Pilot operation of desalination plant

14 December 2010

Councilors and Senior Officials tasting the first desalinated water

Recent rains have brought some relief to the town of Plettenberg Bay, however the water situation remains critical ahead of the imminent December holiday period and the expected tourist and non-resident home-owner influx.

As the construction of the emergency desalination plant adjacent the Beacon Isle hotel in Plettenberg Bay draws to a conclusion, the Project Engineers are hopeful that the plant will be ready for a test commencing on the 3^rd December 2010 and, if all runs to plan, the plant will start producing water by the 7^th December 2010. The abstraction points will remain accessible during the commissioning test period.Construction will be discontinued over the December holiday period to all Robberg Beach and the Piesang Estuary to be fully accessible and enjoyed by tourists and residents. The Finishing-off of the structure will be undertaken in the new year.

It was originally envisaged that water would be abstracted from beach wells located on Robberg Beach. A test well inserted at the commencement of the construction phase in September 2010 however revealed that not only was the beach sand too shallow for vertical wells but the native sand was too fine and impervious to allow horizontal wells and achieve the required 10ℓ/s/well supply rate. Whilst the horizontal beach wells could have been expanded and redesigned to overcome these challenges, the costs and time losses proved to be prohibitive factors.

 

In seeking a feasible alternative, the Engineer’s considered expanding the horizontal beach wells on Robberg Beach, moving the wells to Central beach or to the sand bar at the mouth of the Piesang River, or opting for direct abstraction from the sea. The thick sand deposits at the mouth of the Piesang Estuary adjacent Central Beach were quickly identified as the most suitable alternative. Initial investigations revealed that there was adequate depth of sand to allow for the sinking of vertical abstraction wells and the coarser sand deposits allowed the required abstraction rates to be achieved whilst delivering a good quality filtered water supply. The project immediately redirected its direction to this new abstraction option.

Seven abstraction wells were sunk approximately 8.5 m into the mouth of the Piesang River and tested for water productivity and water quality over a two month period. This testing phase revealed that the three boreholes located closest to the estuary were abstracting water with a lower salinity than the other four, raising concern that estuary water was being abstracted more so than seawater. It was also observed that all boreholes showed increasing water salinity with increasing depth. As a result of these findings, the three low salinity containing boreholes were moved closer to the remaining four and all of the boreholes were redeployed to the maximum depth of 12 m. It is hoped that this new abstraction well configuration will intersect the seawater intrusion and have little affect on the water levels and water quality of the estuary. This is dependent largely however on the structure and morphology of the basement rock of the estuary mouth.

The move from Robberg Beach into the Piesang River gave rise to concerns around the possible environmental impact that water abstractions from this location could have on the estuary by both the public and authorities. To address public concern a fortnightly meetings have been held throughout the construction phase with public representatives from the Plettenberg Bay Community, the Environmental Forum, and the Plettenberg Bay Rates Payers Association, to discuss project progress and environmental issues associated with the construction and operational phases of the project.

The public and authorities concerns were provisionally supported by the findings of project marine ecologist, Dr. Barry Clark, who undertook a preliminary assessment of the Piesang Estuary and an evaluation of the potential impact that abstraction may have on the system. Dr. Clark found that the Piesang Estuary was in a surprisingly good condition, contrary to belief that the Estuary was in a poor state due to urban discharges and leachate run-off from the landfill, and reported that this was a conservation worthy and valuable system to both Plettenberg Bay and all South Africa.

 

In response to the growing environmental concern, the Bitou Municipality arranged a specialist workshop which was held on the 3^rd November 2010. The workshop delegates are all recognised experts in the fields of marine and estuarine ecology, groundwater, hydrology, water and marine engineering. The specialists reviewed and discussed the information and findings to date and applied their own experience to the situation. After some debate and deliberation, the conclusion was that there was evidence to support three possibilities with regard to the origin of abstracted water; these are that the abstraction well field has a good interconnection to the ocean and / or a good interconnection to the estuary. If a strong connection between the abstraction well field and the ocean can be demonstrated, the current location and configuration is likely to be vindicated an acceptable long term solution. The specialists noted that the Estuary is a very dynamic hydrological system. However, no certainties or conclusions could be drawn on the basis of the information and observations made to date about the potential environmental impact that ongoing abstractions may have on the estuary.

The delegates agreed that the best solution was to run the plant for a three or four month pilot phase. This would allow the Bitou Municipality to provide adequate water over the peak demand season, allow plant performance to be tested, and the response of the estuary to be carefully monitored under actual operational conditions. The information collected during this pilot phase will then be used to inform a decision as to whether the abstractions from the Piesang River will be an acceptable long term solution or whether an alternative abstraction option should to be pursued.

 

The monitoring programme will involve daily measurements of salinity and water levels in two up-steam locations in the Piesang Estuary. The Department of Water Affairs have assisted in this monitoring programme by establishing a continuous water level logger at the Otto du Plessis Bridge. Ten monitoring wells have also been established in a concentric circle around the abstraction well field and water levels and salinity will be monitored daily over the pilot operations phase.

Whilst the specialists generally agreed that a three month pilot phase should not cause irreversible damages to the estuary the adoption of a precautionary approach was advised. Further to this the specialists agreed on a set of thresholds or “red flag” scenarios that, if transgressed during the pilot operations phase, should be reassessed and may require additional mitigation measures or discontinuation of abstractions to prevent irreversible damages to the Piesang Estuary. These thresholds are:

• A drop in estuary water level by 200 mm below the lowest recorded level measured thus far;
• If the estuary mouth remains closed for a period of more than 3 months;
• If salinity rises above that of seawater (35 ppt or 55 ms/cm) in the water at the bottom of the estuary at Otto du Plessis Bridge.

Dr. Clark and his diving team have also undertaken a biological survey of the ocean floor and reef systems in the vicinity of the planned brine discharge outlet. Follow-up surveys will be undertaken during the operational phase to reveal the extent of environmental impacts on the local marine ecosystem around Beacon Isle and Robberg Beach. A continuous water quality logger will also be deployed between the discharge point and the Blinders reef to monitor salinity fluctuations and other water quality parameters.

The information derived from the pilot phase operations monitoring will be used to guide and inform decisions about the future configuration and operation of the plant, will serve to improve the understanding the functioning of the Piesang Estuary system and the potential environmental impacts associated with the desalination plant.

The Bitou Municipality appeals to all residents and visitors to be mindful of the water situation over the coming months and be wise with water usage as the emergency water situation isn’t behind us yet.

Patrick Killick (Aurecon)

LMR NGOQO

MUNICIPAL MANAGER

MUNICIPAL NOTICE NO: 207/2010