Anaerobic Digestion: Effects on Particle Size and Dewaterability

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  • Anaerobic Digestion: Effects on Particle Size and DewaterabilityAuthor(s): Desmond F. Lawler, Yoon Jin Chung, Shiaw-Jy Hwang and Barbara A. HullSource: Journal (Water Pollution Control Federation), Vol. 58, No. 12 (Dec., 1986), pp. 1107-1117Published by: Water Environment FederationStable URL: http://www.jstor.org/stable/25043143 .Accessed: 14/07/2014 19:59

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  • FOCUS

    Anaerobic digestion: Effects on particle size and dewaterability Desmond F. Lawler, Yoon Jin Chung, Shiaw-Jy Hwang, Barbara A. Hull

    Anaerobic digestion is commonly used in wastewater treat

    ment plants to stabilize sludge produced in primary and sec

    ondary treatment, and to reduce the volume of solids for ultimate

    disposal. Solid/liquid separation (secondary digestion, thickening or dewatering, or a combination of these) follows anaerobic

    digestion to complete volume reduction. Nevertheless, no clear

    picture of the effects of anaerobic digestion on subsequent de

    watering (or other solid/liquid separation processes), or on the

    particle characteristics that influence dewatering, emerges from

    previously-published research.

    The objectives of this research were to study the effects of anaerobic digestion (including design and operational variables of digesters) on particle characteristics and dewaterability of typ ical municipal wastewater sludges and to relate the particle char

    acteristics to sludge dewaterability. These objectives were achieved through operation of carefully-controlled laboratory digesters under various conditions.

    BACKGROUND

    Digestion. Anaerobic digestion is currently thought of as a

    three-stage process that involves hydrolysis of solid and complex soluble organics by fermentative bacteria, oxidation of volatile

    acids and alcohols to acetate and hydrogen, and C02 and CH4

    production by methanogenic bacteria.1'2 Design and operational variables that significantly influence digester performance include

    temperature, solids loading rate, solids detention time, and in

    fluent sludge concentration and composition (the relative mix

    ture of primary and second sludges). High-rate digesters usually

    are designed3 for 10 to 20 days detention time, influent solids

    concentrations of 3 to 8%, volatile solids loading rates of 1.6 to

    6.4 kg/m3 d, and operating temperatures of 30? to 40?C. Anaerobic digestion and dewaterability. Previous studies on

    the effects of digestion on dewatering present a confusing picture.

    Approximately equal numbers of studies show that digestion

    improves dewaterability or makes it worse. Rudolphs and Heu

    kelekian4 reported, on the basis of batch tests a well-digested

    sludge had better drainability on a small sand filter than raw

    sludge and that drainability improved with increased digestion time. Brooks et al.5 showed that digested sludge had better de

    waterability (lower specific resistance) than activated sludge or

    mixtures of primary and activated sludges. Haug et al.6 found

    that without thermal pretreatment, digestion significantly im

    proved dewaterability of primary sludge and a 1:1 mixture

    (weight of volatile solids basis) of primary and activated sludges but had little effect on dewaterability of activated sludge alone. A U. S. Environmental Protection Agency (EPA) manual7 states that anaerobic digestion of primary sludge alone produces clearer

    digester supernatant and a more easily dewaterable sludge than

    when biological sludge is added. Kini and Nayak8 showed that

    digested sludge had better dewaterability (measured in vacuum

    filtration tests) than primary sludge and that dewaterability im

    proved as digestion proceeded.

    However, other authors reported that dewaterability deteri

    orated with anaerobic digestion. Pearson and Buswell9 found that as digestion proceeded, sludge drainability was reduced.

    Others10,11 concluded from plant experiences that digested ac

    tivated sludge was more difficult to dewater than other sludges. Karr and Keinath12 reported that anaerobically digested sludge was more difficult to dewater (measured by capillary suction

    time and specific resistance) than either primary or activated

    sludge.

    Particle characteristics and dewaterability. Many factors have

    been reported to influence sludge dewatering characteristics. A

    review of previous work concluded that particle size was clearly

    the most important factor that influences dewaterability, and

    that many other factors reported were important because of their

    effect on particle size.12 In experimental work reported in the same paper, sludge samples were fractionated through a series

    of steps that included sieving, flocculation and settling, and

    membrane filtration at three pore sizes. After re-mixing sludges

    using different proportions of their fractions, researchers found

    that different types of sludge made in similar proportions had

    equal dewaterability, and that the "supracolloidal" fraction (ap

    proximately 1 to 100 /im) had the most significant effect on

    dewaterability. Particle size distribution was not directly mea

    sured. Instead the fractionation technique was used to estimate

    size distributions.

    Digesters should be designed and operated not only for

    sludge stabilization, but also to improve

    subsequent dewaterability.

    Knocke and co-workers13,14 reported direct measurements of

    particle size distributions and their effects on dewaterability for metal hydroxide and other water treatment plant sludges. A good correlation occurred between particle surface area determined

    from size distributions and the Kozeny equation applied to spe cific resistance. Particle size distributions (1 to 80 /im) of two

    anaerobically digested sludges showed that although most of the

    particles, on a number basis, were in the smallest sizes measured,

    the volume (or mass, assuming constant density with size) was

    spread almost uniformly across a log diameter scale throughout

    that range.15 No correlation between the size distributions and

    thickening or dewatering measurements was attempted in that

    study.

    December 1986 1107

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  • Lawler et al._

    EXPERIMENTAL DESIGN

    To define the interrelationships among anaerobic digestion, particle characteristics, and dewaterability, laboratory-scale high

    rate anaerobic digesters were operated under various conditions.

    Feed strength, feed composition, and solids detention time were chosen as primary control variables, and each was varied in

    separate experimental sets while the others were held at a stan

    dard or control value. Feed composition refers to the percent

    by weight of activated sludge in the influent; the remainder was

    primary sludge. The specific values used for each reactor in each

    set are given in Table 1 ; exceptions to the control values were

    necessary in some cases as indicated.

    In Set 1, the detention time was 21 days for all reactors; this was expected to be the standard, but the resulting solids loading rates were too low. Ten days was chosen as the standard detention

    time after Set 2; this change accounts for the apparent duplication between Sets 1 and 4 in varying the influent solids concentration. For the reactor in Set 3 with 100% activated sludge as the influent, it was not possible to achieve 3.5% solids concentration by gravity thickening; therefore the influent solids concentration for this reactor was 2.5%. Also in Set 3, one reactor was operated at 4?C

    in an attempt to test the effect of mixing with little or no digestion. The two-stage operation in Set 5 separated the acid phase from the methane production phase in two reactors in series. These

    reactors were operated with detention times of 2 and 8 days for the acid-phase and methane-phase reactors, respectively; the

    short detention time for the acid-phase is consistent with the

    findings of Eastman and Ferguson.16

    Digester design. The laboratory digesters were acrylic cylinders (20-cm I.D.) with a liquid volume of 6.0 L and a gas space of

    approximately 1.5 L. Plastic ball valves (1.27-cm I.D.) were used

    as ports on the top and side for introduction and withdrawal of

    sludge, respectively; the large size was used to minimize floe

    breakup. Gas was collected in inverted bottles submerged in an acid brine solution (10% NaCl and 1.0 N H2S04) and measured

    through a wet test meter. Continuous mixing was accomplished with a stirrer (20 rpm) that consisted of three submerged flat

    impellers and a fourth impeller with a curved face protruding above the liquid surface to prevent scum layer development. Baffles (2 cm) were installed throughout the depth at 90-deg intervals.

    Feed sludge. The entire sludge volume used in each set of

    experiments was obtained at one time from a municipal treat ment plant in Austin, Tex.; influent sludge for the laboratory reactors was the same throughout each set but was different for

    different sets. Primary and activated sludges were obtained sep

    arately to control the mixture ratio. Sludge was passed through a screen ( 1.27-cm mesh) to remove large materials, mixed at the

    desired ratio after measurement of the total solids content of

    each, and stored at 4?C in full 1-L plastic bottles to minimize air contact before use.

    Operation. For initial start-up of the laboratory digesters, active

    digesting sludge was obtained from a local municipal treatment

    plant. After each experimental set, the contents of all the digesters were combined, mixed, and divided again so that all digesters had identical conditions for the start of the next set.

    Sludge was withdrawn from each digester just before each

    feeding, with the amount (equivalent to the amount to be added) determined by the detention time. Feeding was done once per

    day during Set 1 and twice daily thereafter. Batch feeding was

    preferable to continuous feeding to avoid floe breakup in lab

    oratory positive displacement pumps. All digesters were operated in constant-temperature rooms at the desired temperatures (35?C

    and 4?C). To have the same average velocity gradient in the

    4?C reactor as in those at 35?C, the mixing speed was increased to 33 rpm.

    Monitoring. Digestion was monitored by measurements of

    the quantity and composition of the gas produced and by analysis of sludge samples for pH, total solids, volatile solids, and chemical

    oxygen demand (COD). Alkalinity and volatile acids are reported elsewhere.17 Gas production was monitored twice daily by re

    leasing the collected gas through the wet test meter. The gas was

    occasionally measured for carbon dioxide and methane content

    using a gas Chromatograph. Twice each week pH was measured

    immediately after sludge withdrawal; calibration was performed with buffers stored at the temperature of the sludges to be mea

    sured. Total and volatile solids concentrations were determined

    by Methods 209A and 209E, respectively, in "Standard Meth

    ods."18 COD was determined by the dichromate reflux method

    slightly modified from Method 508A in "Standard Methods."18 Instead of a sample volume, a weight of sludge (5 to 15 g) was

    chosen and then diluted to 20 mL. Hence, COD data were in units of milligrams of COD per kilogram of sludge (mg/kg). Because the density of wet sludge is nearly that of water ( 1 kg/ L), the COD results are approximately equal to mg/L.

    Measures of dewaterability. Two common measures of de

    waterability, specific resistance and capillary suction time (CST), were used throughout this research. Despite their common use,

    no standard methodology exists, especially for specific resistance.

    The methods used in this research reflect the findings of Ka

    vanaugh19 and Christensen and Dick.20,21 The CST device was

    used with the small stainless steel reservoir ( 1.8-cm I.D. and 2.5

    cm height). CST depends on the solids concentration, but one

    Table 1?Research design.

    Parameter varied*

    Standard

    value Variations

    Set 1: Feed strength5, % total solids

    Set 2: Detention time, days Set 3: Feed composition, weight % activated sludge Set 3: Temperature, ?C

    Set 4: Feed strength, % total solids

    Set 5: Staged operation

    3.5

    10

    20

    35

    3.5

    Single

    2.0

    7

    0

    4

    2.0

    2-Stageb

    5.0

    14

    40

    5.0

    6.5

    21

    100b

    6.5

    1 All reactors were operated at the standard values for all parameters except for the one parameter varied in each set. 3 Reactors with exceptions to the standard values; see text.

    1108 Journal WPCF, Volume 58, Number 12

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  • _Focus on Dewatering

    objective in this research was to compare dewaterability of dif ferent sludges with different concentrations. CST was measured both at the existing solids concentration of the sample and at a

    standardized value, chosen as 2.1% total solids. This was chosen

    because most digested sludge samples from the reactors fed with 3.5% raw sludge had this concentration in the early part of the research.

    For specific resistance, the experimental apparatus consisted

    of a B?chner funnel (7.6-cm I.D.), a 100-mL graduated cylinder with a vacuum port installed, a stainless steel screen (700-jum

    openings), filter paper (Whatman no. 2, 9-cm diameter), and a

    stopwatch. These details are provided because specific resistance

    measurements depend somewhat on the equipment used. The

    filter paper was positioned above the screen and sealed against the walls of the funnel by wetting with distilled water and ap

    plying a vacuum. A sample of sludge (usually 50 mL) was poured into the funnel and measurements of the volume of filtrate (V) at different times (/) were taken until the rate of flow was very slow. All specific resistance tests were run at the original sample concentration.

    Specific resistance (rs, in m/kg) is calculated by the following formula:

    2PA2b r*

    =- (1)

    /JLW

    Where:

    P = the applied vacuum (Pa), A = the filter area, cm2,

    fi = the viscosity, g/cm s, and

    w = the mass of solids in the filter cake per unit volume of

    filtrate, g/cm3.

    Unit conversions are necessary to obtain the desired units. The

    pressure and area values were constant throughout the experi

    ments and the viscosity was taken as that of water at the measured

    temperature. The values of b and w were calculated using in

    formation from the plot of t/Vversus Fas shown in Figure 1. The slope of the straight portion of the graph is the value b (s/ cm6). The value of w was calculated as the product of the initial total solids (TS) concentration of the sludge and the ratio of the

    0 10 20 30 40 50

    FILTRATE VOLUME (cm3)

    Figure 1?Plot used in determination of specific resistance. (Data shown

    are from effluent of standard reactor in Set 3.)

    initial volume of the sample to the volume of the filtrate at the end of the straight portion on the plot ( Vf on Figure 1 ). In this

    work, the total solids concentrations were used rather than sus

    pended solids because they were routinely measured in this re

    search; the error is less than 5% and consistent for all samples. Preliminary experiments to test the validity of specific resis

    tance measurements showed that the measurements were re

    producible, independent of solids concentration (0.79 to 2.36%

    sludge tested), and independent of the volume of sample used

    (30 to 70 mL tested). Nevertheless, some difficulties were en countered in determining the b and Vf values for high concen tration sludges and activated sludges. For high concentration

    sludges, the filter sometimes clogged, so that water no longer

    flowed through the cake and accumulated above the cake. Such

    ponding destroys the accuracy of the measurement because Vf is underestimated, so that specific resistance is also underesti

    mated. To prevent this ponding, the applied sludge volume was

    reduced to decrease the solids applied to the filter. A second

    difficulty, encountered particularly when measuring the specific resistance of activated sludge, was previously defined by Chris tensen and Dick21 as "nonparabolic behavior." In such a test,

    no well-defined straight line portion exists on the t/ V versus V

    plot, so that neither the b nor Vf values are known well. No

    solution was found for this problem during this research; mea

    surements that were subject to these errors are noted.

    Particle size distributions. The particle size distribution mea surements were made using an electrozone type instrument

    equipped with a logarithmic amplifier and a 100-channel pulse height analyzer. Twelve and 30 pm apertures were used on all

    samples, together with two ( 140 and 280 ^m) or three (100,200, and 560 pm) other aperture sizes. With this equipment, ex

    tremely detailed measurements of the particle size distributions were obtained over a broad size range, with a lower limit of detection of 0.66 pm. Procedures for measurement are reported

    elsewhere.22,23

    RESULTS

    Digester performance. Representative data for gas production,

    volatile solids content, and total solids concentration are shown

    in Figure 2. These data are from experimental Set 4 in which the influent solids concentration was varied between reactors.

    Results from the reactor fed 6.5% influent sludge are not shown because a mechanical failure occurred after 20 days. Although that digester performed well before the failure, it never operated successfully after the repair, apparently because of the high load

    ing rate. Results from the reactor fed 3.5% influent sludge are omitted from the solids measurements on the figure for the sake of clarity; in all cases, the results were between those shown for the 2% and 5% reactors and followed the same trends. The results for all three measures shown in Figure 2 indicate that reasonably steady conditions were achieved within 15 days of start-up. Gas

    production was approximately proportional to the influent solids concentration. Influent sludges had total solids concentrations

    slightly less than their nominal values. The results for both types of solids measurements show that significant digestion (destruc tion of solids) occurred.

    The performance of all digesters in this research is summarized in Table 2. The averages reported were obtained using data after 30 days of operation, except in the case of the 6.5% reactor

    which failed before that time. Several observations can be made

    December 1986 1109

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  • Lawler et al.

    0 10 20 30 40 50 60 TIME (days)

    Figure 2?Monitoring of digester performance in set 4 with varying in

    fluent solids concentration. (Detention time 10 days, temperature 35?C, 20% by weight activated sludge in influent.)

    from the data in this table. First, again with Set 4 as an example, the reductions in total solids, volatile solids, and COD for the

    digesters fed 2.0, 3.5, and 5.0% total solids all reflect excellent

    digestion, and show little variation between the reactors. The

    data for the reactor fed 6.5% sludge, taken between 10 and 20

    days, do not suggest that the reactor would have failed.

    Second, within each set of experiments, the same trend of

    little variation in any of the percent reductions can be seen. This

    suggests that digester performance is quite insensitive to the

    changes imposed. The principal exception to this is Set 3, in which the digester with 100% activated sludge as influent exhib ited notably poor performance and the digester that received

    only primary sludge (0% activated sludge) performed better than the digesters that were fed mixtures. The small amount of diges

    tion in the 4?C reactor in Set 3 and the first stage digester in Set 5 was expected; the similarity between these two reactors in all

    measures taken in this research suggests that the 4?C reactor

    behaved as a slow first-stage reactor.

    Third, both the total solids and volatile solids reductions were less in Sets 1 and 2 than in the other sets. The result for the total

    solids was expected because the volatile solids content was low

    in the influent sludge for those two sets (62 and 55% of the total

    influent solids in Sets 1 and 2, respectively, compared to 78 to

    84% in the remaining three sets). The lower volatile solids re

    duction in Sets 1 and 2 suggests that inorganic (nonvolatile) solids shield volatile solids and slow or prevent digestion of some

    volatile solids.

    Despite the exceptions and variations noted, the major con

    clusions from the data in Table 2 are that laboratory digester performance was excellent and was relatively insensitive to vari

    ations in operation.

    Effects of digestion on dewaterability. Determining the effects

    of anaerobic digestion on dewaterability was one objective of

    this study. The data for specific resistance during Set 4 for three reactors are shown in Figure 3. For influent sludges, specific

    resistance measurements at the different concentrations are ap

    proximately equal, which confirms that this measure is inde

    pendent of concentration. The results for reactors with influent

    sludges at 2.0 and 3.5% total solids concentration are typical for

    this experimental work. The most important observation is that

    the effluent specific resistance was much less than that of the

    influent; digestion significantly improved the dewaterability of these sludges.

    The results shown in Figure 3 for the reactor with 5% total

    solids in the influent are quite atypical, but nevertheless illustrate

    two points. First, the variation for effluent samples was the worst

    for any reactor in this research. These samples often exhibited

    the ponding effect or nonparabolic behavior, or both, which are

    the two significant causes of uncertainty in the measurement of

    specific resistance. These errors lead to an underestimation of

    the true specific resistance. Second, the average effluent specific

    resistance (omitting the first two points because steady-state per

    formance was not yet achieved) was worse than the influent for

    this reactor. When considered with the results for the other two

    reactors in Set 4, the confusion in the previously published lit

    erature about the effects of digestion on dewaterability becomes

    understandable. Performance of the three digesters, as measured

    by the parameters listed in Table 2, was equally good, so that

    digestion performance alone cannot explain the variation in de

    waterability.

    The average results for dewaterability for all of the experi

    mental conditions are shown in Table 3; CST values reported are those from the standard solids concentration of 2.1%. Ratios

    of the effluent to influent specific resistance and capillary suction

    time are almost all less than 1 ; in those cases, anaerobic digestion

    improved dewaterability. Of the 21 cases reported, there are six

    exceptions to this rule, all from digesters that were considered

    to be operated under stressed conditions. They are from the

    digesters in Sets 1 and 4 with the highest influent solids concen

    tration, the digesters in Set 3 with 40 and 100% activated sludge in the influent, the digester operated at 4?C, and the first stage

    digester in Set 5. Performance of the latter three was significantly worse than the others; the first three achieved satisfactory diges

    tion as measured by solids and COD reductions, but were near

    the limits of operation of those digesters that did not perform well. It is clear that good digestion generally leads to improved

    dewaterability, poor digestion leads to worse dewaterability, and

    dewaterability is more sensitive to variations in digester design

    and operation than digestion itself.

    Trends in dewaterability within each set of experiments are

    enlightening. Dewaterability of the digested sludge in Sets 1 and

    4, worsened with increasing concentration. The differences be

    tween the digesters fed 2.0 and 3.5% total solids are not statis

    1110 Journal WPCF, Volume 58, Number 12

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  • Focus on Dewatering

    Table 2?Effects of operating variables on digester performance.

    VS loading,

    kg/m3d pH

    %TS reduction

    % VS reduction

    % COD reduction

    m3 CH4/kg VS added

    Set 1: Total solids concentration, %

    2.0

    3.5

    3.5

    5.0

    6.5

    Set 2: Detention time, days 7

    10

    14

    21

    Set 3: % Activated sludge 0

    20

    40

    100 Set 3: Temperature, ?C

    4

    Set 4: Total solids concentration, %

    2.0

    35

    5.0

    6.5

    Set 5: Staged operation Standard

    1 st stage 2nd stage Overall 2-stage

    0.58

    0.98

    0.98

    1.41

    1.91

    2.67

    1.87

    1.34

    0.89

    2.86

    2.62

    2.57

    2.02

    2.73

    1.59

    2.75

    3.94

    5.30

    2.73

    13.7

    2.84

    2.73

    7.4

    7.4

    7.4

    7.5

    7.5

    7.2

    7.2

    7.2

    7.3

    7.2

    7.2

    7.4

    7.2

    7.1

    7.3

    7.4

    7.1

    7.3

    5.3

    7.3

    34.8

    34.3

    30.7

    30.3

    34.8

    28.6

    27.7

    29.8

    30.1

    56.1

    48.5

    48.4

    25.0

    12.2

    49.6

    54.6

    52.5

    53.9

    50.2

    15.4

    36.7

    46.5

    52.9

    50.4

    49.1

    50.1

    52.5

    47.0

    48.5

    50.8

    52.6

    63.4

    56.8

    54.9

    31.9

    6.9

    58.3

    60.3

    59.4

    59.6

    58.8

    16.6

    45.4

    54.5

    60.4

    59.8

    58.7

    58.6

    58.5

    53.8

    56.3

    56.9

    59.8

    64.0

    63.1

    58.6

    33.8

    2.2

    61.6

    61.9

    60.3

    55.9

    61.5

    4.3

    52.6

    54.7

    0.43

    0.45

    0.47

    0.46

    0.48

    0.42

    0.43

    0.44

    0.45

    0.52

    0.47

    0.46

    0.23

    0

    0.52

    0.48

    0.50

    0.44

    0.003

    0.48

    0.51

    I_I-1-1-1-1_I 0 10 20 30 40 50 60

    TIME (days)

    Figure 3?Dewaterability of raw (influent) and digested (effluent) sludges in set 4. (Detention time 10 days, temperature 35 ?C, 20% by weight

    activated sludge in influent.)

    tically significant but differences between the other reactors are

    significant. The trend of decreasing specific resistance with in

    creasing concentration for the feed sludges in Set 1 is thought to be caused by measurement error; the final procedures for

    measurement of specific resistance were not adopted until the

    end of that set. This trend is not found in the CST results for

    Set 1 nor is it statistically significant in the specific resistance

    results for Set 4. Within Set 2, the difference between the dewaterability of

    feed sludge and digested sludge was statistically significant. However, the trend of increasing specific resistance with longer detention time in the digested sludges was not statistically sig nificant at the 0.05 level. This trend is not as obvious in the CST

    measurements. It is important to note that both measures of

    dewaterability for the influent sludge were low in this set in

    comparison to the subsequent sets, apparently as a result of the

    low volatile solids content.

    Within Set 3, the results are quite complex. The results of

    each reactor after digestion were different, and the best dewa

    terability was achieved by the reactor that received the standard

    mixture of 20% activated sludge and the remainder primary

    sludge. As previously noted, in reactors that received 0% and

    20% activated sludge digestion improved dewaterability, whereas

    in the others dewaterability deteriorated. In both the first stage digester in Set 5 and the reactor at 4?C

    in Set 3, dewaterability became significantly worse with digestion,

    apparently because digestion was incomplete in both reactors.

    The nearly identical response of these two reactors is further

    indication that the digester operated at 4?C performed as a slow

    December 1986 1111

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  • Lawler et al.

    Table 3?Sludge dewaterability: effects of digestion.

    Specific resistance Capillary suction time

    Influent,

    1013

    m/kg

    Effluent,

    1013

    m/kg

    Effluent:influent

    ratio Influent, s Effluent, s

    Effluent:influent

    ratio

    Set 1: Total solids concentration, %

    2.0

    3.5

    3.5

    5.0

    6.5

    Set 2: Detention time, days 7

    10

    14

    21

    Set 3: % Activated sludge 0

    20

    40

    100 Set 3: Temperature, ?C

    4

    Set 4: Total solids concentration, %

    2.0

    3.5

    5.0

    Set 5: Staged operation Standard

    1 st stage 2nd stage overall 2-stage

    12.38

    10.77

    10.77

    9.42

    8.40

    8.38

    13.34

    16.67

    15.00

    15.24

    16.67

    23.30

    22.80

    19.55

    16.06

    16.06

    32.83

    16.06

    6.81

    9.81

    6.76

    7.52

    8.50

    2.96

    3.33

    4.85

    6.28

    10.56

    3.15

    19.36

    26.37

    30.46

    4.89

    5.76

    26.83

    1.01

    32.83

    0.41

    0.41

    0.55

    0.91

    0.63

    0.80

    1.01

    0.35

    0.40

    0.58

    0.75

    0.79

    0.19

    1.29

    1.73

    1.83

    0.21

    0.25

    1.37

    0.06

    2.04

    0.01

    0.03

    162 186 186 188 189

    125

    197 242 210 262

    242

    139 155 188

    145 145 283 145

    41 93 44

    151 218

    32 35 35 45

    115 54

    266 371

    484

    45 60

    296

    27 283

    16 16

    0.26

    0.50

    0.24

    0.80

    1.15

    0.25

    0.28

    0.28

    0.36

    0.64

    0.22

    1.27

    1.42

    2.00

    0.32

    0.38

    1.57

    0.18

    1.96

    0.06

    0.11

    first-stage reactor. Dewaterability after the second stage is slightly better than that of the standard reactor in the same set, although the difference is only statistically significant for the CST. The results of the two-stage operation suggest, as expected from the

    ory, that the first stage broke down large particles into smaller

    fragments, and that these fine particles were subsequently con

    verted into soluble products and methane in the second stage.

    Particle size distribution results, presented subsequently, support

    this conclusion.

    The ratios of both specific resistance and CST lead to the same conclusions and show the same trends with each variation

    in operating conditions; nevertheless, there is not good numerical

    agreement between the two measurements. This disagreement stems from fundamental differences in the two measurements.

    Kavanagh19 mathematically demonstrated that, for samples

    measured at their original concentrations, the CST (corrected

    by subtracting the CST of pure water) should be proportional to the product of the specific resistance and w.19 These results

    confirm that relationship; when plotted as shown in Figure 4, the slope was 1.09 which compares favorably with the predicted value of 1.0. The relationship also makes clear that CST is a

    measure of dewaterability that focuses on the volume of water

    removed per unit time, whereas specific resistance focuses on

    the mass of solids that can be dewatered per unit time. Because

    dewatering devices must be designed both with sufficient hy draulic loading capacity and sufficient solids handling capacity, both measures (and their trends in this work) are important.

    o

    160

    15.5

    ISO

    145

    140

    135

    130

    n-r

    Regression Analysis

    Slope? 1.09 R2 - 0.88

    n-r

    05 1.0 1.5 2.0 25 3i0

    LOG CST1

    Figure 4?The relationship between sp?cifie resistance and capillary suction time. (CST in seconds, modified by subtracting the CST of water;

    specific resistance in m/kg, influent and effluent sludges from Set 4

    shown.)

    1112 Journal WPCF, Volume 58, Number 12

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  • _Focus on Dewatering

    Effects of digestion on particle size distributions. The detailed

    particle size distributions measured in this research yield several

    insights into the destruction of solids during digestion and the effects of digestion on dewaterability. The destruction of solids of different sizes is most clearly illustrated by the volume dis

    tribution; a few examples are shown in Figure 5. The area under the curve between any two sizes (log diameter values, log dp) represents the volume concentration (/?m3/cm3) of particles in

    that size range. Influent sludge for the standard reactor (20% by weight activated sludge) had a bi-modal distribution. The ma

    jority of the particle volume (mass) was located in the range 10

    /xm < dp < 100 ?mi ( 1.0 < log dp < 2.0), but a significant fraction in the measured size range less than 2.5 ?urn (log dp < 0.4). This

    distribution was typical of all sludges studied with the standard mixture of primary and activated sludge. The 100% activated

    sludge influent, also shown in Figure 5, was significantly different, with a much greater fraction of its particle volume in the large size range, as might be expected.

    The destruction of particles in digestion is obvious in the re sults from the standard reactor shown in Figure 5; a significant reduction of the volume concentration occurred in both parts

    of the bi-modal distribution. Overall, most of the destruction occurred in the large sizes because that is where most of the

    particle volume was located. However, the fractional reduction

    in the small particle size range was even greater. This overall

    reduction throughout the entire particle size range was typical

    of most reactors in this study.

    As might be expected from the digestion and dewatering re

    sults, the destruction of particles in the reactor fed 100% activated

    sludge did not conform to the standard pattern, as indicated in

    40

    30h

    E

    o

    I

    ?? tr

    20 h

    60

    50

    40

    30

    20

    10

    20% Activated Sludge Reactor

    o Influent o Effluent

    cb

    *?

  • Lawler et al.

    verts small particles into gaseous forms. The same presumably

    occurs in single-stage digesters, but the separation of the two

    steps in the two-stage process makes it clear.

    Average values of the total surface area concentration for each

    digester are shown in Table 4; ratios of the effluent to influent

    values are included for comparison. Also shown are the specific surface area concentrations and ratios for this parameter. Results

    in the latter columns of Table 4 are discussed subsequently. Several trends are obvious in the results for the surface area

    concentrations. First, the ratios of the effluent to influent values

    are almost all less than 1, which indicates that the surface area was decreased through digestion. Again the exceptions are the reactor fed 100% activated sludge (Set 3), the reactor operated at 4?C (Set 3), and the first-stage reactor (Set 5).

    Second, the results from Sets 1 and 4 indicate that the reduc tion in surface area lessened as the influent solids concentration

    increased. The results for the reactor fed 5% sludge in Set 4

    suggest that this reactor was under stress; apart from the three reactors with ratios greater than 1, this reactor had the highest ratio of effluent to influent surface areas. The stressed condition of the reactor is clear from the particle size distribution mea

    surements (and dewaterability data) even though it was not per

    ceptible in the overall performance indicated by the measures

    reported in Table 2.

    Third, the data from Set 3 with varying sludge composition showed trends similar to the dewatering results: the standard reactor had the greatest destruction of surface area and the ac

    tivated sludge reactor had no overall reduction. In the latter

    case, some surface area reduction occurred in the particles larger

    than 2 /?m, but the surface area increased in the smaller size

    range. The small particles produced from the large particles could not be digested well under the operating conditions; perhaps a

    longer detention time would lead to better destruction.

    Fourth, a significant increase in the surface area occurred in

    the reactor operated at 4?C. Operation of this digester was de

    signed to test the effects of mixing alone on dewaterability and

    particle size distributions, but the monitoring results suggest that this reactor operated as a slow first-stage (acid-forming) reactor;

    therefore this study failed to isolate the effects of mixing. The relationship between particle size distributions and de

    waterability. The similarity in trends of the dewaterability and surface area distribution results suggests that surface area is a

    principal determinant of dewaterability. The Kozeny equation provides a basis for testing this relationship quantitatively. As shown in a previous study,24 the Kozeny equation can be written

    in the form

    KS0\\-e) ...

    rs =-i- (2) ? Pp

    Where

    rs =

    specific resistance, m/kg, =

    fractional voidage of cake,

    K = Kozeny constant,

    Table 4?Particle surface area: effects of digestion.

    Surface area concentration Specific surface area

    Influent,

    107 Mm2/cm3

    Effluent,

    107 Mm2/cm3

    Effluentinfluent

    ratio

    Influent, Effluent,

    1011 ?tm2/g 1011 /tim2/g

    Effluentinfluent

    ratio

    Set 1 : Total solids concentration, %

    2.0

    3.5

    3.5

    5.0

    6.5

    Set 2: Detention time, days 7

    10

    14

    21

    Set 3: % Activated sludge 0

    20

    40

    100 Set 3: Temperature, ?C

    4

    Set 4: Total solids concentration, %

    2.0

    3.5

    5.0

    Set 5: Staged operation Standard

    1 st stage 2nd stage overall 2-stage

    20.04

    35.24

    35.24

    50.48

    63.33

    27.68

    41.24

    40.32

    39.43

    25.10

    40.32

    24.92

    40.26

    59.51

    40.74

    40.74

    42.21

    40.74

    8.12

    20.36

    15.60

    27.17

    41.77

    13.85

    14.84

    12.45

    15.40

    19.27

    10.81

    18.56

    25.20

    48.18

    6.33

    18.16

    44.46

    17.36

    42.21

    21.91

    21.91

    0.41

    0.58

    0.44

    0.54

    0.66

    0.50

    0.54

    0.45

    0.56

    0.47

    0.27

    0.47

    1.00

    1.19

    0.25

    0.45

    0.75

    0.43

    1.04

    0.52

    0.54

    10.64

    10.75

    10.75

    10.77

    10.22

    8.19

    11.76

    12.27

    11.91

    10.09

    12.27

    11.80

    12.28

    12.29

    11.78

    11.78

    14.69

    11.78

    6.25

    8.74

    6.87

    7.99

    9.90

    5.50

    6.05

    5.23

    6.46

    12.60

    5.00

    11.24

    13.74

    15.83

    6.89

    12.19

    20.72

    9.52

    14.69

    11.78

    11.78

    0.59

    0.81

    0.64

    0.74

    0.97

    0.68

    0.74

    0.64

    0.79

    1.07

    0.47

    0.94

    1.36

    1.29

    0.58

    0.99

    1.69

    0.81

    1.25

    0.71

    1.00

    1114 Journal WPCF, Volume 58, Number 12

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  • Focus on Dewatering

    pp =

    particle density, g/cm3, and

    S0 =

    surface area per unit volume of particles, pm2/pm3.

    As shown in Equation 2, the specific resistance is related to the

    particle density and, through S0, particle size and shape. No

    attempt was made to determine the shape of the particles in this

    study; in the analysis of measurements from the electrozone

    particle counter, it is assumed that the particles are spheres. The

    few particle density measurements made in this research were

    considered unreliable. However, it is reasonable to assume that

    neither the particle shape nor particle density varied as much

    as the size distributions among the sludges used in this study. With that assumption, differences in specific resistance between

    samples should be primarily related to differences in S0, ac

    cording to the Kozeny equation.

    S0 could be obtained as the ratio of the total surface area

    concentration to the total volume concentration from the particle

    size measurements. However, because of some unreliability in

    the 560-/im aperture measurements (which affects the total vol

    ume significantly but not the surface area because it measures

    the largest particles), it was more reasonable to normalize the

    surface area distributions by the measured solids concentrations.

    The resulting value is termed the specific surface area (As), and

    is related to S0 by Equation 3

    S0 = Aspp (3)

    This substitution requires the assumption that the particle density was constant across the size range.

    Specific surface area distributions were plotted for all samples (Figure 7). The area under such curves represents the total surface

    40

    30

    5 'o q:

  • Lawler et al.

    equation (including the assumptions about density in the analysis noted previously) confirms that the particle surface area is a

    principal determinant of sludge dewaterability.

    SUMMARY

    Relationships among anaerobic digestion, dewaterability, and

    particle size distributions were investigated and the relationships between pairs of the three factors have been presented. Figure 9 ties all three topics together. The form of the plot is suggested by the Kozeny equation and relates the square of the specific surface area to specific resistance; digestion is accounted for by using the ratios of the effluent to influent for these quantities.

    Three lines are drawn on the figure in addition to the data

    points that represent each reactor. The horizontal and vertical dotted lines represent no change through digestion in specific resistance and specific surface area, respectively. All but a few

    reactors fall within the box formed by these two lines, which demonstrates that both specific resistance and specific surface area were reduced by digestion. If the changes in specific resis tance brought about by digestion were accounted for entirely by the changes in the specific surface area, the points would fall on the line with a slope of 1. Results for virtually all reactors fall

    reasonably close to this line, especially when the level of uncer

    tainty in the four measures that are necessary for each point and

    the squaring of the abscissa values are considered. The only ex

    ceptions are from the three digesters with excellent dewaterability omitted from the analysis of Figure 8 and from the digester fed 5% solids in Set 4; as noted earlier, there was considerable un

    certainty in the specific resistance measurements for that digester.

    The figure makes clear that digestion generally improves de

    waterability and does so by the selective destruction of small

    particles that contribute relatively more surface area than mass

    in sludge samples. When digesters are stressed, they can still accomplish some

    digestion but can create rather than destroy small particles. The four highest points on Figure 9 correspond to the reactors that

    were under the most stress from a biological viewpoint, the first

    Asz , digested/As2 ,

    raw

    Figure 9?Interrelationships among anaerobic digestion, dewaterability, and particle size characteristics.

    stage reactor, the 4?C reactor, the reactor fed 100% activated

    sludge, and the reactor fed 5.0% sludge in Set 4.

    Implications for full-scale digesters. This research was carried out in small laboratory digesters under well controlled conditions, but some implications for design and operation of full-scale di

    gesters are obvious. Most digesters are followed by dewatering devices, and dewatering is usually improved by polymer addition. It is reasonable to think that the optimal polymer doses are pro

    portional to the surface area of particles because it is primarily polymer interactions with the surface of the particles that causes

    effective flocculation. The effects, therefore, of digester operating variables on particle characteristics will affect polymer usage and costs.

    Digesters should be designed and operated not only for sludge stabilization, but to improve subsequent dewaterability; the range of operating conditions that can lead to good digestion is wider than the range that can lead to good dewaterability. Designers should recognize that saving space by a reduction in detention time or by an increase in influent sludge concentration might ultimately be more costly; larger dewatering units and more

    polymer are likely to be necessary to process the sludge after

    digestion. Also, digestion of primary and activated sludges should not be separated.

    Operators might find that dewatering problems are reduced

    by better operation of digesters. The greater sensitivity of de

    waterability than digester performance to operational variations

    suggests that dewaterability measures should be added to the list of monitoring tools, and could yield earlier warning of stress in a digester.

    CONCLUSIONS In carefully-controlled laboratory reactors, anaerobic digestion

    generally improved sludge dewaterability. Operating variables

    of digesters, including detention time, influent solids concentra

    tion, and the mixture ratio of primary and activated sludges, influence digested sludge dewaterability. Dewaterability is more

    sensitive to digester operating variables than is digestion itself.

    Dewaterability is commonly measured by either CST or spe cific resistance, but there are fundamental differences in these

    measures. CST is more aptly suited for plants whose dewatering

    devices are limited by their ability to process the volume of

    sludge; specific resistance is better in cases where the limitation

    is in processing the mass of solids in the sludge. Anaerobic digestion changes the particle size distribution of

    sludges. When digestion works well, particles of all sizes are de

    stroyed, but there is a preferential removal of particles of small

    sizes, a consequent loss of specific surface area, and therefore

    an improvement in dewaterability. When digestion does not

    work well, large particles are destroyed but small particles are

    created with a consequent gain in specific surface area, and

    therefore a worsening in dewaterability. Specific surface area

    (jum2/g) is the principal determinant of sludge dewaterability, as

    expected from the Kozeny equation.

    ACKNOWLEDGMENTS Credits. The research reported in this paper was supported

    by the U. S. Environmental Protection Agency, Office of Re search and Development, under grant R-810147-01-0. The con

    tents of the paper do not necessarily reflect the views and policies of the Environmental Protection Agency. Donald F. Carey was

    project officer.

    1116 Journal WPCF, Volume 58, Number 12

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  • _Focus on Dewatering

    Howard M. Liljestrand and Joseph F. Malina, Jr. suggested details of the measurement methodology and operation of lab

    oratory digesters. James Stewart built the laboratory digesters and improved their design. Frank Husley solved many opera tional problems with the measurement instruments and digesters.

    This paper was presented at the 57th Annual Conference of the Water Pollution Control Federation, New Orleans, Louisiana

    (September 30 to October 4, 1984). Authors. Desmond F. Lawler is an associate professor in the

    Department of Civil Engineering at the University of Texas, Austin. At the time the research was performed, Yoon Jin Chung,

    Shiaw-Jy Hwang, and Barbara A. Hull were graduate students

    in the Department of Civil Engineering at the University of

    Texas, Austin. Dr. Chung is now an assistant professor in the

    Department of Environmental Engineering at Ajou University in Suweon, Korea. Ms. Hwang is an engineer with the Los An

    geles County Sanitation District, Los Angeles, Calif. Ms. Hull is an engineer with Turner, Collie, and Braden, Inc., Austin,

    Tex. Correspondence should be addressed to Dr. Lawler at the

    Department of Civil Engineering, ECJ 8.6; University of Texas at Austin; Austin, TX 78712.

    REFERENCES 1. McCarty, P. L., "History and Overview of Anaerobic Digestion."

    Second International Symposium on Anaerobic Digestion, Treve

    munde, Germany (1981). 2. Speece, R. E., "Anaerobic Biotechnology for Industrial Wastewater

    Treatment." Environ. Sei. TechnoL, 17, 9, 416A (1983). 3. Metcalf and Eddy, Inc., "Wastewater Engineering: Treatment, Dis

    posal, and Reuse." Second Ed., McGraw-Hill, Inc. New York, N. Y. (1979).

    4. Rudolfs, W., and Heukelekian, H., "Relation between Drainability of Sludge and Degree of Digestion." Sew. Works J., 6,6, 1073 (1934).

    5. Brooks, R. B., "Heat Treatment of Sewage Sludge." Water Pollut.

    Control (G. B.), 69, 1,92(1970). 6. Haug, R. T., et ai, "Effect of thermal pretreatment on digestibility

    and dewaterability of organic sludges." J. Water Pollut. Control Fed.,

    50, 1,73(1978). 7. U. S. Environ. Prot. Agency, "Process Design Manual: Sludge

    Treatment and Disposal." EPA-625/1-79-011, U. S. EPA, Municipal Environmental Research Laboratory, Cincinnati, Ohio (1979).

    8. Kini, A. D., and Nayak, S. L., "Optimizing Vacuum Filtration of

    Sewage Sludge." Filtr. Sep., 17, 4, 313 (1980). 9. Pearson, E. L., and Buswell, A. M., "Sludge Ripeness Studies." Ind.

    Eng. Chem., Analytical Edition, 3, 4, 359 (1931). 10. Morris, R. H., "Polymer Conditioned Sludge Filtration." Water

    Works & Wastes Engineering, 2, 3, 68 (1965). 11. Bacon, V. W., and Dalton, E. F., "Chicago Metro Sanitary District

    Makes No Little Plans." Public Works, 97, 11, 66 (1966). 12. Karr, P. R., and Keinath, T. M., "Influence of the particle size on

    sludge dewaterability." /. Water Pollut. Control Fed., 50, 8, 1911

    (1978). 13. Knocke, W. R., et al, "Vacuum Filtration of Metal Hydroxide

    Sludges." J. Environ. Eng. Div., Proc. Am. Soc. Civ. Eng., 106, EE2,

    363(1980). 14. Knocke, W. R., and Wakeland, D. L., "Fundamental Characteristics

    of Water Treatment Plant Sludges." /. Am. Water Works Assoc,

    75, 10,516(1983). 15. Faisst, W. K., "Characterization of Particles in Digested Sewage

    Sludge." In "Particulates in Water." M. C. Kavanaugh, and J. O.

    Leckie, (Eds.), Advances in Chemistry Series 189, American Chem

    ical Society, Washington, D. C, 259 (1980). 16. Eastman, J. A., and Ferguson, J. F., "Solubilization of paniculate

    organic carbon during the acid phase of anaerobic digestion." J.

    Water Pollut. Control Fed., 53, 3, 352 (1981). 17. Chung, Y. C, "Interrelationships Among Anaerobic Digestion,

    Sludge Dewaterability, and Particle Characteristics." Ph.D. Disser

    tation, Dept. Civil Eng., Univ. of Texas at Austin (1985). 18. "Standard Methods For the Examination of Water and Wastewater."

    15th Ed., Am. Public Health Assoc. (1981). 19. Kavanagh, B. A., "The Dewatering of Activated Sludge: Measure

    ment of Specific Resistance to Filtration and Capillary Suction

    Time." Water Pollut. Control (G. B.), 79, 3, 388 (1980). 20. Christensen, G. L., and Dick, R. I., "Specific Resistance Measure

    ments: Methods and Procedures." J. Environ. Eng. Div., Proc. Am.

    Soc. Civ. Eng, 111, 3, 258 (1985). 21. Christensen, G. L., and Dick, R. I., "Specific Resistance Measure

    ments: Nonparabolic Data." J. Environ. Eng. Proc. Am. Soc. Civ.

    Eng., 111,3,243(1985).

    22. Lawler, D. F., et al, "Particle behavior in gravity thickening." J.

    Water Pollut. Control Fed., 54, 10, 1388 (1982). 23. Lawler, D. F., et al., "Changes in Particle Size Distributions in Batch

    Flocculation." J. Am. Water Works Assoc, 75, 12, 604 (1983). 24. Gale, R. S., "Filtration Theory With Special Reference to Sewage

    Sludges." Water Pollut. Control (G. B.), 66, 6, 622 (1967).

    December 1986 1117

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    Article Contentsp. 1107p. 1108p. 1109p. 1110p. 1111p. 1112p. 1113p. 1114p. 1115p. 1116p. 1117

    Issue Table of ContentsJournal (Water Pollution Control Federation), Vol. 58, No. 12 (Dec., 1986), pp. 675a-700a, 1081-1152, 701a-730aVolume InformationFront MatterDepartmentsAbstracts [pp. 688a, 690a]Market Place [pp. 694a-695a]Recent Books [pp. 696a-697a]Washington Notebook [pp. 698a-699a]

    EditorialThe Search for Signs of Progress [p. 1081-1081]

    Letters [pp. 1082-1083]MonitorLos Angeles: 'A New Era Begins' [pp. 1084-1091]

    FeaturePhosphate Detergents: A Closer Look [pp. 1092-1100]

    WPCF Issue Paper [pp. 1101-1104]WPCF Committee ViewpointPretreatment: Are We Headed in the Right Direction? [pp. 1105-1106]

    Focus on: Sludge DewateringAnaerobic Digestion: Effects on Particle Size and Dewaterability [pp. 1107-1117]Effects of Mean Cell Residence Time and Particle Size Distribution on Activated Sludge Vacuum Dewatering Characteristics [pp. 1118-1123]

    Process DesignRe-Evaluation of Launders in Rectangular Sedimentation Basins [pp. 1124-1128]Hydraulic Studies and Cleaning Evaluations of Ultraviolet Disinfection Units [pp. 1129-1137]

    Water QualityThe Effects of Advanced Wastewater Treatment on River Water Quality [pp. 1138-1144]

    DepartmentsProduct Guide [pp. 702a, 704a, 706a, 708a, 710a, 712a, 714a]New Equipment, New Literature [pp. 703a, 705a, 707a, 709a, 711a, 713a]People [p. 730a-730a]

    Back Matter

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