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Treatment of domestic wastes in an aerated lagoon and polishing pond
~,', .ac~ R~*e.grch
Vol. '4, pp .t 3 to 2.9 Pergamon Press 19"5 Printed in Great Brttain.
TREATMENT OF DOMESTIC WASTES IN AN AERATED LAGOON AND POLISHING POND E. BALASttA a n d H . SPERi;I:R
[. Balasha-Z. Jaton. Consuhing Engineers. P.O. Box 1727. Haifa. Israel
(Receiccd 12 September 19741
NOMENCLATURE
Se So
K t X,. s,. a t~
k T (t~ O_, Q ~ a' h'
The study was sponsored by the National Council for Research and the Watcr Commission of the Government.
= soluble effluent BOD ling I- t) = influent BODImgl -t) = overall removal rate coet~cient = detention time {daysl = aeration volatile suspended solids (rag 1- t) =&,s, = sludge yield coefficient [(g VSS produced)/(g BOD removedl] = auto-oxidation rate coefficient (days- t) = BOD removal rateeoetficient(l, mg -t x day) = basin temperature { CI = temperature coclticient = oxygen requirement in the aeration system (kgO: day- t) = Itow rate (m 3 day- t) = volume of the aerated lagoon {m31 = oxygen consumed per unit substrate removed = endogenous rate cocllicient Iday- t l
THE NFITANYATREATMFNT PLANT The Netanya Treatment Plant was designed to treat 2000 m 3 d a y - ~ and it serves a population of 15.000 20.000. The plant consists of an aerated lagoon followed by a polishing pond. both built of earthwork. The aerated lagoon has a volume of 14.000 m 3 and a depth of 4 m, and it is equipped with two surface aerators of 25 h.p. each. The sewage is pumped into the aerated lagoon at a constant rate through inlets located underneath the two aerators. The polishing pond has to improve the etfluent quality and to serve as a reservoir for irrigation. It has a water surface of 0"8 ha and a depth of I m. The etfluent is used for irrigation of grazing crops 8-9 months yr- ~.
INTRODUCTION Sewage treatment in small and medium sized communities in Israel is done at present mostly in stabilization ponds. Duc to increasing population densities, ecologic problems and the scarcity of water in the country, stabilization ponds are no longer in most cases adequate. As a result, aerated logoons are being considered as one of the possible alternative methods. An aeratcd lagoon treatrnent plant has been constructed and put into operation by the town of Netan~a in July 1971. The ptant has been buih to solve a specific problem of one part of the city and also to serve as a prototype, test and study facility for larger phmts to be built throughout the cot, ntry. For one year. starting July 1972. a systematic study has been carried out in the plant. The purpose of this stud.~ was to verify that the process was suitable for the typical local sewage which is of relatively high strength, and if found suitable, to develop design criteria and establish the numerical values of the various coefficients appearing in the process formulae.
METHOD OF" FII.J+D STUDY Four series of tests were made on the plant, each car-
ried out during a different .~ason of the ,,ear so that the ambient temperature was relativel,, constant for the duration of each test. During each test series the flow rate was constant and it was changed from onc series to another, The conditions under which the plant was operated are presented in Table 1. More than 2000 tests and measurements werc done during the study period. 500-600 tests in each series. The parameters monitored were: flow rates, mixed liquor temperature, pH, D.O., BOD, COD, suspended solids, settling characteristic of the mixed liquor. nitrogen compounds. ABS detergents, sulfides and algae count. All figures presented below are bascd on average values of the numerous results obtained. 43
E. BALASHa, and H. Spt R m R
44
Table 1. Operating conditions
Jul? -September
Tc,~t ~crics I1 No,,ember- December
111 Januar.~
Ma~ oh-April
2700
2 I00
270t)
1200
I
Flo~ rate (m ~ day- t l Detention time in aerated lagoon {da~s) Mixed liquor temperature (:C)
5.2
6-8
28
5.2
19
RESULTS A N D DISCI_:SSION
lV
11.5
13
18
tivclv unitorm in each series, having an average of 200300 mg 1- t. These values are higher than those found and cited in literature, usuall? up to 150. This is a resuh of the strong and more concentrated raw influent. Class!hc'ation Q/tht' Ioqooll. Most of the investigators define an aerobic pond as a pond where the mixing intensit.', is sutficicnt to maintain solids in suspension. On the other hand. in a facultative pond. most of the solids settle on the bottom. H e c k ~ d e r [2] presents quantitative relationships between pond ;'olume and power level required to ensure complete mixing, which are between 20 W m -a in a pond having a volume of 500m 3 and down to 1 0 W m -3 in a pond having a volume of 2000m 3. Othcr authors claim that 4 5 W m - 3 is enough to maintain complete mixing. The Nctan.',a Plant has a power input of 2.7 W m -3 in a t4.000 m 3 aerated ktgoon. On the basis of results obtained, this aerated lagoon may be classified as standing somewhere between an aerobic pond and a facultative one although the aerobic character is deftniteb dominant. The fact that the influent inlets arc
Mi.\'in.q in the aerated lagoon Vertical and horizontal profiles of temperature. D.O. and suspended solids were established, as shown in Table 2. Temperature. In each test series the temperature was tbund to be uniform for all of its profiles. Eckenfelder [ 1] states that a completely mixed lagoon will have a uniform temperature profile while a plug-flow type lagoon is characterized by a linear temperature decrease. Dissolrcd oxj',qen. In each test series the D.O. was higher near the aerator and decreased with increasing distance from the aerator in depth and in length. The valt, es of D.O. were always above zero and they increased with the liquor's decreasing temperature, at constant aeration intensity. The effect of detention time on D.O. concentration can.be seen from series IV when the detention time was I 1.5 and the D.O. above 4"0 mg 1- i. Su.~pe,ded solids. The suspended solids concentrations in the mixed liquor were found to be rela-
Table 2. Profiles in the aerated lagoon Test series II
I
Distance from aerator (m) Depth of sampling (m) Temperature (°C) Dissolved oxygen (mg1-1 ) Total suspended solids (mg 1- t) Volatile suspended solids (mg I- ~)
Ill
3-22
22-35
3-22
22-35
0-15-4.0 28
0-15-4-0 28
0.1_5-4.0 19
2-0-0-1
0-4-0-1
2.4-1-0
1.3-1-I
215-296
248-265
254-2870*
190--244 210-224
203-2190"
3-22
0-15--4.0 0-15-4-0 19 13
IV 22-35
3-22
22-35
0-15-4-0 13
0.15-4.0 18
0-15-4-0 18
3.3-19
2.0-1-9
5.2-4.0
4.0-4-0
127-485
250-16000*
250-750
15(~5600"
t60-165
104-285
230-11300*
210-380
115-4030"
127-142
* Only in isolated spots and with a thickness not exceeding 15 cm.
Treatment of domestic wastes
45
Table 3. BOD removal efficiency Test series Total BOD in Influent (mg 1- ~t Soluble BOD in Effluent (mg I- ~) Percentage of removal on the soluble BOD basis Total BOD in Etttuent ling 1- ~) Percentage of removal on the total BOD basis BOD in supernatant after 2 h settling (rag 1- 1) Percentage of removal after 2 h- ~ settling
z
IIO
•
I
[1
III
IV
300 8
400 9
423 13
400 6
97". 110
102
98",, 74"~
68" 0
82";,
30
28
56
33
90";
93",,
87',i,
92'!,
Se 1 So = 1 + krX,.t"
t
la
--"
98". 73
64'?0
4
JI
97"0 137
(2)
The average values of K r and k r, relating to soluble eltluent BOD. as found for each test series are presented in Table 4. K r is a function of temperature [-3].
On
K r = K2o'0~r-2°L
70
A plot of the above equation using the least-squares analysis is shown in Fig. 2. The values obtained from these curves are: K,,, = 6-7 d a y - t and k2o = 0-03 I. m g - t day.
|0
S
6
7
|
S
10
11
12
1)
OETENTIQN TIME (daTsl
Fig. t. B O D removaL vs detention time,
The values of the slopes are 1.03 and 1.04, as a resuh 0~ can be established as 1.035.
located underneath the aerators. "probably has a considcrable effect on the existence of this condition.
toe h7 m o
Pe~:lbrmance of the aerated lagoon
-1.40
BOD femoral. The average values of percentage of BOD removal are listed for each test series in Table 3. For the purpose of B O D removal predictions. cur~es showing the relations between B O D removal and detention time in the aerated lagoon are presented in Fig. I. Remm'al rute coeJficients. The basic design formula
/J ] 1IT, L7,1.04 (v-ze] 0.$0
// z , ! ,¢'//
$o -
-TO
I
I
kT
-
~ -5
O.lO 0
5
10
T-ZO[oC]
(1)
1 + Krt
i
/ . / /
-L|O
a rc : Se
(3)
Fig, 2. Determination of the coefficients K:o. k_~oand 0t.
Table 4. Rate coefficients Test series Kr (day- 11 k r (1. rag- t day)
1
I1
I11
IV
9.7 0-037
6-4 0.033
5.6 0.023
5.1 0-028
The rate coefficient. Kr. is a function of temperature [3].
46
E. BALASHAand H. SI,t RUH~ This equation, plotted b~ using the least-squares analysis, is shown graphically in Fig. 4. The values of the coefficients a and /, obtained from this figure are:
I3"C Z~O
,,,]
ZOO
a
0-6
g VSS produced g BOD removed
h = 0"06 day- i
tOO
i
4
i
i
i
1',
;l
|EIENTION
u
TIM| ( d a y s l
Fig. 3. Suspended solids concentration in the mixed liquor ~s detention time. The values of K obtained, fall close to experimental values in other investigations. Fleckseder[2] has reported a value of K.,, as 8"1 on the BOD basis, and 6'2 on the T O C basis. The values of kz, reported in rel~rences [4. 5] vary between 0.02-0.04. The coefficient 0,, according to Eckenl'clder I6]. depends on the kind of treatment and on the suspended solids concentration. For activated sludge and extended aeration the value of this coefficient is near I-0. For a thcultative lagoon where the suspended solids concentration is low, the values vary between 1.065 and 1.085 [7- 10]. Suspemled solids. The concentration of suspended solids as a function of detention time. as found in this study is presented in Fig. 3. The biological suspended solids concentration is expressed by the relationship: aSr X,. = - I +hi
(4)
and after transformation it may be expressed as:
i
s,
- = - h + a x
- - -
t
{tZ
=
,
,
X,.x
,
(5)
t"
e
o
o
/
r
Coctiicient a represents the accumulation of volatile suspended solids in the aerated system and expresses the synthesis of biological solids [6]. The value of a is characteristic of the nature of the sewage. For domestic waste the reported values are between 0.5 and 0.8 l-6. 1 I. 12]. Coet'ficicnt h expresses the cellular autooxidation rate and its values cited in the literature vary between 0"05-0-08 da'~- 1. BOD and s.s. rclurioushil,. The results indicate that the greater part of the BOD in aerated efttuent was in the form of suspended solids, or biomass, and most or the soluble BOD was removed in a short period. The total BOD in the mixed liquor depends on the conccntration of volatile suspended solids as given by equation: B O D , . . , = S,. + cX,,. [6) The average values of c as obtained in this study are presented in Table 5. Settlin~l I,rOlwrtics ql thc mixed liquor. Settling curves of tile total suspended solids to be used in design procedurc were established and shown in Fig. 5. It is clca) that the settling properties are improved as the sludge ages. Ef'l'ect of temperature. It is generally agreed that under certain operational conditions the temperature of the mixed liquor influences the BOD removal efficiency. Bartsch and Randall [13] have stated that a threshoM temperature (14 C)exists, above which, the influence of temperature is not significant. Sawyer [8] prescnts data showing that the temperature has no influence on BOD removal efficiency when the detention time is longer than 4-5 days. In this study a clear correlation between the temperaturc and the BOD removal efficicnc.x was not determined. O.Y.l'qt'lt rcquiremeltt. The oxygen requirements for the BOD metabolism and endogenous respiration of the biomass can bc expressed as:
02 = a'S,O + b'X,Y
(7)
and aftcr translbrmation: I
1.4
.
O2 --- h' + a' - & X, l" X,.t
(8)
-Lilt
Fig. 4. Determination of coefficients a and h.
The equation (8). plotted b,, using the least-squares analysis, is pre~'nted graphically in Fig. 6.
.4.7
T r c a t l ' n e n l o[" d o m c , , t i c v,a-,tcs
Table 5. The dependence of BOD on VSS in the mixed liquor
S
\
Test series
Total elf]uent BODlmgI-~l
(mgl-~l
I II
i l0 102
~ 9
105 164
III Iv
137 73
13 6
229
Imgl ~i
c tcalculatedl
135
o.53
1)-57 0.54 0.50
The ~alues of the coefficient c to be used in the prediction of total BOD. is I].lirf~ constant and has an a~crage of I).54.
Nitro~lcn c(mll~Ottltd.'< The concentr~.ttions o[" nitrogen compounds ,,,,'ere 54-73 mg 1-~ as N. most of them being in the Iorm of ammonia, and organic nitrogen. The presence of nitrates in the cltluent, up to 8 mg las N. testifies that a process of nitrification exists althocigh :.it a low rate. Detcr~lcms. Onlv A BS t5 pc detergents are in rise. arid their concentration in the raw sewage varies from 20 to 26 m g l - ~ . It was found that 65"o of them were removed in the aerated lagoon. Foam was never accumulated in the aeration pond. This phenonlenon may be explained b,, the high S.S. concentration of the mixed liquor.
.~ tOO iO ii
i|
~
sa
-~
4O
~
3a
~,
za
l
litlil
•
:
In
ZO
31
41
SO
|a
II
70
I
....... * .......
I,',-.-~--
II
la|
lflltll
Ill
II l l l l l
Ill Ilililii]
Fig. 5. Settling curves of suspended solids in the mixed liquor. The values received are: d = 0.44
g oxygen constimed g BOD removed
h' = 0.25 d a y As known the coefficients a' and h' are characteristic of the nature of the sewage and are independent of the type of treatment. The values of ~f reported by other investigators for domestic wastes vary between 0.36 and 0.63. while the values b' vary between 0.13 and 0-28 [6, I1.12].
0.4
] _~..= I"
LI
0.l$ , 0.44,
~, t"
U
I i
l 'i
1.4 S, 1.,'~
[dill'il
Fig. 6. Determination of the coefficients a' and h'. W.R. %1 - - D
77'eatmem C[ficiclw.l' ~!f dw I,olishiml poml BOD aml suspemled solids. The polishing pond is 400 m long and it was found that most of the suspended solids settled near tile entrance to the pond. The concentration of st, spcnded solids in the final cltluent is very close to the wdues obtained in the laboratory settling tests after 2 h (see Fig. 5). BOD and S.S. removal in the polishing pond as well as total efficiency of removal in the plant is presented in Table 6. Profih> of I,olishin{l I,oml. A profile of the polishing pond is given in Table 7. Aklae. The dominant algae were Euglena, Phacvs, and Oscillatoria. In oxidation ponds in this country. the dominant algae is usua[l~ Chlordla, which was found in small concentrations in the polishing pond. The value of general algae concentration IASU) were 10.000-20.000 in the polishing pond which is only 20 per cent of concentration encountered in conventional oxidation ponds, treating raw or settled sewage. Dissohed o.vj'Oe~t. The D.O. concentration increases with increasing distance from the inlet to the pond. At the farthest part of the pond. the D.O. concentration reached 20 mg I- ~. These findings verify the fact that no odour nuisance has been monitored for the 2 years of plant existence, in spite of the lhct that sludge or biomass was collected in the pond for over 2 ,,ears. as will be discussed later.
E. BALASHA and H SPIRtWR
48
Table 6. Treatment etticicnc} in the plant Ra~ influent mg I- ~ Total BOD Soluble BOD Total S.S. Volat. S.S.
3(D 423 -25t>344 210-2S0
Acrol,ic lagoon effluent mg 1- ~ "0 removal
Polishing pond e~tlucnt mg I- t % removal
73.137" 6-- 13 172 270 135-230
24--42
~,7 69
90-92
5S $6+ 52 -5;
66- 6s 62-67
75--'7 73 75
08-76 97-98 --
Final cfltucnt ",, removal
* Including BOD of the biomass. + Including Algae. Table 7. Profile of the polishing pond Distance from entrance Ira) Depth of .v~mpling (m) pH Total solids (g I- t) Dissolved oxygen (mg 1- ~)
5 I00 0.20 6-9 28 39 0.2-2.5
100- 200
0.80 0.20 6.9 7.7 S-o 44.5--48-6 0.0,S Nil 5.0--7.3
No sulfides were found in the polishing pond. except at its bottom, near the inlet where the maximum concentration found was 0.4 mg l- I its S. Sludge accumulation. All the sludge accumulated at the first third of the polishing pond. and examination of samples taken a{ this part of the pond show an average sludge concentration of '4.-5",,. Theoretically the amount of solids accumulated in the polishing ponds during the operation period should have been about 170 tons, but measurements actually taken show a lower value of about 40 tons.--It is evident that the solids which settled in the polishing pond undergo some kind of biological digestion. The behavior of the sludge and absence o f o d o u r nuisance lead to the conclusion that the sludge was partially stabilized leaving the aerated lagoon. The additional stabilization of the sludge in the polishing pond took place in the presence ofdissolved oxygen, its can be seen from Table 7. Lack of oxygen and traces of sulfides were found only at the bottom in the inlet section of the pond. The sludge accumulation and its disposal create a problem that should be taken into consideration while designing this kind of plant. In order to find out the adequate method of the sludge disposal more investigations concerning its characteristics and behavior should be carried out. CONCLUSIONS
I. A treatment plant consisting of aerated lagoon and polishing pond was found to be suitable for the treating of domestic sewerage of fairly high strength. Its eMuent is stable and adequate for irrigation purposes. 2. The aerated lagoon having a power input of 2-7 W m -~ had a character close to a fully mixed aero-
0-so 7-2 7-4 7.,~ 122 0. I-3.0
200 300 0.21) 7.9 S.I 0.06 II) 14
I).s/) 7-4 7.6 I)-0S 1.5 4.5
300 400 0.20 7.8 8-2 0.03 17 20
0%0 7.6-7-S 0-05 5.5-9.0
bic basin. It seems that 3-4 W m - 3 would create a fully aerobic pond. in basins having a volume or" above 10.~X)0m 3. In case a definite facultative ktgoon is desired, probably lower values than 2"0 W m -3 should be applied. 3. The relativcly high inlluent BOD concentration--4(X)mg 1-~. results in high S.S. content in the lagoon--250 and above at 5 7 days of aeration. 4. The kinetic coefficients are similar to those found in aerated lagoons operating with higher power input and lower BOD concentration in the influent. 5. The longer the detention period, the lower thc values of B O D and S.S. in the aerated ellluent. Most of the soluble B O D is removed in short aeration periods. Expected BOD and S.S. values can be estimated from the curves presented in Figs. 1 and 3. 6. The total BOD in the lagoon effluent is expressed by the relation S,. + 0-54 .\',. while S,. and X,. can be found using the aboxe mentioned curves or the basic formulas substituting the relevant coefficients found in this study. 7. The settling characteristics of thc iterated effluent were found to be very good and the total BOD of the supernatant is low. 8. The polishing pond contribution to the treatment was in removing the settleable solids and by imparting additional stabilization to the plant ellluent. These are beside the fact that this pond can be used as operational storage for ettluent utili/ation. 9. The sludge accumt, lated in the polishing pond for two years did not release any obnoxious odours or create an} nuisance. Its specific volume decreased and it seems that it undergoes some biological decornposition in addition to physical thickening.
Treatment of domestic wastes 10. It is r e c o m m e n d e d that due to the high solids content in the aerated lagoon effluent more studies and investigations be continued in order to determine the sludge behavior a n d characteristics concerning its ~va~s of disposal. REFERENCES
[I] Eckenfelder W. W. Jr. et al. (1972) A rational design procedure for aerated lagoons treating municipal and industrial wastewaters. Presented at the 6th International Water Pollution Research ConJl. 18-23 June. Pergamon Press. Oxford. [2] FIeckseder H. R. and Malina J. F. Jr. (1970) Performance of aerated lagoon process. Center for Res. in Water Resources. Univ. of Texas. Austin. [3] Eckenfelder W. W. and Ford D. k. 11970) 14~tter Poll,tion and Control: Experimelztal Procedures for Process Design. The Pemberton Press. Texas. [4] Von Der Erode W. (19631.4dt'al~ces it~ Biolo,qical Wasw Treatment. Pergamon Press, New York. [5] Wt, hrmann K. (1954) High rate activated sludge treatment and its relation to stream sanitation. J. War. Pollut. Control Fed. 26, (t }.
49
[6] Eckcnfelder W. V~'.. Jr. 119671 Comparaticc Biolo~lical ~tsre Treatmeslr Dcsiqn. A.S.C.E. Sanit. Eng. Di~. SA6. [7] Breimhurst L. H. (19701 Mimre.~ora 4er~ttt'¢l Po~d Study. Serco. Sanitar? Engineering Labs. Minneapolis. Minnesota. [8] Saw.~er C. N. 11967~ New concepts in aerated lagoon design and operation. In .qdcancc~ iJz ~,tcr Q,aliry {mprot'elllcttt Vol. 1. Lni~ersit~ of Texas. [9] Carpenter W. L.. ~'[ al. 11968) Temperature relationship in aerobic treatment and disposal of pulp and paper wastes. J. l,Vat. Pollut. Co~trol Fed. [10] Mancini J. L. and Barnhart E. L. (19681 Industrial waste treatment in aerated lagoons. J. War. Pollur. Control Fed. Eli] Eekenfelder W. W. Jr. l1970) Water qualit? engineering for practicing engineers. In P~'oli.'~siolzal Eiz~liJlecl'illy Career Det:elopment Series. Barnes & Noble. New York. El2] Eckenfetdcr W. W. Jr. {1966) lmhl~trial ~tt~.l" Polhltioll Comrol. McGraw-Hill. New York. {13] Bartsch E. H. and Randall C. W. 11971) Aerated lagoons--a report on the state of the art. J. 14~tt. Poll, t. Control Fed. 43, 699.
Vol. '4, pp .t 3 to 2.9 Pergamon Press 19"5 Printed in Great Brttain.
TREATMENT OF DOMESTIC WASTES IN AN AERATED LAGOON AND POLISHING POND E. BALASttA a n d H . SPERi;I:R
[. Balasha-Z. Jaton. Consuhing Engineers. P.O. Box 1727. Haifa. Israel
(Receiccd 12 September 19741
NOMENCLATURE
Se So
K t X,. s,. a t~
k T (t~ O_, Q ~ a' h'
The study was sponsored by the National Council for Research and the Watcr Commission of the Government.
= soluble effluent BOD ling I- t) = influent BODImgl -t) = overall removal rate coet~cient = detention time {daysl = aeration volatile suspended solids (rag 1- t) =&,s, = sludge yield coefficient [(g VSS produced)/(g BOD removedl] = auto-oxidation rate coefficient (days- t) = BOD removal rateeoetficient(l, mg -t x day) = basin temperature { CI = temperature coclticient = oxygen requirement in the aeration system (kgO: day- t) = Itow rate (m 3 day- t) = volume of the aerated lagoon {m31 = oxygen consumed per unit substrate removed = endogenous rate cocllicient Iday- t l
THE NFITANYATREATMFNT PLANT The Netanya Treatment Plant was designed to treat 2000 m 3 d a y - ~ and it serves a population of 15.000 20.000. The plant consists of an aerated lagoon followed by a polishing pond. both built of earthwork. The aerated lagoon has a volume of 14.000 m 3 and a depth of 4 m, and it is equipped with two surface aerators of 25 h.p. each. The sewage is pumped into the aerated lagoon at a constant rate through inlets located underneath the two aerators. The polishing pond has to improve the etfluent quality and to serve as a reservoir for irrigation. It has a water surface of 0"8 ha and a depth of I m. The etfluent is used for irrigation of grazing crops 8-9 months yr- ~.
INTRODUCTION Sewage treatment in small and medium sized communities in Israel is done at present mostly in stabilization ponds. Duc to increasing population densities, ecologic problems and the scarcity of water in the country, stabilization ponds are no longer in most cases adequate. As a result, aerated logoons are being considered as one of the possible alternative methods. An aeratcd lagoon treatrnent plant has been constructed and put into operation by the town of Netan~a in July 1971. The ptant has been buih to solve a specific problem of one part of the city and also to serve as a prototype, test and study facility for larger phmts to be built throughout the cot, ntry. For one year. starting July 1972. a systematic study has been carried out in the plant. The purpose of this stud.~ was to verify that the process was suitable for the typical local sewage which is of relatively high strength, and if found suitable, to develop design criteria and establish the numerical values of the various coefficients appearing in the process formulae.
METHOD OF" FII.J+D STUDY Four series of tests were made on the plant, each car-
ried out during a different .~ason of the ,,ear so that the ambient temperature was relativel,, constant for the duration of each test. During each test series the flow rate was constant and it was changed from onc series to another, The conditions under which the plant was operated are presented in Table 1. More than 2000 tests and measurements werc done during the study period. 500-600 tests in each series. The parameters monitored were: flow rates, mixed liquor temperature, pH, D.O., BOD, COD, suspended solids, settling characteristic of the mixed liquor. nitrogen compounds. ABS detergents, sulfides and algae count. All figures presented below are bascd on average values of the numerous results obtained. 43
E. BALASHa, and H. Spt R m R
44
Table 1. Operating conditions
Jul? -September
Tc,~t ~crics I1 No,,ember- December
111 Januar.~
Ma~ oh-April
2700
2 I00
270t)
1200
I
Flo~ rate (m ~ day- t l Detention time in aerated lagoon {da~s) Mixed liquor temperature (:C)
5.2
6-8
28
5.2
19
RESULTS A N D DISCI_:SSION
lV
11.5
13
18
tivclv unitorm in each series, having an average of 200300 mg 1- t. These values are higher than those found and cited in literature, usuall? up to 150. This is a resuh of the strong and more concentrated raw influent. Class!hc'ation Q/tht' Ioqooll. Most of the investigators define an aerobic pond as a pond where the mixing intensit.', is sutficicnt to maintain solids in suspension. On the other hand. in a facultative pond. most of the solids settle on the bottom. H e c k ~ d e r [2] presents quantitative relationships between pond ;'olume and power level required to ensure complete mixing, which are between 20 W m -a in a pond having a volume of 500m 3 and down to 1 0 W m -3 in a pond having a volume of 2000m 3. Othcr authors claim that 4 5 W m - 3 is enough to maintain complete mixing. The Nctan.',a Plant has a power input of 2.7 W m -3 in a t4.000 m 3 aerated ktgoon. On the basis of results obtained, this aerated lagoon may be classified as standing somewhere between an aerobic pond and a facultative one although the aerobic character is deftniteb dominant. The fact that the influent inlets arc
Mi.\'in.q in the aerated lagoon Vertical and horizontal profiles of temperature. D.O. and suspended solids were established, as shown in Table 2. Temperature. In each test series the temperature was tbund to be uniform for all of its profiles. Eckenfelder [ 1] states that a completely mixed lagoon will have a uniform temperature profile while a plug-flow type lagoon is characterized by a linear temperature decrease. Dissolrcd oxj',qen. In each test series the D.O. was higher near the aerator and decreased with increasing distance from the aerator in depth and in length. The valt, es of D.O. were always above zero and they increased with the liquor's decreasing temperature, at constant aeration intensity. The effect of detention time on D.O. concentration can.be seen from series IV when the detention time was I 1.5 and the D.O. above 4"0 mg 1- i. Su.~pe,ded solids. The suspended solids concentrations in the mixed liquor were found to be rela-
Table 2. Profiles in the aerated lagoon Test series II
I
Distance from aerator (m) Depth of sampling (m) Temperature (°C) Dissolved oxygen (mg1-1 ) Total suspended solids (mg 1- t) Volatile suspended solids (mg I- ~)
Ill
3-22
22-35
3-22
22-35
0-15-4.0 28
0-15-4-0 28
0.1_5-4.0 19
2-0-0-1
0-4-0-1
2.4-1-0
1.3-1-I
215-296
248-265
254-2870*
190--244 210-224
203-2190"
3-22
0-15--4.0 0-15-4-0 19 13
IV 22-35
3-22
22-35
0-15-4-0 13
0.15-4.0 18
0-15-4-0 18
3.3-19
2.0-1-9
5.2-4.0
4.0-4-0
127-485
250-16000*
250-750
15(~5600"
t60-165
104-285
230-11300*
210-380
115-4030"
127-142
* Only in isolated spots and with a thickness not exceeding 15 cm.
Treatment of domestic wastes
45
Table 3. BOD removal efficiency Test series Total BOD in Influent (mg 1- ~t Soluble BOD in Effluent (mg I- ~) Percentage of removal on the soluble BOD basis Total BOD in Etttuent ling 1- ~) Percentage of removal on the total BOD basis BOD in supernatant after 2 h settling (rag 1- 1) Percentage of removal after 2 h- ~ settling
z
IIO
•
I
[1
III
IV
300 8
400 9
423 13
400 6
97". 110
102
98",, 74"~
68" 0
82";,
30
28
56
33
90";
93",,
87',i,
92'!,
Se 1 So = 1 + krX,.t"
t
la
--"
98". 73
64'?0
4
JI
97"0 137
(2)
The average values of K r and k r, relating to soluble eltluent BOD. as found for each test series are presented in Table 4. K r is a function of temperature [-3].
On
K r = K2o'0~r-2°L
70
A plot of the above equation using the least-squares analysis is shown in Fig. 2. The values obtained from these curves are: K,,, = 6-7 d a y - t and k2o = 0-03 I. m g - t day.
|0
S
6
7
|
S
10
11
12
1)
OETENTIQN TIME (daTsl
Fig. t. B O D removaL vs detention time,
The values of the slopes are 1.03 and 1.04, as a resuh 0~ can be established as 1.035.
located underneath the aerators. "probably has a considcrable effect on the existence of this condition.
toe h7 m o
Pe~:lbrmance of the aerated lagoon
-1.40
BOD femoral. The average values of percentage of BOD removal are listed for each test series in Table 3. For the purpose of B O D removal predictions. cur~es showing the relations between B O D removal and detention time in the aerated lagoon are presented in Fig. I. Remm'al rute coeJficients. The basic design formula
/J ] 1IT, L7,1.04 (v-ze] 0.$0
// z , ! ,¢'//
$o -
-TO
I
I
kT
-
~ -5
O.lO 0
5
10
T-ZO[oC]
(1)
1 + Krt
i
/ . / /
-L|O
a rc : Se
(3)
Fig, 2. Determination of the coefficients K:o. k_~oand 0t.
Table 4. Rate coefficients Test series Kr (day- 11 k r (1. rag- t day)
1
I1
I11
IV
9.7 0-037
6-4 0.033
5.6 0.023
5.1 0-028
The rate coefficient. Kr. is a function of temperature [3].
46
E. BALASHAand H. SI,t RUH~ This equation, plotted b~ using the least-squares analysis, is shown graphically in Fig. 4. The values of the coefficients a and /, obtained from this figure are:
I3"C Z~O
,,,]
ZOO
a
0-6
g VSS produced g BOD removed
h = 0"06 day- i
tOO
i
4
i
i
i
1',
;l
|EIENTION
u
TIM| ( d a y s l
Fig. 3. Suspended solids concentration in the mixed liquor ~s detention time. The values of K obtained, fall close to experimental values in other investigations. Fleckseder[2] has reported a value of K.,, as 8"1 on the BOD basis, and 6'2 on the T O C basis. The values of kz, reported in rel~rences [4. 5] vary between 0.02-0.04. The coefficient 0,, according to Eckenl'clder I6]. depends on the kind of treatment and on the suspended solids concentration. For activated sludge and extended aeration the value of this coefficient is near I-0. For a thcultative lagoon where the suspended solids concentration is low, the values vary between 1.065 and 1.085 [7- 10]. Suspemled solids. The concentration of suspended solids as a function of detention time. as found in this study is presented in Fig. 3. The biological suspended solids concentration is expressed by the relationship: aSr X,. = - I +hi
(4)
and after transformation it may be expressed as:
i
s,
- = - h + a x
- - -
t
{tZ
=
,
,
X,.x
,
(5)
t"
e
o
o
/
r
Coctiicient a represents the accumulation of volatile suspended solids in the aerated system and expresses the synthesis of biological solids [6]. The value of a is characteristic of the nature of the sewage. For domestic waste the reported values are between 0.5 and 0.8 l-6. 1 I. 12]. Coet'ficicnt h expresses the cellular autooxidation rate and its values cited in the literature vary between 0"05-0-08 da'~- 1. BOD and s.s. rclurioushil,. The results indicate that the greater part of the BOD in aerated efttuent was in the form of suspended solids, or biomass, and most or the soluble BOD was removed in a short period. The total BOD in the mixed liquor depends on the conccntration of volatile suspended solids as given by equation: B O D , . . , = S,. + cX,,. [6) The average values of c as obtained in this study are presented in Table 5. Settlin~l I,rOlwrtics ql thc mixed liquor. Settling curves of tile total suspended solids to be used in design procedurc were established and shown in Fig. 5. It is clca) that the settling properties are improved as the sludge ages. Ef'l'ect of temperature. It is generally agreed that under certain operational conditions the temperature of the mixed liquor influences the BOD removal efficiency. Bartsch and Randall [13] have stated that a threshoM temperature (14 C)exists, above which, the influence of temperature is not significant. Sawyer [8] prescnts data showing that the temperature has no influence on BOD removal efficiency when the detention time is longer than 4-5 days. In this study a clear correlation between the temperaturc and the BOD removal efficicnc.x was not determined. O.Y.l'qt'lt rcquiremeltt. The oxygen requirements for the BOD metabolism and endogenous respiration of the biomass can bc expressed as:
02 = a'S,O + b'X,Y
(7)
and aftcr translbrmation: I
1.4
.
O2 --- h' + a' - & X, l" X,.t
(8)
-Lilt
Fig. 4. Determination of coefficients a and h.
The equation (8). plotted b,, using the least-squares analysis, is pre~'nted graphically in Fig. 6.
.4.7
T r c a t l ' n e n l o[" d o m c , , t i c v,a-,tcs
Table 5. The dependence of BOD on VSS in the mixed liquor
S
\
Test series
Total elf]uent BODlmgI-~l
(mgl-~l
I II
i l0 102
~ 9
105 164
III Iv
137 73
13 6
229
Imgl ~i
c tcalculatedl
135
o.53
1)-57 0.54 0.50
The ~alues of the coefficient c to be used in the prediction of total BOD. is I].lirf~ constant and has an a~crage of I).54.
Nitro~lcn c(mll~Ottltd.'< The concentr~.ttions o[" nitrogen compounds ,,,,'ere 54-73 mg 1-~ as N. most of them being in the Iorm of ammonia, and organic nitrogen. The presence of nitrates in the cltluent, up to 8 mg las N. testifies that a process of nitrification exists althocigh :.it a low rate. Detcr~lcms. Onlv A BS t5 pc detergents are in rise. arid their concentration in the raw sewage varies from 20 to 26 m g l - ~ . It was found that 65"o of them were removed in the aerated lagoon. Foam was never accumulated in the aeration pond. This phenonlenon may be explained b,, the high S.S. concentration of the mixed liquor.
.~ tOO iO ii
i|
~
sa
-~
4O
~
3a
~,
za
l
litlil
•
:
In
ZO
31
41
SO
|a
II
70
I
....... * .......
I,',-.-~--
II
la|
lflltll
Ill
II l l l l l
Ill Ilililii]
Fig. 5. Settling curves of suspended solids in the mixed liquor. The values received are: d = 0.44
g oxygen constimed g BOD removed
h' = 0.25 d a y As known the coefficients a' and h' are characteristic of the nature of the sewage and are independent of the type of treatment. The values of ~f reported by other investigators for domestic wastes vary between 0.36 and 0.63. while the values b' vary between 0.13 and 0-28 [6, I1.12].
0.4
] _~..= I"
LI
0.l$ , 0.44,
~, t"
U
I i
l 'i
1.4 S, 1.,'~
[dill'il
Fig. 6. Determination of the coefficients a' and h'. W.R. %1 - - D
77'eatmem C[ficiclw.l' ~!f dw I,olishiml poml BOD aml suspemled solids. The polishing pond is 400 m long and it was found that most of the suspended solids settled near tile entrance to the pond. The concentration of st, spcnded solids in the final cltluent is very close to the wdues obtained in the laboratory settling tests after 2 h (see Fig. 5). BOD and S.S. removal in the polishing pond as well as total efficiency of removal in the plant is presented in Table 6. Profih> of I,olishin{l I,oml. A profile of the polishing pond is given in Table 7. Aklae. The dominant algae were Euglena, Phacvs, and Oscillatoria. In oxidation ponds in this country. the dominant algae is usua[l~ Chlordla, which was found in small concentrations in the polishing pond. The value of general algae concentration IASU) were 10.000-20.000 in the polishing pond which is only 20 per cent of concentration encountered in conventional oxidation ponds, treating raw or settled sewage. Dissohed o.vj'Oe~t. The D.O. concentration increases with increasing distance from the inlet to the pond. At the farthest part of the pond. the D.O. concentration reached 20 mg I- ~. These findings verify the fact that no odour nuisance has been monitored for the 2 years of plant existence, in spite of the lhct that sludge or biomass was collected in the pond for over 2 ,,ears. as will be discussed later.
E. BALASHA and H SPIRtWR
48
Table 6. Treatment etticicnc} in the plant Ra~ influent mg I- ~ Total BOD Soluble BOD Total S.S. Volat. S.S.
3(D 423 -25t>344 210-2S0
Acrol,ic lagoon effluent mg 1- ~ "0 removal
Polishing pond e~tlucnt mg I- t % removal
73.137" 6-- 13 172 270 135-230
24--42
~,7 69
90-92
5S $6+ 52 -5;
66- 6s 62-67
75--'7 73 75
08-76 97-98 --
Final cfltucnt ",, removal
* Including BOD of the biomass. + Including Algae. Table 7. Profile of the polishing pond Distance from entrance Ira) Depth of .v~mpling (m) pH Total solids (g I- t) Dissolved oxygen (mg 1- ~)
5 I00 0.20 6-9 28 39 0.2-2.5
100- 200
0.80 0.20 6.9 7.7 S-o 44.5--48-6 0.0,S Nil 5.0--7.3
No sulfides were found in the polishing pond. except at its bottom, near the inlet where the maximum concentration found was 0.4 mg l- I its S. Sludge accumulation. All the sludge accumulated at the first third of the polishing pond. and examination of samples taken a{ this part of the pond show an average sludge concentration of '4.-5",,. Theoretically the amount of solids accumulated in the polishing ponds during the operation period should have been about 170 tons, but measurements actually taken show a lower value of about 40 tons.--It is evident that the solids which settled in the polishing pond undergo some kind of biological digestion. The behavior of the sludge and absence o f o d o u r nuisance lead to the conclusion that the sludge was partially stabilized leaving the aerated lagoon. The additional stabilization of the sludge in the polishing pond took place in the presence ofdissolved oxygen, its can be seen from Table 7. Lack of oxygen and traces of sulfides were found only at the bottom in the inlet section of the pond. The sludge accumulation and its disposal create a problem that should be taken into consideration while designing this kind of plant. In order to find out the adequate method of the sludge disposal more investigations concerning its characteristics and behavior should be carried out. CONCLUSIONS
I. A treatment plant consisting of aerated lagoon and polishing pond was found to be suitable for the treating of domestic sewerage of fairly high strength. Its eMuent is stable and adequate for irrigation purposes. 2. The aerated lagoon having a power input of 2-7 W m -~ had a character close to a fully mixed aero-
0-so 7-2 7-4 7.,~ 122 0. I-3.0
200 300 0.21) 7.9 S.I 0.06 II) 14
I).s/) 7-4 7.6 I)-0S 1.5 4.5
300 400 0.20 7.8 8-2 0.03 17 20
0%0 7.6-7-S 0-05 5.5-9.0
bic basin. It seems that 3-4 W m - 3 would create a fully aerobic pond. in basins having a volume or" above 10.~X)0m 3. In case a definite facultative ktgoon is desired, probably lower values than 2"0 W m -3 should be applied. 3. The relativcly high inlluent BOD concentration--4(X)mg 1-~. results in high S.S. content in the lagoon--250 and above at 5 7 days of aeration. 4. The kinetic coefficients are similar to those found in aerated lagoons operating with higher power input and lower BOD concentration in the influent. 5. The longer the detention period, the lower thc values of B O D and S.S. in the aerated ellluent. Most of the soluble B O D is removed in short aeration periods. Expected BOD and S.S. values can be estimated from the curves presented in Figs. 1 and 3. 6. The total BOD in the lagoon effluent is expressed by the relation S,. + 0-54 .\',. while S,. and X,. can be found using the aboxe mentioned curves or the basic formulas substituting the relevant coefficients found in this study. 7. The settling characteristics of thc iterated effluent were found to be very good and the total BOD of the supernatant is low. 8. The polishing pond contribution to the treatment was in removing the settleable solids and by imparting additional stabilization to the plant ellluent. These are beside the fact that this pond can be used as operational storage for ettluent utili/ation. 9. The sludge accumt, lated in the polishing pond for two years did not release any obnoxious odours or create an} nuisance. Its specific volume decreased and it seems that it undergoes some biological decornposition in addition to physical thickening.
Treatment of domestic wastes 10. It is r e c o m m e n d e d that due to the high solids content in the aerated lagoon effluent more studies and investigations be continued in order to determine the sludge behavior a n d characteristics concerning its ~va~s of disposal. REFERENCES
[I] Eckenfelder W. W. Jr. et al. (1972) A rational design procedure for aerated lagoons treating municipal and industrial wastewaters. Presented at the 6th International Water Pollution Research ConJl. 18-23 June. Pergamon Press. Oxford. [2] FIeckseder H. R. and Malina J. F. Jr. (1970) Performance of aerated lagoon process. Center for Res. in Water Resources. Univ. of Texas. Austin. [3] Eckenfelder W. W. and Ford D. k. 11970) 14~tter Poll,tion and Control: Experimelztal Procedures for Process Design. The Pemberton Press. Texas. [4] Von Der Erode W. (19631.4dt'al~ces it~ Biolo,qical Wasw Treatment. Pergamon Press, New York. [5] Wt, hrmann K. (1954) High rate activated sludge treatment and its relation to stream sanitation. J. War. Pollut. Control Fed. 26, (t }.
49
[6] Eckcnfelder W. V~'.. Jr. 119671 Comparaticc Biolo~lical ~tsre Treatmeslr Dcsiqn. A.S.C.E. Sanit. Eng. Di~. SA6. [7] Breimhurst L. H. (19701 Mimre.~ora 4er~ttt'¢l Po~d Study. Serco. Sanitar? Engineering Labs. Minneapolis. Minnesota. [8] Saw.~er C. N. 11967~ New concepts in aerated lagoon design and operation. In .qdcancc~ iJz ~,tcr Q,aliry {mprot'elllcttt Vol. 1. Lni~ersit~ of Texas. [9] Carpenter W. L.. ~'[ al. 11968) Temperature relationship in aerobic treatment and disposal of pulp and paper wastes. J. l,Vat. Pollut. Co~trol Fed. [10] Mancini J. L. and Barnhart E. L. (19681 Industrial waste treatment in aerated lagoons. J. War. Pollur. Control Fed. Eli] Eekenfelder W. W. Jr. l1970) Water qualit? engineering for practicing engineers. In P~'oli.'~siolzal Eiz~liJlecl'illy Career Det:elopment Series. Barnes & Noble. New York. El2] Eckenfetdcr W. W. Jr. {1966) lmhl~trial ~tt~.l" Polhltioll Comrol. McGraw-Hill. New York. {13] Bartsch E. H. and Randall C. W. 11971) Aerated lagoons--a report on the state of the art. J. 14~tt. Poll, t. Control Fed. 43, 699.