1. Problem using reactor analysis in design. Design a multi-pass baffled chlorine contact tank for a wastewater treatment plant serving town with a projected population of 30,000. ------- Effluent ---------------- Influent Sketch of a three-pass baffled chlorine contact tank. The Recommended Standards for Wastewater Facilities (10-States plus Ontario) include: An average daily consumption of 380 L/cap/day. The chlorine contact basin has a minimum contact time (or hydraulic residence time = V/Q) of 15 min at the maximum hourly flow and meets the state /provincial effluent standards. For some jurisdiction the minimum detention time may be as high as 45 minutes. To allow for maintenance there should be multiple tanks, and the system should be able to adequately treat 100% of the average flow with one tank out of operation. (in this case use two tanks which is the norm) The two tanks together should be able to achieve the effluent standard under peak hourly flow conditions (Each taking 1/2 the flow). The peak hourly flow factor (factor to multiply by the average flow to obtain the peak hourly flow) will be at least lpeak hourly 18+ VP =- were P is the population in thousands laverage 4+VP Assume in this case the standard is 2000 faecal coliforms counts (FC) per liter, and the influent to the basin will have 10 FC/L. Based on what you have seen at other plants and the Metcalf & Eddy book you find that the key tank sizing criteria include: at least 3 passes should be used (some use up to 7 passes). So Volpass = Total Vol./ (# of passes) = ( Length (of pass) * width (of pass)* depth) / (#passes) all the passes generally have the same size the total length (i.e., from the multiple passes) to width ratio for each pass is at least 20:1 and preferably 40:1 or higher. The tendency is that with a greater number of passes you shorten the length of the individual pass. So [Length (of pass) * (#passes) / width (of pass) ] > 20 the cross section of the pass have a width to depth ratio ranging from 1:0.5 to 1:2. Ax =(width of pass)*(depth) so Ax ranges from 0.5*width to 2*width assume the sides of the tank will have a freeboard (extra depth) of at least 30 cm to avoid overflow. Assume that each pass can be simulated as a CSTR with some dead (inactive) volume, the active volume fraction of 0.85. So if you choose a three pass system it can be simulated as three equal size CSTR reactors in series. For the selected chlorine dose, the kinetics of disinfection are assumed to be IFC= -10 5. FC where ric is the rate of faecal coliform bacterial destruction (counts/m/d); and FC is the faecal coliform bacterial (counts/m?). TASK: a) Check if a three pass tank will reduce the concentration sufficiently to meet regulatory requirements. If not expand the number passes to meet the requirement. b) Design the contact tank, i.e. select the dimensions of the tank, the number of passes, the total hydraulic residence time. c) How many passes would be required if the effluent standard is 200 faecal coliforms counts (FC) per liter