On the importance of mixing characterization and application in the water treatment process

Floc separation depends on the physical properties of the floc, such as size, size distribution, density and settling velocity. These properties are determined by environmental factors (such as the composition and concentration of impurities) and by chemical (Gonzalez-Torres et al. 2017; Filipenska et al. 2019) and mixing conditions (Bache & Rasool 2001; Coufort et al. 2007; Xu et al. 2010) during flocculation. Therefore, the optimization of these conditions is necessary.

Chemical conditions are still the primary interest. Thus, optimization of the coagulant dose is common. The optimization of pH is less frequent; the importance of this step has been explained in Naceradska et al. (2019). However, the optimization of mixing conditions (i.e., mixing intensity and flocculation time) is usually neglected. Despite this, there are several cases of drinking water treatment plants (DWTPs) where mixing optimization resulted in more efficient separation and thus a more efficient overall treatment process within the DWTP modernization (e.g., Svetla nad Sazavou, Kutna Hora, Milence, CZ).

Therefore, we would like to show and emphasize in this paper that controlling floc properties by optimizing mixing leads to a high separation efficiency of flocs and is as essential as coagulant dose and pH optimization. We propose general guidelines for designing flocculation parameters in water treatment.

The properties of flocs determine their separation efficiency. The only variables that can be chosen and thus set to tailor flocs with suitable properties are coagulant type and mixing intensity. In particular, the role of mixing is often underestimated both in research and in DWTP operation. Therefore, optimization of mixing for a given separation technology is crucial and necessary. Another significant difficulty is inaccurate quantification of mixing conditions. The mixing intensity is very often characterized by the rotation frequency of a certain impeller in revolutions per minute (rpm). However, rpm units provide no information about hydrodynamics. It is, thus, always necessary to give the value of the global velocity gradient in research papers dealing with flocculation.

The velocity gradient is a fundamental quantity for mixing intensity characterization in various mixing devices (Equations (2)–(7)). In contrast, the frequently used units of rpm are insufficient for quantification of mixing intensity because they prevent comparison of different mixing devices.

Flocculation and separation reach the best results when mixing has a few different stages. Each mixing stage has its purpose. The first stage, homogenization mixing, is applied immediately after dosing the coagulant, and its primary purpose is to achieve efficient and rapid dispersion of the coagulant in the treated volume of water. Homogenization mixing is achieved either hydraulically (under plug flow), e.g., by using different static mixers, or mechanically (by back mixing), e.g., by using different impellers (Bratby 2006), and has high intensities of G > 300 s⁻¹ (Morrow & Rausch 1974). In contrast, the purpose of subsequent flocculation mixing is to form flocs that are removable by commonly used separation processes, such as sedimentation, flotation and filtration. Thus, flocculation mixing allows collisions between the particulate pollutants and the coagulant and ensures the binding of these components into flocs. Since each of the separation processes requires different floc properties, different mixing intensities are needed, and flocculation mixing can be further divided into rapid and slow. However, this division is a rough representation, and the flocculation mixing intensity should be adapted solely to the required floc properties.

In practice, rapid flocculation mixing is often mistaken for homogenization mixing. However, the purposes of the two processes are different. Rapid flocculation mixing is a technological process in the initial phase of flocculation. The applied velocity gradients are in the range of approximately 100–400 s⁻¹ (Polasek 2007). Mixing with higher intensities does not further affect floc properties (Mutl et al. 2006). The selected mixing intensity should be applied until the floc properties reach a steady state, i.e., the floc size and density do not change any further (Polasek 2007). Literature data on necessary flocculation times vary considerably due to differences in the definition of rapid mixing, inconsistencies in the way the velocity gradients are calculated, and different ways of evaluating the mixing efficiency. Rapid flocculation mixing produces small and dense flocs that are effectively separable by sand filtration (Ngo et al. 1995; Pivokonsky et al. 2011; Bubakova & Pivokonsky 2012).

Slow flocculation mixing is characterized by velocity gradients ranging from 20 to 100 s⁻¹ with flocculation times ranging from approximately 5 to 30 min (Polasek 2007). Slow flocculation mixing should follow rapid flocculation mixing if large and rapidly settling flocs are required. Nevertheless, the usual practice is the application of slow flocculation mixing immediately after homogenization mixing. Such mixing produces flocs with unsatisfactory properties; they are heterogeneous in size, irregular and very porous with a low settling velocity (Coufort et al. 2005; Pivokonsky et al. 2011; Bubakova et al. 2013; Filipenska et al. 2019). The technological process consisting of homogenization mixing and rapid and slow flocculation mixing produces large flocs with high settling velocities (Polasek 2011; Wang et al. 2011), which are effectively separable by sedimentation.

The efficiency of floc separation is influenced by floc properties (size, size distribution, shape, density and settling velocity) that can be controlled by adjusting mixing intensity. Based on the amount and properties of formed flocs, we can distinguish two types of separation: single-stage and double-stage separation.

Direct filtration requires flocs that penetrate the entire volume of the filter bed and exhibit sufficient adhesion ability onto the filter cartridge surface (sand). Small flocs, generally <50–60 μm with high density, have been proven to be ideal for direct filtration separation (Ngo et al. 1995; Bache & Gregory 2010; Pivokonsky et al. 2012). In contrast, large flocs (>100 μm) of irregular shape and relatively low density are not suitable for sand filtration separation because they are not able to penetrate deep into the filter bed. They clog the sand filter rapidly, which causes a rapid pressure drop, and filter cycles (filter run times) are shortened to a minimum (Pivokonsky et al. 2011; Bubakova & Pivokonsky 2012).

The pressure profiles in a filter bed during filtration of suspensions (flocs) prepared at different velocity gradients are illustrated in Figure 1. With increasing mixing intensity, the filtration efficiency increases. Large flocs prepared by slow flocculation mixing penetrate only the upper layers of the sand filter. The filter capacity is not fully utilized, and the filter cycles are short. When the mixing intensity increases, the floc properties change, and the flocs penetrate deeper into the filter medium. The sludge capacity of the filters increases significantly, and the filtration cycles are extended (Polasek & Mutl 2002).

The velocity gradients and residence time required to form a suspension separable by filtration roughly correspond to the rapid flocculation mixing values (G = 100–400 s⁻¹) (Pivokonsky et al. 2011; Polasek 2011).

When a suspension is heterogeneous in size, direct filtration is inappropriate. Therefore, floc separation has to proceed in two successive steps, sedimentation and filtration. Large flocs (>100 μm) with high density and resistance to tangential forces are ideal for separation by sedimentation (Edzwald 1995; Polasek & Mutl 2005). The remaining smaller flocs continue to the filtration step.

It has been suggested (Polasek 2011) and confirmed at several restored DWTPs that large flocs with high density can be prepared by applying a sequence of rapid and slow flocculation mixing (Mixing Purpose: Slow flocculation mixing). The formation of large flocs can be supported using a flocculation aid at the end of a rapid flocculation mixing phase. After homogenization with the flocculation aid, slow flocculation mixing follows.

During dissolved air flotation (DAF), flocs are trapped on the surface of air bubbles that lift them to the water surface. Therefore, small flocs of tens of μm, preferably 25–50 μm, with a low density close to the density of water are required (Edzwald 2010). Even large (>50 μm) low-density flocs formed by low-velocity gradients (G = 10 s⁻¹) are removable by flotation with high efficiency (Vlaski et al. 1997). The reason might be that flocs are broken into sizes suitable for flotation when they enter the contact zone where velocity gradients are 10–100 times higher than those used for floc formation (Bache & Gregory 2010).

Since flotation requires only very small flocs, the residence time in flocculation is usually up to 10 min (Valade et al. 1996; Edzwald 2010). Thus, flocculation does not proceed to a steady state and usually continues in the flotation unit.

The separation of flocs by flotation is often compared with sedimentation. DAF is reported to be more effective than sedimentation in treating water containing cyanobacteria, algae, and humic substances or a low content of turbidity-forming particles (clay minerals and aluminosilicates). The reason for the high flotation efficiency of these pollutants is the low density of the flocs formed by their coagulation (Edzwald 2010). However, this assertion is based on the erroneous assumption that the separation of flocs formed under the same mixing conditions was compared (Khiadani et al. 2013).

It is necessary to realize that the properties of flocs are also affected by their composition. Thus, applying the same velocity gradient to different components (impurities/coagulants) often results in flocs with different properties (Filipenska et al. 2019). This phenomenon can be seen in Figure 2. This fact must also be considered when choosing a separation method. For example, flocs created from a ferric coagulant and cellular organic matter (COM) from Microcystis aeruginosa are suitable only for sedimentation filtration due to their size. If an aluminium coagulant is used instead, even flotation filtration and direct filtration are possible when the mixing parameters are suitably optimized (Filipenska et al. 2019).

In practice, the contaminant concentration also determines the separation method. In the case of raw water with low contamination requiring a small amount of coagulant, a small amount of suspension is produced. Thus, single-stage separation by sand filtration is possible. In the case of highly contaminated raw water requiring large coagulant doses, a large amount of suspension will be produced. Thus, double-stage separation by sedimentation and sand filtration is necessary. In conclusion, when selecting a separation method, it is necessary to take into account the type and concentration of impurities present in the raw water and the type of coagulants used.

Mixing conditions are a key parameter controlling floc properties. Mixing optimization can significantly improve floc separation efficiency, which leads to a decrease in DWTP operating costs. Mixing intensity is very often characterized by impeller rotation frequency (rpm), although rpm units provide no information about hydrodynamics. The usage of the global velocity gradient is more convenient and correct. The global velocity gradient can be calculated from the torque or power number of an impeller, or in the case of hydraulic mixing, from the pressure drop. With increasing velocity gradient, flocs are smaller and more compact. For single-stage separation by filtration, rapid flocculation mixing must be applied after homogenization to produce small and dense flocs that penetrate deep into the filter bed. Double-stage separation by sedimentation and filtration requires the application of homogenization, rapid flocculation mixing and slow flocculation mixing to produce large flocs with high sedimentation velocities. Double-stage separation by flotation (DAF) and filtration require small flocs with a low density. Such flocs are obtained either by rapid flocculation mixing after homogenization, shortening the flocculation time or breaking large flocs in the DAF contact zone. Nevertheless, floc properties are also influenced by the type and concentration of impurities and coagulants, which must also be taken into account when mixing is optimized for a given separation method. The ranges of velocity gradients G recommended for different mixing types and needed for the formation of flocs suitable for different separation steps are summarized in Table 1.

This work was supported by the Czech Science Foundation (project number 18-05007S) and the Czech Academy of Sciences (institutional support RVO 67985874).

All relevant data are included in the paper or its Supplementary Information.