Consolidated Edison Company of New York (Con Edison) provides electric, gas and steam service to 3.5 million customers in New York City and Westchester County, New York. Con Edison has five steam-generating plants in New York City that provide steam for heating residential and commercial buildings in Manhattan. The water used to generate the steam requires demineralization treatment to prevent scaling in the boilers and the steam distribution network.
Con Edison’s East River steam generating facility (East River) is alongside the East River on the East Side of Manhattan. This facility has two systems to treat feed water for the boilers that generate the steam. One is a conventional mixed bed cation and anion demineralization system and the other is a more modern system using reverse osmosis (RO) and electrodeionization (EDI).
Demineralization Processes
Total dissolved solids (TDS) consisting of ions such as calcium, magnesium, silica, aluminum and other metals can precipitate at elevated temperatures and pressures within the boilers, resulting in the formation of scale and deposits. These deposits reduce heat transfer efficiency and can cause corrosion leading to boiler tube failures. The scaling ions or TDS must therefore be reduced to very low levels well below 1 ppm to protect the boiler equipment and allow efficient operation. For high purity water treatment, the water quality target is typically expressed as conductivity, which is proportional to the TDS level but easier to measure at very low TDS levels. The target treated water conductivity for the feed to the mixed bed demineralization system at East River was 50 µS/cm.
A mixed bed demineralization system consists of a strong acid cation exchange resin for removing cations such as calcium, magnesium and sodium and a strong base anion exchange resin to remove anions such as bicarbonate, sulfate and chloride. This ion exchange treatment occurs in mixed bed vessels where the resins are combined. At some point when the resins are loaded with ions, the vessels need to be taken off-line and regenerated with mineral acid and sodium hydroxide for the cation and anion resins, respectively. The higher the feedwater TDS, the more often these vessels need to be regenerated for a given volume of ion exchange resin.
Another demineralization treatment system used by Con Edison is reverse osmosis. This method removes over 99% of the TDS followed by EDI, which is used as a polishing process to achieve the treated water conductivity target. Reverse osmosis membranes are semipermeable and reject most ions, but a small percentage of ions can still diffuse through the membranes requiring additional polishing to meet high purity treatment targets. EDI is a process used for polishing RO permeate where an electric field is applied across membranes to remove ions and polar species.