Biosolids project

The picture gallery on this page shows the steps in processing sludge to biosolids, and the application of biosolids to land. See the explanatory text below the clickable thumbnail pictures.

Sludge lagoons

Following anaerobic digestion, digested sludge is discharged to dewatering lagoons. There are a total of five. lagoons (some of which are operated in pairs) covering an area of about 7.2 hectares. Sludge is allowed to settle and supernatant flows back through the treatment system. It takes about 5 years to fill a lagoon, after which it is dewatered/drained for a further five years or so until it is solid enough for excavation (Solids content about 18 to 20 %), and is then considered to be Biosolids.

Windrowing of biosolids

On excavation from the Lagoon, biosolids are spread on a paddock for further drying for up to two years. During this time they are they are turned two or three times, formed into windrows, and dry to about 40% to 60% solids.

Land application

Biosolids were spread on the trial Sandy Point area in March 2010, using a silage spreader. The application rate was limited to 300Kg nitrogen per hectare, which equated to 20mm biosolids at 50% average total solids.  The trial site is 7.4 hecatares.

The application area is intended for planting in native shrubs/trees and developed as a passive recreational area, which will include walking, cycling and horseriding tracks etc.

Groundwater flowing through the site is being monitored to assess the level of contamination from the biosolids. Provided levels of contamination are within acceptable limits, resource consents will be sought for the application of biosolids to other parts of the Sandy Point reserve.