Pond volume

We want to estimate how much of the trifluoroacetate (TFA) would diffuse into the pore space of a small pond (volume V, area A) within one year. 

Due to an accidental spill, a significant amount of TCE (see just above) has been introduced into a small, well-mixed pond (volume V = 1 x 104 m3, total surface area A = 5 x 103 m2, T = 15°C). Measurements carried out after the spill, during a period of 1 week, showed that TCE was eliminated from the pond by a first-order process with a half-life of 40 h. Because export of TCE by the outflow of the pond can be neglected, and because it can be assumed that... 

AXIAL-FLOW CONVEYOR CENTRIFUGES. In 1970 Schnittger showed that the actual residence time in a conveyor centrifuge is much less than that given by Eq. 30.81). He proposed a model in which liquid flows in a thin boundary layer over an essentially stagnant liquid pond. The critical separation takes place in this layer, for once a particle moves from the boundary layer into the pond, it is effectively removed unless later resuspended mechanically. A thin boundary layer gives better separation than a thick one. The total pond volume is not important the pond needs to be no deeper than necessary to ensure that the compacted solids are well below the surface. 

The amount of land required varies as well, not only as a function of the amount of production that is anticipated, but also on the type of culture system that is used. It may take several hectares of static culture ponds to produce the same biomass of animals as one modest size raceway through which large volumes of water are constantly flowed. Constmction costs vary from one location to another. Local labor and fuel costs must be factored into the equation. The experience of contractors in building aquaculture facihties is another factor to be considered. 

Anaerobic ponds are loaded such that anaerobic conditions prevail throughout the Hquid volume. One of the major problems with anaerobic ponds is the generation of odors. The odor problem can frequentiy be eliminated by the addition of sodium nitrate at a dosage equal to 20% of the appHed oxygen demand. An alternative is the use of a stratified facultative lagoon, in which aerators are suspended 3 meters below the Hquid surface in order to maintain aerobic surface conditions, with anaerobic digestion occurring at the lower depths. 

With a depth of 5 ft, total volume of the pond would amount to a 45.5-h holdup, which is more than the 24-h holdup required to maintain a fairly constant discharge temperature throughout the day. 

With the suggestion that the last common genetic ancestor is a hyperthermophile, the role of temperature on the origins of life is important. The lower temperature limit in water is limited by the phase transition from liquid to ice. This is a problem because the density of ice is lower than that of water and the increase in volume on freezing will cause the cell structure to become disrupted in the same way that pipes burst in the winter. The lower limit for bacterial growth reported so far is -20°C, which is the temperature at which intracellular ice is formed. Adaptation to the cold requires a considerable salt content to depress the melting point of water the Don Juan Pond in Antarctica, which has a saturated CaCE solution, preserves the liquid phase at temperatures as low as —53°C. 

The background of this study is the investigation at the site Bielatal (see Daus et al. this volume). The arsenic concentrations in the seepage water of this tailings pond are high (up to 4 mg/1) and natural arsenic precipitation processes are incomplete. The neutral pH, the presence of both arsenite and arsenate (sum 1 mg/L), and the oxygen saturation of the water are the boundary conditions of the described experiments. 

The pilot and experimentation plant at the Pohla mine site was built in a eoncrete pond (part of the former mine operation), with a footprint of 475m and a total volume of 830m. After the installations were fitted, the usable volume was 415m. The installations divided the concrete pond into seven treatment basins (eells) whieh served various purposes ... 

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. 

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 required surface area of a pond is dependent upon both the volume of concentrate and the evaporation rate. Surface area, A (m ), can be estimated using the following equation ... 

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 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). 

Rainwater is generally of good guality, free from contaminants, and relatively low-pH, making it suitable for use on all types of plants. Surprisingly large volumes can easily be harvested from roofs via gutter downspouts and stored in rain barrels, tanks, and ponds. 

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