Impact of Joule Heating and pH on Biosolids Electro-Dewatering

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Impact of Joule Heating and pH on Biosolids Electro-dewatering Tala Navab-Daneshmand, Raphaël Beton, Reghan J. Hill, and Dominic Frigon Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/es5048254 • Publication Date (Web): 15 Dec 2014 Downloaded from http://pubs.acs.org on December 20, 2014

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Environmental Science & Technology

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Impact of Joule Heating and pH on Biosolids

2

Electro-dewatering

3

Tala Navab-Daneshmand1, Raphaël Beton1, Reghan J. Hill2 and Dominic Frigon*, 1

4 5

1

Department of Civil Engineering and Applied Mechanics, McGill University, 817 Sherbrooke

6 7

St. West, Montreal, Quebec H3A 0C3, Canada 2

Department of Chemical Engineering, McGill University, 3610 University St., Montreal,

8

Quebec H3A 0C5, Canada

9

* Email: [email protected], Telephone: +1-514-398-2476, Fax: +1-514-398-7361

10 11 12

ABSTRACT Electro-dewatering (ED) is a novel technology to reduce the overall costs of residual

13

biosolids processing, transport and disposal. In this study, we investigated Joule heating and pH

14

as parameters controlling the dewaterability limit, dewatering rate, and energy efficiency.

15

Temperature-controlled electrodes revealed that Joule heating enhances water removal by

16

increasing evaporation and electro-osmotic flow. High temperatures increased the dewatering

17

rate, but had little impact on the dewaterability limit and energy efficiency. Analysis of

18

horizontal layers after 15 min ED suggests electro-osmotic flow reversal, as evidenced by a

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shifting of the point of minimum moisture content from the anode toward the cathode. This flow

20

reversal was also confirmed by the pH at the anode being below the isoelectric point, as

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ascertained by pH titration. The important role of pH on ED was further studied by adding

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acid/base solutions to biosolids prior to ED. An acidic pH reduced the biosolids charge while

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simultaneously increasing the dewatering efficiency. Thus, process optimization depends on

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tradeoffs between speed and efficiency, according to physicochemical properties of the biosolids

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microstructure.

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INTRODUCTION Biosolids production and its potential health risks are important concerns for

29

municipalities. High biosolids water content from mechanical dewatering (65-85% w/w) leads to

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high transport and disposal costs.1 Additionally, the recovery of biosolids as soil amendments

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demands US-EPA microbiological Class A or B or equivalent status.2 Electro-dewatering (ED) is

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an attractive new technology for enhancing the sustainability of biosolids handling. By applying

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an electric field to mechanically dewatered biosolids, ED can achieve 35% w/w water content

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while using less than 25% of the energy required for thermal drying;3 it can also reduce bacterial

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pathogen indicators to meet US-EPA regulations.4

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During ED with an open-sided unit,4 two phenomena participate in the dewatering

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process: electro-osmosis and evaporation. Electro-osmosis is the transport of water from pores

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and interstices by viscous and/or molecular interactions. The surface charge density of colloidal

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biosolids particles induces electro-osmotic flow when subjected to an electric field.5 This water

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transport occurs in parallel with electrolysis, producing hydrogen ions at the anode and hydroxyl

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ions at the cathode. The production and transport of ions inside the biosolids matrix affects the

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composition of the biosolids cake that is sandwiched between the electrodes.4, 6 Concurrently

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with electro-osmotic flow, Joule heating increases the temperature. High temperatures may

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facilitate dewatering by reducing the water viscosity and enhancing evaporation.7 Ultimately, the

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cumulative effects of electro-osmosis and evaporation determine the extent of dewatering.


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Beyond temperature changes, ED subjects the dewatering cake to changes in pH. The

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effects of biosolids pH and particle surface charge on biosolids dewaterability have been studied

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in different contexts.5, 8-11 A common pH reference is the isoelectric point, which corresponds to

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zero surface charge density; it is reported to occur at pH values between 1 and 3 for municipal

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wastewater biosolids.5, 10, 12 At pH levels slightly above the isoelectric point, biosolids particle

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surfaces have a low negative charge, and the biopolymer matrix is most compact.13 At pH values

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far from the isoelectric point, high charge densities (positive charge at low pH and negative

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charge at high pH) enhance repulsion and, hence, expand the matrix. The expanded structure

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would present a higher resistance to flow by increasing the biosolids specific surface area.13

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Despite these initial descriptions of temperature and pH on ED, theoretical concepts

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linking the observed dynamics and the engineering of the process remains to be properly defined

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for practitioners. Here, we propose three concepts. First, the dewaterability limit is the mass of

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remaining water in a biosolids sample per mass of total solids when electro-osmotic flow stops.

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This is the ultimate moisture content achievable by the ED process. Second, the dewatering rate

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is the mass of water removed per dewatering time. As it relates to the overall treatment rate of

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ED units, dewatering rate negatively correlates with the capital investment in a treatment facility.

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Third, dewatering energy efficiency is the mass of water removed per unit of electrical energy

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consumed. This negatively correlates with the operational costs of ED units. The main objective

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of this study is to describe the impact of temperature and pH on the ED process in terms of the

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defined parameters: dewaterability limit, dewatering rate, and energy efficiency.

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To fully understand these concepts, they need to be defined with respect to the

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classification of water contained in biosolids: free, capillary, vicinal, and hydration.14 Free water

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is not bound to the solids matrix; capillary water is loosely bound to the matrix by mechanical


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adsorption; vicinal water is more tightly bound to the matrix by hydrogen bonding; and, finally,

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water of hydration is bound within the solids biopolymer matrix by relatively strong chemical

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bonds (and can be removed only by thermal energy). The binding energy of water to the solids is

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inversely related to water activity, which is the ratio of the vapor pressure of water in equilibrium

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with biosolids to the saturated vapor pressure of pure water. Classical S-shaped water adsorption

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isotherms related water activity and moisture content, and they can be interpreted according to

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the discrete water distribution discussed above.15, 16

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The binding strength of water to a biosolids matrix plays a significant role in

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dewatering.17 By hindering water flow and determining the necessary energy requirement to

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remove water, the binding strength of water to the matrix impacts the dewaterability limit,

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dewatering rate, and energy efficiency. Furthermore, electrical conductivity is related to water

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activity, because water activity also measures the availability of the water molecules to transport

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ions as electrical current.18 While water distribution in biosolids has been studied in relation to

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other dewatering processes,14-17 so far it has not been examined for ED. Therefore, ED dynamics

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need to be defined with respect to biosolids water activity and its related adsorption isotherm.

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This study investigated the dewaterability limit, and the dewatering rate and efficiency of

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ED biosolids. The impact of Joule heating on the fractions of water removed by electro-osmosis

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and evaporation was experimentally determined via a temperature controlled ED unit. Here,

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mass and energy balances were used to compute the evaporated and remaining water during the

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tests. Next, the effects of the initial biosolids pH and ionic strength on ED rate and efficiency

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were evaluated by the addition of acid, base or neutral salts. Finally, because of the vertical

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stratification in water activity and pH between the anode and the cathode, these effects were

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analyzed by dividing the cake into four horizontal layers.


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MATERIALS AND METHODS Biosolids. The secondary waste activated sludge biosolids were obtained from the

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centrifugal dewatering units at a wastewater treatment plant near Montréal without a primary

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clarifier. The cationic polymers Flo-CA475 and Flo-CA4800 were added before the dissolved air

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floatation and centrifuge dewatering units (1-4 kg/ton-TS and 12-21 kg/ton-TS; SNF Canada

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Ltd., Trois-Rivières, Québec, Canada). Dewatered biosolids samples were collected, brought to

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the laboratory on ice, and then stored at 4 °C for up to 4 d. Throughout the study, total solids,

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pH, and apparent resistivity of biosolids were in the range 14.1-18.5% w/w, 6.6-7.5, and 1000-

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2000 Ohm/m, respectively.

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Electro-dewatering. The laboratory ED unit was an Ovivo model CINETIK® CK-Lab

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(Boucherville, Québec, Canada, Figure S1). In these experiments, a direct current was applied

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with the maximum voltage and current set at 60 V and 6.5 A, respectively. The effect of high

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temperatures on dewatering was examined using temperature controlled (i.e., cooled) electrodes

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(see details in Navab-Daneshmand et al., 2012).4 For the layered ED tests, a 53-µm pore-size

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Spectra Mesh™ nylon filter was used as the partitioning material (Spectrum® Laboratories Inc,

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Rancho Dominguez, CA). This nylon mesh has been shown to have no significant impact on

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energy consumption, water removal, and temperature.4 Voltage, current, pressure, removed

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filtrate mass, cake thickness, electrical energy consumption, biosolids cake temperature, anode

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temperature, and cathode temperature were measured at 2 s intervals. To study the influence of

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the initial pH and ionic strength on ED efficiency, 4-10 mL of 0.5 M NaCl, HCl, or NaOH

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solutions were added to the untreated biosolids samples and mixed in with a spatula. To test the

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apparent resistivity (resistance divided by cake thickness) in each horizontal layer of the


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dewatered cake, four layered biosolids samples were electro-dewatered for 15 min. After ED,

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samples from the same layers in different ED tests were mixed thoroughly and electro-dewatered

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for another 5 min.

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Total Solids, Water Activity and Moisture Sorption Isotherms. Standard Method No.

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2540-B was used to measure biosolids dryness.19 Moisture sorption isotherms were constructed

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by drying 10 wet g of biosolids at 105 °C for 2 to 8 h. Biosolids were then transferred to water

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activity plastic containers (ROTRONIC®, Switzerland). The containers were immediately sealed

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(to avoid moisture exchange) and left on the bench until they reached room temperature (20 °C).

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Water activity was measured using the manometric HygroLab Set 2 water activity meter

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equipped with an AW-DIO probe (ROTRONIC®, Switzerland) according to the manufacturer’s

125

instructions. Water activity was recorded as 1 if condensation was present on the container walls

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before the test. Final biosolids dryness was measured after the water activity measurements.

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Water activity versus moisture content (mass of water per mass of dry solids) data from

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these tests were analyzed using established isotherm models.16, 17, 20, 21 To identify the best

129

sorption isotherm, the logarithmic forms of various models for moisture content were fitted to

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experimental data using non-linear optimization within the SAS® software package (PROC

131

MODEL, SAS Institute Inc., Cary, NC). Models were compared by the mean relative error

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(MRE).17 Analyses showed that among the suggested isotherms, the Oswin (eq 1), and

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Guggenheim-Anderson-de Boer (GAB, eq 2) models provided better fits to the data.16, 17, 20, 21

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While the Oswin isotherm is empirical, the GAB isotherm has physico-chemical interpretations:

135 136

Oswin model: =

GAB model: = (

)(( ) )


(1) (2)

6

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where aw is the water activity, Mw is the moisture content, C and n are empirical temperature

138

dependent constants, Xm is the moisture content at full monolayer coverage of adsorption

139

substratum, Cb is the ratio of the water-water to water-substratum binding energies, and k is an

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empirical constant.16, 20, 21

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Biosolids pH and Surface Charge Density. Biosolids pH was measured following

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Navab-Daneshmand et al.4 using an accumet® gel-filled AgCl combination electrode (Fisher

143

Scientific, Canada). For pH titrations, 30 mL of a 0.5 M KCl solution was added to 30 g of wet

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biosolids. Different volumes of a 0.5 M HCl or NaOH solution were added to separate beakers to

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achieve concentrations between 0-120 mM. The total volume was adjusted to 130 mL by adding

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DDI (distilled and then deionized) water. Suspensions were stirred on a stir plate for about 30

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min, after which the pH was recorded.

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Biosolids surface charge density was measured using the colloid titration technique

149

described by Morgan et al.22 Briefly, 2 wet g of biosolids were homogenized in 40 mL DDI

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water with an Ultra-Turrax® S10N-10G disperser (IKA® Works Inc., Wilmington, NC); 0.5 mL

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of the homogenized suspension was diluted in 49.5 mL DDI water; 2 mL of Polybrene 0.002 N

152

(Sigma-Aldrich, Milwaukee, WI) and a few drops of toluidine blue indicator were added. The

153

mixture was then titrated against PVSK (Sigma-Aldrich, Milwaukee, WI). The mixture pH was

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manipulated by adding 0.5 M NaOH or HCl to the mixture before adding Polybrene.

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Mass and Energy Balances. Considering the biosolids dewatering cake as the control

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volume, and assuming that no solid leaves the control volume by filtration, the water mass

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balance is:

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−

−

=

(3)


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where mF is the mass of filtered water, mV is the mass of vapor, mW is the mass of remaining

160

water in the biosolids, and t is the dewatering time. Similarly, the energy balance is:

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−

() −

() +

!

+

"

= # $ %$

&

+ #' %

&

+

( ()

(4)

162

where CpS and CpL are heat capacities at constant pressure for solids and liquid water [J/(kg K)];

163

CpS was considered 900 J/(kg K) at 20 ºC. HL(T) and HV(T) are the enthalpies of liquid and vapor

164

water [J/kg] at temperature T [K] evaluated using constant heat capacities for liquid and vapor

165

water, respectively (i.e., CpL and CpV). UL(T) is the internal energy of liquid water [J/kg] at

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biosolids temperature T. Qe is the electrical power [J/s], and Qh is the total heat loss (to the air,

167

anode and cathode, eq 5) [J/s].

168

Qh = βh (Qhair + Qhanode + Qhcathode), with each term in the form Qh* = A*h*( Tcake− T*) (5)

169

where the asterisk (*) identifies either air, anode or cathode, A is the interfacial cross-sectional

170

area between the cake and the heat sink of interest [m2], and h is the heat transfer coefficient (h =

171

10, 19, and 16 J/(m2 K s) for air, titanium anode, and stainless steel cathode).23 Note that for the

172

cooled tests, Qhanode and Qhcathode also account for heat losses due to the flow of ice-cold water in

173

the electrodes. To account for these losses, the constant )" was adopted as a fitting parameter to

174

ensure that the calculated value for the mass of remaining water in the biosolids (mW) at the end

175

of each test equaled the measured mass of remaining water in the cake.

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Protonation/Deprotonation Equilibria. An analytical model was fitted to the

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experimental data using chemical reactions for acid and base dissociation. The fitting was

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performed to determine the apparent acid (Ka) and base (Kb) dissociation constants for the

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surface functional groups. It was assumed that the weak acid/base contributions in the ionic

180

solutions were only from the surface functional groups, denoted as aH and bOH. Strong acid and

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strong base (AH and BOH, respectively) were assumed both on surfaces and added to the


8

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182

solution during titration. The model is based on the following solution electroneutrality (eq 6)

183

and water and acid/base equilibrium conditions (eqs 7-9): *+ , + *- , + *. , = */ , + *0 , + * ,

184

(6)

185

[H+][OH−] = 10−14

(7)

186

*+ , =

(8)

*/ , =

187

188

*1 ,

21345 67

*81 , 213(29345) 6

(9)

where [a0] = [aH] + [a−] and [b0] = [bOH] + [b+]. Combining eqs 6-9 furnishes eq 10:

189

190

191

:

*1 ,

21345 67,1

+

*2 ,

21345 67,2

+ *- ,
?@A!

L

+ B10 (E FG) + *- ,HI>!J>= =

*81 , 213(29345) 6,1

+ *0 ,M

=>?@A!

+ N10 FG + *0 ,OI>!J>=

(10)

192

where [a0] = [aH] + [a−], [b0] = [bOH] + [b+], and the subscripts refer to the concentrations in

193

the aqueous solution or on solid surfaces. Eq 10 was solved numerically for Ka and Kb by fitting

194

to the pH titration data. The pH time series from a base titration were fitted with the following

195

exponential relaxation:

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P = PQ + (PQ − PR )S

U T

(11)

197

where pH∞ is the final stabilized pH calculated from the empirical model in eq 10, pH0 is the

198

initial pH after NaOH addition, t is time [min], and τ is the exponential-relaxation time constant

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[min]. Finally, particle surface charge density (V@ ; Coulomb/g-TS) and Donnan potential (WX ; V)

200

were calculated using eqs 12 and 13:


9


201

202

V@ = N*- , − *0 ,O=>?@A! − L WX =

*1 ,

21345 67,1

−

*2 ,

21345 67,2

+

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*81 , 213(29345) 6,1

N*Y3 , *Z[ , *Y2 ,*Y\ ,O]^_`7ab B R345 R3(29345) *Y3 , *Z[ ,H N*Y3 ,*Z[ ,*Y2 ,*Y\ ,O]^_`7ab BR345 R3(29345) *Y3 , *Z[ ,H

M

=>?@A!

7c^bd^]

7c^bd^]

(12)

(13)

203

where {[A−]-[B+]}surface is a constant used to adjust the correspondence between the pH and

204

colloidal titration data; it was acquired by fitting colloid titration data and the calculated

205

coefficients from eq 10.

206 207 208

RESULTS AND DISCUSSION Impact of Temperature. During non-cooled (i.e., normal) ED operation, the temperature

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of the dewatering cake rapidly increased to 95.7 ± 1.9 °C (± standard deviation) after 6 min

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(Figure 1a). Energy consumption and water removal were linear with time until a water removal

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plateau was reached after approximately 10 min (Figure 1a). With the cooled electrodes, the

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temperature remained below 47 °C throughout the 15 min tests (Figure 1b). Comparing the two

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systems, the test with the non-cooled electrodes consumed more electrical energy than the one

214

with cooled electrodes (45.3 ± 2.4 W h and 32.0 ± 0.7 W h, respectively, Figures 1a and b). This

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difference corresponds to the non-cooled system removing more water (71.2 ± 1.1% vs. 49.6 ±

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2.2% in non-cooled and cooled ED, respectively, Figure 2).

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The ED temporal dynamics were analyzed using the mass and energy balance equations

218

(eqs 3 and 4) to calculate the evaporated and remaining fractions of water during 15-min ED

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(Figures 1d and e). The remaining and evaporated water are not displayed for the 60-min cooled

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experiments (Figure 1f) because )" was not constant with time. In the non-cooled system, )"

221

(1.3 ± 0.3) was not significantly different from 1.0 and the differences are likely due to the errors


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in measurements and the specific h coefficients considered for the heat sinks. The high )" value

223

for the cooled system accounts for the heat loss to the flow of ice-cold water that is not

224

considered in eq 4.

225

226 227

Figure 1. (a) and (b) Consumed energy, biosolids temperature, and apparent resistivity;

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(d) and (e) remaining water, filtered water and evaporated water, for the non-cooled (left) and

229

cooled (middle) electro-dewatering systems with 15 min dewatering time. (c) Consumed energy,

230

biosolids temperature, and apparent resistivity; and (f) filtered water, for the non-cooled electro-

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dewatering system with 60 min dewatering time. Values are the average from three replicates.

232

The standard errors for the data were below 12%, except for the evaporated water in the non-

233

cooled system where it reached 54% over the period 0-10 min. Non-cooled (a and d) and 15-min

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cooled (b and e) experiments were performed one year apart from the 60-min cooled (c and f)

235

experiments. )" were 1.3 ± 0.3 for non-cooled (d), and 8.5 ± 0.9 for 15-min cooled ED (e).

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In the non-cooled system, electro-osmotic dewatering and evaporation began after 1 min,

238

increased linearly, and stopped after 10 min and 8 min, respectively (Figure 1d). From the initial

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biosolids water content in the non-cooled system, 50.1 ± 0.6% was removed by electro-osmosis

240

and 21.1 ± 0.9% by evaporation (Figure 2a). In the cooled ED system, however, electro-osmosis

241

was slower, never reached a plateau during the 15-min cycle, and removed 36.6 ± 1.3% moisture

242

as filtrate (Figures 1e and 2a). Furthermore, evaporation was not detected until 8 min, and

243

ultimately accounted for 13.0 ± 1.8% of the total moisture removed (Figures 1e and 2a). Note

244

that this evaporation may include artifacts due to water retained by capillarity in the filtration

245

mesh. Even with these uncertainties, higher temperatures due to Joule heating seem to enhance

246

water removal by increasing the evaporation and electro-osmotic flow rates. When cooled ED

247

was tested over a 60-min period, a dewatering plateau was observed after about 50 min (Figure

248

1f). The intermittent water flow in the electrodes during these experiments control the

249

temperature around 20 °C and, hence, caused the observed oscillating temperature and resistivity

250

profiles.

251

252 253 254

Figure 2. (a) Water fractions in biosolids after 15 min non-cooled and cooled electrodewatering; and (b) dewaterability limit after reaching a plateau after 15 min and 60 min in non-


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cooled and cooled electro-dewatering, and dewatering rate and efficiency after 10 min of non-

256

cooled and cooled electro-dewatering. Bars represent the standard error from three replicates.

257 258

The impact of temperature on ED performance can be analyzed in terms of the three

259

performance parameters defined earlier. First, the dewaterability limits after reaching a water

260

filtration plateau were 1.28 ± 0.02 and 1.64 ± 0.01 g-remaining water/g-TS for the non-cooled

261

(15 min) and cooled (60 min) ED systems, respectively (Figure 2b). This suggests that higher

262

temperatures did not impact the dewaterability limit once sufficient dewatering time has elapsed

263

under each condition. Second, the dewatering rate was significantly higher during the non-cooled

264

ED compared to the cooled system (9.15 ± 0.04 g/min vs. 4.85 ± 0.06 g/min, Figure 2b). Finally,

265

the dewatering efficiencies were not significantly affected by different temperatures (Figure 2b).

266

Note that dewatering rates and energy efficiencies were calculated at 10 min. The 10 min

267

reference time was chosen because it is the practical full-scale dewatering time recommended for

268

this unit (Ovivo, Boucherville, Québec, Canada).

269

Cake Physicochemical Gradients. Since vertical gradients in different physicochemical

270

parameters have been previously shown to form in the dewatering cake,4 their relationship to the

271

dewaterability limit was further investigated. Biosolids samples were divided into four horizontal

272

layers and then electro-dewatered using the non-cooled system. Just before reaching the water

273

removal plateau at 10 min (Figure 1d), Layer 1 (the layer closest to the anode) was the driest

274

with 0.508 ± 0.018 g-water/g-TS (i.e., 66.2 ± 0.9% w/w total solids, Figure 3a). After dewatering

275

for 15 min, however, Layer 2 became the driest with 0.622 ± 0.376 g-water/g-TS (i.e., 65.3 ±

276

15.3% w/w total solids). To study the apparent resistivities, samples from each layer of 15 min

277

ED cake were tested in the ED unit. Although no further water was removed during these tests


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(data not shown), the apparent resistivity was measured and reported (Figure 3c). The apparent

279

resistivity in the top layers (Layers 1 and 2) was 3-4 times higher (Figure 3c) compared to Layers

280

3 and 4. Thus, the dewaterability limit may be due to the low moisture and high resistivity in the

281

layers closest to the anode.

282

Impact of pH on Dewatering Rate and Efficiency. The changes in pH are expected to

283

affect the biosolids surface charge and, hence, ED dynamics. To study the impact of biosolids pH

284

on the dewatering rate and efficiency, HCl and NaOH were added to biosolids samples to adjust

285

the initial pH (Figure 4a). The ionic strength was also controlled by the addition of NaCl (Figure

286

4b). Departures from neutrality (i.e., increase or decrease of initial pH) reduced the dewatering

287

rate (Figure 4d). Comparing the no-additive neutral biosolids with the NaCl-added samples, the

288

main parameter that affects the dewatering rate is the ionic strength (decreasing with increasing

289

ionic strength). However, biosolids with lower initial pH demonstrated higher dewatering rates

290

compared to biosolids with higher initial pH values. Conversely, the dewatering efficiencies

291

were mainly affected by the initial pH (Figure 4e). Biosolids with lower initial pH showed higher

292

dewatering efficiencies than those with higher initial pH, an effect also observed by others.5, 9

293


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294 295

Figure 3. (a) Moisture content, and (b) pH after different electro-dewatering cycles in

296

four horizontal layers of biosolids. (c) Apparent resistivity of layers 1, 2, 3, and 4 after 15 min

297

electro-dewatering, measured by averaging the apparent resistivity of each layer after electro-

298

dewatering for another 5 min. In panel (a), bars represent the range of two replicates for the 8, 9

299

and 10 min electro-dewatering cycles, and the range of four replicates for the 15 min electro-

300

dewatering cycle. In panel (b), bars represent the standard error from three replicates (pH data

301

were redrawn with permission from Navab-Daneshmand et al.).4


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302 303

Figure 4. (a) Initial and final pH, (b) added ionic strength, (c) final moisture content, (d)

304

dewatering rate, and (e) dewatering efficiency for biosolids without and with added NaCl, HCl or

305

NaOH after 15 min electro-dewatering.


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306 307

Figure 5. Biosolids (a) pH titration, (b) surface charge density by colloid titration, (c) and

308

Donnan potential. In panels (a) and (b), pH was adjusted by the addition of 0.5 M HCl or NaOH.

309

For colloid titration, the measured surface charge densities were divided by a factor of 23.7 to fit

310

the calculated data. The Donnan potential was calculated based on HCl and NaOH solutions

311

added to biosolids during pH titration for two different KCl concentrations (0.3 and 0.5 M).

312 313

The impact of biosolids charge properties affecting dewatering rates and efficiencies at

314

different pH were characterized by pH and charged-colloid titration (Figure 5). When biosolids


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315

were titrated with HCl, the pH stabilized quickly, producing a robust titration curve (Figure 5a).

316

However, when NaOH was added, the pH did not stabilize within 6 h testing (Figure S2a). We

317

therefore used the HCl titration data to model the biosolids pH titration and estimate the surface

318

charge densities using eqs 10 and 12 (Table 1). The resulting titration model was used to

319

estimate the stable pH (PQ ) when NaOH was added, and to calculate a pH relaxation-time

320

constant (τ) for each test via eq 11 (Figure S2b). These relaxation-time constants increased

321

linearly with pH (Figure S2b), and may be valuable for future mathematical modeling of the

322

dewatering cycle. An isoelectric pH of 4.05 was measured by colloid titration and eq 12 (Table

323

1). Here, the net charge densities measured by colloid titration were approximately 24 times

324

higher than those obtained by the pH titration method. This discrepancy has been explained by

325

the non-stoichiometric polymer complex precipitation during colloid titration.24

326 327

Table 1. Model fit coefficients for eqs 10 and 12. Analytical model coefficients Equation 10 a0 Ka,0 a1 Ka,1 b0 Kb,0 Equation 8 {[A−]-[B+]}surface

Experimental data calculated values pH titration (Figure 5a) 5.37×10−2 mol/L 2.46×10−3 4.65×10−2 mol/L 1.23×10−5 1.03×10−1 mol/L 5.21×10−7 Colloid titration (Figure 6b) −4.50×10−2 mol/L

328 329


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330


The dewatering rate and efficiency can now be explained with respect to the pH

331

dependency of measured charge densities. The effective attraction between the anode and

332

biosolids increases with the biosolids charge. This charge can be interpreted in terms of a

333

Donnan potential, which responds to the pH and ionic strength (eq 13 and Figure 5c). It provides

334

a good explanation for the variations in dewatering rates, since small additions of salt (i.e.,

335

increase of ionic strength) reduced the dewatering rate (Figures 4b and d) due to compression of

336

the diffuse layers of biosolids counter charge.

337

Dewaterability Limit: pH and Water activity. Dewatering reached a plateau after 10

338

min ED (Figure 1d), possibly due to changes in pH reversing electro-osmotic flow.25 Based on

339

the pH-dependent charge density, biosolids became positively charged when pH < 4 (Layer 1,

340

Figures 3b and 5b), thereby reversing the electro-osmosis driving force close to the anode. This

341

is also evident from the moisture content in Layer 1, increasing between 10 and 15 min ED,

342

while Layer 2 continued to dewater (Figure 3a).

343

Furthermore, the layers closer to the anode (i.e., Layers 1 and 2) measured 3-4 times

344

higher apparent resistivity (Figure 3c), indicating a lower capacity for Layer 1 and 2 to conduct

345

electrical current after 15 min ED. Since electrical conductivity is related to water activity,18 we

346

measured the water adsorption isotherm (Figure 6).16, 17, 20, 21 While the Oswin and GAB models

347

fitted the data satisfactorily, as observed by others,17, 20 the GAB model provided a better fit to

348

the data (Figure 6 and Table 2). Comparing the dryness of the different layers to the isotherm

349

suggests that the dewatering plateau occurs when the water activity falls below 0.95,

350

corresponding to moisture contents less than 0.82 g-water/g-TS (i.e., < 55% w/w total solids,

351

Figure 6). Under these conditions, capillary and free water have been removed and the remaining

352

water is strongly associated with the biosolids (Figure 6).15 After reaching the dewatering plateau


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353

(Figure 1d), the moisture content in Layer 1 did not change, whereas the moisture content in

354

Layer 2 fell below 0.11 g-water/g-TS, suggesting that it became electrically insulating and,

355

therefore, unable to sustain continued ED. This is consistent with a previous study that showed a

356

direct relationship between water activity and conductivity, where matrices with water activities

357

below 0.95 were nonconductive.18

358

359 360

Figure 6. The moisture sorption isotherm in thermally dried biosolids samples, and in the

361

four horizontal layers of electro-dewatered biosolids cake, after 8, 9, 10 and 15 min cycles. Lines

362

are the best fits for the Oswin and GAB isotherms. Numbers next to symbols for 10 and 15 min

363

ED cycles represent the horizontal position of the cake layer, with 1 being the closest to the

364

anode and 4 being the closest to the cathode (as shown in Figure 3). The values for the electro-

365

dewatered samples are averages of two experiments with the range of replicates below 1.4% and

366

15.7% for water activity and moisture content, respectively. Interpretations of water fractions are

367

according to Vaxelaire et al.16


20

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368 369 370

Table 2. Sorption isotherms model fit coefficients and error criteria (eqs 1 and 2 solved with

371

experimental data in Figure 6) Coefficients C N Xm Cb K MREa a Mean relative error

Oswin model 0.076 0.837

0.579

GAB model

0.041 17.031 1 0.573

372 373 374

In summary, this study shows that Joule heating during ED enhances water removal from

375

biosolids by evaporation and electro-osmotic flow mechanisms. While high temperatures

376

significantly increase the dewatering rate, they do not affect the dewaterability limit and

377

dewatering efficiency. The spatial and temporal evolution of the cake, as revealed by moisture,

378

resistivity, and pH profiles, suggests a localized reversal of the electro-osmosis driving force and

379

an increase in electrical resistance. The resulting flow reversal reflects a pH-induced change in

380

the biosolids charge due to hydrogen-ion production at the anode. Thus, at long times, the cake

381

becomes driest within, rather than at the anode, i.e., the driest layer migrates from the anode

382

toward the cathode. These dynamics are accompanied by an increase in electrical resistance, due

383

to lower water activity, which further increase resistance to electro-osmotic flow. ED with

384

various initial pH values showed that an initially acidic pH increases dewatering efficiency. This

385

is because the matrix adopts a low negative charge that reduces the hydrodynamic resistance to


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386

electro-osmotic flow. Note that pH values far from the isoelectric point decrease the dewatering

387

efficiency, because the stronger mutual particle repulsion seems to increase hydrodynamic

388

resistance. These inferences might be explained by an increase in the specific surface area of the

389

matrix that accompanies an increase in the biosolids surface charge. Finally, increasing the ionic

390

strength at constant pH decreases the dewatering rate. This can be explained by a compression of

391

the diffuse layers of the biosolids counter charge; and is evident, in part, by the effect of the ionic

392

strength on Donnan potential, which we ascertained from a pH titration model. These

393

observations and the accompanying titration data will assist the development of a mathematical

394

model that accurately captures the observed spatial and temporal dynamics.

395 396 397

ACKNOWLEDGEMENTS The Natural Sciences and Engineering Research Council of Canada’s Collaborative

398

Research and Development program and Ovivo provided funding for this study. The authors

399

thank Frederic Biton (Ovivo), Bruno Desmarais (Ovivo), Alain Silverwood (Ovivo), Céline

400

Gagnon (Aquatech), and Gilbert Samson (Régie d’Assainissement des Eaux du Bassin LaPrairie)

401

for their technical support and useful comments.

402 403 404

SUPPORTING INFORMATION AVAILABLE This information is available free of charge via the Internet at http://pubs.acs.org/.

405 406

REFERENCES

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Impact of Joule Heating and pH on Biosolids Electro-Dewatering

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