Influencing Factors On Struvite Precipitation And Its Fertilizer Effect

The Study of struvite recovery from wastewater has got a lot of significant progress, butthe quantitative analysis of the products is still not solved effectively. The impacts of K⁺andCa²⁺on struvite crystallization have not been studied systematically. Furthermore, struvite isproposed as a potential slow-release fertilizer, but there is no corresponding theoretical study,which is based on the migration, transformation and dissolution rate of struvite. This papercontains the above-mentioned legacy issues, focusing on experimental work, and obtains thefollowing results.1. The impacts of K⁺, Ca²⁺on struvite crystallization.The experiments show that:1)At low pH （≤7.5）, the presence of K⁺（195mg/L）, willimprove the thermodynamic driving force of the reaction, thus speeding up the formation ofstruvite, and if the amount of K⁺is too large（273mg/L）, the acceleration effect disappears.2)At high pH （≥7.5）, the acceleration effect of K⁺on struvite crystallization disappears, butthe formation of MgKPO4·6H2O is promoted.3)At low pH （≤7.5）, if [Ca²⁺]≤30mg/L orn（Ca）:n（Mg）≤1:8, there is no obvious negative effects on the formation of struvite.Otherwise, Ca²⁺will not only reduce the product of struvite, but also inhibit the removal ofphosphorus in the system.4) The presence of K⁺does not affect the formation of calciumphosphate precipitation.5) In the system of [K⁺]=195mg/L, pH <9.0, the presence of Ca²⁺has no significant impact on the formation of MgKPO₄·6H₂O. pH>9.0, if [Ca²⁺]≤30mg/L orn（Ca）:n（Mg）≤1:8, there is a little positive effects on the formation of MgKPO4·6H2O,Otherwise, Ca²⁺will inhibit the formation of MgKPO₄·6H₂O.2. The supplementary research on electrochemical deposition method which canaccelerate struvite crystallization.The electrochemical deposition can speed up the formation of struvite at low pH, butthere are still some problems, such as low phosphorus removal rate（≤30%）, warmingsignificantly. The thesis focuses on the above two problems to try to improve theelectrochemical deposition device. The results showed that:1) The electrochemical depositionunit which is added a small volume of alkali adding and mechanical mixing zone is a feasibleand ideal device.2) The system added2.93g/L NaCl need a40.7%off voltage, nearly doesnot heating up, and has no negative effect on struvite crystallization.3. This paper systematically studied the dissolution rate of struvite as a fertilizer, themigration and transformation of struvite in soil and the corresponding plant cultivation.The experiments show that:1)Struvite has a good slow-release property. The maximumphosphorus concentration of leaching solution （12.3mg P/L） is got after5days’ leaching.And a higher release level （8.0010.00mg P/L） can be maintained for about33days. Theleachate phosphorus concentration is still maintained at2.004.00mg P/L in the laterleaching days.2) The migration of struvite is significantly lower than the migration ofordinary phosphate fertilizer.3) The Ca2-P content of the soil which is applied struvite and leached54days, increases several times, and considerable amount of Olsen-P （125mg P/kg）can meet the demand of plant growth for a long term.4) The weak acid water can promote thedissolution of struvite.