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Water quality improvement of piggery effluent

Wrigley, Timothy John (1999) Water quality improvement of piggery effluent. PhD thesis, Murdoch University.

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An experimental wetland system was constructed to upgrade the water quality of piggery wastewater. Transplanted aquatic macrophytes were successfully established within the wetland. The wetland experiment was concluded prematurely because of continual pipework blockages which prevented wastewater flow into the wetland.

The precipitate in the pipework was identified as Struvite, MgNH4PO4.6H2O, a precipitate widely associated with wastewaters and kidney stones.

The formation of struvite in the piggery wastewaters indicated that struvite may be an important process in the removal of nutrients.

X-ray diffraction of lagoon sludge confirmed that struvite precipitation occurred in the three wastewater lagoons at the piggery.

Magnesium, essential for the formation of struvite, entered the waste water system via groundwater used for wash down. Magnesium was also an important food supplement in the feed provided for the pigs. Magnesium coupled with ammonia and phosphorus at pH values greater than nine (9) can produce struvite. Up to 250 tonnes/annum of struvite could be removed from the final piggery effluent under correct pH and dosing conditions. Struvite is a valuable slow release fertilizer, and its formation and removal from wastewaters may be important in the reduction of eutrophication in receiving waters.

Laboratory-scale batch and continuous anaerobic digestion of piggery effluent was undertaken. The removal of struvite from solution by chemical dosing with MgSO4 after digestion was determined.

During anaerobic digestion of piggery wastewater, COD was reduced by 50%- 90%, as measured by biogas production. Concentrations of PO4-P in the supemate increased during batch digestion (21 to 33 mg/L) while a marked reduction in PO4-P concentration of the supemate from the continuous digestion (21 to 10 mg/L) occurred. The addition of MgSO4 to the supernate reduced PO4-P concentrations from 33 to 7 mg/L (batch) and from 10 to 0.1 mg/L (continuous). Struvite, aphthitalite (K3Na(SO4)2) and thermardite (Na2SO4) were precipitated from solution.

A computer model was developed to describe struvite solubility chemistry; this included the electro-neutrality equation which allowed for greater variability in the input components. Relationships between the major ions were retained without approximation. The model results fitted data from both our laboratory results and published values. Equilibrium constants which markedly affect the output were identified and solubility constants derived.

The computer model of the solution chemistry of struvite was then improved. Firstly, the algorithm was smaller and faster with ammonia as the prime calculation point in the liquid phase. Secondly, the incorporation of dissolved magnesium hydrogen phosphate (dMHP) in the model significantly increased concentrations in solution. Thirdly, estimates of activity coefficients were included.

These improvements made a 5-10% improvement in the fit. This flexible modeling procedure allowed for the ready inclusion of all possible species. The addition of associated ammonium phosphates improved the fit. Estimates of the association constants were included. Applications of the model include waste-water treatment and the formation of kidney stones.

Item Type: Thesis (PhD)
Murdoch Affiliation: Division of Science and Engineering
Notes: Note to the author: If you would like to make your thesis openly available on Murdoch University Library's Research Repository, please contact: Thank you.
Supervisor(s): McComb, Arthur
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