Ibrahimi, Farshad (2001) Lagoon effluent treatment using grass filtration bays. Research Master thesis, Victoria University of Technology.

Abstract

The Western Treatment Plant (WTP), operated by Melbourne Water, is identified as a major contributor of nutrient to Port Phillip Bay, especially in winter. To minimise eutrophication and maximise beneficial uses of the receiving waters of the Bay, the Environmental Protection Authority (EPA) of Victoria has issued a more stringent waste discharge licence for WTP. Therefore, Melbourne Water has proposed a number of strategies to improve effluent quality into the bay and meet its new waste discharge licence requirements. These strategies include provision of lagoon treatment followed by grass filtration of all dry weather flows in winter. This thesis presents the results of a project established to determine the waste treatment efficiency, especially for nitrogen removal, and an optimal hydraulic loading rate for the proposed winter grass filtration system when fed with effluent from lagoons at WTP. The project involved hydraulic data acquisition and twice-weekly monitoring of influent, effluent, and wastewater quality within the bays for seven trial Italian Ryegrass filtration bays during a 22-week winter period from May to early October 1997. Four hydraulic loading rates of 20, 30, 40, and 50 mm/day, were used in the trials. There was a significant amount of cross flow between some ofthe grass filtration bays used in the trials. Also, infiltration losses were higher than the values expected on the basis of hydraulic conductivity values from studies conducted on soils in the area of the trials. On average, about 5.8 mm/day of flow was lost through infiltration while the net effects of rainfall and evapotranspiration account for about 1.2 mm/day of losses. The high infiltration losses may be due to the root channelling and cracks in the soil. The trials showed a temporal trend in most of the monitored parameters. Ammonia, total nitrogen, total phosphorus, and BOD removals were generally higher during the first half of the trial period, while effluent colour was poorer during the first three and last five weeks of the monitoring period. The relatively higher ammonia and phosphorus removals during the first half of the trials were attributed to higher plant uptake and lower influent mass loading. Although nitrification/denitrification is identified as generally the most important nitrogen transformation and removal mechanism on the bays, its relative importance does not vary significantly with time during the entire grass filtration period. The poor colour levels are due to decaying organic matter left on the bays during present or previous grass filtration treatment. Dissolved oxygen concentration ofthe effluent also showed a temporal trend similar to that observed for ammonia, total nitrogen and phosphorus. This is due to ageing and reduced photosynthetic activity by the Italian Ryegrass, resulting in a lower rate of oxygen production on the bays during the second half of the trials. Apart from the first three weeks of the trials, most of the other effluent parameters generally remained fairly constant throughout the trials.

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