Ind. Eng. Chem. Res. 2008, 47, 4387–4393
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PROCESS DESIGN AND CONTROL Alternate Cycles Process for Municipal WWTPs Upgrading: Ready for Widespread Application? Paolo Battistoni,*,† Francesco Fatone,‡ Emanuela Cola,† and Paolo Pavan§ Institute of Hydraulics and Transportation Infrastructures Engineering Faculty, Marche Polytechnical UniVersity, Via Brecce Bianche, 60131 Ancona, Italy, Department of Science and Technology, UniVersity of Verona, Strada Le Grazie 15, Ca` Vignal 37134 Verona, Italy, and Department of EnVironmental Sciences, UniVersity of Venice “Ca` Foscari”, Calle Larga Santa Marta, Dorsoduro 2137 Venice, Italy
The interest in the biological alternating anoxic/oxic reactors to upgrade existing wastewater treatment plants has been recently renewed, thanks to the use of reliable automatic control systems. To discuss the treatment capabilities of the alternating systems, with particular relation to the municipal WWTPs’ upgrading, this study exploits the experience gained in this field through numerous years of R&D. At first a pilot plant study and then a full scale alternating plant were used to supply experimental data that support the final schematic of methodology. The pilot experimentation was based in the real loading conditions of the italian municipal WWTPs. Real municipal sewage was used to feed the plant in six steady-state periods, applying nitrogen loading rates ranging from 0.03 to 0.1 kg N m-3 d-1. Major problems with the nitrogen removal performances occurred in the case of overaeration of the activated sludge tank, corresponding to influent low-loaded wastewater. The experimental durations per day of the anoxic and oxic phases were in good agreement with a simplified mathematical model, which was validated by full scale data and was finally considered useful for the upgrading design. The maximum treatment capacity of the process in terms of nitrogen loading rates was estimated in the range 0.10-0.16 kg N m-3 d-1 according to the different rates for biological nitrification and denitrification, which influence the oxic and anoxic durations per day. Moreover, the reliability of the control device used for the experimentation was proved through the statistic analyses of the performed cycles, which were in agreement with the actual nitrogen removal performances. Finally, the schematic of methodology shows how easy and consolidated the upgrading of existing wastewater treatment plants by the alternate cycles process could be. Introduction Upgrading of existing wastewater treatment plants (WWTPs) may be necessary for a variety of reasons. In recent years, in consequence of the normative developments, more stringent requirements for nutrients discharge are resulting in a need to upgrade treatment capabilities for large and small WWTPs located in sensitive areas. With particular concern to the Italian scenario, a recent survey by the Italian Environmental Protection Agency1 has pointed out that a large part of the municipal wastewater are treated by a huge number of small municipal WWTPs with (a) treatment capacity under 2000 population equivalent (PE) and (b) operating biological processes for only BOD oxidation and ammonia nitrification. To improve plant nutrient removal efficiency and/or reduce treatment costs, a number of alternatives may be practicable.2 Among the different choices, the alternating oxic/anoxic process with the automatic control of the intermittent aeration on the basis of redox potential, pH and/or dissolved oxygen3–7 may be not only viable but also very effective and easy to implement. The present renewed interest in the alternating systems with respect to a conventional multizone scheme is proved by worldwide * To whom correspondence should be addressed. Tel.: 39 071 2204530. Fax 39 0712204528. E-mail:
[email protected]. † Marche Polytechnical University. ‡ University of Verona. § University of Venice “Ca` Foscari”.
researchers;8,9 from a practical viewpoint, four main advantages may be identified: (1) the high performances in carbon and total nitrogen removal; (2) the possibility for the supervisory control of the treatment system; (3) the power requirements lower than the conventional processes; and (4) the complete recovery of the existing structures.4,5,8,10 A process control system, called alternate cycles process, has been studied and developed by Battistoni et al.,5,10,11 and to date has reached its almost widespread full-scale application, being applied in or proposed for numerous Italian WWTPs with treatment capacities from 700 to 700 000 PE. The technology is therefore mature to outline a methodology for its implementation for the upgrading of existing plants. The objective of this paper is to present experimental results in a way that demonstrates that the alternate cycles system is ready for wide application by an easy-to-implement methodology, especially for upgrading existing municipal WWTPs. In fact, the relation with the real cases is understandable observing the experimental choices, such as process parameters and influent loading, which were based on the present italian scenario concerning municipal wastewater treatment. In particular, at first a pilot scale and then full scale experimental results were used to validate a method to evaluate the maximal nitrogen loading rates (NLRs) treatable in the AC WWTPs. Finally, a schematic of upgrading methodology was outlined as common practice for plants upgrading by the AC process.
10.1021/ie070109g CCC: $40.75 2008 American Chemical Society Published on Web 06/07/2008
4388 Ind. Eng. Chem. Res., Vol. 47, No. 13, 2008
Figure 1. Influent flowrate: typical hourly fluctuation. Table 1. Nitrogen Loading Rates and Per Capita Bioreactor Volume run
loading
1 2 3 4 5 6
high medium-high medium medium-low low diluted (wet weather)
NLR (kg TN m-3 day-1) Vsp (Lreactor/capita) 0.103 0.079 0.062 0.052 0.039 0.028
117 152 192 233 304 434
Table 2. Operating Parameters of the Pilot Experimentation NLR (kg TN m-3 MLSS alk -1 (g L-1) run pH (mg CaCO3 L ) day-1) 1 2 3 4 5 6
7.8 7.7 7.9 7.8 7.9 7.8
440 395 464 436 447 358
0.103 0.079 0.062 0.052 0.039 0.028
4.2 ( 0.9 4.2 ( 1.1 4.0 ( 0.3 3.8 ( 0.4 3.4 ( 0.8 3.6 ( 0.6
MLVSS/ SRT MLSS (day) SVI 0.67 (0.02 0.68 (0.02 0.67 (0.02 0.66 (0.02 0.63 (0.02 0.62 (0.01
16 16 16 15 12 11
149 171 141 158 173 141
Experimental Section Pilot Plant. The bench-scale pilot plant had a basic configuration, commonly used in small WWTPs. It was composed of one activated sludge tank (volume of 23 L), where the AC process was performed, and one secondary clarifier (volume of 5.2 L). Real municipal wastewater was used to feed the plant. Different nitrogen and carbon speciation (soluble and particulate) in the influent were reached according to the following procedure: the raw wastewater was settled for 2 h, and the clarified supernatant and settled solids were then separately collected. Both were stored at 4 °C for no longer than 4 days. Feeding was daily prepared mixing fractions of settled solids and supernatant wastewater according to certain ratios. Also, the feeding flow rate was automatically adjusted hourly in order to study the answer of the alternating treatment plant to loading fluctuations typical of municipal systems (see Figure 1 for a typical profile of the influent flowrate). Process Monitoring and Control: Online and LaboratoryAnalyzed Parameters, Control System Architecture. According to the AC process, the durations of the alternating oxic-anoxic phases are automatically determined by a patented control device,11 which operates an online data processing of DO (dissolved oxygen) and ORP (oxidation-reduction potential) signals. In particular, the ammonia exhaustion during the aerobic phase is identified by a flex point on the DO or ORP profile versus time, whereas the nitrates exhaustion in the anoxic phase is identified by a bending point in the ORP profile versus time, at the end of the denitrification phase. Further, set points for DO and ORP values and the time-out for the length of the aerobic and anoxic phases are set to better manage the process when the bending points are not present on the DO and ORP profiles.12 The protocol for the data processing basically operated according to the following steps: (1) a controller of analogical
signals (Mod µDIC, Chemitec Srl Italy) converted the online analogical DO and ORP to digital; (2) an external personal computer managed the digital signals; (3) the real time DO and ORP values were displayed as database and graphics. The final database is then the input for further software of statistic data processing as described below. The chemical-physical characteristics of the main streams were determined from daily averaged samples according to the Standard Methods,13 and the readily biodegradable COD (chemical oxygen demand) was calculated according to Mamais et al.14 Moreover, the single anoxic-aerobic cycles and, consequently, the reliability of the control device were periodically investigated in detail, by directly analysis of the nitrogen forms in the biological reactor every 10 min and over the two following anoxic-oxic phases. End-Reason of the Cycles. Data-processing software was purposely engineered to evaluate the reliability of the control system. Once the operator selected the target period, the software processed the before mentioned database of the analogical and digital online signals giving basically two types of information: (1) the time lengths of the aerobic and anoxic phases that alternated into the activated sludge tank; (2) the end-reason that led the automatic control device to switch from aerobic to anoxic phase and vice versa. Such end-reasons can be diverse: (a) set point on maximum time, (b) set point on maximum DO or ORP, (c) detection of the ammonia bending point (optimal condition), for the aerobic phase; (A) set point on maximum time, (B) set point on minimum ORP, (C) detection of the nitrates breakpoint (optimal condition), for the anoxic phases. As for the output of the data-processing software, it included (1) average, maximum, and minimum values of the phases time lengths, (2) the proportional times of aerobic and anoxic phases, calculated as percentage on the analyzed period; (3) the endreasons that led to the blowers switching on or off, again calculated as percentage. Nitrogen Mass Balance. Nitrogen mass balance was used to objectively evaluate the treatment potential of the AC process in terms of nitrogen removal from wastewater. The balance was calculated according to eq 1, whereas the nitrification and denitrification performances were studied according to four parameters: the nitrifying efficiency referring to the total incoming nitrogen (En, in eq 2) and to the amount of the only form of nitrogen that can be nitrified (Enn, in eq 3); the nitrogen removal efficiency referring either to the total incoming nitrogen (Ed, in eq 4) or to the nitrified nitrogen, NOx-N (Edd, in eq 5). LTNden)LTNin-LTNqw-LTNout
(1)
where LTNden ) total denitrified nitrogen mass loading (kg/ day); LTNin ) total nitrogen mass loading in the influent (kg/ day); LTNqw ) total nitrogen mass loading in the waste activated sludge (kg/day); and LTNout ) total nitrogen mass loading in the effluent (kg/day). En(%) ) (LTNnit⁄LTNin)100 ) [(LTNden+LNOX-NoutLNOX - Nin)/LTNin]100
(2)
Where LNOx-N out ) NOx-N mass loading in the plant effluent (kg/day) and LNOx-N in ) NOx-N mass loading in the plant influent (kg/day). Enn(%) ) [LTNnit/(LTKNin+LTKNras-LTNqwLNnb org out)]100
(3)
Where LTKNin ) total Kjeldahl nitrogen mass loading in the influent (kg/day); LTKNras ) total Kjeldahl nitrogen mass loading in the return activated sludge (kg/day); and LNnb org out
Ind. Eng. Chem. Res., Vol. 47, No. 13, 2008 4389 Table 3. Chemical Physical Characteristics of Influent Sewage in the Pilot Experimentation run
NLR (kg TN m-3 day-1)
TSS (mg L-1)
COD (mg L-1)
sCOD (mg L-1)
rbCOD (mg L-1)
TKN (mg N L-1)
TN (mg N L1)
TP (mg P L-1)
0.103 0.079 0.062 0.052 0.039 0.028
406 361 364 253 344 195
436 409 482 353 357 268
91 91 109 107 74 59
52 50 68 67 29 22
62 49 54 45 38 20
65 52 58 47 40 28
6.5 5.4 6.4 5.4 4.9 2.0
1 2 3 4 5 6
Table 4. Chemical Physical Characteristics of the Treated Effluent in the Pilot Experimentation rRun 1 2 3 4 5 6
TSS (mg L-1)
COD (mg L-1)
SCOD (mg L-1)
NOx-N (mg L-1)
NH4-N (mg L-1)
TKN (mg N L-1)
TN (mg N L-1)
PO4-P (mg P L-1)
TP (mg P L-1)
2.0 2.0 2.5 2.4 1.5 1.2
33 35 35 33 32 35
27 29 25 24 25 28
6.1 3.5 6.2 5.9 5.0 3.0
5.3 2.4 2.7 2.3 1.1 0.2
8.3 4.5 4.7 4.3 3.8 2.9
14.3 8.0 10.9 10.2 8.8 5.9
2.1 1.6 2.5 2.3 1.6 1.0
3.0 2.2 2.9 2.6 2.0 1.4
) not biodegradable organic nitrogen mass loading in the effluent (kg/day). Ed(%) ) (LTNden/LTNin)100
(4)
Edd(%) ) [LTNden/(LTNden+LNOX-Nout)]100
(5)
Results and Discussion Typical Bioreactor Volumes in Italian Municipal WWTPs. To contextualize the experimental activity with the real Italian scenario of the municipal wastewater treatment, we analyzed selected and representative municipal WWTPs to characterize the suspended growth biological systems and, in particular, to know the typical bioreactor volumes with respect to the plant potentiality. This basic survey pointed out that the major part of the plants for small communities (treatment capacity less than 10 000 PE) adopt the biological process for BOD oxidation and ammonia nitrification and can actually rely on specific reactor volume (Vsp) of about 200 L per capita; larger plants (more than 20 000 PE) adopt a biological predenitrification-nitrification configuration and actually use 100-130 L per capita. Furthermore, in the zones drained by combined sewers systems and/or affected by groundwater infiltration in the collection pipelines, WWTPs might have to cope with diluted wastewaters, and the bioreactor may operate with Vsp up to 350-400 L per capita. To better understand the loading conditions of the municipal WWTPs, we can relate the Vsp to the NLRs according to eq 6, which also includes the per capita daily discharges of pollutants NLR )
LTNin TNU ) V Vsp
respectively, were withdrawn from the same full-scale plant. Also, the operating parameters (Table 2) were chosen in a way to be similar to those commonly adopted for the operation of full-scale systems. In fact, the MLSS content was lowered for diluted and low loaded runs, which are usually observed in relation to long wet hydraulic flows and require faster sludge settling velocities. In parallel, the sludge retention times (SRTs) were 15-16 days for the more loaded periods, 11-12 days over the rest of the experimental runs. However, the SRTs were always sufficient to guarantee the suitable fraction of autotrophic bacteria for the potential complete ammonia nitrification at the actual process temperature. The influent municipal wastewater came from a combined collection system; therefore, the COD/TKN ratio ranged from 6.7 to 9.5 and the readily biodegradable COD (rbCOD) was from 8 to 19% on the total COD (see Table 3 for the main average physical-chemical characteristics).
(6)
Where TNu ) total nitrogen unit loading factor (typical: 12 g TN capita -1 day-1); Vsp ) specific per capita volume of the activated sludge tank (liters per capita); LTNin: load of total nitrogen in the influent (kg TN day-1); V ) reaction volume of the biological tank (L). Understanding the General Behavior of Alternating Anoxic/Oxic Systems: The Pilot Study. In light of the typical NLRs often occurring in municipal WWTPs, in the pilot experimentation, six steady-state periods were carried out by step increases of the NLRs from 0.03 to 0.1 kg N m-3 day-1 (Table 1). The plant was fed always with real municipal sewage adequately mixed, according to the method mentioned in the experimental section. Both raw wastewaters and activated sludge, to continuously feed and initially seed the plant,
Figure 2. (a) Actual nitrification and denitrification efficiencies (Enn and Edd). (b) Overall nitrification and denitrification efficiencies (En and Ed).
4390 Ind. Eng. Chem. Res., Vol. 47, No. 13, 2008
Figure 3. (a) Typical cycles plots during high nitrogen loading. (b) Typical cycles plots during low nitrogen loading.
Figure 4. Oxidation and anoxic phase time length with nitrogen mass loading. Figure 5. Comparison of experimental and theoretical results.
The average effluent characteristics are given in Table 4 and show the impact of different NLRs on the AC process. However, generally, the effluent quality met good quality standards within the experimental range. The removal of biodegradable COD was always quite complete and the effluent sCOD confirmed that basically only the nonbiodegradable organic carbon was effluent. Otherwise effluent NH4-N was satisfying for NLRs up to 0.079 and worsened sensitively when this loading was increased to 0.103. Besides the influent characteristics, obviously the nitrogen removal performances depended on: (1) the actual nitrification Table 5. Ranges for Nitrification and Denitrification Rate in the Full Scale Plant
avg min max SD
Kn (kg NH4-N/kg MLVSS d)
Kd (kg NH4-N/kg MLVSS d)
0.10 0.03 0.18 0.02
0.03 0.02 0.08 0.02
and denitrification rates, (2) the durations of aerobic and anoxic phases. The obtained nitrogen removal performances are shown in panels a and b in Figure 2. Within the range of NLRs studied, three different conditions can be pointed out: (1) from 0.02 to 0.04 kg N m-3 day-1, where overaeration occurred, influencing the denitrification; (2) from 0.04 to 0.08 kg N m-3 day-1, where the maximal performances are reached; (3) from 0.08 to 0.1, where the performances are decreasing, but are still suitable to reach the high-quality standard of the effluent. The impact of low- and high-influent loadings is easily understandable even after a quick examination of the online patterns of DO and ORP. Panels a and b in Figure 3 show the typical profiles for medium-high and low-diluted strength wastewaters, respectively. The durations of the aerobic-anoxic phases were heavily influenced by the overaeration phenomena. The air supply was unchanged run by run, and the number of cycles per day was
Ind. Eng. Chem. Res., Vol. 47, No. 13, 2008 4391
Figure 6. Durations of the aerobic and anoxic phases in a full-scale AC WWTP.
outline the relationship between oxic and anoxic phases durations and NLRs. Generally, the system had longer denitrification times with respect to the nitrification (Figure 4). Nitrification in the range 300-350 min day-1 is required for NLRs higher than 0.05 kg TN m-3 day-1, whereas low loadings (0.028 kg TN m-3day-1) correspond to minimum nitrification and maximum denitrification time. Figure 4 confirms the behavior of the AC process already observed by the DO and ORP curves (panels a and b in Figure 3). Starting from low NLRs, the lower loadings can well-represent the hydraulic overloading of wet weather conditions, when overaeration reduces the rbCOD that should be used for the following anoxic biological denitrification. Depending on the nitrification and denitrification rates, the durations of the anoxic and oxic periods reached a plateau for NLRs higher than 0.05 kgTN m-3 day-1. This is a fundamental aspect at the basis of plant up-grading desing. In fact, once known, the durations of the aerobic and anoxic phases reached in the plateau of the curves, the NLRs can be fixed depending on the required standard for the treated effluent and the process rates. Therefore, because the existing plants can be analyzed to determine the actual nitrification rate, eq 7 gives the NLR that can effectively be treated by the alternate system NLRtreatable ) Kntaerobic_plateauMLVSS
Figure 7. Denitrification efficiency (Edd) and “end-reason” for the anoxic phases.
Figure 8. Schematic of activities to upgrade municipal WWTPs through the alternate cycles process.
generally guided by the denitrification rates. Therefore, about 17-19 cycles per day were performed with high loadings; on the contrary, no more than 7-8 cycles were observed for low NLRs, when an important source of biodegradable carbon was actually wasted during the overaeration in the aerobic phase. With concern to the number of cycles per day, one should observe that the number of blowers’ ON-OFF (or mixers’ OFF-ON) is not critical for these machines. The analysis of all the cycles are used to generalize the impact of the influent loadings on the alternating system and let one
(7) -3
-1
Where NLRtreatable ) nitrogen loading rate (kg N m day ), which can be biologically removed in the alternating reactor; Kn ) nitrification rate (kg N (kg MLVSS)-1 day-1); taerobic_plateau ) maximal aerobic time per day, reached in the plateau of the profile “aerobic times” vs “NLRs”; MLVSS ) biomass concentration (kg m-3). Of course, the taerobic_plateau cannot be determined always by pilot test. So, in the following part of the paper, a simplified mathematical model5 is validated using both pilot and full-scale results. Once validated, this may be considered a good tool to know if the alternate cycles process can be applied to upgrade existing municipal WWTPs. Validation of the Simplified Mathematical Model through Pilot Plant Results. Once known the general behavior of the alternating system through the pilot experimentation, the experimental durations of the oxic and anoxic phases were compared with the theoretical values coming from the simplified mathematical model (eqs 8 and 9) proposed by Battistoni et al.5 Here the nitrification rate is constant, whereas the denitrification one, always slower than the nitrification, changes for different runs according to the carbon availability and its biodegradability degree. tn )
QNH4,in tc V KnX
(8)
td )
QNH4,in tc V KdX
(9)
Where Q ) influent flow-rate (m3 day-1); Kd ) maximal denitrification constant (day-1); NH4,in ) ammonia concentration (mg L-1); Kn ) maximal nitrification constant (day-1); X ) mixed liquor suspended solids (mg L-1); V ) volume of the biological tank (m3); and tc ) time length of the cycle anoxic plus oxic phase (day). To validate the simplified model, we measured the maximal nitrification and denitrification rates daily as maximal specific utilization rates of ammonia and nitrate by respirometry tests according to Kristensen et al.15 (Table 5).
4392 Ind. Eng. Chem. Res., Vol. 47, No. 13, 2008 Table 6. Typical Initial Set-Points in AC Plants plant under-loaded MAX DO aerobic phase ORP anoxic phase time length
plant overloaded MIN
MAX
6-7 mg/L (reason: oxygen 0.3-1 mg/L 3-4 mg/L(reason: these values saturation levels may be can be reached only at night reached before the ammonia time, when the loading is exhaustion) lower) 100-250 mV (reason: the -50 to -150 mV (reason: the 0-100 mV ammonia breakpoint may nitrate knee may occur in a occur in a wide range of ORP) wide range of ORP, but the cycles are rather high) according to actual nitrification and denitrification rates
Applying the simplified mathematical model, we found a good agreement between the experimental and theoretical results (Figure 5). The better agreement with the model was found out for the aerobic phases, whereas the anoxic had greater discrepancy especially in case of low loadings and consequent overaerated conditions and consequent decrease in the C:N ratio for the heterotrophic denitrification. This phenomenon was due to the imprecision of applying always the maximal denitrification constant rate, even when the overaeration caused a shortage of readily biodegradable carbon for the denitrifying biomasses. On the other hand, the nitrification rate was rather invariant and well-corresponded to that measured by the respirometry tests. Therefore, one can state that once the nitrification rate is known, through simple ammonia utilization rate (AUR) tests, and the duration of the aerobic phase, the simplified model could be used to determine the NLRtreatable. However, the real implementation of the alternate cycles process requires safety factors. For instance, in this study, the proposed design strategy would lead us to choose a maximum NLR of 0.1 kg TN m-3 day-1, corresponding to 117 Lreactor per capita. However, the result from the model should be adjusted by a safety factor ranging between 1.3 and 1.5, in order to (1) provide the AC plant of the sufficient treatment capacity also in case of seasonal and/or irregular overloadings, (2) not operate with too large Vsp, so to avoid overaeration phenomena. This specification means that a plant can be appropriately upgraded by the AC process if the available bioreactor volume is (1) higher than the value calculated through the simplified mathematical model; (2) not in excess of 130-150% of the Vsp calculated through the actual nitrogen removal rates. However, although the first case requires additional reaction volume to perform the alternating process, the second requires only the adjustment of the activated sludge aeration with respect to the influent loadings, for instance, providing the blowers with adequately programmed frequency regulators. Validation of the Simplified Mathematical Model on a Full-Scale Case Study. Because denitrification rates may change depending on different availabilities of carbon substrate and its degrees of biodegradability, aerobic and anoxic times can change accordingly. As a result, the taerobic_plateau might increase and higher NLRs might be adopted for the upgrading design. The selected, representative, full-scale upgrading is the Viareggio municipal WWTP,16 where the nitrification and denitrification rates were 0.04 and 0.08 kg N (kg MLVSS)-1 day-1, respectively, and involved the durations of aerobic and anoxic phases shown in Figure 6. The aerobic time per day in the full-scale case study was about 1100 min and allowed a design NLR of 0.15 kg N m-3 day-1, corresponding to a Vsp of about 80 L per capita. This nitrogen loading has been really treated successfully from the full-scale plant and can clearly confirm the theoretical calculations.
MIN 0.3-1 mg/L
∼-200 mV (reason: the denitrification can be very fast and low ORP may occur before nitrate exhaustion)
In light of the before discussed validations, one can say that the simplified model can be used if the following three assumptions are valid: (1) the influent C/N ratio is higher than 6-7; (2) the maximal aerobic time, resulting from the model, is 1000-1100 min per day; (3) the maximal nitrification and denitrification rates are reliable and refer to the activated sludge and the actual sewage characteristics. Reliability of the Control Device. The diagnosis of the control system reliability was done analyzing all the cycles performed and interpreting these results in parallel with the nitrogen removal performances, found by means of the nitrogen mass balance. In particular, because the denitrification performances were very unstable during the experimentation, the statistic data of the anoxic phases are used to evaluate the reliability of the control device. Figure 7 shows the before mentioned “end-reasons” together with the actual denitrification efficiencies. Figure 7 shows that the higher is the optimal condition for the phase-changing, the higher are the denitrification performances. In particular, owing to overaeration phenomena the process was less efficient for lower NLRs, where the anoxic times were not sufficient to complete the denitrification. Therefore it can be observed that in the case of excess of activated sludge aeration, the performances of total nitrogen removal are severely affected. On the other hand, too high NLRs may involve unacceptable effluent ammonia. Methodology for the up-Grading. In the light of the experimental results before presented and to the experience gained by the wide application of the AC process, it is possible to outline a general methodology to upgrade the municipal WWTPs through this technology. Figure 8 shows a schematic of the upgrading procedure which is following commented point by point: (1) WWTP “desktop” analysis: This analysis basically consists of a review of the historical data in order to identify the problem processes and allow the evaluator to target the field testing. (2) WWTP “field” analysis: In addition to the test emerging from the “desktop” analyses, the actual nitrification and denitrification rates are determined by simple respirometry test. (3) Identification of WWTP upgrade needs: Once the plant capacity is known by the preliminary analyses, the upgrade needs and priorities are itemized. (4) Simulation of alternating processes in field conditions: Using the actual nitrification and denitrification rates, the simplified mathematical model is used to determine the NLRtreatable. Also, a consolidated simulation tool like the ASIM2d is used to know the possible behavior of the intermittent process in the existing plant. At this step, a lot of attention is paid to the reactor configuration, which is set as a series of CSTRs, depending on the geometry of the activated sludge tank (squared,
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rectangular, circular, or annular) and the location of the submerged mixers. (5) Modification of the WWTP: If the Vsp and the aeration systems are adequate, minor modifications of the existing structures are necessary. Besides the installation of the submerged mixers to keep the activated sludge suspended during anoxic phases, usually only works aimed at the optimal distribution of the flows are necessary (i.e., flow dividers, baffling, etc.). Moreover, modifications of the electric switchboard are required to have the appropriate interface with the AC control device. If not already present, it is a good practice to install a frequency regulator to control the aeration systems. (6) Installation of the control instrumentation and initial system setting: a couple of DO-ORP sensors are necessary in each CSTR. The initial DO and ORP set-points are usually set on the basis of the influent characteristics and their relation to the biological process (see Table 6 for common initial ranges). (7) Definitive assessment of the alternate cycles control system: the initial setting parameters for the control system are adjusted according to the first trial operation. By this action, the system is definitely set in relation to the actual loading and process conditions. (8) Installation of the plantwide control system: once the AC process is routinely operating, a software for the whole plant monitor and control is usually installed. This allows one to reach the supervisory control and manage a network of AC WWTPs. Conclusions The study allows for the main following remarks: (1) In light of the present actual loading conditions for the italian municipal wastewater treatment plants, the alternating anoxic-oxic process has the potential to upgrade a huge number of existing plants and effectively reduce the total nitrogen content in the final discharge to the water bodies. (2) The calculation of the maximal nitrogen loading rate treatable in alternating oxic-anoxic WWTPs is reliable using the actual nitrification rate as input data and a simplified mathematical model as calculation tool. (3) Depending on the nitrification and denitrification rates, a volume of 80-÷120 L of reactor per capita was found suitable to perform nitrogen removals high enough to meet the European standard for discharge in sensitive water bodies; (4)the flexibility of the process enables to face the possible seasonal low loaded (diluted) conditions, which are common in case of combined sewers systems; (5) The statistical analysis of the cycles is a good tool to have a quick, but sufficient, online and remote knowledge of the process behavior and, indirectly, of the influent main characteristics. (6) The present state of development of the alternate cycles technology allows for its safe application for the upgrading of numerous municipal wastewater treatment plants according to a simple and consolidated procedure.
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ReceiVed for reView January 17, 2007 ReVised manuscript receiVed April 16, 2008 Accepted April 16, 2008 IE070109G
