The Study Of Recovering Phosphorus In The Form Of Struvite From Dewatering Sludge Filter

The technique of recycling phosphorus in the form of struvite from sludge dewatering filtrate is one of the best ways to recycle phosphorus, because it not only can separate phosphorus from wastewater, but also can use struvite for agriculture and industry. Because of those reasons, the technique of recycling phosphorus in the form of struvite from sludge dewatering filtrate is becoming the hot point researched into by many domestic and foreign scientists. Taking synthetical and real sludge dewatering filtrate as subject,and taking stirrer with six oars as sequencing stirring reactor, the goal of this study is to find out the effects from the water quality of synthetical and real sludge dewatering filtrate, the operation conditions of sequencing stirring reactor and crystal catalyst on producing struvite.The results got from reaserching on synthetical sludge dewatering filtrate are as follows: the recycle rate of PO₄^3- increase with initial PO₄^3-, Mg²⁺ and NH⁴⁺ concentrations rising. The recycle rate of PO₄^3- has line relationship with the natural logarithm of initial PO₄^3-, Mg²⁺ or NH⁴⁺ concentration. The initial Mg²⁺ concentration is a limiting factor to PO₄^3- recycle, so the initial concentration of Mg²⁺ should be not less than that of PO₄^3-. The optimal pH values is 10.24 under the reaction condition. When the initial concentration ratio of NH⁴⁺ and PO₄^3- is in 2.0².2, the purity of struvite crystallization would be up to 95.0%.When stirring speed changed to 350rpm from 50rpm, the recycle concentrations stayed still macroscopically. Longer reaction time is able to make struvite crystals bigger, so enough long reaction time should be necessary for struvite.After researching on the effect of the water quality of real sludge dewatering filtrate, the operation conditions of sequencing stirring reactor and crystal catalyst on recycling struvite under the condition of the same PO₄^3-,Mg²⁺ and NH⁴⁺ initial concentrations, the results are as follows: The recycle rate of PO₄^3- can be more than 90.0% only when the same PO₄^3-, Mg²⁺ and NH⁴⁺ initial concentrations is over 5.00mmol/L. Under the condition of the same PO₄^3-, Mg²⁺ and NH⁴⁺ initial concentrations in the real sludge dewatering filtrate, the best pH value of struvite crystallization is about 10. Mg²⁺,NH⁴⁺ and PO₄^3- with the same concentration are not able to produce precipitation including more than 90% struvite, unless the initial Mg²⁺ and PO₄^3- concentration are over 5.00mmol/L, and the initial NH⁴⁺ concentration are over 12.00 mmol/L. Ca²⁺ can not only react with PO₄^3-, but also react with OH-, so Ca²⁺ can restrain struvite crystallization. In real sludge dewatering filtrate, stirring speed have few effect on recycling Mg²⁺,NH⁴⁺ and PO₄^3-. With several diatomite quantity, the recycle concentrations of Mg²⁺,NH⁴⁺ and PO₄^3- are not diffrente. Differente struvite quatitifies do not affect Mg²⁺,NH⁴⁺ and PO₄^3- recycling concentration, but struvite crystal catalyst material make the new crystals smaller.The results of producing struvite in real sludge dewatering filtrate is that pH of sludge dewatering filtrate rise after stirring without any reagent addition; however, it is not enough to form struvite; When the initial concentration of Mg²⁺ and PO₄^3- increases in the same ratio, the recycle concentrations of Mg²⁺,NH⁴⁺ and PO₄^3- go up too; Mg²⁺ of 5.00mmol/L not only can make the recycle rate be more than 90%, but also can stop Ca²⁺ reacting with PO₄^3-. When pH values ranged from 10.0 to 10.5, the highest recycle concentration of NH⁴⁺,Mg²⁺ and PO₄^3- can be obtained; The more lower turbidity of sludge dewatering fitrate, The more better the effect of recycling struvite; When revolution speeds ranged from 50rpm to 250 rpm, the recycle concentrations of Mg²⁺, NH⁴⁺and PO₄^3- changed slightly, but the recycle concentrations of NH⁴⁺ are higher than that of Mg²⁺ and PO₄^3-; The suitable reaction time was 40⁸0min;Turbidity and remained PO₄^3- concentration decreased gradually with precipitation time prolonged, and precipitation time should not be shorter than 20min; Differente struvite quantity do not affect Mg²⁺,NH⁴⁺ and PO₄^3- recycle concentration, but struvite crystal catalyst make the new crystals smaller.