the
606
surface rheological properties of all the constituent phases. The formation, stability, and
607
mechanical properties of foams depend on the interfacial physico-chemical characteristics;
608
further influenced by operational parameters, physical features and process characteristics
609
(v) Bubble nucleation defined by bubble sizes and stability and (vi) Subsequent foam
610
formation and its bubble size distribution and stability.
611
4
612 613
Ac ce p
604
Discussion This review has discussed the main fundamental factors and mechanisms in
explaining AD foaming. Much of the fundamental phenomena involved have been 30
Page 31 of 48
extended from other three-phase foam literature, as no direct investigations have been
615
reported on mechanistic understanding of foaming using the digester content matrix and
616
under AD conditions. Nevertheless, they form the current basic approach to understand AD
617
foaming. Though several sources of bubble formation have been reviewed for finding a
618
fundamental explanation for foaming, none of them led to a distinct answer as the
619
interactions between uncontrolled and controlled factors are numerous. In AD several
620
commonly used methods of foam creation are in effect. Bubble nucleation certainly plays a
621
role in AD foaming though the type of nucleation is under question. The type of nucleation
622
may be different for the two types of foam. Pre-existing gas nuclei or microbubbles are a
623
possibility given the nature of the digester contents but have not been detected in digesters
624
unlike other foams. Relative instantaneous concentrations of CO2 and CH4 may contribute
625
to foam dynamics in the digester though it has not been implicated to be a cause or
626
contributor to foaming thus far in popular literature. Due to the presence of methane,
627
bubble nucleation processes are not spontaneous and require a mechanism in order to
628
initiate or enhance the foaming cause. This may also be one of the reasons that contributors
629
are required for the foaming episode to manifest. The overall discussion emphasizes the
630
role of mixing as a contributor to AD foam. Mixing creates favorable conditions for AD
631
foaming by: (i) entrapping bubbles into which the gas formed can diffuse and serve as
632
nucleation sites for bubbles (ii) providing the necessary free energy thus lowering the
633
energy barrier leading to a rapid rise in the headspace volume due to foam.
Ac ce p
te
d
M
an
us
cr
ip t
614
634 635
Acknowledgements 31
Page 32 of 48
The authors gratefully acknowledge financial assistance for this work from Water
636 637
Environment Research Foundation (WERF), Alexandria, VA - Project INFR1SG10.
639
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640
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Figure captions
867
Fig.1. Schematic of general three-phase foam stabilized by solid particles.
868
Fig.2. Formation of AD foam bubbles and role of the controlled and uncontrolled factors.
869
Fig.3. Growth of bubbles in rapid volume expansion. Increasing size of bubbles rather than
870
new bubbles being formed.
871
Fig.4. Possible factors and relationships leading to bubble and foam formation. Mechanistic
872
and fundamental aspects of AD foam.
874 875 876
d
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877 878 879 42
Page 43 of 48
880 881
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882 883
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Table 1. Classification of the causes of foaming [32]. Classification Causes Sludge feed characteristics Surface active agents in feed sludge Foam causing filaments in feed sludge Digestion process-related Organic loading aspects – overload and inconsistent characteristic loading PS:WAS solids feed ratio to digester VFA production - Imbalances between the successive hydrolysis, acidogenesis and methanogenesis, upstream fermentation in the WWTP Contributors Gas production rate/withdrawal variations Digester operating conditions Temperature; pressure changes Mixing intensity and patterns Digester configuration, shape and Digester shape and configuration physical features Sludge withdrawal and gas piping
890 891
Ac ce p
te
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M
889
892 893 894 895 43
Page 44 of 48
Ac ce p
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M
an
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cr
ip t
896 897 898 899 900 901 902 903 904 905 906 907 908 909 910 911 912 913
914 915 916 917
Figure 1. Schematic of general three-phase foam stabilized by solid particles.
44
Page 45 of 48
Ac ce p
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M
an
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918 919 920 921 922 923 924 925 926 927 928 929 930 931 932 933 934 935
936 45
Page 46 of 48
937 938 939
Figure 2. Formation of AD foam bubbles and role of the controlled and uncontrolled factors.
ip t
Bubble Forming Zone
Explosion leading to Foam
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Bubble Size
Critical Diameter Size
942 943
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941
Bubble Death Zone
Figure 3. Growth of bubbles in rapid volume expansion. Increasing size of bubbles rather than new bubbles being formed [adapted from 42].
Ac ce p
940
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M
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Time
46
Page 47 of 48
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947
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946
Figure 4. Possible factors and relationships leading to bubble and foam formation. Mechanistic and fundamental aspects of AD foam [adapted from 92].
Ac ce p
944 945
47
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