Abstract
The increasing concern over environmental pollution caused by excessive nutrient (phosphorus and nitrogen) discharge from livestock and poultry farms has led to a growing interest in the development of efficient and sustainable nutrient recovery technologies. This thesis focuses on the development and implementation of an efficient and cost-effective electrochemical process for the recovery of phosphate (PO43-) and ammonium ions (NH3–N) from anaerobic digested poultry litter as struvite (MgNH4PO4‧ 6H2O) - a slow-release fertilizer with market value.In Chapter 3, preliminary testing was conducted to study the behavior of an electrolytic reactor during struvite precipitation. A double chamber electrolytic reactor with a magnesium anode was investigated to remove NH4+ and PO43- from synthetic wastewater by producing struvite. The effects of mixing speed, pH, and applied current on struvite yield, NH4+, and PO43- removal efficiencies were first evaluated using a factorial design. Then, the two most significant parameters were further optimized using Central Composite Design coupled with Response Surface Methodology. A 5.7-fold increase in struvite yield was achieved by increasing the applied current from 0.1 to 0.5 A. The three regression equations generated by the CCD/RSM design with applied current and mixing speed as the two independent parameters were highly correlated with the response variables (struvite yield, NH4+ and PO43- removal efficiencies). The desirability analysis showed the best operating condition: current, 0.5 A and mixing speed, 414 rpm, for the reactor system, under which the optimal struvite yield and NH4+ and PO43- removal efficiencies were 4.75 g/L, 93.0%, and 58.4%, respectively.
In chapter 4, due to the shortcomings and high construction cost of the double chamber electrolytic reactor, an innovative air-lift electrolytic reactor (ALER) with a magnesium anode was developed for PO43- and NH3–N recovery from anaerobically digested poultry wastewater through struvite precipitation. Under constant current (12.4 A/m3) and superficial velocity (107.42 m/h), the maximum removal efficiencies of PO43- and NH3–N were 99.9% and 97.3%, respectively. Kinetic modeling revealed that PO43- and NH3–N removal from the liquid phase by the ALER followed a pseudo-first-order reaction model, with rate constants (k) of 0.0188 min-1 and 0.0143 min-1, respectively. The recovered precipitate was characterized with FTIR, XRD, and SEM-EDS which also identified that precipitates as high-purity (88.5%) struvite with a mean particle size of 142.95 μm. Mean particle size and size distribution were found to be linearly increased with superficial upflow velocity. The electrical energy consumption per order (EEO) of the BCER was 0.824 kWh/m3, and the space-time yield is achieved at 0.988 kg/m3 h with a corresponding operating cost of 3.87 $/m3.
The limited mineralization of phosphorus to phosphate during the anaerobic digestion process poses a significant challenge in the development of a cost-effective and environmentally beneficial method for nutrient recycling from anaerobically digested poultry wastewater. Thus, a study (Chapter 5) was conducted to investigate the influence of organic acids on phosphorus solubilization from ADPW, followed by its recovery as struvite using a single chamber bubble column reactor, without the need for chemical additives. The impact of seeding on the efficiency of PO43- and NH3-N recovery, as well as size distribution of recovered precipitates from the acid pre-treated ADPW was also evaluated. The tests conducted using oxalic acid achieved complete solubilization of phosphorus, reaching 100% extraction efficiency from the ADPW wastewater at pH 2.5. The maximum removal efficiency of phosphate and ammonia-nitrogen from the ADPW were 88.87% and 90.10%, respectively. The addition of struvite seed to the reactor increased PO43- removal efficiency by 9.6% and 11.52% with 5 and 10 g/L of seed, respectively. The value of the kinetic rate constant, k, increased from 0.0176 min-1 (unseeded) to 0.0198 min-1, 0.0307 min-1 and 0.0375 min-1 as the seed loading rate increased from 2, 5, and 10 g/L, respectively. Concurrently, the average particle size also rose from 75.28 μm (unseeded) to 82.12 μm (9.08%), 125.72 μm (67.01%), and 148.95 μm (97.86 %), with the addition of 2 g/L, 5 g/L, and 10 g/L of struvite seed, respectively. Results from XRD, EDS and dissolved chemical analysis revealed that the product obtained from the recovery process was a multi-nutrient fertilizer consisting of 94.7% struvite with negligible levels of heavy metals.In chapter 6, significant contributions were made towards the development of model-based optimization strategies for controlling the particle size distribution (PSD) of struvite precipitates during nutrient recovery in electrochemical reactors. For the first time, kinetic parameters describing the mechanisms (nucleation, crystal growth rate, and agglomeration) involved in the formation of struvite from ADPW were estimated simultaneously from experimental data.
Overall, this research contributes to the development of sustainable solutions for nutrient recovery from anaerobic digested manure. The utilization of electrolytic reactors offers a promising approach for the efficient, and environmentally friendly retrieval of nitrogen and phosphorus, supporting the transition towards a circular economy and resource-efficient wastewater management practices.