Alexandra Wastewater Sludge Dewatering Plant

Central Otago District Council (CODC) owns and operates a wastewater treatment plant at Graveyard Gully. The plant provides wastewater treatment for the domestic and commercial wastewater within Alexandra. The plant is an extended aeration oxidation ditch system provided with a mechanical aeration system. The resident population in Alexandra is approximately 5,000 people and increases to approximately 8,000 during the peak Christmas holiday period.

To minimise transport and disposal costs, historically sludge from Alexandra’s Wastewater Treatment Plant (WWTP) was dried on sand drying beds. However drying beds function poorly in cold winters and sludge dewatered in geotextile bags had created odour problems.

The sludge in general was Grade b under the New Zealand Biosolids Guidelines due to the cadmium, copper, mercury and zinc concentrations. The consented method of disposal was to the Victoria Flats landfill near Queenstown.

Sludge dewatering trials by centrifuge were undertaken in the summer and winter of 2011 to determine future targets for dry solids of the dewatered sludge, polymer usage and centrate solids content.

In October 2011 CODC advertised for a Design and Build Contract for the sludge dewatering equipment with acceptance of Contract in December 2011. The dewatering equipment had 24 week manufacture and delivery lead time to the site.

As beds become unusable throughout winter and no future use of geotextile bags was proposed a tight project timeframe of seven months had to be met to implement a centrifuge based sludge dewatering plant.

A separate Head Contract for construction of civil, mechanical installation, electrical and telemetry control work was accepted in February 2012 and the complete dewatering plant commissioned in June 2012.

This paper describes the key elements of the project and the Road Map approach to contract management interfaces for project delivery.