The recovery of wastewater for industrial uses presents many challenges for the design and operation of water treatment equipment. Since the term wastewater can be applied to a wide range of water sources from municipal sewage to storm runoff to industrial discharge water, the water characteristics may have great variability. As such, the equipment to treat wastewater must be robust and forgiving. When these wastewaters are used to produce ultra high purity boiler feedwater, the challenges become magnified.
This paper describes a water treatment system, fed with municipal wastewaters, for the production of high purity boiler feed makeup water at a combined cycle gas-fired power plant, and the experiences of the plant operator after five years of continuous operation.
This fully automatic water treatment system consists of micro-media filtration, reverse osmosis and a short bed ion exchange demineralizer/polisher, and requires minimal operator attention and maintenance.
Typically, wastewater has higher levels and a wider variety of dissolved solids and metals, and potentially high levels of organic material, both natural and man-made, especially where municipal wastewater is concerned. With increasing environmental concerns and restrictions, it can be challenging to treat these wastewaters for mere discharge, let alone for reuse in industrial or municipal applications. Greater restrictions on the use of fresh water for industrial purposes and the diminishing supply from existing fresh water sources have necessitated some innovative ways to reuse these wastewaters.
But until recently, their use for the production of high purity boiler feedwater had not been considered. In the power generation industry, particularly in areas where fresh water is scarce, this is now being considered as an alternative.
At a 600 MW combined cycle gas-fired power plant in northern Mexico, they have been using wastewater from the nearby municipality as their sole source of service and boiler feedwater since the plant began operations in 2002.
The original boiler feed makeup water treatment system consisted of two-stage sand filters, three-stage reverse osmosis (RO) and electro-deionization (EDI) polishers. During the first three years of operation, the water treatment system struggled to meet both the quality and capacity requirements of the power plant. The RO units fouled rapidly and required frequent cleanings, sometimes more than once per month, and this led to inconsistency in the output from the EDI system both in terms of quality and quantity.
Additional capacity was required and the operational difficulties and boiler makeup water quality issues needed to be resolved.