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Concentrate Minimization via Pellet Softening - Process Evaluation and Pilot Study

Project ID: 2444
Principal Investigator: Saied Delagah
Research Topic: Desalination and Water Treatment
Priority Area Assignments: 2012 (Advanced Water Treatment), 2013 (Advanced Water Treatment), 2014 (Advanced Water Treatment)
Funded Fiscal Years: 2012 and 2013
Keywords: concentrate management, pellet softener, zld, zero liquid discharge

Research Question

Can reverse osmosis (RO) concentrate streams be further dewatered sustainably and cost effectively to reduce the volume of concentrate in need of disposal and recover additional water trapped in the concentrate? RO concentrate stream disposal costs add a considerable expense to capital and operation and maintenance (O&M) costs of RO plants. Reducing the volume of concentrate generated reduces this additional cost and frees additional untreated water trapped in the concentrate streams.

RO concentrate streams cannot be treated further in the RO process because of the fouling potential of the concentrate and the expense associated with its disposal and/or management. The scaling potential of the remaining stream increases significantly as the feedwater is concentrated in an RO process. Precipitation occurs as the stream is supersaturated, causing irreversible membrane damage. Efficient formation of ionic species and removal of precipitate is crucial to minimizing concentrate stream volumes with further RO treatment. The scaling potential of the concentrate stream is reduced as ionic precipitates are removed from the concentrate stream.

Pellet softening using sand and/or seed crystals in a fluidized bed reactor aids in the formation of ionic crystals and removal of the precipitate from the concentrate stream. This technology requires further analysis to study its efficiency and associated costs for effective concentrate management.

Need and Benefit

Membrane processes are becoming a technology of choice for the water purification industry. Purification of sea and brackish water creates new sources of water that enhance the nation's conventional water supplies. The mission of Reclamation is to manage, develop, and protect water and related resources in an environmentally and economically sound manner in the interest of the American public (Reclamation Web site). Desalination of water assists Reclamation in achieving its mission with greater success and is becoming critical for Reclamation to achieve its mission in managing and developing water supplies.

The concentrate generated from membrane desalination contains the particulates and salts that were removed from the feedwater. The volumes of these concentrate streams can be anywhere from 50 percent of the total feed for seawater desal and up to 15 percent of the total feed for brackish water desal. Disposal or management of these concentrate streams represents a significant waste disposal cost and loss of additional valuable water (15 to 50 percent of wasted concentrate) contained in the concentrate stream. Concentrate disposal is more difficult in inland areas where the ocean cannot be used as the ultimate disposal site. Costly evaporation ponds are used in inland areas to reduce the volume of concentrate. Other unsustainable methods of dealing with concentrate are discharge to surface water/wastewater treatment plants and deep well injection, which are costly and unsustainable .

There is a need for minimization of the concentrate stream volume that is achieved by using the appropriate processes to induce the desired affect in the concentrate. Water recovered from the precipitated salts can be recycled to the head of the treatment system, thereby increasing recovery of the whole process. Treatment technologies such as pellet softening that can sustainably and cost effectively allow viable concentrate management should be investigated.

This proposal is being submitted under the water supply (WS) #2 research output area within the Science and Technology (S&T) program of Reclamation. WS #2 is defined as the enhancement of water supplies through desalination and water treatment by developing and improving cost- effective desalination and other water purification technologies to expand water supplies. To meet these program goals and stated objectives, funding and resources must be applied towards creation of new technologies that can reduce the cost of desalination and expand water supplies. Various studies have, currently and in the past, been funded by Reclamation to study the concentrate disposal issue.

Mickley and Associates studied the current state of practice and regulations of membrane concentrate disposal. A pilot plant study was awarded by the S&T program in FY03 that investigated the effect of an innovative process in recovering water from RO concentrate streams. There have been other concentrate disposal projects that Reclamation and S&T have funded in the past. Reclamation has been and is currently supporting the search for solutions to the concentrate disposal problem. Our proposal will effectively help to achieve Reclamation and S&T's goals by testing pellet softening to reduce calcium carbonate fouling potential in concentrate streams.

Recently, Reclamation assisted in a report entitled Desalination and Water Purification Technology Roadmap in which a summary of the current and future challenges facing the water supply were summarized and suggestions for research and development (R&D) to create new technologies to face these challenges were made. Research in concentrate disposal issues was identified as one of the most important areas for adoption and progress of desalination technologies for water supply expansion especially for inland desalination. Our proposal is focused on evaluating a unit process that can sustainably and cost effectively reduce concentrate volumes.

Contributing Partners

None

This information was last updated on April 24, 2014
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