Power station almost self-sufficient in process water use

By Garry Craig, Andrew Layson

The Eraring Power Station water reclamation plant (WRP) in New South Wales, Australia, has saved almost US$ 3.7 million, with the biggest cost savings in water charges and downstream ion exchange regeneration chemical costs within the past two years.

The WRP provides 3.8 megalitres (Ml) of reclaimed water per day to the 2,640-MW coal-fired electric power station. The plant treats secondary sewage from a local treatment works, using a combination of microfiltration (MF) and reverse-osmosis (RO) technology from USFilter Memcor Products. Last November, Eraring extended its service contract with Veolia Water (USFilter's sister company) for another three years.

In 2002 and 2003, the power station used 2,824 kl of reclaimed water per day or 66% of total water used on-site. The extremely dry weather during the past nine months caused this volume to be lower than in previous years.

The drought-like conditions similarly reduced effluent supply during this period.

To date, energy company costs for both CMF and RO units come to a little more than US 5¢/kl of reclaimed water produced. Water savings are expected to increase to US$ 986,500 per annum as more secondary effluent becomes available. Annual savings in the station's demineralising plant (lower salt loading results in a higher throughput and fewer regenerations) are steadily estimated at US$ 52,600.

The MF and RO systems at the Eraring Power Station are somewhat unique because they still contain some of the original membranes installed almost eight years ago. According to Lisa Clarke, development engineer involved in research and development at Memcor Australia Pty Ltd (a division of USFilter), this is the longest operation of membranes in an effluent reuse application anywhere in the world.

The "B" and "C" CMF units are still operating with most of the original membranes. During the last few years of operation, minor pin repairs were made to the "A" and "B" CMF units, indicating their membranes may soon need to be replaced. The "A" CMF membranes were replaced in August 2003 after 8-1/2 years of operation for an estimated US$ 78,900. The station is trying to prolong membrane life in the "B" unit by replacing faulty modules with good modules taken from the "A" unit, thereby deferring the costs of upgrading the "B" unit by as much as 18 months. Membranes in the "C" unit have not needed replacement yet. They are the same age as the unit (3-1/2 years old).

Eraring is following a similar replacement schedule for the "A" and "B" RO units. During the plant upgrade in 1998, Eraring took the best membranes from the "A" RO unit and installed them in the "B" RO unit. Those in the "A" RO unit were simply replaced. Membranes in the "A" and "B" RO units are almost 5 and 8-1/2 years old, respectively. With newer membranes, the "A" RO array is producing water of much lower conductivity than the "B" RO array.

"Despite the varied age of the membranes, the reclaimed water produced contains far fewer impurities than did the previously used potable water," Clarke said.

In addition, both RO arrays were only recently chemically cleaned — more than 12 months after the previous cleaning.

The WRP operates infrequently at full capacity (3.75 Ml/day), such as when the Dora Creek buffer storage dam level rises above 35% of capacity. When this level decreases, the plant is run at only 50% capacity by operating a single RO unit (usually the "A" RO array) in addition to all CMF units, but at lower flow-rates. The station runs briefly the out-of-service RO unit every four days to flush the membranes. The plant is not expected to operate fully and continuously until 2010 when full secondary effluent supply is available.

Authors' Note
Garry Craig is chemical team leader at the Eraring Power Station, owned and operated by Eraring Energy in Dora Creek, New South Wales, Australia. Andrew Layson is general manager with Memcor Australia Pty Ltd's Membrane Technology Centre and is based out of Windsor, New South Wales, Australia.

• An automatic pressure decay test is regularly used to monitor the membrane integrity of each CMF unit.
• A sonic test is used to better identify the spent membrane.
• The filtrate side of the compromised sub-module is manually isolated so it ceases to filter.
• The compromised sub-module is then pin-repaired.