Case Study SA-OM
SEQUENCING BATCH REACTOR WASTEWATER TREATMENT
The Gansbaai Wastewater Works
Sequencing Batch Reactor Technology:
Nereda Wastewater Treatment System
Nereda is an advanced biological wastewater treatment technology which uses aerobic granular
biomass for treatment of wastewater. The technology was introduced into South Africa by a local
engineering and project management group Royal Haskoning DHV (formerly SSI).
Background:
Royal Haskoning DHV (RHDHV) claim that the Nereda process can be used to treat both domestic and
industrial wastewaters to produce final discharge water that conforms to effluent standards as set down by
the Department of Water Affairs. The technology is based on aerobic sludge granules which not only settle
at a much higher rate than activated sludge found in conventional biological nutrient removal (BNR) plants,
but also allows for a much higher microbial density than that found in conventional activated sludge plants.
The increased efficiency of the process therefore allows for very compact treatment works.
It is also claimed that as an added advantage, one of the features of the large granules used in the system is
that anaerobic conditions exist inside the granules, allowing simultaneous phosphate removal and
nitrification, while chemical oxygen demand removal can be achieved through aerobic treatment.
Furthermore, the various treatment processes occur in a single reactor, greatly simplifying construction of
the plant and reducing the overall footprint of the works. In turn, the simplified construction and reduced
number of unit processes results in significant cost savings, around 25 to 35% lower than those for
conventional plants according to RHDHV.
The energy requirements of these works are also lower than for conventional BNR plants, savings as high as
35% being possible compared to conventional wastewater works. In addition, adequate phosphate removal
is generally possible without the addition of chemicals, further reducing operational, consumable and
energy costs. The Nereda system is also highly automated, allowing operation of the plant with only limited
operator input.
The very first full scale domestic wastewater treatment plant to employ the Nereda technology was
commissioned by the then SSI in Gansbaai in the Western Cape of South Africa. This plant was constructed
on the site of the existing Gansbaai Wastewater Works and took about a year to construct and has a design
capacity of 4.5 ML/d. It consists of three sequencing batch reactors (SBR) operating simultaneously as
opposed to a conventional system in which there is generally only one reactor with a constant flow. Based
on the success of the Gansbaai plant, a second 5 ML/d Nereda plant has been constructed at Wemmershoek
WWW in Stellenbosch, also in the Western Cape.
Description of Process:
The process consists of aerobic sludge granules developed in sequencing batch reactors, without carrier
materials. Short feeding periods are selected to create feast and famine periods based on the presence
(feast) or absence (famine) of organic matter in the sewage and in this way the appropriate microorganisms needed to form granules are achieved.
When organic contaminant concentrations in the sewage are high, the granule-forming organisms can
store the organic matter in the form of poly-β-hydroxybutyrate to be consumed in the “famine period”,
providing an advantage over filamentous organisms.
RHDHV further claims that the Nereda system provides flexibility, being able to deal with fluctuating
conditions and thereby capable of withstanding shock or toxic loadings. In addition, the aerobic conditions
on the surface of the granules, combined with the anaerobic conditions that exist within the granules, allows
for COD reduction, phosphate reduction and nitrification to occur simultaneously.
Potential Interventions

Careful control of the plant is required in order to achieve the treatment objectives. This is achieved
through the use of PLC and the only labour required is for checking of the head of works, control of
chlorine stocks and ground keeping.
Range of potential Savings
The Nereda technology allows energy savings as high as 35% compared to conventional BNR plants, while
construction costs are around 20 to 40% lower than that for conventional works. In addition, chemical
addition for phosphate removal is not required.
Specific energy usage (kWh/(pe.year)
45
40
y = 81351x-0,7736
R2 = 0,9142
35
30
25
20
15
10
5
0
0
10.000
20.000
30.000
40.000
50.000
60.000
70.000
80.000
90.000
Pollution load (p.e. à 136 gTOD)
Figure 1: Energy Consumption during the verification on the Nereda Process at Gansbaai.
Case Study SA-OM
SEQUENCING BATCH REACTOR WASTEWATER TREATMENT
Case Study SA-OM
Ref
Response nformation, description and remarks
1
Location:
Gansbaai, Overstand Municipality in the Western Cape, South
Africa. Urban with a population density of 650/km2.
Wemmershoek, Stellenbosch Municipality, Western Cape, South
Africa. Rural to urban with a population density of 1.2/km2.
2
Sector:
Wastewater
3
Works Owner or Operator:
The works are owned by the respective municipalities:
Gansbaai – Overstrand Municipality
Wemmershoek – Stellenbosch Municipality
4
Size:
Gansbaai WWW – 4.5 ML/d
Wemmershoek WWW – 5 ML/d
5
Energy Provider:
The municipalities purchase bulk electricity from Eskom, the national
electricity agency. Electricity tariffs are expected to treble electricity
costs over the next 7-10 years and there has already been a 25%
increase over the past 3 years. The increased demand for electricity
countrywide and the resultant power outages that occur during periods of
high demand is also of concern.
6
Process:
The process involves aerobic granular activated sludge in sequential
batch reactors.
7
Component:
The process includes the entire wastewater treatment process.
8
Motivation for the case study:
The option of a Nereda system at Gansbaai wasnot motivated by energy
savings, but rather by spatial limitations, The site was only big enough to
accommodate a conventional wastewater works with a capacity of 2
ML/d.
The Nereda process was chosen at Wemmershoek because it was the
most cost effective in terms of construction and also provided the best
option in terms of ease of operation, due to the high level of automation.
However in both cases the energy savings gained in operational energy
costs were seen as an added motivation.
9
Process/Plant changes:
Both plants are Greenfield sites, so in both cases the entire plants were
constructed as new.
10
Civil/Physical Changes:
The Nereda system has allowed for a much smaller footprint compared
to that of conventional treatment plants. The number of unit processes
is reduced and so the carbon footprint of the works is also much
smaller.
The effluent quality can be manipulated by adjusting plant operation.
For example good phosphate and ammonia removal can be achieved
through correct control of the plant. Nitrate removal (denitrification) can
be achieved as well, but careful plant tuning is then required.
11
Operational Changes:
The entire plant is controlled by PLC and the only labour required is
for checking the head of works, ensuring that chlorine stocks are
adequate and for ground keeping.
12
Risksand Dependencies:
The risks posed by the Nereda system are claimed to be low, certainly
lower than conventional wastewater treatment plants since the number
of unit processes is reduced and there are fewer mechanical and
electrical devices. The Nereda plants have only feed pumps and
blowers, so the operating equipment is greatly reduced compared to
conventional works. In addition, PLC monitoring makes operation more
controlled and the alarm systems that form part of the PLC ensure that
any problems are quickly identified.
13
Implementation:
The plant is constructed under normal contract procedures, but with a
proprietary PLC programme. The plants have been constructed by
outside contractors with RHDHV (SSI) supervision and contract
management. RHDHVownthe rights to the PLC programme.
14
Energy Efficiency gains:
Energy savings are usually around 25% relative to that of conventional
wastewater treatment plants, but can be as high as 35%. If one uses
typical energy consumption figures for activated sludge of around
0.35kWh/m3 (EPRI 2002) then the energy saving is around 0.088 to
0.11kWh/m3.
15
Cost/Benefit analysis:
The payback period will be 35% shorter than for construction of
conventional wastewater treatment works, since construction costs are
around 35% less. Once the energy savings are added in as well as the
fact that chemical addition is no longer required, the payback time is
reduced still further.
16
Project review:
The technology is fairly new, only around 5 years old, so has been
developed using the latest technology available.
17
Confidence grade:
High. The confidence level is high, since some systems have already
been in use for some 4 years already and have operated successfully.
18
Reference/Information source:
Paul Gaydon, RHDHV, Pietermaritzburg.
References:
EPRI Report, “Water and Sustainability: US Electricity Commission for Water Supply and Treatment – the Next
Half Century”, Technical report by EPRI Project Managers R Goldstein and W Smith, March 2002.