Retrofitting Water Treatment Plants

In water treatment, the process of flocculation can play a pivotal role in meeting required water quality standards. Today, water providers face increasing concerns over water regulation, pollution, and budgets. Retrofitting existing water treatment facilities with advanced flocculation technology is fast becoming a proven solution to meet evolving regulatory requirements and community needs at a reasonable cost. This paper explores the significance of flocculation in water treatment; diving into principles, applications, and current technologies. Analyzing recent retrofits and case studies provides insights into the practical implementation of upgrading existing water treatment infrastructure.

2. Defining Flocculation

Flocculation constitutes suspension and gentle collision of particles in water to form larger aggregates known as floc all while avoiding sedimentation. Floc must achieve a specific size to facilitate removal during sedimentation of the water treatment process. The effectiveness of flocculation largely relies on the introduction of chemical agents, such as cationic polymers, during the coagulation stage. These chemicals, typically positively charged, serve to neutralize the negatively charged particles in the water. This neutralization promotes the agglomeration of particles into larger floc.

Successful flocculation is a delicate balance of adequate mixing to encourage the collision and aggregation of floc, while simultaneously minimizing shear forces to prevent disintegration. This equilibrium ensures the formation of stable floc that settle out effectively, optimizing the overall efficiency of the treatment process.

3. Current technologies

Coagulation, flocculation and sedimentation are interconnected processes which rely on one another. While this paper focuses on flocculation retrofits, it’s important to acknowledge the advancements in using improved coagulants and flocculants engineered for better performance and cost-effectiveness. These chemicals are selected based on their efficiency in neutralizing suspended particles and promoting floc formation. While this paper doesn’t dive deeper into this topic, choosing the best coagulant for different source water qualities remains a critical component in water treatment.

When retrofitting current technologies into existing systems, basin design, flows, transition areas, and inlet/outlet locations are a few important factors that must be carefully examined. By considering the overall hydraulics and flow velocities of a current system, the best technology can be selected for replacement of old methods, and improvements can be made to optimize the flocculation process.

Presently, a wide variety of flocculator methods are installed in the U.S. and abroad. Common types of flocculators include: hydraulic baffle walls, vertical and horizontal paddle wheels, bladed impellers, walking beams, and hyperbolic impellers.

Several factors must be considered to determine the ideal equipment for the treatment plant. Major criteria for evaluation include maintenance requirements, adjustable velocity gradients, power usage, energy transfer, redundancy, and floc quality:

• Maintenance: Paddle wheels and walking beams have parts, such as submerged and bottom bearings located below the water surface, which require basin drainage for maintenance. In contrast, all maintenance for bladed impeller and hyperbolic flocculators is performed above the water surface. Hydraulic baffle flocculators typically require the lowest maintenance, as they have no mechanical parts. However, periodically the entire train must be taken offline and drained for cleaning sediment buildup.

• Adjustable Velocity Gradient: Hyperboloid and bladed impeller flocculators offer the highest level of treatment flexibility, by adjusting the rotational velocity to optimize the velocity gradient. Velocity gradients with vertical and horizontal paddle wheels and walking beams are typically set during design by adjusting the number of paddles. The rotational speed may be adjusted if the units are installed with variable frequency drives. However, speed changes can lead to excessive settling in the floc basins. Hydraulic baffles offer little or no process flexibility as they are designed for one flowrate without adjustment. By being able to adjust the velocity gradient, an operator has the ability to adjust the flocculators, seasonally for temperature, flows, or when treatment chemicals are changed.

• Power Usage: Except for hydraulic baffle flocculators, all other types utilize gearboxes and motors, indicating similar power consumption levels due to velocity gradient requirements per stage. Due to a higher pumping capacity, hyperboloid flocculators may be operated at lower speeds than dictated by velocity gradient alone, providing some energy savings.

• Redundancy: Bladed impeller, vertical paddle wheels, and hyperboloid flocculators offer the highest redundancy if a unit is down. Horizontal paddle wheels have the least amount of redundancy, as multiple units typically operate on one drive.

• Energy Transfer: Shear forces can impact floc quality when energy transfer between the flocculator and the floc particles is not taken into consideration. The hyperboloid flocculator has equal pressure distribution over the shape of the mixer body which acts to minimize shear forces. The acceleration path of the particles is very long, indicating a more gradual transfer of mixing energy to the floc particles. With bladed impellers, the energy from the mixer is transferred over a much shorter acceleration path, meaning higher shear forces. Additionally, pressure developed during floc acceleration is spread only over the surface area of 3 or 4 blades which can increase shearing as well.

• Floc Quality: Properly operated flocculators of all types are known to produce settleable floc. Horizontal Paddle wheels, although an older technology with more maintenance and higher levels of sedimentation, are also recognized for producing good settling floc. Hyperboloid flocculators have an advantage balancing appropriate mixing energy with low shear forces due to the shape and flow patterns.

Computational Fluid Dynamics (CFD) modeling can be a beneficial tool for the design and retrofit of flocculators. CFD enables precise optimization of mixing parameters, further enhancing flocculation efficiency. It aids in identifying short circuits, flow issues, and understanding of the influence of baffles and openings throughout the flocculation stages. By understanding flow, basin design, and overall hydraulics, areas of high shear can be avoided. CFD modeling can even help select the most suitable equipment for flocculation by modeling the different flocculator technologies under the same basin hydraulics and design.

4. Floc Properties

Understanding floc properties is crucial for optimizing flocculation. Floc, formed through the aggregation of suspended particles, exhibit a range of characteristics that influence their behavior and settling dynamics within the treatment system.

One key property of floc is size distribution, which directly impacts settling rates and sedimentation efficiency. Larger floc generally settle more rapidly than smaller ones, aiding in their removal from the water during the sedimentation process. Other factors which significantly affect settling behavior include structure, density, porosity and surface area of the floc. Additionally, floc with a loose, open structure tend to settle more slowly than those with a denser, compact structure. Internal porosity and surface area influence adsorption capacity and interaction with contaminants, further affecting treatment efficiency.

Required velocity gradients and mixing intensities also play an important role in the formation of floc. Velocity gradient is defined as the square root of the ratio of power loss by shear per unit volume of fluid to the viscosity of the fluid. Although it represents the energy level per volume unit, this parameter is also used to predict the intensity of shear and represents the intensity of shear applied to the fluid, which influences the flocculation dynamics by affecting the rate of particle collisions and floc formation.

G = Velocity Gradient or Mixing Intensity (s^¯¹)
P = Power (kW)
V = Tank Volume (m^³)
µ = Viscosity (Pa*s)

While the velocity gradient quantifies the amount of mixing energy imparted to the fluid, it does not fully describe the manner in which this energy is introduced or distributed within the flocculation basin. Understanding the distribution of shear energy and its impact on floc formation requires a more nuanced approach. Specifically, assessing how power is introduced and distributed in the basin. Considering factors such as flow patterns, turbulence, and the geometry of the mixing equipment, this can provide a deeper insight into the flocculator’s performance.

For example, energy dissipation can vary significantly depending on whether the mixing system generates uniform or localized shear, since it affects how efficiently the energy is transferred to the floc particles. By analyzing the spatial distribution of shear and energy within the flocculator, one can better understand how these factors influence floc size, structure, and settling characteristics. Therefore, integrating the concept of energy distribution with the velocity gradient offers a more comprehensive view of flocculator performance and allows for optimization of the flocculation process.

Understanding the properties of floc, including size distribution, structure, density, and velocity gradient, is
essential for designing, operating and retrofitting efficient flocculation systems. By carefully assessing these
parameters and integrating innovative technologies such as hyperboloid flocculators, treatment plants can
achieve superior removal of suspended solids and contaminants.

5. The Hyperboloid Flocculator

6. Real World Applications

Built new in 2015 to replace an existing water treatment plant, the City of Annapolis, Maryland installed hyperboloid flocculators replacing old bladed impellers from the existing plant. Annapolis reported several significant improvements from the previous system which were attributed to the new equipment. The lead
operator highlighted increased treatment capacity in a smaller footprint, particularly praising the hyperboloid flocculators. The City now uses significantly less coagulant for water treatment; approximately 30% less alum resulting in cost savings. The operator also reported that less lime is needed to neutralize the acidifying affect of alum as well. Treated water quality improved with iron levels decreasing from 1.0 ppm in the old system to 0.5 ppm in the new system, despite
reduced coagulant usage. The City of Annapolis noted the reliable performance and minimal maintenance requirements of the new equipment. Sediment accumulation in tanks is minimal and is accredited to the flow patterns generated by the hyperboloid flocculator confirming prior CFD modeling. CFD modeling was performed on four different load cases that considered minimum, average, and maximum flow rates with different rotational speeds. The modeling confirmed that the design would handle all flow rates with the proposed basin layout. The CFD modeling also concluded that even at lower rotational speeds, there would be reliable mixing through the stages, with no areas of high shear.

In 2019, the City of Bellevue, Ohio’s Water Filtration Plant operating since 1935, replaced old horizontal paddle flocculators with hyperboloid flocculators. Bellevue Operations indicated several notable changes and positive impacts in the survey. Bellevue’s respondents noted enhanced first-stage treatment process while operating more quietly than past installed equipment. While they did not observe a reduction in coagulant dosage, they do now have the ability to feed powdered activated carbon (PAC) at the flocculation stage which represents a significant operational enhancement. The City of Bellevue also obtained improved treated water quality, with decreased turbidity in the treated water.

In 2013, Hickory, North Carolina Public Utilities and Systems retrofitted one treatment train of the City of Hickory Water Treatment Plant. Four bladed impeller flocculators were replaced with four hyperboloid flocculators. The survey results of the adoption of hyperboloid flocculators reveal several positive results. Operators reported significantly less settling of solids within the hyperboloid train than the trains with bladed impellers. The design of the basins prevents isolation of the hyperboloid flocculators for dosage adjustment which makes it difficult to determine any changes in chemical dosage. Flow to the hyperboloid flocculators remains consistent with the other trains. While data on treated water quality isn’t tracked for each basin, it’s noted that the flocculation basins with hyperboloid units have less sediment buildup over time compared to the bladed impellers. Although flows are not separated for each train, a recent collection of data indicated lower turbidity in the sedimentation basin following the hyperboloid flocculators at 0.22 NTU average compared to the sedimentation following the bladed flocculators at 0.31 NTU average.

The City of Atlanta, Georgia Chattahoochee Water Treatment Plant, has begun the upgrade of their existing horizontal paddle wheels to the hyperboloid flocculators. The first train to be installed did not initially achieve the expected settled water turbidity. An in-depth study including CFD modeling of the basins with both the hyperboloid flocculators and the old paddle style was conducted to pin-point the issue. It was discovered that the original basin design had significant areas of short circuiting, which was leading to sub-optimal performance. At the manufacturer’s recommendation, baffles were added, indicated by the modeling, to create a serpentine flow and eliminate short-circuiting. This adjustment aimed to improve flow dynamics within the basins containing the hyperboloid flocculators. The correction of the hydraulic short-circuits has led to the upgraded train producing the best settled water turbidity of the plant. While most of the other five trains average 0.55 NTU to 0.90 NTU, the retrofitted train consistently averages 0.15 to 0.50 NTU, even after wet-weather events.

The Houston East Water Purification Plant upgraded their horizontal paddle wheels with hyperboloid flocculators in 2020. Since installing the hyperboloid flocculators, there has been a noticeable improvement in treated water quality, with management and operators expressing high satisfaction. Although there was a switch from Ferric and Lime to PACl, making direct cost comparisons challenging, the new system has significantly reduced the amount of sediment in the tanks, with only about 1 inch of corner sediment compared to the previous 10 feet of sludge buildup. Flow rates have remained unchanged, and no data is available to compare treated water quality between the old and new systems.

Outside of the US in the Czech Republic, the Bzenek Ground Water Treatment Plant replaced horizontal paddle wheels with hyperboloid flocculators to trial in 2007 which led to a complete replacement in 2009. The trial was designed to objectively compare the effectiveness and performance of hyperboloid flocculators versus horizontal paddle wheels. Initially, the same speed was set for the Invent mixers, which was then varied – both increased and decreased. Different speeds were subsequently adjusted for the front and rear mixer pairs. At the start of the trial, the front mixer pairs operated at a higher speed to optimize floc formation in the rear mixers and prevent their disruption. Later, the front pairs were taken out of service while the rear pairs operated, and vice versa. Analysis of the results indicated that maintaining the same speed across all four hyperboloid flocculators created optimal conditions, as a higher shear at the flocculation tank inflow made increased speed of the front pairs unnecessary.

During the trial, power consumption for both flocculators was monitored, allowing for calculations of power requirements, energy consumption, and estimated annual operating costs for the mixers. In conclusion, the results indicate that higher speeds (up to 20 rpm) lead to increased floc destruction, shorter retention times in the flocculation area and sedimentation tank, and higher filtering velocities for total iron, total manganese, and calcium carbonate. The optimal speed for the hyperboloid flocculators was inversely related to WTP output: higher outputs require a minimum speed of 6 rpm, while lower outputs can use a maximum of 13 rpm. Analysis shows that setting all four hyperboloid flocculators to the same speed of 10 rpm achieves optimal conditions, enhancing flocculation
for this water source.

Comparative analyses from samples taken between November 26 and December 21, 2007, demonstrate that hyperboloid flocculators in train 4 outperformed paddle wheels in train 2 in terms of efficiency: total iron reduced by approximately 0.41 mg/l, total manganese by 0.10 mg/l, along with turbidity and color also seeing significant decreases. This results in longer runs at a higher filtering velocity and an annual savings of about 82% in power consumption when replacing paddle wheels with hyperboloid flocculators.

7. Conclusion

8. Literature

Droste, R. L., & Gehr, R. (2018). Theory and practice of water and wastewater treatment (2nd ed.). Hoboken, NJ: John Wiley & Sons.

Metcalf & Eddy Inc., Tchobanoglous, G., Burton, F. L., Tsuchihashi, R., & Stensel, H. D. (2014). Wastewater Engineering: Treatment and Resource Recovery (5th ed.). New York, NY: McGraw-Hill Education

MWH. (2012). Water treatment: Principles and design (3rd ed.). Hoboken, NJ: John Wiley & Sons.

Sawyer, C. N., McCarty, P. L., & Parkin, G. F. (2003). Chemistry for environmental engineering and science (5th ed.). New York, NY: McGraw-Hill Education.