10th International Conference on Hydroinformatics HIC 2012, Hamburg, GERMANY

USE OF HYDRODYNAMIC MODELS FOR OPTIMUM DESIGN OF STABILIZATION PONDS: CASE STUDY RUBIALES, COLOMBIA

CARLOS VELEZ (1), LEONARDO ALFONSO (2), ARLEX SANCHEZ (1) (1): PhD research fellow IWSG Department, UNESCO-IHE, Westvest 7, 2611AX Delft, Netherlands (2): Post-doc researcher, IWSG Department, UNESCO-IHE, Westvest 7, 2611AX Delft, Netherlands

The design of stabilization ponds for the treatment of wastewater is traditionally based on empirical guidelines. Even more the hydraulic design is normally over simplify. However, the performance of a pond will highly depend on the proper hydraulic design. This paper present an approach that combines the traditional empirical design with the use of modelling tools to predict the performance of the ponds. The approach proposed is tested in a case study for the design of an anaerobic pond in Colombia. The results show that with the approach proposed the design of the pond can be optimized. INTRODUCTION Waste stabilization ponds are widely used to treat wastewater from urban, agricultural and industries. They are known for their relative simplicity of construction and operation when compared with activated sludge technology. Stabilization ponds are a cost-effective alternative if the topography and climate conditions are favourable and the land for construction is available. However, this reputation of simplicity may hinder the reality of complex physic-chemical and biological processes which performance may be affected by the hydraulic design of the system (Shilton and Harrison 2003).

In general, the hydraulic design of stabilization ponds are based on rough ‘rules of thumb’ (Shilton and Harrison 2003). In other words it follows a empirical iterative approach as describe by Harremoës and Rauch (1999). The structures are designed and built on simplified assumptions and their performance is subsequently evaluated through monitoring. When the monitoring system proves that the performance is inadequate, then an improved plan of action is implemented, for instance the use of directional walls (i.e baffles) to modify the flow pattern. An alternative is to use the prediction design approach. In the prediction-design approach, models play an essential role in the prediction of performance and evaluation of competing alternatives for design (Harremoës and Rauch 1999).

With the standardisation of the activated sludge model (ASM1) in the 1980s (Henze, et al. 1999) there has been and increase number of use of prediction-design approach for wastewater treatment plants (WwTP) design. However, most of the research and applications have been done on the assessment of WwTP designs based on activated sludge process. In addition, optimization of designs using mathematical models favours the detail descriptions of biological processes (i.e. ASM1) but simplified the hydraulic design of the

reactors for instance by assuming a complete mix reactor. One of the few studies that address the hydraulic design in stabilization ponds is the guidelines developed by Shilton and Harrison (2003). In the guidelines, the authors highlight the importance of modelling tools to assess the hydraulic performance of the ponds. Recently researchers have become interested in the application of computational fluid dynamics as tools to assess the hydraulic performance of WwTP systems.

This paper presents a prediction design approach for the hydraulic design of stabilization ponds. A set of performance indicators is selected and with the support of a computational fluid dynamic tool they are estimated for each of the competing design alternatives. The approach is implemented in a non-trivial case study where the shape of the ponds has to be adapted to the irregular topography of the terrain, contrary to traditional rectangular shape of the constructed ponds. DESIGN OF STABILIZATION PONDS Depending on the influent wastewater and the effluent quality required, stabilization ponds may use one ore more types of ponds where different processes take place, i.e., anaerobic ponds, facultative ponds and maturation ponds. The primary treatment takes place at anaerobic ponds that are generally designed to remove suspended solids and a fraction of the dissolved organic matter. Such ponds are generally between 2 and 5 m depth and are designed for high organic loads (between 100 and 300 gBOD5/m3/d), which makes them to be the smallest ponds of a treatment system. The average retention times of anaerobic ponds are between 0.85 and 1.5 days (Mara and Pearson 1998).

The main factors that influence the efficiency of anaerobic ponds are a) the characteristics of the waste water (organic load, suspended solids, temperature, etc); b) the environmental conditions of the site; c) the design load; d) flow conditions of the reactor; e) the hydraulic retention time. For factors a), b) and c) there exist important developments and design guides for different organic load and climatic conditions (Mara 1997).

However, due to the simplified geometry of anaerobic ponds (rectangular area and uniform, flat bottom) and the low flow velocities occurring in these systems, the analyses of factors d) and e) are carried out by assuming simplified flow conditions. The guidelines of Shilton and Harrison (2003) suggest considering the following factors in the hydraulic analysis of ponds: a) pond geometry; b) location of inlets and outlets; c) inflow and its temporal variations; d) wind and temperature, and e) position and length of directional walls if they exist. There are a number of ways of sizing ponds. Loading rates give a ratio of, for example, BOD to pond area. Alternatively we can use the ideal flow equations, as mentioned previously, to calculate the retention time required. Regardless of which approach is used, they all have a common weakness – they take no account of the physical configuration. Flow Types in Reactors and Design Criteria In theory, there are two ideal reactor types: those with completely mixed flow and plug-flow. In the design of the firsts an instantaneous mix between the mass and the water is

assumed, while in the design of the second there is no longitudinal diffusion when water goes through the pond. In the latter, the pollution particles tend to get out the pond in the same sequence as they get into it, which means that the retention time tends to the theoretical retention time.

Nevertheless, in reality, stabilization ponds do not operate as plug-flow but as a non ideal plug-flow with high longitudinal dispersion (in other words a combination of the two ideal types of flows). This means that, in general, stabilization ponds do not operate with the hydraulic retention time they are designed for. This difference between theory and practice can be explained by the presence of short-circuits (preferential path that has the shortest distance between inflow and outflow, so that retention times are insufficient for removal processes) and dead-zones (space that is not reached by the normal flow, so that the effective volume for treatment is reduced).

Therefore, the hydraulic design of a stabilization pond requires that the average retention time Ta be as close as possible to the theoretical retention time Tr, avoiding in that way the formation of short-circuits (Xs) and dead-zones (Xd). The flow type analysis can be done using the equations of no ideal plug-flow. The degree of dispersion can be estimated using the dispersion index (d) as an indicator. If d tends to zero, then the flow tends to be plug-flow and if it tends to infinite then the flow is mixed. In practice, d, Xs and Xd are estimated by means of tracer experiments in the field (Levenspiel 1999). Nonetheless, for designing purposes, these indicators must be predicted and for this reason mathematical models are needed. METHODOLOGY In general, the methodology is based in a combination of the empirical iterative approach and the prediction-design approach. Therefore, first a preliminary design of the system is defined based on loading rates and theoretical retention times. The position of the inlet and outlet are defined based on available guidelines for hydraulic design (Shilton and Harrison 2003). Then, the preliminary design obtained is used to build a mathematical model of the system. Since, this is a design exercise there is no need for model calibration. However the structure and stability of the model must be proved. Once the model is able to represents the behaviour of the system, is then used to predict the hydraulic of the pond. A set of hydraulic performance indicators (PI) are selected and with the help of digital particle tracers they are predicted for each alternative. In other words, the model is used to simulate tracer experiments; as if they were carried out in the field and the estimated PI are used to assess the hydraulic behaviour of competing design alternatives.

In this research a sound mathematical model was needed to represent the hydraulic behaviour of the ponds. For this purpose the software Delft3D open source (from Deltares) was used. One of the main characteristics of the software is the possibility to represent the hydrodynamics in 2 or 3 dimensions of any water system. In addition, it can be used to represent the transport of substances and digital tracers.

Definition of Performance Indicators The PIs are defined based on the theory of hydraulics in reactors described above and presented in Table 1. The number of tanks in series is also included, in order to compare the results with this indicator commonly used in WwTP evaluations. The indicators are calculated based on the methodology by Levenspiel (1999) for tracer experiments.

Other indicator included is the percentage of the pond whose horizontal velocities are below the critical velocity for which sediments are dragged (Vc). For the considered case study, the critical velocity was set to Vc = 0.05 m/s. Additional information as the length of a path followed by a particle and the total length of directional walls is also included for comparison of alternatives.

Table 1. Selected performance indicators used as optimization criteria Indicator ID unit Expected tendency Reported values* Theoretical retention time Tr day Average retention time Ta day Ta==>Tr Dead-zones Xd - Xd==>1 Short-circuits Xs - Xs==>0

Dispersion index d - d ==>1

Single ponds = 1 to 4 Ponds in serie = 0.1 to 1

Dispersion coefficient D m2/s 0.1 to 100 Number of tanks in series N - Length of directional walls Lb m Length of particle path Area with v > critical velocity area>Vc % Vc = 0.05m/s

*As reported in (Tchobanoglous, et al. 2003) CASE STUDY: RUBIALES, COLOMBIA The case study consists in the design of two stabilization ponds in serie for Rubiales, Colombia. One of the constraints of the design is the request to use the characteristics of the terrain to flood an area and limit the construction cost of the system. This implies that the system designed has to adapt to the terrain and not as the conventional approach were, the terrain is heavily modified to fit the rectangular shapes of stabilization ponds. The shape of the pond and depths, after flooding the area, is presented in Figure 1. The resulting ponds are prompt to have short circuiting and dead-zones. Therefore, the aim of the research is to apply the methodology proposed to find the best hydraulic design of the two ponds. In this paper the optimal design refers to the configuration of inlet, outlet and baffles such that the flow indicators are within the ranges described in the Table 1. Model description for the pond The pond L2 (Figure 1) consists of a 20661 m3

reservoir modelled with 220 cells distributed in 20 cells in north-south direction and 11 in east-west direction (Figure 1).

Figure 1. Grid configuration for pond L2

The analysis of the flow patterns is carried out by analyzing pond L2 with and without

directional walls and for maximum and average flow. The characteristics of the scenarios are summarized in Table 2 and Figure 2. Table 2 describe the basic information of the pond: area, volume, flows and retention times. Figure 2a shows the base scenario that consists of the preliminary design pond with out directional baffles. Figure 2b present one of the scenarios assessed with baffles. Note, that more scenarios were evaluated but they are not included in this paper. In the map of Figure 2b are included dark lines that describe the configuration of the baffles used to direct the flow. The maps in Figure 2 also include lines in lighter colour (blue dashed line) that describe the path generated by the model for the particle tracer. The length of the path of the tracer is used to estimate the coefficient of dispersion (D).

Table 2. Description of ponds and design flows Characteristic Unit L2

Volume m3 20662 Surface area m2 8076 Maximum flow (MF) m3/s 6.67 Theoretical retention time max flow days 0.04 Average flow (AF) m3/s 4.44 Theoretical retention time average flow days 0.05

a. Base scenario L2-0-MF b. Scenario with baffles L2-1-MF

Figure 2. Scenarios for ponds with and without directional walls

RESULTS AND DISCUSION Analysis of base scenario The base scenario corresponds to the preliminary design. The hydraulic indicators estimated for this scenario are presented in the third column of Table 3. In general, the PIs

show that the pond has a very poor hydraulic performance. The average retention time is only 20% of the theoretical retention time. The additional indicators may explain that the short retention time of the pond is associated with the significant volume of dead-zones (Xd =1.76 > 1). This result was expected due to the shape and configuration of the pond. As can be seen in Figure 2a, in the bays of the pond the model register average horizontal velocities near to cero. That means that the volume of the bays do not contribute to the useful volume of the pond. Another reason that explain the poor hydraulic performance is the presence of short-circuits (Xs =0.19 > 0). The path delineated by the particle track (blue line for scenario base in Figure 2a) shows the preferential path of the flow and it follows the deepest part of the pond.

The dispersion index (d = 1.3 > 1) and the number of tanks in serie (N = 1.2 < 2) suggest that this pond will work as a no-ideal reactor with trend to complete mix reactor flow. This is coherent with the type of flow expected in a one reactor treatment plant, as suggested by Tchobanoglous, et al. (2003). As a consequence of the poor hydraulic performance of this preliminary design, it is expected that this pond will have removal efficiencies bellow the values expected. For this reason in this design is needed to include baffles and adjust the location of the inlet and outlet according with. The objective is to block the preferential paths and direct the flow through the pond, thus, the volume available for treatment is used at maximum possible.

Table 3. Summary of results for selected indicators Indicator Unit L2-0-MF L2-1-MF

Tr day 0.04 0.04 Ta day 0.01 0.03 Tp day 0.01 0.02 Xd - 1.76 1.54 Xs - 0.19 0.29 D - 1.29 0.64 D m/s2 10.98 7.25 N - 1.2 1.83

L_bafl m 0.0 108.1 Dist_traz m 97.5 182.9 Area>Vc % 91.5 65.4

Analysis of scenario with baffles The hydraulic PIs for the scenario with 108 m of baffles are presented in the four column of Table 3. In general, the indicators show an improvement in the flow in the pond. In terms of average retention time (Ta), the baffles increase two times the retention time with respect to the base scenario. In other words, Ta estimated for the scenario with baffles is 75% of the theoretical time (Tr) which is a significant improvement.

The dead-zones are also reduced according to the values of the index (Xd). This may be due to the channels created by the baffles that direct the flow to the bays thus using more of the volume available. Even thought, the Xd index is still more than one, indicating the occurrence of depth zones. That can be a consequence of new dead-zones form between

the corners of the baffles and the border of the pond. The short circuiting index (Xs) shows a small increment. That may be associated with the increment in the flow velocities in channels form by the baffles. The increment in velocities can be observed in the velocity map presented in Figure 3.

The dispersion index (d=0.81<1) shows that the flow in the pond with baffles tend to be a non ideal plug-flow with high longitudinal dispersion. This type of flow can be also explained because the channels form with baffles. Thus, increasing the use of the available volume in the pond and increasing the retention time. Figure 3 presents the results of the curve E (normalized concentration of tracer at the effluent of the pond) and the maps of average depth velocity. When Figure 3a and Figure 3c are compared, it can be observed the effect of baffles in terms of type of flow. The flow change from a completely mix reactor (Figure 3a) to a reactor that tend to be a two tanks in serie, in other words, it tend to be plug-flow (Figure 3c).

The velocity maps show a significant change between scenarios. In the bays, the flow velocities increase from near cero (Figure 3b) to 0.05 m/s (Figure 3d). It is also critical the acceleration of flow in the first canal, which may disturb the settling processes. Therefore, additional measures in the design may be required; for instance deepening the channel will reduce the flow velocity. The outlet has a negative effect in the flow, leaving a significant part as a depth zone. Therefore, with the addition of the baffles is needed to modify the location of the outlet.

Pond L2, Inflow = 6.67 m3/s (MF) a) E-curve, scenario L2-0-MF b) Velocity map scenario L2-0-MF

c) E-curve, scenario L2-1-MF d) Velocity map scenario L2-1-MF

Figure 3. E-curves and velocity maps for the two design alternatives

In general, the methodology proposed allow the designer to replicate the information that generated by a tracer study. This model based tracer study generates a significant amount of information that allows understanding the behaviour of the pond and the

competing alternatives. It is clear that the pond with out directional walls will have a poor hydraulic performance and therefore a inefficient treatment of the wastewater. With 108 m of baffles, the retention time is near the theoretical retention time, therefore increasing the chance of the pollution components of the wastewater to receive the proper treatment. The dispersion index and number of tanks in series show that the flow tends to become a plug-flow with high horizontal mix. Even though is not the ideal flow it is better than the complete mix reactor when the interest is the removal of organic matter.

CONCLUSIONS AND RECOMENDATIONS

The methodology proposed is a promising approach to support the design of wastewater treatment plants base on stabilization ponds. Valuable information can be generated and correction actions can be evaluated in the model platform following a prediction-design approach.

In traditional design of ponds the location of baffles and the length of them follow empirical guidelines. However, the results of this research show that they may have contradictory effects. For instance, the baffles may eliminate dead-zones form at the edges of the ponds but it may create others in the corners of the baffle. Or the channels form with baffles may increase the velocity and disturb the treatment process. Thus, the selection of the baffles could be express as a mathematical optimization problem with multiple objectives. Significant experience has been accumulated in the optimum design of wastewater system by using for instance multi-objective evolutionary algorithms. A further research, could explore this possibility.

Future research should also include the description of the wastewater treatment process and indicators for investment and operational cost.

REFERENCES

[1] Harremoës, P.; Rauch, W.,(1999). "Optimal design and real time control of the integrated urban run-off system", Hydrobiologia, Vol.410 p.p. 177.

[2] Henze, M.; Gujer, W.; Mino, T.; Matsuo, T.; Wentzel, M. C.; Marais, G. v. R.; van Loosdrecht, M. C. M.,(1999). "Activated sludge model no. 2d." Water Science and Technology, Vol.39 (1), p.p. 165~182.

[3] Levenspiel, O., (1999). "Chemical reaction engineering third edition", Sons, J. W. Ed. New York.

[4] Mara, D.; Pearson, H., (1998). "Design manual for waste stabilization ponds in mediterranean countries. " In Lagoon Technology International; : Leeds, England.

[5] Shilton, A.; Harrison, J., (2003). "Guidelines for the hydraulic design of waste stabilisation ponds", In Institute of Technology and Engineering Massey University: Palmerston North NEW ZEALAND.

[6] Tchobanoglous, G.; Burton, F. L.; Stensel, D.; Metcalf & Eddy, (2003). "Wastewater engineering: Treatment and reuse , inc." McGraw-Hill Ed.