阅读说明：本技术 一种废弃物资源化利用方法及其在控制农业面源污染中的应用 (Waste resource utilization method and application thereof in controlling agricultural non-point source pollution ) 是由 王鹏飞 王书航 王坤 姜枫 崔冠楠 于 2020-12-14 设计创作，主要内容包括：本发明提供一种废弃物资源化利用方法及其在控制农业面源污染中的应用,所述方法包括：先将废弃物预处理得到以磷计磷酸盐浓度大于70mg/L且以氮计氨氮浓度大于60mg/L的废水,然后分别将废水、碱液和镁盐溶液从反应器下端进料,同时将反应器顶部的上清液回流至反应器下端,与废水、碱液和镁盐溶液混合,形成流化反应区；反应初始,向流化反应区加入作为晶种的鸟粪石颗粒,设定流化反应区的空隙率为0.8～0.9,计算上清液的回流流量；保持回流流量及废水和镁盐溶液的进料量不变,继续反应直至流化反应区的空隙率下降到0.7以下,移出流化反应区内的大粒径鸟粪石颗粒。该方法可实现农业面源污染的控制。(The invention provides a method for recycling wastes and application thereof in controlling agricultural non-point source pollution, wherein the method comprises the following steps: firstly, pretreating wastes to obtain wastewater with the phosphate concentration of more than 70mg/L in terms of phosphorus and the ammonia nitrogen concentration of more than 60mg/L in terms of nitrogen, then respectively feeding the wastewater, alkali liquor and magnesium salt solution from the lower end of a reactor, simultaneously refluxing supernate at the top of the reactor to the lower end of the reactor, and mixing the supernate with the wastewater, the alkali liquor and the magnesium salt solution to form a fluidized reaction zone; at the beginning of the reaction, adding struvite particles serving as seed crystals into a fluidized reaction zone, setting the void ratio of the fluidized reaction zone to be 0.8-0.9, and calculating the reflux flow of supernatant liquid; keeping the reflux flow and the feeding amount of the wastewater and the magnesium salt solution unchanged, continuing the reaction until the void ratio of the fluidization reaction zone is reduced to be below 0.7, and moving out the struvite particles with large particle size in the fluidization reaction zone. The method can realize the control of agricultural non-point source pollution.)

1. A method for recycling wastes is characterized by comprising the following steps: pretreating wastes to obtain wastewater with the phosphate concentration being more than 70mg/L in terms of phosphorus and the ammonia nitrogen concentration being more than 60mg/L in terms of nitrogen, then respectively feeding the wastewater, alkali liquor and magnesium salt solution from the lower end of a reactor, simultaneously refluxing supernatant liquid at the top of the reactor to the lower end of the reactor, and mixing the supernatant liquid with the wastewater, the alkali liquor and the magnesium salt solution to form a fluidized reaction zone;

at the beginning of the reaction, adding struvite particles serving as seed crystals into the fluidized reaction zone, setting the void ratio of the fluidized reaction zone to be 0.8-0.9, and calculating the reflux flow of supernatant liquid;

keeping the reflux flow and the feeding amount of the wastewater and the magnesium salt solution unchanged, continuing the reaction until the void ratio of the fluidization reaction zone is reduced to be below 0.7, and removing the struvite particles with large particle size in the fluidization reaction zone.

2. The method of claim 1, wherein the void ratio of the fluidized reaction zone is a ratio of a volume occupied by the liquid in the fluidized reaction zone to a volume of the fluidized reaction zone.

3. The method as claimed in claim 1 or 2, wherein after the struvite particles with large particle size in the fluidized reaction zone are removed, the struvite particles with small particle size automatically descending to the fluidized reaction zone from the upper part of the reactor are used as seed crystals to continue the reaction until the void ratio in the fluidized reaction zone is reduced to below 0.7 again, the struvite particles with large particle size in the fluidized reaction zone are removed, and the process is repeated.

4. The method for recycling waste as resources according to any one of claims 1 to 3, wherein the reactor comprises the fluidized reaction zone, the transition zone and the settling zone in sequence from bottom to top, and the cross sectional area of each zone increases in a stepwise manner from bottom to top.

5. The method of claim 4, wherein the upward flow velocity of the liquid in the settling zone is less than 2.0 x 10^-3m s^-1。

6. The method for recycling waste according to any one of claims 1 to 5, wherein struvite particles as seed crystals added at the beginning of the reaction have a particle size of 0.8 to 2.0mm and a porosity of 0.1 to 0.2.

7. The method for recycling waste as resources according to any one of claims 1 to 6, wherein supersaturation degree in the reactor is controlled during production, and when the top of the reactor is continuously turbid, the feeding amount of the alkali liquor is reduced.

8. A struvite particle with a large particle size produced by the method for recycling waste of any one of claims 1 to 7.

9. The large-particle-size struvite particles of claim 8, wherein the large-particle-size struvite particles have a particle size of 3 to 4mm and a porosity of 0.01 to 0.05.

10. The application of the method for recycling the wastes according to any one of claims 1 to 7 in controlling agricultural non-point source pollution.

Technical Field

The invention relates to the technical field of waste treatment, in particular to a waste resource utilization method and application thereof in controlling agricultural non-point source pollution.

Background

Phosphorus is a non-renewable resource, is one of three nutrient elements required by plant growth, and is also an essential element of all biological life activities on the earth. The existing phosphorus utilization mode, namely phosphorite mining, fertilizer processing and applying, plant absorption, digestion by animals and human beings through food, excretion of unabsorbed parts, gathering into water and refuse landfills, is not sustainable, and phosphorus gathered into the water can cause pollution to the water. On the other hand, the quick-acting fertilizer applied in the farmland is easy to run off, farmland runoff becomes an important pollution source for eutrophication of most lakes, and the substitution of the slow-release fertilizer for the quick-acting fertilizer is an effective way for solving the problem, but the slow-release fertilizer is higher in price at present and is not high in use enthusiasm of farmers.

In order to increase the sustainable utilization of phosphorus, the recovery of phosphorus in wastes (wastewater and solid wastes) must be carried out, and the method is also an effective measure for treating the problem of lake eutrophication. The waste contains phosphate and ammonia nitrogen with higher concentration/content, and MgCl is added₂NaOH, which is a commonly used method for recovering phosphorus from wastes by struvite, and the principle of the method is that phosphate ions, ammonium ions and magnesium ions in waste water or pretreated solid wastes react and precipitate under the alkaline condition to generate struvite (MgNH)₄PO₄·6H₂O), struvite is a slow release fertilizer which can be absorbed and utilized by plants. The waste is used as the raw material to produce the slow release guanite, so that the cost can be obviously reduced.

However, the existing process mostly uses a continuous flow mixing reactor or a fluidized bed reactor, the recovered product is single crystal form of struvite powder or small-particle-size struvite particles with low compressive strength, the packaging, the transportation and the application are inconvenient, and the struvite particles with large particle size and slower nutrient salt release rate can be obtained through subsequent granulation procedures, so the cost is increased.

Disclosure of Invention

Aiming at the defects in the prior art, the invention provides a waste resource utilization method and application thereof in controlling agricultural non-point source pollution, so that phosphorus and ammonia nitrogen in the waste are converted into struvite particles with large particle size (about 3-4 mm), high purity (96-97%), high strength and low porosity, and the struvite particles can be directly used as slow release fertilizer to be applied to control agricultural non-point source pollution.

The invention adopts the following technical scheme:

the invention provides a method for recycling wastes, which comprises the following steps: pretreating wastes to obtain wastewater with the phosphate concentration (calculated by phosphorus) being more than 70mg/L and the ammonia nitrogen concentration (calculated by nitrogen) being more than 60mg/L, then respectively feeding the wastewater, alkali liquor and magnesium salt solution from the lower end of a reactor, simultaneously refluxing supernatant at the top of the reactor to the lower end of the reactor, and mixing the supernatant with the wastewater, the alkali liquor and the magnesium salt solution to form a fluidized reaction zone;

at the beginning of the reaction, adding struvite particles serving as seed crystals into the fluidized reaction zone, setting the void ratio of the fluidized reaction zone to be 0.8-0.9, and calculating the reflux flow of supernatant liquid;

keeping the reflux flow and the feeding amount of the wastewater and the magnesium salt solution unchanged, continuing the reaction until the void ratio of the fluidization reaction zone is reduced to be below 0.7, and removing the struvite particles with large particle size in the fluidization reaction zone.

The main function of the refluxed supernatant in the invention is to provide ascending flow, so that particles in the fluidization reaction zone can be suspended without sedimentation, and phosphate ions and ammonium ions in the wastewater and magnesium ions in the magnesium salt solution react at a proper pH (usually 7.5-9.5) in the fluidization reaction zone. The research of the invention finds that the mode of refluxing the supernatant at the top of the reactor and controlling the reflux flow can integrate the formation, growth and polymerization of the struvite single crystal into a whole, so that the struvite crystal grows and polymerizes on the surface of the added seed crystal in the form of a compact crystal polymer, and in the reaction form, by keeping the feeding amount and the reflux flow unchanged and simply controlling the index of the void ratio of the fluidized reaction zone, the struvite particles with large particle size, high purity, high strength and low porosity can be stably obtained and can be directly used as slow-release fertilizer for application.

The waste can be digestive juice generated in the anaerobic digestion process of sludge in a town sewage treatment plant, high-phosphorus high-ammonia nitrogen wastewater such as production wastewater of a dairy processing plant and the like, and solid waste with high content of phosphorus and high ammonia nitrogen such as sludge in the town sewage treatment plant (shown in a specific embodiment mode), livestock manure and the like. When solid waste is used as a raw material, the solid waste is pretreated into high-phosphorus high-ammonia nitrogen wastewater by acidolysis and other methods, namely the concentration (by phosphorus) of soluble phosphate in the wastewater is more than 70mg/L, and the concentration (by nitrogen) of ammonia nitrogen is more than 60 mg/L.

The void ratio of the fluidization reaction zone is the ratio of the volume occupied by the liquid in the fluidization reaction zone to the volume of the fluidization reaction zone.

The relationship between the reflux flow rate of the supernatant and the void ratio of the fluidized reaction zone in the present invention is shown in formula (1):

in the formula, Q_r(m³The flow rate is/s); q_tot(m³The flow is the sum of the inflow flow and the backflow flow; q_in(m³The flow rate is the sum of the flow rate of wastewater and the flow rate of magnesium salt solution, the flow rate of alkaline solution is very small relative to the flow rate of wastewater, the flow rate of magnesium salt solution and the flow rate of backflow, and the flow rate is ignored and mainly used for adjusting pH; d_f(m) is the fluidized reaction zone diameter; epsilon is the void fraction of the fluidized reaction zone; k is a radical of_fCorrection coefficient for tube wall effect, z is index, U₀(m/s) is the particle free settling velocity, k_fCalculated by formula (2), z by formula (3), U₀Calculated by the formulas (4) and (5):

ρ_p＝ρs(1-ε_p)+ρ_lε_p (6)

in the formula (d)_p(m) is the struvite particle diameter; rho_l(kg m^-3) Is liquid density, which can be approximated as the density of water; rho_p(kg m^-3) Calculated for the effective density of the struvite particles from equation (6), where ρ_s(1.7×10³kg m^-3) Is true density of struvite,. epsilon_pIs struvite particle porosity; μ (Pa · s or kg m)^-1s^-1) Is the kinetic viscosity of water; g is acceleration of gravity, 9.8m s^-2；C_DThe drag coefficient was determined from the Reynolds number. Due to the U contained in the Reynolds number₀Calculate U₀Multiple iterations are required.

The method comprises the steps of carrying out reaction initiation, substituting the diameter and the porosity of struvite particles which are added and used as seed crystals and the porosity of a preset fluidization reaction zone into the formula to calculate backflow flow, wherein in the operation process, along with the increase of particle size, the porosity of the fluidization reaction zone is gradually reduced, and when the porosity is too small, the struvite particles are not enough in strength and are easy to break. The research of the invention finds that when the void ratio of the fluidized reaction zone is reduced to below 0.7, the particles with large particle size are moved out of the reactor, and the particle size and the strength of the obtained struvite can reach the comprehensive optimum. In the specific operation process, the particle diameter and the porosity can be measured by periodically removing a small part of particles, and the porosity is calculated by the formula so as to judge whether the removal time is the removal time.

Further, after the struvite particles with large particle size in the fluidization reaction zone are removed, taking the struvite particles with small particle size which automatically descend to the fluidization reaction zone from the upper part of the reactor as seed crystals to continue to react until the porosity of the fluidization reaction zone is reduced to below 0.7 again, removing the struvite particles with large particle size in the fluidization reaction zone, and repeating the steps in this way.

The invention utilizes the small-particle-size struvite particles automatically settled from the upper part of the reactor as the seed crystals to continue production, so that the reactor can keep continuous operation and production, no additional seed crystals are required to be added, the cost is saved, and the working procedure is simplified.

In a specific embodiment of the invention, the reactor sequentially comprises the fluidized reaction zone, the transition zone and the settling zone from bottom to top, and the cross-sectional area of each zone is gradually increased from bottom to top. The reactor is favorable for the method of the invention and the control of the production process.

Further, in the settling zone, the upward flow velocity of the liquid is less than 2.0 x 10^-3m s^-1。

The small struvite crystals can not be completely settled due to the overlarge ascending flow velocity, and part of unsettled small particles enter the bottom end of the reactor along with the backflow water, so that the problems of water inlet blockage, reduction of the growth rate of the seed crystal particles in the reactor, reduction of the particle size of the product and the like are caused. Wherein the rising flow velocity (v)_pM/s) is calculated as shown in equation (7):

in the formula, D_p(m) is the settling zone diameter.

Preferably, the struvite particles which are added at the beginning of the reaction and used as the seed crystals have the particle size of 0.8-2.0 mm and the porosity of 0.1-0.2. The struvite particles with the particle size and the porosity within the range are proper in size and strength as the seed crystals, can be prevented from being crushed in a reactor, are favorable for fast growth to obtain the target large-particle-size struvite particles, and simultaneously leave a certain space to form a compact crystal polymer on the outer layer of the seed crystals, so that the overall strength of the obtained large-particle-size struvite particles meets the requirement.

Further, the supersaturation in the reactor is controlled during production, and the feed amount of the lye is reduced when the reactor top continues to be turbid. Preferably, the supersaturation degree in the reactor is controlled to be 1.5-3 in the production process.

The invention also provides struvite particles produced by the waste resource utilization method.

According to the method, the particle size of the obtained struvite particles is 3-4 mm, and the porosity is 0.01-0.05. Compared with struvite obtained by treating wastewater by a conventional struvite crystallization method, the struvite particles obtained by the method have the advantages of large particle size, high strength, low porosity and high purity, can be directly used as slow-release fertilizer, do not need to be granulated, and save the cost.

The invention provides a resource utilization method of wastes, which can simply and controllably convert phosphorus and ammonia nitrogen in the wastes into struvite particles with large particle size (about 3-4 mm), high purity (96-97%), high strength and low porosity, and can be directly used as slow-release fertilizers for application to control agricultural non-point source pollution.

Drawings

In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly described below, and it is obvious that the drawings in the following description are some embodiments of the present invention, and those skilled in the art can also obtain other drawings according to the drawings without creative efforts.

FIG. 1 is a schematic view of a reactor used in the method for recycling waste provided by the embodiment of the present invention;

FIG. 2 is a schematic view of a fluidized reaction zone in operation of a reactor in an embodiment of the present invention;

FIG. 3 is a scanning electron micrograph (left) and a cross section (right) of the surface (left) and cross section of struvite particles recovered after 28 days of reaction in an example of the present invention;

FIG. 4 is a photograph of struvite particles recovered after 35 days of reaction in an example of the present invention;

FIG. 5 is a scanning electron micrograph of the surface (left) and cross-section (right) of struvite particles recovered after 35 days of reaction in an example of the present invention.

Detailed Description

In order to make the objects, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are some, but not all, embodiments of the present invention. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.

Examples

The embodiment provides a method for recycling wastes, and particularly relates to a method for producing large-particle-size struvite particles by using sludge of a domestic sewage treatment plant as a raw material, wherein the sludge of the domestic sewage treatment plant is subjected to acidolysis and filtration by adding a sulfuric acid solution to pH 5, the phosphate concentration (calculated by phosphorus) in the obtained filtrate is 464mg/L, and the ammonia nitrogen concentration (calculated by nitrogen) is 1493 mg/L.

In this example, a reactor as shown in FIG. 1 was used for the reaction, the effective volume of which was 3.33L, and which was mainly divided from bottom to top into a fluidized reaction zone (referred to as a fluidized zone in the figure, having a diameter of 2.2cm), a transition zone and a settling zone (having a diameter of 13.6cm), and the supernatant in the settling zone was refluxed to the fluidized reaction zone together with wastewater, MgCl and MgCl₂The solution was mixed with NaOH solution. A schematic of the fluidized reaction zone during reactor operation is shown in fig. 2, from which it can be seen that struvite particles are in suspension.

The reactor operating parameters were: the wastewater flow rate is 0.72L/h (corresponding to the hydraulic retention time of 4.6h), the reflux flow rate is 96L/h, and MgCl is adopted₂Flow 0.27L/h (MgCl)₂Solution concentration (calculated by Mg) 1380Mg/L), pH 7.6, and seed crystal 40g of struvite particles with the particle size of 1.4-2.0 mm (average particle size is 1.7mm, and porosity is 0.19). Under the condition, the initial void ratio of the fluidized reaction zone is 0.84, and the flow velocity of the settling zone is 1.85X 10^-3m s^-1. Along with the prolonging of the running time, the struvite particles in the reactor gradually increase and the void ratio gradually decreases, when the running time reaches 28 days, the struvite particles increase to 3.1mm,the porosity is reduced to 0.05, and the porosity of the fluidized reaction zone is 0.72 according to the formula, and the porosity is still more than 0.7, so that the struvite particles with the maximum particle size can be continuously remained in the reactor for growing. And when the time reaches 35 days, the struvite particles are increased to 3.5mm, the porosity is reduced to 0.04, the calculation is carried out according to a formula, the void ratio of the fluidized reaction zone is 0.71, the void ratio is still more than 0.7, the struvite particles with the maximum particle size can be continuously remained in the reactor to grow, and the struvite particles in the fluidized reaction zone are completely removed until the void ratio is reduced to be below 0.7.

Scanning electron micrographs of the surface (left) and cross section (right) of the struvite particles recovered after 28 days of reaction are shown in FIG. 3. A photograph of the struvite particles recovered after 35 days of reaction is shown in fig. 4; the surface (left) and cross-sectional (right) scanning electron micrographs of the struvite particles are shown in fig. 5. As can be seen from the figure, the particles consist of a relatively loose seed particle in the center and a dense crystalline polymer of struvite in the outer layer which increases the strength of the particle and decreases the porosity.

Through detection, the elemental compositions of the struvite particles recovered after the reactor is operated for 28 days and 35 days are shown in table 1, and the purity is about 96-97%.

Table 1 struvite particle element content (%) -recovered after 28 and 35 days of reactor operation

From the results, the struvite particles obtained by the method disclosed by the embodiment of the invention have the advantages of large particle size (3-4 mm), high purity (96-97%), high strength and low porosity, are not easy to break, are convenient to transport, package and apply, have low nutrient release rate, and are excellent slow-release fertilizers.

Finally, it should be noted that: the above examples are only intended to illustrate the technical solution of the present invention, but not to limit it; although the present invention has been described in detail with reference to the foregoing embodiments, it will be understood by those of ordinary skill in the art that: the technical solutions described in the foregoing embodiments may still be modified, or some technical features may be equivalently replaced; and such modifications or substitutions do not depart from the spirit and scope of the corresponding technical solutions of the embodiments of the present invention.