~ Pergamon

PI!: S0273-] 223(98)00721-5

Wal. Sci. Tech. Vol. 38, No. 8-9. pp. 435-442,1998.IAWQ

© 1998 Published by Elsevier Science Ltd.Printed in Great Britain. All rights reserved

0273-1223/98 $19'00 + 0'00

EFFECT OF AUTOTHERMALTREATMENT ON ANAEROBICDIGESTION IN THE DUAL DIGESTIONPROCESS

Amanda Ward*, H. David Stensel**, John F. Ferguson**,Gregory Ma*** and Stan Hummel***

* CH2M HILL, P.O. Box 91500, Bellevue, WA 98009, USA** University of Washington. Seattle, WA, USA*** King County Department ofNatural Resources, Seattle, WA, USA

ABSTRACT

The performance of two laboratory mesophilic anaerobic digesters was studied to determine if the claims forimproved mesophilic anaerobic digestion following autothermal pre-treatment are valid and if the benefitsclaimed are affected by the mesophilic digester SRT. One digester was fed sludge taken after a full-scaleautothermal reactor and the other was fed sludge from the same plant just before the autothermal treatmentunit. Autothermal treatment did not significantly increase volatile solids destruction kinetics duringsubsequent anaerobic digestion. The VS destruction rate was 2.7 to 2.8% per day with or without autothermaltreatment. Autothermal pre-treatment did not affect methane production per unit VS destroyed at anaerobicdigester SRTs ranging from 6 to 12 days. In the digester fed autothermal sludge, higher alkalinity and lowerVFA concentration produced a lower VFA to alkalinity ratio, indicating the autothermal pretreatment has thepotential to make anaerobic digester operation more stable compared to anaerobic digestion only.Autothermal pretreatment consistently reduced fecal coliform to below detection limits and fecal coliformremained below detection limits during anaerobic digestion. © 1998 Published by Elsevier Science Ltd. Allrights reserved

KEYWORDS

Aerobic thermophilic; anaerobic mesophilic; Class A biosolids; dual digestion; pathogens.

INTRODUCTION

To protect public health, the U.S. Environmental Protection Agency (USEPA) has enacted 40 CFR part 503governing land application of biosolids. These regulations protect public health by regulating biosolidspathogen density and requiring vector attraction reduction before land application. The USEPA Part 503regulations classify biosolids in two categories, Class A and Class B, based on pathogen density and landapplication restrictions. Class B sludge has a fecal coliform density less than 2 x ]06 MPN/g and landapplication is restricted. Class A sludge requires that fecal coliform be reduced even further to <],000MPN/g fecal coliform and that Salmonella be <3 MPN/4 g. Since Class A levels allow unrestricted use ofbiosolids, many facilities are changing sludge treatment processes to produce Class A biosolids.

435

436 A. WARD et al.

A dual digestion process consisting of an autotherma] pasteurization step before a mesophilic digester is onemethod to produce Class A biosolids. The dual digestion process uses a relatively short autothermal contacttime, typically about 24 hours, before mesophilic anaerobic digestion. Pure oxygen or air is provided)n thefirst step to support aerobic exothermic biological reactions that increase sludge temperature to 50-60AC forpathogen kill. There are 8 known facilities in the U.S. that use the dual digestion process. The ability of thedual digestion system to reduce pathogens is well documented (Baier and Zweifelhofer, 1991; Zweifelhofer,1985, Pagilla et ai., 1996). Besides meeting the goal of pathogen reduction to produce Class A biosolids,researchers have claimed that the aerobic stage of the process also enhances anaerobic digester efficiency.Based on a pi lot study, Pagilla et ai. (1996) reported that a dual digestion system achieved 6 percentagepoints higher VS destruction than a control digester operated at the same total SRT. However, a statisticalevaluation of the data was not given to support this claim. Other claims made by Pagilla et ai. were, higherpercent methane, and lower H2S in the digester gas, lower gas production per unit VS destruction, and lowerNocardia levels in the anaerobic digester. Messenger and Ekama (1993) claimed higher gas production andmore stable anaerobic digestion in a pilot scale dual digestion system operated in South Africa.

The goal of the research was to determine if the claims for improved mesophilic anaerobic digestionfollowing autothermal treatment are valid, and if the benefits claimed are affected by the mesophilic digesterSRT. The performance parameters of interest were better volatile solids destruction, greater methaneproduction, and more stable digestion. The performance of two laboratory mesophilic anaerobic digesterswas studied over an 8-month period to observe the effect of autothermal treatment. One was fed sludge takenafter a full-scale autothermal reactor (I-day SRT at 65A.C) at the Tacoma, WA central wastewater treatmentfacility (WWTF) and the other was fed sludge from the same plant taken just before the autothermaltreatment unit. The digesters were operated at identical SRTs, which were varied in the range of 4 to 14 daysto determine if the effect of autothermal treatment would be more noticeable as a function of SRT.

METHODOLOGY

Two laboratory mesophilic digesters were operated; one fed autothermal reactor sludge (AT digester), andone fed sludge (primary plus secondary) without treatment (control digester). The laboratory mesophilicdigesters consisted of 4 liter glass bottles with a 21 sludge volume, and were sealed with rubber stoppers.Each digester had one 0.5 inch sludge sampling ports and one 0.75 inch gas sampling port. A section oftubing was fitted over the sludge sampling port and a pinchcock clamp was used to prevent flow. The gassampling port was fitted with a ground glass stopcock and septa. A Tygon tubing gas line connected thedigester headspace to a 41 graduated manometer with acidic brine (pH <2) solution. The digesters werelocated in a 35°C constant temperature room. The reactors were mixed for five minutes every 30 minutesusing magnetic stirrers (9" x 9", Cole Parmer) with 3-inch stir bars. The digesters were operated ascompletely stirred reactors with no recycle, so that the system SRT and HRT were equal.

The AT and control digesters were seeded with sludge from the Tacoma Central WWTF mesophilicdigester. The digesters were batch fed an equal volume of sludge every 24 hours ± 2 hours, with effluentsludge removal before feeding. Digesters were operated at SRTs of 4, 6, 8, 10, 12 and 14 days. Digesterswere operated for a minimum of 3 SRTs at each detention time to ensure steady state operation. Thefollowing influent and effluent parameters were measured 2 to 3 times per week: total solids (TS), volatilesolids (VS), dissolved VS, total and soluble COD (TCOD, SCaD), volatile fatty acids (VFA), ammonianitrogen (NH3-N), fecal coliform, and alkalinity concentrations and percent methane of the digester gas. Thediss~lved VS was obtained by the same method as VS, but with a centrate from the centrifuged samples, sothat It represented the loss due to soluble organics. The difference between VS and dissolved VS is termedVSS concentration and represents the 550°C ignition losses due to only solids. The digester pH and gasproduc~io~ w~re measured daily. VFA and percent methane were measured by gas chromatography withflame IOnIZatIOn and thermal conductivity detectors, respectively. The other parameters were measuredaccording to Standard Methods (APHA, 1989).

Autothermal treatment 437

RESULTS

Effect of autothermal treatment on slud&e characteristics

To understand how autothermal treatment affects anaerobic digestion, its effect on raw sludge characteristicsis presented first, in Table 1. Results from Student T-tests are included to indicate the significance of thedifferences in parameters observed before and after autothermal treatment. TS, VS, and VSS concentrationswere reduced 11%, 12% and 17%, respectively, in the AT reactor. The T-test results indicate that thesechanges are significant at a 95% confidence interval. Approximately 6% of the raw sludge VS was dissolvedVS and 11 % of the autothermal sludge VS was dissolved VS. However, the TCOD concentration was notsignificantly changed by autothermal treatment.

SCOD, VFA (as COD), and other SCOD (O-SCOD) concentrations increased 2.6, 1.2, 1.4 gil respectively inthe autothermal reactor, indicating that sludge solids were hydrolyzed and fermented. O-SCOD is defined asany SCOD that is not short chain VFA. T-tests showed that these increases were significant. The dissolvedVS concentration also increased 1 gil in the AT reactor. A 1 gil dissolved VS is equivalent to an increase of2.1 gil COD (when converted using the average influent and effluent sludge TCODNSS ratio of 2.1), whichis somewhat consistent with the measured SCOD increase of 2.6 gil. Alkalinity increased 1.4 gil and pHincreased 0.8 standard units, probably due to the buffering of the ammonia released (300 mg/l) during aminoacid fermentation.

Table t. Changes in Tacoma WWTF raw sludge characteristics following autothermal treatment (65°C, 1•day SRT).

Y (10.35)

Y (10. 17)

+0.8 S.u.+300

+2.6 (40%)+1.2+1.4+1.4

6.6±.3500±2oo._--_._--_._------_....__._._-_.....--

S Raw Sludge AT Reactor sludge Change T-test± st dev ± st dev YIN (P)-_.__..__._._---_ __._--,._ _ _."_..__._..-.. -,,, - , _-_ _.._._-----,_ _--------_ _.._._- _._ _.__ _--_._._--,.._ _-----_ _..- ----_..__ .._-----------~--

VS (%) 125 3.1±O.5 2.7±O.8 -0.4 (12%) Y (10· )TS (%) 125 3.5±O.5 3.1±O.5 -0.4 (11 %) Y (10.6)

VSS (%) 49 2.9 2.4 -0.5 (17%) YDissolved VS (gIL) 49 2±1 3±1 +1 Y (.01)TCOD (gIL) 30 51±11 48±9 -3 N(O.3)TCOD:VSS 1.9 2.3SCOD (gIL) 110 6.2±1.5 8.8±1.2VFA as COD (gIL) 52 1.5±.6 2.7±.6O-SCOD 4.7 6.1Alkalinity 59 1.6±O.9 3.0±0.9(as CaC03, gIL)PH 194 5.8±.3

__NH3-N (m~l- ~...3 ~~~QO

S-Number of samples takeny-the difference is significant (P<O.05)N-the difference is not significant (P>O.05)

Effect of autothermal treatment on anaerobic di&estion solids destruction performance

TS and VS in the digester feed ranged from 2.8 to 3.8%, and 2.2 to 2.7% respectively. As indicated by Tablet, the AT digester received a lower VSS concentration and higher dissolved organic concentration. Thedigester volatile solids loading decreased from approximately 6 kg VS/m3-day at a 4-day SRT to 2 kgVS/m3-day at a 14-day SRT. Typical anaerobic digester volatile solids loadings are 1.9 to 2.5 kg VS/m3-day(WEF, 1992). At 4-, 6-, 8-, and 10-day SRTs the digesters were loaded above this range. The TCOD to VSratio in the raw and autothermal reactor sludge was approximately the same (±O.3) at all SRTs, and ranged

from 1.7 to 2.2 gig.

438 A. WARD et ai.

VS destruction efficiency is compared in Table 2 between the control and AT digesters and between the dualdigestion system and the control digester to determine if VS destruction efficiency was enhanced byautothermal treatment. The control and AT digesters were operated at the same SRT, so that the VSdestruction efficiency could be compared to determine if the autothermal treatment improved digestion VSdestruction kinetics as claimed in the literature. The dual digestion system SRT was thus always one daygreater than the control digester SRT.

The digester's average percent VS destruction increased from about 30% at a 4-day SRT to about 55% at the14-day SRT. The percent destruction efficiency was within 1-2 percentage points for most SRTs for the ATand control digesters. At the 10-day SRT, the average % VS destruction was 4 percentage points higher forthe AT digester versus the control digester, but the T-test evaluation determined that this difference, alongwith the others, was not statistically significant. For all control digester SRTs, except for the 6-day SRT, thedual digestion system, with its I-day longer total SRT, had a higher % VS destruction efficiency by 3 to 7percentage points. These differences were significant for most of the test SRTs (4,10,12 and 14 days).

Table 2. Average percent VS destruction across the dual digestion system (AT reactor and AT digester), ATdigester, and control digester at each operating SRT (35°C)

% VS reduction T-test Evaluation, YIN (P)

AT System AT Digester Digestion AT system vs. AT digester vs.

SRT S ±st. dey ± st. dey only i Control Digester Control DigesterI

± st. dey i

4 4 30±3 31±4 Y (0.002) N (0.5)

5 4 38±16 11 34±4 34±6 N (0.15) N (0.5)

7 11 33±68 12 40±7 42±12 N (0.2) N (0.4)

9 12 45±910 22 52±6 48±6 Y (0.001) N (0.1)11 22 55±712 42 53±7 52±11 Y (0.05) N (0.5)13 42 58±1514 51 55±6 57±7 Y (0.006) N (0.1)15 51 60±9

S-Number of samples takenY-the difference is significant (P<O.05)N-the difference is not significant (P:>O.05)

The data in Table 2 are shown graphically in Figure 1. Error bars are not shown on the figure because theyoverlap and could not be distinguished. In general, the VS destruction efficiency increased linearly with SRT~n all three systems and the data are fit by a linear regression. The slope of the dual digestion system data fitIS 2.8% VS destroyed per day, while the slope of the AT and control digester fits was 2.8 and 2.7% VSdestroyed per day, respectively, indicating that the VS destruction rate was similar in all three systems.Autothermal treatment did not improve VS destruction kinetics.

However, the ~esul~s shown in Figure I indicate that the overall VS destruction efficiency is a little higherfor the dual dIgestIOn system compared to only digestion, when compared at the same SRTs. The figureshows that at the same total SRT, the % VS destroyed is 1.4 percentage points higher for the dual digestionsystem compared to the control digester. The T-test comparisons for individual SRTs shown in Table 2s~ggest that in general this difference is significant. This difference in VS destruction efficiency is due to ahIgher VS destruction rate in the I-day autothermal treatment step, since the VS destruction rates in theanaerobic digesters was similar with or without autothermal treatment.

Autothermal treatment 439

70

60ic0

5013~! 40

:i~.. 30=•'0>

20

... ':-...... ~.."

Dual ~tXln 0 .. , ~~.::' 0

System /,.Regression Line " ;III

.. 'Limifc V__.. ~..-- ,"0., or ector, ' AttracllOn ReductXln

",'" ..AT and Coltrol DigesterRegrcssi:m Lines

o AT Digester

• Coltrol Digester :I

.. Dual DigestionSystem

10

15105

0+-----+---_-+- ---1

oSRT(days)

Figure I. Effect of SRT on Percent VS destruction in dual digestion system (AT reactor 65°C and AT digester), ATdigester, and control digester (35°C). The SRT for the dual digestion system includes I day for the autothermal

treatment.

The effect of autothermal treatment on methane production in anaerobic digestion

The AT and control digester methane production per unit of VS destroyed (m3 CHJkg VS destroyed) aresummarized in Table 3. The unit methane production was slightly higher in the AT digester than the controldigester at a 14-day SRT and much higher at the 4-day SRT. Both comparisons are statistically significant.The percent methane in the digester gas is also shown. The methane production varied from 0.7 to 0.9 m3

CH4/kg VS destroyed and was not statistically different for the AT and control digester at 6-, 8-, 10- and 12•day SRTs.

The percent methane was slightly higher in the AT digester gas than in the control digester gas by 1 to 2percentage points (statistically significant) for the 6-, 8-, and 14-day SRTs, and was the same in bothdigesters at the 10- and 12-day SRTs. At a 4-day SRT, the methane production in the control digesterdropped to 0.2 m3 CHikg VS destroyed and the percent methane dropped to 50%, while the AT digestermethane production was 0.5 m3 CHJkg VS destroyed and the AT digester gas was 59% methane. The dropin methane production and percent methane in the control digester was probably due to the low digester pH(6.3) and high VFA concentration (7,000 mg/I). The improved efficiency of the AT digester at the 4-daySRT was most likely due to its higher and more stable pH (7.0).

Table 3. Methane production (m3/kg VS destroyed) and percent methane in AT digester and control digester---... -;J-.--..-----------.- --.--------,--------- - --- -----

SRT ~iH,lk~E;;~~:troye~-i~~h~fT~~~:t:~ideyT-test (Pl(da s) 01 ester 01 ester I 01 ester 01 ester

4 O.5±O.l 0.2±O.1 Y (0.05) 59±1 50±2 Y(10' )6 0.7±O.1 0.9±O.1 N (0.1) 64±3 62±1 Y(1O-3)8 0.7±O.2 0.7±O.2 N (0.3) . 63±2 63±2 Y(O.04)10 0.7±O.1 0.8±O.2 N(O.l) I 63±1 63±1 N(O.4)

__._ ~~..__ ~;~:;~_ ~:~:~;__.. ; /~~L_I_~:::;~:__ ~~o41)__The effect of autothermal treatment on VFA to alkalinity ratio in anaerobic digestion

The VFA to alkalinity ratio in the digesters were compared as an indicator of digester stability. The VFA toalkalinity ratio is the ratio of the amount of organic acid accumulation relative to the available buffer. It is

440 A. WARD et al.

often used as an indicator of anaerobic digester stability. The EPA reported that a digester VFA to alkalinityratio of above 0.5 indicates that a digester is unstable (USEPA, 1992). A summary of the VFA and alkalinityconcentrations in the control and AT digesters along with the corresponding VFA to alkalinity ratios foreach SRT is shown in Table 4. Because the AT digester had lower VFA concentrations and higher alkalinityconcentrations at all SRTs. the AT digester had consistently lower VFA to alkalinity ratios (with theexception of the 12-day SRT), indicating the AT digester was more stable than the control digester.

Table 4. Average VFA (mgll as COD) and alkalinity (mg/l as CaC03) concentrations and VFA: Alkalinityratio in control and AT digesters for each SRT

---- ..--.--.--.-..-------- .. --- -----····---·--------·-·--·-...,.-,·-----------·-----·T·---·---..- ..- ..---- -----..-.--VFA ! AlkalinitY.... .VFA/Alk _

_. SR..ll~ays),_· "-A-T- ..~Co~troL[AT-Control ...f\T ~<:>..!!~~~I__4 2,000 5,800 I 3,500 3,200 0.6 1.86 400 900 I 5,300 3,800 0.13 0.538 300 800 ! 5,000 3,200 0.06 0.2510 400 700 1 5,200 3,600 0.08 0.212 100 30 i 4,800 3,400 i 0.021 0.00914 30 30 I 5,000 4,600 0.006 0.007

Effect of autothermal treatment on pathogen density

The dual digestion system consistently met Class A pathogen density requirements. In seven samplingevents fecal coliform was not detected in the AT reactor sludge (<10 MPN/g). Fecal coliform density in thecontrol and AT digesters are shown in Figure 2. Fecal coliform density in the AT digester was consistentlybelow the Class A limit and most densities were below the detection limit of 10 MPN/g, except for threesamples taken after an accidental injection of raw sludge into the AT digester. After the raw sludge injection,fecal coliform density dropped to below detection limits in three weeks. The control digester fecal coliformdensity varied from 103 to 106 MPN/g, below the 2 x 106 MPN/g Class B limit and above the Class Abiosolids limit.

IE-009

i-i:I- AT Di......

II. -.-Conlrol

Di....cr

Gass B umt

, ..

------ -.7 "V , --\ ,

.Il "~J~

OassA umt

Ope/lllOr Error

Detection uml (10 MPNl2) ,/

---&-G-O lr-E1 0 G--g

IE.oo7/14196 813196 8nJ196 9/12196 1012196 10122196 11111196 12/1196 12121196

Figure 2. Fecal coliform density and 95% confidence intervals for AT and control digesters (35°e). Values belowthe 10 MPN/g detention limit are also shown.

DISCUSSION

Some volatile solids destruction, presumably due to oxidation and solubilization, occurred in the autothermaltreatment step. Hydrolysis and fermentation in the AT reactor increased the sludge SCaD concentration by2.6 gIl and the VFA concentration by 1.3 gIl. Haner et ai. (1994) observed similar results in a lab scale ATreactor (I-.day SRT, 61°C). Haner et al. reported a 2 gil increase in the dissolved organic carbon (DOC)concentration a.nd a 1 gil increase in VFA concentration. Haner (1994) claimed that particulates arehydrolyzed dUrIng autothermal treatment because autotherrnal microbes release an extracellular enzyme

Autothermal treatment 441

under oxygen limiting conditions. He also claimed that fermentation occurs because oxygen limitingconditions in the AT reactor create micro-anaerobic environments where fermenters can convert SCOD toVFA.

Figure I showed that the dual digestion system VS destruction efficiency was 1.4 percentage points higherthan the control digester VS destruction efficiency for the same total SRT from 4 to 15 days. This increasedVS destruction efficiency is smaller than the 6 percentage point increase in VS destruction efficiencyreported by Pagilla et al. (1996). They compared VS destruction efficiency between a pilot scale dualdigestion system operated at a 14-day SRT to a control mesophilic digester operated at the same total SRT.Our results indicate that the small increase in VS destruction efficiency for the dual digestion system is notdue to improved VS destruction rates in anaerobic digestion following autothermal treatment, but due tomore rapid solids hydrolysis in the autothermal step due to the higher temperature. Pagilla et aI. did notreport the percent VS reduction in their autothermal treatment step, nor the statistical significance of theirhigher VS destruction efficiency difference, so it is not known if their higher VS destruction efficiency in thedual digestion system was due to the autothermal operating condition.

Autothermal treatment did not significantly increase methane production, either. Methane production (m 3/kgVS destroyed) was not statistically different in the AT digester versus the control digester at 6-, 8-, 10-, or12-day SRTs and was only slightly higher at the AT digester than in the control digester at a 14-day SRT(0.7 vs. 0.8 m3/kg VS destroyed).

Autothermal treatment did increase the percent methane in the digester gas. The percent methane wasslightly higher (I to 2 percentage points) in the AT digester gas than the control digester gas at 4-,6-,8- and14-day SRTs. Pagilla et al. (1996) observed similar results in a pilot scale dual digestion system. Pagilla etal. showed that methane production was the same in the control and AT digesters (0.5 m3/kg VS destroyed)at a 14-day SRT at 35°C, but that the percent methane was 4 percentage points higher in the AT digester gasthan the control digester gas, 65 versus 61 %.

The effect of autothermal treatment was significant at a very low mesophilic digester SRT. At a 4-day SRT,the AT digester produced more than twice as much methane (per kg VS destroyed) as the control digesterand the percent methane in the AT digester gas was 9 percentage points higher than in the control digester.Methanogenic bacteria may have started to wash out of the control digester. Two to four days is the acceptedminimum SRT for methanogenic bacteria at 35°C (Lawrence and McCarty, 1969). In the AT digester,methanogenic bacteria may have been able to grow faster, and not get washed out at the 4-day SRT becausethe pH was 0.7 standard units higher in the AT digester than the control digester. The higher pH was likelydue to a higher alkalinity that was produced by ammonia release in the autothermal treatment.

CONCLUSIONS

I. Autothermal treatment did not significantly increase volatile solids destruction kinetics duringsubsequent anaerobic digestion at SRTs of 4 to 14 days. The VS destruction rate was 2.7 to 2.8% perday with or without autothermal treatment.

2. For the same total system SRT, autothermal pre-treatment plus digestion (dual digestion) resulted ina slight increase (1.4 percentage points) in overall % VS destruction, compared to digestion only.

3. Autothermal pre-treatment increased methane production per unit VS destroyed slightly (0.1 m3

CH4/kg VS destroyed) at a 14-day SRT, but did not affect methane production at 6-, 8-, 10- and 12-

day SRTs.

4. Autothermal pre-treatment slightly increased (1 to 2 percentage points) the digester gas percentmethane at 4-, 6-, 8- and 14-day SRTs, but did not affect the percent methane at 10- and 12-day

SRTs.

442 A WARD et al.

5. Higher alkalinity and lower VFA concentration produced a lower VFA to alkalinity ratio in thedigester fed autothermal sludge, indicating that autothermal treatment has the potential to make theAT digester operation more stable, especially at low operating SRTs.

6. Autothermal pre-treatment before anaerobic digestion reduced fecal coliform to below detectionlimits.

REFERENCES

American Public Health Association (1989). Standard Methods for the Examination of Water and Wastewater. 17 th edition. NewYork APHA

Baier, U. and Zweifelhofer, H. (1991). Effects of aerobic thermophilic pretreatment. Water Science and Technology, 23, 56.Haner, A, Mason, C. and Hamer, G. (1994). Death and lysis during aerobic thermophilic sludge treatment: characterization of

recalcitrant products. Water Research, 28, 4.Lawrence, A and McCarty, P. L. (1970). Unified Basis for Biological Treatment Design and Operation. Journal of Sanitary

Engineering, SA 3, 757.Messenger, J. R. and Ekama, G. A. (1993). Evaluation of the dual digestion system, Part 1 Overview of the Milnerton Experience.

Water S.A., 19, 3.PagilIa, K., Craney, K. and Kido, W. (1996). Aerobic thermophilic pretreatment of mixed sludge for pathogen reduction and

Nocardia control. Water Environment Research, 68, 7.USEPA (1992). Technical Support Document for Reduction of Pathogens and Vector Attraction in Sewage Sludge. EPA/822/R•

93-004.Water Environmental Federation (1992). Manual of Practice Number 8 for Design of Municipal Wastewater Treatment Plants,

Vol. I and Vol. 2, Book Press, Brattleboro, VT.Zweifelhofer, H. P. (1985). Aerobic thermophilic/anaerobic mesophilic two stage sewage sludge treatment: Practical experience in

Switzerland. Conservation and Recycling, 8, I.