Switching small WWTPs from extended to intermittent aeration

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					  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
                                                                                                           Emanuela Cola
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: fatone@sci.univr.it;
in the critical winter period, the decrease in the filamentous bacteria total number, a net
                                                                                                           Giacomo Carletti*
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: p.battistoni@univpm.it;
Key words     | alternate cycle processes, filamentous, gravity flux, O&M costs, removal efficiencies                 e.cola@univpm.it
                                                                                                            Interuniversity Consortium “Chemistry for the
                                                                                                               Environment” (INCA),
                                                                                                           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

doi: 10.2166/wst.2008.333
 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
                                                                               21           21
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
                                                         month    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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