Abstract
Final Settlement Tanks (FSTs) often limit the efficiency of a wastewater treatment plant. If capacity growth or more stringent compliance is needed, it is prudent to determine the performance-enhancing effect of an adaptive inlet system in the FSTs by means of flow simulations (i.e. Computational Flow Dynamics) before making a decision to install additional tanks and/or filter systems.
It is demonstrated that the use of adaptive inlets can significantly enhance the capacity and performance of FSTs, enabling treatment plants to better react to changing flow and load conditions and thereby to improve compliance or to reduce the works needed to address growth and changing consents. A case study from Dresden-Kaditz Wastewater Treatment Plants illustrates how the use of adaptive inlets has successfully addressed performance challenges without the need to build additional clarifiers.
Keywords
Adaptive; Blanket; Capacity; CFD; Clarifier; Inlet; Retrofit; Settlement,
Introduction
FSTs are often the main interface point to the environment for wastewater treatment plants and therefore have a particularly high impact on the environmental performance of the entire wastewater treatment plant. The interdependency of fluctuating flow and load, and the fixed geometry of an FST inlet structure, can affect both the retention of suspended solids and the hydraulic loading capacity that can be achieved (Armbruster & Barth 2017).
This paper explores how the use of adaptive inlets can deliver significantly enhanced performance and capacity in FSTs, resulting in a range of capital and operational benefits.
Current State – Working with Fixed Inlets
Due to the wide range of loads regularly encountered in wastewater treatment plants, resulting in extremely different sludge levels in FSTs, hydrograv developed height-variable inlet systems for FSTs that automatically optimise to suit the current flow and load conditions. With these systems, the position and opening width of the tank inlet between diffusion drum and McKinney baffle are continuously optimised using an algorithm to match the tank sludge level.
The efficiency of FSTs is dominated by fluid mechanical processes and the tank geometry, and is affected by the following influences:
- The concentration of the incoming sludge, which determines sludge density and in turn impacts flow dynamics.
- The settling behaviour of the sludge, as measured by the sludge volume index (SVI).
- The flow rate of sludge being drawn off and returned through the activated sludge plant, both of which affect inflow rates to the secondary clarifier.
Conventional tank inlets are fixed and therefore always discharge the sludge-water mixture at a pre-determined height. However, the bigger the distance between the sludge level and the inlet height, the more disturbance occurs within the sludge blanket and mixing zone.
If the inlet is above the sludge level, the inflow stream passes through the clear water and the incoming sludge emits particles into the clear water in a turbulent way. Since activated sludge hardly settles as individual particles, sludge introduced into the clear water via the density waterfall is often discharged via the clear water outlet and leads to increased suspended solids discharge values of the wastewater treatment plant.
If the discharge takes place below the top of the sludge level, individual bacteria remain trapped in the sludge and do not cloud the clear water. This phenomenon is called “flake filter effect”. However, in this situation the sludge swirls up under turbulence and thus the efficiency of the tank is strongly impaired. The internal volumetric flow rates increase massively and the tank is subjected to significantly higher loads than the rated load of flow, MLSS and SVI would suggest (Armbruster, 2004).
It should be noted that a deep inlet structure lowers suspended solids in the activated sludge by unnecessarily strong sludge displacement, and thus reduces the capacity for nitrogen decomposition. In addition, the sludge level rises and the hydraulic capacity and operational safety against sludge drift is reduced.
The Case for Adaptive Inlets
The task of a wastewater treatment plant is not only to achieve the best achievable effluent values temporarily for a more or less randomly selected loading combination, but also to try and secure them at all times for any realistic loading. The use of fixed inlets in FSTs inhibits the ability of many treatment plants to meet these objectives.
Using adaptive inlets enables FSTs to react to changing flows and loads by moving the inlet position to one which limits disturbance within the sludge blanket and clear water zone, thus maximising the effective operation of the tank. It also ensure that during dry weather conditions, the sludge blanket level can be reliably reduced and the clear water zone maximised, thus giving the tank the maximum possible solids handling capacity in advance of a storm event (Benisch et al. 2018).
The effective use of adaptive inlets has been proven through its deployment in over 250 tanks across over 100 projects globally.
Assessing the Potential Benefits of Adaptive Inlets
Clearly the installation of adaptive inlets represents a significant investment, so their use is generally considered as part of a design process to address a growth driver, tightening consents or compliance issues within a wastewater treatment plant. Before reaching a decision to invest, it is generally advisable to analyse their effectiveness and estimate their benefits in a virtual environment.
To this end, Computation Flow Dynamics (CFD) is carried out on the FST, both in its existing state and with the use of an adaptive inlet system. This tool enables the flow patterns in FSTs to be considered under both Dry Weather Flow (DWF) conditions and during Full Flow to Treatment (FFT) situations.
Analysis of the CFD outputs then enables reliable comparison of likely sludge blanket levels and mixing zones within the FST, and also to quantitively estimate the likely performance improvement. According to the application and project objectives, this from there can include improvements to hydraulic capacity and/or the removal performance of suspended solids (and by inference non-dissolved phosphorus).
The CFD analysis enables an optimum geometry to the adaptive inlet to be determined, and provides a basis for the design of a control algorithm linked to works flow.
Dresden Case Study
During the design of an upgrade to the activated sludge stage of the Dresden-Kaditz Wastewater Treatment Plant, flow simulations were used to analyse the extent to which the suspended solids in the activated sludge could be increased by rigid or height-variable modifications to the inlet structures of the FSTs, while at the same time meeting the treatment consent for the plant.
The secondary clarification stage of the plant consisted of six circular tanks with a diameter of 48.5 m and a 2/3 depth of approx. 4.62 m (Fig. 1). The diffusion drums had a diameter of 6.0 m. The inlet opening in a concrete centre column was 2.56 m above the base, with an opening width of approx. 1.15 m and vertical lamellas within the drum. The tanks were equipped with full-span scraper systems and the clear water was drawn off via submerged slotted pipes.
Figure 1: Secondary clarifier of the Dresden-Kaditz wastewater treatment plant – cross section.
The original proposal to address the capacity shortfall included the construction of two additional clarifiers with the same geometry and equipment as the six existing tanks.
A comprehensive statistical analysis of the operating data was conducted prior to the CFD simulations, in order to determine the loading range as well as possible. From this, relevant loading cases for low, medium and high loadings were derived, and for each of these the tanks were evaluated both in their initial state and with adaptive inlets (Armbruster & Barth 2017).
For the most common loading combination “medium load” (50 %-percentile of the sludge volume) an optimised inlet structure configuration was derived. For the “low” loading case (10 % percentile of the sludge volume), the functional safety against flake discharge with increased suspended solids was demonstrated. And at “high” load (95 % percentile of the actual sludge volume occurring at the wastewater treatment plant) the functional safety against sludge discharge was proven.
To illustrate the benefits of an adaptive inlet system, the CFD outputs for “high” load conditions clearly show how the height of the clear water zone can be increased from 0.5m to approximately 2m, and the depth of the ‘mixing zone’ can be dramatically reduced (Fig. 2).
With the fixed inlet, the clear water zone is simply too small and rising sludge from the mixing zone is liable to reach the outlet. However, the adaptive inlet stays above the denser sludge zones, limiting mixing and ensuring that a much deeper clear water zone is available.
Figure 2a: CFD output for “high” load in wet weather conditions – MLSS 3250 mg/l and SVI 150 ml/g – using existing fixed inlet arrangement.
Figure 2b: CFD output for “high” load in wet weather conditions – MLSS 3250 mg/l and SVI 150 ml/g – using an adaptive inlet configuration.
Following the analysis, the operator of Dresden-Kaditz WWTP elected to install adaptive inlets across the full bank of secondary clarifiers. In the solution, the height of the inlets can be modulated by up to 2.9m as determined from the CFD outputs. Works to each tank took approximately three weeks, although due to the need to perform works in autumn/winter and one tank at a time, the equipment took 14 months to install across all tanks.
Following completion of the tank upgrades, operating results showed that the adaptive inlet systems achieved significantly better values for turbidity and suspended solids, as well as for COD and phosphorus. With the likely increase in MLSS by the operators, and the reduced sludge displacement in mixed water, the modification is also expected to have a positive effect on nitrogen discharge concentration.
Figure 3: Secondary clarifiers at Dresden-Kaditz WWTP. Left: pre-existing condition, right: during modification.
Figure 4 shows the effects of the modification of the inlet structures on the effluent turbidity of the wastewater treatment plant. After the modification, the average monthly turbidity decreases to around 2 FNU continuously, which is only one third of the values before the project. In addition, TSS and Phosphorus removal performance improved, with TP seeing a continuous reduction by approximately 0.2 mg/l compared to previous concentrations without changes to the precipitant dosing regimes. In time, the operator was able to reduce the consumption of precipitant without risking TP compliance.
Figure 4: Turbidity results (monthly mean values) in the treatment plant effluent from before, during and after the modifications to the clarifiers.
Conclusion
Overall, the retrofitting of adaptive inlets to the clarifiers at Dresden-Kaditz WWTP succeeded in maintaining and even improving treatment performance at the plant without the need to construct additional clarifiers as originally intended. The capital cost saving of this was estimated to be in excess of €4 million with some Opex benefits also realised.
Acknowledgements
The authors wish to thank Stadtentwässerung Dresden (Dresden City Drainage Board) for their co-operation with this scheme, and for their permission to use data and imagery from the project.
References
Armbruster, Dr. M. and Barth, Dr. M. (2017) Wastewater Treatment Plant – Potentials of Modern Inlet systems for Secondary Clarifiers. WWT Modernisierungreport – Wasser & Abwasser Kommune, Huseen-Medium GmbH Berlin.
Benisch, M., Neethling, J.B., Hammer, G. and Armbruster, Dr. M (2018) Can a Variable Clarifier Inlet Reduce Tertiary Treatment Requirements at Metro Wastewater Reclamation District’s Robert W. Hite Treatment Facility?. Proceedings of the Water Environment Federation’s Technical Exhibition and Conference (WEFTEC) 2018, New Orleans, U.S.A.
