United Arab Emirates University College of Engineering Dept Civil & Env Eng Name Student ID# CIVL375: Water and Wastewater Technology 2nd Semester 05-06 Section Mid-Term Exam Max Grade 20 Time 1:15 (3.0 pts) This problem is related to the schematic below which shows a flow diagram of a domestic wastewater treatment plant Select the correct answer Wastewater BS TS GC AT PS SS Chlorine Treated wastewater CT F BS=bar screen AT=aeration tank CT= contact tank AC TS= traveling screen GC= grit chamber SS= secondary settling tank F= filtration PS=primary settling tank AC= activated carbon a The type of particle settling encountered in the grit chamber is (are) (i) Type I settling (ii) Type II settling (iii) Type III settling (iv) Type III and IV settling b The type of particle settling encountered in the primary settling tank is (are) (i) Type I settling (ii) Type II settling (iii) Type III settling (iv) Type III and IV settling c The type of particle settling encountered in the secondary settling tank is (are) (i) Type I settling (ii) Type II settling (iii) Type III settling (iv) Type III and IV settling d The objective of aeration in the aeration tank is to (i) increase dissolved oxygen (ii) reduce the pH (iii) remove undesirable gases (iv) increase the pH e The activated carbon reactor shown is designed as a (i) batch reactor (ii) plug flow reactor (iii) continuous-stirred tank reactor (iv) packed bed reactor f If the primary settling tank of this treatment plant is 12-ft deep and the tank receives a flow rate of million gallon per day The minimum weir length that should be provided for this tank is: (i) 10000 ft (ii) 100 ft (iii) 12 ft (iv) 0.01 ft (1.5 pts.) In the design of a wastewater treatment plant, the quality of the effluent and the capacity of the plant are basic requirements for the design On what basis each of these two requirements is determined? (2.0 pts.) The US EPA states that “bacterial quality criteria for drinking water from public supplies require not more than total coliform/100 ml as the arithmetic mean of all water samples examined per month, with no more than coliforms/100 ml in any sample if the number of samples is ≤ 20/month or no more than coliforms/100 ml in 5% of the samples if the number of samples exceeds 20/month" The effluent of a water treatment plant that serves 15,000 people was sampled at regular intervals over a month period for coliform testing and the results are shown in the table below: Sample number Coliforms/100 ml 1 3 10 11 12 13 14 15 16 Based on the above results, are the number of samples tested and the effluent bacterial quality acceptable? Explain (3.0 pts) The cumulative volume of water consumption of a residential town that has 90,000 is presented below An elevated water tank is to be built so that water is pumped from a water treatment plant at a constant rate to the tank and is then transferred by gravity to the residents Cummulative flow, million gallon 0 12 16 20 24 T ime, hr a (2.0 pts) What is the design capacity of the storage tank? b (0.5 pts) What is the pumping rate from the water treatment plant into the tank? c (0.5 pts) Based on the given data, what is the average daily per capita water consumption in the town? (2.5 pts) What detention time is needed to remove 99% of a contaminant in a plug flow reactor with a reaction rate constant of 1.5 per day? (3 pts) A sedimentation tank 3.5-m deep has a flow-through velocity of m/hr and an over-flow rate of 0.92 m3/(hr.m2) What is the length of the tank? (2.0 pts) A secondary settling tank is 4-m deep and receives a flow rate of 7400 m3/day with suspended solids concentration of 2,200 mg/l a (1 pts) What would be the sludge depth in the tank for a settled sludge with a suspended solids concentration of 12,000 mg/l? b ( pts) If the detention time required to reach the solids concentration in part a is hr, what is the surface area needed to achieve this? (3.0 pts) Using settling trajectories determined by the settling column analysis shown in the figure below, find the overall percent removal for a settling basin 3.0-m deep with a settling time of 30 minutes Note that solid lines indicate percent removal of suspended solids 0.5 75% 1.0 60% 1.5 Depth (m) 2.0 15% 30% 45% 2.5 3.0 10 20 30 Time (min) 40 50 60 POTENTIALLY USEFUL INFORMATION GLUMRB Recommendations Rapid mixing: Detention time≤ 30 sec Slow mixing (flocculation): Detention time > 30 Velocity through tank= 0.5-1.5 ft/min Paddle velocity= 0.5-3 ft/sec Sedimentation: Detention time ≥ hrs Velocity through tank ≤ 0.5 ft/min Weir loading≤ 20,000 gpd/ft Over-flow rate= 500-800 gpd/ft2 C = C o + k (V / Q) C = e − k (V / Q ) Co  4kt R  a = 1 +  Pe   C 4ae ( Pe / 2) = Co (1 + a ) e ( aPe / ) − (1 − a ) e ( − aPe / 2) ∑ tC t= ∑C C o H o = Cu H u ∑t C − t = ∑C σ2 v= Q Ax σ θ2 = σ2 t2 vo = Q As 0.5 and Pe = uL/D σ θ2 = Ac = Q vo D D − 2( ) (1 − e −uL / D ) uL uL At = Q × tu Ho Number of samples taken for coliform testing based on population served by treatment facility