Power stations use large quantities of water and have substantial impacts on the environment. A power station consisting of $5$ units each with a generating capacity of $650$ MW $($megawatt$)$ uses wet evaporative cooling to cool the water from the condensers.

$($a$)$ Calculate the volume of water used daily $($in m$3/$d$)$ by the power station if the specific water consumption is $1\mathrm{.}85$ L$/$Kwh$.(5)$

$($b$)$ Assume that the water flow to each cooling tower is $7800$ L$/$s and that the water is cooled from $40$ to $25$ oC$.$ Losses from the system include evaporation, windage and blow$-$down. Evaporation is estimated at $2\%$ loss of the recirculating flow per $5$oC drop in temperature; windage $($very small water droplets lost from the cooling tower$)$ is $0\mathrm{.}5\%$ of the recirculating flow. The cooling water make$-$up equals the sum of losses. Calculate the volume of blow$-$down and the volume of make$-$up water if the TDS $($Total Dissolved Solids$)$ in the cooled water leaving the cooling tower is allowed to increase to a level of three times that in the make$-$up water. $(8)$

$($c$)$ The blow$-$down from the system has a TDS of $1500$ mg$/$L and is not allowed to be discharged. It has been decided to desalinate and reclaim the blow$-$down for reuse in the system. Both reverse osmosis $($RO$)$ and electrodialysis reversal $($EDR$)$ are considered to desalinate the water. Describe the main differences between the two processes and do an evaluation of each process for this application. Which process would you recommend? Motivate your answer. $(8)$

$($d$)$ It has been decided to install an RO plant to desalinate the blow$-$down from the cooling tower and to recycle the desalinated water to the cooling system. The RO plant has a TDS rejection of $95\%$ and the water recovery is $80\%.$ Draw a flow diagram of the system and calculate the volume of permeate and brine if the feed is $9\mathrm{.}50$ ML$/$d$.$ Calculate the TDS concentration in the permeate and the brine if the feed TDS is $1500$ mg$/$l$.(12)$

$($e$)$ Comment on the optimum membrane array configuration that would provide the highest permeate flow rate. $(5)$

$($f$)$ The permeate is recycled to the make$-$up of the cooling water system. This reduces the amount of fresh water make$-$up required to maintain the TDS in the water that enters the cooling tower below $1500$ mg$/$l$.$ Draw a flow diagram of the system, draw up a water$-$ and TDS mass balance for the cooling system incorporating the permeate recycle and calculate the volume of make$-$up required to maintain the TDS below $1500$ mg$/$l in the system assuming that conditions of evaporation and windage losses remain unchanged. The TDS of the make$-$up water is $250$ mg$/$l$.(12)$

$($g$)$ Calculate the area of an evaporation dam $($in ha$)$ to dispose of the brine from the RO plant if the average annual rainfall is $760$ mm$/$a and the annual average evaporation rate is $1150$ mm$/$a$.$ The water level in the dam is not allowed to exceed $1\mathrm{.}85$m after $3$ years.