Evaporation pond

Another mining process involves the recovery of sodium carbonate decahydrate from alkaline ponds. EMC mines this material from its solar evaporation pond using a bucket wheel dredge. The decahydrate slurry is dewatered, melted, and processed to soda ash. 

Lake Texcoco. Lake Texcoco, a few miles northeast of Mexico City, is in the lowest part of the Valley of Mexico. The lake is mostly dry and alkaH is recovered from brine weUs that have been drilled into the underlying stmcture. The brine is concentrated first in a spiral flow solar evaporation pond and further in conventional evaporators. This strong brine is carbonated and then cooled to crystallize sodium bicarbonate which is subsequently filtered and calcined to soda ash. Purity of this product is similar to Magadi material (9,29). 

Soil Clay is the primary construction material for settling basins and waste-treatment evaporation ponds. Since there is no single type of clay even within a given geographical area, shrinkage, porosity, absorption characteristics, and chemical resistance must be checked for each application. 

Mathematical models have also predicted a low volatility for methyl parathion (Jury et al. 1983 McLean et al. 1988). One study using a laboratory model designed to mimic conditions at soil pit and evaporation pond disposal sites (Sanders and Seiber 1983) did find a high volatility from the soil pit model (75% of the deposited material), but a low volatility for the evaporation pond model (3. 7% of the deposited material). A study of methyl parathion and the structurally similar compound ethyl parathion, which have similar vapor pressures, foimd that methyl parathion underwent less volatilization than ethyl parathion in a review of the data, the reduced level of volatilization for methyl parathion was determined to be due to its adsorption to the soil phase (Alvarez-Benedi et al. 1999). 

Soil redox also strongly affects solubility of the compounds of other trace elements in arid soils. Amrhein et al. (1993) found that Fe, Mn, Ni and V in an evaporation pond soil were more soluble under reducing conditions. Han and Banin (2000) reported that after one year of saturated incubation, the solubility of Fe, Mn, Co, V, Ni, Cu and Zn in two Israeli arid soils with 0.5-23% CaC03 increased, while the solubility of Cd decreased with time. During saturated incubation, soil pH in highly calcareous arid soil containing high content of carbonates decreased. In a loessial soil from Israel, Han and Banin (1996) reported that soil pH decreased from 8.0 to 7.0-7.4 over saturated incubation. With the decrease in Eh over incubation, the parameter pe+pH also decreased from initial values of 12-13.6 to 4 after initial 7-9 days of saturation incubation. 

Fujii, R. 1988. Water-Quality and Sediment-Chemistry Data of Drain Water and Evaporation Ponds from Tulare Lake Drainage District, Kings County, California, March 1985 to March 1986. U.S. Geol. Surv. Open-File Rep. 87-700. 19 pp. Avail, from U.S. Geological Survey, Federal Center, Building 810, Box 25425, Denver, CO 80225. 

This book examines five methods used for concentrate management, namely disposal to surface water, disposal to sewerage, deep well injection, land applications and evaporation ponds. In particular, the book focuses on the design, siting, cost, and environmental impacts of these methods. While these methods are widely practiced in a variety of settings already, there are many limitations that restrict the use of certain disposal options in particular locations. 

Surface water Deep well injection Spray irrigation Evaporation pond... 

The relative increase in cost for surface water disposal, deep well injection, spray irrigation (land applications) and evaporation ponds can be seen in Fig. 3.1. It can be seen that surface water disposal is the cheapest alternative, and it has a strong economy of scale as concentrate volume increases. Deep well injection also has a strong economy of scale, but this method s high construction costs means that it is only becomes feasible with a high enough disposal volume. Evaporation ponds have a poor economy of scale, and it can be seen that the overall cost increases rapidly with volume. This is due to the large amounts of land that are required as the volume of concentrate increases. 

Ahmed, M., Shayya, W.H., Hoey, D., Mahendran, A., Morris, R., Al-Handaly, J. Use of evaporation ponds for brine disposal in desalination plants. Desalination 130(2), 155-168 (2000) Mickley, M. Treatment of Concentrate, Desalination and Water Purification Research and Development Program Report No. 155. U.S. Department of the Interior, Bureau of Reclamation, Denver (2009)... 

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. 

Fig. 7.1 Cross sectional view of the inner bank of an evaporation pond showing depth and freeboard...

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. 

When determining the evaporation rate for an evaporation pond in the absence of a corrected pan evaporation rate, an approximation of 70% of the freshwater evaporation rate is considered reasonable (Mickley 2006). 

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. 

Suggested dimensions for evaporation ponds are given by Singh and Christen (2000), and these can be seen in Fig. 7.5. The banks surrounding the pond should be a minimum of 1 m high and 2.4 m wide to allow for vehicular access. To reduce erosion, it is recommended that the slope of the inner bank should be 1 5 and the outside bank a slope of 1 2. 

The feasibility of an evaporation pond is determined by the volume of concentrate, the cost of land and the ambient evaporation rate. They are most cost-effective in areas with low rainfall, high evaporation rates, and where large expanses of land are available at low cost (Mickley 2009). This best suits inland desalination plants in regional and remote areas where these conditions can be met. 

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). 

The major cost factors of an evaporation pond include pond liners, land preparation, excavation and clearing, site surveying, bank construction, pumps, control systems, disposal of precipitated solids, maintenance and geotechnical investigation of the site (Singh and Christen 2000). Of these, pond liners typically represent the greatest cost (Nicot et al. 2009). 

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