Ind. Eng. Chem. Res. 2002, 41, 2805-2811
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Addition of an External Carbon Source To Enhance Nitrogen Biological Removal in the Treatment of Liquid Industrial Wastes Paolo Battistoni,*,† Raffaella Boccadoro,† Laura Innocenti,‡ and David Bolzonella‡ Institute of Hydraulics, University of Ancona, Via Brecce Bianche, I-60131 Ancona, Italy, and Department of Science and Technology, University of Verona, Strada Le Grazie 15, I-37134 Verona, Italy
This paper deals with the optimization of biological nitrogen removal in the treatment of liquid industrial wastes. In particular, the use of an external carbon source in a two-step alternate oxic-anoxic process with separate biomass has been investigated. A 4-month experimental work analyzing both carbon and nitrogen removal and enhancing the latter through acetic acid addition as a second step at the beginning of the anoxic phase was performed. Nitrogen mass balance, cycle analysis, and a typical trend of dissolved oxygen and oxidation reduction potential (ORP) are used as tools to evaluate the success of the method and to understand the exact role of the two steps and the effect of carbon addition. The approach to using a two-step treatment with separate biomass does not reveal satisfactory performances in nitrogen removal if the nitrification is mainly confined to the second step, because enough carbon is not always available. The implementation with an external carbon source allowed a high performance and showed a typical flex point in the ORP trend. The comparison among ORP slopes does not produce any way to estimate the carbon addition: on the other hand, a useful tool for saving on managing cost can be the carbon addition when it is clear that the ORP does not reach a 0 mV level in a prefixed time, after the anoxic phase has started. All N-oxide (NOx-N) concentrations in the effluent have been rationalized in a mass balance for nitrogen providing a prevision of the final effluent quality in relation to the process performances. Introduction The treatment of industrial wastewater requires a proper combination of chemical, physical, and biological processes to avoid the inhibitory, toxic, or recalcitrant effect of heavy metals and organic compounds. The chemical-physical pretreatments have to guarantee the role of finishing the step to the biological treatment.1 In platforms for the treatment of liquid wastes, the situation is particularly complex because of the variability of the amount and characteristics of the treated wastewater. To solve these problems, each treatment step must always fix its particular target; that is, the chemical-physical treatment has to effectively remove the pollutants which can set up the following biological process. This has to reduce the biodegradable carbon compounds and remove the nutrients. The use of a completely stirred equalization basin, as a first step, is generally considered as a minimum measure to get a continuous feeding and a constant concentration of the substrates. Besides the chemical-physical treatments and the flexible organization of the plant,2-4 also the biological aspects have to be considered. Generally, the high salinity of the wastewater has to be controlled to prevent a possible reduction in biological nitrification. This is obtained, for example, by using acidic wastewater to neutralize alkali solutions. Another important aspect to consider in the biological step is the nitrogen high loading. High concentrations of ammonia in the incoming stream of the biological treatment can be controlled * To whom correspondence should be addressed. Tel: +39 071 2204530. Fax: +39 071 2204528. E-mail: idrotre@ popcsi.unian.it. † University of Ancona. ‡ University of Verona.
by struvite (magnesium ammonium phosphate) precipitation.4,5 Among the available biological treatment processes, it was demonstrated that the adoption of a two-step aerobic-anoxic process with separate biomass can be an interesting alternative to retrofitting an existing two-step biological plant managed as an extended aeration process, allowing one to obtain a good performance in carbon and nitrogen removal.6 The startup of the retrofitted plant appeared as the largest removal of carbon and nitrogen occurred in the first step, while the second one simply exerted a finishing function. Despite this evidence, a long-term analysis of a full-scale industrial waste treatment platform confirmed this solution as a successful idea to optimize biological nitrogen removal, though low nitrification and denitrification rates mainly characterize the process.7 As a matter of fact, nitrification was the limiting step in the first tank, while denitrification was limited in the second one, because of the low carbon availability; this can lead to the risk of exceeding nitrogen law standard in the effluent, especially when high nitrogen mass loading entered the plant. In this paper the use of acetic acid as an external carbon source to support the denitrification process and the anoxic conditions in the second step is experimented. Although this is a common solution in activated sludge processes with preor post-denitrification basins, its use in an alternating cycle process is not typical and the carbon source addition protocol must be optimized to reduce reagent costs. Materials and Methods Wastewater Treatment Plant Layout and Description. The platform for the liquid industrial wastes treatment was managed by a private company, the SEA,
10.1021/ie010828+ CCC: $22.00 © 2002 American Chemical Society Published on Web 04/30/2002
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Figure 1. Flow scheme of the biological section of the treatment plant. Table 1. Surface, Volume, and Electromechanical Devices of the Biological Section of the Treatment Plant process incoming flow rate (Qin) equalization basin (EQU) oxidation step (OX1) mixer (M1) secondary settler of OX1 oxidation step (OX2) mixer (M2) secondary settler of OX2 recycling flow rate (Qr)
surface (m2)
volume (m3)
42 57
126 400
flow rate (m3 day-1)
mixer (kW)
blower (kW)
air flow rate (N m3 h-1)
19-32
780-1180
19-32
780-1180
128 3.4 19 100
53 400 3.4
20
50
whose main activity was represented by environmental services for regional and national industries. The industrial liquid wastes mainly came from food, tannery, paint, and galvanic industries. The plant was placed in Ancona (middle Italy), and the applied process was organized according to the flow scheme in Figure 1. A receiving station was used for checking wastes conferred by trucks. Accepted wastes were collected in a stocking station before classification. Two days was the maximum time to ascertain the presence of toxic compounds in the wastes; the maximum daily flow rate was 150 m3 day-1. The completely biodegradable industrial wastes were directly sent to the equalization basin and then to the biological treatment, while the other wastes, which were the main part, were physically chemically pretreated before sending them to the equalization basin. A complete description of the platform layout was presented in work by Battistoni et al.6,7 Biological Process. A two-step biological process with separate biomass operated in the alternate cycles mode was used (Figure 1); the two step sizes (400 m3 each one) guaranteed a high hydraulic retention time (HRT ) 2.5 days) even when operating at the maximum flow rate (160 m3 day-1). The biological treatment reactors were both managed as an alternate cycle process (oxic-anoxic) by turning the aeration system on and off in order to give a cyclic variation of dissolved oxygen in each tank. The oxygen demand for both steps was satisfied by two air blowers (total power of 64 kW), with a third blower (9 kW) as stock equipment. They were able to supply up to 2360 N m3 h-1 of air to fine bubble spargers. The two aeration basins included a settling section (50 m3; see Table 1) which worked with a hydraulic surface loading of 0.3 m h-1. Wasteactivated sludge was gravity-thickened before sending it to the dewatering station; the two blowers were used both to satisfy the oxygen demand and to run some utilities (i.e., sludge recycling). A recycle of the final effluent to the first biological process was provided (Qr ) 100 m3 day-1), even though, in an alternate oxicanoxic process, it was not theoretically necessary. It was
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sometimes used when a high concentration of nitrates was present in the effluent of the second bioreactor. The process was monitored on the basis of the dissolved oxygen concentration (DO) and the oxidation reduction potential (ORP) and pH, conductivity, and temperature values. The aerobic and anoxic phases were time controlled in each step according to a protocol providing 150 min for oxidation and 45 min for denitrification in OX1 and 105 min for oxidation and 180 min for denitrification in OX2; no adjustments were performed during the 4-month experimental period. Acetic acid [80% (w/w) purity] was used as a readily biodegradable chemical oxygen demand (RBCOD) source. It was fed in the OX2 step at a flow rate of 1 L min-1 for 10 min, at the beginning of the anoxic phase, with an amount equivalent to 50 kg of acetic acid day-1 (that is about 53 kg of COD) on the basis of five cycles per day. The addition (53 kg day-1) was the same one calculated (52.5 kg day-1) on the basis of 6 kg day-1 of nitrogen to be completely denitrified supposing a request of 7.5 kg of COD/kg of NO3-N (8.8 kg of commercial acetic acid per kg of NO3-N denitrified) to support the denitrification step.8 Analysis. RBCOD measurements were performed following two methods: the first is the simplified procedure reported by Mamais et al.,9 where the soluble nonbiodegradable COD is measured in the plant effluent as soluble COD; the second is based on aerobic batch tests as described by Ekama et al.10 The wastewater chemical-physical characteristics were measured as described in the Standards Methods.11 The specific uptake rates of ammonia (AUR), nitrates (NUR), and oxygen (OUR) were measured according to Kristensen et al.12 Nitrogen Mass Balance. The nitrogen mass balance was calculated according to eq I, while the nitrification and denitrification performances were studied according to four parameters: the nitrifying efficiency referring to the total incoming nitrogen (Eni in eq II) and to the amount of the only form of nitrogen that can be nitrified (Enni in eq III); the nitrogen removal efficiency referring
Ind. Eng. Chem. Res., Vol. 41, No. 11, 2002 2807 Table 2. Operational Conditions of the Biological Process OX1
OX2
period
Qw (m3 day-1)
T (°C)
SRT (days)
MLSS (mg L-1)
TVS/TSS
T (°C)
SRT (days)
MLSS (mg L-1)
Qw (m3 day-1)
TVS/TSS
Qr (m3 day-1)
Feb ’99 Mar ’99 Apr ’99 May ’99
16 15 20 19
13 17 19 25
23 26 19 20
6533 6850 8903 7230
0.7 0.8 0.7 0.7
12 16 18 25
>300 >300 179 191
3354 3697 3224 3080
0.7 1.3 2.2 2.1
0.6 0.7 0.7 0.7
35 9.9 0.0 36
either to the total incoming nitrogen (Edi in eq IV) or to the nitrified nitrogen, NOx-N (Eddi in eq V).
LNtotden,i ) LNtotin,i - LNqwi - LNtotout,i
(I)
where LNtotden,i ) total denitrified nitrogen mass loading, LNtotin,i ) total nitrogen mass loading in the influent, LNqwi ) total effluent nitrogen mass loading in the wasted biological sludge, LNtotout,i ) total nitrogen mass loading in the effluent, and subscript i ) 1 and 2 for the first and second steps, respectively.
Eni ) (LNtotnit,i/LNtotin,1) × 100
(II)
where Eni ) nitrifying efficiency upon total incoming nitrogen, LNtotnit,i ) total nitrified nitrogen mass loading, LNtotin,1 ) total nitrogen mass loading in the influent to the first reactor, and subscript i ) 1 and 2 for the first and second steps, respectively.
Enni ) [LNtotnit,i/(LTKNin,1 + LTKNqr LNqw1+2 - LNnb org out,2)] × 100 (III) where Enni ) nitrifying efficiency upon the nitrificable incoming nitrogen, LNtotnit,i ) total nitrified nitrogen mass loading, LTKNin,1 ) total Kjeldahl nitrogen mass loading in the influent of the first reactor, LTKNqr ) total Kjeldahl nitrogen mass loading in the recycle flow rate, LNqw1+2 ) total effluent nitrogen mass loading in the wasted biological sludge, LNnb org out,2 ) nonbiodegradable organic nitrogen mass loading in the final effluent, and subscript i ) 1 and 2 for the first and second steps, respectively.
Edi ) (LNtotden,i/LNtotin,1) × 100
(IV)
where Edi ) denitrifying efficiency upon total incoming nitrogen, LNtotden,i ) total denitrified nitrogen mass loading, LNtotin,1 ) total nitrogen mass loading influent to the first reactor, subscript i ) 1 and 2 for the first and second steps, respectively.
Eddi ) [LNtotden,i/(LNtotden,1+2 + LNOxNout)] × 100 (V) where Eddi ) denitrifying efficiency upon the denitrificable nitrogen, LNtotden,i ) total denitrified nitrogen mass loading, LNtotden,1+2 ) total denitrified nitrogen in the first and second reactors, LNOxNout ) NOx-N mass loading in the effluent, subscript i ) 1 and 2 for the first and second steps, respectively. Aerobic-Anoxic Cycle Analysis. The DO and ORP signals, online recorded during the process management, showed the same trend on a weekly basis. In general, their trend was typical of alternating oxicanoxic conditions, but the signal behavior in any cycle was found to be related to the final conditions of the previous one (i.e., high or low DO and ORP at the end of oxidation) and to those of the feed.13,14 For a cycle
analysis, it is necessary to evaluate the process feasibility. The adopted criterion was that the plant configuration could be assessed on the basis of the percentage of the performed cycles with respect to the hypothesized ones and of the percentages of efficient cycles with respect to those performed or hypothesized. Both criteria help to understand whether the cycle time planning can be reliable. According to this approach, a cycle can be considered efficient when the ORP in anoxic conditions reaches negative values, because this means that the denitrification process is certainly performed, even though this condition does not involve a complete denitrification.15 During an oxic phase, a cycle can be considered efficient when the ORP overcomes 0 mV. All of the data were stored in a PC station. Results and Discussion The average operational conditions of the biological process during the 4-month experimental period (FebMay 1999) are reported in Table 2. The biological process showed different values of the sludge retention time (SRT) in OX1 and OX2 steps: while in OX1 the average SRT was 22 days (range 19-26 days), wasting 15-20 m3 day-1 of activated sludge, in OX2 a very high SRT was observed even with a low to null value of the wasted sludge flow rate. Also the mixed liquor suspended solids (MLSS) concentrations were completely different: the values ranged between 6500 and 8900 mg L-1 in OX1 and between 3100 and 3700 mg L-1 in OX2. All of these considerations allow one to classify the OX2 step as an extended aeration process and to point at OX1 as a reactor subject to the variation of the biomass concentration because the primary sedimentation was absent. Finally, the process temperature fluctuated according to the external temperature, and a gap of 1 or 2 °C between the first and second steps could be found: this was because of a dissimilar aeration mode in the two reactors. In fact, the aeration times were 77% and 37% of the whole time for the first and second steps, respectively. Carbon Removal. Carbon loads are shown in the flow scheme of Figure 1, where acetic acid is added as the external carbon source in the second step. The addition could not be performed in a continuous mode but only during the anoxic phase and when NOx-N in the effluent was increasing in concentration. The statistical parameters for COD (Table 3) were indicated because acetic acid, when added, represented from 5% to 40% of the COD total loading coming from the first step effluent. Thus, its role was, besides nitrogen removal enhancement, to sustain biomass growth. An unusual trend was observed in Mar 1999 when a relevant fraction of the incoming COD passed throughout the biological section. The plot of the COD concentration versus time (Figure 2) confirmed this behavior: this was due to the loss of a significant amount of nonbiodegradable COD in the chemical-physical pretreatments (i.e., the Fenton process). The process per-
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Table 3. Flow Rates and Statistical Parameters of COD Loading EQU - COD
OX1out - COD
OX2out - COD
period
average Qin (m3 day-1)
average (kg day-1)
s.d. (kg day-1)
average (kg day-1)
s.d. (kg day-1)
acetic acid as COD (kg day-1)
average (kg day-1)
s.d. (kg day-1)
Feb ’99 Mar ’99 Apr ’99 May ’99
128 139 124 129
562 578 523 546
218 345 319 185
99 179 73 90
24 108 41 37
22 10 24 35
87 171 41 59
38 86 20 17
Figure 2. Concentration and distribution of COD within the biological process. Table 4. Concentrations (mg L-1) of Nitrogen Compounds within the Process EQU average Feb ’99 Mar ’99 Apr ’99 May ’99
40 27 46 64
OX1out s.d 26 13 34 28
average NH3-N 2.8 13 40 27
OX2out s.d
average
s.d
4.6 10 24 14
0.22 9 1.3 0.6
0.24 11 2.6 0.4
Feb ’99 Mar ’99 Apr ’99 May ’99
0.46 0.56 0.23 0.03
0.23 1.0 0.35 0.03
NO2-N 0.14 0.07 0.07 0.14
0.30 0.07 0.08 0.27
0.09 0.66 0.40 0.22
0.07 1.3 0.79 0.30
Feb ’99 Mar ’99 Apr ’99 May ’99
2.5 1.1 0.61 0.69
1.4 0.84 1.0 0.35
NO3-N 1.0 0.95 0.92 0.94
0.62 0.91 1.8 0.23
3.7 1.2 6.3 3.9
3.5 1.0 6.0 4.7
formances were evaluated in terms of COD removal for each step according to the total amount entering the plant. The OX1 step exerted the main role, with a COD removal ranging from 450 to 463 kg of COD day-1, while the OX2 step has a finishing role with a COD removal in the range of 34-66 kg of COD day-1; this confirms what was previously outlined both during the start-up and a 1-year plant management.6,7 Generally, 80% COD removal was achieved in the OX1 step. In some cases, it can be stated that the OX2 step could support COD loads higher than those finalized to nitrogen removal enhancement: in this way a future substitution of acetic acid, an expensive carbon source, with a liquid waste characterized by a high COD to TKN ratio, as a cheaper source, may be possible. Nitrogen Removal. The average nitrogen concentrations in the different sections of the plant are reported in Table 4. As a matter of fact, the influent ammonia concentration showed a great variability because of the different types of wastewater which were processed daily; the months with medium (Feb and Mar 1999) and high (Apr and May 1999) concentrations of
Figure 3. Concentration and distribution of ammonia within the biological process.
ammonia in the incoming flow rate can be recognized (see spots 1 and 2 in Figure 3). Clearly, in these months a different organization of the two steps of the biological process was required. Despite these variations, the plant overall performance can be well estimated because low to null NO2-N and very low NO3-N concentrations in the effluent were found (Table 4). A deeper examination of what really happened in the plant was shown in the plot of ammonia concentration (Figure 3), where a complete visualization of data variation is shown. A first qualitative function of the two steps could be summarized as follows: a complete nitrogen nitrification was observed in OX1 when ammonia nitrogen in the incoming flow rate was lower than 30 mg of N L-1, while a consistent role of the OX2 step was clear at higher values (see spot 2 in Figure 3). The comparison between these data and the ones reported in Table 4 allows one to assume that the methodology used to control the ammonia maximum content, that is, struvite precipitation in the effluent of the physical-chemical treatment,4 was a successful way for a proper management of the biological plant and for the accomplishment of nitrogen standard limits when, as a rule of thumb, the ammonia concentration exceeds 100 mg L-1. The loss in “nonbiodegradable” COD, previously observed in Mar 1999, is in accordance with a loss in ammonia nitrification (see spot 1 in Figures 2 and 3). In fact, some toxic and inhibitory compounds for nitrifying biomass may sometime inflow the biological step, although a chemicalphysical and biological characterization of the liquid wastes was performed. Moreover, the pretreatment step could occasionally fail. A possible solution for this kind of problem lies in the use of an online control system: an OUR analyzer should be positioned in the influent stream, before the equalization tank, to determine the real toxicity of some liquid wastes. The nitrogen mass balance highlighted the real performances of the process using the four performance indexes En, Enn, Ed, and Edd (see the Materials and Methods section). In particular, the nitrification process is normally well described by En, but in this case, the consistent amount of nitrogen linked to nonbiodegrad-
Ind. Eng. Chem. Res., Vol. 41, No. 11, 2002 2809 Table 5. Nitrogen Mass Balance
Table 7. OX2 Anoxic Phase Analysis
Nitrification Process OX1 + OX2 month Feb ’99 Mar ’99 Apr ’99 May ’99
OX1
OX2
En (%) Enn (%) En (%) Enn (%) En (%) Enn (%) 41 8 39 45
100 42 100 87
35 5 15 19
84 26 39 36
6 3 24 27
16 17 61 51
Denitrification Process OX1 + OX2 month Feb ’99 Mar ’99 Apr ’99 May ’99
OX1
OX2
Ed (%) Edd (%) Ed (%) Edd (%) Ed (%) Edd (%) 40 7 33 43
88 69 83 92
37 6 15 19
82 55 38 41
3 1 18 24
6 14 45 51
period in 1999
no. of realized cycles
efficient cycles/ realized cycles (%)
Jan 1-15 Jan 16-31 Feb 1-15 Feb 16-28
75 80 75 65
8 38 28 42
Mar 1-15 Mar 16-31 Apr 1-15 Apr 16-30 May 1-15 May 16-31 June 1-15 June 16-30
75 80 75 75 75 80 75 75
57 59 32 85 87 86 79 80
acetic acid addition no addition no addition no addition addition for 2/3 of the considered period no addition no addition full addition full addition full addition full addition full addition full addition
Table 6. Nitrogen Mass Loading and Oxic Phase Analysis in OX1
period Feb ’99 Mar ’99 Apr ’99 May ’99
efficient cycles/ nitrogen no. of no. of realized mass loading that can be hypothesized realized cycles of NH3-N nitrified (kg day-1) cycles cycles (%) (kg day-1) 207 229 222 229
211 243 177 138
91 79 38 59
5.6 4.0 6.5 8.6
8.6 5.8 10.6 15.3
able COD calls for the term Enn (Table 5), which excludes the TKN concentration in the plant effluent, to make the exact role of the biomass clear. In other words, even though En for OX1 + OX2 is not higher than 45%, a complete nitrification was observed (Enn of 87-100%; Table 5). The nitrogen nitrification happened in some cases almost completely in OX1 (84% in Jan 1999) though it was more frequently performed by the OX2 step: Apr 1999, 61%; May 1999, 51%. Also the denitrification process can be better discussed on the basis of Edd, rather than Ed, because its physical meaning stands in the performance in the denitrification process related to the available NOx-N. Therefore, Edd describes the efficiency of the process but nitrogen removal was actually linked to the nitrification efficiency (En and Enn). In this case, the results (Table 5) showed that the denitrification was either performed rather completely in OX1 (Feb 1999, Edd of 82%) or distributed between OX1 and OX2 (Apr and May 1999). Occasionally, as observed in a previous work,7 the OX1 denitrifies apparently more than all of the amount of NOx-N produced (Feb 1999, Ed of 37% and En of 35%), but this was due to the feedback of the effluent recycle. The reasons for this behavior must be searched in mass loading variations or in an anomalous running of the alternating cycle. The analysis of OX1 behavior during the nitrification process (Table 6) revealed that quite all of the hypothesized cycles were performed during Feb and Mar 1999, while the realized cycles were much less than the hypothesized ones in Apr and May 1999. Furthermore, the cycles were efficient only in Feb 1999 (91%) and Mar 1999 (79%), and a strong reduction of the conditions necessary for nitrification was registered in Apr 1999 (efficient cycles 38%) and May 1999 (59%). This remark gives an explanation for the nitrification performances previously observed in OX1, while no significance can be ascribed to ammonia mass loading changes. Actually, the experimental kinetic constant rate was always higher than 0.01 kg of NH3-N
Figure 4. Examples of DO and ORP profiles versus time without acetic acid addition.
kg of MLVSS-1 day-1, a value that determines an average amount of nitrogen to nitrify equal to 17 kg day-1, which is higher than any ammonia or nitrogen mass loading to the OX1 step (Table 6). The analysis of the denitrification process was performed considering a time range wider than that related to the research work with the aim of confirming the effect of carbon addition through a larger number of data. The realized cycles have not always turned out efficient (ORP at the end of anoxic phase e 0 mV); however, the percentage of efficient cycles over realized cycles must be considered to be linked to the addition of acetic acid. As a matter of fact, when the external carbon source becomes a constant operational condition, a high percentage of efficient cycles is obtained (from Apr 16-30 to June 1999, in Table 7). A more inclusive knowledge of the process can be strengthened by the DO and ORP plots in the OX2 step during a whole day without and with acetic acid addition. In the first case (Figure 4) at the end of the oxidation phase the ORP reached approximately +260 mV (DO 4-6 mg L-1); despite an anoxic phase length up to 180 min, the ORP decreased to an end value ranging between +20 and +70 mV. Even though they cannot be considered conditions that can exclude the denitrification of NOx-N, no flex points in the ORP profile were observed. On the other hand, when acetic acid was added (Figure 5), a decrease of ORP in the anoxic phase up to -500 mV and a clear flex point were observed, showing the depletion of NOx-N. The DO profile in Figure 5 can appear unusual: the DO concentration was determined by the change of the blower power according to an online control of the oxygen set too high (6 mg of O2 L-1). The comparison among ORP trends in the anoxic phase (Figures 4 and 5) suggested the procedure for an automatic addition
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Figure 5. Examples of DO and ORP profiles versus time with acetic acid addition.
Figure 7. Prediction of effluent nitrates in different operative conditions.
while the last is clear with a further amount of NOx-N to denitrify (LNOxNout,1). Equation VII is obtained by transforming eq VI using the definitions of En2 and Ed2 previously fixed in eqs II and IV, respectively.
LNOxNout,2 ) LNtotnit,2 + LNOxNout,1 - LNtotden,2 (VI)
Figure 6. NOx-N pattern versus process performance.
of the external carbon source considering that the scenario is constituted by a time interval in which the process must be, however, performed and by the logical necessity to save an expensive carbon source. The last observation is that the slope of the initial anoxic decrease of the ORP profile is not sufficient to distinguish between a situation where the carbon source to promote the denitrification process is enough or not; hence, it cannot be used to predict what will happen. For these reasons the only condition to carry out the addition is in the case where after 30 min the ORP has not reached 0 mV yet. Effluent Quality Prevision. To predict the quality of the biological plant final effluent, expressed as the NOx-N concentration, a relation between the NOx-N concentration and the process performances in nitrogen removal was preliminary carried out. In Figure 6 the monthly average results were plotted, as the NOx-N concentration, versus the ratio of nitrification and denitrification efficiency. A generalization of the process behavior was carried out through the analysis of the long term running. The process performances obtained during 14 months of management revealed two main linear correlations according to only the nitrified nitrogen loading in OX2. The first is applicable in the low to null range (0.3-1.0 kg day-1 of NOx-N mass loading produced in the OX2 step) and the second in the 2-5.5 kg day-1 one. In both cases the same good variance explanation (r2 ) 0.89) was found. This empirical situation can be theoretically explained with the NOx-N mass balance in the OX2 step (eq VI), from which the NOx-N in the effluent of the OX2 step can be derived as a function of the process performance ratio (eq VII). Certainly, the NOx-N in the final effluent will be dependent on the OX2 and OX1 performances. The first is expressed as the ratio between Ed and En,
where LNOxNout,2 ) NOxN mass loading in the effluent of the second reactor, LNtotnit,2 ) total nitrified nitrogen in the second reactor, LNOxNout,1 ) NOxN mass loading in the effluent of the first reactor, and LNtotden,2 ) total denitrified nitrogen in the second reactor.
NOxNout,2 ) -a(Ed2/En2) + b
(VII)
where a ) LNtotnit,2(1000/Qin) and b ) LNtotnit,2 LNOxNout,1(1000/Qin). Equation VII is a linear model which allows the prediction of the NOx-N concentration in the effluent as a function of the ratio Ed/En for the second step (OX2), when a value of LNtotnit,2 is fixed. According to this model, the experimental data represented in Figure 7 (dots), and previously explained in an empirical mode, are now justified on the basis of a theoretical one for different values of LNtotnit,2 (from 0.3 to 5.5 kg day-1). Conclusions The addition of an external carbon source to enhance nitrogen removal in the biological treatment of liquid industrial wastes has been studied. A two-step alternate oxic-anoxic process with a separate biomass was utilized, and the main results can be summarized as follows: 1. The strategic use of a two-step process with a separate biomass is characterized by the possibility of using the second step when the first is not sufficient. This condition is not always guaranteed because critical conditions of biomass growth in the second step are common and the addition of an external carbon source helps in maintaining a minimum level of biomass. Furthermore, to enhance nitrogen removal, the substitution of external sources with wastes characterized with a high COD to TKN ratio is suggested as a cheaper solution. 2. The nitrogen removal is successfully managed if the control of the influent ammonia loading coming from the pretreatment of wastes is exerted; however, nitrification either can happen entirely in the first step or can be distributed between the first and second steps.
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An important role is exerted by the real nitrification conditions, and it can be verified through the number of efficient cycles (ORP > 0 mV) on the total number performed in oxic conditions. 3. The nitrified nitrogen in the first step is entirely denitrified, while the one in the second step needs an external carbon source to be denitrified because not enough carbon is normally derived from the first one. 4. The process monitoring allows one to confirm through the flex point of ORP the complete nitrogen denitrification exerted by carbon addition at the beginning of the anoxic phase. 5. To save on the use of an expensive carbon source, a feasible protocol is to organize the carbon addition when the ORP probe has not reached 0 mV in a fixed time. 6. The nitrogen mass balance of the second step allows the prediction of nitrates concentration in the plant effluent. Literature Cited (1) Eckenfelder, W. W., Jr.; Musterman, J. L. Treatment and pretreatment requirements for industrial wastewater in municipal activated sludge plants. Water Sci. Technol. 1994, 29 (9), 79-88. (2) Andreadakis, A. D.; Kalergis, C. M.; Kartsonas, N.; Anagnostopoulos, D. Determination of the impact of toxic inflows on the performance of activated sludge by wastewater characterisation. Water Sci. Technol. 1997, 36 (2-3), 45-52. (3) Haas, C. N.; Vamos, R. J. Hazardous and industrial waste treatment; Prentice Hall: Englewood Cliffs, NJ, 1995; p 384. (4) Battistoni, P.; Boccadoro, R.; Bolzonella, D.; Latini, F. Optimisation of chemical and physical pretreatment in an industrial wastewater treatment plant. Ind. Eng. Chem. Res. 2001, 40 (21), 4506. (5) Tunay, O.; Kabdash, I.; Orhon, D.; Kolcak, S. Ammonia removal by magnesium ammonium phosphate precipitation in industrial wastewaters. Water Sci. Technol. 1997, 36 (2-3), 779782.
(6) Battistoni, P.; Morini, C.; Pavan, P.; Latini, F. The Retrofitting of an extended aeration process to optimise biological nitrogen removal in liquid industrial wastes. Environ. Technol. 1999, 20, 563-573. (7) Battistoni, P.; Boccadoro, R.; Pavan, P.; Bolzonella, D. The monitoring of two steps aerobic-anoxic process with separate biomass to enhance performances in the treatment of liquid industrial wastes. Environ. Technol. 2002, 23, 73-84. (8) Isaac, S. H.; Henze, M. Controlled carbon source addition to an alternating nitrification-denitrification wastewater treatment process including biological P removal. Water Res. 1995, 29 (1), 77-89. (9) Mamais, D.; Jenkins, D.; Pitt, P. A rapid physical-chemical method for the determination of readily biodegradable soluble COD in municipal wastewater. Water Res. 1993, 2, 195-197. (10) Ekama, G. A.; Dold, P. L.; Marais, G. v. R. Procedures for determining influent COD fractions and the maximum specific growth rate in activated sludge systems. Water Sci. Technol. 1986, 18, 91-114. (11) APHA. Standard methods for the examination of water and wastewater, 16th ed.; American Public Health Association: Washington, DC, 1985. (12) Kristensen, G. H.; Jorgensen, P. E.; Henze, M. Characterization of functional microorganism groups and substrate in activated sludge and wastewater by AUR, NUR and OUR. Water Sci. Technol. 1992, 25, 43-57. (13) Craig, Q.; Peddie, C.; Donald, S.; Mavinic, C.; Jenkins, J. Use of ORP for Monitoring and Control of Aerobic Sludge Digestion. J. Environ. Eng. ASCE 1990, 116 (3), 461-471. (14) Zipper, T.; Fleischmann, N.; Harberl, R. Development of a new system for control and optimisation of small wastewater treatment plants using oxidation reduction potential (ORP). Water Sci. Technol. 1998, 38, 307-314. (15) Beccari, M.; Passino, R.; Ramadovi, R.; Vismara, R. Rimozione di azoto e fosforo dai liquami; Hoepli, Milano, Italy, 1993.
Received for review October 9, 2001 Revised manuscript received February 13, 2002 Accepted March 7, 2002 IE010828+
