Degree Granting Department
Civil and Environmental Engineering
Sarina J. Ergas
anaerobic digestion, concentrated animal feeding operations (CAFO), ion exchange, life cycle assessment, struvite
Swine production represents approximately 40% of the world's meat production, and swine wastes contain high concentrations of organic matter, nitrogen (N) and phosphorus (P). Swine production is intensifying as meat demand increases and concentrated animal feeding operations (CAFOs) are becoming increasingly common, making it difficult to treat the waste generated. A system for holistic treatment of swine waste produced in CAFOs was investigated in this study that sustainably generates energy and recovers N and P as saleable fertilizers. The system uses anaerobic digestion (AD) for methane production and solids stabilization, followed by precipitation of struvite (MgNH4PO4*6H2O) and recovery of N by ion exchange onto natural zeolites. This process is expected to mitigate both eutrophication of receiving waters and greenhouse-gas emissions while generating products that meet agronomic nutrient demands; however, the economic and environmental sustainability remains unknown. The objectives of this study were to: (1) evaluate water quality and the fate of nutrients and ions in each step in the proposed system through pilot and bench scale experiments, (2) evaluate content/quality of struvite precipitates formed in wastewater treatment processes, (3) assess basic composition of zeolite materials that are being considered for use as IX materials, (4) quantify the environmental impact of the proposed system, and (5) estimate the economic benefits and costs of the proposed system.
The results of a bench scale evaluation of the system show that although water quality greatly improves throughout the treatment process, the effluent water quality has high concentrations of COD (2,803 mg O2/L) and E. coli (106.3 CFU/100ml). This limits reuse options for the reclaimed water, however a variety of on-farm applications may be suitable.
During struvite precipitation, the recovery efficiency of SRP was 87% (60 mg/L recovered); however, although measurements that take into account P in suspended solids show a lower recovery efficiency, they also show higher mass recovery (77% efficiency, 66 mg/L recovered). N recovery during struvite precipitation showed a similar trend, with 49% of TN and 7% of NH4-N being recovered. Struvite recovery can only occur from NH4-N and soluble reactive P. The additional recovery observed is likely due to adsorption of the nutrients onto the precipitate. Therefore, to accurately measure and report recovery, measurements of N and P that take into account suspended solids should be used. In most wastes, magnesium is the limiting constituent for struvite formation, but for swine AD effluents, P is the limiting constituent. Therefore, a higher soluble P concentration would increase recovery potential. The majority of the remaining N and P as well as a significant amount of potassium (K) were recovered during IX.
Six struvites from commercial processes as well as our bench-scale experiments were assessed and compared by X-ray diffraction, SEM imaging, and SEM-EDX scans. All samples were confirmed as struvite by XRD, however they varied widely in crystal size and shape. The elemental composition of the samples was similar; however, struvite formed from phosphate mining waste had higher amounts Mg and P, indicating more pure struvite formation. The presence of impurities in some samples was likely due to the reactor design and solids separation methods.
XRD was also used to confirm the identity of zeolites. Three clinoptilolites had similar crystal size and elemental composition except for Zeosand [reg] which showed a surface roughness, which likely contributes to higher cation exchange capacity. Chabazite has smaller crystal size and larger pores than clinoptilolite, which also likely contributes to its higher capacity.
Life cycle assessment (LCA) was used to evaluate the environmental sustainability of the system and the results suggested that environmental benefits were provided across almost all impact categories. Two alternatives for raising the pH in struvite precipitation (NaOH addition vs. aeration) and two alternatives for zeolite IX materials (chabazite vs. clinoptilolite) were assessed, but there were negligible differences between alternatives. The system was also assessed at a medium and large scale, and the large scale was more environmentally friendly across all categories. Operational impacts were significantly greater than construction impacts; therefore, the environmental impact of the system can be accurately assessed by only including operation.
A life cycle cost assessment (LCCA) was also performed on the system and showed a payback period of 39 years for a medium sized system and 15 years for a large size. This, however, is when compared to a "business-as-usual" scenario and does not consider renewable energy credits or government grants. Furthermore, although a larger system is more economically beneficial, this must be balanced with quality of animal care. From a cost standpoint, IX recovery using chabazite is not recommended and struvite precipitation using aeration is more economically beneficial than NaOH addition.
Scholar Commons Citation
Amini, Adib, "Sustainable Energy and Nutrient Recovery from Swine Waste" (2014). Graduate Theses and Dissertations.