Pond Area

Figure 12-25 provides a rapid method of determining the pond-area requirements for a given coohng duty. Di and Do are the approaches to equilibrium for the entering and leaving water, °F V Js trie wind velocity, mFh product PQ represents the area of the pond surface, ft /(gal-min) of flowto thepond. The P factor assumes a pond with uniform flow, without turbulence, and with the water warmer than the air. 

Keywords Costs Environmental concerns Evaporation pond design Evaporation rate Liners Pond area Pond banks Pond depth Social impacts... 

The design of an evaporation pond must take into account both the volume of concentrate from the plant and the evaporation rate at the selected site. The prevention of salinity in surrounding areas and the contamination of nearby potable aquifers is of great importance and must be carefully considered during the design phase. The major design factors to consider are the pond area, depth, liners and bank size. 

The minimum depth of a pond is directly proportionate to the rate of evaporation. This depth needs to allow for increases in volume, the precipitation of salts, as well as for rainfall and waves. It is estimated that the best evaporation rate can be achieved with pond depths of 0.03-0.45 m, however ponds with depths of up to 1.02 m have been shown to have reasonable evaporation rates (Mickley 2006). Similarly to pond area, a safety factor can be applied to the calculated minimum pond depth to increase the capacity and prevent the pond from overflowing. This extra depth will depend upon the expected additional discharge volume at the beginning of plant operation (Mickley 2006), and the ambient conditions during winter, at which time the pond may store water rather than reduce its volume (Ahmed et al. 2000). 

While the volume of concentrate can be determined based on plant capacity and recovery, the evaporation rate at any given site varies with climate. To determine the evaporation rate of fresh water at certain locations, a standard pan evaporation measurement is taken. Evaporation pans are small, open air pans filled with water from which losses in water due to evaporation are measured. Standard size Class A evaporation pans are most commonly used, which are 1.207 m in diameter and 0.25 m in depth. The daily change in depth, minus any rainfall, is used to determine the evaporation rate in mm/day. This rate takes into account the effects of climate on evaporation rate, but corrections for pond area and salinity must be made when determining the evaporation rate of a specific evaporation pond. 

The banks around evaporation ponds can be built from the existing soil and excavated earth. It is suggested that a layer the topsoil be removed from the pond area, and the subsoil underneath be used to form the inside of the bank. The outer slope of the bank can be covered in the removed topsoil, as this promotes the regrowth of vegetation (Singh and Christen 2000). An example of this is shown in Fig. 7.4. 

Evaporation ponds have a poor economy of scale and while they are economic for small waste flows, the largest feasible volume of concentrate is typically no greater than 5 MOD (Glater and Cohen 2003). Moreover, if the evaporation rate is low during the cooler months, the pond area may increase to an unfeasible size. In such instances, alternative disposal methods or concentrate storage options should be considered (Mickley 2009). 

Based on projected area of PBR tubes. Based on actual pond area. 

The outer face of an impounding dam should be planted with trees. Deep roots help to stabilize the soil. The silted-up settling pond areas should likewise be planted with trees or otherwise be used as pasture or arable land. Other possible uses are as sports fields or recreational facilities, as such areas are usually very flat. 

When substantial pond areas have thus filled up, planting on them should commence as soon as they are sufficiently firm and trafficable. 

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