This article was downloaded by: [Northeastern University]On: 23 November 2014, At: 11:52Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House,37-41 Mortimer Street, London W1T 3JH, UK

Environmental Technology LettersPublication details, including instructions for authors and subscription information:http://www.tandfonline.com/loi/tent19

Anoxic‐aerobic digestion of waste activated sludge: PartI‐solids reduction and digested sludge characteristicsC. J. Jenkins a & D. S. Mavinic ba Ministry of Environment , Nanaimo, B.C., Canadab Environmental Engineering Group, Department of Civil Engineering , University of BritishColumbia , B.C., Canada , V6T 1W5Published online: 17 Dec 2008.

To cite this article: C. J. Jenkins & D. S. Mavinic (1989) Anoxic‐aerobic digestion of waste activated sludge: PartI‐solids reduction and digested sludge characteristics, Environmental Technology Letters, 10:4, 355-370, DOI:10.1080/09593338909384752

To link to this article: http://dx.doi.org/10.1080/09593338909384752

PLEASE SCROLL DOWN FOR ARTICLE

Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in thepublications on our platform. However, Taylor & Francis, our agents, and our licensors make no representationsor warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Anyopinions and views expressed in this publication are the opinions and views of the authors, and are not theviews of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should beindependently verified with primary sources of information. Taylor and Francis shall not be liable for any losses,actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoevercaused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content.

This article may be used for research, teaching, and private study purposes. Any substantial or systematicreproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in anyform to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http://www.tandfonline.com/page/terms-and-conditions

Environmental Technology Letters, Vol. 10, pp. 355-370©Publications Division Selper Ltd., 1989

ANOXIC-AEROBIC DIGESTION OF WASTE ACTIVATEDSLUDGE: PART I - SOLIDS REDUCTION AND DIGESTED

SLUDGE CHARACTERISTICS

C. J. JenkinsMinistry of Environment, Nanaimo, B.C., Canada

D. S. Mavinic*Environmental Engineering Group, Department of Civil Engineering,

University of British Columbia, B.C., Canada, V6T 1W5

(Received 5 December 1988; in final form 30 March 1989)

ABSTRACT

This research demonstrated the feasibility of incorporating, at regularintervals, non-aerated conditions in aerobic sludge digestion. Part I of this paperoutlines the benefits associated with solids reduction and digested sludgecharacteristics. Three digestion modes (anoxic-aerobic, aerobic with lime controland aerobic) were investigated at 20 °C and 10 °C. For each mode and temperature,solids retention times (SRTs) of 10, 15 and 20 days were examined. The resultsshowed that anoxic-aerobic digestion yielded comparable percent solids reduction,despite using only 42 percent as much air. Anoxic-aerobic digestion also maintainedneutral mixed-liquor pH levels at 'no cost'. In addition, significant nitrogenremoval (up to 32 percent) and reduced phosphorus release were realized.

INTRODUCTION AND LITERATURE REVIEW

Advances in wastewater treatment have increasingly improved the quality ofsewage treatment plant effluents. Accordingly, regulations have strengthened to thepoint where secondary treatment is often required. Many of these secondary treatmentfacilities produce waste-activated sludge. Thus, treatment schemes for waste-activated sludge have had to deal with a dual problem: increasing quantity combinedwith decreasing quality.

Aerobic digestion is ideally suited to the treatment of most waste activatedsludges, especially in small plants. Carried out in a separate reactor, aerobicdigestion relies on extended aeration to "produce a biologically stable end-productsuitable for disposal or subsequent treatment in a variety of processes" (1). Notencumbered by the capital cost and delicate operation of anaerobic digestion, aerobicdigestion has been the most suitable choice of wastewater systems treating small tomedium discharges (2). At present, there are approximately 25 aerobic digestersoperating in British Columbia, Canada. Since most of these digesters are locatedoutside and are not heated, there is a need for studies that reflect low temperatureoperating conditions.

It has been reported that aerobic digestion has a number of advantages. Adamsand Eckenfelder (1) summarized the advantages associated with aerobic digestion whencompared to anaerobic digestion, citing improved supernatant quality, formation ofstable end-products, and equal volatile solids reduction for secondary sludges.However, a disadvantage is the higher energy costs associated with aerobic digestion,due to aeration requirements.

355

Dow

nloa

ded

by [

Nor

thea

ster

n U

nive

rsity

] at

11:

52 2

3 N

ovem

ber

2014

Another potential drawback to aerobic digestion is the resulting drop in mixed-liquor pH (MLpH). pH levels as low as 3.8 have been reported in the literature.While many researchers have maintained that these low MLpH levels do not adverselyaffect solids reduction, Anderson and Mavinic (3) demonstrated that maintainingneutral MLpH, by lime buffering, improved solids reduction considerably. Even ifthis is not the case> maintaining a. low MLpH (besides sub-optimum conditions forbacterial activity) produces a number of potential disadvantages related to digestedsludge characteristics and supernatant quality.

The concept of anoxic-aerobic sludge digestion incorporates, at regularintervals, non-aerated periods during aerobic digestion. This produces a digesterwhich cycles between anoxic and aerobic conditions. The main objective of this studywas to assess the acceptability of anoxic-aerobic digestion, in comparison to aerobicdigestion with and without lime addition. It was felt that cycling of the air wouldhave at least two benefits: (1) less air consumption, and (2) maintenance of aneutral pH as a result of alkalinity consumption and production by way ofnitrification-dehitrification processes. The first benefit would translate intolower operating costs, and the second benefit could result in better digested sludgecharacteristics and supernatant quality.

Very little literature exists, to date, on the development of anoxic-aerobicsludge digestion. As far as the authors are able to discern, work conducted byWarner et al. (4) was the first use of regular, non-aerated periods, during aerobicdigestion of waste activated sludge. Previous researchers may have, on occasion,experienced anoxic conditions during periods of sludge settling and production ofsupernatant (5); however, the condition was not reported as such.

Warner et al. (G) discussed the application of the general activated sludgemodel as set out by Dold et al. (7) and extended by van Haandel et al. (8) to anoxic-aerobic digestion of waste-activated sludge. The laboratory experiments Involvedcycling of air to produce aerobic, then anoxic conditions, at regular time intervalsfor an extended time frame. Some of the conclusions drawn from this work are:

1. Aerobic digestion of waste-activated sludge, with alternating periods of anoxicand aerobic conditions, is as efficient with regard to volatile solidsdestruction as purely aerobic digestion provided: i) the anoxic cycles do notconstitute more than 50 to 60% of the total retention time, and ii) the durationof a single anoxic cycle is less than 3 hours;

2. For digesters with a 50% anoxic time, the nitrate generated by nitrification inthe aerobic cycle was denitrified during the anoxic cycle; and

3. By denitrifying the nitrate generated, the alkalinity, and hence the pH, in thedigester remains stable.

Dold et al. (9) further proposed that anoxic-aerobic digestion showed promise interms of cost savings, in terms of approximately 20% less oxygen required over purelyaerobic digestion. In addition, due to nitrification-denitrification cycling, aneutral pH was maintained, thus negating the need for chemical (e.g., lime) addition.

Peddie and Mavinic (10) reported VSS reduction to be the same (24% at 20 °C) forpurely aerobic, aerobic with lime addition and anoxic-aerobic sludge digestion.Investigations were carried out at pilot-scale (300 litres), utilizing continuousflow-through reactors and incorporating a post thickening clarifier with recycle.Anoxic-aerobic cycle times of 3 hours were used. Air was used for mixing,consequently, air flow rates were 90 - 103 L/m3 min., far exceeding the 20 - 40 L/m3

min. recommended by Metcalf and Eddy (11).

In reviewing the previous research conducted into aerobic digestion, it isevident that considerable attention has been devoted to a better understanding of thevarious processes involved. At first, primary, mixed primary-chemical and chemicalsludges were investigated under a variety of conditions. With the increase inbiological waste treatment facilities, came the need for digestion of waste-activated

356

Dow

nloa

ded

by [

Nor

thea

ster

n U

nive

rsity

] at

11:

52 2

3 N

ovem

ber

2014

sludges. These biologically active sludges lent themselves ideally to aerobicdigestion and the need for an even greater understanding of the aerobic digestionprocess. Investigations into improvements and enhancements of the process continued.The need for more research, in terms of design and satisfactory operation of aerobicdigesters, became evident. The idea that aerobic digestion could be made moredesirable, through the incorporation of anoxic periods, deserved to be furtherinvestigated and became the focal point of this study.

EXPERIMENTAL METHODS

Six identical, 6.0 L inverted-bell jars were employed throughout the study. Thedigesters were split into two sets of three. Each set consisted of one anoxic-aerobic, one lime control and one aerobic control digester. Anoxic-aerobicconditions were achieved through the use of air solenoids. The air was "on" for 2.5hours and "off" for 3.5 hours. Thus, air supply was cycled over a period of 6.0hours, resulting in four cycles per day. Air flow rates were maintained between7.5 - 12 L/m3 min. for all digesters. Dissolved oxygen (DO) levels were maintainedbetween 2.0 and 4.5 mg I"1 (for the anoxic-aerobic digesters, a maximum DO greaterthan 2.0 mg I"1 was reached for each air on cycle). In order to maintain a pH levelabove 6.8 within the lime control digesters, a daily aliquot of an 80 g 1"1 lime(Ca[OH]2) slurry was added. The corresponding average doses were 28, 29 and16 mg l~x of lime per litre of sludge per day for Runs 1, 2 and 3, respectively.

Runs 1 and 2 were conducted at 20 i 0.5 °C (within the laboratory) and Run 3 wasconducted at 10 °C (within a walk-in environmental chamber). By wasting 300, 400 and600 ml of digested sludge per day and replacing it with the corresponding volume ofraw sludge, solids retention times (SRTs) of 20, 15, and 10 days, respectively, wereinvestigated at each temperature.

Fresh secondary raw sludge was collected daily from the University of BritishColumbia Pilot Plant (biological phosphorus removal process), transferred to thelaboratory and allowed to settle; this elevated the solids levels, before addition tothe digesters. Daily solids levels (TSS and TVSS) were determined on the raw sludge,as well as the wasted sludge, using a modified version of Standard Methods (12);namely, 25 ml aliquots of MLSS were transferred to 50 ml centrifuge tubes and spun at2500 rpm for 15 minutes. After centrifuging, the samples were poured onto washed,fired and weighed Whatman 934AH glass fibre 5.5 cm dia. filters and vacuum filtered.Drying took place at 104 °C over 24 hours and volatilization took place at 550 °C forat least 1 hour. Nevertheless, whenever possible, all analyses were conducted inaccordance with Standard Methods (12).

Overall solids mass balances were obtained using the following (5):

Overall change in mass=Amount in - Amount out + Net change within the system(Feed) (Effluent) (increase/decrease in

digester solids) ....(1)

Accordingly, TSS and TVSS values were summed over the entire length of each run andpercent solids reductions were calculated as:

Solids reduced - 9 ^ 3 1 1 ^ ^ ^ x 10Q%

357

Dow

nloa

ded

by [

Nor

thea

ster

n U

nive

rsity

] at

11:

52 2

3 N

ovem

ber

2014

RESULTS AND DISCUSSION

The investigation of anoxic-aerobic sludge digestion was undertaken to determinethe "acceptability" of the process, when compared directly to aerobic digestion, withand without lime addition, in terms of a number of parameters:

1. Digestion kinetics, including solids reduction and determination of endogenousdecay coefficients;

2. Digesting sludge characteristics, including TSS and TVSS levels, daily pHvalues, nitrogen and phosphorus balances, alkalinity destruction and production,chemical oxygen demand (COD) and qualitative observations;

3. Supernatant characteristics, including nitrification, nitrogen forms anddenitrification, phosphorus forms, COD and alkalinity;

4. Application of continuous monitoring using ORP and DO, to show nitrification anddenitrification, as well as the reproducibility of the ORP versus time profile;and

5. An overall rating system..

Items 1 and 2 above are discussed in this paper, while the remaining items arethe subject of Part II. Throughout, anoxic-aerobic digester refers solely to non-aerated/aerated periods of 3.5:2.5 hours respectively, based on a 6 hour cycle and 4cycles per day.

A - Digestion Kinetics

1. Solids Reduction

Using the approach outlined, solids balances, in terms of total suspended solids(TSS) and total volatile suspended solids (TVSS), were calculated. Solidsoperating levels can be found in Table 3.

Given that the air flow rates were about equal in all digesters, and that theresulting dissolved oxygen levels remained above 2.0 mg I"1 (for the anoxic-aerobic reactors this value was, in general, reached or exceeded by the end ofthe aeration period), the only variables considered to have an affect on theperformance of the digesters, in terms of solids reduction, were the solidsretention time (SRT), the mixed-liquor temperature and the resulting mixed-liquor pH (MLpH). Table 1 is a summary of the percent reduction in TVSS for allSRT's and temperatures. Figures 1 and 2 reveal the trends in digesterperformance.

Table 1. Summary of Percent TVSS ReductionBased on an Overall(l) Mass Balance

ModeC(4)L(5)A(6)

2023.28.28.

903

20 °CSRT(3)1518.820.018.4

Temperature(2)

1014.G19.019.1

2018.017.013.2

10 °CSRT15

14.816.812.2

107.

10.6.

097

(1) Single balance period equal to 3 SRTs.(2) Liquid temperature in °C.(3) Solids retention time in days.(4) Anoxic-aerobic digester.(5) Constant aeration with daily lime slurry addition.(6) Constant aeration.

358

Dow

nloa

ded

by [

Nor

thea

ster

n U

nive

rsity

] at

11:

52 2

3 N

ovem

ber

2014

For Figure 1, the percent TVSS reduction was calculated on an overall massbalance basis, for the entire length of a given run and acts as the independentvariable. The product of the SRT and the digestion temperature has beencalculated and acts as the dependent variable. As expected, there is anincrease in percent TVSS reduction with an increase in the SRT-temperatureproduct. This is true for the experimental and both control digesters. Whilethe anoxic-aerobic digester and the lime control show parallel performance (limeslightly above), the purely aerobic digester exhibits a greater rate of increasein digestion of TVSS with increasing time and temperature. However, the aerobicdigester lags behind both the anoxic-aerobic and the lime control until the SRT-temperature product exceeds 220 and 380, respectively. Over the range plotted,this relationship appeared to be linear.

cou_3

• D<U

ceCO00

l ~

"cQJ

u<5CL

30

28

26

24

22

20

18

16

14

12

10

8

6

4

2

0

Anoxic—AerobicLime ControlAerobic Control

lime

onox/aero'

aero

Figure

Linear Regression Equation

Anoxic-Aerobic y = 0.04753 x + 5.4853Lime ControlAerobic Control y

0.04902 x +0.06412 x +

7.60091.8906

R Squarec

0.835510.906750.88045

— i —100 200 300 400 500

Degree-Days (° C * days)Performance Curve Based on Percent TVSS Reductionat Various Combinations of SRT and Temperature.

Co

•o0J

c_coCO1

cOJu_.0)CL

30

28

26

24

22

20

18

16

14

12

10

8

20 °C

10°CAnoxic—Aerobic

___ Lime Control

îft Aerobic Control

Figure 2.

10 15 20 30

Solids Retention Time, SRT (days)Percent TVSS Reduction Versus SRT at 20 t and 10°C.

359

Dow

nloa

ded

by [

Nor

thea

ster

n U

nive

rsity

] at

11:

52 2

3 N

ovem

ber

2014

In an effort to separate the combined affects of temperature and- SRT, Figure 2is presented. The slope of the curves show that there was improved performancewith increasing SRT. As well, there was greater percent reduction found at20 °C than at 10 °C, for all digesters, at all SRTs. This was expected. It wasobserved that the anoxic-aerobic digesters performed at approximately the samelevel as either of the controls. Therefore, it appeared that cycling the airflow (3.5 hours off:2.5 hours on) did not greatly influence the percent TVSSreduction.

From the trends observed in Figures 1 and 2 it was apparent that the limecontrols consistently yielded the greatest percent TVSS reduction. Theseresults act as a good indication that percent TVSS reductions, for the threedigestion modes, were quantitatively different, yet qualitatively similar.

Cycling of the air supply also realized another major benefit, namely, aconsiderable savings in the amount of air required for stabilization. Toillustrate this point, the percent TVSS reduction values presented earlier havebeen divided by the amount of air sparged. For the lime and aerobic controls,the amount of air was assumed to be unity, while for the anoxic-aerobicdigesters the amount of air was 0.42 unity. Thus, at 20 °C and 10 °C, thepercent TVSS reduction per unit air supplied for the anoxic-aerobic digesterwere 56.9, 44.7, 34.8, 43.0, 35.1 and 16.6, respectively. The lime and aerobiccontrol values remained the same as presented in Table 1. In all cases, basedon a per unit of air supplied, the anoxic-aerobic digesters out performed eitherof the controls by 150 to 200%.

Caution should be exercised, however, when-comparing the results in this manner.The qualitative observation remains that, on a mass balance basis, the limecontrols exhibited the greatest percent TVSS reduction. In addition, the factthat there was a 58% 'savings' in the amount of air supplied does notnecessarily imply that there would be similar cost savings. The anoxic-aerobicdigesters required the installation of timers and air solenoids, as well asmixing energy. At this time a cost analysis was not performed; however, itseems reasonable to assume that there could be significant cost savingsassociated with anoxic-aerobic digestion. Warner et al. (6), estimated apotential cost saving of 30% for anoxic-aerobic digesters (having an on offratio of 1:1) over conventional aerobic digesters. Based on the resultsobtained herein, this estimate seems attainable.

Initially, it seemed counter intuitive that roughly the same amount ofstabilization could be achieved using less air (and that, on a unit basis, theanoxic-aerobic would be vastly superior). Nevertheless, a few of explanationsare postulated. One explanation is that the anoxic-aerobic digesters were moreefficient in use of the supplied air; that is, when the air came on, thedissolved oxygen level in the anoxic-aerobic digesters was very low and, thus,initially (until the residual DO exceeded 1.0 mg I" 1), the driving force wasvery high. It was speculated that this high driving force partially accountedfor greater oxygen transfer efficiency during aerated periods. With the DOlevel in both the controls being maintained above 2.0 mg l~x, the driving forcewas not as great; consequently, oxygen was 'wasted'.

A second, possible explanation was that, during the non-aerated periods, thedenitrifiers were able to use nitrates produced during the aerated period forenergy. Thus, there continued to be endogenous decay, despite the absence ofmolecular oxygen. During aerated periods, while oxidation took place in theabsence of an external food source, micro-organisms were able to endogenouslydecay as expected.

360

Dow

nloa

ded

by [

Nor

thea

ster

n U

nive

rsity

] at

11:

52 2

3 N

ovem

ber

2014

2. Endogenous Decay Coefficients

Endogenous decay coefficients for all digesters, at all SRTs and temperatures,were calculated. For the purpose of determining endogenous decay coefficients,the semi-continuous digesters were treated as daily, batch systems.

The choice of endogenous decay coefficients and the expected percent of timethat that value can be achieved may prove useful during the design stage. Fordiscussion purposes, the use of the 50% of time-less-than-y value is suggestedIn Figure 3, starting at the 50% less-than-y-value on the x-axis, a verticalline was drawn until it reached the curve denoting the type of digester ofinterest; from that intersect, a horizontal projection to the y-axis yielded theanticipated endogenous decay coefficient. This has been done for each digester.A summary of these results is presented in Table 2 and revealed that the median,endogenous decay coefficients for the anoxic-aerobic and lime control digestersbased on TVSS, were larger than those based on TSS. However, the opposite wastrue for the purely aerobic digesters. No trend with SRT was apparent.

XJ

• ^

X I

OO

o

Q

en

Oco"OcLÜ

0.110.100.090.080.070.060.050.04

0.030.02

0.010.00

-0.01-0.02-0.03-0.04-0.05-0.06-0.07-0.08

• Anoxie-Aerobic+ Lime Control° Aerobic Control

i

0 20 40 60 80

Percent of Time less—than Y—Value

Figure 3. Endogenous Decay Coefficients for 20 day SRTat 20°C - TVSS Basis.

100

361

Dow

nloa

ded

by [

Nor

thea

ster

n U

nive

rsity

] at

11:

52 2

3 N

ovem

ber

2014

Table 2. Medlan(l) Endogenous Decay CoefficientBased on TVSS and TSS Measurements

C(2)

L(3)

A(4)

TVSS

TSSTVSS

TSSTVSS

TSS

200.0185

0.01610.0229

0.01450.0168

0.0192

20 °CSRT15

0.0144

0.01330.0132

0.00910.0198

0.0165

Temperature

100.0168

0.00930.0277

0.01750.0204

0.0232

200.0104

0.00720.0130

0.00830.0100

0.0092

10 °CSRT15

0.0104

0.01180.0156

0.01410.0076

0.0114

100.0106

0.00720.0135

0.00940.0099

0.0108

(1) Value equal to or less-than 50 of the time.(2) Anoxic-aerobic digester.(3) Constant aeration with daily lime slurry addition.(4) Constant aeration.

Plots such as Figure 3 also provided information about the changes that occurdue to changes in SRT and digestion temperature. It was observed that themedian values, as well as the maximum and minimum values for each of the threetypes of digesters, increased with increasing SRT and temperature. Also, theoverlapping of the points indicated that 'equivalent1 performance could beanticipated for these three digestion modes.

Using the 50-%-less-than values provided in Table 2, Streeter-Phelps temperaturesensitivity coefficients were determined from the slope of the semi-logarithmplots (Figures 4 - 6 ) . No explanation was found for the anomalous point at 10°C,using lime, in Figure 5. No trend with SRT was noticed. Average temperaturesensitivity coefficients of 1.046, 1.067 and 1.076 were calculated for theanoxic-aerobic, lime control and aerobic control digesters, respectively. Thissuggested that, over the temperature range examined, anoxic-aerobic digestionwas least affected by liquid temperatures.

c

üeuo

ou0>

O

oc(ÜCnO"O

c

U.UJib -

0.0251 -

0.0200 -

O.0158 -

0.0125 -

0.0100 -

0.0079 -

0.0063 -

—-A—

• \ e

9 = 1.053

Anoxie—Aerobic

Lime Control

Aerobic Control

= 1.058

\ e = 1.059

20 10

Temperature (°C) .

Figure 4. Temperature Sensitivity Coefficients for 20 day SRT.

362

Dow

nloa

ded

by [

Nor

thea

ster

n U

nive

rsity

] at

11:

52 2

3 N

ovem

ber

2014

3.0251

0.0063

Anoxic-Aerobic

Cn\

ffic

ieC

oeis

D

ecay

uOc

End

og«

0.0200 -

0.0]58 -

0.0126 -

0.0100 -

0.0079 -

e =

e =

1.100 *

1.033 •A

Lime Control

Aerobic Control

20 10

Temperature-(°C)

Figure 5. Temperature Sensitivity Coefficients for 15 day SRT

0.0398

—'

fici

ent

Coe

fiD

ecay

c

oc

Lu

0.0316 -

0.0251 -

0.0200 -

0.0158 -

0.0126 -

0.0100 -

0.0079

\ 0 = 1.075

G = 1.047

Anoxie—Aerobic

Lime Control

Aerobic Control

= 1.075

Figure 6.

20 10

Temperature (°C)

Temperature Sensitivity Coefficients for 10 day SRT.

363

Dow

nloa

ded

by [

Nor

thea

ster

n U

nive

rsity

] at

11:

52 2

3 N

ovem

ber

2014

B - Digested Sludge Characteristics

1. Daily TSS and TVSS levels

The average, standard deviation, maximum and minimum value for both TSS and TVSSlevels were determined and are presented in Table 3.

Table 3. Summary of Daily TSS and TVSS Levels

SRTParameter

mg I"1 Mode Ave.Std.Dev. Max. Min.

TSS

20°C

C(2)L(3)A(4)

TVSS

20 days

FCLA

TSS

10°C

FCLA

TVSSFCLA

7960634962516131

538218191438

9268687667006876

6616588459004772

6227481945094886

424156170324

7340521649685340

5212444042403808

8555711272677076

794143164242

9984750876047540

6228675269566532

6587535954015469

616117139197

7732566856765844

4784506451325080

TSS

20-C

FCLA

TVSS

15 days

FCLA

TSS

10°C

FCLA

TVSSFCLA

7538620764645856

1240126210183

12584650472206352

54565904•61205484

5798466046504597

921107143147

9640485650525008

4232438443524312

8555766475687633

794129135158

9984782477807852

6228737273007328

6587582957045953

61691

100123

7732596858686116

4784559655005716

TSS

20°C

FCLA

TVSS

10 days

FCLA

TSS

10-C

FCLA

TVSSFCLA

7960697668356649

538166172196

9268736870967056

6616670464086232

6227533050785198

424131135143

7340561652805492

5212510448004780

8555770674597552

794173239162

9984802078567912

6228734869727296

6587581955785760

616157204134

7732610859366000

4784551651725520

(1) Raw sludge feed.(2) Anoxic-aerobic digester.(3) Constant aeration with daily lime slurry addition.(4) Constant aeration.

364

Dow

nloa

ded

by [

Nor

thea

ster

n U

nive

rsity

] at

11:

52 2

3 N

ovem

ber

2014

The precision of the method used to determine the solids levels in general, wasvery good; on two separate occasions, with a sample size of 9, the relativeerrors in the TVSS levels were 1.24% and 0.61%, respectively. In addition, theaverage, relative error of daily duplicate samples throughout all three runs wasless than 1.4%.

Since no effort was made to control the daily raw sludge to a set value, it isnot unexpected that there was considerable variability in the daily TSS and TVSSlevels. This was true for both the raw.sludge and the digested sludge exitingthe digesters on a daily basis.

Daily Mixed-Liquor pH Levels

No attempt was made to control the pH level in the anoxic-aerobic digesters orthe aerobic digesters. Figure 7 shows the typical variation in daily mixed-liquor pH levels experienced throughout a single run. A daily dose of limeslurry was added to the lime control digester, in order to maintain the pH near7.0.

The average MLpH level in the anoxic-aerobic digesters was 7.12 + 0.07. Thus,anoxic-aerobic digesters showed a remarkable ability to 'naturally1 control thepH level. This was the case for both temperatures, as well as for the threeSRT's investigated. One benefit of this self-regulating pH process was apparentduring the discussions concerning the TVSS reduction, namely, that at lower SRT-temperature products, the anoxic-aerobic digester out-performed the aerobiccontrol.

The average, minimum daily MLpH level in the lime control digesters was 6.81 +0.12. While the standard deviation is larger than that for the anoxic-aerobicdigesters, it can be explained as a function of operator reliability and dailyslug dosing. There was considerable variation in the MLpH levels in the aerobiccontrol digesters. For example, the average MLpH was 5.25 +.0.42 and rangedfrom 6.71 to 4.24. While the aerobic controls did not seem to suffer greatly interms of solids reduction, it cannot be assumed that large fluctuations in MLpHlevels were completely tolerable. For example, sudden changes in the MLpHlevels within the aerobic control were associated with nitrite build-up and lackof alkalinity consumption, due to inhibition of Nitrobacter. A more detailedaccount of this phenomenon appears in Part II of this paper.

An explanation of why the lime control out-performed (in terms of solidsreduction) both anoxic-aerobic and aerobic digesters cannot be founded solely onthe basis of pH control. Additional benefits associated with a neutral digesterpH, in terms of supernatant quality, are outlined in Part II.

xQ.

XCL\_

oDg-_iI

"O• Anoxic-Aerobic+ Lime Controlo Aerobic Control

Figure 7.

Daily Mixed —Liquor

pH Levels for 20 day

SRI at 20°C.

24-Sep 01-Oct 08-0ct 15-Oct 22-Oct 29-Oct 05-Nov 12-Nov 19-Nov

Date

365

Dow

nloa

ded

by [

Nor

thea

ster

n U

nive

rsity

] at

11:

52 2

3 N

ovem

ber

2014

3. Nitrogen Balance

In all cases, there was a decrease in the levels of Total Kjedhal Nitrogen (TKN)and the percent N through digestion. Variations in the TKN levels did notfollow a trend with respect to SRT. However, the reduction in TKN was greaterfor all digesters at 20 °C than at 10 °C. Average values of 0.086 and 0.094 gTKN/g TVSS were calculated for the raw sludge at 20 °C and 10 °C, respectively,and are within the range reported in the literature (5). The unit values forall digesters were quite similar, with overall averages at the two temperaturesof 0.082 and 0.092 g TKN/g TVSS. These values represented decreases of only4.1% and 2.1%; due to the relative errors associated with their determination,this was not considered significant. In order to determine more closely thefate of nitrogen through the various digestion modes, the following mass balanceequation was employed:

Overall N balance = Total N in - Total N outi Change in N in digester ....(3)

While Koers (5) found that there was considerable nitrogen removal, it should benoted that the only route for nitrogen removal at this level of investigationwas through denitrification. He observed that the process of turning off theair and daily decanting "most assuredly resulted in anoxic conditions"; thus, anexplanation for the denitrification could easily manifest itself.

Table 4 presents the amount of nitrogen removal that can be expected. It wasfound that most of the nitrogen could be accounted for in the lime and aerobiccontrols. Since the air was always on, it is speculated that there was verylittle denitrification taking place in these digesters. However, since theanoxic-aerobic digesters experienced anoxic conditions on a regular basis, itwas not unreasonable to expect a significant amount of denitrification, andhence nitrogen removal. As such, nitrogen removals up to 30% may be anticipatedthrough the use of anoxic-aerobic digestion.

Table 4. Nitroaen(l) Removal Data

T

20

10

(1)(2)(3)(4)(5)(6)(7)(8)

SRT Mode

C(6)20 L(7)

A(8)C

°C 15 LAC

10 LAC

20 LAC

°C 15 LAC

10 LA

Nitrogen expressedEquals sum of bothEquals sum of TKN

! Ni n

(2)

899489948994670867086708467846784678

104821048210482466646664666654265426542

as mg 1"1

No.,(3)

685066637337449445054612398738253854799382678384380637563769578855715742

as N.

t N;;:Nou

m214423311657221422032096591853824248922152098860910897754971800

TKN and NO« added.wasted.

Nitrogen gas evolution assumed toBased on Na/TKNm x 100.Anoxic-aerobic digester.Constant aerationConstant aeration.

with daily lime

be (f) -

t NO» .„,

(a)27

2533150024

145512679.885377539

172011451163455514781704

(g).

slurry addition.

N*(4)

2043-2531312189747829586-4343

245049595384925634274019096

Removal(5)%

22.7-2.81.532.611.112.412.5-0.90.923.44.79.118.25.57.3

11.32.91.5

366

Dow

nloa

ded

by [

Nor

thea

ster

n U

nive

rsity

] at

11:

52 2

3 N

ovem

ber

2014

4. Phosphorus Balance

The percent phosphorus (P) levels in the aerobically digested sludge, as evidentin Table 5, were consistently less than those in the raw sludge, constitutingaverage decreases of 15.4 and 3.14 % at 20 and 10 °C. Moreover, the greatestdecrease in percent P content of the sludge was found to be 19.9%, for theaerobic digester at a 15 day SRT and 20 °C.

In contrast, the average percent P for the anoxic-aerobic and the lime controlsincreased approximately 5% and 9%, respectively, during digestion. This canreadily be explained for the lime controls by the formation of calciumphosphate, which is insoluble and became a portion of the solids. Increases inpercent P for the anoxic-aerobic digesters were consistently apparent and maypartially be explained, since denitrifiers have been shown to be responsible forphosphorus accumulation (13). In addition, it is speculated that thecontinuation of anoxic-aerobic cycling (experienced in the biological phosphorusremoval process from which the sludge was obtained) promoted phosphorusretention within the digesting sludge. However, to confirm this, more work isnecessary.

The observation that the unit TP per unit TVSS (g TP/g TVSS) increased (whencompared to the values for the raw sludge) for all the digesters at bothtemperatures can be explained. During digestion, there was a decrease in thelevels of TP as well as a decrease in the TVSS levels; however, the decrease inthe TVSS content was greater than the decrease in the TP content. This was anencouraging result, since it suggested that, during aerobic digestion(especially for the anoxic-aerobic and lime control) there was an attempt madeby the microorganisms to retain P. A mass balance of phosphorus was conductedin a manner similar to that done for nitrogen; within experimental error, allthe phosphorus can be accounted for with a recovery range of 77 - 99%.

Table 5. Average Phosphorus LevelsWithin Raw and Digested Sludge

Parameter

. TPmg I"1

%P

TP/TVSS

Mode

F(l)C(2)L(3)A(4)FCLAFCLA

20

296284287285

3.653.854.242.940.0480.0590.0640.058

20°C

SRT

15

305248260253

3.863.884.073.090.0530.0550.0560.055

Temperature

10

3052962942833.843.874.093.560.0490.0560.0580.054

20

299269279277

3.443.663.753.240.0450.0500.0520.051

10°C

SRT

15

305293284280

3.253.643.633.060.0440.0500.0500.047

10

300293283282

3.563.723.823.640.0470.0500.0510.049

(1) Raw sludge feed.(2) Anoxic-aerobic digester.(3) Constant aeration with daily lime slurry addition.(4) Constant aeration.

367

Dow

nloa

ded

by [

Nor

thea

ster

n U

nive

rsity

] at

11:

52 2

3 N

ovem

ber

2014

5. Alkalinity Consumption and Production

The raw sludge contained, on average, 214 mg I"1 as CaCO3 alkalinity. Ingeneral, this value would be considered low; yet in areas where there is verylittle natural alkalinity in the water supply, such as Vancouver, BritishColumbia, this value is normal. In order to maintain favourable conditionswithin the pilot-plant, alkalinity, in the form of sodium bicarbonate, is added.

As with pH, no trends in digested sludge alkalinity were found for the anoxic-aerobic digesters, either with respect to SRT or temperature. The levels, onaverage, were approximately 20% less than those in the raw sludge and, ingeneral, matched the levels maintained in the lime controls. Ordinarily,alkalinity is consumed at the rate of 7.14 mg I"1 (as CaCO3) per mg I"1 ofammonia N oxidized to nitrate. Yet, the first step, ammonification, results ina gain of 3.57 mg I"1 of alkalinity as CaCO3 per mg I"

1 of N ammonified (6).Therefore, when there is no subsequent denitrification, a net loss ofalkalinity, of 3.57 mg I"1 (as CaCO3) per mg I"1 ammonia N, results. Theresults of this work indicated a slightly lower net loss value of 3.07 mg I"1 ofalkalinity (as (CaCO3) per mg I"1 ammonia N solubilized) could be expected.However, the alkalinity data was not exhaustive and the authors do not disputethe theoretical value.

In the case when denitrification does take place, such as with anoxic-aerobicdigestion, an alkalinity gain of 3.57 mg l~x (as CaCOa) per mg I"1 of nitrate Nreduced can be expected. Thus, over one complete cycle, there is an alkalinitygain of 3.57 mg l"x (as CaCO3) during ammonification, a loss of 7.14 mg l"x (asCaCO3) during nitrification, and a gain of 3.57 mg I"1 (as CaC03) duringdenitrification. Consequently, there is no net loss of alkalinity duringanoxic-aerobic digestion, provided there is complete denitrification.

In this study, the aerobic controls consumed almost all alkalinity present inthe raw sludge. In general, the levels in the aerobic control wereapproximately one tenth those in the raw sludge. This is an overall average andreflects the fact that, on several occasions, there was incomplete conversion ofnitrites to nitrates, resulting in less alkalinity being consumed. This isdiscussed in more detail in the nitrification section of Part II.

6. Chemical Oxygen Demand

Trends in the mixed-liquor COD levels matched those found for percent TVSSreduction. However, since COD levels were measured only occasionally, anyparallels are observational and may not withstand closer investigation. Thelime control digesters were found to be most effective at COD removal,regardless of SRT or temperature.

Averages of 1.42 and 1.43 g COD/g TVSS for the raw sludge and all digestedsludge, respectively, were calculated. The small difference confirmed thespeculation that the COD reduction paralleled the TVSS reduction. This isconsistent, since the majority of the volatile solids would be carbon compoundsand COD is an indirect measure of the carbon content.

The traditionally accepted C:N:P ratio of 100:5:1 (11) was not observed. Aratio of 100:6.3:3.5 was found. This was not unusual, considering the sludge'sorigin. . Coming from a biological nutrient removal facility, it seemspredictable that the sludge would have higher portions of both N and P. Also,the ratio of N to P was decreased significantly, suggesting that there were highlevels of P in the sludge.

368

Dow

nloa

ded

by [

Nor

thea

ster

n U

nive

rsity

] at

11:

52 2

3 N

ovem

ber

2014

CONCLUSIONS

Based on the results of this experimental work the following conclusions can bedrawn:

1. ' Anoxic-aerobic digestion compared favourably to aerobic digestion, with andwithout lime addition. The use of non-aerated and aerated conditions in theratio of 3.5:2.5 hours per six hour cycle allowed for equal percent TVSSreduction when compared with aerobic digestion. Lime control digestersconsistently showed greater percent TVSS reduction. However, the differencesbetween the three digestion modes, in terms of percent TVSS reduction, werewithin experimental error.

2. The incorporation of non-aerated periods during aerobic digestion resulted in asignificant savings in the amount of air required for comparable percent TVSSreduction. Anoxic-aerobic digestion consumed only 42% of the air required byeither the lime or aerobic controls.

3. Temperature sensitivity was different for each digestion mode. Between 20 °Cand 10 °C, temperature sensitivity coefficients of 1.046, 1.067 and 1.076 werefound for anoxic-aerobic, lime control and aerobic digesters, respectively.This indicated that anoxic-aerobic sludge digestion was least affected bytemperature and considerably less than the standard 6 = 1.072.

4. Anoxic-aerobic digesters showed an ability to control the mixed-liquor pH (MLpH)levels that was superior to daily lime dosing. Neutral MLpH conditions weremaintained in the anoxic-aerobic digesters, despite low alkalinity levels in theraw sludge. Aerobic digestion afforded large drops in HLpH, but this did notseem to inhibit solids reduction. Low (generally 4.2 - 5.5) MLpH levels did,however, effect the overall acceptability of the process.

5. A reduction in total TKN content of the sludge can be expected, whicheverdigestion mode is employed. There was no trend in the amount of TKN reductionwith respect to SRT; however, greater reduction occurred at 20 °C than at 10 °C.The average levels of 0.086 and 0.094 g TKN/g TVSS for the raw sludge at 20 °Cand 10 °C, were maintained throughout digestion. Anoxic-aerobic digestionshowed an ability, through denitrification, to remove nitrogen as nitrogen gas.As much as 32% of the raw sludge nitrogen may be converted to nitrogen gas.

6. Phosphorus release took place during all three digestion modes. Anoxic-aerobicand aerobic digestion with lime addition showed the most promise, with respectto retention of percent phosphorus within the solids.

7. Alkalinity was consumed during aerobic digestion. The incorporation of regularnon-aerated periods resulted in the production of alkalinity, such that therewas no net loss of alkalinity during anoxic-aerobic digestion. The dailyaddition of lime indirectly maintained the alkalinity level within the limecontrols.

8. Reductions in COD levels paralleled TVSS reductions. A ratio of 1.42 g COD/gTVSS was observed throughout digestion. The ratio of C:N:P was 100:6.3:3.5,before and after digestion.

ACKNOWLEDGEMENTS

This work was partially supported by a grant from the Natural Sciences andEngineering Research Council of Canada (NSERC), as well as a U.B.C. Graduate ResearchAssistantship awarded to the senior author, Mr. Jenkins. The authors also wish toacknowledge the excellent technical assistance provided by the staff of the U.B.C.Environmental Engineering Laboratory.

369

Dow

nloa

ded

by [

Nor

thea

ster

n U

nive

rsity

] at

11:

52 2

3 N

ovem

ber

2014

REFERENCES

1. C.E. Adams, Jr. and W.W. Eckenfelder, Jr. Sludge Treatment ed. W.W. Eckenfelder& C.J. Santhanam. Maneel Dekker, Inc. New York, 279-305 (1981).

2. L.G. Rich, Water and Sewage Works. 124, 94 (1977).3. B.C. Anderson and D.S. Mavinic, Journal Water Pollution Control Federation, 56,

7, 889-897 (1984).4. A.P.C. Warner, G.A. Ekama and G.v.R. Marais, Res Rep W 48, Dept of Civil Eng,

Univ. of Cape Town, (1983).5. D.A. Koers, Ph.D. Dissertation, The University of British Columbia, Vancouver,

B.C. (1979).6. A.P.C. Warner, G.A. Ekama and G.v.R. Marais, Wat. Sci and Tech. 17., 8, 1475-1488

(1985).7. P.L. Dold, G.A. Ekama and G.v.R. Marais, Prog Wat Tech. 12, 47-77 (1980).8. A.C. Van Haandel, G.A. Ekama and G.v.R. Marais, Water Research. 15., 1153-1152

(1981).9. P.L. Dold, G.A. Ekama and G.v.R. Marais, Inst, and Cont. of Wat. & Wastewater

Treatment and Transport Systems, Proc. 4th IAWPRC Workshop, Houston & Denver,Ed. R.A.R. Drake, Pergamon Press (1985).

10. C.C. Peddie and D.S. Mavinic, Proceedings. CSCE Annual Conference, Calgary,Alta., May 25-27 (1988).

11. MeteaIf & Eddy, Inc., Wastewater Engineering: Treatment, Disposal. Reuse. 2ndEdition. New York: McGraw-Hill (1979).

12. Standard Methods for the Examination of Water and Wastewater. 16th ed., AmericanPublic Health Association, Inc., New York (1986).

13. Y. Comeau, K.J. Hall, R.E.W. Hancock and W.K. Oldham, International Conference:New Directions and Research in Waste Treatment and Residuals Management,University of British Columbia, Canada, 1, 327, (1985).

370

Dow

nloa

ded

by [

Nor

thea

ster

n U

nive

rsity

] at

11:

52 2

3 N

ovem

ber

2014