The proposed desalination plant for Hout Bay would be built on an open gravel area near the boat yards on Harbour Road, according to a tender document.
A raw-water abstraction station would be built near the Fish on the Rocks restaurant.
The conceptual layout, prepared by GLS Consulting as sub-consultants to WorleyParsons also shows that the brine discharge pipeline would run to the existing marine outfall for brine discharge further down from West Fort.
The City of Cape Town has identified Hout Bay as well as Granger Bay and Dido Bay as possible sites for desalination plants.
On Thursday August 17, Mayor Patricia de Lille announced several developments in the City’s water resilience plan in the wake of the crippling drought.
She said the City had started procuring and commissioning various augmentation schemes to find up to 500 million litres of new water a day.
The technologies include groundwater extraction, desalination from land-based containers, desalination from barges, marine-based desalination and water reuse.
“The urgency to get to these various technologies at the various locations is well understood. The City now has an advanced understanding of the requirements of each site, and how to connect new technologies into the existing water network,” Ms De Lille said.
Dam storage levels are at 34.2% with usable water at 24.2%.
The tender for the desalination plants closed on Wednesday this week. The contract period is two years from the date of commencement.
In its invitation to tender, the City stressed that it wanted to establish the sites as soon as possible, and it would permit 24/7 construction.
Labour would be sourced from local communities, in accordance with the City’s supply chain regulations.
According to the documents, the plant would yield four million litres of new water a day, and the containerised plant, pump station and generators would be located within the harbour complex on the open gravel parking area south of the main entrance. The proposed plant footprint is 40m x 70m.
The injection rate is 3.3 million litres a day, which will primarily be supplied into the Hout Bay reservoir zone normally supplied from the Hout Bay reservoir situated on the eastern mountain slopes of Hout Bay.
The raw-water abstraction pipe works would be located south east of the proposed plant, off the harbour breakwall. The raw-water abstraction pump unit would be located on the breakwall with the abstraction point about 75 metres offshore.
Onshore, the raw-water abstraction pipe works towards the plant would be above ground mostly, following Harbour Road along the southern sidewalk. The onshore raw-water abstraction pipe works would be held in position by sandbags along the route.
The document also states that all road crossings would be underground and access to various properties would be arranged for by means of steel/concrete ramps “where possible”.
“The potable water produced will be injected into the existing network west of the proposed site. Brine produced in the process will be discharged via a dedicated brine discharge pipe into the existing sewer reticulation network west of the site.”
Justin O’Riain, a Hout Bay resident and professor in behavioural ecology and conservation at UCT, said he was familiar with some of the potential impacts of a desalination plant on marine life.
“The most obvious threat is mortality associated with organisms being sucked against the intake filters. This threat is mostly to smaller marine organisms like small fish species and jelly fish. Microscopic animals such as zooplankton pass through the filters but are killed in the process that separates the freshwater from the seawater,” he said.
“It is possible to avoid most marine life mortality by placing the intake pipe below the substrate with the sandy seabed acting as a macro-filter. However this entails more cost and perhaps more time than the City has before Day Zero. It should also be noted that there are many other factories in Hout Bay that currently abstract seawater onto land and thus the abstraction of seawater onto land is not a new phenomenon in Hout Bay.”
The other obvious impact, he said, was the release of processed seawater back into the sea.
“This water is referred to as brine and is approximately twice as salty as the seawater that went into the desalination plant. However, the proposal in Hout Bay is to mix the brine with our waste water that is currently dispersed into the ocean beyond the Sentinel.
“A recent presentation by the City of Cape Town revealed minimal impact of waste water on marine organisms at the outfall site. There were greater impacts for animals in the harbour which had higher levels of toxic pollutants such as PCBs (industrial chemicals).”
He said adding salty water to polluted freshwater would dilute the brine and reduce the potential impact of excessive saltiness on marine organisms associated with the desalination plant.
“When one considers the enormous ecological impacts of removing too much water from rivers, damming them and polluting them with sewage, the potential impacts of desalination seem much less. Abstracting groundwater may also have far greater long-term impacts to the terrestrial ecosystems we rely on for most of our food.
“On balance, I think desalination is the lesser of many of the evils we impose on the environment in constant abuse of freshwater resources. One can only hope that the current crisis forces us to rethink our relationship with freshwater rather than simply re-engineer ourselves out of trouble and continue to abuse this finite resource.”
Ward councillor Roberto Quintas welcomed the proposal.
“Considering the global climate changes, of which we in Cape Town are experiencing the affects of first hand, we have to acknowledge that water scarcity and lower winter rainfall may not be a temporary weather pattern, but our new normal,” he said.
“As such, I am fully supportive of any measures taken to ensure ongoing water supply and am extremely pleased that a potential desalination plant in Hout Bay would provide local jobs and contribute to the water security of the City.”
Len Swimmer, chairperson of the Hout Bay Residents and Ratepayers’ Association, said a desalination plant was “long overdue”.
“We are in a dry climate. All the run-off water from Table Mountain goes into the sea, and it should have been collected years ago, but now the mayor is looking at this option,” he said.
Fish on the Rocks owner Alfred Broderick said a desalination plant was “something we all need” during the dry spells.
“It’s obviously at the proposal stage, but I am not anti it,” he said.
Having the abstraction station located near his business could even assist in bringing down crime in the area, something he and fellow restaurateurs in the harbour have battled with.
“My only concern would be whether the access roads to the harbour would be affected as pipes are laid. Access roads to the harbour are limited, and roadworks over the weekends or during the summer months when the tourists are here could be a problem when we’re busy. But for me, they sooner they start on the plant the better.”