Comparative tests of the efficiency of the transition from the hydraulic flocculation chambers to the mechanical ones at the wastewater treatment plant of Severodvinsk

V.M. ULCHENKO¹, I.Iu. BOIKO²

1. V.M. Ulchenko, Deputy Technical Director

2. I.Iu. Boiko, Process Engineer

Flocculation chambers (FC) of water supply plants are designed to create favorable conditions for the formation and aggregation of flakes during the coagulation treatment, and to form conditions for their further sedimentation in the sedimentation tanks. The FC efficiency determines not only the operation of the sedimentation tanks but also the subsequent filtering facilities and, as a result, the quality of the discharged treated water.

During the reconstruction of the water supply plants in Severodvinsk (WWTP-2) to intensify the flocculation process, the hydraulic FCs were reconstructed into mechanical ones with the installation of frame mixing devices with wall turbulizers in accordance with the patented technical solution [1].

After the reconstruction, a detailed assessment of the effect of the rotary speed of the mixers in the FC on the efficiency of the clarification process was carried out, the results of which are described in this paper.

Characterization of flocculation chambers. Water in the wastewater treatment plants, after mixing with reagents, is supplied through the pipes for the first stage of treatment to horizontal sedimentation tanks with built-in FC (see Fig. 1). Each sedimentation tank has one chamber 4.9 m long, 5.8 m wide. The design height of the water level in the flocculation chambers is 3.8 m.

Fig. 1. A scheme of the flocculation chamber (FC) with a frame mixer, Ecopolymer PE

1 – FC water inlet; 2 – distribution perforated pipe; 3 – EMT-1-304 frame mixer; 4 – wall-mounted turbulizer; 5 - electric mixer drive.

Fig. 2. The bridge with mixer drives and control cabinets

A partition wall with a spillway, separating the chamber from the sedimentation zone, is made in the form of a concrete inclined wall 3 m high, forming the width of the chamber at the bottom equal to 2.6 m. The efficient volume of one flocculation chamber is 85 m³. The retention time of water in the chamber is 25-30 minutes, depending on the flow.

During the reconstruction, the FCs were equipped with mixing equipment.

Characteristics of mixers installed in the flocculation chambers

EMT-1-304PP frequency-regulated frame mixers, Ecopolymer, were installed according to the reconstruction project of WWTP-2 in Severodvinsk, performed by My Project in 2014. For their installation, the project provides a bridge running along all 12 FCs and serving as a support for mixers (see Fig. 2). The bridge accommodates control cabinets for the mixers, each for 3 mixers.

Technical characteristics of the mixers in the FC are presented in Table 1.

The effective surface of the installed mixers is 4.2 m². With a diameter of 3.48 m and a rotation speed of 3.6 rpm, the average energy dissipation of the mixer surface is 102 J/(m²·h). The frame mixer allows evenly distributing the introduced energy in the water volume, and achieving maximum aggregation of the flakes, and at the same time minimizing their destruction, which is of particular importance in treating highly-colored low-turbidity water.

Table 1. Technical characteristics of the mixers

Experimental procedure

A production experiment to assess the efficiency of the mechanical FC was carried out in February-March 2016 on a separate line of WWTP-2, consisting of a single mixer, three FCs and three sedimentation tanks (No. 10-12). The layout of the facilities, the points of reagent injection and sampling are shown in Fig. 3.

Fig. 3. The layout of the clarification line used in the production experiment, with the indicated points of control

The test was carried for the "turbidity" and “color", total 97 determinations were done, 21-28 in each series.

The water from point 2 (after the sedimentation tanks) was analyzed in a stirred sample, and the water taken from the WWTP (point 1) in the clarified sample, after 2 hours of sedimentation.

The influence of the mixing mode on the flocculation process was evaluated in 4 series, with varying mixing conditions in separate sedimentation tanks to exclude the influence of any structural features of the flocculation chambers and sedimentation tanks on the kinetics of clarification of the treated water. The characteristics of the mixing devices of flocculation chambers for a series of experimental works are presented in Table 2.

Table 2. Operating characteristics of the mixers of the flocculation chambers during the experiments

For the comparative evaluation of the operation of flocculation chambers with mechanical mixing, the following modes were chosen: without mixing and two modes with mixing of different intensities. The difference was provided by supplying the electric mixer drive non-adjusted frequency (50 Hz) and adjusted frequency up to 30 Hz, which corresponds to the maximum radial rotation speed of the point of the mixing devices furthest from the shaft equal to 0.6 and 0.36 m/s, respectively.

In series 1 and 3, a chamber in the sedimentation tank No. 10 was used as a classical hydraulic FC, and in series 2 and 4 - the FC of the sedimentation tank No. 12.

The intervals between water sampling were 1-2 hours, and the duration of each series was 7-12 hours.

In series 1 and 2, sampling was performed at point 2 (see Fig. 1), at the outlet of clarified water from the sedimentation tank, and in series 3 and 4, sampling was performed directly from the FC at point 1 (see Fig. 1). Water samples in series 3 and 4 were taken by a bathometer at a depth of 1.5 m from the surface at the outlet of the central part of the FC, after which sedimentation was carried out in a 1 l beaker for 2 hours. Then, a sample of standing water was taken from a glass at a depth of 30 mm, to determine the parameters "turbidity" and “color".

The concentration of the "contact medium" of the flocculation chambers of the sedimentation tanks was assessed, depending on the rotation speed of the mixer.

The study of water was carried out using the RD/890 Hach photoelectric colorimeter, based on the integrated color (No. 19) and turbidity (No. 95) determination programs.

Raw water characteristics

The quality of water supplied for treatment during the period of the experiments is shown in Table 3. Water had high color, low turbidity, low alkaline reserve and extremely low temperature. It should be noted that it was very low water temperature in winter that caused work to increase coagulation efficiency through the reconstruction of the FC.

Table 3. Characteristics of water coming from the water source

Reagents and doses used

The reagent treatment conducted at the plant applies a coagulant – aluminum sulfate, a basifying reagent – soda, and a flocculant – Praestol 650TR.

Mixing of solutions of reagents with water takes place in a two-level corridor mixer with mechanical mixing devices, which are installed both in the coagulant injection zone and in the flocculant injection zone.

Preliminary test coagulation was carried out and optimum doses of treatment agents, maintained by automatic dosing stations, were determined.

The last series showed a change in the quality of the incoming water. In this situation, based on operational experience, it would be optimal to increase the dose of coagulant and reduce the pH working point from 6.1 to 5.7 to reduce color. However, since this would lead to an increase in the solubility of aluminum compounds and its residual concentration in treated water after passing through the high-rate filters (during the experiments it did not exceed 0.19 mg/l), the coagulation conditions were left unchanged. As can be seen from Table 6, 7, this led to an increase in color and turbidity in the standing water and in the mud load on the high-rate filters. However, these values, as well as the concentration of dissolved aluminum were normal in treated water.

Doses of reagents, which during all tests were maintained automatically, are presented in Table 4.

Table 4. Reagent dosing

Results of the study

In the course of the first two series of experiments, the effect of mixing the quenched FC on the concentration of the sediment layer ("contact medium") was studied. Samples of the medium were taken directly from the FC, the concentration of suspended solids was assessed for "turbidity". The average results are shown in Table 5.

Table 5. Dependence of the turbidity of the medium in the FC on the mixing conditions

The highest content of suspended particles (for turbidity) was in the FC, operating as hydraulic, with a stopped mixer: 239-248 mg/l; the lowest - with a mixer operating at maximum speed (at a current frequency of 50 Hz), 219-228 mg/l. There is a tendency to decrease in the concentration of the contact medium in the flocculation chamber with an increase in the mixer rotation speed.

The best result for the color of the standing water was obtained at a radial speed of the mixer equal to 0.36 m/s (Table 6), for three series of four.

Table 6. Dependence of the color of the clarified water, sampled after the FC, on the mixing conditions

The average color value is a bit higher after the FC with a mixer at a radial speed of 0.6 m/s, and the worst result of water clarification was observed after the flocculation chamber operating without additional mixing. It should be assumed that an increase in the rotation speed up to 0.6 m/s leads to partial destruction of the flocs formed.

The results are graphically presented in Table 4.

Fig. 4. Comparative characteristics of the residual color of the settled water at different speeds of the mixers in the FC

It should also be noted that the difference in color between water after hydraulic and mechanical FCs retained in the sedimentation tank reaches 25-30%, in favor of mechanical mixing, and when settled in laboratory conditions, the difference decreases up to 15% due to the increase in water settling efficiency after hydraulic FC.

The results of the study of the turbidity of water having passed the flocculation chamber with different rotation speeds of the mixers after settling are presented in Table 7.

The best result for the turbidity of the settled water was obtained after the FC with a mixer operating at a radial rotation speed of 0.36 m/s. A slightly higher the average value of the turbidity of water is observed at a radial speed of 0.6 m/s, and the worst result of water clarification is after the hydraulic FC. Thus, the results obtained are similar to the above described dependence for color.

The results are graphically presented in Table 5.

Fig. 5. Comparative characteristics of the residual water turbidity

Settling in the sedimentation tank results in greater difference in the "turbidity" between the hydraulic and mechanical flocculation chambers than in the “color" index, and reaches 50-60% in favor of the mechanical one. And in the case of settling in the laboratory conditions, the difference in turbidity between the best result for the FC and the hydraulic chamber does not exceed 10%.

mechanical flocculation chambers promote more effective release of carbonic acid formed as a result of alkalizing water with soda, while flotation of coagulated impurities, both in the FC and in the sedimentation tank, is less active. Perhaps, the flotation of impurities by carbon dioxide in the sedimentation tank is the main factor that negatively affects the results of operation of the hydraulic FCs in comparison with the mechanical ones, as demonstrated by the studies conducted at Severodvinsk WWTP.

Mechanical devices for mixing in flocculation chambers have certain advantages and can significantly reduce the load on the filter structures. The main indicator that determines the duration of the filter cycle of the high-rate filters is turbidity. If the turbidity of the sedimentation tanks decreases by 50%, a similar increase in filter cycle can be expected in the high-rate filters and, consequently, a reduction in energy consumption for flushing, as well as the consumption of technical water for own needs by waterworks.

Summary

The comparative study of the hydraulic and mechanical flocculation chambers has shown the process advantage of the latter.

The mixers of the flocculation chamber must be able to change the rotation speed in order to adjust this parameter to certain conditions, subject to the properties of the water supplied for treatment and the reagents used. It was determined that the optimum radial rotation speed of the mixer of the flocculation chamber of Severodvinsk WWTP-2 at low water temperature should be about 0.36 m/s.

The use of mixing devices in the flocculation chambers will allow reducing the mud load on filter structures by up to 50%, and, consequently, significantly reducing energy consumption, as well as the loss of treated water for process needs - washing of the high-rate filters.

Mechanical mixers cause a decrease in the concentration of the "contact medium" in the flocculation chambers, but the results of the FC operation show that this factor is not critical since additional energy that promotes aggregation of the particles is supplied to the chamber.

The mixers of the flocculation chambers contribute to a better release of dissolved gases in the water, in this case – carbon dioxide, formed during alkalization, which reduces the subsequent flotation of impurities in the sedimentation tanks.

References

Meshengisser Yu.M, Zhurba M.S, Pelykh S.N., Ulchenko V.M. Kamera khlopieobrazovaniia (A flocculation chamber) // Patent RU 146147 U1, 4.03.2014