865 Q IWA Publishing 2008 Water Science & Technology—WST | 58.4 | 2008
Switching small WWTPs from extended to intermittent
aeration: process behaviour and performances
Anna Laura Eusebi, Giacomo Carletti, Emanuela Cola, Francesco Fatone
and Paolo Battistoni
The paper presents the one year results obtained by a small wastewater treatment plant (WWTP) Anna Laura Eusebi
located in a decentralized area. The plant was organized, reusing current structures, introducing Paolo Battistoni
Department of Science and Technology,
an equalization tank, an anoxic selector and the alternate cycles (AC) as technology in the University of Verona, Strada Le Grazie 15,
biological process. The experimentation data processed show the excellent quality of the efﬂuent `
Ca Vignal, 37134 Verona,
with high removal efﬁcencies for all the macropollutants. Also, the anoxic selector effect allows, E-mail: email@example.com;
in the critical winter period, the decrease in the ﬁlamentous bacteria total number, a net
improvement of the settling behaviour with an average solid maximum ﬂux value of Francesco Fatone
3.8 kgMLSSm22 h21 and a good mixed liquor settleability. The alternate cycles process ﬂexibility Institute of Hydraulics and Road Infrastructures,
Marche Polytechnic University,
consents to well manage the high ﬂuctuations of the inﬂuent loadings. The costs comparison of Via Brecce Bianche, 60100 Ancona,
AC process and extended aeration conﬁrms the sustainability of the upgrading. E-mail: firstname.lastname@example.org;
Key words | alternate cycle processes, ﬁlamentous, gravity ﬂux, O&M costs, removal efﬁciencies email@example.com
Interuniversity Consortium “Chemistry for the
via delle Industrie 21/8 Marghera, Venice,
A recent survey (APAT 2005) has been carried out to analyze maintenance (O&M); (2) low capital and operating costs; (3)
the Italian scenario concerning the municipal wastewater enhanced nutrient removal capacity.
treatment. This document reports that 15,000 WWTPs are One of these viable technologies is the alternate cycles
operating in Italy, being able to treat more than 70% of the process, adopted in numerous WWTPs, which beneﬁts
civil sewage wastewaters, and that the majority has design achieve convenience and reliability have been widely
treatment capacity below 2,000 PE. Today many water public demonstrated (Battistoni et al. 2000, 2008). However there
companies would divest these small plants and collect the are still unknowns aspects such as the impact on the
wastewater to large centralized facilities. However, one process behaviour (i.e. sludge settleability, best equalization
should consider the difﬁculties connected with this policy strategies, effect of the anoxic selector) (Olsson et al. 2005).
(i.e. high investment cost to sewers construction for long In this paper will be presented the results of a case study
distance, mountain and rural zones, small towns spread on made in a small plant, located in a decentralized area in the
the hills) and the advantages produce by the innovative central Italy, with treatment capacity of 2,500 –3,000
technologies. Infact, the small plants could rely on con- population equivalent (PE). The most suitable ﬂow-scheme,
solidated technologies that allow (1) easy operation and the sludge settling properties according to different
866 A. L. Eusebi et al. | Switching small WWTPs from extended to intermittent aeration Water Science & Technology—WST | 58.4 | 2008
equalization solutions and the process removal efﬁciencies The alternate cycles process
will be evalueted in one year experience. Finally the
As for the process engineering, the most important action was
industrial sustainability of the system will be evaluated on
the adoption of the alternate cycles process. This is an
the basis of the involved costs.
automatically controlled biological process where the inter-
mittent aeration of the suspended activated sludge bioreactor
is managed on the basis of on-line signals of dissolved oxygen
MATERIAL AND METHODS (DO) and oxidation reduction potential (ORP), which are
The plant processed in real time by a patented control device. In
particular the ammonia disappering during the aerobic phase
The present case study comes from a retroﬁtting of an is identiﬁed by an optimal condition on the DO or ORP proﬁle
existing plant. In the original project phase the plant was versus time, and nitrates disappearing in the anoxic phase is
designed with a potentiality of 3,300 PE. The previous ﬂow identiﬁed by the ORP proﬁle versus time at the end of the
scheme deﬁned a biological process operating the extended denitriﬁcation phase. Further, set points for DO and ORP
aeration only for carbon while ammonia oxidation is values and the time-out for the length of the aerobic and anoxic
obtained according to enviromental condition. Instead, phases are adopted to avoid over aeration to better manage the
the new conﬁguration provided the adoption of AC process process (Paul et al. 1998; Battistoni & Chemitec 1999).
to carbon and nitrogen remove, a new secondary clariﬁer,
to assure the hydraulic ﬂexibility, and the conversion of the
old longitudinal clariﬁer in an anoxic selector, an equaliza-
Sludge settling characteristics
tion basin, and a pre-thickener (Table 1). The equalization
basin, that can operate both in- and off-line mode, was The WWTPs in decentralized areas may involve critical
suitable to face the overloads and was used, also, from the conditions for the sludge settling properties. Here, in fact, low
local farmers to dispose low quantities of liquid agro-wastes temperatures of the liquor can enhance the growth of
to be treated. ﬁlamentous organisms in the activated sludge. In this case
The resulting block ﬂow diagram, in actual condition, is study, the metabolic-based anoxic selector was applied as
shown in Figure 1. Furthermore, the ﬂow-scheme (Figure 1) simple and effective solution before the alternating reactor.
was upgraded to meet the needs of the decentralized area The real effect of this tank was investigated according to two
where the plant is located and to recovery of existing plants conﬁgurations: (1) in-line equalization and anoxic
structures. selector; (2) off-line equalization and without anoxic selector.
Table 1 | Basins and units dimensions in previous and actual condition
Previous condition Actual condition
Equalization basin Volume (m ) – 150
Anoxic selector Volume (m ) – 15
Type process Total oxidation Alternate cycles
Volume (m3) 480 480
Submerged aerator (n8) 2 3
Submerged aerator (kWtot) 22 38
Mixer (n8) – 2
Biological reactor Mixer (kWtot) – 8.2
Geometric characteristic Rectangular Circle
Secondary clariﬁer Volume (m3) 238 270
Pre-thickener Volume (m3) – 62,5
867 A. L. Eusebi et al. | Switching small WWTPs from extended to intermittent aeration Water Science & Technology—WST | 58.4 | 2008
Figure 1 | Block ﬂow diagram after the up-grading.
The gravity ﬂux curve was periodically determined and the ﬂowrates, three periods have been identiﬁed. It is possible
maximum value (kgMLSSm22 h21) was taken as index of the to evaluate both the common seasonal ﬂuctuations,
settling characteristics of the activated sludge. The solid ﬂow normally observable in small wastewater systems, and the
curve determination was made in a laboratory cylinder plant behaviour to new industrial discharges.
(volume 2 L) by successive dilutions and readings, to regular The plant treated from 202 to 340 kgCOD per day in the
time intervals, the solid liquid interface (Metcalf & Eddy three different periods (I: WWTP start up; II: dry period
2005). The bacteria type and length were determined by with mass loads peaks; III: long wet period). This range
optical microscope. The ﬁlamentous number was evaluated underlines the incidence of dry and wet season in a small
using the Jenkins method (Jenkins et al. 1993). WWTP. Kjeldahl Nitrogen has a different behaviour during
the year. It increases steadily from 15 to 41 kg N per day and
this is due not only to the seasonal variation but expecially
RESULTS AND DISCUSSION to the new industrial discharges. In accordance with these
conditions, it is possible to observe a population equivalent
Inﬂow, outﬂow and process performances
increasing, both on COD basis and on TKN basis. In dry
The data tracking, during the year, identiﬁed the strong weather periods the COD/TN ratio was 12, but during long
seasonal variation of the inﬂuent characteristics (Table 2). wet periods it decreases up to 7. This phenomenon was due
On the basis of daily values of incoming mass loads and also to the relevant nitrates incoming linked to the
Table 2 | Inﬂuent characterization and operative conditions
Qin m3 d21 Qr Qw L TSS kg d21 L COD L TKN COD TN TKN NH4-N MLSS mg L21 MLVSS % SRT d PE CODbasis PE TKNbasis
I A.V. 747 513 24 196 202 15 12 1.4 3,825 66 34 1,926 1,209
S.D. 228 186 14 124 101 7 715
II A.V. 547 480 16 311 341 24 12 1.4 3,232 63 28 3,250 2,012
S.D. 138 1 13 246 239 10 1,044
III A.V. 782 593 11 206 271 41 7 1.4 2,593 67 22 2,578 3,437
S.D. 275 172 7 128 110 21 971
A.V.: Average Value; S.D.: Standard Deviation.
868 A. L. Eusebi et al. | Switching small WWTPs from extended to intermittent aeration Water Science & Technology—WST | 58.4 | 2008
inﬁltration into the sewers system of rainfall runoff through The positive effect of the anoxic selector use (conﬁgur-
the cultivated ﬁelds. During the whole period, biological ation 1) was more evident at low temperature (January –
process operative conditions were almost stable, MLSS April) when the plant can suffer more from the enhanced
ﬂuctuated from 3.8 to 2.6 kg m with a volatile solids growth of ﬁlamentous bacteria.
fraction always higher than 60%; the sludge retention time This considerable effect was clear (Figure 3) pointing
decreases from 34 to 22 days during the three periods. out that the average ﬁlamentous total number in the
However the process did not suffer from these conditions conﬁguration 1 (January – February) is 2,200 nummg21
thanks to its ﬂexibility. It was able to guarantee always high against 3,000 nummg21 in the conﬁguration 2 (March –
performances during the dry weather period (II), but also April). The selector achieve was, further, highlighted
during long wet periods (III), the removal efﬁciencies allowed evaluating the temperature increase from conﬁguration 1
to obtaine an efﬂuent that meets the stringent italian law limits. to conﬁguration 2.
The efﬂuent good quality was conﬁrmed also by the The periods with conﬁguration 1 (May – July) and
nitrogen mass balances used to calculate the nitrogen removal conﬁguration 2 (August– September) explained a signiﬁcant
efﬁciencies. The nitriﬁcation and denitriﬁcation performances decrease in the ﬁlamentous total number and length of
were studied according to four parameters: the nitrifying them. The selector inﬂuence became no plain for the
efﬁciency referred to the total incoming nitrogen (En) and to impressive temperature enhance up to 238C.
the amount of the only form of nitrogen that can be nitriﬁed The same results were clariﬁed evaluating the maximum
(Enn); the nitrogen removal efﬁciency referred either to the value of gravity ﬂux in each phase considered (Figure 4),
total incoming nitrogen (Ed) or to the nitriﬁed nitrogen, plotted together with the temperature of the liquor.
NOxZN (Edd). The calculation considers, by analytical As a matter of fact, maximum gravity solid ﬂux
determination, the nitrogen into the wasted activated sludge decreased from 3.8 kgMLSSm22 h21 at average 108C, with
(WAS). The performances were reached in the range of conﬁguration 1, to 2.8 kgMLSSm22 h21 at average 138C,
69%–88% and 67%–77% respectively for the ammonia with conﬁguration 2, showing a meaningful worsening
nitriﬁcation and nitrates denitriﬁcation. A summary of all gravitational behaviour. In the next periods the selector
these evidences is reported in the following Table 3. effect and the temperature increase, added their contri-
Also the percentile analysis, regarding the efﬂuent butions and the maximum gravity solid ﬂux arrived to
nitrogen forms, is reported (Figure 2) in order to better 4.5 kgMLSSm22 h21 at average 198C.
underline effectiveness of the proposed process. Up to 90th
percentile TN was under the law limit, nitrogen ammonia
Validation of the control device
under 3.5 mg L21 and nitrates nitrogen lower than
14 mg L21. The points at 95th percentile are singular events. As showed in the ﬁrst part of the paper, the process control
automation based on the ORP and DO proﬁles could be not
Process behaviour: sludge settling characteristics
reliable when the plant is overloaded or overaerated. In
The main ﬁlamentous species characterization deﬁned the these situations, in fact, the optimal condition (OC) could
bacteria Type 0581 presence of 35%, Type 0092 of 33% and be not detectable. In these cases other two control device
Parvicella of 17%. are expected and can cause the change of the phase: the
Table 3 | Total removal efﬁciencies and nitrogen mass balance
Total removal efﬁciency TN mass balance
TSS% COD% TKN% Ptot% Ntot% Enn% En% Edd% Ed%
I A.V. 86.9 83.0 92.2 79.9 70.3 94.2 69.2 72.4 66.8
II A.V. 97.6 94.4 95.0 75.9 80.6 94.8 88.0 84.0 77.5
II I A.V. 95.0 87.8 95.2 74.8 73.7 94.7 81.0 76.3 72.3
869 A. L. Eusebi et al. | Switching small WWTPs from extended to intermittent aeration Water Science & Technology—WST | 58.4 | 2008
Figure 2 | Percentile analysis for the efﬂuent nitrogen forms.
set-points basis and the time length basis. Also these two the large part of the year. On the contrary the percentage
last control methods allow the ﬂexibility of the process. decreased up to almost 40% during long wet periods when
Considering the high ﬂuctuations of the inﬂuent loadings in the COD/N ratio was too low for the nitrates load increase.
small systems, all the conditions just described can be Simultaneously, the time length of anoxic phase enhanced
observed in Figures 5 and 6. up to 120’, validating the COD lack for the denitriﬁcation
The reliability of the control device was studied one process. Aerobic phase (Figure 6) shows the same beha-
year long through the statistical analysis of 5,943 cycles. viour as the anoxic one. The phases ended always in OCs
Figure 5 shows the percentages of successful detection of (80 –90%) during the dry weather with peak mass loads.
the optimal conditions together the time length of the Instead, in the long wet period the OC was detected in
anoxic phases. The plot of the ﬁgure shows that the OCs lower percentage but the phases were performed in a short
were detectable permanently with a success over 85% for time mainly due to over aeration. In these periods however
Figure 3 | Liquid temperature and total number and length of main ﬁlamentous species.
870 A. L. Eusebi et al. | Switching small WWTPs from extended to intermittent aeration Water Science & Technology—WST | 58.4 | 2008
Figure 4 | Gravity solid ﬂux of the activated sludge and liquor temperature.
the performances of the plants were always guaranteed This practice is used to knock down the O&M costs which
(Table 3) and the respect of the law limits were ﬁrmly must be kept under the low public fee for the municipal
reached. wastewater treatment. Often it is not suitable to meet the
recently required high standard to discharge the treated
water. Nowadays it is possible to consider a reliable fee of
The operation and maintenance costs
0.37 e m23 (about 2.8 e PE21 month21 considering 250 L
In central Italy the small plants are usually controlled on- PE21 d21 of drinking water supplied and discharged).
site from the supervising staff once every 7 – 14 days. This In the case study, the sustainability of the technology has
management is mainly due to the small plants dimensions been evaluated also in terms of O&M costs. More details are
but does not always determine a good efﬂuent quality. given for the energy consumptions, which involved the
Figure 5 | Successful detection of anoxic phase optimal condition.
871 A. L. Eusebi et al. | Switching small WWTPs from extended to intermittent aeration Water Science & Technology—WST | 58.4 | 2008
Figure 6 | Successful detection of aerobic phase optimal condition.
major part of the O&M costs. In particular the overall using a remote on-line supervision and the AC process
installed power was about 90 kW and the speciﬁc energy lends itself excellently (Battistoni et al. 2003a,b).
consumptions were 0.8– 1.2 kWh m23 depending on the
inﬂuent loadings. The distribution of installed power and of
O&M can be split up in to the items shown in Table 4.
On the basis of the real data along one year operation,
the plant had an average treatment capacity of almost 2,600 The paper has presented a detailed overview on a small
PE and the overall O&M costs were 2.9 e PE month . municipal WWTP located in an Italian hilly and decen-
The previous year, with a capacity of almost 2,000 PE the tralized area. The upgrading plant design was done to meet
same costs were 3.05 e PE 21
. Then, one can the local needs. As a general result, both the ﬂow-scheme
observe a light O&M cost decrease, despite the higher and the automatically controlled biological process were
incoming loads. However the cost can be lower installing suitable to reach high standard efﬂuent and sustainable
appropriate air diffusers instead of the actual submerged O&M cost.
aerators. Further economic advantages can be enhanced The off-line equalization can be an optimal solution to
meet the seasonal needs of the local farmers. In fact it can
be used for the disposal of low amount of liquid agro-wastes
Table 4 | Distribution of the O&M costs which are mixed together with the overﬂows and feed the
plant avoiding shock loadings for the activated sludge.
Distribution of installed power % O&M costs %
The effect of the anoxic selector before an intermittent
Sludge dewatering 16
reactor was validated and proved to be of great importance
Head pumping station 5 Process control by analysis 6
especially with low weather temperature.
Screening and degritting 2 Routine maintenance 8
The process control automation allowed high standard
Equalization basin 5 Sludge disposal 12
efﬂuent and sustainable O&M costs.
Secondary clariﬁer 9 Extra-routine maintenance 17
The process control algorithm was reliable also for a
Mixer for the AC tank 9 Energy consumption 57
plant which treated very variable inﬂuent loadings. In the
Blowers for the AC tank 54
rural areas, during wet periods, a ﬂexible system was able to
872 A. L. Eusebi et al. | Switching small WWTPs from extended to intermittent aeration Water Science & Technology—WST | 58.4 | 2008
face, as far as possible, the inﬂuent nitrates derived from the Battistoni, P. & Chemitec, 1999 Metodo e dispositivo di
inﬁltration of rainfall runoff into the cultivated ﬁelds. controllo di un processo di trattamento biologico, a
cicli alternati, di acque reﬂue. Italian Patent No
The alternate cycles process, locally and in remote
control, allows us to know the status of the plant and of the Battistoni, P., De Angelis, A. & Bolzonella, D. 2003a An
efﬂuent quality, representing a real and suitable control for automatically controlled alternate anoxic-oxic process
the decentralized WWTPs. for small municipal wastewater treatment plants. Ind.
Eng. Chem. Res. 42, 509– 515.
Battistoni, P., Boccadoro, R., Bolzonella, D. & Marinelli, M. 2003b
An alternate oxic-anoxic process automatically controlled.
Theory and practice in a real treatment plant network. Water
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