Design and simulation of a reject water recycling system for the reverse osmosis plant : case study : Coca-Cola Century bottling company-Mbarara plant.

Abstract

Reverse osmosis (RO),is a system that purifies water by removing about 98% of total dissolved solids, organics and heavy metals. However, it rejects large quantities of water with a raised content of minerals normally called concentrate (reject water). At present, RO technology has the actual producing water rate in the RO process as only about 50-75%, so it still faces serious discharge problem of rejected water (Shahalam, Al-rashidi and Abusam,2010).

The main objective of this project was to design and simulate a recycling system for the reverse osmosis reject water for Coca-Cola Century Bottling Company-Mbarara branch. The project encompassed a review of literature about the reject water and recycling systems, characterizing and quantifying this reject water, design of the recycling system components for reject water, simulation of the recycling system and finally economic evaluation of the project.

The project was carried out using different engineering principles, methods and equations. The reject water was characterized using laboratory tests; the quantity was got from primary data as an average of several months. The system was designed per component where: water pump and distribution pipes were obtained using Darcy-Welsbach and continuity equation, the storage tank was sized using the peak discharge of reject water. The caustic soda dosing pump was obtained using the chemical dosage equation. The simulation was done in Arduino and Proteus. Finally, the economic evaluation was done using benefit cost ratio analysis and payback period method.

It was found out that the reject water parameters, total dissolved solids (121<500mg/l), electrical conductivity, turbidity (O.2<10NTU) were within the required national standards of portable water apart from PH0 which indicated acidity. A PH correction system was put to neutralize the reject water with caustic soda. The average quantity of reject water discharged per day was 25M3 and peak discharge as 40M3, so the storage tank had a volume of 40M3. The project was found to be economically feasible since the benefit cost ratio was 2.58 greater than 1 and payback period of 2 years

The design once implemented, it will correct the PH of reject water to portable water standards and eliminate the cases of corrosion in pipes and tank: hence very safe for floor cleaning, car washing and toilet flushing. This will reduce on the mains water demand and costs for the plant.