Coal, Emissions

Water Treatment On Wheels

Issue 9 and Volume 109.

Design options and combinations of fixed and mobile demineralization equipment give power plant operators the flexibility to continually optimize their water treatment system to meet rapidly changing needs.

Robert T. Taylor, Jr., GE Infrastructure, Water & Process Technologies

Today’s power generation marketplace has become very competitive. In this environment, the company that can produce the lowest cost power sells the most, makes the most profit and wins the game. Contract water treatment services are one tool power companies are using meet their water treatment needs and increase their competitive position in the marketplace.

Water treatment service providers have evolved from emergency mobile demineralizer suppliers to solution providers that can offer a flexible platform to integrate mobile or fixed membrane and media technologies, optimized to meet site-specific client needs. Water treatment service contracts let the power industry eliminate upfront capital outlay for equipment, and transfer operational and performance risk to experts in the field.

Service contracts also offer clients the flexibility to continually optimize their water treatment systems to adapt to changes in demand for treated water quality or quantity, incorporate future water treatment technologies as they evolve, and meet site-specific environmental requirements.

This article classifies water treatment service contracts for demineralized (DI) water into four categories, and presents associated design, economic and operational advantages to power plant designers, constructors, owners and operators.

Service Contract Categories

Service contracts for high purity water production can be structured for terms of 1 to 15 years for continuous, seasonal or emergency use. Service contracts in the power industry typically range in flow capacity between 50 and 1,000 gpm, and designs can employ all mobile equipment, building/container enclosed fixed equipment, or a combination of fixed and mobile equipment to meet client needs. Equipment designs used by leading global service providers to meet the variable needs of the power industry can be generically classified into four categories:

  1. Mobile trailer-mounted demineralization
  2. Mobile trailer-mounted primary treatment and mobile demineralization
  3. Fixed primary treatment and mobile demineralization
  4. Fixed primary treatment and primary demineralization and “exchange” demineralization polishing

In each class, the assets used to produce the treated water are owned by the service provider. Consumables and replacement parts are also normally provided by the service company. While client operation and monitoring of mobile demineralizer equipment is typical, the service provider provides the skilled operational support needed for membrane treatment technologies, assumes operational risk and offers system performance guarantees.

Mobile Trailer-Mounted Demineralization – Mobile trailer-mounted ion exchange demineralization has reliably serviced the power industry for the last 40 years. In the early years, mobile demineralizers were primarily used as a “911” service – supplying emergency support for capital equipment failures. Today, new combustion turbine (CT) plant designs use mobile demineralizers as a facility’s sole water treatment plant. Simple cycle combustion turbine designs use mobile demineralizers to meet their full complement of DI water needs – for power augmentation using inlet air fogging, evaporative cooling, and nitrogen oxide control (NOx). In some cases, combined-cycle combustion turbine makeup designs also use mobile demineralizers as a facility’s sole water treatment plant. The most common drivers for such services are restrictions on water consumption or discharge, and eliminating thermal processes for zero discharge.

Mobile RO Trailers from GE Infrastructure, Water & Process Technologies.
Click here to enlarge image

Designs using mobile demineralizers present advantages to CT power plant developers because a mobile demineralizer is a true zero-discharge treatment system (liquid and solid), and no industrial wastewater discharge permit is needed. Eliminating industrial wastewater generation and discharge can help mitigate local community concerns and facilitate CT plant development and permitting. The most successful new plants designed for mobile demineralizer treatment involve consultation with the service supplier in the design stage to address required footprint and utilities, instrumentation for monitoring, plant operational responsibilities and, most importantly, to provide adequate demineralized water storage tank volume.

Mobile Primary Treatment and Mobile Demineralization – An expanded range of technologies available on a mobile platform make integrated mobile services possible. Mobile primary treatment typically consists of reverse osmosis (RO) and supporting pretreatment equipment – filtration, softening, activated carbon – along with chemicals. Integrating mobile demineralizers (DI) with mobile pretreatment allows service providers to deliver a complete mobile water treatment system to process raw river water into high purity demineralized water.

RO technology, in combination with mobile demineralization, presents advantages to improve treatment economics on water sources with greater than 200 to 300 ppm total dissolved solids (TDS). The mobile RO-DI design is favored over mobile DI for installations with one or more of the following characteristics:

  • Continuous water demand at the design flow
  • Wastewater discharge/treatment capabilities to handle the RO reject
  • Medium to high TDS supplies
  • Ion exchange treatment alone cannot meet specification for total organic content (TOC)

In general, an integrated mobile platform presents advantages to clients that need continuous DI water production for a defined period, typically for one peak season (winter or summer). Integrated mobile services require longer advanced notice and prescheduled service because RO equipment is a limited rolling stock inventory item and requires more preparation time by the service provider. The service supplier must be able to predict and schedule tangible business to present the most favorable economics, manage equipment inventory, and commit to equipment availability. Integrated mobile services for peak power facilities or for seasonal supplemental support at coal-fired stations typically include a minimum service period commitment.

Click here to enlarge image

Fixed Primary Treatment and Mobile Polishing – Like mobile primary treatment, fixed primary treatment is conventionally RO and the supporting pretreatment equipment and chemicals. Fixed primary treatment can be favored over integrated mobile systems for installations with one or more of the following characteristics:

  • Plants that operate in the winter and summer peaking seasons
  • Plant where custom-engineered pretreatment equipment is required for primary treatment
  • Plants operated at baseload (with mobile demineralizer polishing in service year-round)

For seasonal operations, the fixed primary treatment design provides the ability to rapidly produce demineralized water. That’s because the fixed equipment is already onsite with utilities connected and because it eliminates mobile pretreatment equipment preparation, delivery, setup time and charges. The only mobile component is the mobile demineralizer, which is simply delivered to the plant to produce demineralizer water on an as-needed basis when the power plant is dispatched for generation. This “asset-light” design for fixed equipment (compared to the fourth category) minimizes installed assets on the ground to produce demineralized water. Contracts are set up with a fixed fee to cover equipment amortization, plus a variable charge for operations, process chemicals and mobile demineralizers.

Since the fixed assets are small, contracts generally have a lower percentage of fixed costs, with a higher variable percentage compared to “asset heavy” systems with the same flow capacity. The low fixed costs are attractive to CT plants that operate in both winter and summer peak because, during off-peak or non-operational seasons, use of mobile equipment allows removal and cost savings.

The fixed primary treatment design with mobile DI polishing is also successfully used for plants in baseload operation and plants that require customized pretreatment designs to treat challenging water sources, such as tertiary municipal wastewater. Economics for baseload service are influenced by cost centers such as water quality (TDS) and proximity to the service provider’s regeneration facility. This fixed primary treatment design with mobile trailer-mounted demineralizer polishing is currently in service at numerous fossil and nuclear facilities throughout the world.

Fixed primary treatment designs can also use offsite regenerated naked mixed bed polishers to polish RO permeate. However, trailer polishing designs offer performance advantages and reduced operational risk to industry over use of naked mixed bed polishers. If there is a problem or failure of the RO system, the RO unit can be partially or completely bypassed with feed sent to the mobile demineralizer with no deterioration in effluent quantity. Naked mixed bed polishers carry a higher operational risk, because they do not have ample ion exchange capacity to provide this level of redundancy.

Fixed Primary Treatment, Primary Demineralization and Mobile Polishing – This design is more “asset heavy” because a large percentage of the treatment equipment is fixed and remains at the facility for the contract term. Fixed components can consist of pretreatment processes, RO, gas transfer membranes (GTM) and electrodeionization (EDI). The mobile component consists of polishing demineralizers, conventionally 40-80 cubic foot mixed bed vessels, which are transported for chemical regeneration at a remote facility.

The economics of this design can offer a lower evaluated cost per unit volume ($/kgal) for facilities in baseload operation. “Asset heavy” designs have a high percentage of fixed costs for capital recovery, consumables and operations, with a smaller variable fee percentage charged during DI production. As with the fixed primary treatment designs in category three, typical contract terms for this asset-heavy design are 5 to 10 years. In both categories, use of standardized equipment designs for the fixed equipment enables service providers to present favorable economics for contract terms as short as five years. This capability is not available with custom engineered equipment because these assets cannot be readily redeployed, and all costs for this equipment are recovered over the contract term.

Discussion of system designs and the associated economic, operational and commercial advantages for this class of service have been well documented in other work.

Although mobile trailer-mounted equipment is not used in daily operation with this design, the importance of mobile demineralizer and integrated mobile services capabilities should not be overlooked. When considering a service supplier, an important question to ask is, “How will that supplier ensure performance guarantees for water quantity and quality when problems occur?” Suppliers may offer guarantees, but a piece of paper will not help their system produce one extra drop of water when a partial or complete system failure occurs. True service companies have a global backup fleet of mobile equipment to support and meet their performance guarantees.

About the Author:

Robert Taylor is Global Development Director for GE Infrastructure, Water & Process Technologies and works with clients to develop build-own-operate solutions for industrial water requirements. He has more than 15 years of experience in industrial water treatment and previously held technical and marketing positions with Ecolochem and Ionics.