Sludge Drying Through Hydrophobic Membranes

15 Sludge Drying Through Hydrophobic Membranes Solmaz Marzooghi, Steven K. Dentel DEPART ME NT OF CIVIL AND ENVIRONMENTAL ENGINEERING, UNIVERSITY OF DELAWARE, NE WARK, DE, USA

1. Introduction 1.1

Water Reclamation and Waste Management

Water conservation and water reuse would seem to go hand in hand in areas where there is an inadequate water supply. Yet, when water is conserved, the strength of wastewater increases. This presents a challenge for water reuse, because removal efficiencies must be increased. Even the treatment of sludge is crucial, because concentrated return streams place further demands on purification processes. This link between water reuse and wastewater treatment is seldom recognized. Approximately 6.9 million US tons (dry solid weight) of municipal wastewater sludge were produced in the United States in 1998,1 projected to increase to 7.6 million and 8.2 million US dry tons in the years 2005 and 2010 (6.9 million and 7.4 million metric dry tons), respectively.2 It is important to note that most sludges are originally about 3% solids, or 97% water, although this number varies widely. This means that the actual mass of sludge is over 250 million metric tons per year. Before safe disposal of the sludge, the moisture content must be reduced sufficiently to facilitate the following land application, composting, or incineration. The more water we attempt to separate from these solids, the more difficult it becomes. In addition, this water must be re-treated because the sludge contaminants do not partition completely into the solids fraction. The thickening and dewatering processes employed to separate the water also require costly conditioning agents (e.g., polymers) and energy-intensive filtration or centrifugation processes, such that sludge treatment and disposal can represent 40–50% of the total wastewater treatment cost, in both capital and operation costs. This does not even include the cost of re-treating the highly concentrated supernatants and filtrates generated in the sludge separation processes. Population growth and more extensive sewerage have led to increases in sludge production worldwide, such that biowaste management in an economically and environmentally acceptable manner has become an increasing concern for modern societies. Water Reclamation and Sustainability. http://dx.doi.org/10.1016/B978-0-12-411645-0.00015-8 Copyright © 2014 Elsevier Inc. All rights reserved.

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This suggests a need for new approaches to solid–liquid separation for wastewater sludges.

1.2

Environmental Protection against Spread of Pollution

On account of rapid population growth, the world’s need for sanitation systems has significantly increased. Yet, globally, 260 million people (40% of the world’s population) do not have access to safe sanitation.3 Because safe disposal of excreta is of critical importance for health and welfare, improved sanitation facilities are warranted, defined as facilities that hygienically separate human excreta from human contact, such as flush/ pour flush toilets or latrines connected to a sewer, septic tank, or pit. However, because of the costs and the water supply requirements, community sewerage systems are not practical in many rural regions and developing countries. In widespread areas worldwide, simple latrines are the only “improved” sanitation facility that is economically attainable. Latrines, however, must be properly constructed and used. The use of simple pits can be inadequate, leading to unsanitary disposal of infected human feces and contamination of groundwater and other sources of water. Such pits can provide the sites and opportunities for flies and mosquitoes to lay their eggs, to breed, or to feed on the exposed material and to spread infections. If overloaded, they must be emptied before the material is stabilized, leading to unsafe conditions for workers. However, if excreta are properly dealt with in well-designed and well-managed latrines, this can provide a hygienic and satisfactory solution for communities that otherwise would lack adequate sanitation systems. An unsafe on-site sanitation system is illustrated in Figure 1.4 An unprotected sanitation system allows pollutant and pathogen release to the environment through soil or during high-water-table conditions. As a consequence, the transmission of diarrheal diseases (including cholera), schistosomiasis, and hepatitis increases, and every year, food and water tainted with fecal matter cause up to 250 million cases of diarrhea among children under the age of 5, resulting in 1.5 million child deaths.3 The consequences,

Pollution

FIGURE 1 Spread of pathogens and pollution through soil or during high-water conditions.4

Chapter 15 • Sludge Drying Through Hydrophobic Membranes 363

therefore, can be devastating for human health as well as the environment. Even in urban areas, where household and communal toilets are more prevalent, 210 million people use toilets. There is, consequently, an urgent need for protection of the environment from the release of pathogens or parasites through subsurface transport, utilizing a barrier between the subsurface environment and the fecal waste. In addition, privies must be emptied out. As a result, the sanitation workers are exposed to various health risks because of the fecal sludge that has not been stabilized properly.

1.3

Proposed Solution to These Aforementioned Problems

To address the problems mentioned in the previous section, there is a need for an affordably sustainable solution that incorporates a variety of aspects such as biowaste management, water reclamation, and environmental protection. That is, the system should be designed or modified in such a way that it could function globally and sustainably, as the problems are not limited to a specific area or period of time. The proposed solution, in broad terms, is to enclose the sludge system with a hydrophobic membrane. The system’s fundamental principal rests on the hydrophobic property of the membrane, causing it to retain the liquid and the contaminants dissolved within the sludge. Additionally, the membrane’s permeability to water vapor would allow for sludge drying and stabilization.

1.3.1 Biowaste Management Coupled with Water Reclamation This study investigated the potential adaptation of the membrane distillation (MD) process as a low-cost and sustainable approach to sludge stabilization. The central feature of MD is the use of a hydrophobic membrane as a barrier between the feed and permeate. Since the membrane is hydrophobic, water and any contaminants dissolved in water cannot pass through the membrane pores, although vapor passage is allowed. The process relies on a vapor pressure gradient to extract water vapor from the feed to the permeate side. The evaporated moisture is condensed outside the membrane and produces contaminant-free distillate. The process is described in more detail in Section 1.4. Incorporating a membrane process similar to MD into sludge treatment is a promising technique to regain water from sludge, not only more efficiently but also with a high quality. Distillate from an anaerobically treated sludge through a hydrophobic membrane has shown conductivity similar to that of distilled water.

1.3.2 Modified On-Site Sanitation Systems for Environmental Protection To be successful and sustainable, the proposed modification to conventional sanitation systems must satisfy a number of criteria: • •

It should provide surface and subsurface environmental protection It should allow nonmanual drainage of the fecal sludge when filled and aged

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•

• • • •

WATER RECLAMATION AND SUSTAINABILITY

Aging, through condensation and drying of the fecal waste, combined with pathogen inactivation because of dehydration and ammonia release, should stabilize the sludge Workers should thus be able to remove the stabilized material from the modified latrine with significantly less exposure to pathogens The process should be affordable for less affluent populations The process should be simple in both operation and maintenance The process should be sustainable

Herein, it is postulated that MD can meet these criteria. The research was intended to assess these possibilities.

1.4

MD Technique

The processes explained in the previous sections can be considered a novel utilization of MD. In this regard, a brief introduction to MD is provided to explain why it can be applied in this way. The MD process is an emerging technique for desalination of salt water. In this process, in general, microporous hydrophobic membranes act as a barrier between the feed (i.e., warm saltwater) and permeate (i.e., cool distillate). Because of their hydrophobic properties, breathable membranes do not allow passage of liquid water or any contaminants within the water, but do allow transport of water vapor. In other words, hydrophobic membranes have pores that fill only with air or water vapor, because the materials have high contact angles, which create nonwetting surfaces. Unlike conventional membrane separation processes, which are driven by hydrostatic pressure, a vapor pressure gradient drives the water vapor through the pores, in the direction of higher to lower vapor pressures. In MD, this vapor pressure gradient is typically provided by a temperature difference between the feed and the permeate sides of the membrane. Even a small temperature difference can create this gradient in vapor pressure and the consequent moisture transport in the form of water vapor. The evaporated moisture from the feed side then can be condensed on the other side of the membrane to produce contaminant-free water. Figure 2 shows MD function schematically.

1.4.1 MD Adapted for Sludge Drying We hypothesized that the same principle could be applied to sludge drying and stabilization, with the membrane in direct contact with the sludge under, around, or, perhaps, over the waste material (as in a composting process) to provide a complete enclosure. In this application, the heat from intrinsic biodegradation or from solar heat may provide the thermal driving force for water vapor transport, even though the membrane exterior reaches a condition of complete saturation. Verifying this hypothesis would suggest a potential application as a low-cost and sustainable approach to waste stabilization, particularly for sludges and fecal wastes.

Chapter 15 • Sludge Drying Through Hydrophobic Membranes 365

Cold feed (e.g., saltwater)

Hydrophobic membrane

FIGURE 2 Membrane distillation schematic performance.

Vapor

Condensed vapor

Warm permeate (Contaminant–free water)

1.5

Prior Research on Sludge Drying

It is critical to understand the drying mechanism for the material and process of interest to improve performance and potentially reduce costs. Several theoretical models have been developed in the study of drying processes for various materials. In the classic liquid diffusion model proposed by Sherwood,5,6 the gradient of the liquid content is considered to drive the moisture transfer and Fick’s law is assumed to hold. The predicted moisture content profiles, however, were not consistent with the observed values. The term “liquid diffusion” is misleading7; however, some limitations of the diffusion equation are observed, such as direct proportionality of the rate of flow to the concentration gradient when you consider the liquid content gradient rather than vapor pressure gradient as a driving force for moisture transport. Subsequently, a vapor diffusion model for drying of food materials was presented, assuming vapor diffusion as the only mechanism of internal moisture transfer.8 A desorption isotherm was used to describe the correlation between the sorptive moisture and the partial vapor pressure in the gas phase. Henry9 proposed an evaporation– condensation theory, which was followed by Harmathy’s10 theory for simultaneous mass and heat transfer during the declining-rate period of a drying process. The derived equations were the same as for the vapor diffusion model. Diffusion was also used as the principal moisture transport mechanism by numerous subsequent researchers. Efremov11,12 proposed a general solution to Fick’s diffusion equation, consisting of two separate solutions, associated with the periods of constant and of the falling rates of convective drying. The application of the model, however, depends very much on the parameters that are expected to be predicted by the model itself, such as the constant drying rate and the final drying time. This so-called two-period model, therefore, is considered a descriptive model rather than a predictive one. Despite the modifications offered to the model in subsequent works,13–15 the functionality of the model is based on empirical fitting parameters that vary with the experimental conditions. That is, the

366

WATER RECLAMATION AND SUSTAINABILITY

parameters in the formulations are quantified to fit the model to the experimental data, rather than representing the physical factors controlling the drying kinetics of the materials. The studies focused primarily on drying patterns of tobacco leaves, various food materials, bricks, fibrous materials, and animal manure. With respect to wastewater sludge, there are few studies on drying kinetics and associated phenomena. Leonard et al.16 and Leonard and Crine17 investigated the drying rate of cylindrical activated sludge pellets. They observed two falling-rate periods and developed models for drying rates. Vaxelaire and Puiggaii18 proposed a similar model for a belt conveyer dryer with air cross-flow. Ferrasse et al.19 investigated heat, momentum, and mass transfer during drying of municipal sludge in an indirect agitated sludge dryer. Irregular decay of moisture content was observed, which implied multiple falling-rate periods during the drying process. Reyes et al.20 also studied drying kinetics of wastewater sludge; similarly, a constant drying rate followed by two falling-rate periods was identified from results of a laboratory drying tunnel with parallel airflow at different temperatures and air velocities. The similar diffusion equation-based models were fit to the obtained experimental data. Nevertheless, for membrane drying of sludge, an appropriate model is expected to account for all impacts of operating conditions associated with the materials, as well as the outside environment including temperature, relative humidity (RH), and the impact of the hydrophobic membrane cover. The models mentioned above fell short of predicting drying rate as a function of the imposed conditions. Development of an improved model that incorporates these phenomena and improves our understanding of these processes would be important from both an environmental and an economic standpoint. Therefore, a fundamental model—the so-called stagnant film model—is developed here, in order to predict the drying rate as a function of controlling factors so that the simulation can be flexibly adapted for various drying conditions. This model is validated along with the other drying models for comparison.

2. Mass Transfer Modeling During Sludge Drying 2.1

Sludge Drying Theory

Sludge contains the solids and colloids separated from water, in addition to the substances produced from biological and chemical processes. The water within the sludge can be characterized based on the manner and degree of association with the sludge’s solid fraction, which leads to a distribution of properties such as vapor pressure and enthalpy.21,22 As a simplification, this distribution can be divided into two main types of water: free water that is neither attached nor influenced by the solid fraction and bound water whose properties are altered by association with the solid particles, through various forces. The bound water can be subdivided depending on the nature of this association: interstitial moisture may be bound physically by capillary forces, surface

Chapter 15 • Sludge Drying Through Hydrophobic Membranes 367

moisture is held by adsorptive attraction, and finally, intercellular and chemically bound water is not removed by drying.23 It should be clear that these groupings are somewhat arbitrary, since the polarity of available surfaces is ultimately responsible for adsorptive, capillary, and chemical interactions that favor hydration and water retention. Drying, in general, involves the application of heat to vaporize water and remove water vapor from the material. Sherwood5 proposed three general mechanisms within the overall drying process. Mechanism 1: evaporation of the liquid at the solid surface, during which the resistance exerted against the internal diffusion to the solid surface is relatively small, compared with that against diffusion from the surface. Mechanism 2: evaporation at the solid surface where, unlike Mechanism 1, the internal resistance to diffusive flux of the moisture is greater than the resistance to the evaporation from the surface of the material. Mechanism 3: this mechanism is associated with the evaporation taking place inside the solid where the internal resistance is considerable, as compared with the total resistance to the moisture diffusion. Although Sherwood was mainly addressing the drying of slabs of material such as fiberboard, these three mechanisms apply somewhat to the drying behavior of sludge. If sludge drying occurs under fairly constant operating conditions and the material is sufficiently wet (i.e., the moisture content is well above a critical level), a so-called constant drying-rate period will be observed, corresponding to removal of free water by Mechanism 1. Approaching the critical moisture level, a “falling-rate period” is then seen as the moisture content begins an asymptotic approach to an equilibrium level. This equilibrium moisture content depends on the RH of the air and the nature of the materials to be dried. The falling rate can be attributed to the increased resistance to bound water removal and transport by Mechanisms 2 and 3. The above phenomena can be graphically illustrated with typical drying curves. Figure 3 shows the general pattern observed in air-drying of sludge exposed to various differences between the interior sludge temperature and the outside air temperature (0
, 2
, 5
, and 10
C), with a hydrophobic membrane separating the air and sludge phases. The vertical axis shows sludge weight relative to the initial value, with values beginning at unity and trending toward very low values on the time axis, when almost complete moisture removal is attained. The process is more rapid with greater applied temperature differences. In all cases, the figure clearly shows a linear region representing the constant drying-rate period, followed by a nonlinear segment reflecting the falling-rate period.

2.1.1 Constant-Rate Period During the constant drying-rate period, evaporation takes place at the surface of the material and it is related to free water evaporation. The theory behind the various drying schemes during evaporation is rationalized quantitatively in the following sections. However, it suffices to mention that drying in the constant-rate period simulates the

368

WATER RECLAMATION AND SUSTAINABILITY

1

Normalized sludge moisture content

0.9 1

0.8

0.8

0.7

0.6

0.6 0.5

0.4

0.4

0.2

0.3 0 0 2 4 6 Time (day)

8

0

2

4

6

8

10

0.2 0.1 0

ΔT(°C)

FIGURE 3 Typical characteristic sludge drying curves based on experimental results: sludge drying through a hydrophobic membrane.

evaporation of water from a free liquid surface as the moisture is sufficient to form a continuous liquid film on the solid surface so that the sludge drying is mainly attributed to the evaporation of water vapor from the liquid film to the surface air film. In this regard, the rate of drying during the constant-rate period is controlled by the rate at which water vapor can diffuse through the surface air film. The rate, therefore, is largely controlled by factors that influence the surface air film thickness. External conditions including the ambient temperature, RH, and the media diffusivity as well as the sludge temperature determine the film thickness.

2.1.2 Falling-Rate Period The nonlinear segments of the drying curve are indicated as the falling-rate period where sludge no longer contains free water and bound water begins to evaporate. The bound water evaporation does not resemble free water evaporation as it is strongly controlled by the moisture content of the material. As a consequence, even under constant external conditions, the rate of drying drops as the drying proceeds and the moisture content of sludge decreases.

2.2

Driving Force for Moisture Transfer from Sludge

In order for the drying process to occur, the water vapor pressure in the material has to be higher than that in the surrounding environment. Vapor pressure is a function of

Chapter 15 • Sludge Drying Through Hydrophobic Membranes 369

temperature and RH. RH, itself, is defined as water vapor pressure relative to the saturated vapor pressure at a prescribed temperature (Eqn (1)). RH ¼

Pv Pv

(1)

where RH denotes RH, Pv is the vapor pressure, and Pv is the saturated vapor pressure at a prescribed temperature. An RH of 100%, therefore, indicates that the air is holding all the water it can at the current temperature, and any additional moisture at that point will result in condensation. The saturated vapor pressure of water is readily obtained through the empirical Antoine equation: 
3841 Pv ¼ exp 23:238  T  45

(2)

where Pv is the vapor pressure in Pascal and T is the temperature in Kelvin. Equations (1) and (2) express that at saturation, the vapor pressure is only a function of temperature. As long as the sludge contains free water, the RH and, therefore, the vapor pressure inside the sludge are in the saturated state. The vapor pressure, accordingly, will be only a function of temperature and obviously will stay constant at a fixed temperature. If the membrane exterior is not exposed to complete saturation state, (i.e., water or saturated vapor) the gradient in RH itself will drive the moisture out. Otherwise, the temperature gradient will be the only contributor to moisture evaporation during sludge drying.

2.3

Quantitative Modeling of Moisture Transport during Sludge Drying

Developing a modeling framework for drying kinetics allows process simulation and facilitates design calculations. Also, it is required to attain optimal operating conditions. Mass transfer is one of the main processes taking place during drying. Knowledge of moisture diffusivity is required to design and model the mass transfer processes including drying, adsorption, and desorption.

2.3.1 Diffusion Equation with Flux-Type Boundary Conditions In order to simulate the drying process using Fick’s equation, the values of the diffusion coefficient of the moisture must be known. In this regard, the concept of an effective diffusion coefficient (Deff) has been adapted to describe the drying material diffusivity to the moisture movement. Equation (3) is the general expression of Fick’s second law. Setting the corresponding boundary conditions, it can be used to describe the entire drying process. vW ¼ VðD VW Þ vt

(3)

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WATER RECLAMATION AND SUSTAINABILITY

Assuming one-dimensional isotropic diffusion, Eqn (3) becomes vW v2 W ¼ Deff 2 vt vx

(4)

where W is the moisture content (grams per grams (g/g)), t is the drying time (seconds), x is the location (meters), and D is the diffusivity (meter squared per second). The boundaries and initial conditions are defined as follows: BC1 :

x ¼ 0 J ¼ D BC2 :

vW ¼ kðW   W Þ vx

x¼L

vW ¼0 vx

(5) (6)

Equations (5) and (6) describe diffusion at the surface of the materials (x ¼ 0) and at the bottom (x ¼ L), respectively, where W* is the equilibrium moisture content (g/g), L is the thickness in meters, and k denotes the kinetic desorption coefficient in meter per second. Using the Laplace transform, the analytical solution to the second-degree differential equation is obtained.13 W  W x pffiffiffiffiffiffiffiffiffiffi ¼ erf W0  W  2 Deff t

!

! rffiffiffiffiffiffiffiffi  k k2 t x þ pffiffiffiffiffiffiffiffiffiffi þ exp xþ t erfc k Deff Deff 2 Deff t Deff


(7)

Equation (7) describes spatial and temporal variations in sludge moisture. Assuming that the evaporation in sludge drying takes place at the surface of the material, the drying kinetic equation is expressed as a function of time as follows: rffiffiffiffiffiffiffiffi!
2  W  W k t ¼ exp t erfc k Deff W0  W  Deff

(8)

Equation (8) satisfies the initial conditions such that at the beginning of the process when t ¼ 0, W ¼ W0 where W0 is the initial moisture content (g/g). After quite a long time (t / N), the moisture approaches the equilibrium moisture content (W ¼ W*). To reduce the number of unknown parameters, the so-called characteristic time (in seconds) has been defined as s¼

Deff pk2

(9)

and the drying kinetic equation can then be expressed as rffiffiffiffiffiffi!
 W  W t t ¼ exp erfc W0  W  ps ps

(10)

In essence, Eqn (10) describes the diffusive flux of moisture. In case there is air motion above the drying material, convective effects on mass transfer need to be taken into account. In order to incorporate the convective mass transfer of moisture into the

Chapter 15 • Sludge Drying Through Hydrophobic Membranes 371

model, the power function of the argument in Eqn (10) is introduced.13 The following expression then provides the drying model: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
nffi!

n  W  W 1 t 1 t erfc ¼ exp p s p s W0  W 

(11)

To employ Eqn (11) we need to guess two fitting parameters s and n. The estimated parameters will highly depend on the nature of the materials to be dried and the operational conditions. The factors, however, are not directly included in the model and any variations in them will be reflected in terms of the fitting parameters. The dependency of the drying kinetics on the driving force guiding the process is vaguely described in the proposed model so that any change in the respective conditions (e.g., temperature, RH, or drying medium diffusivity) is not directly translated in the equation. More important, calculation of the temporal moisture content (i.e., W) requires the knowledge of the final moisture content, which is expected to be predicted by the model itself. Thus, the model fails to offer a solid prediction and is considered a descriptive model rather than a predictive one. Also, no fundamental support was provided for inclusion of the power term (n) as being appropriately indicative of the air motion influence on drying. The aforementioned reasons impose limitations on considering the application of the model for membrane drying of sludge. A separate fundamental model to be described in the next section has been developed to predict the drying rate and design the system thereupon. The validity and solidity of the model will be discussed further in the validation part.

2.3.2 Two-Period Model The two-period model can be applied when drying takes place in both constant- and falling-rate periods. The model initially was developed for simulating convective drying of capillary porous fiber materials,11,12 and it was later used to describe drying of diluted organic suspensions.24 The two-period model proposed by Efremov11,12 is expressed in the following form: W ¼ W0

pffiffiffiffi
 

t N0 s p tf  t þ 1  erf 1  N0 W0 2W0 s

(12)

where W and W0 are the instantaneous and initial moisture contents, respectively (g/g); N0 is the drying rate during the constant drying period (1/s); t is the drying time and tf is the final drying time (seconds); and s is the characteristic time (seconds). The first term in Eqn (12) expresses the first period of drying termed constant drying period, while the second term represents the falling drying period. The constant drying term is characterized by the constant drying rate, N0 ¼ dW/dt. The term associated with the falling-rate period is obtained by applying the Laplace transform to the equation of diffusion with the boundary conditions in the form of a constant concentration on the material surface.

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WATER RECLAMATION AND SUSTAINABILITY

The parameter s, the so-called characteristic time, is calculated experimentally from the drying parameters. Considering the fact that at the end of drying time, the moisture content reaches the equilibrium value, Eqn (12) becomes: W ¼ W0


1  N0

t W0

 þ

pffiffiffiffi N0 s p 2W0

(13)

and the characteristic time is expressed as 
2 W0  W  s ¼ pffiffiffiffi tf  N0 p

(14)

where W* is the equilibrium moisture content (g/g). The characteristic time, as can be inferred from the mathematical formation, is a function of the drying conditions and the nature of the materials to be dried. Also, it requires determination of the final drying time and moisture content, as well as the intensity of drying at constant-rate period, which are expected to be predicted by a suitable modeling framework rather than quantifying experimentally.

2.3.3 Stagnant Film Model The major problems with the previously mentioned models are the lack of reliable prediction based on the parameters controlling drying behavior of the materials and the great dependency on the experimental quantities or fitting parameters instead. However, an adequate modeling framework incorporates all the controlling conditions in drying behavior of the materials and provides a complete prediction directly based on the imposed condition. For this purpose, we have used a stagnant film model, which predicts the molar drying rate (moles per meter squared second) depending on the temperature and bulk vapor pressures across the membrane (Eqn (15)). This model gives a quite reliable prediction of the rate of mass transfer when fluid flows through a membrane at steady state. The fluid (air) in immediate contact with the fixed surface can be said to be stagnant, while there is a net motion of flowing fluid (vapor) away from the evaporating surface.25 Briefly, the model originates from the steady-state diffusion equation for a binary gas system when only one of two species is diffusing. In such a system, vapor and air are assumed to be the diffusing and the stagnant species, respectively. Equation (15) describes the diffusive flux of the vapor through the stagnant air film, NA (moles per meter squared second). NA ¼


 DAB P P  pA1 ln P  pA2 RTavg d

(15)

where R is the ideal gas constant (Joule per mole Kelvin); Tavg is the average temperature across the film (Kelvin); PA1 and PA2 are the water vapor pressures inside and outside the stagnant film, respectively; and P is the total air and water vapor pressure in Pascal. The parameter d denotes the stagnant film thickness (meter) and DAB (meter squared per second) is the diffusivity of component A (water vapor) in component B (air).

Chapter 15 • Sludge Drying Through Hydrophobic Membranes 373

In the presence of the membrane layer as a porous and tortuous medium, the effective diffusion coefficient (Deff) is: Deff ¼ DAB

ε s

where ε and s are the porosity and tortuosity of the medium, respectively. Incorporating the effective diffusion into Eqn (15), the model becomes: NA ¼


 DAB Pε P  pA1 ln RTavg ds P  pA2

(16)

Equation (16) can be rearranged in the form of NA ¼


 P Dwaterair@Tave 1 P  pA1 ln R l Tave P  pA2

(17)

where l¼

ds ε

(18)

The stationary air creates a film resistance to the motion of vapor such that in the presence of the membrane, this resistance is considered an overall resistance of the air filled inside the pores of the membrane and the membrane resistance itself. The parameter l (meter) indicates the overall resistance. A helpful and simplifying result from this model is that the overall resistance was found not to be specific to the hydrophobic breathable membrane type (it can generally be quantified as 0.01 m), allowing a general prediction of performance based on membrane area and temperature difference. As long as the sludge contains free water (i.e., the dry base moisture content is over 30%), the RH—and, therefore, the vapor pressure inside the sludge—are in a saturated state. The principal feature of the modeling framework is its ability to account for all factors influencing the drying behavior of the materials so that it is sufficiently flexible to be used for wide ranges of temperatures and relative humidities, as well as to incorporate the membrane effect on mass transfer during the drying process.

3. Materials and Methods 3.1

Membranes

To test the technology, potentiality of the membranes typically used in the MD process was examined. Various hydrophobic membranes from different manufacturers, based on either Polytetrafluoroethylene (PTFE) or Polyvinylidene fluoride (PVDF) with mean flow pore sizes ranging from 0.2 to 6.0 mm, were incorporated in the experimental setup. Table 1 shows characteristics of the membranes largely used in the experiments. The membranes were exposed to direct contact with sludge and their responses in terms of expelling the vapor, durability, and fouling were examined. Interestingly, the water vapor

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WATER RECLAMATION AND SUSTAINABILITY

Table 1

Capillary Flow Porometry Pore Sizes for Membranes Useda Capillary Flow Porometry Results Mean Flow Pore Size

Bubble Point

Frazier Air Permeability

Sample

n

(mm)

s

(mm)

s

(cfm/ft2)

S

Gore Cover (PTFE) Pall TF-200 (PTFE)

3 6

0.236 0.186

0.002 0.007

6.379 0.758

0.579 0.002

0.08 0.17

0.01 0.05

a

Membrane tests all used the same 25 mm Diameter. Sample size: 1100 Millipore plates with a 30 mesh stainless steel support screen and a #2-210 O-ring. Machines were precalibrated to obtain the correct 1 ohm table calibration. GalwickÔ wetting fluid was used in all tests. Mean flow pore size is the micron rating where the air flow is 50% above this value and 50% below this value, the bubble point is the largest pore size, and Frazier air permeability is at 0.500 Water Column pressure. Note that n is the number of samples and s is the standard deviation.

flux rates of these materials are all quite similar. However, thermal insulation, tensile strength, and cost characteristics must also be taken into account.

3.2

Sludge

In all experiments, anaerobically digested sludge was used, and it was assumed to be a surrogate for fecal sludge or primary sludge because it is more stable and reproducible and also contains much lower levels of pathogens relative to raw fecal sludge or primary sludge. The samples were collected from the wastewater treatment plant in Wilmington, Delaware, USA.

3.3

Experimental Setup

The technology was tested in well-controlled laboratory experiments. First, sludge drying with and without a membrane cover was examined. Moisture loss and temperatures were monitored over the drying time. Figure 4 shows how a preliminary batch test is performed with the membrane in contact with sludge on one side and air on the other side. The tests were performed using a membrane “envelope” with one side made of the breathable membrane and the other made of impermeable polyethylene plastic. The latter side was placed on a precision hot plate or water bath at 0, 2, or 10
C above ambient temperature. The envelope was weighed periodically to track moisture loss. Figure 4 shows the use of this method with a membrane area of 10  10 cm, filled initially with 100 ml of anaerobically digested sludge. The envelope setup was evolved into membrane-covered containers to test larger samples. Figure 5 shows the configuration and the heating system, which is composed of a hot plate and a thermocouple to monitor the temperatures inside and outside the membrane. The experimental devices described served as the basis for the work. Temperature control and thermocouple accuracies limited the usable DT to values of 2
C or greater.

Chapter 15 • Sludge Drying Through Hydrophobic Membranes 375

FIGURE 4 (A) Injecting sludge into a PTFE membrane—covered enclosure. (B) Sludge isolated by the membrane. (C) Initial sludge appearance from the backside of the enclosure (thickness: 1 cm). (D) Dried sludge after 3.9 days with a 2
C difference between inside and outside of the enclosure. The aluminum foil acts as a sealer to prevent leakage.

In reality, for the actual scales, drying would require larger temperature gradients, but compost or latrine processes could rest on smaller temperature gradients, resulting in slower fluxes so that laboratory scale setups are not able to meet it. The models, therefore, are built up to provide reliable estimations to longer term processes with slower rates.

3.4

Experimental Conditions

To ensure consistency of the ambient temperature and RH, the samples were located in a climate-controlled room. The applied temperature gradients were 0, 2, and 10
C.

FIGURE 5 Cylindrical plastic containers with a diameter of 13 cm are covered with a triple-layer hydrophobic membrane Gore Cover (PTFE). The temperatures on both sides of the membrane are monitored using thermocouples. RH is also recorded with an RH meter. A close-up of the heating system is also depicted in the picture on the right-hand side.

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WATER RECLAMATION AND SUSTAINABILITY

In the membrane enclosure configuration, as controls, heat or membrane was removed from some samples to examine the impact of heating and membrane on the drying characteristics. In addition, for comparison with the sludge drying results, water evaporation experiments under the same conditions were conducted.

3.5

Measurements

The temperature and RH inside and outside of the membrane were measured and recorded by thermocouples and RH meters equipped with data loggers, respectively. Moisture loss and drying rates were obtained simply by periodic removal of the enclosures with the contents for weighing and then reintroducing them into the drying apparatus.

3.6

Additional Tests—Investigating the Influence of Outside Media Resistance

The moisture removal measurements were ultimately intended to determine the stagnant film resistance against mass transfer to incorporate the parameter into the modeling framework. This resistance might not only be controlled by the resistance exerted by the membrane matrix and the air filled within the membrane pores but also might be influenced by the media adjacent to the exterior wall of the membrane. This hypothesis was tested in experiments employing a circulator that distributed the air surrounding the outside of the membrane so that the ambient air film resistance was reduced (Figure 6). The tests without any membrane cover were included to examine the circulator and membrane cover impacts.

FIGURE 6 Containers filled with the sludge and water, as well as with and without the membrane cover. An air circulator is dispersing the surrounding air to decrease the ambient air resistance against the moisture transfer.

Chapter 15 • Sludge Drying Through Hydrophobic Membranes 377

4. Results and Discussion 4.1

Sludge Drying Characteristic Curves

Results of drying experiments can be graphically presented in numerous ways. In addition to a simple plot of moisture content vs time, data can be presented as percentage change vs time, drying rate vs time, or cake dryness (percentage solids) vs time. Instead of time, the abscissa can also be percentage solids or percentage moisture. Herein, the moisture content (normalized as a fraction of the initial value) will be plotted vs time, to illustrate drying patterns in a consistent way.

4.1.1 Moisture Content Curve Figure 3 shows typical data of normalized moisture content vs drying time. In this case, the apparatus shown in Figure 4 was used, employing a Gore Cover (PTFE) membrane. At the start of each experiment, wet, warm, and soupy sludge was on the topside of the membrane, while cooler air was at the bottom. Temperatures in the sludge and air were monitored with thermocouples, and differences in temperature are reported as DT. The vapor pressure on the sludge side was computed assuming 100% RH at the measured temperature, while the vapor pressure on the air side was determined from RH and temperature measurements. In one experiment, DT ¼ 2
C, when the sludge was cooled such that the temperature gradient was reversed. Figures 7 and 8 show the experimental drying curves in the presence and absence of the membrane cover, respectively. As expected from the aforementioned literature data, drying of the initially wet, soupy sludge takes place mostly at a uniform rate until the moisture content (mass of water/mass of dry sludge) is quite small (i.e.,
4.2

Quantification of the Resistance to the Mass Transfer

The controlled drying experiments conducted in the absence of the membrane (Figure 8) revealed that membrane presence does not change the drying pattern, but it does retard the drying rate. The decreased rate stemmed from the added resistance associated with the membrane layer. The resistance is readily calculated, fitting the experimental drying rates to the drying rate obtained through the stagnant film model. That is, the experimental drying rates are obtained through the slope of the characteristic drying rate. The rate values are compared with the predicted ones with the model in iterative procedures to find the proper l in the stagnant film formulation. The estimated l values for drying with a membrane cover converged to 0.01 m. The null hypotheses of having a mean value of 0.01 m for l associated with the membrane cannot be rejected at the 95% confidence interval with p > 0.05 (i.e., 0.085 for twotailed test). The l values corresponding to the different temperature gradients are

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WATER RECLAMATION AND SUSTAINABILITY

FIGURE 7 Experimental data showing sludge moisture loss with respect to drying time in the presence of the Gore Cover.

FIGURE 8 Experimental data showing sludge moisture loss with respect to drying time without a membrane cover.

Chapter 15 • Sludge Drying Through Hydrophobic Membranes 379

Table 2 l Values Obtained Experimentally at Various Temperature Gradients during Sludge Drying Through the Hydrophobic Membrane DT (K)

l (m)

2 2 10

0.0112 0.01003 0.0111

listed in Table 2. The values correspond to the experimental results illustrated in Figure 9.

4.3

Outside Media Contribution to the Mass Transfer Resistance

Additional experiments were conducted to determine the resistance l when the air was vigorously mixed with a circulating fan on the air side of the membrane. The corresponding results presented in Table 3 and Figure 10 indicate that there was no significant difference in l when air was mixed or stationary. The results indicate that the measured l value represents the membrane resistance alone and is not influenced by boundary layer impacts in the air. 1 ΔT = +10 ºC ΔT = +2 ºC ΔT = –2 ºC ΔT = +10 ºC - Model ΔT = +2 ºC - Model ΔT = –2 ºC - Model

Normalized sludge moisture content

0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0

1

2

3 4 Time (day)

5

6

7

FIGURE 9 Normalized moisture content experimental data compared with the stagnant film model predicted results. The experimental data are indicated by marker symbols, and the results are associated with drying through the hydrophobic membrane.

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WATER RECLAMATION AND SUSTAINABILITY

Table 3 l Values Associated with Additional Experiments to Test Outside Media Resistance When Drying Occurs in the Presence and Absence of a Circulating Fan as well as the Hydrophobic Membrane Sample

Membrane Cover

Outside Air Condition

l (m)

Sludge Sludge Water Water Sludge Sludge Water Water

Gore Cover Gore Cover Gore Cover Gore Cover No cover No cover No cover No cover

No circulator With circulator No circulator With circulator No circulator With circulator No circulator With circulator

0.0137 0.01247 0.00992 0.00941 0.00741 0.00391 0.00686 0.00311

On the other hand, when the same comparison was performed without the membrane cover, results suggested that the fan reduced the outside air film resistance in this case. Thus, the membrane controls transport resistance when it is present.

4.4

Membrane Fouling

Sludge can contain various organic compounds that form deposits on the membrane surface and cause fouling. Therefore, direct contact of the sludge can lead to fouling and potentially constrain process efficiency. However, the intensity differs significantly from membrane to membrane, and it can be controlled. Among the various types of tested membranes, the Gore Cover exhibited the most promising response to direct contact with the sludge, as no attachment or accumulation of dried sludge crusts at the membrane surface was observed. Figure 11 shows the sludge shrinkage while the drying

FIGURE 10 Estimated mass transfer resistance for sludge and water samples in the presence and absence of a membrane cover as well as a circulating fan.

Chapter 15 • Sludge Drying Through Hydrophobic Membranes 381

FIGURE 11 Hydrophobic membrane response to direct contact with the sludge while it is losing moisture through the membrane pores.

system was in operation, and Figure 12 demonstrates how the membrane surface remained intact after the sludge achieved the desired level of dryness at the end of the process. It suggests a faint possibility of deposition within the pores and the consequent flux decline. As mentioned previously, because of hydrophobicity of the membrane, only vapor should directly contact the membrane surfaces and pores. Thus, deposition should not occur within the pores and will be less intimately associated with the membrane surfaces. In addition, drying in this system proceeds at a relatively slow pace, preventing compaction of any aggregates deposited on the membrane surface, lessening their effect on transport resistance. This was proven through experimental observations of stable flux of water vapor throughout the drying process as well as consistent flux at each run of experiments conducted with the same membrane. The

FIGURE 12 Hydrophobic membrane surface after the sludge has achieved the desired level of dryness.

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WATER RECLAMATION AND SUSTAINABILITY

accurate estimation of durability of the membrane, however, takes scaled up field examinations.

4.5

Model Validation

Although the stagnant film model is preferable to predict the drying rate in this study, validity and utility of the other candidate models mentioned in Section 2.3 are also discussed.

4.5.1 Diffusion Equation with Flux-Type Boundary Conditions Applying the diffusion equation with flux-type boundary conditions (Eqn (11)) requires estimation of two fitting parameters: the characteristic time, s, and the power, n. The experimental data produced from the drying process under various experimental conditions were fit to the model results to find the corresponding s and n (Figures 13 and 14, and Table 4). The surface area used was 0.01 m2 for all experiments and the equilibrium moisture content was very small (i.e., 0.0001 g/g). MR is calculated as MR ¼ (W  W*)/ (W0  W*)(obtained from Eqn (11)). The diffusion model seems to describe the drying process successfully, and the power, n, appears to converge to a single value of 2.5 for the sludge, regardless of the experimental conditions. The characteristic, however, does not exhibit any meaningful trend, or at least the modeling framework fails to explain the variation fundamentally. That is, it is not clear how s changes with the controlling conditions, so the model cannot be used for predicting the moisture reduction for different conditions imposed on the system.

FIGURE 13 Moisture reduction data fit to the diffusion model. The experimental data are indicated by marker symbols, and the results are associated with drying time through the hydrophobic membrane.

1 ΔT = +10 ºC ΔT = +10 ºC - Model ΔT = +2 ºC ΔT = +2 ºC - Model

0.9 0.8

ΔT = –2 ºC ΔT = –2 ºC - Model

0.7

MR

0.6 0.5 0.4 0.3 0.2 0.1 0

0

1

2

3 4 Time (day)

5

6

7

Chapter 15 • Sludge Drying Through Hydrophobic Membranes 383

1

ΔT = +10 ºC ΔT = +10 ºC - Model ΔT = +2 ºC

0.9 0.8

ΔT = +2 ºC - Model ΔT = –2 ºC ΔT = –2 ºC - Model

0.7

MR

0.6 0.5 0.4 0.3 0.2 0.1 0 0

1

2

3

4 Time (day)

5

6

7

8

FIGURE 14 Moisture reduction data fit to the diffusion model. The experimental data are indicated by marker symbols, and the results are associated with drying in the absence of a membrane cover.

4.5.2 Two-Period Model Parameters associated with the final drying state (e.g., final drying time and the equilibrium moisture content) and also drying rate during the constant-rate period appear in the two-period model formulation. As stated previously, the model describes kinetics of drying using the parameters that are expected to be predicted by the modeling framework itself. This imposes restrictions on modeling the process for full-scale design as the characteristic time depends on the sample size (Eqn (14)). However, the required parameters are determined experimentally here to check consistency of the moisture

Table 4 Estimated Fitting Parameters for the Diffusion Model under Various Conditions DT (
C)

Membrane Cover

n

s (day)

2 2 10 2 2 10

Gore Cover Gore Cover Gore Cover Not covered Not covered Not covered

2.5 2.5 2.5 2.5 2.5 2.5

7.83814 4.71871 3.64307 6.55282 3.46625 2.14998

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WATER RECLAMATION AND SUSTAINABILITY

Table 5 Corresponding Experimental Constant Rate and Final Drying Time Used for Validation of the Two-Period Model DT (
C)

Membrane Cover

N0 (1/day)

tf (day)

2 2 10 2 2 10

Gore Cover Gore Cover Gore Cover Not covered Not covered Not covered

0.18 0.30 0.38 0.18 0.38 0.64

6.81 6.04 4.24 6.44 3.16 1.80

content results obtained with the two-period model, relative to those produced during laboratory tests. Incorporating the observed values for the final drying time and the drying rate during the constant-rate period for different experimental conditions (Table 5), the results produced by the model match well with the experimental data (Figures 15 and 16). The surface area used was 0.01 m2 for all experiments, and the equilibrium moisture content was very small (i.e., 0.0001 g/g).

1

ΔT = +10 ºC ΔT = +2 ºC ΔT = –2 ºC ΔT = +10 ºC - Model ΔT = +2 ºC - Model

Normalized sludge moisture content

0.9 0.8 0.7

ΔT = –2 ºC - Model 0.6 0.5 0.4 0.3 0.2 0.1 0 0

0.5

1

1.5

2

2.5

3 3.5 4 Time (day)

4.5

5

5.5

6

6.5

7

FIGURE 15 Normalized moisture content data fit to the two-period model. The experimental data are indicated by marker symbols, and the results are associated with drying through the hydrophobic membrane.

Chapter 15 • Sludge Drying Through Hydrophobic Membranes 385

1 ΔT = +10 ºC

Normalized sludge moisture content

0.9

ΔT = +2 ºC ΔT = –2 ºC ΔT = +10 ºC - Model

0.8

ΔT = +2 ºC - Model

0.7

ΔT = –2 ºC - Model

0.6 0.5 0.4 0.3 0.2 0.1 0 0

0.5

1

1.5

2

2.5

3 3.5 4 Time (day)

4.5

5

5.5

6

6.5

7

FIGURE 16 Normalized moisture content data fit to the two-period model. The experimental data are indicated by marker symbols, and the results are associated with drying in the absence of the membrane cover.

4.5.3 Stagnant Film Model The stagnant film model (Eqn (17)) has the capability to predict the drying rate incorporating all factors influencing the process. The resistance exerted by the membrane layer (i.e., l ¼ ds/ε) is the characteristic examined in this modeling effort, and it was found to be independent of the experimental conditions and converged to a single value of 0.01 m. The nonspecificity of the characteristic property is consistent with the corresponding equation, lending credence to the model. In addition, the excellent match between data and model prediction in Figure 9 supports the validity of the model (although such correspondence cannot prove model validity a priori). Quantities corresponding to the elements of the models are tabulated in Table 6. The surface area was 0.01 m2, which was used to convert the drying rate obtained from the model in mole per meters squared second to grams per day. It should be noted that for the dilute sludge samples, drying mostly proceeds in the constant-rate mode during which the RH of the sludge is consistently in the saturated state and independent of the moisture content. Once the sludge runs out of free water, the RH is no longer 100%, but becomes moisture content dependent. This correlation is determined through a desorption isotherm that is specific to the material. Accordingly, the assumption of constant RH holds true only for the constant drying mode. Applying the stagnant film model for the rest of the drying period requires the use of a desorption isotherm for the specific material to be dried and

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Table 6 Values Corresponding to the Plots Illustrated in Figure 9 and l values in Table 2 Membrane Cover

T1 (K)

T2 (K)

Tave (K)

DT (T1 – T2) (K)

PA1 (Pa)

PA2 (Pa)

D (m2/s)

NA (g/d)

Gore Cover Gore Cover Gore Cover Not covered Not covered Not covered

305 309 306 299 305 313

307 307 296 301 303 303

306 308 308 300 304 308

2 2 10 2 2 10

4465.831 5588.94 4726.507 3147.62 4465.83 6947.62

3250.145 3950.176 2232.06 1594.52 1194.46 1592.62

2.72E-05 2.75E-05 2.65E-05 3E-05 3E-05 3E-05

18.8383 28.977 31.218 16.1814 34.5868 58.0317

solving for the drying rate numerically. Reducing the moisture content to this level (i.e., about 30%) would be desirable, but, in a practical sense, is not likely to be attained. Thus, the focus is on the constant mode of drying where the membrane is rate limiting.

5. Technical Considerations and Practical Applications 5.1

Full-Scale Design

Full-scale design of the modified on-site sanitation system, as one important application of the process, is possible with the established models. Depending on the location where the system is placed, the outer side of the membrane, if used as a pit toilet enclosure, might be in contact with either air or water. If the pit is only within the unsaturated zone of the surrounding soil, the air would be within its porous volume. The membrane might also cover upper sludge surfaces and be in direct contact with air. However, if the pit descends into a zone of soil saturation, the membrane would contact water within the porous volume of the soil. In some applications, such as flooding or tidal areas, the membrane might be in contact with free water. In general, in real-world applications, it might be expected that some fraction of a membrane enclosure could be in contact with air, and the remainder with water. Typical pit latrine dimensions are presented in Figure 17.4 In an on-site sanitation system, as long as the water table is well below the pit latrine, the vapor pressure outside the pit latrine is equal to the vapor pressure of the soil (which is a function of the temperature and RH of the soil, here assumed to be 30
C and 0%, respectively). If the membrane-enclosed pit latrine is completely surrounded by water or fully saturated soil, a temperature gradient will still cause moisture loss from the fecal sludge. In practice, the internal conditions will be determined by the extent of fecal sludge warming during biodegradation or by a supply of external heat (e.g., passive solar) to control the temperature. In some cases, the pit may be partially within a

Chapter 15 • Sludge Drying Through Hydrophobic Membranes 387

FIGURE 17 Typical pit latrine design criteria. L ¼ 1.2 m, W ¼ 1.1 m, and D ¼ 2.1 m.4

water table, but partially above it, giving an intermediate rate of moisture loss. Figure 18 shows the drying rate as a function of the water table depth relative to the latrine and ground level, with the color bar indicating temperature gradients from 2
C to 14
C. This might be imagined as the effect on drying as a water table rises from below the pit latrine. The membrane-lined walls of the pit latrine will start to become exposed to the saturated zone as the water table begins to rise; then the RH of the outside area takes the value of 100%. This causes a considerable drop in the drying rate. Predicting the vapor transfer rate in different modes of drying enables us to estimate the capacity of the sanitation system. As a case in point, with application of a 2
C temperature gradient when the water table is well below the pit latrine base and the RH of the surrounding soil is assumed to be 30%, the moisture transfer rate is predicted to be 180 14 160 12

Drying rate (l/day)

140 120

10

100

8

80

6

60

4

40 2

20

0

0 –20 –0.5

–2 0 0.5 1 1.5 2 2.5 Water table elevation relative to pit base (m)

FIGURE 18 A drying rate profile at different water table depths and temperature gradients. The color bar indicates the temperature gradients from low to high. Assumed latrine pit dimensions as in Figure 17.

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WATER RECLAMATION AND SUSTAINABILITY

64.35 l/day. Taking a simplistic approach to the problem in the absence of local information, the system is assumed to receive 520 g per person per day wet feces in an African area.4 Assuming 85% moisture content (wet base) for the feces,26 moisture input is calculated to be 0.44 l per person per day. Expecting this amount of input allows approximately 146 people to utilize the on-site sanitation system daily.

6. Conclusions The hydrophobic breathable membrane provides a significant rate of drying while serving as a barrier between sludge and air. The membrane does not make any alteration to the general drying pattern, but does slow down the flux rate to some extent. Three mass transfer models including the diffusion, two-period, and stagnant film models were studied and validated as candidates for putting the drying process on a quantitative basis. Among the models, the stagnant film model presents a mechanistic prediction incorporating all parameters controlling the drying process. As long as the sludge contains free water, it is at 100% RH and the moisture transport is limited by the membrane area. The vapor pressure inside the membrane, therefore, is not controlled by the moisture content of the sludge and the stagnant film model predicts a constant rate of drying. The parameter l, defined as the resistance to the mass transfer across the membrane, was quantified as 0.01 m for the membrane utilized in the laboratory. It is important, however, to note that the quantity stems from the virtual stagnant film exerting resistance against the molecular diffusion from the feed to the permeate side, so that the thickness in Eqn (18) represents the virtual film thickness rather than the membrane physical thickness and membrane presence results in increased resistance. Applying the stagnant film model to MD practice, characterizing the membrane physical properties (i.e., thickness, porosity, and tortuosity), and using Eqn (18) to calculate l values may produce misleading results. The inconsistency will be even greater if the principle is applied to hydrodynamic conditions such as direct contact MD, where feed and permeate flow through the channels to create mixing, which decreases the film resistance. In technical consideration of a pit latrine design, as one practical application of the process, since the pit latrine is mostly at a sufficient level of moisture, the simulation presented as a drying-rate profile based on the stagnant film model should provide a reliable prediction for the rate of moisture loss from the pit, depending on the applied temperatures and water elevation from the pit base. Exposure to the saturated state slows down the drying rate. However, drying still takes place at a reasonable pace, enabling significant removal of water over timescales of relevance to latrine pits. Rates of water removal predicted from our measurements suggest that a pit latrine lined with a breathable membrane can remove moisture at a rate sufficient to equal or exceed the likely loading from a population of several families.

Chapter 15 • Sludge Drying Through Hydrophobic Membranes 389

Acknowledgments This research was funded by the Bill and Melinda Gates Foundation. The authors thank Dr Paul T. Imhoff for his continued support.

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23. Tsang KR, Vesilind PA. Moisture distribution in sludges. Water Sci Technol 1990;22(12):135–42. 24. Benali M, Kurda T. Thermal dewatering of diluted organic suspensions: process mechanism and drying kinetics. Dry Technol 2002;20(4):935–51. 25. Bird RB, Stewart WE, Lightfoot EN. Transport phenomena. revised 2nd ed. John Wiley & Sons; 2007. 26. Chaggu EJ. Sustainable environmental protection using modified pit-latrines. Wageningen: Sectie Milieutechnologie, Wageningen University; 2004.