KR20210047422A - Method for collecting Phosphate from sludge dewatering liquor using slow corrosion of magnesium bead - Google Patents

Abstract

The present invention relates to a phosphate recovery method from sludge desorption filtrate using corrosion of magnesium metal, comprising the steps of: supplying sludge desorption filtrate and magnesium (Mg) metal; causing a struvite crystallization reaction for the sludge desorption filtrate; separating the sludge desorption filtrate crystallized in a struvite crystallization reactor with solid and liquid; and extracting phosphate from the crystallized sludge desorption filtrate.

Description

Method for collecting Phosphate from sludge dewatering liquor using slow corrosion of magnesium bead}

The present invention relates to a method of crystallizing struvite and recovering it from a sludge release filtrate generated in a sewage treatment process by a struvite crystallization method.

In general, the sewage treatment process is largely composed of a water treatment system and a sludge treatment system, and the supernatant and filtrate generated in the sludge treatment system such as a digester, a concentration tank, and a dewatering tank are returned to the water treatment system and then treated. Reject water is a high-concentration mixed waste liquid generated from the sludge concentrated filtrate, sludge dewatering filtrate, etc. generated during the sludge treatment process in the sewage treatment process.

The backwash water is only 1.2% of the total flow, but the load of the sewage treatment plant increases by 10%, SS 31%, TN 21%, and TP 22%, respectively. However, the C/N ratio of domestic sewage is relatively low compared to 28:5 in the United States and 30:6 in Japan, which is 20:8 in the case of the confluence type, and it has a problem that nitrogen and phosphorus cannot be sufficiently treated without the injection of additional external carbon sources. .

In particular, in the case of small and medium-sized sewage treatment plants, there is more difficulty in maintaining the efficiency of the water treatment process compared to large-scale treatment plants because the generation of backwash water is not continuous and the frequency of occurrence is not constant. In particular, in the case of small and medium-sized sewage treatment plants, there is more difficulty in maintaining the efficiency of the water treatment process compared to large-scale treatment plants because the generation of backwash water is not continuous and the frequency of occurrence is not constant.

In the present invention, struvite is crystallized from a sludge depleting filtrate by using reaction characteristics due to magnesium metal corrosion, and the precipitated struvite is recovered using a ceramic filter.

The present invention is a technology capable of recovering phosphorus from the desorption filtrate of anaerobic digestion sludge _{, and according to the use of chemicals such as MgCl 2} and MgO, the molar ratio setting according to the object properties, pH control, and auxiliary facilities such as inverters and pumps are provided. The purpose of this is to provide a method of recovering struvite crystals and phosphorus by using a corrosion reaction using magnesium metal, taking into account that the scale of the facility and equipment is increased as necessary and the economic feasibility is reduced.

The present invention uses magnesium corrosion reaction to ^{supply Mg 2+} necessary for the formation of struvite crystals and increases pH due to hydrogen generation reactions due to corrosion reactions. Thus, struvite and struvite from the desorption filtrate are not used without chemicals. Phosphorus can be recovered.

When struvite crystals of the desired appropriate particle size are formed, the struvite in the form of sediment is recovered using a ceramic filter. The suspended struvite precipitate is filtered using a ceramic filter, and the filter adsorbed on the ceramic filter is used. True bite is recovered through the backwashing process.

1 is an XRD analysis result of magnesium metal.
2 is a result of a reaction experiment (Exp. A) to confirm a metal corrosion reaction of magnesium (Mg).
3 shows the results of a reaction experiment (Exp. B) for the desorption filtrate.
4 is an ORP measurement result for reaction experiments (Exp. A, B, C).
_{5 is a result of NH 4} -N, PO ₄ -P and pH for the reaction experiment (Exp. C).
6 shows the results of Ca and Mg for the reaction experiment (Exp. C).
7 is a saturation index result through Visual MINTEQ for the reaction experiment (Exp. C) result (physical and chemical data).
8 is an XRD analysis result of a precipitate for a reaction experiment (Exp. C).
9 is a SEM-EDS analysis result of the precipitate for the reaction experiment (Exp. C).
10 is a result of particle size analysis of a precipitate for a reaction experiment (Exp. C).
Fig. 11 is a process chart of filtering and recovering the precipitate (struvite) generated after the reaction experiment is completed.

In general, there are various techniques for recovering phosphorus from the desorption filtrate of anaerobic digested sludge, such as crystallization and adsorption, but the crystallization method is most suitable economically and technically. In the crystallization method, phosphate is ^{crystallized with a divalent metal such as Mg 2+} or Ca ²⁺ under a specific pH condition, and recovered as MAP (Magnesium ammonium phosphate) or HAP (Hydroxyapatite) crystals. Among them, the MAP method is known to be suitable for the desorption filtrate of anaerobic digested sludge because it has little effect of alkalinity and can also recover ammonia nitrogen.

The representative process for high purity phosphorus recovery is known as the struvite crystallization process. Struvite is ^{a crystal in which Mg 2+} , NH ⁴⁺ , and PO ₄ ^3- are combined in a molar ratio of 1:1:1 and is known as Magnesium Ammonium Phosphate (MAP). Struvite crystallization process can treat nitrogen and phosphorus at the same time, and because the reaction time is short, it requires a small site area, and it has the advantage of improving the overall treatment efficiency of the sewage treatment plant because it can be treated in conjunction with the biological treatment process.

According to the present invention, a reaction experiment (Table 2, Exp. A) was conducted to apply a corrosion reaction of a magnesium (Mg) metal (FIG. 1) as a magnesium source for precipitation of struvite. As a result of checking the pH, ORP, EC, and Mg ²⁺ ions over time, the pH rapidly increased to 8.94 for 1 minute of the reaction, and after reaching the equilibrium state after 10 minutes of the reaction, the pH appeared to be 10.82. Mg metal is a metal having a high hydrogen evolution reaction (HER), and the standard reduction potential _{is reported to be -2.37V nhe} (25℃).

The ORP according to the Mg corrosion reaction was found to be -123.8mV at 120.4mV and the EC at 0.6 to 12.99 mS/m as a reducing environment. ^{In addition, there was no change in Mg 2+} ion concentration according to the stirring speed (30, 200 rpm) (FIG. 2). ^{In aqueous solution, Mg metal generates Mg 2+} , OH ^- ion dissolution and Mg(OH) ₂ precipitate through corrosion reaction such as Eq (1) ~ (3). The pH and ORP increase and decrease exponentially by corrosion reaction, respectively, and reach equilibrium through Eq (3). _{Mg(OH) 2} generated by the corrosion reaction can prevent metal surface corrosion because the actual corrosion potential of Mg is lower than that of the standard reduction potential and film formation on the metal surface.

_{Mg(OH) 2} supersaturated by the corrosion reaction ^{consumes Mg 2+} ions in the form of hydroxides like Eq (4) and (5), and after each reaction experiment, Mg is a metallic luster. ) Could be observed to disappear.

[Equation]

In general, chlorine ions promote corrosion, whereas PO ₄ -P and Ca ions are known to prevent corrosion due to compound formation and adsorption. The properties of the desorption filtrate used in the present invention (Table. 1) contain salts that can accelerate the corrosion of Mg metal. As a result of the reaction characteristics experiment (Table. 2, Exp. B) according to this, the pH is the reaction. It rapidly increased after 5 minutes, and the PO ₄ -P removal rate was 100% at 10 minutes of reaction time.

PO ₄ -P delayed the corrosion reaction of Mg by ^{Cl −} _{, and after the removal of PO 4} -P, the corrosion reaction was accelerated, indicating that Mg ²⁺ ions were rapidly increased (Fig. 3). The increase in pH according to the hydrogen generation reaction of Mg _{affects the removal of PO 4} -P by the formation of Ca and Mg compounds, and the physical reaction according to the agitation speed change (30rpm vs 200rpm) promotes the hydrogen generation reaction, FIG. 4 Likewise, it plays a role of generating spontaneous corrosion of Mg.

It is said that impurities or second phases contained in the reaction solution act as an anode and promote galvanic corrosion of Mg. However, since the precipitates formed through the reaction are adsorbed on the Mg surface to inhibit corrosion or affect the efficiency of struvite recovery, it is necessary to adjust the stirring speed and use air bubbles to set the conditions for adsorption and aggregation of the metal surface of the precipitate. It is expected to be needed. In addition, the physical reaction according to the stirring speed affects the Mg ²⁺ ion consumption reaction Eq (4) and (5) of _{the Mg(OH) 2 precipitate generated by corrosion.}

_{The desorption filtrate was subjected to the removal of PO 4} -P according to Mg metal corrosion as the target (Table. 2, Exp. C) (FIG. 5). As the reaction proceeded, the pH was 8.19 to 8.87, and the ORP was 4.21 to -316.2 mV (Table 3).

The _{removal of PO 4} -P was calculated by applying the first order kinetic (R ² : 0.969, 0.996, 0.963) and k constant was 0.124 min ^-1 (Exp. C-1), 0.471 min ^-1 (Exp. C-2), 0.733 min ^-1 (Exp. C-3), and the _{removal rates of PO 4} -P were 46.46%, 97.73% and 100%, respectively. Previous studies using Mg metal (1) Graphite-Mg metal air bubbling column using galvanic corrosion (PO ₄ -P: 96.3%, pH 8.91, 15min), (2) Mg alloy plate (PO ₄ -P: 96.4%, pH 9.17, 150min), and the reaction rate was found to be low compared to the results of this study. After the corrosion reaction, Mg ²⁺ and OH ^- ions were dissolved and _{reacted with NH 4} -N and PO ₄ -P in the desorption filtrate, and white and brown precipitates were confirmed after the reaction experiment by conditions was completed. As such, Mg metal corrosion forms struvite crystals through Eq (6) as a magnesium source.

[Equation]

Due to the corrosion reaction, Mg gradually increases according to the reaction time, and the _{concentration of NH 4} -N and PO ₄ -P decreases by reacting with the struvite causative ion. ^{However, the Cl-} ion that promotes the corrosion reaction generates excessive Mg, which exceeds the theoretical molar ratio of Struvite (N(1): P(1): Mg(1)), and the loss of Mg and the struvite crystal are newberyite (pksp). ; 5.51), a phase change occurs. As the precipitate is formed, the formation of compounds such as _{Mg 3} (PO ₄ ) ₂ , MgHPO ₄ ·3H ₂ O, Mg ₂ (OH) ₃ Cl·4H _{2 O, etc. increases the PO 4} -P removal efficiency (up to 100%). On the other hand, it was found that the _{removal rate of NH 4} -N (up to 8.84%) was relatively reduced, and NH ₄ -N was consumed in the conversion reaction (NH ₄ -N ₃ -N, up to 3.87%) and to form struvite crystals.

Struvite crystallization reaction is highly dependent on pH, and the formation reaction (Eq 6) delays crystal formation due to ^{H +} _{ion generation, but the conversion reaction of ionic species (NH 4} -N, PO ₄ -P) and corrosion reaction of Mg Through the pH buffer. In addition, an increase in pH (8 -> 9) decreases _{the removal rate of NH 4} -N (90%, and increases HPO ₄ ^{2- as H +} decreases. This reaction promotes the generation of precipitates and corrosion). Struvite crystals are formed due to the supply of Mg ²⁺ _{and consumption of PO 4} -P and NH _{4 -N in the reaction (Fig. 8).}

phosphate is ^{_{monovalent (H 2 PO 4 -)}} , divalent (HPO 4 2-), and exists as ^{_{trivalent (PO 4 3-), pH}} 5-6.5 (H 2 PO 4 -) in accordance with the pH environment, pH 7- 9 (HPO ₄ ^2- ) is said to be dominant, respectively. As a result of confirming the distribution of species using Visual MINTEQ 3.1, divalent (HPO ₄ ^2- ) is dominant in 87.24% (pH 8.19), and after corrosion reaction, HPO ₄ ^2- is due to pH change and compound formation. They are 89.74% (pH 8.41, Exp. C-1), 79.09% (pH 8.67, Exp. C-2), and 66.26% (pH 8.87, Exp. C-3), respectively. PO ₄ -P is a reactant (Ca, Mg, NH ₄ -N) and a supersaturated compound through precipitation reaction is ACP (Amorphous calcium phosphate, Ca ₃ (PO ₄ ) ₂ :am2, SI: 1.93), TCP (Tricalcium phosphate, Ca ₃ (PO ₄ ) ₂ :beta, SI: 2.60), OCP(Octacalcium phosphate, Ca ₄ H(PO ₄ ) ₃ :3H ₂ O(s), SI: 2.31), Hydroxyapatite(HAP, Ca ₅ ( PO ₄ ) ₃ (OH), SI: 11.01) and Struvite (MAP, MgNH ₄ PO ₄ ·6H ₂ O, SI: 0.39) (Fig. 7).

Ca contained in aqueous solution causes competition and interference to form struvite crystals, and affects the formation of Mg-P, Ca-P compounds and struvite crystals through various reaction mechanisms. Table of the results of checking the Ca/Mg molar ratio according to the corrosion reaction. It appeared like 4.

Ca removal rate is Exp. C-1 (83-84%), Exp. C-2 (65-84%), Exp. C-3 (32~73%) rapidly increased in the initial reaction and then decreased with the passage of reaction time (Fig. 6).

Due to the corrosion properties of Mg, Ca reacts with PO ₄ -P predominantly over Mg to form Ca-P compounds such as ACP (pksp; 28.25), TCP (pksp; 28.92) and HAP (pksp; 58.7). , Depending on the reaction time, the Ca/Mg molar ratio (Ca/Mg> 1/4) increases, so that Mg gradually _{reacts with NH 4} ⁺ and PO ₄ -P to form MAP (pksp; 13.26). As the corrosion reaction proceeds, _{the saturation index of MAP increases as the consumption of PO 4} -P of Mg is increased than that of Ca, while the unsaturation and saturation index of Ca-P compound decreases. Due to the difference in thermodynamic energy, MAP reaches an equilibrium state predominantly than the amorphous form of ACP, thereby affecting the crystalline form. In addition, it was confirmed that the Ca-P compound formed at the beginning of the reaction repeated precipitation and dissolution due to the physical reaction according to the corrosion reaction and the stirring speed, and as the reaction time progressed, the Ca ions were re-dissolution. (Fig. 6).

As a result of performing XRD analysis on the precipitate after completion of the reaction, the Ca-P compound was found to be amorphous, and only struvite crystals could be identified (Fig. 8). Through the reaction experiment (Table 2, Exp. C), Ca and PO ₄ -P rapidly decreased to form a precipitate, but due to the formation of an unstable Ca-P compound, it was decomposed and dissolved to form a crystal and to exist in an amorphous form. do. Struvite crystals are formed due to the supply of Mg and _{the consumption of NH 4} -N due to corrosion, but the dissolution of Ca affects the struvite crystal strength (peak, 15-16 ^o , 16-17 ^o , 21-22 ^o ). I could confirm.

As a result of grasping the crystallization characteristics of the sediment (Table. 2, Exp. C) (Fig. 9), MAP and solid sediments of a rod and irregular shape were identified. In other words, it was confirmed that the precipitates were not formed sequentially through competition between Ca and Mg, but were generated simultaneously to maintain the nucleation and crystal growth of the Ca-P compound and MAP having a small particle size. In addition, as a result of comparing the rod-shaped uniform precipitate and the irregular precipitate, solid precipitates containing impurity ions (K, Na, Cl, etc.) are characterized by adsorption or aggregation on irregular crystal surfaces rather than uniform crystals. can confirm.

As a result of observing the particle size distribution characteristics according to the reaction time, D[4, 3] and Dv(50) are shown in Table. As shown in Fig. 5, it was confirmed that the particle size of the precipitate was reduced.

As a result of checking the characteristics of particle size distribution (Fig. 10), Exp. C-1 has a uniform particle size distribution, while Exp. As the reaction time progresses, the particle size of C-2 decreases and Exp. It was confirmed that it became similar to C-3. As such, Ca ions contained in the desorption filtrate _{react quickly with PO 4} -P and have an effect on removal through precipitation, but it was confirmed that it has an effect on the particle size distribution and particle size of struvite.

The struvite recovery process (Fig. 11) is operated as follows.

1. The desorption filtrate is filtered with SS 500ppm or less using a ceramic filter, and the filtered treated water is transferred to a Struvite reactor containing Mg balls.

2. In the Struvite reaction tank, Mg balls flow using a blower. Mg reacted with the desorption filtrate generates precipitates due to corrosion reaction, and moves to the ceramic filter reactor.

3. The desorption filtrate in which the precipitate (struvite) is formed is transferred to a ceramic filter reaction tank and filtered, and crystallized struvite can be recovered using ceramic filter filtration.

The Mg metal corrosion reaction according to the present invention makes it possible to recover struvite from the desorption filtrate by raising the pH and supplying magnesium. It will be affected, and the PO ₄ -P recovery rate according to the corrosion reaction could be recovered 100% through the reaction experiment (Exp. C).

Of the struvite causative ions, PO ₄ -P is rapidly reduced while forming various compounds through a competitive reaction between Ca and Mg, whereas NH ₄ -N is consumed in the conversion reaction and formation of struvite crystals, resulting in a relatively low removal rate. . In addition, while Ca ions rapidly decrease to form precipitates, struvite crystal formation decomposes unstable Ca-P compounds due to thermodynamic energy differences, and re-dissolves Ca ions and forms crystals using _{dissolved PO 4 -P.} However, MAP crystals of irregular shape are formed. In addition, it was confirmed that the re-dissolved Ca affects the particle size and distribution characteristics of the MAP crystal.

Claims (3)

Supplying a sludge desorbing filtrate and a magnesium (Mg) metal;
Causing a struvite crystallization reaction to the sludge desorption filtrate;
Solid-liquid separating the sludge desorption filtrate crystallized in the struvite crystallization reactor; And
A method for recovering phosphate from the sludge desorption filtrate using corrosion of magnesium metal, comprising the step of extracting phosphate from the crystallized sludge desorption filtrate. The method according to claim 1,
Phosphate recovery method from the sludge desorption filtrate using corrosion of magnesium metal further comprising the step of adjusting the supply flow rate and nitrogen loading rate of the sludge desorption filtrate supplied to the struvite crystallization reaction tank. The method according to claim 1,
A method for recovering phosphate from the sludge desorption filtrate using corrosion of magnesium metal, characterized in that a certain amount of air is supplied in addition to the sludge desorption filtrate and the magnesium metal.

Images