Stormwater
There are a variety of sources of stormwater which Ravensdown is required to manage at the Dunedin Works: hillside runoff, road runoff (SH88, Ravensbourne Road), and rainfall on site. Stormwater is collected using a system of sumps, drains and open channels, and then managed in four different ways depending upon location:
[1] Via Outfall One in the Acid Plant, discharging into the Otago Harbour (with pH monitoring and automated caustic dosing as and when necessary).
[2] Via an open drain (known as “Channel B”) along the back of the storage sheds, past B Despatch and into the loader wash. Alternatively, it is possible to open a bypass valve at Channel B so that the water bypasses collection in the loader wash and flows directly to join the Moller Park drain. The loader wash area also directly receives some roof water and yard runoff from the B Despatch and Storage sheds area. From the loader wash it is pumped directly into the stormwater ponds, OR in an occasional pump failure / high flow / heavy rainfall scenario it will overflow from the loader wash and pass through an enclosed drain to join the Moller Park drain which discharges to the Otago Harbour.
[3] Via three collection sumps along the harbourside length of the site from where it is gravity-fed into an underground surge chamber. From there a pumping station transfers accumulated stormwater to the greenwater ponds (a.k.a. the stormwater holding basin). Any overflow from the greenwater ponds will occur via a v-notch weir (or optionally via a drain valve) at the north end of the seaside pond and joins an enclosed drain which feeds into the Moller Park drain which discharges to the Otago Harbour. In an extreme rainfall event it is possible that the stormwater pumping station may be unable to keep up with the volume of collected stormwater and there may be a discharge directly to the Otago Harbour via Outfall 3. This overflow will generally be after the ‘first flush’ has been diverted into the integrated stormwater collection system (ISCS). Note: the previously active Outfall 4 and Outfall 6 are now both decommissioned, i.e. there is no discharge possible via either of these outfalls.
[4] Via accumulation within bunds on site, most notably the 500T tanks bund and the acid circulation area in the Acid Plant and the 2000T tank bund near Manufacture. This stormwater can be acidic in nature (low pH) due to residual acidity from trace amounts of sulphuric acid within the bund. This bund water is manually pumped from the bund for disposal. Bund water from the 2000T tank bund near Manufacture is pumped to nearby drainage and hence to the loaderwash, i.e. the latter part of [2] above. When the Acid Plant is operational, excess bund water from the 500T tanks bund and acid circulation area that cannot be reused within the Acid Plant operation is discharged via Outfall One, i.e. [1] above. When the Acid Plant is not operational, or when there is insufficient cooling water volume, excess bund water from the 500T tanks bund is pumped to the ISCS, i.e. [3] above. Both Outfall 1 and the ISCS have automated caustic dosing systems in place to neutralize acidity.