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Phosphorus removal and septic tank effluent renovation with sand amended with waste residues

Cheung, Kwai-Chung (1998) Phosphorus removal and septic tank effluent renovation with sand amended with waste residues. PhD thesis, Murdoch University.

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On-site wastewater treatment and disposal systems consisting of a septic tank and soil infiltration trenches are widely employed as an economical effluent disposal option in areas without a wastewater collection network. Infiltration systems utilising peat or sand have been reported to be inefficient in reducing phosphorus (P) concentration to acceptable levels. The present study examined a range of industrial waste residues as amenders to improve P sorption in sand. A series of experiments was undertaken to assess their comparative P sorption capacities and then to examine the performance of selected amenders for renovation of septic tank effluent.

P sorption capacities of materials including Spearwood sand, Merribrook loamy sand and several industrial waste residues (acidic precipitator and bottom fly ash. alkaline lagoon and precipitator fly ash, black oxide and neutralised red mud) were evaluated first by batch sorption isotherms and then by a repeated contacting test. Ranking of adsorbents by the two methods are inconsistent. Batch sorption isotherm is a crude way of estimating the P sorption capacity of sorbents. Ranking of adsorbents based on these results did not always reflect the expected performance of the sorbents under actual infiltration operation. On the other hand, repeated contacting with P solution gave a more realistic estimation. The results of subsequent column experiments showed a similar ranking.

Among the materials tested, alkaline precipitator fly ash exhibited the greatest sorbed P. Alkaline lagoon fly ash was found to be less effective than alkaline precipitator fly ash and neutralised red mud. Relatively high application rate of lagoon fly ash appeared necessary for sustained P removal.

Kinetic studies of the various adsorbent materials were undertaken to elucidate the time course of the adsorption process. P sorption by the adsorbents was found to occur in two distinct stages. The first stage involved relatively rapid P uptake and lasted up to several days. A slower phase of P sorption followed which in some cases continued for many months. For all materials tested, with the exception of Merribrook loamy sand the fast reaction accounted for a large proportion (>70%) of the total P uptake at different initial P concentration. P sorption in the rapid sorption stage appeared to be related to the soluble calcium (Ca) content of alkaline precipitator fly ash and red mud gypsum. The time course of P sorption could be described by the Elovich equation and by a kinetic extension of the Freundlich isotherm expression for contact times up to 16 days.

Continuous infiltration studies using laboratory columns (30 cm) were carried out to assess renovation of septic tank effluent by percolation through Merribrook loamy sand, Spearwood sand, lagoon fly ash and sand amended with 30 or 60% (w/w) lagoon fly ash or 20% (w/w) neutralised red mud. The removal of chemical oxygen demand (COD) was high in all column effluents (71 to 93%). Extent of nitrification was high in Spearwood sand, Merribrook loamy sand and 20% red mud amended Spearwood sand. however, actual removal of nitrogen (N) by nitrification and denitrification was high in columns containing lagoon fly ash.

Unamended Spearwood sand possessed only minimal capacity for P sorption. Merribrook loamy sand and red mud amended sand effected complete P removal through the study period. Significant P leakage occurred from lagoon fly ash amended sand columns following six to ten weeks of operation. Neither lagoon fly ash or red mud caused any studied heavy metal contamination including manganese (Mn), lead (Pb), zinc (Zn), cadmium (Cd) and chromium (Cr) of effluent. It can be concluded that Merribrook loamy sand is much better natural soil than Spearwood sand as a filter medium. The addition of lagoon fly ash enhanced the removal of P in Spearwood sand but the efficiency was lower than with red mud amendment.

A further set of column studies were conducted to investigate septic tank effluent renovation by sand amended with alkaline precipitator fly ash. Spearwood sand was amended with 0, 5, 15 and 30% (w/w) precipitator fly ash. Amending Spearwood sand with alkaline precipitator fly ash significantly improved the removal of N and P from infiltrating septic tank effluent. P breakthrough occurred in sand amended with 5% precipitator fly ash after week 8. P was not detected in the columns containing 15% and 30% precipitator fly ash over investigation period. N removal increased with increasing fly ash content. However significantly enhanced levels of Ca, aluminium (Al) and selenium (Se) were present in the effluent from columns with amended sand. There could be of concern with reference to water quality criteria for ground water receiving the infiltrated effluent. It was concluded that the precipitator fly ash could be a suitable amendment for coarse sand to provide better effluent quality at an application rate of around 15%. An adequate dilution factor may be needed at the receiving water body in order to avoid groundwater contamination from Ca, Al and Se.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): School of Biological and Environmental Sciences
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): Venkitachalam, Hari and Scott, William
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