Understanding the impacts of supersaturation and seed crystal loading on struvite precipitation kinetics and collection efficiency

To valorize nutrient rich streams at wastewater treatment plants, installations of full-scale phosphorus removal with struvite precipitation processes is increasing. Utilities are increasingly understanding the necessity for higher levels of nutrient removal due to the regulatory pressures and the up and coming stringent discharge limits. To subsidize some of the operational costs and reap environmental and economic benefits while installing advanced biological nutrient removal processes, utilities are also migrating to nutrient recovery processes focused especially on struvite crystallization and recovery. Extractive nutrient recovery processes are usually coupled with enhanced biological phosphorus removal processes to reduce the scaling potential of resulting phosphorus rich streams. The increased availability of phosphorus in sludge handling liquors during anaerobic conditions, along with the presence of magnesium and ammonium ions, can lead to unwanted precipitation of struvite at even slightly basic pH conditions. To get rid of unwanted encrustation due to struvite, crystallization techniques to recover struvite from such streams require extensive capital investments. Utilities are employing these side stream phosphorus recovery processes without a clear understanding of the advantages and drawbacks of different reactor designs and process configurations. Although all full-scale processes can achieve a basic level of phosphorus removal (>80%), the supplementary benefits to utilities in terms of product quality, production rate, phosphorus handling capacity, operational costs, etc. can significantly vary. In this study, segregation between secondary crystal growth and fines generation during struvite crystallization was explored. The effects of initial supersaturation and seed loading concentrations on phosphorus removal kinetics and struvite solids distribution were studied using batch experiments. The kinetics of struvite precipitation with high seed loadings was also investigated and compared using literature data and modeling techniques. Our results show that phosphate removal was dependent on initial supersaturation and not the mass of seed crystals in the reaction vessel. Struvite fines represented the majority of struvite solids that were formed in all but 2 experiments. Assuming that is true for full-scale processes as well, capital and operational costs, and the associated risks must be accounted for due to the formation, capture and loss of struvite fines before re-introduction into the biological process. Experimental results also show that the percentage removal of total phosphate associated with struvite fines and the kinetic rate of phosphate removal are both dependent on initial supersaturation and seed loading concentrations. The rate of struvite mineral formation must be able to capture the available surface area for crystal growth to take place. Current modeling techniques need to segregate fines and seed growth kinetics to better reflect the reality of full-scale processes which work with larger size struvite crystals and much larger seed loading concentrations