Cooled and desalinated thermal water utilization in the Podhale heating system

Abstract

Distribution water that circulates in heating installations must meet certain parameters depending on the technical requirements set by the manufacturers of the equipment through which it flows. It should not cause scaling of boilers, should not corrode parts of the installation and should not foam. This paper examines the proposition the used of geothermal water to replenish network water losses in the largest Polish geothermal heating system located within the Podhale Basin reservoir. The area in question has considerable geothermal water resources while at the same time exhibiting a fresh water deficit. Only part of the total volume of geothermal water (up to 5 m[3]/h) was used and treated; this is roughly equivalent to the district heating system demand for treated fresh distribution water (ca. 550 m[3]/month). The treatment uses membrane processes within the framework of a integrated arrangement including ultrafiltration (UF) and two independent reverse osmosis stages (RO-1 and RO-2) connected in series. Artesian pressure was used to a certain extent during the treatment, which makes it possible to reduce the power required for the distribution pump, decreasing electrical power consumption by ca. 0.7-0.9 kW. Thanks to the fact that the temperature of the water to be treated is higher than the public supply, the thermal capacity requirement was reduced (~30 kW) and both water treatment and degassing processes are more efficient. Iron content was reduced from ca. 4 to 0.013 g/m[3], total hardness was reduced from 13.5 eq/m[3] to <0.02 eq/m[3], alkalinity was reduced from 4.45 eq/m[3] to <0.01 eq/m[3] and phosphate content was reduced from 0.03 to <0.006 g/m[3]. Thus the requirements set forth in the standards have been met and the treated geothermal water can be used to replenish network water losses in the district heating system following pH adjustment and degassing.

The PN-85/C-04601 Polish Standard does not specify any requirements concerning the admissible con­centrations of, inter alia, chloride and sulphate ions, i.e. anions that affect water corrosivity, mostly pitting corrosivity. These ions form soluble compounds with metals and thereby inhibit the formation and precipitation of metal oxides. The use of the membrane technology discussed here in the desalination of geothermal waters, also made it possible to achieve high retention coefficients - 97% after RO-2 for chlorides and 99% for sulphates already after RO-1. [PUBLICATION ABSTRACT]