Marine Ecology - Natural resources & the environment

Potential Impacts of a
Seawater Intake Structure
and a Brine Disposal System
for the Proposed Desalination
Plant at Mile 6, Swakopmund
Marine Ecological Aspects
Nina Steffani & Andrea Pulfrich
Environmental Services(Pty) Ltd
Marine Ecological Aspects
Key Sources for Potential Impacts on the Marine Environment:
 Construction of intake and brine disposal structures
 Permanent presence of these structures
 Discharge of high-salinity brine
Intake Structures:
Alternative 1: Direct open water intake structure with caisson at 8 or 10 m
depth (intake mid-depth) 800 m to 1500 m offshore. Connected to onshore
pump station through a pipeline that is trenched through the surf zone and
beyond the surf zone placed on the sea bottom. On the shore, the pipeline
will run underground into the pump station.
Alternative 2: Breakwaters extending 147 m out to sea to create a stilling
basin of ~2 m water depth with an open channel connecting the stilling basin
with the intake pump station.
Discharge Structure:
~ 60% of the seawater will be returned to the sea as brine via a 600 m long
pipeline. The brine will have approximately double the salinity of seawater.
Marine Ecological Aspects
Main Environmental Issues (Intake and Discharge Structures):
Construction Phase:
 High vehicular traffic on beach
 Accidental spillage or leakage of fuel, chemicals, or lubricants
 Increased turbidity and resettling of sediments during excavation
 Destruction of biota in the excavation area (inter- and subtidal)
 Disturbance to marine life from blasting
 Disturbance to seabirds
Operational Phase:
 Permanent presence of intake and discharge structures results in loss of
original substratum but provides new settlement ground (e.g. artificial reef)
 Changes in hydrographical conditions and sediment transport; changes
in wave patterns may affect beach morphology; due to sedimentation
dredging may be required in future (breakwater)
 Impingement and entrainment of marine organisms
 Avoidance behaviour of fish, marine mammals, loss of potential feeding
or breeding ground
Marine Ecological Aspects
Main Environmental Issues (Brine Discharge):
 Elevated salinities in the mixing zone and potential seafloor spreading
 Discharged effluent potentially having a higher temperature than the
receiving environment
 Residual chlorine and chlorination by-products in the effluent (if no
dechlorination step)
 Co-discharge of constituents with the brine (e.g. increased turbidity from
backwash water and discoloration near outlet from FeCl3, heavy metals
from corrosion processes, antiscalants may bind nutrients and ions
dissolved in seawater, cleaning solutions for membrane maintenance may
interfere with natural processes of dissolved seawater constituents)
 Removal of particulate matter with the feedwater from the water column
where it is a significant food source, as well as changes in phytoplankton
production due to changes in nutrients, water column structure and mixing
processes (can cause algal blooms)
 Direct changes in dissolved oxygen content due to the difference
between the ambient dissolved oxygen concentrations and those in the
discharged effluent (e.g. due to dechlorination with sodium bisulfite), and
indirect changes in dissolved oxygen content of the water column and
sediments due to changes in phytoplankton production as a result of
nutrient input
Marine Ecological Aspects
 Desktop study based on existing literature
 Field-work to establish type of community around brine outlet (thin
sediment cover over bedrock, visual inspection by diving required)
 International guidelines provide target values (e.g. ANZECC, DWAF,
World Bank) for e.g. salinity, residual chlorine
 Based on target values and guidelines assess impact of brine plume
footprint determined by modelling (CSIR)
Marine Ecological Aspects
Information Requirements:
 Exact design of both intake structure alternatives: type of seawater
intake (screen type, mesh size, intake volume, intake velocity, etc.)
 Construction activities (temporary jetty, blasting schedule, etc.)
 Exact composition of brine (concentration of residual chlorine,
concentrations of co-discharged constituents, discharge volumes, outfall