Method for recovering phosphorus in sludge

Abstract

The invention discloses a method for recovering phosphorus in sludge, which comprises the following steps: S1, adding citric acid into sludge, and uniformly mixing to obtain treated sludge; s2, the treated sludge is conveyed into a fermentation device for aerobic fermentation, and fermented sludge is obtained; s3, conveying the fermented sludge into a separation device for separation to obtain dewatered sludge and filtrate; s4, magnesium oxide is added into the filtrate, magnesium ammonium phosphate crystals are obtained in a crystallization and precipitation mode, and therefore phosphorus in the filtrate is recycled. According to the method, the release efficiency of phosphorus in the sludge can be improved while the energy consumption and the dosage are reduced, and efficient recovery of sludge resources is realized.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of sludge treatment and resource processing, in particular to a method for recovering phosphorus from sludge. BACKGROUND

[0002] Phosphorus is a non-metallic mineral resource that is difficult to regenerate, and it can participate in the life activities of animals and plants and has important utilization value in industry and agriculture. The concentration of nitrogen and phosphorus in sludge from municipal wastewater treatment plants is much higher than that in general sewage / wastewater, which provides the possibility for the recovery of phosphorus from sludge, making sludge a potential recyclable phosphorus ore resource.

[0003] In the prior art, struvite method or calcium phosphate method is mainly used to recover phosphorus from sludge concentrate or sludge anaerobic digestion liquid, but this method is only suitable for sludge with high water content and is not suitable for general concentrated sludge. Moreover, the above method is relatively single, and the release efficiency of phosphorus in sludge is not high. For general concentrated sludge, heat treatment method, acid-base method or microwave method are used to recover phosphorus from sludge, but the energy consumption of heat treatment method and microwave method is high, and the economy is poor. The acid-base method needs to use chemicals such as sulfuric acid, hydrochloric acid or sodium hydroxide, which not only increases the cost, but also has the risk of secondary pollution. The above methods all have obvious limitations. Therefore, it is necessary to develop a method for efficiently recovering phosphorus from sludge to avoid resource waste. SUMMARY

[0004] The present application aims to solve the technical problems of high energy consumption, high risk of secondary pollution and low release efficiency of phosphorus in sludge in the prior art. The present application provides a method for recovering phosphorus from sludge, which combines citric acid assisted aerobic fermentation technology with ammonium magnesium phosphate recovery method, thereby reducing energy consumption and drug dosage while improving the release efficiency of phosphorus in sludge, so as to realize efficient recovery of sludge resources.

[0005] To solve the above technical problems, the present application discloses a method for recovering phosphorus from sludge, which comprises the following steps:

[0006] S1: adding citric acid to the sludge, and mixing uniformly to obtain treated sludge;

[0007] S2: feeding the treated sludge into a fermentation device for aerobic fermentation to obtain fermented sludge;

[0008] S3: feeding the fermented sludge into a separation device for separation to obtain dewatered sludge and filtrate;

[0009] S4: adding magnesium oxide to the filtrate to obtain ammonium magnesium phosphate crystals by crystallization and precipitation, thereby recovering phosphorus from the filtrate.

[0010] Preferably, in step S1, the amount of citric acid added is 10-50 g / kg dry weight, based on the dry matter weight of the sludge.

[0011] Preferably, step S2 includes:

[0012] The treated sludge is transported to the reaction zone of the fermentation unit;

[0013] Air was introduced into the reaction zone and aerobic fermentation was initiated at room temperature and pH 6.5-7.5.

[0014] The phosphorus dissolution concentration in the reaction zone was monitored in real time. At the end of fermentation, the phosphorus dissolution concentration was greater than 400 mg / L.

[0015] Preferably, in step S2, the air aeration rate is 300-600 L / min·kg dry weight based on the dry matter weight of the sludge; the temperature during the aerobic fermentation of the sludge is monitored, and the aerobic fermentation time of the sludge at a temperature of 40-60 ℃ is controlled to be 12-48 h.

[0016] Preferably, in step S3, the dewatered sludge is subjected to anaerobic digestion or aerobic composting.

[0017] Preferably, step S4 is performed as follows:

[0018] Add magnesium oxide to the filtrate until the pH of the filtrate is 8.5-9.5;

[0019] Stir the filtrate at room temperature to react;

[0020] The precipitate was obtained by crystallization precipitation.

[0021] The precipitate was washed and dried to obtain magnesium ammonium phosphate crystals.

[0022] Preferably, in step S4, the amount of magnesium oxide added is calculated based on the molar ratio of Mg in magnesium oxide to phosphorus in the filtrate being 1.8-3.8:1.

[0023] Preferably, in step S4, the stirring reaction time is 30-90 min.

[0024] Preferably, the phosphorus content in the magnesium ammonium phosphate crystals is >12wt%, and the purity of the magnesium ammonium phosphate crystals is ≥90%.

[0025] Preferably, the fermentation device is a fermentation reactor, and the separation device is a filter press or centrifuge; and / or the dry matter content in the sludge is 20-50 g / L, the total phosphorus content is 15-40 g / kg; and / or the phosphorus recovery rate in the sludge is >90%.

[0026] Compared with the prior art, the beneficial effects of the present invention include:

[0027] (1) The recovery method provided by the present invention combines citric acid pretreatment with aerobic fermentation technology, which greatly promotes the effective dissolution and release of solid phosphorus in sludge. Aerobic fermentation technology can release biobound phosphorus through microbial metabolism, and citric acid can release chemically bound phosphorus as a chelating agent. The two almost cover all major phosphorus forms in sludge, maximizing the total release rate and recovery potential of phosphorus, thereby significantly improving the recovery efficiency of phosphorus in sludge.

[0028] (2) The recycling method provided by the present invention uses aerobic fermentation technology. Aerobic microorganisms can release biological heat during aerobic fermentation to meet the temperature rise requirements during the reaction process without external heating, thereby reducing reaction energy consumption and improving the economic efficiency of the recycling process.

[0029] (3) The magnesium oxide used in the recovery method provided by the present invention can adjust the pH value of the reaction system while providing magnesium ions to the reaction system, without the need to add strong base, thus reducing the cost of reagents.

[0030] (4) The recycling method provided by this invention uses citric acid, a natural organic acid, to replace traditional inorganic strong acids for pretreatment, reducing the consumption of acid-base neutralization reagents in subsequent treatment. Moreover, compared with traditional inorganic strong acids, citric acid has better biodegradability and can be completely biodegraded in aerobic fermentation and subsequent treatment without producing toxic or harmful residues or byproducts, thus avoiding the introduction of chloride ions or sulfate ions that may cause secondary pollution. Detailed Implementation

[0031] The following specific embodiments illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. Although the description of the present invention is presented in conjunction with preferred embodiments, this does not mean that the features of the invention are limited to these embodiments. On the contrary, the purpose of describing the invention in conjunction with embodiments is to cover other options or modifications that may be derived based on the claims of the present invention. To provide a deep understanding of the invention, many specific details will be included in the following description. The invention may also be implemented without using these details. Furthermore, to avoid confusion or obscuring the focus of the invention, some specific details will be omitted in the description. It should be noted that, unless otherwise specified, the embodiments and features in the embodiments of the present invention can be combined with each other.

[0032] If a quantity, concentration, or other numerical value or parameter is expressed as a range, preferred range, better range, or a series of upper and lower limits, it should be understood that this document has specifically disclosed all ranges consisting of any pair of upper or preferred or better values ​​of that range and lower or preferred or better values ​​of that range, regardless of whether such ranges are disclosed separately. Furthermore, when a range of numerical values ​​is mentioned herein, unless otherwise stated, the range should include its endpoints and all integers and fractions within the range.

[0033] To make the objectives, technical solutions, and advantages of the present invention clearer, the embodiments of the present invention will be described in further detail below.

[0034] The present invention discloses a method for recovering phosphorus from sludge, which includes the following steps:

[0035] S1: Add citric acid to the sludge and mix well to obtain treated sludge.

[0036] S2: The treated sludge is transported to a fermentation device for aerobic fermentation to obtain fermented sludge.

[0037] S3: The fermented sludge is transported to a separation device for separation to obtain dewatered sludge and filtrate;

[0038] S4: Add magnesium oxide to the filtrate and obtain magnesium ammonium phosphate crystals by crystallization precipitation, thereby recovering phosphorus from the filtrate.

[0039] Specifically, the dry matter content in the sludge is 20-50 g / L, and the total phosphorus content is 15-40 g / kg. The phosphorus recovery rate in the sludge is >90%.

[0040] Further, in step S1, the amount of citric acid added is 10-50 g / kg dry weight, based on the dry matter weight of the sludge. Preferably, the amount of citric acid added is 20-30 g / kg dry weight.

[0041] Using the above technical solution, citric acid, as an organic acid, can provide H+ to the reaction system, lowering the pH of the sludge and thus directly dissolving some of the phosphate precipitate. It can also form stable complexes with phosphorus-binding metal ions in the sludge, improving the phosphorus release efficiency. Furthermore, as a biodegradable organic carbon source, citric acid can rapidly initiate microbial activity, improving the efficiency of the fermentation process. Simultaneously, citric acid is completely biodegradable during aerobic fermentation and subsequent treatment, producing no toxic or harmful residues or byproducts, avoiding the introduction of chloride or sulfate ions that could cause secondary pollution. Moreover, adding 10-50 g / kg dry weight of citric acid can ensure high phosphorus recovery and purity in the sludge while controlling reaction costs.

[0042] Further, step S2 includes: transporting the treated sludge to the reaction zone of the fermentation device; introducing air into the reaction zone and starting aerobic fermentation at room temperature and pH 6.5-7.5; monitoring the phosphorus dissolution concentration in the reaction zone in real time, and at the end of fermentation, the phosphorus dissolution concentration is greater than 400 mg / L.

[0043] Specifically, in step S2, based on the dry matter weight of the sludge, the air aeration rate is 300-600 L / min·kg dry weight; the temperature during the aerobic fermentation of the sludge is monitored, and the aerobic fermentation time of the sludge at a temperature of 40-60 ℃ is controlled for 12-48 h. Preferably, the air aeration rate is 450 L / min·kg dry weight, and the aerobic fermentation time of the sludge at a temperature of 45-55 ℃ is controlled for 24 h.

[0044] Specifically, the fermentation device is a fermentation reactor.

[0045] By adopting the above technical solution and using phosphorus dissolution concentration as the reaction endpoint, the stability of sludge output quality can be ensured. Furthermore, the synergistic effect of air aeration rate, fermentation temperature, and fermentation time can stably produce high-concentration phosphorus-containing filtrate with low energy consumption and in a short time, thereby improving the stability and reliability of the process.

[0046] Furthermore, in step S3, the dewatered sludge is subjected to anaerobic digestion or aerobic composting.

[0047] By adopting the above technical solutions, the residue from which phosphorus has been extracted can be further converted into biogas or compost through anaerobic digestion or aerobic composting, which reduces the environmental burden and improves the overall utilization rate of sludge.

[0048] Specifically, the separation device is a filter press or a centrifuge.

[0049] Further, step S4 is performed as follows: magnesium oxide is added to the filtrate until the pH of the filtrate is 8.5-9.5; the filtrate is stirred and reacted at room temperature; a precipitate is obtained by crystallization; the precipitate is washed and dried to obtain magnesium ammonium phosphate crystals.

[0050] Specifically, in step S4, the amount of magnesium oxide added is calculated based on a molar ratio of Mg in magnesium oxide to phosphorus in the filtrate of 1.8-3.8:1. Preferably, the amount of magnesium oxide added is calculated based on a molar ratio of Mg in magnesium oxide to phosphorus in the filtrate of 3:1.

[0051] Specifically, in step S4, the stirring reaction time is 30-90 min. Preferably, the stirring reaction time is 60 min.

[0052] Specifically, the phosphorus content in magnesium ammonium phosphate crystals is >12wt%, and the purity of magnesium ammonium phosphate crystals is ≥90%.

[0053] Traditional magnesium ammonium phosphate precipitation methods typically require the separate addition of a magnesium source and an alkali source to provide Mg. 2+ The pH value is adjusted. This application uses magnesium oxide to complete magnesium ion supply and pH adjustment in one step, reducing the amount of reagents needed and lowering reaction costs. Furthermore, magnesium oxide is a milder alkaline substance, posing lower operational safety risks and avoiding the corrosiveness and hazards of strong alkalis. Simultaneously, the selection of the magnesium / phosphorus molar ratio, pH range, and reaction time creates excellent reaction conditions for the formation of magnesium ammonium phosphate crystals, improving product recovery and purity.

[0054] The technical solution of the present invention will be further described below with reference to specific embodiments and comparative examples:

[0055] Example 1

[0056] S1: Take 100 L of sludge with a dry matter content of 35 g / L and a total phosphorus content of 20 g / kg in a mixing tank, add citric acid to the sludge at a rate of 25 g / kg dry weight, and mix evenly to obtain the treated sludge.

[0057] S2: The treated sludge was transported to the reaction zone of the fermentation reactor, and air was introduced into the reaction zone at an aeration rate of 300 L / min·kg dry weight. Aerobic fermentation was started at room temperature, and the temperature of the reaction zone was monitored during the aerobic fermentation of the sludge, controlling the aerobic fermentation at 50 ℃ for 24 h. The phosphorus dissolution concentration in the reaction zone was monitored in real time using a flow injection analyzer. At the end of aerobic fermentation, the phosphorus dissolution concentration in the sludge was approximately 520 mg / L.

[0058] S3: The fermented sludge is transported to a filter press or centrifuge for separation, yielding dewatered sludge and approximately 80 L of filtrate. The phosphorus concentration in the filtrate is approximately 560 mg / L. The filtrate is then further processed, and the dewatered sludge undergoes either anaerobic digestion or aerobic composting.

[0059] S4: Magnesium oxide was added to the filtrate until the pH reached 9.0. The amount of magnesium oxide added was calculated based on a 2:1 molar ratio of Mg in magnesium oxide to phosphorus in the filtrate. The filtrate was stirred at room temperature for 60 min, and then a precipitate was obtained by crystallization. After washing and drying, 0.57 kg of magnesium ammonium phosphate crystals were obtained. The phosphorus content in the magnesium ammonium phosphate crystals was 12.6 wt%, and the purity of the magnesium ammonium phosphate crystals was 90%. The phosphorus recovery rate from the sludge was calculated to be 92%.

[0060] Example 2

[0061] S1: Take 100 L of sludge with a dry matter content of 40 g / L and a total phosphorus content of 36 g / kg in a mixing tank, add citric acid to the sludge at a rate of 40 g / kg dry weight, and mix evenly to obtain the treated sludge.

[0062] S2: The treated sludge was transported to the reaction zone of the fermentation reactor, and air was introduced into the reaction zone at an aeration rate of 450 L / min·kg dry weight. Aerobic fermentation was started at room temperature, and the temperature of the reaction zone was monitored during the aerobic fermentation of the sludge, controlling the aerobic fermentation at 55 ℃ for 36 h. The phosphorus dissolution concentration in the reaction zone was monitored in real time using a flow injection analyzer. At the end of aerobic fermentation, the phosphorus dissolution concentration in the sludge was 530 mg / L.

[0063] S3: The fermented sludge is transported to a filter press or centrifuge for separation, yielding dewatered sludge and approximately 80 L of filtrate. The phosphorus concentration in the filtrate is approximately 580 mg / L. The filtrate is then further processed, and the dewatered sludge undergoes either anaerobic digestion or aerobic composting.

[0064] S4: Magnesium oxide was added to the filtrate until the pH reached 9.5. The amount of magnesium oxide added was calculated based on a 3:1 molar ratio of Mg in magnesium oxide to phosphorus in the filtrate. The filtrate was stirred at room temperature for 60 min, and then a precipitate was obtained by crystallization. After washing and drying, 1.05 kg of magnesium ammonium phosphate crystals were obtained. The phosphorus content in the magnesium ammonium phosphate crystals was 12.6 wt%, and the purity of the magnesium ammonium phosphate crystals was 90%. The phosphorus recovery rate from the sludge was calculated to be 92%.

[0065] While the present invention has been described with reference to certain preferred embodiments, those skilled in the art should understand that the above description is a further detailed explanation of the invention in conjunction with specific embodiments, and should not be construed as limiting the specific implementation of the invention to these descriptions. Various changes in form and detail can be made by those skilled in the art, including several simple deductions or substitutions, without departing from the spirit and scope of the invention.

Claims

1. A method for recovering phosphorus from sludge, characterized in that, The recycling method includes the following steps: S1: Add citric acid to the sludge and mix well to obtain treated sludge; S2: The treated sludge is transported to a fermentation device for aerobic fermentation to obtain fermented sludge; S3: The fermented sludge is transported to a separation device for separation to obtain dewatered sludge and filtrate; S4: Add magnesium oxide to the filtrate and obtain magnesium ammonium phosphate crystals by crystallization precipitation, thereby recovering phosphorus from the filtrate.

2. The recycling method as described in claim 1, characterized in that, In step S1, the amount of citric acid added is 10-50 g / kg dry weight, based on the dry matter weight of the sludge.

3. The recycling method as described in claim 1, characterized in that, Step S2 includes: The treated sludge is transported to the reaction zone of the fermentation device; Air was introduced into the reaction zone and aerobic fermentation was initiated at room temperature and pH 6.5-7.5; The phosphorus dissolution concentration in the reaction zone was monitored in real time, and the phosphorus dissolution concentration was greater than 400 mg / L at the end of fermentation.

4. The recycling method as described in claim 3, characterized in that, In step S2, based on the dry matter weight of the sludge, the aeration rate of the air is 300-600 L / min·kg dry weight; the temperature during the aerobic fermentation of the sludge is monitored, and the aerobic fermentation time of the sludge at a temperature of 40-60 ℃ is controlled to be 12-48 h.

5. The recycling method as described in claim 1, characterized in that, In step S3, the dewatered sludge is subjected to anaerobic digestion or aerobic composting.

6. The recycling method as described in claim 1, characterized in that, Step S4 is performed as follows: Add magnesium oxide to the filtrate until the pH of the filtrate is 8.5-9.5; Stir the filtrate at room temperature to react. The precipitate was obtained by crystallization precipitation. The precipitate was washed and dried to obtain magnesium ammonium phosphate crystals.

7. The recycling method as described in claim 6, characterized in that, In step S4, the amount of magnesium oxide added is calculated based on the molar ratio of Mg in the magnesium oxide to phosphorus in the filtrate being 1.8-3.8:

1.

8. The recycling method as described in claim 6, characterized in that, In step S4, the stirring reaction time is 30-90 min.

9. The recycling method as described in claim 6, characterized in that, The phosphorus content in the magnesium ammonium phosphate crystals is >12wt%, and the purity of the magnesium ammonium phosphate crystals is ≥90%.

10. The recycling method as described in claim 1, characterized in that, The fermentation device is a fermentation reactor, and the separation device is a filter press or a centrifuge; and / or the dry matter content in the sludge is 20-50 g / L, and the total phosphorus content is 15-40 g / kg; and / or the phosphorus recovery rate in the sludge is >90%.