CN108217620A - A kind of method for using sea water as magnesium source processing anaerobic fermented liquid and preparing guanite - Google Patents

Abstract

The invention relates to a method of using seawater as a magnesium source to treat anaerobic fermentation liquid to prepare struvite; firstly, colloids and particles in seawater are removed; seawater passes through a three-stage nanofiltration system, and the concentrated water produced is used as a magnesium source; after nanofiltration The concentrated water magnesium source produced after treatment enters the alkali-added precipitation system to remove part of Ca ²⁺ ; then the supernatant, anaerobic fermentation liquid and lye of the precipitation system are added to the struvite preparation system to produce struvite precipitation. The use of cheap seawater, especially in offshore areas, uses a three-stage nanofiltration system to prepare magnesium sources. From the perspective of long-term development, it greatly reduces the cost of magnesium sources added to the preparation of struvite; through the use of struvite The method removes nitrogen and phosphorus, which can not only remove nitrogen and phosphorus nutrients, reduce the harm to the environment, but also realize resource utilization, and use it as a slow-release fertilizer, which can increase the potato yield by 40%. After removing nitrogen and phosphorus, the anaerobic fermentation liquid of excess sludge can also be reused as a carbon source in sewage treatment plants, reducing the impact of nitrogen and phosphorus on the system and reducing treatment costs.

Description

A method for preparing struvite by using seawater as a magnesium source to treat anaerobic fermentation broth

technical field

The invention relates to a method for recovering nitrogen and phosphorus in anaerobic fermentation liquid during wastewater treatment and preparing struvite by adding magnesium source, in particular to a method for preparing magnesium source by using seawater through a three-stage nanofiltration system.

Background technique

In the reuse process of municipal excess sludge anaerobic fermentation liquid as a carbon source for sewage treatment, some studies have found that the ammonia nitrogen content in the excess sludge anaerobic fermentation liquid is about 150 mg/L, and the phosphorus content is about 160 mg/L; In the liquid, the ammonia nitrogen content is about 200-2000mg/L, and the phosphorus content is about 20-400mg/L; in the landfill leachate, the ammonia nitrogen content is about 100-3000mg/L, and the phosphorus content is about 1-70mg/L. The content of nitrogen and phosphorus in the anaerobic fermentation broth is relatively high, if not handled properly, it will cause secondary pollution to the environment. By adding magnesium source, excess nitrogen and phosphorus can be recovered in the form of struvite in the form of precipitation, which can not only reduce the negative impact on the treatment process brought by the reuse of fermentation broth as carbon source, but also realize resource utilization. Struvite (MgNH ₄ PO ₄ 6H ₂ O, or MAP) is a white crystal that is insoluble in water. Known and applied multi-element slow-release fertilizers use struvite precipitates as slow-release fertilizers, which not only have high fertility, but also because they are multi-element fertilizers, the utilization rate is greatly improved compared with single chemical fertilizers. No matter how much fertilizer is applied, there will be no crop burning caused by nutrient accumulation, and struvite will not adsorb heavy metal elements in the soil, and will not harm crops when fertilizing. In addition, it can also be used as a feed additive, chemical reagent, fire retardant for structural products, etc. The study found that it can increase potato yield by 42%.

Magnesium sources need to be added during the preparation of struvite. There are many sources of magnesium. Magnesium chloride and magnesium hydroxide are commonly used. The latter is not only used as a source of magnesium, but also undertakes the important task of adjusting pH. The dissolving time of magnesium chloride is very long; and although the reaction of magnesium chloride is not slow, the cost is very high. If a method can be adopted to reduce the cost of magnesium sources, the method of removing nitrogen and phosphorus by struvite will be more widely used. The average value of magnesium ion content in seawater is 1280mg/L. How to separate magnesium ion from seawater containing relatively harmless counter anions (chloride ion is about 20000mg/L, sodium ion is about 11000mg/L) is a problem. Very challenging job.

Nanofiltration membrane technology is a new type of membrane separation technology developed in the past 10 years. Due to its special pore size range and special treatment (such as compounding and charging) of nanofiltration membranes, nanofiltration membranes have special characteristics. It has a high removal performance for divalent and multivalent ions and organic compounds with a molecular weight between (200 and 1000), while the removal rate for monovalent ions and small molecules is relatively low. Most nanofiltration membranes are charged. Using the special ion separation performance of nanofiltration membranes, the relative separation of divalent and monovalent ions in seawater can be achieved. The order of interception for ions is: SO ₄ ^2- >Mg ²⁺ >Ca ^{2 +} >Na ⁺ >Cl ⁻ .

There are many studies on nanofiltration membrane technology in the field of seawater desalination and water purification. In the field of seawater desalination, it is generally used as pretreatment, which can greatly reduce the hardness and TDS content of seawater, and solve the scaling pollution existing in the traditional seawater desalination process. In the field of water purification, it is mainly used for high-quality advanced treatment of urban recycled water, advanced treatment of urban water supply, and household water purifiers. The disposal of concentrated water has always been a difficult and hot research point. However, using nanofiltration membrane technology to treat seawater, the concentrated water produced is used as a magnesium source, and there are few related studies on the removal of nitrogen and phosphorus in the anaerobic fermentation broth to produce struvite precipitation; Quantity plays a very critical role, and there are fewer related reports, and the development of nanofiltration membrane technology is constantly updated. The present invention uses nanofiltration membrane technology to process seawater. In order to obtain a high content of magnesium source, a nanofiltration membrane product with high removal rate of Mg ²⁺ ions, relatively low removal rate of Ca ²⁺ , and high recovery rate of produced water is selected.

Contents of the invention

The present invention relates to a method for reducing the cost of magnesium source required for preparing struvite, which can use magnesium ions in seawater as magnesium source nearby, and provides a method for preparing magnesium source; A certain proportion forms struvite precipitation, which is utilized as a resource. Use cheap and easy-to-obtain seawater through pretreatment and three-stage nanofiltration system. The nanofiltration membrane adopts low-pressure nanofiltration membrane, the model is DF8040-R30/365, the water production rate is 37.5m ³ /d, and the magnesium sulfate desalination rate is ≥97%. The desalination rate of calcium chloride is 30% to 50%, the desalination rate of sodium chloride is 30% to 50%, the recovery rate of product water is 75% to 90%, and the prepared concentrated water is used as a magnesium source. Then add alkali precipitation system to reduce the influence of Ca ²⁺ on struvite precipitation. The precipitated supernatant, lye, and anaerobic fermentation liquid are added to the struvite preparation system according to the optimal reaction conditions to obtain the struvite precipitate.

Technical scheme of the present invention is as follows:

A method of utilizing seawater as a magnesium source to treat anaerobic fermentation broth to prepare struvite, at least consisting of the following processes:

1) Remove colloids and particles in seawater;

2) Seawater passes through a three-stage nanofiltration system, and the concentrated water produced is used as a source of magnesium;

3) The concentrated water magnesium source produced after the nanofiltration treatment enters the alkali-adding precipitation system to remove part of Ca ²⁺ ;

4) Add the supernatant, anaerobic fermentation liquid and lye of the precipitation system to the struvite preparation system to produce struvite precipitation.

Said step 1) After the seawater is treated by the multi-media filter and the security filter of the pretreatment system, the seawater is pumped into the nanofiltration system by a high-pressure pump.

The step 2) three-stage nanofiltration system, the concentrated water of the first stage is used as the water inlet of the second stage, and the concentrated water of the second stage is used as the water inlet of the third stage, and the number of membrane elements of the pressure vessel in the latter stage is more than that of the upper stage The number of membrane elements in one section is small.

In step 2), the pressure vessel arrangement ratio of the three-stage nanofiltration system is preferably 4:2:1.

Preferably, the filter medium of the media filter is quartz sand and activated carbon, and the method of pressure filtration is adopted.

Preferably, the pore size of the filter element of the security filter is 5 μm, so that the effluent sedimentation index SDI<5.

Nanofiltration membrane DF8040-R30/365 is preferred, MgSO ₄ desalination rate ≥ 97%, CaCl ₂ desalination rate 30% ~ 50%, NaCl desalination rate 30% ~ 50%, product water recovery rate 75% ~ 90%, concentrated water Fresh water is returned to the sea as a source of magnesium.

The preferred addition and subtraction precipitation system is to add lye sodium hydroxide to react with Mg ^{2 +} and Ca ²⁺ ions in the concentrated water generated by the three-stage nanofiltration system to remove part of the calcium ions and reduce the impact on struvite precipitation.

The preferred struvite preparation system is by adding anaerobic fermentation broth, magnesium source and lye sodium hydroxide for pH adjustment, according to pH 8-11, Mg/N 1.0-2.0:1, P/N=0.7- 1.3. The stirring time is 2min to 5min under the reaction conditions, and the struvite is prepared.

The supernatant of the struvite preparation system is recovered and used as an additional carbon source for the sewage treatment plant or other applications.

The specific instructions are as follows:

Seawater is rich in magnesium ions, the average value is 1280mg/L, and the content of calcium ions in the sea is about 400-500mg/L. First, the colloid and particulate matter in the seawater are removed through the multimedia filter 1 and the security filter 2, and then the seawater is pumped into the three-stage nanofiltration system 4 through the high-pressure pump 3, and the three-stage nanofiltration system 4 uses the nanofiltration membrane DF8040-R30/ 365, the fresh water returns to the sea, and the content of magnesium ions in the concentrated water is about 12416mg/L, and the content of calcium ions is about 1200mg/L; adding sodium hydroxide solution to the alkali precipitation system 6, to a certain extent removes part of the calcium ions, A magnesium ion solution with a final content of about 10 g/L was obtained. Anaerobic fermentation liquid, magnesium source and lye are added to the struvite preparation system 5, according to the reaction of pH 8-11, Mg/N 1.0-2.0:1, P/N=0.7-1.3, stirring time 2min-5min Conditions are carried out, and struvite precipitation is finally obtained, the removal rate of ammonia nitrogen is 75% to 90%, and the removal rate of phosphorus is 70% to 88%. The purity of struvite will vary according to different properties of the anaerobic fermentation liquid, generally greater than 70%. The resulting supernatant may be used as a carbon source for reuse in sewage treatment plants, or for other applications according to actual conditions.

The beneficial effect of the present invention is that the magnesium source is prepared by using cheap seawater, especially in offshore areas, by adopting a three-stage nanofiltration system. From the perspective of long-term development, the cost of magnesium source added to the preparation of struvite is greatly reduced. ; By adopting the method of struvite to remove nitrogen and phosphorus, it can not only remove nitrogen and phosphorus nutrients, reduce the harm to the environment, but also realize resource utilization, and use it as a slow-release fertilizer, which can increase the potato yield by 40%. After removing nitrogen and phosphorus, the anaerobic fermentation liquid of excess sludge can also be reused as a carbon source in sewage treatment plants, reducing the impact of nitrogen and phosphorus on the system and reducing treatment costs.

Description of drawings

Figure 1: A process schematic diagram of a method of using seawater as a magnesium source to treat anaerobic fermentation broth to prepare struvite.

1. Multi-media filter, 2. Security filter, 3. High-pressure pump, 4. Three-stage nanofiltration system, 5. Struvite preparation system, 6. Alkali-added precipitation system.

Detailed ways

This method is suitable for the recovery of nitrogen and phosphorus from the waste liquid with relatively high nitrogen and phosphorus content in the anaerobic fermentation liquid, and the method of struvite precipitation is used to recover nitrogen and phosphorus, especially for the preparation of magnesium sources required in offshore areas by using seawater. The preparation method adopts three-stage nano filter system for preparation. The obtained magnesium source has high content and high purity, which reduces the cost of purchasing the magnesium source.

Below in conjunction with accompanying drawing 1 and embodiment the present invention is further described:

As shown in Figure 1, a method of using seawater as a magnesium source to treat anaerobic fermentation broth to prepare struvite includes pretreatment, nanofiltration system, alkali addition precipitation system and struvite preparation system.

The pretreatment includes a multimedia filter 1 and a security filter 2 . The filter medium of multimedia filter 1 is quartz sand and activated carbon, and pressure filtration is adopted; the pore diameter of filter element of security filter 2 is 5 μm, so that the effluent sedimentation index SDI<5.

The nanofiltration system includes a high-pressure pump 3 and a three-stage nanofiltration system 4 . The pressure vessel arrangement ratio of the three-stage nanofiltration system 4 is 4:2:1; the nanofiltration membrane is DF8040-R30/365, the desalination rate of MgSO ₄ is ≥97%, the desalination rate of CaCl ₂ is 30%-50%, and the desalination rate of NaCl is 30% % to 50%, and the recovery rate of product water is 75% to 90%.

The multimedia filter 1 goes in and out from the top, and the outgoing pipeline is connected to the upper part of the security filter 2, and the pipeline from the bottom of the security filter 2 is pumped to the three-stage nanofiltration system 4 through the high-pressure pump 3, and the concentrated water It flows into the alkali-adding precipitation system 6 through the pipeline, and the fresh water is collected by the water production pipe and returned to the sea. The precipitated struvite is discharged as a slow-release fertilizer, and the supernatant can be reused as a carbon source or used in other applications.

A method of utilizing seawater as a magnesium source to treat anaerobic fermentation broth to prepare struvite, at least consisting of the following processes:

5) Remove colloids and particles in seawater;

6) Seawater passes through a three-stage nanofiltration system, and the concentrated water produced is used as a source of magnesium;

7) The concentrated water magnesium source produced after the nanofiltration treatment enters the alkali-adding precipitation system to remove part of Ca ²⁺ ;

8) Add the supernatant, anaerobic fermentation liquid and lye of the precipitation system to the struvite preparation system to generate struvite precipitation.

Including pretreatment system, nanofiltration system, alkali addition precipitation system 6 and struvite preparation system 5, seawater is pumped into nanofiltration by high pressure pump 3 after being treated by pretreatment system multimedia filter 1 and security filter 2 system, the nanofiltration system adopts a three-stage nanofiltration system 4, and the concentrated water produced is treated with an alkali-added precipitation system 6, and then added to a struvite preparation system 5 to form struvite precipitation.

The method for removing colloids and particles in seawater is characterized in that seawater passes through a multimedia filter 1 and a security filter 2 to remove colloids and particle pollutants in the incoming water. The filter medium of the multimedia filter 1 is quartz sand and activated carbon, and pressure filtration is adopted; the pore diameter of the filter element of the security filter 2 is 5 μm, so that the effluent sedimentation index SDI<5.

The three-stage nanofiltration system 4 is characterized in that the concentrated water of the first stage is used as the feed water of the second stage, and the concentrated water of the second stage is used as the feed water of the third stage. In order to ensure the normal feed water flow rate, the membrane The fouling speed is guaranteed to ensure the service life of the membrane. The number of membrane elements in the latter stage is less than that in the upper stage. The typical arrangement ratio of pressure vessels is 4:2:1. Select nanofiltration membrane DF8040-R30/365, MgSO ₄ desalination rate ≥ 97%, CaCl ₂ desalination rate 30% ~ 50%, NaCl desalination rate 30% ~ 50%, product water recovery rate 75% ~ 90%, concentrated water as Magnesium source, fresh water returns to sea.

The addition and subtraction precipitation system 6 is characterized in that by adding lye sodium hydroxide and the Mg ²⁺ and Ca ²⁺ ions in the concentrated water produced by the three-stage nanofiltration system 4 to react, remove a part of calcium ions, and reduce the impact on birds. Effects of bezoar sedimentation.

The struvite preparation system 5 is characterized in that by adding anaerobic fermentation liquid, magnesium source and lye sodium hydroxide for pH adjustment, the pH is 8-11, Mg/N is 1.0-2.0:1, The reaction conditions are P/N=0.7-1.3, and the stirring time is 2min-5min, and the struvite is prepared and utilized.

Example 1:

When the struvite preparation system is carried out according to the reaction conditions of pH 8, Mg/N 1.9:1, P/N=0.7, and stirring time 2 minutes:

Take the excess sludge anaerobic fermentation liquid of a sewage treatment plant that processes 50,000 tons of sewage a day as an example. The excess sludge produced every day is about 300 tons, of which 10 tons of excess sludge are subjected to anaerobic fermentation. The ammonia nitrogen content is 150mg/L, and the phosphorus content is 160mg/L. As shown in Figure 1, first remove the colloids and particles through the multi-media filter 1 and the security filter 2, and then use the high-pressure pump 3 to pump seawater into the three-stage nanofiltration system 4 to process seawater to prepare magnesium source, MgSO ₄ desalination rate ≥ 97 %, CaCl ₂ desalination rate is 30% to 50%, NaCl desalination rate is 30% to 50%, product water recovery rate is 75% to 90%, concentrated water is used as magnesium source, and fresh water is returned to the sea. The content of magnesium ions in the concentrated water produced is about 12416 mg/L, and the content of calcium ions is about 1200 mg/L; sodium hydroxide solution is added to the alkali-adding precipitation system 6 to remove part of the calcium ions to a certain extent, and the final content is about 10 g/L solution of magnesium ions. In the struvite preparation system 5, it is carried out according to the reaction conditions of pH 8, Mg/N 1.9:1, P/N=0.7, and stirring time 2 minutes. It is necessary to add about 4000g of magnesium source and use about 0.45 tons of seawater, that is, 10 tons. The anaerobic fermentation liquid needs about 0.45 tons of seawater to prepare magnesium source. The struvite produced is detected by XRD instrument, and the pattern matches the standard pattern of struvite completely, indicating that the main component is struvite, which is about 10kg after weighing , the proportion of phosphorus in struvite is calculated according to 13%, and the removal rate of phosphorus is 82%.

Example 2:

When the struvite preparation system is carried out according to the reaction conditions of pH 10, Mg/N 1:1, P/N=1.15, and stirring time 3 minutes:

Take the excess sludge anaerobic fermentation liquid of a sewage treatment plant that processes 50,000 tons of sewage a day as an example. The excess sludge produced every day is about 300 tons, of which 10 tons of excess sludge are subjected to anaerobic fermentation. The ammonia nitrogen content is 150mg/L, and the phosphorus content is 160mg/L. As shown in Figure 1, first remove the colloids and particles through the multi-media filter 1 and the security filter 2, and then use the high-pressure pump 3 to pump seawater into the three-stage nanofiltration system 4 to process seawater to prepare magnesium source, MgSO ₄ desalination rate ≥ 97 %, CaCl ₂ desalination rate is 30% to 50%, NaCl desalination rate is 30% to 50%, product water recovery rate is 75% to 90%, concentrated water is used as magnesium source, and fresh water is returned to the sea. The content of magnesium ions in the concentrated water produced is about 12416 mg/L, and the content of calcium ions is about 1200 mg/L; sodium hydroxide solution is added to the alkali-adding precipitation system 6 to remove part of the calcium ions to a certain extent, and the final content is about 10 g/L solution of magnesium ions. In the struvite preparation system 5, it is carried out according to the reaction conditions of pH 10, Mg/N 1:1, P/N=1.15, and stirring time of 3 minutes. It is necessary to add about 4200g of magnesium source and use about 0.48 tons of seawater, which is 10 tons. The anaerobic fermentation broth requires about 0.48 tons of seawater to prepare magnesium source. The struvite produced is detected by XRD instrument, and the pattern matches the standard pattern of struvite completely, indicating that the main component is struvite, which is about 10.7 after weighing. kg, the proportion of phosphorus in struvite is calculated as 13%, and the removal rate of phosphorus is 84%.

Example 3:

When the struvite preparation system is carried out according to the reaction conditions of pH 11, Mg/N 2:1, P/N=1.3, and stirring time 5 minutes:

Take the excess sludge anaerobic fermentation liquid of a sewage treatment plant that processes 50,000 tons of sewage a day as an example. The excess sludge produced every day is about 300 tons, of which 10 tons of excess sludge are subjected to anaerobic fermentation. The ammonia nitrogen content is 150mg/L, and the phosphorus content is 160mg/L. As shown in Figure 1, first remove the colloids and particles through the multi-media filter 1 and the security filter 2, and then use the high-pressure pump 3 to pump seawater into the three-stage nanofiltration system 4 to process seawater to prepare magnesium source, MgSO ₄ desalination rate ≥ 97 %, CaCl ₂ desalination rate is 30% to 50%, NaCl desalination rate is 30% to 50%, product water recovery rate is 75% to 90%, concentrated water is used as magnesium source, and fresh water is returned to the sea. The content of magnesium ions in the concentrated water produced is about 12416 mg/L, and the content of calcium ions is about 1200 mg/L; sodium hydroxide solution is added to the alkali-adding precipitation system 6 to remove part of the calcium ions to a certain extent, and the final content is about 10 g/L solution of magnesium ions. In the struvite preparation system 5, it is carried out according to the reaction conditions of pH 11, Mg/N 2:1, P/N=1.3, and stirring time 5 minutes. It is necessary to add about 4100g of magnesium source and use about 0.47 tons of seawater, which is 10 tons. The anaerobic fermentation broth needs about 0.47 tons of seawater to prepare the magnesium source. The struvite produced is detected by XRD instrument, and the pattern matches the standard pattern of struvite completely, indicating that the main component is struvite, which is about 10.3 after weighing. kg, the proportion of phosphorus in struvite is calculated as 13%, and the removal rate of phosphorus is 83%.

Claims (10)

1. a kind of method for using sea water as magnesium source processing anaerobic fermented liquid and preparing guanite, is at least made of following process：

1) colloid and particulate matter in seawater are removed；

2) seawater is by three sections of nanofiltration systems, and the concentrated water of generation is as magnesium source；

3) the concentrated water magnesium source generated after nanofiltration is handled into alkaline chemical precipitation system, removes part Ca²⁺；

4) supernatant of settling system and anaerobic fermented liquid and lye are added in into guanite preparation system, generates guanite and sink It forms sediment.

2. the method as described in claim 1, it is characterized in that the step 1) seawater passes through pretreatment system more medium filter After cartridge filter processing, seawater is pumped into nanofiltration system by high-pressure pump.

3. the method as described in claim 1, it is characterized in that three sections of nanofiltration systems of the step 2), the concentrated water of first segment is as Two sections of water inlet, the water inlet of the concentrated water of second segment as third section, the membrane component quantity of back segment pressure vessel is than the preceding paragraph Membrane component quantity is few.

4. method described in claim 1, it is characterized in that the pressure vessel arrangement of three sections of nanofiltration systems of the step 2) is than being 4: 2:1。

5. the method described in claim 2 it is characterized in that the filter medium of medium filter uses quartz sand and activated carbon, uses The mode of pressure filtration.

6. the method described in claim 2 it is characterized in that the filter core aperture of the cartridge filter is 5 μm, refers to water outlet siltation Number SDI<5.

7. the method described in claim 3, it is characterized in that selecting NF membrane DF8040-R30/365, MgSO₄Salt rejection rate >=97%, CaCl₂Salt rejection rate 30%~50%, NaCl salt rejection rates 30%~50%, the production water rate of recovery 75%~90%, concentrated water as magnesium source, Fresh water returns to sea.

8. the method described in claim 2, it is characterized in that plus-minus settling system is to be received by adding in lye sodium hydroxide with three sections Mg in the concentrated water that filter system generates²⁺、Ca²⁺Ion is reacted, and removes a part of calcium ion, reduces what guanite was precipitated It influences.

9. the method described in claim 2, it is characterized in that guanite preparation system be by add in anaerobic fermented liquid and magnesium source with And the lye sodium hydroxide of pH is adjusted, according to pH be 8~11, Mg/N is 1.0~2.0：1st, when P/N=0.7~1.3, stirring Between 2min~5min reaction condition carry out, be prepared into guanite.

10. the method as described in claim 1, it is characterized in that being used as sewage disposal after the supernatant recycling of guanite preparation system The additional carbon or other application of factory.