Biosolids

In producing a higher standard of effluent from the wastewater treatment process, Watercare is removing three times the amount of solid material from the waste stream.

While biosolids are currently being disposed of in on-site landfills, their long-term disposal and sustainable reuse is a challenge, as it is for wastewater utilities internationally. Initial investigations into beneficial biosolid reuse suggested that quarry restoration, composting and forestry merited further consideration.

The Mangere Wastewater Treatment Plant has three main outputs: emissions to the air, water discharges to the Manukau Harbour, and the treated solids or biosolids.

The recent upgrade of the plant has cleaned up the discharge to the harbour but has nearly tripled the quantity of biosolids produced, from 40,000 tonnes in the mid-1990's to 108,000 tonnes a year.

Watercare is using this material to rehabilitate 35 hectares of the former oxidation pond area alongside the plant. Which could later be used for public recreation. The company is also investing in cell lysis technology at the plant, which could help extract more methane in the treatment process and produce drier, more stable biosolids better suited to land rehabilitation.

The latest projections for this site give Watercare seven years to find a new long-term use for biosolids. The company is planning trials on Watercare land adjacent to the plant to test dried biosolids suitability for turf farming, floriculture and pasture rejuvenation. The possibility of using biosolids as a forestry soil conditioner is also under investigation. The company plans to set up a trial with the Forest Research Institute to look at trees' nutrient uptake, surface and groundwater effects, transportation issues, storage and spreading.

Watercare is also participating in an industry working group looking at national environmental standards for biosolids use on land.

Watercare monitors and reports on the plants discharge to the harbour in compliance with the resource consent. Monitoring for metals in compliance with the Australian and New Zealand Environment and Conservation Councils (ANZECC) low-level values. Upgrading the plant has also reduced discharge of pollutants such as ammonia, nitrogen and suspended solids by more them 95 per cent. Stringent new consent conditions, reflecting higher expectations for the new plant, came into effect on 1 October 2003.