Potable water production – new advances

In a second exclusive for World Water, Anthony Bennett of Specialist Technical Solutions Ltd evaluates the use of ultrafiltration and micro-filtration technologies at a number of plants worldwide.

Advances in filtration and separation technologies over recent years have resulted
in the increasing popularity of membrane systems for providing potable quality
water. In this article we review three recent case studies from Koch Membrane
Systems, Zenon Environmental and USFilter where potable water is produced by innovative
process solutions. We evaluate the use of ultra-filtration (UF) to produce high
quality potable water, and the implementation of micro-filtration (MF) technology
to handle a highly variable source water whilst meeting stringent water quality

Alameda UF Plant, USA
In July 2004, Koch Membrane Systems Inc. (KMS) started up their latest potable
water plant – a 38,000 metres3/day UF plant for the Alameda County Water District
(ACWD) in Fremont, USA. The plant is designed to treat both clarified and raw
water from the local South Bay Aqueduct.

‘Alameda County Water District is excited about the start-up of the new membrane
plant … We expect that this plant will provide a reliable supply of high
quality drinking water to our customers for many years to come,’ said Robert
Shaver, ACWD’s Engineering Manager.

KMS was awarded the contract after a 6-month pilot trial utilising raw and
clarified water from the South Bay Aqueduct. During the pilot work, ferric chloride,
powdered activated carbon (PAC) and various polymers were added to both raw
and clarified water to help determine the optimum plant design. KMS concluded
that PAC provided the most effective protection against raw water turbidity

The California DHS certified PMPWTM UF membranes were selected in part due
to their ability to achieve 4-log (99.99%) removal of Cryptosporidium, 4-log
removal of Giardia and 4-log removal of viruses, as well as to meet US Title
22 regulations regarding the treatment of surface water for potable use.

The system comprises KMS 200 mm diameter by 1.8 m long UF cartridges and consists
of six HF-52 racks containing the cartridges. These are shown in Figure 1. Each
cartridge contains over 45 metres2 of the PMPWTM membrane. The membrane is a
hollow fibre of 35 mm diameter, rated to remove particles greater than 100,000
molecular weight.

The membrane plant is designed to operate under 8 different feed water conditions.
During operation with clarified feed water, the plant is intended to operate
in single pass mode. Under raw water conditions, if turbidity spikes occur and
the addition of PAC is required, the plant will automatically switch to recirculation
mode. To minimise operator interface a high degree of system automation was
incorporated, including integrity testing as well as cleaning operations.

Chestnut Ave, Singapore
In December 2003, Zenon Environmental commissioned a 273,000 metres3/day UF
system to ensure the Singapore Public Utilities Board (PUB) could continue to
provide a high quality potable water supply to meet increasing demand at the
Chestnut Avenue Water Works. The system includes enhanced coagulation for the
removal of colour and organics and comprises ZeeWeed® 500d immersed membranes,
supplied in 16 process trains.

When selecting suppliers for this upgrade, PUB worked with consulting engineers
Black & Veatch SEA Pte. Ltd. After deciding that UF was the best available
technology for their application, a 5-month pilot study was conducted with various
UF membrane technologies. Zenon was selected in November 2002. Fibre durability
and the removal of organics to reduce trihalomethane formation potential were
important criteria in the design evaluations.

At commissioning in December 2003, the plant was the largest operational UF
membrane-based potable water plant in the world. The entire system was designed
and commissioned in 13 months. Civil works provided for a future capacity of
478,000 metres3/day but only 273,000 metre3/day of membrane capacity was initially
supplied. The equipment was installed in a small fraction of the space necessary
for conventional treatment. The footprint for the building was 47m x 53m; equivalent
to 190 metres3/day of filtration capacity per square metre of land area.

Compared to conventional treatment, the ZeeWeed® enhanced coagulation process
results in superior colour and total organic carbon (TOC) removal, and requires
less coagulant. The lower chemical dosages result in significantly less treatment
residuals and reduced disposal costs.

Raw water is collected from a reservoir, and filtered through a 1mm fine screen.
As the water enters the plant, alum coagulant is added to aid removal of colour
and organics. Lime is also dosed to increase pH. The water flows by gravity
to flocculation tanks, and from there to the ZeeWeed® system.

Each tank has five ZW500d cassettes and produces filtrate at a flow of 750
to 810 metres3/h. The membranes are reinforced hollow fibres constructed of
PVDF and are operated in an ‘outside-in’ flow. Filtration is achieved by drawing
water to the inside of the membrane fibre by utilising a siphon to create the
vacuum necessary to produce permeate.

The siphon is created by a 9 m difference in elevation between the ZeeWeed®
tank water level, and the product water storage tank. This eliminates the need
for filtration pumps and reduces capital cost, system footprint and energy requirements.

The fibres are aerated intermittently and backpulsed with filtrate four times
per hour to maintain production capacity. Daily automatic membrane integrity
testing to confirm conformance to specifications is undertaken. Chemical cleaning
is fully automatic and contract specifications limit cleaning to nine times
per year. Typical treated water results from the system are turbidity < 0.1 NTU, colour < 5 Hazen units and aluminium < 0.05 mg/l.

Following commissioning, Gerry O’Toole, Project Manager said, ‘Black &
Veatch is pleased to have worked with Zenon on the world’s largest, state-of-the-art
and innovative membrane water treatment plant, which has provided the end user
with the best process solution and lowest capital and operating cost.’

Bendigo MF Plant, Australia
Since USFilter Memcor (www.usfilter.com) commissioned their submerged MF membrane
system at Coliban Water Authority’s (CWA) Bendigo Water Treatment Plant (Victoria,
Australia) in 2002, it has been producing potable water for approximately 110,000
residents. As part of CWA’s 25-year, build-own-operate-transfer project, the
Bendigo facility was designed to meet existing guidelines and anticipate future
regulations. The 125,000 metres3/day facility combines MF, ozonation and biological
activated carbon (BAC).

The plant is faced with a challenging mountain source water with a turbidity
of typically 2.25 NTU, colour of 14.85 Hazen units, a dissolved oxygen content
of 7.63 mg/l and an Algae count of 298 cells/ml. Because of these supply parameters,
CWA incorporated pilot testing as part of the tendering process. CWA imposes
penalties for excursions from any contractual specification including taste
and odour, colour, continuous 2 to 5 micron particle removal and 4-log reduction
for Cryptosporidium oocysts.

Memcor® submerged CMF-S membrane technology was selected for its ability
to consistently meet project requirements. Pilot testing demonstrated that the
plant could filter high and variable turbidities and algae loads. Testing was
initiated in 1998, followed by whole process simulation in 1999. This continued
beyond the commissioning of the full-scale plant in 2002.

Raw water is screened, then lime is dosed to increase pH and carbon dioxide
is added to control alkalinity and corrosion. Aluminium chlorohydrate is then
dosed and rapidly mixed to coagulate colour, metals and suspended solids.

The water is next filtered by the CMF-S membrane system, which consists of
eight cells (six being on duty), each containing 576 submerged membrane modules.
Water enters at the bottom of each cell and is drawn through the porous membranes
by a filtrate pump.

Membranes are backwashed using a combination of filtrate and air to remove
solids from the membrane surface. Chemical cleaning is automatically initiated
when the trans-membrane pressure reaches the maximum design level.

Integrity testing
Membrane performance is continuously monitored by turbidity and particle counting
instrumentation, along with regular integrity testing utilising the automated
pressure decay test (PDT). This allows the detection of a single damaged hollow
fibre. Process control is achieved by monitoring PDT results to ensure consistent
water quality rather than relying on instrumentation.

Further treatment is undertaken in the ozone/BAC system. Ozone is generated
on site and applied to the filtrate to act as an oxidant, breaking down organic
material for further treatment by BAC filtration to reduce the TOC level. The
potable water produced is dosed with chlorine and then ammonia to provide residual
disinfection in the distribution system.

To date, the plant has maintained membrane integrity at least 0.3 log above
the required 4 log level. Filtrate turbidity has been consistently low at <0.1 NTU, averaging 0.02 NTU. Taste and odour removal have met all CWA requirements.

The analysis of the above systems demonstrates that consistently high quality
potable water can be obtained in large quantities from variable feed sources
by utilising membrane technology.

Given the predicted impact of global climate change and the resulting pressure
on potable water supplies, the need for utilising ever more challenging feed
waters is likely to result in the increasing need for advanced filtration and
separation technologies.

Author’s Note: Anthony Bennett is Managing & Technical Director at Specialist
Technical Solutions Ltd, UK.

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