MODELLING THE DYNAMIC RESPONSE OF A SECONDARY CLARIFIER TO A SEA WATER INCURSION AT THE RADFORD STW

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1 MODELLING THE DYNAMIC RESPONSE OF A SECONDARY CLARIFIER TO A SEA WATER INCURSION AT THE RADFORD STW Burt, D. 1, Ganeshalingam, J. 1, Hammond, C. 2 and Egarr, D. 1 1 MMI Engineering Ltd, UK, 2 Hyder Consulting, UK Corresponding Author Tel DBurt@MMIEngineering.com Abstract As part of a recent study of a secondary clarifier optimisation for the Radford coastal Sewage Treatment Works (STW) in Plymouth, a Computational Fluid Dynamic (CFD) model has been used to consider the dynamic performance of the secondary clarifiers during storm flows with sea water incursions. The addition of seawater into the system increases the density of the liquid phase at the influent producing an enhanced gravity current. There is significant site evidence to suggest that a sea water incursion degrades clarifier performance but the mechanisms for this are not clearly understood. This behavior has not previously been investigated with a modelling approach The paper describes the development of a CFD model of the Radford STW final clarifier No 4 to determine the performance under dynamic changes in influent flow including storm influx and a 1% seawater incursion for a duration of up to 1 HRT (Hydraulic Retention Time). Two inlet modifications are investigated; one with an upturned bellmouth and the other with an Energy Dissipating Inlet to test whether dispersing and mixing the load at the influent could exert some control over the enhanced gravity current. Keywords Activated Sludge, Mass Flux Theory (MFT), EDI (Energy Dissipating Inlet), Computational Fluid Dynamics (CFD) Introduction Radford Sewage Treatment Works (STW) is managed and operated by South West Water Ltd and serves a population of approximately 22, from the Plymstock area to the east of Plymouth. Over the next 2 years, it is anticipated that the catchment population will grow by around 8,. This will result in higher average flow and loads for treatment. South West Water Ltd and Hyder Consulting Ltd have carried out a review to identify the asset upgrades required to deal with future growth and historical performance issues, particularly sea water incursion. The source of the sea water is a result of seawater ingress in to the sewer system. This sea water is consequently conveyed to the STW.

2 Figure 1: Aerial View of the Radford STW and low-lying coastal areas (Google Maps, 213) A number of options for extending Radford STW were considered including increasing the volume of the activated sludge aeration tanks, increasing the area of final settlement tanks and improving the characteristics of the activated sludge. All options involve increasing the capacity of the oxygen transfer system, remedial works to reduce saline infiltration and measures to reduce the degree of filamentous foaming. There is significant site evidence to suggest that a sea water incursion degrades clarifier performance but the mechanisms for this are not clearly understood. This behaviour has not previously been investigated with a modelling approach. In the initial phase of Computational Fluids Dynamics modelling, the performance of the existing secondary clarifiers is assessed under steady state conditions to identify potential improvements for current and future flow conditions. The secondary clarifier design was then tested for influent flow profiles approximating a diurnal flow cycle for a storm flow event as well as for a seawater incursion event where an influent flow was contaminated with 1% by volume of seawater. The performance of the secondary clarifier, measured by the effluent concentration and bed depth, were monitored over the period of the storm and seawater incursion event and the following hours up to 6 hydraulic detention time (HRT). This paper mainly discusses the secondary clarifier response to a storm and sea water incursion. Methodology Mass Flux Theory (MFT) is the standard and basic design approach to determine whether a tank has sufficient surface area to settle the solids load at a given process condition. However, Computational Fluid Dynamics modelling has significant

3 61 mm 61 mm 38 mm 23 mm 23 mm 1 mm advantages over 1-dimensional Mass Flux Theory (MFT) by using the actual geometry of the secondary clarifier to resolve the flow patterns within the tank which will include non-ideal flow behaviour such as flow recirculation. The CFD model developed for this work has been extended from the IAWQ drift flux model for activated sludge and water mixtures in secondary clarifiers. The solids settling behaviour is included using a drift flux model; hindered settling is incorporated with the Takaćs double exponential function which is an extension of the Vesilind settling curve. Sludge mixture density follows Larsen, and the sludge rheology model of Bokil & Bewtra is used to describe the non-newtonian behaviour of the liquid-solid mixture in the settled sludge layer. The development of these models were tested and validated against three independent sets of site data. In a CFD model, the fluid dynamics within the secondary clarifier are resolved in 2D or 3D. CFD models can therefore provide contour maps of flow profiles and solids concentration in the secondary clarifiers. The CFD model therefore allows the calculation of the height of the sludge blanket and the effluent suspended solids (ESS) concentration, which are both indicators of how close the tank is to failure i.e. the point at which the blanket is spilt. Neither of these parameters can be determined from mass flux theory. CFD models can be used to test different internal geometry configurations such as the stilling well design which may incorporate a McKinney baffle or Energy Diffusing Inlets (EDIs). Geometry Radford STW has four secondary clarifiers. Of the four secondary clarifiers, the tank with a diameter of 18 m and a side wall depth of 2.3 m was considered and discussed in this paper. This tank is fitted with a diffuser drum of 3.5m diameter and 1.5m depth below top water (TWL). The existing tank (as built) and proposed EDI modification, marked in dark blue, for the CFD model are presented in Figure 2. The EDI has two rows of four ports, each spaced at uniform pitch centres (eight ports in total). 175 mm 9 mm 25 mm 3 mm 18 mm 9 mm 9 mm 175 mm 15 mm 4 Ports 28x28mm 121 mm 4 Ports 28x28mm 165 mm mm 172 mm As built Secondary Clarifier Secondary Clarifier with an EDI device Figure 2: Radford STW Secondary clarifier geometry details with and without proposed EDI modification.

4 V, Settling velocity [m/h] Settling Velocity [m/h] Sludge Characteristics A series of settlement tests were carried out at Radford STW to characterize the hindered settling behavior of the activated sludge. The data was analysed to determine the SSVI3.5 value for the subsequent MFT and CFD analysis. Figure 3(a) shows the Vesilind coefficients from the measured data. Based on these coefficients, the sludge equates to SSVI 15 ml/g for the Pitman and White correlation y = 5.751e -.456x V zs = V o exp(-nx) n =.456 m 3 /kg V o = 5.75 m/h Sludge Concentration [kg/m 3 ] (a) Vesilind coefficients from the measured data Measured Pitman and White Sludge Concentration[kg/m 3 ] (b)sludge settling correlations compared with the experimental data. Figure 3: Radford STW Activated Sludge characteristic Figure 3 (b) shows a comparison of the measured hindered settling velocity with the standard UK Pitman & White correlation for a SSVI3.5 value of 15 ml/g. Hence, all the CFD and MFT calculations were undertaken using Pitman & White settling characteristics to ensure that the results would be representative of the site sludge samples. A limitation of this work is that the effect of the local concentration of sea water is not considered. The main influence on the settling performance of the tank is assumed to be the change in the mixture density at the inlet due to the presence of sea water. Results and Discussion Influent Optimisation - Steady State Simulations Initially, the performance of the existing secondary clarifier was assessed under steady state flow conditions (a) with the existing internal geometry arrangement and (b) with an EDI. The EDI design developed by MMI comprised two rows of 4 counter current discharge ports. This bespoke design of EDI device is discussed in detail in Reference. The EDI induces a swirling (circumferential) component to the flow. The process conditions that were assessed in this phase of work are presented in Table 1. Figure 4 presents CFD results at State Point C. Both the as built tank and the EDI design show very similar performance. The built tank shows that there is a density waterfall and flow re-entrainment which creates stirring. However the depth of the

5 ESS [mg/l] tank prevents transport of high concentrations of solids to the effluent weir. The EDI shows improved sludge consolidation and therefore has a slightly lower sludge bed than the as built tank. Table 1: Influent Optimisation Secondary clarifier performance with and without an EDI design Run Process flow condition QEff / tank QRAS/ tank Inlet MLSS [L/s] [L/s] [mg/l] SSVI 3.5 [ml/g ] ESS [mg/l ] As built Bed Depth [m] EDI design ESS [mg/l ] Bed Depth [m] A FFT B FFT C FFT D FFT E FFT Basecase : As built Case2 : EDI design ESS = 1 mg/l Bed depth = 1.83m Density water fall and re-entrainment Better consolidation of the bed ESS = 1 mg/l Bed depth = 1.95m Effluent Solids Conc. [mg/l] vs Stable ESS As Built EDI Design Figure 4: Contour plots showing the distribution of solids concentration through a 2D slice of the tank on a log scale (1 mg/l Conc. 1 mg/l) for the state point C.

6 Flow [ L/s] When analysed for a range of state points the EDI device was found to be marginally more effective at high flow and solids loading. The EDI design improves the settling performance resulting in a consolidated sludge bed. The EDI enhances mixing in the stilling well by inducing a swirling (circumferential) component to the flow. This usually increases mixing within the stilling well and improves settling performance. Hence the addition of the EDI gives improvements in effluent solids stability. Storm Flow Event - Dynamic Simulation The secondary clarifier with and without an EDI was assessed to determine the performance of the secondary clarifiers for a storm flow event over a 24 hour period. This event is considered with MLSS (35 mg/l), SSVI (12 ml/g) and a constant RAS flow Site Flow per FST, L/s Fitted CFD -Flow, L/s Figure 5: Selected storm flow event for 24 hour period and fitted curve for CFD analysis. Figure 5 shows the influent flow profile representing the storm flow event. During this storm, there is an increase in flow of factor 3 when compared to the flow rate at the beginning of the storm flow event. This increase in flow occurs in a 14 hour period. Figure 6 shows the effect of the dynamic flow on the effluent suspended solids concentration where the ESS spikes up to 27mg/l and 13mg/L for the as built tank and EDI design respectively. Therefore, in this analysis, the effect of sea water is not considered, and the tank performance is assessed for the storm profile only. The peak ESS concentration is approximately double for the existing design than the peak value for the EDI design therefore confirming that for an increase in flow, the tank with an EDI fitted operates better than the existing inlet arrangement. The results also indicate that there is typically a delay of approximately 1 hour between the peak storm flow entering the secondary clarifier and the response of the ESS concentration. Figure 7 shows that the sludge blanket position rises to a minimum of 1.7m below TWL for the as built tank and to 1.9m below TWL for the clarifier with an EDI fitted which

7 Sludge bed depth below TWL [m] Flow, [L/s] Effluent solids [mg/l] Flow, [L/s] confirms that the sludge bed continues to be more consolidated under storm conditions with the EDI. Overall, the secondary clarifier performs well under storm conditions with and without the EDI modification. The EDI acts as a momentum dissipater, providing a degree of protection from a typical storm event, halving the maximum ESS (13 vs 27 mg/l) and keeping the blanket lower in the tank (1.9 vs 1.7 m below TWL) As Built - ESS EDI- ESS Flow [L/s] Figure 6: Effect of storm flow on the ESS concentration for the secondary clarifier with and without an EDI influent modification As Built- SBD EDI- SBD Flow [L/s] Figure 7: Effect of storm flow on the settle sludge bed for the secondary clarifier with and without an EDI influent modification Saline Seawater incursion - Dynamic Simulation A seawater incursion event was assumed such that the influent flow is constant at 52L/s and contaminated with 1% by volume of seawater during the first 2.6 hours (1

8 Effluent solids [mg/l] Sea water [%] hydraulic residence time) of the storm flow. This event is considered with values of MLSS of 35 mg/l, SSVI of 12 ml/g and recycle ratio of.6. The secondary clarifier with and without an EDI was tested to assess the performance of the tank during the seawater incursion event and the following 15.6 hours (6 HRT). Figure 8 shows that both designs discharge substantial effluent solids following the event up to 5 x HRT) as a result of the enhanced density current. The peak effluent discharge is about 2% (34 vs. 381 mg/l) lower for the EDI. The mechanism for the ESS rise is due to the influent density current that lifts the sludge blanket as shown in Figure 9. Both designs took approximately 13 hours (5 x HRT) to recover to consent levels for ESS concentration. The EDI influent modification had only a minor effect reducing peak effluent discharge by 2% relative to the original installed equipment. Hence, the influence of the EDI for the saline intrusion event is less significant As built - ESS EDI - ESS Sea water influent Figure 8: Effect of seawater incursion on the effluent suspended solids concentration for the secondary clarifier with and without an EDI influent modification.

9 Sludge bed depth below TWL [m] Sea water [%] Basecase : As built Basecase : As built T = 2.6 Hours (1HRT) No seawater present at the influent T = 2.6 Hours (1HRT) 1% (v/v) seawater present at the influent Figure 9: Effect of salt incursion on the influent density current and sludge bed within the secondary clarifier As built - Bed depth EDI - Bed depth Sea water influent Figure 1: Effect of seawater incursion on the settle sludge bed for the secondary clarifier with and without an EDI influent modification Figure 1 shows that the sludge bed rises to a minimum of.65 m below TWL for the secondary clarifier with an EDI and to.75m below TWL for the existing tank. Conclusions CFD analysis has been undertaken for the secondary clarifiers at Radford STW, for the existing tank design and retrofitted with an EDI. The CFD model was used to calculate

10 and compare the clarifier performance with respect to effluent solids concentrations and sludge bed depths. The results suggest that the EDI works well as a momentum dissipater but works less well for a seawater incursion. The EDI therefore provides a degree of protection from a typical storm event without sea water ingress, halving the maximum ESS and keeping the blanket lower in the tank. The trend information suggests that the EDI gives improved performance for both steady state behaviour and dynamic loads for the secondary clarifier. However, the absolute differences in effluent solids carry over are not significant for this secondary clarifier except during the peak storm flow. This modelling study confirms that a sea water ingress pulse can significantly degrade final clarifier performance at a coastal STW and that it is difficult to control such incursions with tank inlet modifications alone.

11 References South West Water Radford STW, Supply and demand evaluation, 12 September 212. Ekama, G.A., Barnard, J.L., Gunthert,F.W., Krebs,P., McCorquadale, J.A., Parker, D.S. and Wahlberg, E.J., Secondary Settling Tanks, Theory, Modelling, Design and Operation, International Association of Water Quality, Scientific and Technical Report No 6, (IAWQ), Takács, I., Patry, G.G., and Nolasco, D., A Dynamic Model of the Clarification Thickening Process, Water Res, 25(1), Larsen, P. On the hydraulics of rectangular settling basins, experimental and theoretical studies, Dept of Water Resources Engineering: Lund Institute of Technology, Lund University, Bokil, S.D. and Bewtra, J.K. Influence of Mechanical Blending on Aerobic Digestion of Waste Activated Sludge, Proc., 6th Int. IAWPRC Conf. on Water Pollution Res., Int. Assoc. on Water Pollution and Control, London, , Burt, D.J. Improved Design of Settling Tanks using an Extended Drift Flux model, PhD Thesis, University of Bristol, UK, January 21. Pitman, A.R. Settling Properties of Extended Aeration Sludge, J. Wat. Pollut. Control Fed. 52(3), ,198. White, M.J.D., Settling of Activated Sludge, Technical Report TR11, Water Research Centre, Stevenage, UK, Wahlberg, E.J. and Keinath, T.M. Development of settling flux curves using SVI. J. Wat. Pollut. Control Fed. 6 (12), , 1988.