M Lequerica, R McInnes
Publication Date (Web): 29 March 2016
DOI: https://doi.org/10.21139/wej.2016.017

Bacterial contamination, dissolved oxygen depletion and eutrophication are the most common negative environmental impacts of untreated sewage discharges on rivers and freshwater streams.

The intensification of animal and crop production systems in the last 50 years has increased the amount of phosphorus and nitrogen runoff into freshwater systems, augmenting these previously mentioned environmental impacts. Eutrophication is a particularly difficult problem to manage because of its multiple consequences. Some of the most commonly reported concerns are: growth of algae and aquatic weeds that interfere with water for fisheries, recreation, industry, agriculture and drinking, oxygen shortages caused by senescence and decomposition of the algae, and biodiversity loss.

Nitrogen and phosphorus can be removed from wastewater by means of chemical, physical or biological treatments, but treatment plants with modern nutrient removal technologies are expensive. Several studies in the past have analysed the effects of sewage treatment plants’ (STPs) technology upgrades and their efficiency in removing nutrients that cause eutrophication, finding significant economic benefits from reducing eutrophication. These findings are relevant not only for researchers, but also for environmental authorities, economic policy-makers and governments worldwide. It is important, then, to find the optimum point where cost efficiency meets with efficient nutrient removal.

Significant investment has been made in upgrading municipal STPs in New South Wales (NSW). The aims of this study are to evaluate if: a) STP upgrades have significant effects on nutrient concentrations in effluent; b) a relationship between the size of the STP and the cost of pollutant abatement exists; and c) the upgrades made have significant effects over operations, management and administrative (OMA) costs in the different STPs.

Out of 10 water quality monitoring of local councils’ STPs datasets, submitted by the participating municipal councils, four constituting a representative sample of these STPs were analysed, comparing nitrogen and phosphorus concentrations before and after. The upgrades had no significant effect on nitrogen concentration.

Phosphorus concentration was significantly reduced in three of the four plants after the upgrade. A cost curve was constructed comparing OMA costs with plant capacity equivalent population (EP). Scale economy was found, as OMA cost tends to reduce for every additional unit of plant capacity.

Submitted datasets were found to be incomplete in most cases. Even though a successful monitoring program depends on good quality datasets, these were of poor quality, with no consistency in monitoring time periodicity, lack of monitoring sites’ spatial descriptions, and no weather variability annotations.

These problems may account for much of the variation seen in the statistical analysis performed in this study, suggesting that although some changes might be achieved with the implementation of new technology, they may be hidden behind a layer of ‘noise’, and so for practical purposes, remain unseen.

A standardised monitoring methodology would guarantee that all STPs follow the same quality guidelines, provide researchers with standardised data to perform studies and accurately cross sectional issues.

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