Research on the effects of ferric salt added into aeration tank for phosphorus removal on biochemical system

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.

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.

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.

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+ 3OH−→Fe(OH)3↓ {\rm{F}}{{\rm{e}}^{3 + }}\;3{\rm{O}}{{\rm{H}}^ - } \to {\rm{Fe}}{\left( {{\rm{OH}}} \right)_3} \downarrow

Fig. 1

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+1/2H2O→2/3Fe3+ 1/3Fe(OH)3↓2/3Fe3++2OH−→2/3Fe(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.

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.

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++3HCO3−→Fe(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

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.

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

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

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.