AU2017414754B2

AU2017414754B2 - Algae-removing coagulant for enhancing algae coagulation and degrading algae-containing sediment in visible light simultaneously, preparation method therefor and application thereof

Info

Publication number: AU2017414754B2
Application number: AU2017414754A
Authority: AU
Inventors: Yan Jin; Haiyan PEI; Shasha Zhang
Original assignee: Shandong University
Current assignee: Shandong University
Priority date: 2017-05-18
Filing date: 2017-09-29
Publication date: 2021-02-11
Anticipated expiration: 2037-09-29
Also published as: CN107098453A; AU2017414754A1; WO2018209875A1; CN107098453B

Abstract

An algae-removing coagulant for enhancing algae coagulation and degrading algae-containing sediment in visible light simultaneously, comprising the following component in part by weight: 50-400 parts of N-TiO

Description

AN ALGAE-REMOVING COAGULANT FOR ENHANCING ALGAL COAGULATION AND MEANWHILE PURIFYING ALGAE-CONTAINING SLUDGE UNDER VISIBLE LIGHT

Technical Field

The present invention relates to a drinking water treatment process, and in

particular to an algae-removing coagulant for enhancing algal coagulation and

meanwhile purifying algae-containing sludge under visible light.

Background

In recent years the eutrophication of freshwater has occurred frequently

worldwide, including in China, leading to the proliferation of algae and the formation

of algal blooms, which seriously degrades the quality of raw water. The large-scale

growth of algae changes the physical and chemical characteristics of the water body,

inducing reductions of transparency and dissolved oxygen, and emission of odours. It

not only affects the aquaculture ecosystem, water supply, tourism, and ecological

landscape, but also seriously influences the dailylife of human beings.

In drinking water treatment, the removal of algae mainly depends on the

coagulation process. However, since the algae are buoyant, the algal flocs formed

cannot easily settle, thereby reducing the algal removal efficiency and increasing the

burden on subsequent treatment processes. In addition, there are some species of

harmful algae, such as Microcystis aeruginosa(M.aeruginosa),that can release

microcystins and pose threats to public health. Through coagulation, these harmful algae will be transferred into the sludge and may cause secondary pollution. Therefore, the question of how to enhance the algal removal efficiency while also allowing the algae-containing sludge to be harmlessly discharged is an urgent problem to be solved.

Summary

The present invention attempts to provide an algae-removing coagulant that can

simultaneously solve the twin problems of low efficiency of algae removal and

pollution due to algae-containing sludge.

It is an object of the present invention to overcome or ameliorate at least one of

the disadvantages of the prior art, or to provide a useful alternative.

To solve the issues above, the technical solutions provided by the present

invention are as given below.

Firstly, the present invention provides an algae-removing coagulant for enhancing

algal coagulation and meanwhile purifying algae-containing sludge under visible-light

irradiation, wherein the algae-removing coagulant is composed of 200-250 parts of

N-TiO2 powder and 7.5 parts of polyaluminium ferric chloride (PAFC) by mass; and

wherein the particle size of N-TiO2 in the coagulant is 50 to 150 mesh.

The N-TiO2 described above is prepared as follows. 12 to 18 parts of tetrabutyl

orthotitanate, 18 to 22 parts of ethyl alcohol, 28 to 35 parts of dilute nitric acid and

0.05 to 0.5 parts of urea by mass are mixed. After heating at 80 to 100 °C for 3 to 5

hours and calcining at 400 to 500 °C for 3 to 5 hours, the white solid obtained is

N-TiO 2 .

Secondly, the present invention provides the algae-removing coagulant, which is

prepared by mixing the N-TiO 2 powder and PAFC according to the masses described

above.

Thirdly, the present invention provides the application of the algae-removing

coagulant for enhancing algal coagulation and meanwhile purifying algae-containing

sludge under visible light.

Compared with the prior art, the present invention has the following beneficial

effects:

In the present invention, in comparison to the application of PAFC alone, mixing

with N-TiO2 can reduce the required PAFC dose to 50% and meanwhile enhance the

algal removal efficiency. N-TiO2 powder has negligible impact on the environment

and will not cause secondary pollution. The reduction of the required coagulant dose

can also decrease the content of heavy metals in water, which is beneficial for

improving the water quality. After coagulation, the N-TiO 2 powder in the

algae-containing flocs settles into the sludge. Under visible-light irradiation

practically all algal cells in the sludge are broken within 12 hours, and 85% of the

released cyanotoxins can be degraded after 48 hours' treatment. After treatment, the

sludge biomass is markedly decreased and the quality of water contained in the sludge

is improved, which is beneficial for safe discharge of the sludge and reuse of water

recovered from the sludge.

Brief Description of the Drawings

Preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:

Figure 1 shows the effect of different doses of PAFC on algal removal.

Figure 2 shows the algal removal efficiencies for treatments by PAFC combined

with different doses of N-TiO 2 during coagulation.

Figure 3 shows the algal removal efficiencies for treatments by PAFC combined

with different doses of N-TiO 2 during floc sedimentation.

Figure 4 shows the changes in chlorophyll-a contents during the degradation of

algae-containing sludge under visible light for treatments by different doses of

N-TiO 2 .

Figure 5 shows the changes in microcystin concentrations during the degradation

of algae-containing sludge under visible light for treatments by different doses of

N-TiO 2 .

Description of the Embodiments

It should be noted that the following detailed description is illustrative and is

intended to provide a further description of the invention. All technical and scientific

terms used herein have the same meaning as commonly understood by professionals

in the field that this invention belongs to.

It is to be noted that the terms used herein are for the purpose of describing

particular embodiments and are not intended to limit the exemplary embodiments of

the invention. For the terms used herein, unless the context clearly indicates otherwise,

singular forms are also intended to include plural forms.

The materials and reagents used in the present invention are commercially available unless otherwise specified.

In the present invention, the algal removal efficiency, the algal cell degradation

efficiency, and the microcystin degradation efficiency are calculated as follows:

Algal removal efficiency (%)= (OD6 8oA-OD 6 oB) 8 X 100% / OD6 80 A (1)

A = raw water; B = supernatant after sedimentation; OD6 8 0 = optical densities of the

extracts at wavelengths of 680 nm

Algal cell degradation efficiency (%)= (Chlc- ChlD)x 100% /Chlc (2)

Chl = chlorophyll; C = before treatment; D = after treatment.

Microcystin degradation efficiency (%) = (MCsE-MCsF)X 100% /MCsE (3)

MCs = microcystins; E = before treatment; F = after treatment.

As described in the background, there are deficiencies in the method for removing

algae in drinking water treatment, and the intracellular organic matter released by

algal cells cannot be effectively degraded. In order to solve these problems, the

present invention provides an algae-removing coagulant for enhancing algal

coagulation and meanwhile purifying algae-containing sludge under visible light,

which is composed of 50 to 400 parts of N-TiO 2 powder and 7.5 parts of PAFC by

mass.

The abovementioned algae-removing coagulant in a preferred embodiment of the

present invention is composed of the following components by mass: 200 to 250 parts

of N-TiO 2 powder and 7.5 parts of PAFC.

In the most preferred embodiment of the present invention, the abovementioned

algae-removing coagulant is composed of the following components by mass: 200 parts of N-TiO 2 powder and 7.5 parts of PAFC.

The N-TiO2 is prepared as follows. 12 to 18 parts of tetrabutyl orthotitanate, 18 to

22 parts of ethyl alcohol, 28 to 35 parts of dilute nitric acid, and 0.05 to 0.5 parts of

urea by mass are mixed. After heating at 80 to 100 °C for 3 to 5 hours and calcining at

400 to 500 °C for 3 to 5 hours, the white solid obtained is N-TiO 2

. In order to improve the algal removal efficiency and algal cell degradation

efficiency, the N-TiO2 in a preferred embodiment is prepared by the following

method:

15 parts of tetrabutyl orthotitanate are added to 20 parts of ethyl alcohol by mass.

After stirring, solution A is obtained.

0.05 to 0.5 parts of urea are added to 30 parts of dilute nitric acid. After stirring,

solution B is obtained.

Solution A is slowly added to solution B while stirring, and the pH is adjusted to a

value of 7 through the addition of a sodium hydroxide solution, and then the mixed

solution is heated at 80 °C for 3 hours.

Thereafter, the mixed solution is centrifuged and the supernatant is discarded. The

remaining precipitate is washed with distilled water 3 times, and then calcined at

400 °C to 500 °C for 3 hours. The white powder obtained is N-TiO 2 .

The particle size of the N-TiO 2 powder mentioned above is 50 to 150 mesh; more

preferably, the particle size of N-TiO 2 powder is 100 mesh.

The present invention provides the algae-removing coagulant, which is prepared

by mixing the N-TiO 2 powder and PAFC according to the masses described above.

The present invention provides the application of the algae-removing coagulant

for enhancing algal coagulation and meanwhile purifying algae-containing sludge

under visible light.

The application of the algae-removing coagulant includes the following steps:

Step 1): Adding the algae-removing coagulant to the algae-containing raw water

for coagulation.

Step 2): After sedimentation, the algal flocs completely settle and the algal cells in

the supernatant are efficiently removed.

Step 3): After separating off the supernatant, the remaining algae-containing

sludge is stirred and irradiated under visible light. After 12 to 48 hours' treatment, the

algal cells and cyanotoxins in sludge are efficiently degraded.

Wherein, in step 1, the density of algae in raw water is 105 to 107 cells/mL. In the

coagulation process, the raw water is stirred at 150 to 250 rpm for 1 to 2 minutes, and

then stirred at 30 to 60 rpm for 10 to 20 minutes.

In the algae-removing coagulant, the required PAFC dose is 7.5 mg/L and the

required N-TiO 2 dose is 50 to 400 mg/L. Without adding N-TiO 2, the optimal

coagulation dose of PAFC for algal removal is 15 mg/L. After the algae-removing

coagulant is added to the algae-containing raw water, the small flocs are rapidly

formed during the rapid-stirring period. In this process, intense turbulence is

generated and the turbidity of water increases. During the slow stirring period, with

appropriate turbulence and sufficient sedimentation time (10 to 20 minutes), the flocs

gradually become large and can completely settle after coagulation based on the action of gravity. During the coagulation process, the OD6 8 0 value of the supernatant

(2 cm below the surface) is measured to monitor the removal of algal flocs.

In step 2, the sedimentation time is 10 to 60 minutes. During the sedimentation

period, the small flocs can be transferred to big flocs through collision, and then the

big flocs settle at the bottom to make the supernatant clean. At the end of

sedimentation, the turbidity of supernatant remains stable. The time required for flocs

to completely settle at the bottom can be determined by the OD6 8 0 value of the

supernatant during the sedimentation process.

Due to the algal cells'buoyancy, the high concentration of cells, and the high

negative charge on the surface of algal cells, the algal flocs produced by conventional

coagulants cannot efficiently settle, seriously decreasing the algal removal efficiency.

When appropriate doses of PAFC and N-TiO 2 powder are applied in coagulation, the

N-TiO2 powder can be incorporated into algal flocs to increase the flocs' size and

density. Combining the functions of PAFC and N-TiO2in algal coagulation, the flocs

that form can rapidly settle, which increases the algal removal efficiency.

In step 3, the volume of the discarded supernatant accounts for 93% to 97% of the

total volume of the algal suspension, the light intensity is 3000 to 15000 lux and the

stirring speed is 200 to 800 rpm.

Due to the advantages of non-toxicity, low cost, stable performance and corrosion

resistance, TiO2 is one of the most widely used photocatalysts. However, TiO 2 also has

some limitations: due to the large band gap (3.2 eV) and narrow range of light

absorption (mainly in the ultraviolet region), TiO2 has a low efficiency of solar light utilization and quantum yield, and a high rate of semiconductor carrier recombination.

But when incorporating the element N (nitrogen) into the structure of TiO 2 (N-TiO 2),

the range of light wave lengths absorbed can be enlarged so as to increase the

photocatalytic activity under visible-light irradiation. Thus, N-TiO 2 can efficiently

disrupt algal cells and degrade the released cyanotoxins and organic matter under

visible light.

When the initial algal cells' density is 10' to 10' cells/mL, the algal removal

efficiency can be above 96% by adding 50 to 400 mg/L N-TiO 2 and 7.5mg/L PAFC in

the coagulation process. In the process of degrading algae-containing sludge, all algal

cells are broken within 12 hours' treatment. 85% of the total cyanotoxins are degraded

after 48 hours' treatment.

In order to help professionals in the relevant fields to understand the present

invention more thoroughly, specific embodiments and contrastive examples are

described below.

Embodiment 1

The algae-removing coagulant consists of the following components: 50 parts of

N-TiO2 powder (100 mesh) and 7.5 parts of PAFC by mass.

M. aeruginosais used as the model alga in this embodiment to prepare the

algae-containing raw water. This strain is grown in BG11 medium at constant

temperature (25 C) with a light/dark cycle (12h/12h) under 2000 lux illumination.

The cultures are harvested at the exponential phase of growth.

The coagulation experiments are performed with a six-paddle stirrer in a series of

IL beakers each containing 1L of M. aeruginosaculture. The initial M. aeruginosa

concentration is diluted with deionized water to 1x106 cells/mL M. aeruginosa

solution to obtain the algae-containing raw water. After 7.5 mg PAFC and 50 mg

N-TiO2 are simultaneously added, the M. aeruginosa solution is stirred at 250 rpm for

1 min and 30 rpm for another 30 min. Algal removal efficiencies are measured for

both the coagulation and the sedimentation processes. Samples collected from 2 cm

below the surface are characterised by OD68oto calculate the algal cell density in the

supernatant. The algal removal efficiency reaches a maximum at 1 h after the

completion of coagulation, which is 96% (Figures 2 and 3).

After coagulation and sedimentation, 930 mL of supernatant is removed, leaving

mL of lab-simulated cyanobacteria-containing sludge. The sludge is magnetically

stirred at 500 rpm under visible-light irradiation (8000 lux). After 48 hours' treatment,

41.6% of algal cells are broken while some microcystins still remain intracellularly

(Figures 4 and 5).

Embodiment 2

The algae-removing coagulant consists of the following components: 100 parts of

N-TiO2 powder (100 mesh) and 7.5 parts of PAFC by mass.

M. eruginosa is used as the model alga in this embodiment to prepare the

algae-containing raw water. This strain is grown in BG11 medium at constant

The cultures are harvested at the exponential phase of growth.

concentration is diluted with deionized water to 1x106 cells/mLM. aeruginosa

solution to obtain the algae-containing raw water. After 7.5 mg PAFC and 100 mg

supernatant. The algal removal efficiency reaches a maximum at 1 h after the

completion of coagulation, which is 97% (Figures 2 and 3).

After coagulation and sedimentation, 930 mL of supernatant is removed, leaving

mL of lab-simulated cyanobacteria-containing sludge. The sludge is magnetically

59.9% of algal cells are disrupted while some microcystins still remain intracellularly

(Figures 4 and 5).

Embodiment 3

The algae-removing coagulant consists of the following components: 200 parts of

N-TiO2 powder (100 mesh) and 7.5 parts of PAFC by mass.

M.aeruginosais used as the model alga in this embodiment to prepare the

algae-containing raw water. This strain is grown in BG11 medium at constant temperature (25 C) with a light/dark cycle (12h/12h) under 2000 lux illumination.

The cultures are harvested at the exponential phase of growth.

concentration is diluted with deionized water to 1x106 cells/mLM. aeruginosa

solution to obtain the algae-containing raw water. After 7.5 mg PAFC and 200 mg

below the surface are characterised by OD6 8 0 to calculate the algal cell density in the

supernatant. The algal removal efficiency reaches a maximum at 10 min after the

completion of coagulation, which is 98% (Figures 2 and 3).

After coagulation and sedimentation, 930 mL of supernatant is removed, leaving

mL of lab-simulated cyanobacteria-containing sludge. The sludge is magnetically

stirred at 500 rpm under visible-light irradiation (8000 lux). After 12 hours' treatment,

all algal cells are broken, and after 48 hours' treatment the efficiency of microcystin

degradation is 84.2% (Figures 4 and 5).

Embodiment 4

The algae-removing coagulant consists of the following components: 400 parts of

N-TiO2 powder (100 mesh) and 7.5 parts of PAFC by mass.

Maeruginosaisused as the model alga in this embodiment to prepare the algae-containing raw water. This strain is grown in BG11 medium at constant temperature (25 C) with a light/dark cycle (12h/12h) under 2000 lux illumination.

The cultures are harvested at the exponential phase of growth.

concentration is diluted with deionized water to 1x106 cells/mL M. aeruginosa

solution to obtain the algae-containing raw water. After 7.5 mg PAFC and 400 mg

N-TiO2are simultaneously added, the M. aeruginosa solution is stirred at 250 rpm for

supernatant. The algal removal efficiency reaches a maximum at 10 min after the

completion of coagulation, which is 98% (Figures 2 and 3).

After coagulation and sedimentation, 930 mL of supernatant is removed, leaving

mL of lab-simulated cyanobacteria-containing sludge. The sludge is magnetically

degradation is 87.6% (Figures 4 and 5).

Among embodiments1 to 4, the algae-removing coagulant in Embodiment 3 is

preferred. In embodiments1 and 2, the algal cells and cyanotoxins in sludge cannot be

effectively degraded within 48 hours. In embodiment 4, the dose of N-TiO 2 is excessive and some of the added N-TiO 2 will remain in the supernatant after coagulation, which will decrease the water quality and increase the treatment cost.

In order to increase the algal removal efficiency and decrease the required

coagulant dose while efficiently degrading algae-containing sludge, the present

invention combines and optimizes the photocatalysts and conventional coagulants to

simultaneously enhance algal coagulation and degrade the algae-containing sludge.

Through screening different types of photocatalysts, N-TiO2 is chosen as the preferred

photocatalyst in the present invention. Compared with the coagulation function of

PAFC, N-TiO2 powder can serve as the core of algal flocs during the coagulation

process, which can increase the flocs' density, thereby enhancing the algal coagulation

and sedimentation and solving the issues caused by the buoyancy of algal cells. In

addition, N-TiO2 can effectively disrupt algal cells and degrade the cyanotoxins and

organic matter under visible light.

Based on the characteristics of algae, PAFC is chosen as the preferred coagulant in

the present invention. It combines the advantages of Al and Fe salts, and has a

significant improvement in the morphology of Al and Fe ions. PAFC combined with

N-TiO2 has the best results for the treatment of algae-containing raw water. For the

optimization of dosages, the optimum dose of PAFC is 7.5 parts by mass. Combining

with the optimum dose of N-TiO 2 (best treatment results with the lowest dose), the

algal removal efficiency will decrease when the dose of PAFC exceeds 7.5 parts by

mass. When the dose of PAFC is less than 7.5 parts by mass, the algal removal

efficiency cannot reach the optimum. Preferably, doses of 7.5 parts of PAFC and 50 to

400 parts of N-TiO 2 powder are selected.

It is found that different types and doses of coagulants and photocatalysts have

various results for the treatment of algae in raw water. Thus, Contrastive ExamplesI

to 5 are shown below, but the research is not limited to the following contrastive

examples.

Contrastive Example1

The algae-removing coagulant consists of the following component: 7.5 parts of

PAFC.

Maeruginosais used as the model alga in this example to prepare the

algae-containing raw water. This strain is grown in BG11 medium at constant

temperature (25 °C) with a light/dark cycle (12h/12h) under 2000 lux illumination.

The cultures are harvested at the exponential phase of growth.

concentration is diluted with deionized water to 1x106 cells/mLM. aeruginosa

solution to obtain the algae-containing raw water. After 7.5 mg PAFC is added, the M.

aeruginosasolution is stirred at 250 rpm for1 min and 30 rpm for another 30 min.

Algal removal efficiencies are measured for both the coagulation and the

sedimentation processes. Samples collected from 2 cm below the surface are

characterised by OD68otocalculate the algal cell density in the supernatant. Results

show that the rate of floc formation is slow and the algal removal efficiency during coagulation is only 5 to 6% (Figure 2). Besides that, when settling for 30 minutes after coagulation the algal removal efficiency is only 60%, and after 120 minutes' sedimentation the algal removal efficiency reaches the maximum of just 80% (Figure

3).

After coagulation and sedimentation, 930 mL of supernatant is removed, leaving

mL of lab-simulated cyanobacteria-containing sludge. The sludge is magnetically

stirred at 500 rpm under visible-light irradiation (8000 lux). After treatment, algal

cells cannot be effectively broken and the microcystin degradation efficiency is

negligible (Figures 4 and 5).

In Figure 1, it can be seen that when using PAFC only, the algal removal

efficiency of 7.5 mg/L PAFC is just 60%. As the dose of PAFC increases to 15 mg/L,

the algal removal efficiency is 90%, which is the maximum value.

Contrastive Example2

The algae-removing coagulant consists of the following components: 200 parts of

N-TiO 2 (100 mesh) and 7.5 parts of poly-aluminium-ferric-silicate-chloride (PAFSC)

by mass.

Maeruginosais used as the model alga in this example to prepare the

algae-containing raw water. This strain is grown in BG11 medium at constant

The cultures are harvested at the exponential phase of growth.

concentration is diluted with deionized water to 1x106 cells/mL M. aeruginosa

solution to obtain the algae-containing raw water. After 7.5 mg PAFSC and 200 mg

supernatant. When settling for 30 minutes after coagulation, the algal removal

efficiency is 80%.

Contrastive Example3

The algae-removing coagulant consists of the following components: 200 parts of

TiO2 powder (100 mesh) and 7.5 parts of PAFC by mass.

Maeruginosais used as the model alga in this example to prepare the

algae-containing raw water. This strain is grown in BG11 medium at constant

The cultures are harvested at the exponential phase of growth.

concentration is diluted with deionized water to 1x106 cells/mL M. aeruginosa

solution to obtain the algae-containing raw water. After 7.5 mg PAFC and 200 mg

TiO2 are simultaneously added, the M. aeruginosa solution is stirred at 250 rpm for 1 min and 30 rpm for another 30 min. Algal removal efficiencies are measured for both the coagulation and the sedimentation process. Samples collected from 2 cm below the surface are characterised by OD6 8 0 tocalculate the algal cell density in the supernatant. When settling for 30 minutes after coagulation, the algal removal efficiency is 85%.

After coagulation and sedimentation, 930 mL of supernatant is removed, leaving

mL of lab-simulated cyanobacteria-containing sludge. The sludge is magnetically

49% of algal cells are broken and 48.4% of the microcystins are degraded.

Contrastive Example4

The algae-removing coagulant consists of the following components: 200 parts of

rare-earth-element-doped TiO2 (100 mesh) and 7.5 parts of PAFSC by mass.

The rare-earth-element-doped TiO2 is prepared as follows. Tetrabutyl orthotitanate

is added to 20 mL ethyl alcohol. After 2 hours' stirring, lanthanum nitrate is added and

a sol-gel is formed. Then the sol-gel is dried and calcined at 400 °C for 5 h to obtain

the rare-earth-element-doped TiO 2 .

Maeruginosais used as the model alga in this example to prepare the

algae-containing raw water. This strain is grown in BG11 medium at constant

The cultures are harvested at the exponential phase of growth.

concentration is diluted with deionized water to 1x106 cells/mL M. aeruginosa

rare-earth-element-doped TiO2 are simultaneously added, the M. aeruginosa solution

is stirred at 250 rpm for 1 min and 30 rpm for another 30 min. Algal removal

efficiencies are measured for both the coagulation and the sedimentation processes.

Samples collected from 2 cm below the surface are characterised by OD68oto calculate

the algal cell density in the supernatant. When settling for 30 minutes after

coagulation, the algal removal efficiency is 83%.

After coagulation and sedimentation, 930 mL of supernatant is removed, leaving

mL of lab-simulated cyanobacteria-containing sludge. The sludge is magnetically

4 3 % of algal cells are broken and 45.6% of the microcystins are degraded.

Contrastive Example5

The algae-removing coagulant consists of the following components: 200 parts of

N-TiO2 (100 mesh) and 10 parts of PAFC by mass.

Maeruginosais used as the model alga in this example to prepare the

algae-containing raw water. This strain is grown in BG11 medium at constant

The cultures are harvested at the exponential phase of growth.

concentration is diluted with deionized water to 1x106 cells/mL M. aeruginosa

solution to obtain the algae-containing raw water. After 10 mg PAFC and 200 mg

supernatant. When settling for 30 minutes after coagulation, the algal removal

efficiency is 90%.

In summary, the algae-removing coagulant described in the present invention is

PAFC mixed with N-TiO 2 powder. Compared with applying PAFC only, adding

N-TiO2 can reduce the required PAFC dosage by 50% and increase the algal removal

efficiency to 96%, and the results are better than those obtained in contrastive

examples 1 to 5. The N-TiO 2 powder has little impact on the environment and will not

cause secondary pollution. After coagulation, the N-TiO 2 powder settles into the

sludge with the algal flocs. With the algae-containing sludge being treated by

visible-light irradiation while stirring, all algal cells can be disrupted within 12 hours,

and more than 85% of the cyanotoxins can be degraded after 48 hours' treatment.

Results are better than those of contrastive examples 1 to 5. After treatment, the

sludge biomass is reduced, which can increase the quality of water contained in the

sludge, which is beneficial for the reuse or discharge of sludge. In addition, the proportion of components in the algae-removing coagulant of the present invention is also very critical. When changing the proportion of components, the algal removal efficiency and sludge treating efficiency will be markedly decreased.

The above embodiments are preferred ones in the present invention, and the

present invention is not limited by the above embodiments. Any changes,

modifications, substitutions, combinations, and simplifications are included in the

scope of the present invention.

Claims

1. An algae-removing coagulant for enhancing algal coagulation and meanwhile

purifying algae-containing sludge under visible-light irradiation, wherein the

algae-removing coagulant is composed of 200-250 parts of N-TiO 2 powder and 7.5

parts of polyaluminium ferric chloride (PAFC) by mass; and wherein the particle size

of N-TiO2 in the coagulant is 50 to 150 mesh.

2. The algae-removing coagulant according to claim 1, wherein the coagulant is

composed of the following components by mass: 200 parts of N-TiO 2 powder and 7.5

parts of PAFC.

3. The algae-removing coagulant according to claim 1, wherein the N-TiO2is

prepared as follows:12 to18 parts of tetrabutyl orthotitanate, 18 to 22 parts of ethyl

alcohol, 28 to 35 parts of dilute nitric acid, and 0.05 to 0.5 parts of urea by mass are

mixed; after heating at 80 to 100 °C for 3 to 5 hours and calcining at 400 to 500 °C

for 3 to 5 hours, the white solid obtained is N-TiO 2 .

4. The algae-removing coagulant according to claim 3, wherein the N-TiO 2 in the

coagulant is prepared as below:

15 parts of tetrabutyl orthotitanate are added to 20 parts of ethyl alcohol by mass,

after stirring, solution A is obtained;

0.05 to 0.5 parts of urea are added to 30 parts of dilute nitric acid by mass, after

stirring, solution B is obtained;

the solution A is slowly added to the solution B while stirring, and the pH is

adjusted to 7 with a sodium hydroxide solution, and then the mixed solution is heated at 80 °C for 3 hours; thereafter, the mixed solution is centrifuged and the supernatant is discarded, the remaining precipitate is washed with distilled water 3 times, and then calcined at

400 °C to 500 °C for 3 hours, the white powder obtained is N-TiO 2

.

5. The algae-removing coagulant according to claim 1, wherein the particle size of

N-TiO2 in the coagulant is 100 mesh.

6. A method for preparing the algae-removing coagulant according to any one of

claims 1 to 5, wherein the preparation of the coagulant is performed by mixing the

N-TiO2 powder and PAFC according to the required masses.

7. Use of the algae-removing coagulant according to any one of claims I to 5 in

enhancing algal coagulation and meanwhile purifying algae-containing sludge under

visible light.

8. The use according to claim 7, wherein when the initial algal cell density is 105

to 107 cells/mL, the required dose of PAFC is 7.5 mg/L and the required dose of

N-TiO2 is 50 to 400 mg/Lin the algae-containing raw water.