This study focuses on the effect of rapid mixing on the coagulation efficiency in a full-scale drinking-water treatment plant and discusses the mechanisms involved in the floc-formation process. The results refer to three periods of operation of the waterworks when no mechanical mixing was provided in the tanks for coagulant dosing due to mechanical failure of the rapid mixers. Although a certain deterioration of the subsequent flocculation process was observed, as assessed using the data for suspended solids, turbidity, and chemical oxygen demand, the overall water treatment performance was not affected. This suggests an insignificant role for intense rapid mixing in sweep flocculation during full-scale water treatment and reveals the potential to reduce the required energy costs for mechanical mixers.

INTRODUCTION

Coagulation is a process widely applied in water treatment to remove particulate, colloidal, and dissolved impurities. The common mechanisms of coagulation with hydrolyzing salts are (1) charge neutralization and destabilization of particles and colloids upon electrical double-layer compression and (2) complexation and precipitation, which implicate the enmeshment of impurities in metal-hydroxide precipitates, also known as sweep flocculation. The extent to which these mechanisms contribute to the coagulation process depends on factors such as pH and coagulant dose. Coagulation due to charge neutralization is preferably achieved with the coagulant being in the form of ionic complexes and in lower concentrations, which requires a rapid dispersion of the chemical in water to ensure optimal destabilization. Sweep flocculation prevails when higher chemical dosages are applied, so that the coagulant rapidly precipitates after dissolution and, thus, loses its ability for charge neutralization (Binnie et al. 2002; Duan & Gregory 2003). This mechanism generally gives considerably improved particle removal, especially when the natural organic matter content in water is high, as compared to charge neutralization.

The role of mixing in coagulation during water treatment is usually described as rapid dispersion of a coagulant in water (Randtke & Horsley 2012). Many studies have investigated the importance of rapid mixing in water treatment (Amirtharajah & Mills 1982; Mhaisalkar et al. 1991; Kawamura 1996; Rossini et al. 1999; Yu et al. 2011). Both the turbulence in the tank and the mixing time are important parameters for the process of coagulation and flocculation. High-velocity gradients are usually applied at the point of addition of the coagulant. This ensures quick distribution of the chemical in water and the rapid formation of floc nuclei. However, Edzwald (2013) recently argued that the importance of the mixing intensity for sweep flocculation processes is largely overstated, suggesting that coagulation by precipitation is indifferent to mixing intensity.

As for the importance of mixing itself, the stalling of a rapid mixer during coagulation is expected to have a critical effect on treatment plant performance. The objective of this study was, therefore, to demonstrate that the accidental breakage of mixers in a full-scale treatment plant can be used to study mixing performance and to understand whether and how such disturbances can be compensated for by other treatment steps. Furthermore, the potential for energy savings was evaluated.

DESCRIPTION OF THE STUDY SITE

South-West Waterworks, Moscow

The South-West Waterworks (SWWW) in Moscow, Russia, treats surface water from the River Moskwa using a conventional pre-treatment prior to multi-media filtration, ultrafiltration, and disinfection. The pre-treatment includes sweep flocculation (Figure 1). Monthly average dosing rates of 3–7 g/m³ as Al3+ are applied as coagulant with subsequent flocculation using a polymer. The coagulation and flocculation take place in three rapid-mixing tanks followed by a gently stirred flocculation tank (Schröder & Förster 2011). The coagulant, polyaluminum chloride during the course of this study, is added in the first rapid-mixing tank; pH is adjusted in this tank by sulfuric acid dosing. The second rapid-mixing tank is used for the addition of powdered activated carbon (PAC). In the third tank, a polymer flocculant is added. Flocculation takes place in the subsequent tank, where low turbulence is provided.

Figure 1

Flow diagram of pre-treatment for the full-scale process applied at South-West Waterworks in Moscow.

Figure 1

Flow diagram of pre-treatment for the full-scale process applied at South-West Waterworks in Moscow.

Mixer failures during operation

During the past two years, mixer failure in the coagulant dosing tank occurred at three different times and in three different lines of the water treatment plant. Each time, the mixer shaft ruptured. The reason for the mechanical failure has not yet been fully understood but it was not the subject of this investigation. In Figure 2, a broken mixer (Figure 2(a)) and its position in the treatment train (Figure 2(b)) are shown.

Figure 2

A broken rapid mixer (a) and its position (b) in the SWWW treatment train.

Figure 2

A broken rapid mixer (a) and its position (b) in the SWWW treatment train.

On each occasion, the mixer failures caused a period of operation without rapid mixing in the affected line. Due to production, delivery and import times, it took between three weeks and three months to replace or repair the mixer shaft (Table 1). During this period, no rapid mixing was provided in the respective coagulation tank.

Table 1

Duration of water treatment plant operation with a rapid mixer failure

Coagulation tankStart of failureEnd of failure
Line 2 21 June 2012 01 October 2012 
Line 3 15 December 2013 21 January 2014 
Line 4 19 February 2014 12 March 2014 
Coagulation tankStart of failureEnd of failure
Line 2 21 June 2012 01 October 2012 
Line 3 15 December 2013 21 January 2014 
Line 4 19 February 2014 12 March 2014 

Without the rapid mixer, only low turbulence due to the water in- and outflow was achieved in the respective tank. Each of the rapid-mixing tanks has a hydraulic retention time of 2.3 minutes. As the time for the formation of metal-hydroxide precipitates, including entrapment of colloids, lasts from 1 to 7 s (Bratby 2006), it is anticipated that dispersion of the coagulant and precipitation will be completed before the water enters the second rapid-mixing tank.

It should be noted, first, that the treatment plant operation was not changed due to the mixer problems that occurred. The plant is continuously operated at its full design capacity of 10,417 m³/h 24 h/day throughout the year. Raw water is treated in four parallel lines with equal capacity. The treatment efficiency is typically evaluated using regular laboratory analyses of parameters such as suspended solids, turbidity, and chemical oxygen demand (COD), with KMnO4 as the oxidant, determined following the standard procedures at SWWW Moscow (GOST 3351–74 1974; PND F 14.1:2:4.154–99 1999; PND F 14.1:2:4.254–2009 2009, respectively). The concentration of suspended solids is measured after sedimentation separately for each of the four lines.

RESULTS AND DISCUSSION

In Figures 3, 5 and 6, the monthly average concentrations of suspended solids, turbidity, and COD for each line, as well as the average of all four lines, are shown. Turbidity and COD were measured after multi-media filtration in blended effluents of lines 1 + 2 and lines 3 + 4, respectively. The parameter concentration in the respective line, during the non-mixing periods, is highlighted in each figure. Direct sampling after each mixing step was not possible due to the tank configuration. Therefore, future research is necessary for more detailed analysis of each step.

Figure 3

Monitored concentration of suspended solids after sedimentation in lines 1–4 (30-day mean values), when the rapid mixers in lines 2 (a), 3, and 4 (b) were out of operation.

Figure 3

Monitored concentration of suspended solids after sedimentation in lines 1–4 (30-day mean values), when the rapid mixers in lines 2 (a), 3, and 4 (b) were out of operation.

Figure 5

Monitored turbidity after multi-media filtration in lines 1 + 2 and lines 3 + 4 (3-day mean values), when the rapid mixers in lines 2 (a), 3, and 4 (b) were out of operation.

Figure 5

Monitored turbidity after multi-media filtration in lines 1 + 2 and lines 3 + 4 (3-day mean values), when the rapid mixers in lines 2 (a), 3, and 4 (b) were out of operation.

Figure 6

Monitored chemical oxygen demand (COD) after multi-media filtration in lines 1 + 2 and lines 3 + 4 (3-day mean values), when the rapid mixers in lines 2 (a), 3, and 4 (b) were out of operation.

Figure 6

Monitored chemical oxygen demand (COD) after multi-media filtration in lines 1 + 2 and lines 3 + 4 (3-day mean values), when the rapid mixers in lines 2 (a), 3, and 4 (b) were out of operation.

As can be seen from Figure 3, the concentration of suspended solids in each of the four production lines followed a similar pattern. The minor fluctuations from the average did not exceed ±1 mg/L in 2012 and ±2 mg/L in 2014. A considerable deviation from the average of suspended solids in line 2 can be seen during the period without mixing from July to October 2012 (Figure 3(a)). During this period, a number of storm-water events caused frequent changes in the raw water quality, which led to higher variability in the flocculation. However, a suspended solids concentration of 4–6 mg/L after sedimentation can be classified as normal for the SWWW plant. Unfortunately, it was not possible to measure the actual floc size, but visual observation indicated that the floc size and distribution in line 4 during the operation without rapid mixing were quite comparable to those observed at the same moment of time in line 3 during the operation with a rapid mixer (Figure 4). However, a generally smaller floc size in line 2 without mixing was observed as compared with the standard operation (not shown). Indeed, smaller flocs are believed to be the reason for less stable sedimentation and may, thus, be responsible for higher concentrations of suspended solids in the flocculation tanks, which were observed during the failures in lines 2 and 3.

Figure 4

Flocs formed during waterworks operation in the coagulation tanks of lines 3 (with rapid mixing) and 4 (without rapid mixing).

Figure 4

Flocs formed during waterworks operation in the coagulation tanks of lines 3 (with rapid mixing) and 4 (without rapid mixing).

Conversely, the quality of the multi-media filter effluent hardly deteriorated due to the mixing problems (Figures 5 and 6). Within a range of ±0.2 mg/L, there was no change in COD levels between the two operation modes (Figures 6(b) and 7(b)), suggesting that the coagulation performance was not impaired with respect to the removal of dissolved organic substances. Similarly, turbidity fluctuations in lines 2 and 4, which had mixer failures, did not exceed ±0.2 NTU and were consistent with the values measured in other lines and other periods of time during standard operation (Figures 5(a) and 7(a)). The failure in line 3 caused a significant increase in turbidity in the filter effluent from ca. 0.1 up to 0.8 NTU. However, this did not affect the established filtration conditions, such as the filtration cycle duration or backwash frequency.

These observations suggest that other factors may impair the coagulation efficiency and contribute to an insufficient enmeshment of suspended solids into flocs. Indeed, the water temperature in SWWW may reach 25 °C in the summer months, but typically does not exceed 0.2 °C in winter (between December and February). Although it was previously shown by Xiao et al. (2008, 2009) that low temperature does not impede the hydrolysis of aluminum, the coagulation rate was lower, and a higher turbidity after sedimentation was observed at low temperatures. This may explain the generally higher suspended solids and turbidity values observed in SWWW in the winter months.

Figure 7

Turbidity (a) and chemical oxygen demand (COD) (b) after filtration measured in lines operated with rapid mixing versus lines operated without rapid mixing during the failure periods.

Figure 7

Turbidity (a) and chemical oxygen demand (COD) (b) after filtration measured in lines operated with rapid mixing versus lines operated without rapid mixing during the failure periods.

In order to evaluate the mixing conditions, we calculated the velocity gradients in the waterworks. During operation with rapid mixing, the velocity gradient in the coagulant dosing tank in the SWWW plant is 300 and 225/s during summer and winter, respectively. Taking into account the kinetic energy from the average flow velocity in the tank without mixing, the velocity gradient is reduced to 30 and 20/s during summer and winter, respectively. Using Kolmogorov's theory of the smallest eddy induced by mixing, the mean transport time for convective transport in mixing can be estimated (Crittenden et al. 2005). While the distribution of the chemicals at SWWW takes 0.5–1.5/s with rapid mixing, the transport time increases 10-fold to 5–15/s if no mixing is provided. Although the typical design criteria for rapid-mixing systems suggest velocity gradients in the range of 300–1,000/s, the product of the velocity gradient and average retention time is recommended in a wide range of G × t = 10,000–200,000 (Degremont 1979; Amirtharajah et al. 1991; Grombach et al. 2000; HDR Engineering Inc. 2001; Binnie et al. 2002; Randtke & Horsley 2012; Qasim et al. 2000). In this study, when no mixing was provided, the velocity gradient of less than 30/s, which is typically designed for the flocculation, and the corresponding velocity time product of G × t < 4,000 did not significantly impair the coagulation process. It is therefore assumed that the conventional design values are much too high.

These results suggest that the coagulation process bears great potential for energy savings with regard to rapid-mixing conditions. In particular, the total installed mixing power in the coagulation tanks in SWWW is 36 kW, which equals 3.5 Wh/m³ of treated water. This results in an annual energy demand of more than 300 MWh/a for mixing during coagulation. At current electricity prices in Moscow, annual costs amounting to 760,000 RUB (ca. 19,000 EUR/a as estimated in June 2014) can be saved.

CONCLUSIONS

It can be concluded from this study that rapid mixing in the tank for coagulant dosing is less important for sweep flocculation than is widely considered to be the case. Even though the suspended solids concentration in the sedimentation tank increased when no mixing was applied, sedimentation may still be compromised, as the turbidity and COD in the filtrate were not directly affected by the failures. This suggests that the available hydraulic mixing conditions are already sufficient for fast coagulant dispersion in the tank. Filtration easily compensated for the mixing failure, which strengthens the importance of the multi-barrier concept. Furthermore, in the SWWW, three consecutive rapid-mixing tanks are installed, which allows the use of an alternative dosing point for the coagulant. However, as the coagulation and the subsequent treatment steps remained stable, it proved unnecessary to change the dosing point for the coagulant.

It was suggested that mechanical mixing for coagulation may require more energy than necessary and has a certain potential for optimization in full-scale waterworks, especially when sweep flocculation is applied. Namely, if the coagulation mixing facilities installed in all SWWW in Moscow were to be disabled, the total energy savings would amount to more than 300 MWh/a.

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