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 parameters.

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 spikes.

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.

Conclusions
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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