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+clc;
+//Example 28.7
+//page no 435
+printf("\n Example 28.7 page no 435\n\n")
+//waste water treatment plant 
+//we have  to compare the total nitrogen discharge from the watershed with that of the city 's sewage treatment plant
+q_w=10//flow rate from waste water treatment plant
+c=35//nitoren concentration,mg/l
+m_dot_w=c*q_w*8.34//discharge   from the treatment plant
+printf("\n fdischarge from the treatment plant m_dot_w=%f lb/day",m_dot_w);
+S=8//area of watershed,mi^2 
+r=0.06//rate of rainfall,ml/day
+n=.5//50% rain reaches the sewers
+q=n*r*S*(5280^2/(3600*12))//volumetric flow rate of the runoff
+c_r=9//tota; nitrogen conentration in runoff,mg/l
+rho=62.4///density of water
+m_r=q*c_r*1e-6*(3600*24)*rho//total nitrogen discharge  from runoff
+printf("\n total nitrogen discharge m_r=%f lb/day ",m_r);
+ //since the durinf rain ,the runoff is over 2.5 times that for the tratment plant