Technologies and Costs for Removal of Arsenic From Drinking Water

The use of evaporation ponds and drying beds involve the discharge of brine waste to a pondfor storage and dewatering via the natural process of evaporation. Typically, such ponds are designedto have large surface areas to maximize evaporation of residual water by the sun and wind. Pond sizeis determined by waste flow and storage capacity requirements.As previously mentioned, evaporation ponds and drying beds are used primarily for brinewastes generated by RO and IX. Such processes produce large volumes of liquid wastescharacterized by high levels of total dissolved solids (TDS), for which mechanical dewateringprocesses, such as filter presses, are impractical. Depending on the solids concentration of the brinewaste stream, intermittent removal of solids may be required. For brines with a TDS content rangingfrom 15,000 to 35,000 mg/L, solids will accumulate in an evaporation pond at a rate of ½ to 1½inches per year (DPRA, 1993a). When the depth of the solids in the pond reaches a predeterminedlevel, discharge to the pond is halted and evaporation is allowed to continue until the solidsconcentration of the waste is suitable for disposal.The use of evaporation ponds is extremely land intensive, requiring shallow basins with largesurface areas. This can be an important consideration in densely populated regions. Reverse osmosisprocesses produce very large volumes of liquid wastes which increases land requirements andultimately construction costs. As a result, evaporation ponds may not be suitable for large watersystems utilizing RO. Evaporation ponds and drying beds have few O&M requirements, but are onlyfeasible in regions with favorable climatic conditions, i.e., high temperatures, low humidity, and lowprecipitation (DPRA, 1993a). Waste streams with low TDS concentrations can be discharged to thesame pond for several years (assuming adequate total capacity) before solids accumulation warrantsremoval.4.2.4 Storage LagoonsLagoons are the most common, and often least expensive, method to thicken or dewatertreatment sludges. However, lagoons, like evaporation ponds, are land intensive (DPRA, 1993a).Storage lagoons are lined ponds designed to collect, retain, and dewater sludge for a predeterminedperiod of time. Dewatering occurs through the evaporation and decanting of the supernatant. Lagoonsize is determined based on the volume of sludge that must be handled and the desired storage time.As with evaporation ponds, when a lagoon reaches its design capacity, solids can be removed bymeans of heavy equipment and shipped for disposal.4-4