e:> Pergamon

PH: S0273-1223(97)OO704-X

Wat. Sci. T~c/I. Vol. 36. No. II. pp. 163-170. 1997.C 1997 IAWQ. Pubhshcd by Elsevier ScIence Ltd

Printed in Great Britain.0273-1223197 $17'00 + ()-()O

AN ACTIVATED SLUDGE PROCESSWITHOUT EXCESS SLUDGEPRODUCTION

Yoshio Sakai*, Tetsuro Fukase**, Hidenari Yasui** andMasahide Shibata**

• Japan Sewage Works Agency. 5/41. Shimosasame. 335. Toda•• Kurita Water Industries.Lld. 7-I.Morinosato-Wakamiya. 243-01. Atsugi. Japan

ABSTRACT

An activated sludge process which produces no excess sludge was developed. The process is very simple as asmall amount of return sludge is ozonated and then returned to the aeration tank. The ozonation enhancesbiodegradability of activated sludge. which is biologically oxidized in the aeration tank.

A full-scale plant for treating 45Om3/d of municipal wastewater was constructed and has been operatedsuccessfully for 9 months. The amounl of excess sludge eliminated is directly proponionaJ 10 the amounl ofozone dosed to the sludge. At the ozone dosing rate of 0.034 kgllcg-SS. complete elimination of excesssludge has been achieved when 4 limes more amount of sludge is ozonated than that of the excess sludgeexpected in the treatment without olonation. After 5 months of operation without any withdrawal of excesssludge, small amount of inorganic substances like sand and sill accumulated in the sludge. On the other hand.inen organic substances does not seem to accumulate. As for effluent quality. BOD and nitrogen were kepIgood. Although effluent SS was 2-15 mgll higher compared to a control without ozonation. it has been wellbelow the discharge limit. @ 1997 IAWQ. Pubhshed by Elsevier Science Ltd

KEYWORDS

Activated sludge; excess sludge; full-scale plant; nitrogen; ozone; sewage.

INTRODUCTION

Through the long and continuous history of the activated sludge processes, it has been proved efficient andeconomic ways for the treatment of organic wastewater to remove not only SS and BOD, but also nitrogenand phosphorus. As the result of wide application and utilization of the process, excess sludge productionhas been presenting a serious problem for the operator to dispose of. Owing to the fact that excess sludgeproduction has been considered as an inevitable drawback inherent to the activated sludge processes. and thefailure of sludge minimization in the extended aeration process. many efforts have been devoted on thesludge treatment after withdrawal of excess sludge from the activated sludge processes such as sludgedigestion and dewatering. Recently, incineration andlor sludge melting processes have been taken intoconsideration to solve the problem.

In contrast to the above mentioned counter measures, the authors have found a way of sludge elimination inactivated sludge treatment (Yasui, 1994). The process utilizes ozone such that a small amount of return

163

164 Y. SAKAI ~Ial.

sludge is ozonated and then returned to the aeration tank. The amount of sludge elimination in this process isdependent on the ozone dosing rate and the amount of sludge to be ozonated. From the three year experienceon full-scale pharmaceutical wastewater treatment, the ozone dosing rate must be at least 0.015 kglkg-SSand the amount of return slUdge to be ozonated is about 3 times more than that expected to be withdrawnfrom the process if ozonation stage is not provided for the complete elimination of excess sludge (Yasui,1996).

This report is the result of the application of the sludge elimination process to a full-scale municipalwastewater treatment plant.

MATERIALS AND METHODS

The sludge elimination process is implemented at the Shima sewage treatment center in a central part ofJapan. The Shima center has two oxidation ditch (00) systems and both are operated with intermittentaeration for nitrogen removal. One is provided with an ozone contactor (Ozone train) and the other (Controltrain) is operated without ozone treatment for the comparison of effluent quality and sludge activity. FigureI shows a schematic flow-sheet of the plant.

.. 0W"'''ellmncd,·Mlnu.1ur

rF? IO'ltlnl' L"UutOL'tut U'.unl' ..,"'IN,'r.atur

Figure I. Schematic of~ p1anL

o I:lnll"'~

Flow meIer

The influent sewage does not contain industrial wastewater and comes mostly from nearby hotels andrestaurants. The sewage is at first screened with a fine screen and degritted and then pumped through a flowsplitter into each train.

Table 1. Operational conditions

Ozonc uainrun 1 run 2 run 3

00 volume (m') 8(KI lI(KI IlIKI

Sewalle now rale (m'/day) 420 440 430Return Sludl:e ('ll> of _lie now) 80 75 80Ozone renclOr YIllume (m') 4.5 4.5 4.5Flow rule I007AlRC rCllclor (m'Ilia) 13 211 33

Conlrol Irain

J.(KKI9III

75-80nilnil

The Control train was operated al constant operational condilions and excess sludge was withdrawn to keep the MLSSconcentration at 3,000 mgll. Analyses were made according 10 ns KOI02.

In the Ozone train, a small part of return sludge is pumped to a o:z:one contactor, which have a volume of4.5m3. The waste ozone from the ozone contactor was removed by passing through a serially connectedcatalyst and activated carbon column. Table I shows the operational conditions of the ODs and the ozonereactor. The Ozone train was operated at three different conditions during runs I to 3 in the ozone contactor.In run I, the ozone dosing rate and the amount of sludge fed into the ozone contactor were 0.02 glg-SS and

An ICtivated sludge process 165

200 g-SSlm3-influent sewage, respectively. Those values were increased to 0.04 and 350 for run 2, and 0.04and 510 for run 3. The ozone transfer efficiency was more than 90 % in all runs.

RESULTS AND DISCUSSION

Excess Sludge Produetioll Ozone treatment was staned in May, 1996. The flow rate of influent sewagewas 440 m3/day for Ozone train and 890 m3/day for Control train, so the hydraulic retention time in theactivated sludge tank was 43.8 and 27.0 hrs, respectively. Before operation of the ozone contactor, theexcess sludge production rate of Ozone train was equal to that of Control train in spite of the lower BODloading rate. Excess sludge production is shown in Figure 2. Sludge production in a given time interval wasthe excess slUdge with drawn and corrected for the change The change in MLSS. Effluent SS wa~ notincluded as it was considered negligible.

f 3,(0) I'=~~=;':~::":';;;;;;::;;';';';';';';;~I 1,(0) r--l-7~:;J'C"-t----j~~~~r'~ot~~5!'OOO~~~IO~'OOO~~:lSj.OOO[==20J'0l[JO==25J,roo[L~-:1l~"UOO:::~:J5:"JUl

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f 3,000

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Amc1unl oIUUh:tJ ~C:".;l8C 1m ')

5,000 10,000 15.000 2O,IUI Z5.WO :\(I,IUI J5.IW

MKlUIII u( Irt:elc:d ~t'Wll~ 1m')

~- ,.. '1'- T----~·ll~91sSSlm ~ ".J.~" - 'l• : 08::IM l'li"

~\U sSSlm'}:: 6 '~""'Ir'in I'/ --r

6,(0)5,(0)4,(0)3,(0)2,0001,000

o

o 10,000 2O,(UI :1lI,llll ~,IIIlJ ~I,llll

Amount: oIuealed iCWiCC 1m))

(~I,IUJ 70,lUl

Figure 2. E~cess sludge production during each run. •

166 Y. SAKAI ~t al.

Figure 2 shows cumulative amount of excess sludge production based on the amount of treated sewage.During run I. excess sludge production was about 2/3 of that of Control train. In this period. as ozoneconsumption was 3.6 glm3.sewage and 27 g-SS/m3-sewage was eliminated. 0.133 g-03 was consumed for Ig sludge elimination. From the same calculation. 0.148 and 0.178 goO] were consumed for I g of sludgeelimination in runs 2 and 3. respectively. Excess sludge production was completely eliminated during run 3.when 510 g-SS/mrsewage was fed into the ozone contactor and 0.034 g-Oyg-feed-sludge was consumed.As shown in Figure 3. the amount of sludge eliminated increased with increasing ozone dose.

o 10 UOmne cansumrlinn la·Ot/m). ~ew:l~J

Figure 3. Ozone consumption for sludge elimination.

20

The MLSS concentrations are shown in Figure 4. The MLSS concentrations are almost equal in both trainsbeing kept around 3.000 mgll except in November.

~Runl - 1.- Run: - ~ I Run.1 I -,- IT' ~I' 1_ ",.

1-~- -... V I

,..t- - --I . -0:-o.;;;;I;;~- '1

I " : Conlrol train r J

i s.ooo_4,(01

.5 3,000i2,000

~ 1,000

::I 05t96 M6

Figure 4. MLSS concentrations.

2J'J7

Effluent QUIlIiIy Effluent total nitrogen concentrations and ammonium nitrogen concentrations are shownin Figures 5 and 6. respectively. Although there is no significant deference on ammonium nitrogen. the totalnitrogen concentration from Ozone train is slightly higher. especially in run 3 there the difference to thecontrol was 1.2 mgll. The organic nitrogen was higher because of higher 55 and the nitrate was due toinsufficient operation of the aerators. However. effluent total nitrogen concentration was still low enough. aslow as 4 mgll in the average during run 3. Therefore. it is concluded that ozone treatment does not have anyinhibitory effect on nitrification and denitrification.

.1 30i,~ i---=--~--:"-I+-~lf..--"':""~-utl,

} - 10 1------11-

~ : ----;-tu -- t.~

5196 1/96

Figure S. Result of total nitrogen removal.

An activated sludge process 167

, 10 runl ,un2 ",oJ

r 66

••--!I~ :

5196 llIlJfi II~JIl %197

Figure 6. Effluent NH3 concentrations.

I ZOO"'01 _ • run2_ ,.--

"-

•• 150 )( 'SclC

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~)(li'tOO --'\clC'lCitc'Scic lCM

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i50

0

! 5196 lII96 11196 2197

Figure 7. Total BOD concentrations.

Result of BOD analysis is shown in Figure 7. The influent BOD concentration varied in 60 mgll to 185 mgll.Although BOD removal was fairly good. the effluent from Ozone train was slightly higher during run 3.Because soluble BOD removal was almost the same in two trains, the increase of effluent SS as shown inFigure 8 is thought to be responsible for the BOD deterioralion. Especially. during the end of Decemberthrough mid January. effluent SS increased to 23 mgll as a maximum. The reason for this suddendeterioration is not clear. Although the effluent SS concentration of Ozone train during run 3 was in therange between 4-23 mgll and about 2-15 mgll higher than that of Control train. it was low enough to keepthe discharge limit.

.,.--",ot nm2 ",,,.1

'--

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11~6 2197

Figure 8. Effluent 55 concenuations.

Total phosphorus concentration in the influent was in the range between 2 and 3.5 mgll. The effluent ofControl train was between 1.5 and 2 mgll. indicating an average of 52 %. On the other hand. the phosphorusremoval in Ozone train was comparatively low. If there is no excess sludge production. phosphorus shouldnot be removed. In fact, during run 3. excess sludge had not been withdrawn and phosphorus was notremoved at all.

AcclUftu1lltion of in.1t mtJI.rlG1s As there is no primary sedimentalis in the treatment plant, small sandparticles and silt in the influent sewage might accumulate in the sludge if excess sludge is not withdrawn.The changes of VSS/SS in the sludge are as shown in Figure 9. The VSS/SS of Control train was relativelystable within the range between 0.86 and 0.89. On the other hand. for Ozone train. it had been 0.87. nearlyequal to Control train just before the operation of the ozone contactor and gradually decreased after

168 Y. SAKAI ~l al.

operation had started. During the 9 month operation, VSS/SS decreased to 0.81, indicating small amount ofinorganic substances accumulated.

1.00

t U.9O;;

! 080 k-....~~~~~~i!:~~~~~~~~!II!l-+ ....~~ 0.70 L.:::::::::.:::::::::::JL--L...-L-_LJ

5196 lI.'J6 11196 2/97

Figure 9. Change of VSS 10 55 ralio.

In the influent sewage, average inorganic 55 was 5.1 mgll. If all of the inorganic SS fed into Ozone train isassumed to be accumulated in the sludge, VSSlSS shall be as low as 0.71. The reason for the differencebetween 0.71 and 0.81 is not clear. However, some part of inorganic compounds are thought to besolubilized and then discharged as a part of soluble component in the effluent. As a result, just about 30 % ofinfluent inorganic SS was found to accumulate in the sludge.

The result of the metal analysis in the sludge is shown in Table 2. The sludge in Ozone train showed highercontent for all the metal species analyzed except Mg. Especially, the content of acid insoluble materials, Fe.and AI was more than double or rather triple. However. the difference of heavy metals, Mg, and Ca wascomparatively small. Those results suggest that increase of inorganic content in the sludge of Ozone trainwas mainly due to the accumulation of sand and silt and that the accumulation of those materials was notsignificant.

Table 2. Result of the metal analysis in the sludge

Ca Mg Fe Al P Add ill,,"'....1cm:,I<.,i,d.

Ozone Itain (ash: t75,UOU) 4,500 t.2OO 11.300 11.700 21.700 45,300

Conlnll Itain (11lI: 11 UlOO) 3,600 1.400 3.800 2.9110 18.3011 111.5110

(Heavy metals) T-H, As Zn T-Cr Cu Cd Pb Ni

Ozoneuain 0.32 18 41U 17 230 2.11 37 IW

ConlnlIltain 0.19 li3 240 10 130 1.6 IS 4.0

unil : mgtkg-SS

In Ozone train, a part of return sludge is ozonated before being returned to the activated sludge basin. Afterozonation, most of the ozonated sludge is not solubilized and still exists in a solid state. Therefore. there is apossibility that some part of ozonated sludge may remain in the sludge as an inen organic material togetherwith biologically stable organic substances in the sewage. In order to clarify the accumulation of inenorganic substances in the sludge, oxygen uptake rate of the sludge (OUR) was measured. Figure 10 showsthe maximum oxygen uptake rate of the sludge measured after the addition of peptone and yeast extract ( I :1)to make 1,000 mgll as TOC. The OUR in Ozone train was a little bit lower than that in Control train. Theaverages were 0.35 and 0.51 g-02"g-VSS/day, indicating 32 % lower in Ozone train. This is in goodagreement with the lower BOD loading of the Ozone train.

An activated sludge process 169

~run) run2 run3

-- - ...... ;' -..... I I e:Oi'.l1Ilclrllill· :--<.1"1>6---

b. : Coulrna In;n~j I> .1 6lil

~~~~~~~t.-I>_lL'tf 'rWi -~I>-.Ql!.- ,-

Ii' ....1' ~ -_ •I

j 1.20tf 1.00

1. ~ 0.80:0 ...c ~ 0.60g"'l: 0.40

§ :§ 0.20E 0.00

~ 5')6 R!'J6 l/'J7

Figure 10. Maximum oxygen uptake rate of sludge.

The endogenous respiration rate of the sludge from Ozone train was also lower as shown in Figure II. If alot of ozonated sludge still remains in the mixed liquor. the endogenous respiration rate will be higher. Thelower BOD loading rate might result in the lower endogenous respiration rate as well as the lower OUR inOzone train.

run2===~~:::==== runJ =::A:~:zr:==~0.20

0.15 •. 1>1> I> - ..... • 1>._-1> I> I> 1>1>1> • ~

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7/96 10/96 1m

Figure II. Endogenous respiralion rale of sludge.

In addition, the OUR values were from 0.21 to 0.4 kg-O/kg-MLSS/day and were high enough to removeinfluent BOD, for the BOD loading rate of Ozone train was 0.04 kglkg-MLSS/day at the maximum.

Therefore, it can be concluded that the accumulation of inert materials in the sludge was negligible. Eventhough some inert organic materials may be present in the sludge, the amount does not seem to increasebecause the difference of the OUR between the two trains had maintained constant for months. Thisphenomenon also indicates that biologically stable substances in sewage have been also decomposedconsistently. Possible explanation of the presence is that the accumulated inert materials are not very easy todecompose biologically, then long retention in the sludge must be required. In other words. the presence ofinert materials in the sludge in some degree might be essential to take balance between decomposition andproduction of the inert materials through sludge ozonation. Much amount of inert materials may accumulateunder short SRT operation.

CONCLUSION

The following conclusion is obtained through 9 month operation of a full-scale activated sludge process inwhich small part of return sludge is ozonated and then returned to the aeration tank.

I. With municipal sewage as a feed there was no excess sludge had to be withdrawn.2. The amount of excess sludge elimination is proportional to the amount of sludge fed into the ozonecontactor under appropriate ozone dosing rate.3. Effluent SS was slightly deteriorated but still satisfactory.4. About 30% of influent inorganic SS accumulated in the sludge. After 9 month operation. VSS/SS of theactivated sludge decreased from 0.87 to 0.81.

170 Y. SAKAI er al.

5. Although a small amount of inen organic materials seemed to be present in the sludge. it had nodetrimental influence on the activity of the sludge. The presence of inen organic materials is considered tobe the result of the balance between biological decomposition and production by the ozonation of the sludge.

REFERENCES

Yasui, H. and Shibata, M. (1994). An innovative approach to reduce excess sludge production in the activated sludge process.Wat. Sci. Tech.. 30(9). 11-20.

Yasui, H.. Nalcarnura, K., Shibata, M and Salcai. Y. (1996). A full-scale operation of a novel activated sludge process withoutexcess sludge production. War. Sci. Tech., 34(3-4). 395-404.