Case Study SA-SC
ENERGY RECOVERY DEVICES IN DESALINATION PLANTS
Energy Recovery Devices (ERDs) for Energy recovery at Desalination plants:
VWS South Africa Plants
Desalination plants are infamous for their high electrical consumptions. In South
Africa over 75% of electricity is generated from fossil fuels. The growing need to
lower carbon emission has led to a desire to lower electricity consumption in all
sectors of South Africa. The use of ERDs has shown to reduce the energy
consumption of desalination plants by 20-40%.
Description of Process:
A desalination plant takes sea or brackish water and pumps it through RO membranes at a very
high pressure in order to purify the water. About 50% of a desalination plan’s operation costs are
due to electrical consumption and between 65-75% of that electricity is consumed by the high
pressure pumps.
The streams exiting the membrane are, however, still at a very high pressure. The pressure needs
to be lowered before the water can be further treated or distributed.
All of the ERDs discussed in this case study uses the pressure in the exit streams to recover
energy and lower the stream’s pressure at the same time. The ERDs are mounted just after the RO
membrane trains. The ERDs are relatively small and do not require large changes to the plant
layout.
Two ERDs are mainly used namely, pressure exchangers and hydraulic turbines. Seen in the
photo above (left) is a series of ERI PX-220 pressure exchangers. Pressure exchangers are
usually installed as show in the picture (right).
The ERDs can be incorporated with an existing system, as seen below.
Hydraulic turbines or Pelton wheels are usually installed as show in the picture below.
Membrane
RO1:
RO2:
RO3:
Plettenberg
Bay
Knysna
Amatola
Water
Without
ERD
3.2 kWh/m3
4.1 kWh/m3
4.2 kWh/m3
With ERD
1.45 kWh/m3
2.8 kWh/m3
2.4 kWh/m3
% Energy
Recovery
47.7%
30.3%
43.3%
Without
ERD
3.2 kWh/m3
4.1 kWh/m3
4.2 kWh/m3
With ERD
1.55 kWh/m3
2.8 kWh/m3
2.4 kWh/m3
% Energy
Recovery
48.7%
30.6%
43.3%
Without
ERD
3.2 kWh/m3
-
4.4 kWh/m3
With ERD
1.6 kWh/m3
-
3.1 kWh/m3
% Energy
Recovery
50.0%
-
29.4%
Mossel Bay
RO1 to RO6
uses the
same ERD.
The devices
save up to
30% of the
total plant’s
power
consumption.
The plant
uses 60
MWh/d with
the ERDs.
The energy efficiency of the desalination plant can also be lowered by combining the ERD
technology with renewable energy sources such as wind farms.
Case Study SA-SC
ENERGY RECOVERY DEVICES IN DESALINATION PLANTS
Ref
Response information, description and remarks
Case Study SA-SC
1
Location:
All the plants are located in South Africa. The Mossel Bay,
Plettenberg Bay, Sedgefield and Knysna plants are in the Western
Cape province, on the southern coast of South Africa. The Amatola
Water Board in East London is in the Eastern Cape province, on the
east coast of South Africa.
2
Sector:
1.
2.
3.
4.
5.
Mossel Bay: Clean water (seawater desalination)
Plettenberg Bay: Clean water (brackish and seawater desalination)
Sedgefield: Clean water (brackish and sea water desalination)
Knysna: Clean water (brackish and seawater desalination)
East London: Clean water (brackish water desalination)
3
Works Owner or Operator:
1.
Mossel Bay: The plant is owned by the Municipality of Mossel Bay
and operated by VWS South Africa. The construction of the plant was
financed by the Municipality of Mossel Bay, PetroSA and the National
Treasury of South Africa.
Plettenberg Bay: The plant is owned by the Bitou Municipality and
operated by Veolia Water.
Sedgefield: The plant is owned and operated by the Knysna
Municipality. Grahamtek is responsible for the maintenance of the
plant.
Knysna: The plant is owned by the Knysna Municipality and is
operated by Veolia Water.
East London: The plant is owned by the Amatola Water Board and is
operated by VWS South Africa.
2.
3.
4.
5.
4
Size:
5
Energy Provider:
1.
2.
3.
4.
5.
Mossel Bay: Design flow: 15ML/day
Plettenberg Bay: Design flow: 2.0ML/day
Sedgefield: Design flow: 1.5ML/day
Knysna: Design flow: 2.0ML/day
East London: Design flow: 1.8ML/day
All the plants are provided with electrical power by the national
electricity agency, ESKOM.
6
Process:
1.
2.
3.
4.
5.
7
Component:
1.
2.
3.
4.
5.
Mossel Bay: The plant uses six filters for pre-treatment. The filters
are multimedia filters using sand and pomice. Chlorine is also added
during the pre-treatment, but is removed before entering the RO
membranes. There are six RO units (42% recovery) using 20
membranes each. The final product treatment is done using soda
ash, sodium chlorite and ferrous chlorite dosage.
Plettenberg Bay: The plant uses dual media filters for the pretreatment. Reverse osmosis is achieved by three Desalator® units
(42% recovery). The final product water post-treatment is done
using limestone, soda ash and sodium hypochlorite (NaOCl) dosage.
Sedgefield: The plant uses dual media filters and no chemicals for
the pre-treatment of the water. Desalination with reverse osmosis is
achieved using two RO units (44% recovery) with 3 membranes per
unit. The product water is treated with sodium hypochlorite before
distribution.
Knysna: The pre-treatment is done using dual media filtration.
Reverse Osmosis is achieved using three Desalator ® units (42%
recovery). The product water is post-treated using soda ash and
chlorine dosage.
East London: Bag filtration is used for pre-treating the water.
Reverse Osmosis is achieved using three trains (50% recovery). The
product water is treated using hypochlorite (NaOCl) dosage.
Mossel Bay: Energy is recovered using Hydraulic turbines to transfer
the concentrate pressure to the feed water (boosts feed pressure).
Plettenberg Bay: The plant uses one ERI PX-220 Pressure Exchanger
per train (RO1, RO2 & RO3) to recover energy.
Sedgefield: The plant uses one ERI PX-220 Pressure Exchanger per
train (RO1 & RO2) to recover energy.
Knysna: Energy is recovered using a Pelton Turbine per train (RO1,
RO2 & RO3) to transform the concentrate mechanical energy into
electricity to assist the feed pumps.
East London: The plant uses a PX-180 Pressure Exchanger (ERI) on
two of the trains (RO1 & RO2) and an HPBe-30 Turbine (Fedco) on
the third train (RO3) to recover energy.
8
Motivation for the case study:
ESKOM announced an annual increase in electricity tariffs of 25.34%
starting on the 1st of July 2011. The desire to lower carbon emissions
also led to a desire to lower electricity consumption in order to
comply with international standards.
9
Process/Plant Changes:
The only major changes to the process are the addition of ERDs
(pressure exchangers and/or hydraulic turbines) to the flow system
of the plant.
10
Civil/Physical Changes:
The only civil change needed is a platform to mount the ERDs on.
Such a platform might already exist on some plants as most of the
water flow systems are on the ground or a large floor/platform.
No civil changes are needed for the PX ERDs; they can be directly
mounted to the existing pipe system.
11
Operational Changes:
12
Risks and Dependencies:
13
Implementation:
After the ERDs have been installed the additional operational skills
needed is very low and the current employees will only need to
attend in-house training by the technology suppliers. The
maintenance of the devices will probably be outsourced and
therefore no additional skills development will be required. If the
maintenance, however, is not outsourced, then the employees
responsible for maintenance will need to attend relevant skills
development courses and advanced maintenance programs.





1.
2.
3.
4.
5.
14
Energy Efficiency gains:
15
Cost/Benefit analysis:
Failure of alarm systems
Mechanical failure
Operational inadequacies
Bacterial growth on membrane
Pre-treatment failure
Mossel Bay: The plant was commissioned in September 2011. The
plant was designed by Veolia Water who also outsourced the
construction of the plant. The O&M of the plant is also done by VWS
South Africa for the first three years of operation.
Plettenberg Bay: The plant was commissioned in December 2010.
The plant was designed by Veolia Water who also outsourced the
construction of the plant. The contract with VWS South Africa
includes the O&M of the plant for 3 years (up to January 2014).
Sedgefield: The plant was commissioned in April 2010. The plant was
designed and constructed by Grahamtek Company. The O&M of the
plant is also contracted to Grahamtek.
Knysna: The plant was commissioned in December 2010. The plant
was designed by VWS South Africa who also outsourced the
construction of the plant. The contract with Veolia Water includes
the O&M of the plant for 3 years (up to January 2014).
East London: The plant was commissioned in August 2010. An
existing plant was refurbished by VWS South Africa who also
outsourced any construction done on the plant. The contract with
VWS South Africa includes the O&M of the plant for 13 months (up
to the end of September 2011).
The energy savings ranged from 47.7 % - 50% for the Pressure
Exchanger used at Plettenberg Bay, 30.3% - 30.6% for the Pelton
Turbine used at Knysna, and 29.4% - 43.3% for the East London plant
using a Pressure Exchanger ERD. At Mossel Bay, around 30% energy
savings were measured (the plant uses 60 MWh/d with the eRDs).






Unit tariffs: 2010/11=83.74c/kWh 2011/12=105.35c/kWh
Pelton turbine price
Hydraulic turbines (Mossel Bay)
ERI PX-220
ERI PX-180
FEDCO HPBe-30 Turbine (East London)
16
Project review:
The project demonstrated that significant savings in power
consumption can be achieved in RO plants by including energy
recovery devices in the plant design. The energy savings efficiencies
of the different systems were compared. Unfortunately South Africa
is yet to incorporate this technology.
The projects by VWS South Africa show the need to continually
improve energy recovery.
17
Confidence grade:
Medium confidence grade.
The confidence in methodology and data acquisition is high, but due
to insufficient data (the plants were all relatively new at the time of
reporting the data) this reduces the confidence grade to medium.