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The Power Project uses Porex's microfiltration technology to operate a unique zero-liquid discharge system

Zero-liquid discharge

The High Desert Power Project in California, US, has been in operation since 2003. Peter S Cartwright describes how the power generator uses Porex's microfiltration technology to operate a unique zero-liquid discharge system.

The High Desert Power Project is a large US-based power generator that uses microfiltration (MF) technology to treat and reuse cooling tower blowdown in a zero-liquid discharge process. In operation since 2003, the process consists of a chemical addition to the blowdown water to enable hardness and silica removal by precipitation. The precipitated solids are then continuously removed with microfiltration technology. The cooling tower is fed by clarified California Aquaduct water. Cooling tower blowdown is pumped to a first-stage reaction tank. Ferric sulphate, soda ash, magnesium chloride and sodium hypochlorite (bleach) are added; the pH in this tank is approximately 8.5, which initiates the precipitation of calcium carbonate and silica.

Effluent from the first-stage reaction tank overflows into a second-stage reaction tank, which receives lime and soda ash, raising the pH to 10.5–11.0 to further precipitate calcium carbonate and silica. This tank overflows into a concentration tank that also receives the concentrate (reject) stream from the microfiltration (MF) system. The concentration tank collects precipitated and chemically saturated solids, and the resulting sludge is fed to a sludge-thickener tank.

The suspended solids slurry in the concentration tank is directed to the MF system for (almost) complete removal of these solids. The MF permeate (that portion of the feed stream that passes through the membrane) flows into the pH adjustment tank, where it receives sodium bisulphite to neutralise the chlorine (from the bleach addition) in the tank, and hydrochloric acid to lower the pH to 6.3.

The MF concentrate (that portion of the feed stream that passes across and over the membrane surface) transports suspended solids out of the MF system and back to the concentration tank. Over time, the total solids concentration in this tank increases to about 5%, at which time the solids are pumped to the sludge-thickener tank that feeds a filter press. The dewatered solids are hauled to landfill and the liquid portion is directed back to the concentration tank.

From the pH-adjustment tank, the treated MF permeate is processed with reverse osmosis (RO) technology, and the firststage permeate stream is either returned as make-up to the cooling tower or fed to a polishing RO. The permeate from the polishing RO is directed to a continuous deionisation system to produce boiler feedwater, and the concentrate stream becomes part of the cooling tower make-up water.

The concentrate stream from the first-stage RO is fed to a second-stage RO, with the permeate from this system directed back to the cooling tower as make-up water, and the concentrate stream fed to a crystalliser. The solids from this unit are delivered to landfill, and the crystalliser condensate returned to the cooling tower as make-up. As water evaporates, as in a cooling tower, the level of contaminants in the blowdown water increases significantly.

Technologies and processes

The technologies (RO and continuous deionisation) required to remove dissolved solids are deleteriously affected by suspended solids and slightly soluble salts that precipitate on concentration. Almost all water supplies, with the exception of seawater, are saturated in calcium and magnesium carbonate. Also, many supplies have high concentrations of silica, another relatively insoluble contaminant, as well as sulphate salts.

Conventional lime softening is the traditional water softening process for high volume flows, and involves adding lime and soda ash. The pH increases due to the addition of lime, calcium carbonate, magnesium hydroxide and magnesium carbonate precipitate. Magnesium hydroxide also removes silica via absorption as it precipitates.

The first-stage RO removes dissolved solids with a portion of its permeate supplying the cooling tower with make-up water, and the remaining permeate flow directed to a polishing RO for additional dissolved solids removal, and then further purified in a continuous deionisation unit to produce high-quality boiler feedwater.

The concentrate stream from the firststage RO is fed to a second-stage RO, with its permeate used as cooling tower makeup water, and its concentrate treated in a crystalliser to produce solids for landfill and the condensate used for cooling water make-up. As the MF process so effectively removes suspended solids, the RO concentrate streams could be returned to the system's front end for further treatment.

High reuse results

Each component in this system contributes to the overall success of this unique design. The chemical additions to the reaction tanks result in hardness and silica precipitation, and the MF system continuously removes clarified water from the solids. This treated water is further polished with RO technology to remove salts to either generate cooling tower make-up water or to feed continuous deionisation technology to produce boiler feedwater. The effectiveness of MF technology is underscored by the fact that the RO units can recover a very high percentage of the treated water for complete reuse. In addition, the RO membranes need cleaning no more frequently than every six months.

The sludge resulting from hardness and silica precipitation is dewatered in a filter press and sent to landfill. The liquid stream from the filter press is redirected to the MF system. The concentrated salts from the RO units are rendered insoluble in a crystalliser with these solids sent to landfill. The crystalliser condensate is also used as cooling tower make-up.

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