Delivering Value for Money Water Quality Improvements – Evaluating the Retrofit of Proprietary Treatment Devices

Long term environmental monitoring and reporting has indicated that water quality in Omaru Creek, a small heavily urbanised stream system draining to Tāmaki Estuary, Auckland, is impaired. The high level of impervious surfaces and historic infrastructure development practices in the catchment has led to changes to the natural flow regime and increased pollution sourced from stormwater runoff. These contaminants are derived from land use types that are typically regarded as being high contaminant load generating activities. Large-scale regeneration that has already begun across the catchment – driven by over 11,000 new residential homes - offers a once in a life time opportunity to deliver substantial stormwater related benefits to the local community by upgrading the existing stormwater network. Those benefits aim to enhance the existing environment by not only mitigating for the intensification in development, but also by addressing existing stormwater problems. Benefits include improving water quality by upgrading existing infrastructure and incorporating modern water treatment technology into the existing stormwater network.

The main town centre of Glen Innes has been identified as an area of particular concern within the catchment. It is an area that contains a high level of imperviousness, has high vehicle traffic use and supports commercial and light industry business types. The combination of these conditions is generally indicative of an area with high contaminant load generating potential. Available land in the Glen Innes town centre is a significant constraint to infrastructure renewals and retrofitting treatment ‘at source’, or large space-hungry communal water quality improvement options. ‘At source’ retention/detention areas, bioretention devices and in-pipe storage tanks or filtration devices have limited potential within the town centre. Devices that require minimal space requirements including start of pipe inlet screens and catchpit inserts, in pipe vortex separators or screens and end of pipe trash racks, baskets or netting devices would be more suited.

This paper focuses on the evaluation and assessment of retrofitting options for a range of different water quality improvement devices within the Glen Innes Town Centre aimed at improving water quality in Omaru Creek. The target contaminant types for this work included the removal of gross pollutants and coarse sediments to complement already existing communal treatment devices located lower down the catchment aimed at treating nutrients, metals and other finer contaminant loadings. The paper outlines the development of an approach to proactively determine the contributing drainage subcatchments, calculate the contaminant loading from each subcatchment, and estimate removal rates for a range of retrofit options to the existing stormwater network informed by literature review.

This paper also outlines the development of a methodology to objectively assess and evaluate the suitability of a range of water quality treatment device types as retrofit opportunities at a subcatchment level, potentially relevant to national discussions on proprietary device evaluation. In collaboration with suppliers, a series of different proprietary devices have been evaluated based on their relative pros and cons and treatment performance. Preferred options were recommended based on a cost benefit assessment for the selected devices.