C. Beck, G. Prades and A.-G. Sadowski Ecole du Ge´nie de l’Eau et de l’Environnement de Strasbourg (ENGEES), UPR SHU, 1 quai Koch, 67070 Strasbourg, France (E-mail:
[email protected]) Abstract The principal objective of our study was to optimise a municipal activated sludge wastewater treatment plant (WWTP) to face high organic flows due to viticulture effluents inputs. Treatment file optimization consisted in testing different treatment lines, changing the number and volume of biological basins and clarifiers, with or without a buffer basin upstream, with a view to achieving a better reduction of COD. The actual WWTP biological stage is composed of two aerated basins whose total volume is 1365 m3. The studied cases are successively, the installation of a single basin of 1365 m3, then of several basins whose total volume remains constant and equal to 1365 m3. Another case was also considered, that of an aerated basin followed by a first clarifier and then, by another aerated basin and a second clarifier. All scenarios presented below were evaluated, for standard dry weather conditions and for high organic load conditions, as encountered during the grape harvest period. The method used was to carry out various simulations, using numerical modelling, and to compare the impact of different process line scenarios and management strategies on the activated sludge WWTP efficiencies. Keywords Activated sludge; ASM1 model; biodegradation; COD fractionation; optimisation scenario; wineries effluents
Introduction
Water Science and Technology Vol 51 No 1 pp 81–88 ª IWA Publishing 2005
Activated sludge wastewater treatment plants optimisation to face pollution overloads during grape harvest periods
Wine production processes generate different kinds of pollution mainly associated with solid wastes and liquid effluents. Viticulture effluents are characterized by their high organic content and high seasonal variability. When rejected into a sewage network, these effluents are mixed with domestic wastewaters, that considerably changes fluxes and quality of effluents to treat at wastewater treatment plants (WWTP). These mixed effluents, if not taken into account in process design, often cause activated sludge WWTP malfunctions. The principal objective of our study, situated in the Alsatian context, was to offer persons in charge of the communities and project superintendents routes to better dimensioning and management of wastewater sewage works, so as to cope with the organic matter flows due to viticulture effluents. Methods
The method used was to carry out various simulations, using numerical modelling, to compare different process line scenarios and management strategies of activated sludge WWTP. Experimental site
In order to base our study on a real case for simulations, we chose the Beblenheim activated sludge WWTP (Alsace, France) whose capacity is 20,000 equivalent-inhabitants. This WWTP is confronted with malfunctions due to very high organic flow variations during the periods of grape harvest. The sewer system drains effluents coming from five communes: Beblenheim, Bennwihr, Mittelwihr, Riquewihr and Zellenberg. These are located in the Alsace vineyard area, in the department of Haut-Rhin, to the north of Colmar. The viticulture activity is one of the
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Table 1 Average flows received by Beblenheim WWTP during the year
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Grape harvest period
Normal period (dry weather)
2,100 2,500 1,100 700
1,100 – 295 315
Average flow (m3/day) COD (kg/day) BOD5 (kg/day) Suspended solids (kg/day)
principal activities of the local economy. The population comprises 4493 inhabitants and the viticulture activity produces a volume of about 100,000 hl (10,000 m3) of wine per year. The wastewater treatment line is made up of the following works: 3 a storm basin (volume: 180 m ), not aerated 3 a pumping station (capacity: 200 m /h) a rotary screen an aerated grit chamber, combined with a grease remover 3 a first biological basin (458 m ), functioning as an anoxic zone except during the grape harvest and as a complementary biological treatment basin in the period of the grape harvest, thanks to an aeration power of 16.3 kW 3 an aerated channel (907 m ) equipped with two surface brushes, one providing 9.5 kW, the other 13.8 kW. During the grape harvest period, an hydroejector is used to increase the aeration power by 12 kW 3 2 a final settling tank (scrapped clarifier, volume: 302 m , surface: 195 m ) The sewage sludge treatment line is made up of the following works: 3 3 two sludge silos of respectively 330 m and 200 m a draining table (EMO type), whose dehydration capacity can be occasionally supplemented by a mobile dehydration unit (that is generally the case during the grape harvest period) a slurry spreader for liquid sludge Based on annual reports realized during normal (dry weather) and grape harvest periods, average daily values of volume and mass flows received by the WWTP are as reported in Table 1. For grape harvest periods, the rejected effluent must respect the criteria given in Table 2. Biological and physical models used
For biological processes simulation, we chose the ASM1 model which is increasingly used in the design and management of activated sludge WWTP (IWA, scientific and technical report n 9, 2000). The modelling of the clarifier is carried out by the one-dimensional model of Taka´cs (Taka´cs et al., 1991), describing both decantation and phases separation. GPS-X software (4.02 version) from Hydromantis was used to simulate various process line configurations, with a view to optimising the COD reduction, and to appreciate performance limits due to the clarifier capacity. The kinetic and stoichiometric values used for the biological model were default ASM1 values. Sedimentation properties of sludge were taken into account by the sludge volume Table 2 Maximum concentrations required during grape harvest periods COD
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120 mg/l
BOD5
TSS
40 mg/l
30 mg/l
Table 3 COD fractions percentage for dry weather and grape harvest periods Normal period (dry weather)
85% 1,2% 9,4% 5%
31% 15% 50% 4%
Ss (readily biodegradable COD) Si (inert soluble COD) Xs (insoluble biodegradable COD) Xi (inert solids)
index (SVI), evaluated by measurements on the site. The principal measurement effort was related to the evaluation of flows and to the characterization of effluents to be treated.
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Grape harvest period
Characterization of the mixed effluent
Characterization of mixed effluent was limited to determination of different soluble or particular fractions of COD (Ss, Xs, Si, Xi). Raw influent samples, taken at WWTP entry, were so characterized, using batch reactor tests during a score of days. The dominant viticulture mixed effluent, in term of COD, is mainly formed of soluble substrate, with a high fraction of biodegradable COD. To achieve COD fractionation of wastewater, two batch reactors were installed for each sample, one filled with raw wastewater, the other one containing 1.2 mm filtered wastewater. Raw and filtered COD value were measured in these continuously agitated and aerated reactors during 21 days. The filtrate reactor is only used to calculate the apparent yield of biomass growth with the substrate initially present in the sample. This value is used to calculate the Xs fraction (slowly biodegradable fraction) of the COD. This protocol, described in detail by Stricker (2000), makes the assumption that this apparent yield keeps the same value in the rough water reactor and in the filtrate reactor. The COD fractions (Ss, Si, Xs, Xi) percentage we obtained are reported in Table 3. Normal period values come from previous experiments (Stricker, 2000) for a strictly domestic effluent. The apparent yield of biomass growth found during our experiment was weak compared to actual literature values. It could be interesting to reproduce this experiment on several couples of reactors in order to verify this tendency under similar conditions. During grape harvest period, in terms of COD, the mixed effluent proved to be organically dominant, with mainly readily biodegradable COD (effluent comparable to viticultural effluents). Results and discussion Validation of simulation assumptions
The basic schema used to represent wastewater treatment line in its original and current state is presented below (Figure 1). Before testing optimization scenarios we tried to adjust
Figure 1 Wastewater treatment line flow diagram
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Table 4 Typical flow and concentration values in permanent mode Wastewater parameters
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1,547 m3/day 468 mg/l 250 mg/l 203 mg/l 42.6 mg/l 23.7 mg/l
Daily volume COD TSS BOD5 TKN NH4+
724 kg/day 387 kg/day 314 kg/day 66 kg/day 37 kg/day
operating parameters and validate our approach for stabilized dry weather conditions. This step was realized, using detailed data collected during two days of stabilized conditions, in July 2002. Corresponding effluent concentrations and flow are presented in Table 4. By using experimental values for COD fractions, and typical literature values for kinetic and stoichiometric parameters, simulation results of the current WWTP in permanent mode were verified to offer a good match with experimental values. Permanent mode simulation results were then used to define the initial state of the WWTP in the model, before applying organic loads for a grape harvest period, using COD and incoming flow values from year 2002 as presented in Figure 2. As for permanent simulation results, a good match was found between experimental and simulated COD values during the grape harvest period. Concerning the suspended matters, the simulated values were significantly lower than the measurements. The modeling of the clarifier and the reliability of measurements may be questioned. However, if the WWTP respects rejection levels in permanent mode, this is not the case during the grape harvest period. An increase of the actual aeration power could be a first solution as regards improvement of the treatment efficiency. Wastewater treatment file optimization
Treatment file optimization consists in proposing various ways of organizing the biological stages between them. The most effective configuration will be the most efficient in terms of COD reduction. We currently have two basins whose total volume is 1365 m3. The studied cases were successively, the use of a single basin of 1365 m3, then of several biological basins whose total volume remained constant and equal to 1365 m3. A final case study was later considered, that of an aerated basin followed by a first clarifier and then, by another 3400
4500 COD values (mg/l)
3200
daily flow (m3/day)
3000
3500
2800
COD (mg/l)
3000
2600 2500 2400 2000 2200 1500
2000
1000
1800
500 0 25/9
1600
2/10
9/10
16/10
23/10
30/10
1400 6/11
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Figure 2 Flow and COD values received by Beblenheim WWTP (grape harvest period)
Daily flow (m3/day)
4000
Table 5 Simulated COD values in treated water according to the number of basins ( permanent mode) Standard dry weather conditions Input COD: 468 mg/l Received flow: 1547 m3/day Sludge index: 120 ml/g
1 basin
2 basins
3 basins
4 basins
0.69 20.0 18.7 1.3 7.7 21.8 20.0
0.64 19.9 18.7 1.2 7.2 22.8 20.1 19.9
0.63 19.9 18.7 1.2 7.0 23.8 20.4 19.9 19.9
Sludge recirculation: 100%
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Simulated values at output of the last basin (mg/l) 1.17 Soluble BOD5 Soluble COD 20.9 COD(Si) 18.7 COD(Ss) 2.2 COD(Xs) 12.0 Soluble COD at B1 output Soluble COD at B2 output Soluble COD at B3 output Soluble COD at B4 output
aerated basin and a second clarifier. All scenarios presented below were evaluated for standard dry weather conditions and for high load conditions, as encountered during the grape harvest period. In permanent mode, the received effluent contains 468 mg/l total COD and 203 mg/l total BOD5. In case of high load conditions, the incoming effluent total COD was set to 4240 mg/l and 2120 mg/l total BOD5 was considered.
Number of basins optimization
The principal result which comes from simulations has the advantage, at constant total reactive volume, of a station provided with two aerated basins. Out of the last basin, the soluble BOD5 and soluble COD values are clearly lowered, as passing from one-basin configuration to a two-basin one, for the same total volume. Tables 5 and 6 summarise results obtained with growing numbers of basins. In permanent mode, when effluent has predominantly domestic characteristics, the gain on COD reduction is 41% when using two basins rather than only one. This tendency is confirmed for high organic load simulations, with gains of 84% on BOD5 and 16% on COD. By observing more particularly the fractions of soluble COD, we notice that the inert COD fraction (Si), whatever the fitting of the basins, remains normally stable; only the readily biodegradable COD (Ss) decreases. Table 6 Simulated COD values in treated water according to the number of basins (high organic load conditions) Peak organic load (grape harvest)
1 basin
2 basins
3 basins
4 basins
0.8 38.5 37.1 1.4 23.0 60.7 38.5
0.6 38.4 37.2 1.2 20.6 90.6 39.9 38.4
0.6 38.5 37.3 1.2 20.3 203.7 42.3 39.2 38.5
Input COD: 4240 mg/l Received flow: 2243 m3/day Sludge index: 150 ml/g Sludge recirculation: 120%
Simulated values at output of the last basin (mg/l) 4.7 Soluble BOD5 Soluble COD 45.8 COD(Si) 36.9 COD(Ss) 8.9 COD(Xs) 32.6 Soluble COD at B1 output Soluble COD at B2 output Soluble COD at B3 output Soluble COD at B4 output
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Figure 3 Wastewater process line with two basins and two clarifiers
However, no significant improvements in COD reduction were found when increasing the number of stages to more than two basins. Influence of the presence of an intermediate clarifier
For the same sludge mass in biological reactors, a two-basin WWTP with an inserted clarifier improved the COD treatment performance, compared to a process line with only one clarifier. The schematic diagram of the corresponding process line is presented in Figure 3. The comparison criteria taken into account during these simulations remained the observation of soluble variables at the basin output for two kinds of conditions: permanent mode during dry weather and high organic load conditions during grape harvest period. The concentration of dissolved oxygen was monitored to 2gO2/m3. The estimated SVI value was 120 ml/g in permanent mode and 150 ml/g for peak loads of pollution. Extraction of sludge was operated only from the second clarifier. Sludge from the first clarifier was recycled at the head of the first basin, as sludge from the secondary clarifier was recycled to the second basin. The recycled flow of sludge towards the first basin was chosen equal to 50% of the entering flow, and was fixed at 70% towards the second basin. Obtained results were not easily comparable with the preceding situations, because the sludge mass in different parts of the system is quite different. Nevertheless concerning the sludge mass contained by reactors, similar orders of magnitude are observed (see Table 7). In permanent mode, treatment improvements were significant concerning the soluble substrate fraction. Abatements of 50% on BOD5 and 46% on soluble substrate fraction of COD were found in treated water. For high organic loads, this tendency was also confirmed. The abatement was 50% on BOD5 and 50% on soluble substrate fraction of COD remaining in treated water. Management of COD flows using a buffer basin
The use of a buffer basin, upstream of the WWTP (Figure 4), was planned to mitigate uncertainty about the quantity of future organic pollution.
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Figure 4 Wastewater process line with a buffer basin
Table 7 Comparison of simulated COD values with 1 or 2 clarifiers (high organic load conditions) Peak organic load (grape harvest)
2 basins
2 basins +2 clarifiers
Sludge Index: 150 ml/g Sludge recirculation
120%
to B1: 85% to B2: 85%
Simulated values at output of the last basin (mg/l) Soluble BOD5 Soluble COD COD(Si) COD(Ss) COD(Xs) Soluble COD at B1 output Soluble COD at B2 output
0.8 38.5 37.1 1.4 23.0 60.7 38.5
0.4 36.2 35.5 0.7 9.4 55.3 36.2
Input COD: 4240 mg/l Received flow: 2243 m3/day
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The circular aerated channel of 907 m3 is preserved as well as the 458 m3 basin, which keeps functioning as an anoxic zone, except during grape harvest period, when it is turned into an aeration basin. The third biological reactor is created from the existing clarifier whose civil engineering can offer a volume of 405 m3. Considering the maximum biological flow on the WWTP being 200 m3/h and a Molhmann index value around 120 ml/g, a 250 m2 new clarifier is proposed to be built downstream of the existing stages. To improve the wastewater treatment, a certain amount of sludge is to be recycled from the clarifier to the buffer basin. This basin will be both mixed and aerated, using immerged air blowers in spite of surface aeration systems, because of variable water levels in the buffer basin. COD outputs from the basin will be controlled to limit peak organic loads to acceptable values for downstream stages. Such a process line configuration enabled all observed organic load peaks to be coped with during grape harvest period for year 2002. Rejected effluents concentrations were always lower than authorised limits. Nevertheless, optimisation of the management of the filling and draining of the basin buffer towards the station still remains to be developed. Limits of processing capacity imposed by the clarifier
Incoming winery wastewater effluents increase organic matter flows, that should lead to a degradation of sludge mechanical properties and, as a result, an increase of sludge volume index. Other consequences are a lower value of re-circulated sludge concentration, a higher sludge mass in the clarifier and a lower sludge mass in the aerated reactor. For simulation studies, we varied the SVI value and observed the consequences on the clarifier TSS profile and on treated water COD concentrations. Up to 150 ml/g, we could not observe that increasing the SVI involved any reduction in the process capacity or a deterioration of the rejected effluent quality. For SVI values higher than 150 ml/g, no significant change could be predicted, which seems to not reflect reality. In the immediate future, it would be necessary to develop studies carried out more precisely on the clarifier.
Conclusions
Initially the oxygenation power needed during grape harvest periods requires the installation of an efficient aeration system, particularly in the first reactor. Hydraulic optimisation of the treatment line could be achieved by the installation, with a constant volume, of two reactors instead of one. In addition, the installation of a secondary clarifier between these two reactors
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improves the treatment quality. The setting of a ventilated and agitated buffer tank, upstream of the treatment line, allows us to control the flow of acceptable soluble COD on the wastewater treatment plant and, at the same time, to begin the organic matter treatment. To maintain the wastewater plant in good working order, sludge mechanical properties, as indicated by SVI, should be looked at more precisely, as these seem not to be well taken into account with actual model. Depending on these properties, suspended matter concentration in the biological reactor cannot be unlimited, which should be remembered in the dimensioning or optimising approach. References Badie, F. (1998). Raccordement et traitement collectif mixte des effluents vinicoles. 2e Congre`s International sur le traitement des effluents vinicoles. Bordeaux, Cemagref e´ditions, p. 164–170. Canler, J.P., Alary, G., Perret, J.M. and Racault, Y. (1998). Traitement biologique ae´robie par bassins en se´rie des effluents vinicoles. 2e Congre`s International sur le traitement des effluents vinicoles. Bordeaux, Cemagref e´ditions, p. 178–188. France assainissement (2002). Syndicat intercommunal d’assainissement de Beblenheim et environs. Rapport de vendange 2002. Strasbourg, France assainissement, p. 20. Hydromantis (2001). GPS-X technical reference. (GPS-X version 4.0.). Hydromantis, Canada. Hu, Z., Chandran, K., Smets, B. F. and Grasso, D. (2002). Evaluation of a rapid physical-chemical method for the determination of extant soluble COD, Water Research 36, 617–624. IWA taskgroup on mathematical modelling for design and operation of biological wastewater treatment (2000). Activated sludge models ASM1, ASM2, ASM2d and ASM3, Scientific and Technical Report 9. Racault, Y. (1993). Les effluents des caves vinicoles: e´valuation de la pollution, caracte´ristiques des rejets. Informations techniques du Cemagref n 92, note 4. Stricker, A.-E. (2000). Application de la mode´lisation a` l’e´tude du traitement de l’azote par boues active´es en ae´ration prolonge´e: comparaison des performances en temps sec et en temps de pluie. PhD thesis, Universite´ de Strasbourg I. Taka´cs, I., Patry, G.G. and Nolasco, D. (1991). A dynamic model of the clarification-thickening process. Water Research, 25, 1263–1271.
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