According to equation (1), if the absorbed net radiation in solution, R^^, increases, the 

 evaporation rate will also increase. To increase the absorption of solar radiation in ponds, a dye 

 such as 2-Naphthol Green is often used. 



Bonython (1965) compared evaporation rates between undyed and dyed salt ponds. At 

 Dry Creek in South Australia during the 1948-1949 summer season, 58 acres of dyed salt- 

 crystallizing ponds were compared with 87 acres of undyed ponds supplied saturated brine. 

 Results showed that crystallization of salt in dyed ponds was 15-20% more than in undyed ponds. 

 This indicates that the use of dye contributes significantly to increasing evaporation rates and 

 the subsequent precipitation of salts. 



Salinity 



Salinity is directly related to seasonally variable drainage volume. Tanji and Grismer 

 (1989) showed that in the San Joaquin Valley, the water volume in evaporation ponds is high 

 during winter due to pre-plant irrigation and rainfall, and is low in late summer and fall because 

 evaporation rate exceeds drainage drainage input. Therefore, salinity is high in summer and 

 low in winter. 



Evaporation decreases with increasing salinity as this and many other studies have 

 shown (Bonython, 1965; Janson, 1959; Moore and Runkles, 1968; Salhotra et al., 1959). In 

 addition, salt crusts may form on the water surface with increasing evapoconcen tration resulting 

 in a reduction of evaporation rate (Adams, 1934). Adequate design and management should thus 

 aim at keeping salinity sufficiently low or removing salt crusts to maintain a reasonable 

 evaporation rate. 



To achieve an adequate evaporation rate, the evaporation ponds should be divided into 

 several cells so that waters can be separated according to their salinity range. The water wiUi 

 the lowest salinity should be directed into the first cell and, as evapoconcentration proceeds, it 

 should be conducted to other cells. 



Ormat Process 



Ormat Engineering, Inc, is advancing the Ormat process to enhance evaporation rat«s 

 of dilute brines. This process is used to concentrate Dead Sea brine for minerals recovery in Israel 

 and is also being demonstrated in a USER project to recover energy ft-om solar ponds at El Paso, 



Texas. 



The patented Ormat process involves pumping dilute brine through a large number of 

 nozzles at a height of 30 meters so that the saline water is in contact with dry air. Due to 

 proprietary constraints, details on the Ormat process are not completely known. This process 

 appears to have a high initial capital investment and high operating cost. A potential problem 

 exists of salt drift to adjacent lands. 



Holor of Solution ™r. j i i. i 



The color of solution affects the degree of net radiation absorption. The darker the color 

 of the solution, the higher the absorption of net radiation. As mentioned earlier, the dye 2- 

 Napthol Green is commonly used to alter the color. 



{Suit Precinitation u • i 



Precipitation of evaporites is a function of ion activities , solution temperature, chemical 

 composition and pH. Each factor depends on several interacting variables which interact not 

 only with each other but also with evaporation rate. For example, water temperature is 

 dependent on absorption of net radiation, latent heat transfer and sensible heat transfer. 

 Generally speaking, if water temperature is high, more minerals are dissolved due to an mcrease 

 in the solubility product and the evaporation rate decreases. As far as the efficiency of 

 evaporation ponds is concerned, the manageable factor affecting evaporation rate and salt 

 precipitation is solute concentration and salinity. , ^ , > .. 



Precipitation of salts usually occurs when the ion activity product of solutes exceeds the 

 solubility product of a particular mineral. When evapoconcentration of pond waters causes 



page 9.2 



