1. bookVolume 6 (2021): Issue 2 (July 2021)
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Research on the effects of ferric salt added into aeration tank for phosphorus removal on biochemical system

Published Online: 10 Nov 2021
Page range: 281 - 290
Received: 25 May 2021
Accepted: 23 Jul 2021
Journal Details
License
Format
Journal
First Published
01 Jan 2016
Publication timeframe
2 times per year
Languages
English
Abstract

In this paper, two iron salts, ferrous chloride (FeCl2) and ferric chloride (FeCl3), are directly added into an aeration tank for phosphorus removal, and their effects on the biochemical system are studied; the water quality parameters such as pH and alkalinity are also investigated. The extent of influence of the added iron salts on the pH and alkalinity of aerated solutions is demonstrated to be FeCl3 > FeCl2. When the dosage of iron ions is 20 mg/L, the decrease in pH and alkalinity caused by FeCl3 is 0.5 and 65 mg/L, which is higher than FeCl2 by 2% and 26%. The initial phosphorus removal effect of FeCl2 is worse than that of FeCl3, but after continued aeration and oxidation, the phosphorus removal effect of FeCl2 can be improved; however, the final phosphorus removal effect is basically the same as that of FeCl3 added directly. The results show that FeCl2 is preferred when iron salt is added directly into the aeration tank to remove phosphorus. The proposed scheme can reduce the effect of iron salts on the alkalinity of the biochemical system on the premise of ensuring the phosphorus removal effect of the system, and is conducive to ensuring the stable operation of the biochemical system.

Keywords

Introduction

As the urbanisation process is expedited and people's living standards have improved in China, the quantity of wastewater discharge is also rising year by year [1], and the black odour water body and eutrophication are increasingly becoming a serious concern [2,3,4]. In order to reduce the total wastewater and pollutant discharge, the level-I A discharge standard of GB 18918–2002 has been fixed: The Discharge Standard of Pollutants for Municipal Wastewater Treatment Plant is being implemented comprehensively and may become even more stringent than usual [5,6,7]. This has caused most wastewater treatment plants to upgrade their standard of water quality indexes, such as the total phosphorus and nitrogen, mainly by process adjustment or extension to enhance the nitrogen and phosphorus removal effect [8,9]. In terms of phosphorus removal, there are mainly two approaches: (1) directly adding coagulated phosphorus removing agent into aeration tanks, and (2) extending the coagulative precipitation tank [10,11]. Extending an aeration tank requires large investment, takes a long period of construction, and requires increasing the floor area. On the other hand, adding a phosphorus removing agent directly into aeration tanks requires only minimal investment, and does not increase the land area along with fast reconstruction, and so is widely applied, although its impact on the biochemical system cannot be ignored.

Coagulated phosphorus removing agents mainly include aluminium and ferric salts. Since aluminium ions may have certain effects on microorganisms [12], and the phosphorus removing effect of ferric salt coagulant is better than aluminium salt [13], ferric salt is more widely used in actual phosphorus removing projects. The common ferric salt coagulants are FeCl3, Fe2(SO4)3, FeCl2, FeSO4 and so on [14], among which FeCl3 is frequently used in phosphorus removing projects. In projects that involve removal of phosphorus from wastewater, the FeCl3 used is primarily prepared on the basis of FeCl2. By adding oxidising agent, catalyst and hydrochloric acid, the FeCl2 is oxidised to FeCl3. The added hydrochloric acid accounts for 20% [15,16], so the FeCl3 added will influence the pH and alkalinity of the biochemical system to a certain extent, and will further produce adverse effects on the performance of the biochemical system, especially the sludge nitration system. If FeCl2 is directly added to aeration tanks, it will be transformed into FeCl3 under the oxidisation effect of aeration oxygenation. This not only reduces the hydrochloric acid added and the agent preparation cost but also prevents the effects of FeCl3 on the biochemical system. Recently, there are no reports related to the research on the effects of FeCl2 and FeCl3 added to the biochemical system.

This paper mainly investigates the changes in pH, alkalinity and phosphorus removing effect after adding FeCl2 and FeCl3 into a solution, and studies the effects of FeCl2 and FeCl3 on the biochemical system of aeration tanks, so as to provide a reference for wastewater plants, which directly add coagulated phosphorus removing agents; this will enable us to select the appropriate phosphorus removing agent in upgrading standards and reconstruction projects.

Materials and methods
Experimental device and test water

Experimental drugs used were sodium bicarbonate (AR), sodium carbonate (AR), sodium phosphate (AR), ferric chloride (AR), ferrous chloride (AR), and hydrochloric acid (AR).

Coagulants used were solid ferrous chloride and ferric trichloride; these were placed in a beaker to prepare the solution with iron ionic concentration of 10%.

The experimental device is shown in Table 1.

Main instrument table for testing.

Experimental device Type
Digital electronic analytical balance AUY220
Precise PH meter PHS-3C
Vacuum extractor MC-600D
Stirrer used in coagulation test ZR4-6
Magnetic stirrer MS7-H550-Pro
Silent air pump AP9903A
UV spectrophotometer UVmini-1240

The test water is the simulation solution and the effluent is taken from the aeration tank of a wastewater treatment plant in Qingdao. In studying the effects of two ferric salts, FeCl2 and FeCl3, added to the pH, alkalinity and phosphorus removing effect of aeration tanks, in view of the complex quality of wastewater in wastewater plant, to prevent the disturbance of other elements in the solution to test results, the simulation solution is used for study, and the test results are verified in real wastewater.

Test scheme
Effects of ferric salt added on simulation solution

Prepare a simulation solution with alkalinity of 120 mg/L (calculated with CaCO3, same as below), and measure its initial pH value. Add FeCl2 and FeCl3 (with iron ion content of 10 mg/L, 15 mg/L, 20 mg/L, 25 mg/L, mg/L 30 mg/L and 35 mg/L, respectively) into two groups of solutions, stir them with magnetic stirrer (at a speed of 200 r/min) and aerate (for 30 min), then let it stand still. Next, measure the pH value of the solution, take the supernatant and measure the alkalinity of the solution. Add ferric salt into two groups of solution, respectively, stir them with stirrer (at speeds of 200 r/min 1 min; 100 r/min 5 min), let them stand still and then measure the pH value and alkalinity of the solution.

Prepare a simulation solution with alkalinity of 120 mg/L and total phosphorus content of 4 mg/L. Mix the solution with HCl to make its pH the same as the initial pH of solution in step (1). Operate according to the addition quantity of ferric salt in step (1) and the test steps, filter it and measure the remaining phosphorus content of the filtrate.

Effects of ferric salt added on aeration tank

The effluent from the aeration tank of a wastewater plant in Qingdao is taken and we measured its total phosphorus content as 4 mg/L. Based on the test results of simulation solution, the optimal dosage of ferric ion is determined. The test steps are the same as in Section 1.2.2, and the changes in pH value and alkalinity of the solution and its phosphorus removing effect are observed.

Analytical method

Potentiometric titration in the Water and Wastewater Analysis and Test Method (fourth version) is used to measure the alkalinity content.

The standards specified in the Water quality-Determination of total phosphorus-Ammonium molybdate spectrophotometric method (GB 118993-89) are used to measure the total phosphorus content.

Results and discussion

Activated sludge is the key to the effect of wastewater disposal in aeration tanks, while the pH value and alkalinity of aeration tanks are closely related to the survival, propagation and metabolic function of microorganisms [17,18,19]. Therefore, in order to enhance the phosphorus removing effect without influencing the biological removal of nitrogen, this paper studies the effects of adding FeCl2 and FeCl3 on the biochemical system of aeration tanks through three indexes: the pH value, alkalinity and phosphorus removing effect.

Effects of ferric salt added on pH value

It can be seen from Figure 1 that adding FeCl2 and FeCl3 to the solution and conducting aeration treatment will reduce the pH value of the solution; as the dosage increases, the decreasing trend of pH value becomes more obvious. When the initial pH value of the solution is 8.5, and the ferric ion dosage is 10 mg/L and 35 mg/L, FeCl2 makes the pH value to reduce by 0.2 and 0.9, and FeCl3 makes the pH value to reduce by 0.2 and 1.8, which results in a greater impact on pH. The reason why addition of ferric salt can lead to a decrease in pH is that ferric ions need to consume OH during hydrolysis. The reaction equation of ferric ion and OH is as follows: Fe3+3OHFe(OH)3 {\rm{F}}{{\rm{e}}^{3 + }}\;3{\rm{O}}{{\rm{H}}^ - } \to {\rm{Fe}}{\left( {{\rm{OH}}} \right)_3} \downarrow

Fig. 1

Effects of adding ferric salt on pH value.

Under neural or alkalescence condition, ferrous ions can easily be oxidised to ferric ions. Its chain reaction with OH is as follows: Fe2++1/4O2+1/2H2O2/3Fe3+1/3Fe(OH)32/3Fe3++2OH2/3Fe(OH)3 \matrix{ {{\rm{F}}{{\rm{e}}^{2 + }} + 1/4{{\rm{O}}_2} + 1/2{{\rm{H}}_2}{\rm{O}} \to 2/3{\rm{F}}{{\rm{e}}^{3 + }}\;1/3{\rm{Fe}}{{\left( {{\rm{OH}}} \right)}_3} \downarrow } \hfill \cr {2/3{\rm{F}}{{\rm{e}}^{3 + }} + 2{\rm{O}}{{\rm{H}}^ - } \to 2/3{\rm{Fe}}{{\left( {{\rm{OH}}} \right)}_3} \downarrow } \hfill \cr }

That is, adding 1 mol FeCl2 to oxidise to FeCl3 can consume 1 mol OH less than directly adding 1 mol FeCl3. So, directly addition of FeCl2 into aeration tanks has lesser impact on pH than FeCl3.

The pH value is an important factor that affects biological nitrogen and phosphorus removal. Li Nan [20] et al. found that in acidic conditions, the glycogen-accumulating organisms fight with phosphorus-accumulating bacteria on the substrate, leading to a decrease in biological phosphorus removal performance. In neural or alkalescene conditions, however, the phosphorus-accumulating bacteria account for 60% in activated sludge, which is a dominant bacteria with stable and efficient biological phosphorus removal performance. Nitrifying bacteria react very sensitively to pH, and Li Li et al. [21] discover from research that in neural and alkalescene conditions, it is more conducive for microorganism floccules to assimilate the ammonia nitrogen in the water body. Hence, the neural and alkalescene conditions are the optimal pH range of biological nitrogen and phosphorus removal. When the FeCl2 dosage is 35 mg/L, the pH of solution is 7.6, which is still within the optimal pH range of biological phosphorus removal and ammonia nitrogen assimilation; but when the FeCl3 dosage is 35 mg/L, the pH of solution is 6.7, showing faintly acidic nature, which has a certain effect on biological nitrogen and phosphorus removal.

It can be seen from Figure 1 that adding FeCl2 and FeCl3 to the solution will reduce its pH value, and the pH value of aerated solution is one unit higher than that of non-aerated solution. This indicates that aeration is helpful to increase the pH value of a biochemical system, and to bring down the effects of ferric salt added to the pH value of the biochemical system. It is inferred that the causes of raised pH value of solution after aeration treatment may be that (1) the pH value is significantly positively correlated with the dissolved oxygen concentration in the water, with the correlation coefficient >0.9 [22]. Aeration can elevate the concentration of dissolved oxygen in the water, resulting in a rising pH value of solution; and (2) aeration can release CO2 in the water, leading to a decrease in carbonate and an increase in pH value by one unit [23].

In order to verify the specific reason for rising pH value of aerated solution, the following experiment was carried out. Test I: conduct aeration treatment on pure water, observe the change in pH value of pure water before and after aeration; Test II: fill CO2 in pure water, conduct aeration treatment and observe the change in pH value of pure water before and after aeration. See test results in Table 2.

Pure water test.

Test I Initial pH The pH after aeration

Pure water 6.4 6.3
Tap water 7.7 8.3

Test II Initial pH The pH after filling in CO2 The pH after aeration

Pure water 6.2 5.3 6.4

From the data of Test I, it is observed that the pH value of pure water is basically the same before and after aeration, demonstrating that the dissolved oxygen is not the reason for the rising pH value of solution after aeration treatment. The pH value of tap water rises by 0.6 after aeration treatment. Compared to the change in pH value of pure water, it is inferred that aeration brings some substances out of tap water and leads to the rising pH value of solution. From the results of Test II, it is observed that the initial pH value of pure water is 6.2, the pH value of solution decreases by 0.9 after filling CO2 in the solution and the pH value of solution after aeration treatment has little difference with the initial pH value. These observations indicate that the reason for the raised pH value of solution after aeration treatment is that aeration brings down the CO2 content in the solution.

Effects of ferric salt added on alkalinity

Alkalinity is a vital factor that affects biological nitrogen removal. In the nitration reaction, oxidising ammonia nitrogen into nitrate needs to consume 7.1 g alkalinity, so as to balance the acidity generated in nitration. At the same time, nitrifying bacteria and nitrococcus also consume alkalinity to propagate. The denitrifying bacteria can produce 3.5 g alkalinity every time it reduces ammonia nitrogen [24]. If we want the biological nitration reaction to proceed normally, it is essential to satisfy the remaining alkalinity to be >100 mg/L, and if the alkalinity is insufficient, it is necessary to add alkaline matter to supplement the alkalinity. So, when adding ferric salt to improve the phosphorus removal effect, it is required to consider the consumption of alkalinity by ferric salt. To guarantee the biological nitrogen removal effect, it is necessary to ensure the alkalinity. If the alkalinity is insufficient, it is required to add alkaline matter to supplement the alkalinity.

From Figure 2, it is observed that the initial alkalinity of the solution is 123 mg/L. After adding FeCl2 and FeCl3 to the solution and conducting aeration treatment, the dosage of ferric ion is 10~35 mg/L, and the FeCl2 added makes the solution's alkalinity to reduce by 20 mg/L, 26 mg/L, 33 mg/L, 44 mg/L, 51 mg/L and 59 mg/L. Under the same dosage, FeCl3 consumes twice as much alkalinity as compared to FeCl2. When the dosage of ferric iron is 20 mg/L, FeCl3 consumes >50% alkalinity. From the above data, it is shown that ferric salt added will consume the solution's alkalinity, and the more the dosage is, the more the alkalinity will be consumed; under the same dosage, FeCl3 has a greater impact on the alkalinity. The reasons why addition of ferric salt can bring down a solution's alkalinity are that: (1) ferric ion consumes OH during hydrolysation (see reaction equation in Section 2.1) and (2) ferric iron consumes HCO3, and the reaction equation of ferric ion is as below: Fe3++3HCO3Fe(OH)3+3CO2 {\rm{F}}{{\rm{e}}^{3 + }} + 3{\rm{HC}}{{\rm{O}}_3}^ - \to {\rm{Fe}}{\left( {{\rm{OH}}} \right)_3} \downarrow + 3{\rm{C}}{{\rm{O}}_2} \uparrow

Fig. 2

Effect of adding ferric salt on alkalinity.

In the process by which FeCl2 reacts with O2 to generate FeCl3, partial ferrous ions reform to Fe(OH)3 sediment; therefore, the free ferric ions are fewer in comparison with those obtained pursuant to direct addition of FeCl3, and HCO3 is consumed less. Thus, directly adding FeCl2 into aeration tanks consumes less alkalinity of biochemical system than that of FeCl3.

From Figure 2, we can see that there is little change between the alkalinity of aerated and non-aerated solutions, which indicates that aeration has less effect on the alkalinity change of solution after adding FeCl2 and FeCl3.

Impact of ferric salt added on phosphorus removal effect

In Figure 3, the initial total phosphorus content of the solution is 4 mg/L, and FeCl2 and FeCl3 added can reduce the total phosphorus content of the solution; as the dosage increases, the total phosphorus content of the solution tends to be stable after a decline. Without aeration treatment, the phosphorus removing effect of adding FeCl2 is worse than that of FeCl3; after aeration treatment, the phosphorus removing effect of FeCl2 and FeCl3 does not differ much. When ferric ion dosage reaches to 20 mg/L, the total phosphorus contents of their supernatant are 0.48 mg/L and 0.49 mg/L (<0.5 mg/L), respectively, which meets the level-I. A discharge standard of the Discharge Standard of Pollutants for Municipal Wastewater Treatment Plant (GB18918-2002) (TP≤0.5 mg/L). Therefore, adding ferric salt into aeration tanks directly, whether FeCl2 or FeCl3, can achieve the anticipatory phosphorus removing effect. But FeCl2 added has less impact on the pH value and alkalinity of the solution, with more advantage of cost performance.

Fig. 3

Effect of adding ferric salt on phosphorus removal.

Effects of adding ferric salt on aeration tank

To avoid the effects of organophosphorus content in wastewater, the effluent from the aeration tank of a wastewater treatment plant in Qingdao was taken to verify the effects of adding FeCl2 and FeCl3 on the aeration tank. The initial pH and alkalinity of the solution are 7.1 mg/L and 126 mg/L, and the prepared total phosphorus content is 4 mg/L (the total phosphorus content of raw water is 0.27 mg/L). The dosage of ferric ion is 20 mg/L (see test results in Figure 4).

Fig. 4

Treatment results of effluent from aeration tank with different valence ferric salts.

It is observed from Figure 4 that, after adding FeCl2 and FeCl3 into the solution, the pH value of the solution reduces by 0.2 and 0.5, and the alkalinity reduces by 19.8% and 42.0%. After conducting aeration treatment on the solution, the aerated pH values rise by one unit more than the non-aerated solution, and the alkalinity remains unchanged, which conforms to the impact of simulation solution on pH value and alkalinity. After adding FeCl2 in the solution, the solution is also treated by aeration, and the phosphorus removing effect can reach to 99.7%. At this time, the total phosphorus content of the supernatant is 0.01 mg/L<0.50 mg/L, which meets the level-I A discharge standard of the Discharge Standard of Pollutants for Municipal Wastewater Treatment Plant (GB18918-2002), and the phosphorus removing effect is better than that of non-aerated solution (with removing rate of 60.7%), which approximates to that of directly adding FeCl3 in the solution (with removing rate of 97.2). So, with regard to the premise of guaranteeing the phosphorus removing effect, directly adding FeCl2 has less impact on the biochemical system of an aeration tank than FeCl3.

Conclusions

Adding FeCl2 and FeCl3 may reduce the pH value of an aeration tank. Under the same dosage, FeCl3 has greater effects on pH value than FeCl2. But aeration can improve the pH value of the biochemical system, and lessen the effects of adding ferric salt on the pH value of a biochemical system to a certain extent, so as to lessen the effects of pH value on the biological nitrogen and phosphorus removal performance.

Adding ferric salt will consume the alkalinity of aeration tank and affect the biological nitrogen removal effect. Under the same dosage, FeCl3 consumes more alkalinity than FeCl2, which tendency conforms to the required impact on pH value; though aeration can affect the pH value, it basically does not affect the consumption of alkalinity by FeCl2 and FeCl3.

FeCl3 has a better phosphorus removal effect than FeCl2, so it is a priority selection in coagulated phosphorus removal process. When adding ferric salt in aeration tanks, FeCl2 has a better phosphorus removal effect than FeCl3, so it is a priority selection in directly adding ferric salt in aeration tanks for phosphorus removal, with the ferric ion dosage of 20 mg/L.

Directly adding ferric ions into aeration tanks for phosphorus removal has a certain effect on the pH value and alkalinity of the biochemical system, while its specific impact on the nitrogen removal by nitration and denitrification needs to be further investigated.

Fig. 1

Effects of adding ferric salt on pH value.
Effects of adding ferric salt on pH value.

Fig. 2

Effect of adding ferric salt on alkalinity.
Effect of adding ferric salt on alkalinity.

Fig. 3

Effect of adding ferric salt on phosphorus removal.
Effect of adding ferric salt on phosphorus removal.

Fig. 4

Treatment results of effluent from aeration tank with different valence ferric salts.
Treatment results of effluent from aeration tank with different valence ferric salts.

Main instrument table for testing.

Experimental device Type
Digital electronic analytical balance AUY220
Precise PH meter PHS-3C
Vacuum extractor MC-600D
Stirrer used in coagulation test ZR4-6
Magnetic stirrer MS7-H550-Pro
Silent air pump AP9903A
UV spectrophotometer UVmini-1240

Pure water test.

Test I Initial pH The pH after aeration

Pure water 6.4 6.3
Tap water 7.7 8.3

Test II Initial pH The pH after filling in CO2 The pH after aeration

Pure water 6.2 5.3 6.4

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