Hi all, I am have designed a small CW system for treatment of greywater with a septic tank as pretreatment to settle out solids.
I need to pump from the septic tank to the CW four times a day (for batch loading). I would appreciate some help in calculating the pumping cost. The outline is as follows:
Ian gave a great response and I'd like to add a couple of comments.
1 ) You stated the pump power consumption. Where did this number come from and is it actually the power consumption of the pump? If you know how much power the pump is consuming then you can calculate pump efficiency directly from the power equation.
2 ) How long does the pump run per day? Given the data presented, the cost is Power x Run Hours x CostElectricity/Hour = Cost/Day. Of course this is assuming that the power consumption is the actual consumption of motor (see 3 below).
3 ) A typical electrical motor in the US is about 94.5% efficient in converting electrical power to shaft horsepower (SHP). If you divide 3-phase power by 0.746 this will give you the equivalent horsepower. As the motor is not 100% efficient, you will have to multiply the equivalent horsepower by 0.945 to get the actual horsepower applied to the pump shaft.
4 ) Assuming the power consumption stated is actual as read by a power meter then the simple answer is Operating Cost = Power x Hours x CostPerKw. You don't need anything else to determine cost, however to do this correctly you should evaluate the actual performance of the pump to see if it needs to be replaced/rehabilitated.
If you wish, I can send you the cost calculation Excel spreadsheet that we have developed in the provincial government to calculate pump electricity costs. My email is: kurt.rasmussen@SaskWater.com
the basic equation to determine the power required to pump is P = QWH / e where P is power in KW, Q is your flow rate in m3/s, W = weight of water = 1000 kg/m3 x gravity = 9.81, H = head to pump which comprises the static head that you can measure plus the friction head which you calculate from the flow rate and pipe type and diameter. e is the efficiency of the pump and motor which you could estimate initially as 50%. Although your pump is rated at 0.45 kW, it will probably operate at a lower power consumption which you will determine as above. Making some assumptions - pump for 1 hour 4 times per day gives Q=2m3/h, static head = 4.5m and friction head for 20m length of a 40mm HDPE pipe = 0.12m hence total head = 4.62m. In this case P = 2/3600 x 1000 x 9.81 x 4.62 / 0.5 = 50 W or 0.05 kW. So with these assumptions your pumping cost per day would amount to 4 x 0.05 x N$2 = N$0.4 per day or N$12 per month.
From the description, you do have a pump already installed. Do you have the rating curves of this pump? Would come with the pump. Otherwise specify age of pump and manufacturer/model for a guess. But for a rough estimate use 0.45kwh X N$2/kwh X 1.5 (accounting for inefficiencies and safety mrgin) X no of hours of operation estimated from.specific pump discharge