Characteristics of Residues from Wet Air Oxidation of Anaerobic Sludges

Characteristics of Residues from Wet Air Oxidation of Anaerobic SludgesAuthor(s): A. A. Friedman, J. E. Smith, J. DeSantis, T. Ptak and R. C. GanleySource: Journal (Water Pollution Control Federation), Vol. 60, No. 11 (Nov., 1988), pp. 1971-1978Published by: Water Environment FederationStable URL: http://www.jstor.org/stable/25046847 .

Accessed: 14/07/2014 19:56

Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at .http://www.jstor.org/page/info/about/policies/terms.jsp

.JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range ofcontent in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new formsof scholarship. For more information about JSTOR, please contact [email protected].

.

Water Environment Federation is collaborating with JSTOR to digitize, preserve and extend access to Journal(Water Pollution Control Federation).

http://www.jstor.org

This content downloaded from 109.176.194.207 on Mon, 14 Jul 2014 19:56:13 PMAll use subject to JSTOR Terms and Conditions

http://www.jstor.org/action/showPublisher?publisherCode=wef

http://www.jstor.org/stable/25046847?origin=JSTOR-pdf

http://www.jstor.org/page/info/about/policies/terms.jsp


Characteristics of residues from wet air

oxidation of anaerobic sludges A. A. Friedman, J. E. Smith, J. DeSantis, T. Ptak, R. C. Ganley

ABSTRACT: This study evaluated the wet air oxidation (WAO)

of anaerobic municipal sludges and the anaerobic treatability of

WAO product liquors. The study showed that WAO, followed

by dewatering of residual solids, reduced the volume of anaerobic

sludges by greater than 96%. Reactor temperatures above 250?C

generated microbial conditions that inhibited anaerobic treatment

of soluble products. The optimum WAO operating temperature was 230?C where 65.5% of the volatile suspended solids was de

stroyed and about 60% of the soluble chemical oxygen demand

(SCOD) generated was biodegradable. Anaerobic rotating bio

logical contracts proved effective for removing biodegradable SCOD from WAO liquors. Most of the heavy metals from the

raw sludge were concentrated in dewatered solids. Settled and

dewatered WAO volatile solids had much higher COD contents

than raw anaerobic sludges, regardless of the operating temper ature. / Water Pollut. Control Fed., 60, 1971 (1988).

KEYWORDS: anaerobic processes, rotating biological contac

tor, sludge, residual, destruction, sludge stabilization, wet air ox

idation.

Unlike other wastewater treatment plant sludges, an

aerobically digested sludges have already undergone ex

tensive treatment, leaving organic residues recalcitrant to

further biological degradation. One potential sludge treat ment alternative that reduces volume and mass of residual

anaerobic sludges applies high temperature, partially ox

idizes previously digested sludges (PDS), and biologically converts the resulting soluble organics to methane. Ther

mal conditioning techniques involve heating wastewater

sludges to 170?C to 240?C at pressures of 1725 to 2760 kPa (250 to 400 psig). Greater destruction of organic ma

terial is achieved when oxygen or air is injected into the

reactor, a process termed wet air oxidation (WAO). The

first detailed information concerning WAO treatment of

anaerobic sludges indicates that WAO can destroy 80% of the volatile suspended solids (VSS) and 73% of the chemical oxygen demand (COD).12 To date, only limited

information3 4 is available concerning the suitability of

WAO-treated PDS as a substrate for the generation of

methane. Because WAO treatment produces a high

strength side-stream, its impact must be considered when

recycled. This paper describes solids destruction, residue

characteristics, and critical biodegradability factors that

should be considered in the design of WAO processes for the management of anaerobic sludges. Ideally, WAO

treatment should reduce sludge volume by at least 80% and produce usable substrates for methanogenesis. It was

anticipated that moderate operating temperatures (below

200 ?C) would result in nearly complete destruction ofVSS.

Because the VSS contains almost all the organic matter

present in anaerobic sludges, it is necessary to dissolve or

destroy VSS as the first step to methane generation. WAO

techniques can almost completely destroy primary treat

ment, trickling filter, and waste activated sludges.5"10 However, recycling WAO decant liquors, which contain 6000 to 10 000 mg COD/L,4

'1_14 can increase the oxygen demand on aerobic treatment systems by up to 30%.'2

Consequently, it was hypothesized that WAO would prove

equally efficient for anaerobic sludge destruction. With

high degrees of oxidation, soluble organic products such as acetic acid should remain214 and be amenable to an

aerobic treatment.

The use of anaerobic sludge WAO decant liquors as a

substrate for methane generation is not described in the

literature, although the anaerobic biodegradability of other

municipal and industrial WAO liquors has been stud io 8,i2,i5-i9

Organic removals typically ranged from 60 to 80% at loadings of 5 to 25 kg COD/m3 d. Gas yields were

generally about 0.42 m3/kg COD removed and contained about 70% methane. Others4 have reported on the anaer

obic biodegradability of heat-treated (100? to 225 ?C) waste activated sludge decant liquors. Although liquors treated at 175?C seemed to be a suitable substrate for methane

generation, the use of 200?C and 225 ?C decant liquors severely inhibited gas production and increased reactor volatile acid concentrations to 4500 mg/L (normally con

sidered to be an objectionable level for methanogenesis).

Materials and Methods

Partially digested sludge source and WAO treatment. All PDS was obtained from secondary anaerobic digesters of the Metropolitan Wastewater Treatment Plant, Syra cuse, N. Y. This 80-mgd facility serves an urban area con

taining residential, commercial, and light industrial areas.

A batch wet air oxidation autoclave was used for these

experiments (Figure 1). Operating conditions were chosen to conserve soluble organics for use as a feedstock for

subsequent anaerobic treatment. The 3.78-L batch reactor

featured controllable electric heating, water cooling, and

internal magnetic stirring.1 The reactor vessel was charged

with 2.5 L of PDS. The headspace was brought to equi librium with compressed air at 2070 kPa (300 psig) at room temperature before heating. Experimental variables

November 1988 1971



Friedman et al.

Influant s ami

Siphon Pump

Temperatur and time tl

controller

Headspace --^^_~

Autoc love

Figure 1?Wet air oxidation system.

Compressed air

cylinder

were temperature, time at temperature, and the initial

VSS concentration. Unless otherwise specified, the WAO

experiments described were conducted with an initial 2070-kPa air charge for 1 hour at temperature. The prod ucts of these experiments are annotated with operating temperatures as subscripts (for example, WAO250,

andPDS23o).

Anaerobic rotating biological contactor. Anaerobic ro

tating biological contactors (AnRBC) are effective fixed film pilot plants20,21 and have been used to treat WAO

products. The AnRBC units (Figure 2) consisted of 12

rotating discs (15.2 cm in diameter) coated with biomass and partially submerged in the wastewater stream. Mixing was provided by disc rotation at 20 rpm to encourage gas dissolution from the liquid. Groups of discs were separated into four stages by baffles to prevent short circuiting. Con struction details are reported elsewhere.1 The AnRBCs

were housed in controlled temperature boxes at 35 ?C. Feed was continuously supplied to the first stage of the AnRBC by a diaphragm metering pump, and the liquid volume was maintained at 6.0 L. Influent and effluent

were analyzed daily for COD and other parameters. Flow rates and gas production were measured volumetrically.

Feed consisted of settled, WAO supernatants diluted with

tap water to provide a desired soluble COD (SCOD). Anaerobic batch bioassays. Anaerobic batch

bioassays22"25 were used to determine the biodegradability and toxicity of WAO-treated PDS. Borosilicate serum bottles (160 mL) fitted with butyl rubber stoppers and

crimped aluminum caps served as bioassay reactors. Nor

mally, the bottles were filled with 100 mL of distilled wa

ter/nutrient broth, fresh anaerobic inoculum, and treated PDS. Control blanks consisted of either anaerobic inoc ulum and nutrient broth for determining activity of the

inoculum; or inoculum, broth, and acetic acid COD ( 1

g/g VSS) to prove the viability of the methanogens in the inoculum. The bottles were purged with nitrogen, sealed,

and incubated at 37?C. Gas production and headspace gas composition were monitored periodically using a 10

mL syringe with a 20-gauge needle to remove excess gas that had expelled the syringe plunger. Gas samples were

analyzed for methane, carbon dioxide, and hydrogen. De

pending on the purpose of the bioassay, bottle contents were analyzed for pH, volatile acids (VA), total COD

(TCOD), SCOD, total suspended solids (TSS), and VSS.

Toxicity studies used the batch bioassay technique with 1000 mg glucose/L as the primary substrate.25 Batch

bioassays were used to investigate the effects of formal

dehyde and heat-treated (230? and 270?C) PDS on glucose metabolism. Bottles were monitored for extended periods, frequently in excess of 100 days.

Settling and dewaterability. Both WAO products and untreated PDS were allowed to settle in 2-L graduated cylinders for 16 to 24 hours to simulate retention times in typical decant tanks. The volume occupied by the settled solids was recorded and COD, SCOD, TSS, and VSS were

determined for both supernatant and settled fractions. The dewaterability and residual volumes of settled

WAO residue solids were measured using the B?chner funnel test procedure26 with a vacuum of 20 cm of Hg and 47-mm filter paper. It was necessary to reduce the

size of the test system because only about 100 mL of settled

slurry was available from each batch of WAO-treated PDS.

Analytical techniques. Analyses for TSS, VSS, and COD were performed according to "Standard Methods."27

Samples to be analyzed for VA were acidified and extracted with ethyl ether;2715-pL aliquots and purchased standards were analyzed using a gas Chromatograph with a thermal

conductivity detector. Concentrations of acetic, propionic,

butyric, and other VA were detemined in triplicate. TCOD and SCOD were measured using the chromic acid pro cedure ("Standard Methods" 508C), modified for smaller volumes.27 Formaldehyde was determined by a colori

metric method28 that relies on the reaction of formalde

hyde with chromotropic acid; the colored complex was measured by light absorbance at 570 nm. This method eliminates interfering substances in the complex WAO

by-products. Biogas was analyzed for methane and carbon

dioxide using a thermal conductivity gas Chromatograph with a Poropak Q column. Molecular hydrogen (H2) in

headspace gases was measured. Details of all analytical

procedures are reported elsewhere.1

WAO Solubilization of Anaerobic Sludges Anaerobic digester residual solids commonly consist of

40 to 60% VSS. It is unclear whether the refractory VSS consists of undegraded colloidal organics from the primary

FEED-*^

PRODUCT GAS --

_Q= EFFLUENT?

?v m ^

3

DISKS -BAFFLES

Figure 2?Anaerobic rotating biological contactors.

1972 Journal WPCF, Volume 60, Number 11



_Friedman et al.

and secondary sludges, material produced de novo by an

aerobic digester organisms, or?more likely?particles originating from both sources. Regardless of the source, residual anaerobic sludges exert a high BOD5 (biochemical oxygen demand), typically 3000 to 10 000 mg/L.

PDS destruction by WAO treatment. Initially, the study goal was to determine the WAO operating conditions

necessary to achieve 80% destruction of VSS. Operating temperature proved to be the controlling factor. On the basis of COD measurements, available headspace oxygen was in excess. Figure 3 indicates that VSS and TCOD destruction for anaerobic sludges increased significantly at temperatures between 250? and 270?C. VSS destruction increased from about 60% to more than 80%, and TCOD destruction shifted from less than 30% to more than 45%.

Concomitantly, the SCOD increased by about 25% for a

temperature increase from 250?C to 270?C. Figure 3 also shows that the low molecular weight VA content (acetic acid) of the product SCOD increased dramatically over the same temperature range. This observation is important

because VAs can readily serve as substrates for methan

ogenesis. At temperatures above 270?C, VSS destruction levels off at about 80%. The mean value for 14 VSS de struction experiments at 270?C was 79.3% (a

= 3.3%).

SS destruction for these same 270?C experiments averaged 63% {a

= 5%). Other experiments suggested that 80% VSS

reduction could be achieved at lower temperatures (260?C with a reaction time of 2.5 hours) or more quickly (0.25 to 0.5 hour at 290?C). Based on economic considerations,

| 1500 O

g , g lOOOr Q. Q

uj 500h

O >

0 100

Q UJ > 80

UJ Q 60

Q 40r UJ >

O er >

-1-r?

e formic O ACETIC O PROPIONC

I e S> ?&$rv A VSS A TCO0 O SCOD A^ -180

f**^_

Q (A

>

20h

100

'00 200 300 400 TEMPERATUREfO

60 ~

Q

20 o o u en

.O

Figure 3?Effects of WAO on PDS (bottom) and VA

production (top) as a function of temperature.

SRT (days) SRT(days) 14 34 54 14 34 54

TIME(days) T/MEidoys)

Figure 4?Effect of sludge age on the character istics of heat-treated (HTP) PDS and WAO270 prod

ucts. Total sludge age is shown as SRT at the top of the figure. The experimental period is at the bot tom.

reactor operating conditions of 1.0 hour at 270?C were

selected as the best compromise for 80% VSS destruction. Effects of sludge age on WAO products. High rate an

aerobic digesters are operated with solids residence times

(SRT) of 10 to 30 days.2629 Increased VSS destruction

(digestion) is associated with longer SRTs. Antecedent conditions of anaerobic digestion do not seem to affect

WAO270 product parameters significantly (Figure 4). Fresh PDS with an SRT of 14.5 days was incubated for an ad ditional 48 days at 35?C. Aliquots were withdrawn at dif ferent SRTs, characterized, and subjected to WAO at 270?C. All parameters decreased as a function of increased anaerobic digestion time with the exception of the anaer

obic sludge SCOD, which remained almost constant. The WAO destruction of PDS-VSS (Figure 4) was relatively uniform regardless of SRT and initial VSS concentration.

WAO destruction of VSS averaged 80 ? 2.5%. Even when the digestion SRT was tripled, residual WAO solids were decreased by only 19%. This suggests that the mass of solids remaining for disposal after WAO treatment at 270?C is relatively independent of sludge history. The differences between PDS-TCOD and 270?C-TCOD con centration (Figure 4) represent the oxygen required to de

stroy sludge organic matter. More oxygen will be required

for sludges generated by short anaerobic digester SRTs. PDS solubilization at 230?C. Because 270?C WAO

treatment routinely resulted in 80% destruction of the raw

sludge VSS, this temperature was initially used to produce feedstock (SCOD) for methane fermentation. However, later experiments revealed that the 270?C products con tained a factor that was inhibitory or toxic to biological

methane generation. Consequently, the operating tem

perature was lowered to 230?C to produce a less toxic anaerobic feedstock. This operating temperature still pro vided reasonable levels of VSS destruction and SCOD

generation.

Table 1 presents results of 230?C WAO experiments over 6 months. The PDS used for these studies had an

approximate SRT of 14.5 days. Although TSS and VSS destruction averaged about 52 and 66% respectively, TCOD destruction at 230?C was only about 15%. WAO treatment generated an average of 0.62 g SCOD/g VSS

November 1988 1973



Friedman et al._

Table 1? Effects of 230?C treatment on 1 L of PDS."

Characteriatic PDS PDS.30

Change,

percent

TSS, mg/Lb

VSS, mg/Lb

TCOD, mg/Lb

SCOD, mg/Lb Net SCOD generated,

mg/Lb Paniculate COD, mg/

Lb

Particulate COD/VSS,

mg/mgb SCOD generated/

PDS-VSS, mg/mgb SCOD generated/

PDS-VSS

destroyed, mg/mgb Settled solids volume,

mLc

TSS in settled

fraction, mg/Lc

Supernatant TSS,

mg/Lc Supernatant TCOD,

mg/Lc

22 030 12 540 23 940

390

23 550

1.9

10 720 4 330

20 340 8 120

7 730

12 220

2.8

0.616

0.942

100

125 000

200

9 304

-52

-66

-15

+2 080

+1980

-48

+47

-90

-99

8 Average values for a 6-month period.

b Stirred samples before settling.

c Approximate values for two experiments, following 18 hours

settling.

present in the raw sludge, and about 0.94 g SCOD/g PDS VSS destroyed. These SCOD yields are important because

they limit the amount of potential methane obtainable by WAO treatment at 230?C. It is of interest to note that

230?C treatment produced residual solids having 2.8 g

COD/g VSS while the original PDS had only 1.9 g COD/ g VSS. This indicates that WAO treatment at 230?C pref

erentially affected or altered components of the raw PDS. Further research is necessary to define the chemical and

structural characteristics of the recalcitrant VSS.

Settling Characteristics and Dewaterability of

WAO230 Treated Anaerobic Sludges Settling characteristics. Heat treatment greatly im

proves the dewatering characteristics of waste activated and primary sludges.8,9 WAO treatment of anaerobic

sludges yielded similar responses (Table 2). PDS was au toclaved and allowed to settle in 1-L graduated cylinders.

Most of the TCOD and VSS concentrated in a settled

slurry within 4 hours; settling was complete in 18 hours. A supernatant/solids interface was visible in treated sam

ples while raw PDS would not settle at all. For tempera tures above 250?C, the settled fraction was progressively smaller and the actual VSS concentration remained fairly constant at 3%.

Following autoclaving and settling, residual solids oc

cupied about 10% of the original PDS volume depending on the temperature (Table 2). Settled TSS averaged about 12% by weight for the 230?C treatment (Table 1). (These

values are approximate because of the difficulty in accu

rately measuring the volume of settled solids.) Treatment at 230?C produces a dense settled product that may be suitable for either landfilling or incineration after further

dewatering. Tables 1 and 2 show that 230?C supernatants contained about 90% of the original PDS volume; less than 1% of the raw VSS and TSS concentrations; about a third of the original TCOD; and high levels of SCOD

which could serve as a substrate for methane generation.

In general, the VSS and TSS values of WAO230 supernatant were relatively consistent despite the variability of the raw

anaerobic sludges. However, when the concentration of

organics in the raw PDS increased, TCOD and SCOD tended to increase slightly.

Dewaterability. WAO greatly reduces the volume of final residue from primary and waste activated sludges.4'26

A limited series of preliminary vacuum filtration experi ments was performed with settled WAO solids to deter mine the effects of WAO temperature on overall solids reduction and filter cake characteristics. As shown in Table

3, WAO treatment always reduced sludge volumes sig

nificantly and increased the solids concentration of final filter cakes. Untreated PDS filter cakes averaged about 4%

Table 2?Effect of WAO temperature on distribution of residue solids following settling."

WAO temperature

WAO product PDS 230?C 250?C 270?C 290?C

Characteristics:

Turbidity

TCOD destruction, percent

Settled solids fraction, mLb

Supernatant volume, mLb

Solids volume reduction, percent

Mass distribution of VSS:

Settled solids, mgb

Supernatant solids, mgb Percent VSS, by weight, in settled fraction

Opaque

0

1000 0

0 12 800

1.2

Very

cloudy 15.0

100 900

90

4 060 100

4.1

Turbid

26.7

120 880

88

3 795 44

3.2

Turbid

38.9

100 900 90

2 667 63

2.7

Slightly turbid

45.9

69

931 93

1 934 70

2.9

a 18-hour settling at room temperature.

b Approximate values.




Friedman et al.

Table 3?Effect of WAO temperature on the dewaterability of settled residues.

Treatment,

?C

Sample

volume,

mL

Filtrate

volume,

mL

Minimum

volume

reduction,*

percent

TSS,b

g/L

Approximate solids

concentration in

filter cake, g/L

None

190 210 230 250 270

50 52 50 50 50 50

18 44 42 42 40 36

36 85 84 84 80 72

27 700 76 000 64 700 54 000 47 100 62 000

42 395 404 338 181 206

a Equals (filtrate volume/sample volume) X 100.

b Settled PDS or settled WAO residue solids.

c Equals (sample volume X TSS/1000) + (sample volume

- filtrate volume).

solids, while WAO treated filter cakes were between 18 and 40% solids. These results are promising for applica tions where the final disposal of PDS presents acute prob lems. Minimizing the final sludge cake volume is desirable

when solids are to be incinerated, transported long dis tances to disposal sites, or buried in secure landfills.

Combined settling and dewatering effects. WAO (190? to 270?C) reduced the volume of settled raw PDS by about 90% (Table 2). Vacuum filtration of WAO solids further reduced the volume of the settled solids (190? to 250?C) by an additional 80 to 85% (Table 3). Thus, the total vol ume reduction achieved by thermal treatment and vac

uum dewatering of PDS was more than 98%.

Redistribution of Metals at 230?C

Sludge oxidation by WAO has a strong effect on the release of metals into solution.2'30 The distribution of

metals in either the supernatant or settled fractions of 230?C WAO products is shown in Table 4. Treatment

yielded settled solids occupying 8 to 12% of the original PDS volume. The concentration of metals increased dra

matically in the small settled solids fraction. With the ex

ception of cadmium, settled solids had a 5 to 10-fold

greater concentration of metals than the raw PDS. The mass distribution of metals in the two fractions of WAO230

product indicated that most of the metals, except mercury, were concentrated in the settled solids. Copper and zinc were settled with high efficiencies. Therefore, concentrated metals in residual solids following WAO treatment may

present a problem for land application. Also, while settled solids represented the main sink for heavy metals in these

experiments, metals in the treated supernatant fraction

could present long-term problems if they become con

centrated by subsequent biological treatment processes.

Toxic Properties of WAO Products Anaerobic treatment systems are cost effective for high

strength wastewaters (3000 to 10 000 mg SCOD/L), and

it was anticipated that WAO products would serve as a

good substrate for methane generation. Initially, 270?C was chosen as the best autoclave operating temperature

because 80% VSS destruction with high SCOD yields could

be routinely obtained. However, repeated failures of the batch and continuous flow anaerobic reactors occurred

when settled WA02?o supernatants were used as feed stocks. While the inhibitory factors) present in WAO270 are unknown, published reports on WAO-treated primary and waste activated sludges indicate that toxicity can occur

when product liquors are used as a feedstock for anaerobic

systems.3,4,31 Operating temperatures above 170?C were

most critical. Suggested inhibitors are ammonia,4 nitrog enous residue from natural N-containing components,32

formaldehyde, amino acids, DNA, RNA, and proteins along with H2 produced in excess from conditioned sludge by the early fermenters.31 Inhibitory action may be reduced or reversed by feedstock dilution.4,31

Bioassay experiments were conducted to find WAO conditions that would yield both reasonable VSS destruc tion and a product that would support anaerobic treat

ment. Figure 5 compares the effects of PDS230 and PDS270 on the total gas yield from glucose in batch bioassays at 37?C. Figure 5a shows a classic stimulation-inhibition

pattern with low levels of PDS230 stimulating gas produc tion and higher levels slowing production. Figure 5b shows

much greater repression of gas production with PDS270 and severe toxicity at SCOD concentrations normally as

sociated with anaerobic treatment. PDS230 seemed at least

Table 4?Distribution of metals in raw PDS and

WAO230 products."

Metal content, mg/Lbc

WAO products Percent total

metal in

Raw Settled settled

Metal PDS Supernatant fraction fraction

Hg 0.162 0.124 1.006 36 Ni 4.7 1.5 30.6 64

Cd 2.1 0.6 7.2 52

Cu 24.5 1.2 194.9 94

Pb 8.8 2.2 45.2 66

Cr 22.3 4.8 147.8 74

Zn 32.2 1.0 247.0 96

a pH between 7 and 8; 230?C; 1 hour; 2070 kPa air charge.

b Following 24 hours of settling.

c Average of four samples collected at 1 -week intervals; product

pH between 7 and 8.

November 1988 1975



Friedman et al.

DAYS

Figure 5?Effect of WAO-treated PDS on cumulative

gas production from glucose, a: PDS230 with 1030

mg/L glucose, b: PDS270 with 1030 mg/L glucose.

partially metabolizable at all concentrations while PDS27o

depressed gas production at most concentrations.

Additional bioassays (Figure 6) indicated that at low concentrations of PDS27o (590 mg SCOD/L) a very long adaptation period (greater than 50 days) was required be fore significant methane was produced. In contrast, the

control without PDS27o began methane production im

mediately, apparently from residual nutrients carried over

in the inoculum.

Others31 have shown that methanogenic activity can be

inhibited by formaldehyde. Formaldehyde was detected in WAO270 at levels as high as 117 mg/L by both gas

chromatography and colorimetric methods. A series of

batch bioassays indicated that formaldehyde concentra

tions as low as 40 mg/L inhibited the fermentation of

glucose to methane. Only subinhibitory traces of form

aldehyde were observed in WAO products generated at

PDS270 PDS270

Si

NO ACIDS OBSERVED

- TOTAL GAS METHANE - CARBON DIOXIDE

-O-ACE TIC ?O-PROPWNIC

-BUTYRIC -VALERIC

IOO DAYS

-x-TOTAL GAS /" - - METHANE ?* -Or- CARBON *

DIOXIDE x j+

IOO 150 DAYS

Figure 6?Gas and volatile acid production in me

sophilic anaerobic sludge bioassays with 590 mg

SCOD/L PDS27o> left side = control.

temperatures below 250?C, which is consistent with the observation that PDS230 would support methane genera tion while PDS270 would not. Thus, from a practical standpoint, it was concluded that the target goal of 80%

VSS destruction and maximum methane generation (SCOD removal) are not simultaneously obtainable.

There was some biological adaptation to PDS27o (Figure 6) that required about 45 days. Also, 45 days corresponds to the delayed increases in metabolic activity after an initial

repression of glucose fermentation by PDS270 (Figure 5). These figures illustrate an important factor that should be considered when interpreting batch anaerobic data. The

adaptation to apparent toxicants can require a longer time

than normally used for anaerobic bioassays.25 Again, de

spite the apparent adaptation observed in the bottle bioas

says, repeated attempts to establish continuous flow an

aerobic treatment with PDS27o failed.

Anaerobic Treatment of WAO230 Supernatants

Two AnRBC reactors were used simultaneously at 37 ?C to evaluate the removal of SCOD with PDS230 as the sub strate. AnRBC-1 was operated in a continuous flow mode

with a hydraulic residence time of 24 hours; AnRBC-2 was operated in a batch mode. Feed was prepared by di

luting treated PDS with tap water as necessary. Reactor

pH remained between 6.4 and 7.6 throughout the study.

Continuous flow operation of AnRBC reactors. Feeding schedule, influent SCOD, effluent SCOD, and mass load

ing rates are listed in Table 5. Removal of SCOD during 156 days of operation is shown in Figure 7. Several features

are of special interest. Reasonably constant effluent SCOD was observed for Periods 2 and 4 when the reactor had time to acclimate to the loading. During Period 4, loading

was sharply reduced because of dilution error, but SCOD removal in this period averaged only 60% despite the rel

atively light organic loading of the acclimated reactor. The

maximum removal, 63% in Period 5, was essentially the same as Periods 4 (a low loading level) and 3 (a well

acclimated biofilm). During the latter portion of Period 7 the reactor seemed to be recovering from a rapid increase

in loading rate (similar to recoveries in Periods 3, 4, and

5) when the autoclave failed and PDS230 was no longer available. Consequently, Period 8 became a starvation ep

Table 5?AnRBC-1 continuous flow performance

characteristics at 37?C.

SCOD Influent Effluent

Operating loading, SCOD, SCOD, Removal,

period Days g/d mg/L mg/L percent

1 1-37 ? ? ? ?

2 37-53 3.42 570 328 42 3 54-111 6.03 1005 496 51 4 112-115 2.50 417 165 60 5 116-126 5.38 897 328 63 6 127-137 12.83 2139 994 54 7 138-144 18.73 3122 2169 31 8a 145-152 ? ? ? ?

9 153-157 7.50 1250 1250 0

a Recycled feed, thus no SCOD values.




_Friedman et al.

95 100 105 MO 115 120 125 DAYS OF OPERATION

130 135 140 145 150 155 DAYS OF OPERATION

Figure 7?Continuous flow PDS230 SCOD removal

by AnRBC-1.

isode during which effluent was continuously recycled to maintain the reactor. During Period 9, no further SCOD

removal was observed. The recalcitrant SCOD observed

during Period 9 shows that only 60% of the SCOD in fresh

PDS230 was biodegradable. When operating data from<Periods 3, 5, and 6 are ex

amined, a linear relationship (Figure 8) between the mass of SCOD applied and removed is seen. (Data from Periods

4, 7, and 8 are not included because the reactor was not

acclimated to the feed). The linear response indicates that the reactor had not been loaded to biological failure. The

slope of the line, 0.61, provides additional evidence that

u? 8 O

? 4

MR =-0.41+0.61 MA R*= 0.894

6 8 10 12 MASS SCOD APPLIED, g/d

Figure 8?Mass removal of PDS230 SCOD by contin uous flow treatment in AnRBC-l. MA = mass of SCOD

applied (g/d); MR = mass of SCOD removed (g/d). Periods correspond to times shown on Figure 7.

only about 60% of the PDS230-SCOD is biodegradable un der anaerobic conditions. Finally, these data demonstrate that an anaerobic fixed-film treatment system can provide consistent removal despite daily fluctuations in flow and feed strength.

AnRBC batch operation. AnRBC-2 was also seeded with fresh sludge, but subsequently operated in a batch mode.

During batch operation (Figure 9) 1-L aliquots were re moved daily and the volume replaced with a solution of diluted PDS230 containing 3.24 g of SCOD. This solution was designed to raise the reactor SCOD concentration by about 540 mg/L per feeding. Following day 45, a daily increase of SCOD in the reactor was observed, as shown

by the solid lines on Figure 9. This suggested a long-term accumulation of non-biodegradable material(s) in the unit. It was observed that on occasions when the reactor was

not fed, little or no additional SCOD removal was observed

during the following day. Figure 9 provides additional evidence that PDS230 contained a sizable quantity of re

calcitrant SCOD. The solid lines represent four periods of nearly uniform feeding conditions for which residual SCOD increased at a nearly constant rate. The average

SCOD increase amount to 223 ? 16 mg/L d. Over time, the batch reactor (AnRBC-2) consistently removed only 59% of the PDS23o-SCOD. This was essentially the same

maximum removal rate observed for the continuous flow

reactor (60 to 63%). The sudden SCOD increase on day 73 resulted from a reactor leak.

Conclusions

This study has shown that WAO, followed by the de

watering of residual solids, can be used to significantly reduce the volume of anaerobic sludges (by greater than

96%). If decant liquors are treated by anaerobic methods, the choice of WAO operating condition is critical. The use of high WAO temperatures (above 250?C) may gen erate toxic or inhibitory factors such as formaldehyde; lower temperatures result in reduced VSS destruction and

more residual solids for final disposal. In this study, the

optimum WAO operating temperature to produce feed stocks for anaerobic treatment seemed to be 230?C. At this temperature, 65.5% of the VSS was destroyed. About 0.62 g SCOD was generated per gram VSS in the raw

anaerobic sludge and about 60% of this SCOD was an

aerobically biodegradable.

5000.

4000r

I* r

o 6

After Feeding ~\ o Before Feeding

40 45 50 55 60 65 70 75 80 DAYS OF OPERATION

Figure 9?Removal of SCOD in AnRBC-2 fed PDS230 in a batch mode.

November 1988 1977



Friedman et al. _

Most of the heavy metals present in raw PDS, with the

exception of mercury, were concentrated in the dewatered

solids. Settled or dewatered WAO volatile solids, or both, had a much higher COD content (g COD/g VSS) than raw PDS regardless of the operating temperature. This indicates that the recalcitrant oiganic solids were physically and perhaps chemically very different than those in the

original anaerobic sludges. Anaerobic bioassays for the

biodegradability of WAO products were continued for much longer than the typical 30-day test and with more

frequent chemical analyses than normally reported. These

procedures revealed acclimation to inhibitory WAO

products and a delayed methanogenesis that would have

not been observed otherwise.

Acknowledgments Credits. This work was supported by cooperative re

search agreement No. 704-RIER-BEA-85 with the New York State Energy Research and Development Authority and by funds from the Syracuse University Office of

Sponsored Programs. Zimbro Inc., Rothschild, Wis., pro

vided a wet air oxidation autoclave for this study and

Douglas L. Miller, then Operations Manager, Onondaga County Division of Drainage and Sanitation provided analyses of metals in sludge samples. Portions of the data in this paper were presented at the 42nd Industrial Waste

Conference, Purdue University, West Lafayette, Ind., May 1987.

Authors. A. A. Friedman is a professor of civil engi

neering and J. E. Smith is an associate professor of biology at Syracuse University, Syracuse, N. Y. J. DeSantis is an

environmental scientist with ERM Inc., Westchester, Pa.;

T. Ptak is an environmental engineer with CH2M-HU1

Inc., Gainesville, Fla.; and R. C. Ganley is a managing

engineer with O'Brien & Gere Inc., Syracuse, N. Y. Cor

respondence should be addressed to A. A. Friedman, De

partment of Civil Engineering, Syracuse University, Syr acuse, NY 13244-1190.

References 1. Friedman, A. A., and Smith, J. E., "Methane from Partially

Digested Sewage Sludge." NYSERDA Report No. 86-12, Al

bany, N. Y. (1987). 2. Wu, Y. C, et al, "Wet air oxidation of anaerobically digested

sludge." J. Water Pollut. Control Fed., 59, 39 (1987).

3. LeBrun, T. J., and Tortorici, L. D., "Thermal Treatment of

Municipal Sludges." EPA-600-52-84-104, U. S. EPA (1984).

4. Haug, R. T., et al, "Effect of thermal pretreatment on di

gestability and dewaterability of organic sludges." J. Water

Pollut. Control Fed., 50, 73 (1978).

5. U. S. Environ. Prot. Agency, "Sludge Treatment Disposal

Process Design Manual." EPA 6251-79-011, Municipal En

vironmental Research Laboratory, U. S. EPA, Cincinnati,

Ohio (1979). 6. Brooks, R. B., "Heat Treatment of Activated Sludge." Water

Pollut. Control (GB), 67, 6 (1968). 7. Erickson, A. H., and Knopp, P. V., "Biological Treatment

of Thermally Conditioned Liquors." In "Advances in Water

Pollution Research." Pergamon Press, New York, N. Y.

(1972). 8. Brooks, R. B, "Heat Treatment of Sewage Sludge." Water

Pollut. Control (GB), 69, 92 (1970). 9. Everett, J. G., "Dewatering of wastewater sludge by heat

treatment." J. Water Pollut. Control Fed. 44, 92 (1972).

10. Everett, J. G., "The Effects of pH on the Heat Treament of

Sewage Sludge." Water Res., 8, 899 (1974).

11. Crawford, G. V., et al, "Anaerobic treatment of thermal

conditioning liquors." J. Water. Pollut. Control Fed. 54,1458

(1982). 12. Haug, R. T., et al, "The Anaerobic Filter Treats Waste Ac

tivated Sludge." Water and Sew. Works, 123, 2 (1977).

13. Haug, R. T., "Sludge processing to optimize digestability and

energy production." / Water Pollut. Control Fed., 49, 1713

(1977). 14. Teletzke, G. H., et al, "Components of sludge and its wet

air oxidation products." J. Water Pollut. Control Fed., 39,

994(1967), 15. Flynn, B. L., "Wet Air Oxidation for Black Liquor Recovery."

Chem. Eng. Prog., 72, 66 (1976).

16. Donovan, E., "Pivot-Scale Anaerobic Filter Treatment of

Heat Treatment Liquor." EPA-600/S2-81-114, (1981). 17. Dague, R. R., et al, "Anaerobic Filter Treatment of Recycle

from Thermal Sludge Conditioning and Dewatering." Paper

presented at 53rd Water Pollut. Control Fed. Conf. (1980). 18. Donovan, E. J., "Treatment of High Strength Wastes by An

aerobic Filter." In "Anaerobic Filters: An Energy Plus for

Wastewater Treatment." Argonne National Laboratory,

Chicago, 111. (1980). 19. Hall, E. R., "Biomass Retention and Mixing Characteristics

in Fixed-Film and Suspended Growth Anaerobic Reactors."

Proc. IAWPR Seminar on Anaerobic Treatment, Copenhagen, Denmark (1982).

20. Tait, S. J., and Friedman, A. A, "Anaerobic rotating biological contactor for carbonaceous wastewaters." /. Water Pollut.

Control Fed, 52, 2257 (1980). 21. Podolak, P. L., et al, "Effects of Reduced Partial Pressure

on an Anaerobic Rotating Biological Contactor." Proc. 2nd

Int. Conf. on Fixed-Film Biological Proc, Arlington, Va.

(1984). 22. Hungate, R. E., "A Roll Tube Method for Cultivation of

Strict Anaerobes." Meth. Microbiol 3B, 117 (1969).

23. Nottingham, P. M., and Hungate, R. E., "Methanogenic

Fermentation of Benzoate." J. Bacterio!., 99, 1170 (1969).

24. Miller, T. L., and Wolin, M. J., "A Serum Bottle Modification

of the Hungate Technique for Cultivating Obligate Anaer

obics." Appl Microbiol 27, 985 (1974). 25. Owen, W. F., et al, "Bioassay for Monitoring Biochemical

Methane Potential and Anerobic Toxicity." Water Res., 13,

485 (1979). 26. Metcalf & Eddy Inc., "Wastewater Engineering: Treatment

Disposal Reuse." 2nd Ed., McGraw-Hill, New York, N. Y.

(1979). 27. "Standard Methods for the Examination of Water and

Wastewater," 16th Ed., Am. Public Health Assoc., Wash

ington, D. C. (1985).

28. Bricker, C. E., and Vail, W. A., "Microdetermination of

Formaldehyde with Chromotopic Acid." Anal Chem. 22,

1615(1950). 29. Tchobanoglous, G., and Schroeder, E. D., "Water Quality:

Characteristics, Modeling, Modification." Addision-Wesley,

New York, N. Y. (1985).

30. M?ller, J. A., and Jin Lung Su, W., "Benthal oxygen demands

and leaching rates of treated sludges." J. Water Pollut. Control

Fed, 44, 2303 (1972). 31. Speece, R. E, and Parkin, G. F., "The Response of Methane

Bacteria to Toxicity." Proc. Third. Intnl. Sym. on Anaerobic

Digestion, Boston, Mass. (1983).

32. Stuckley, D.C., and McCarty, P. L., "The Effect of Thermal

Pretreatment on the Anaerobic Biodegradability and Toxicity of Waste Activated Sludge." Water Res., 18, 1343 (1984).




Documents

Characteristics of Residues from Wet Air Oxidation of Anaerobic Sludges