Design of a modified low cost treatment system for the recycling and reuse of laundry waste water

A low cost system was designed for the recycling and reuse of waste water from a laundry unit. The unit was associated with a petroleum refinery in Pakistan. A survey was made to check the quantity and quality of waste water generated. The unit was discharging 1.90 m3/day of waste water into the natural water course. A small size sand and gravel filter was made from 0.00125 m sand, 0.0125 and 0.025 m gravels. The sand and two sizes of gravels were arranged in three layers. Each layer was 0.10 m deep. The three layers were placed in a stainless steel hopper and waste water was passed through the filter. Both, the waste water and treated water were analyzed and compared with tap water. The physical and chemical parameters considered were pH, turbidity, total suspended solids (TSS), total dissolved solids (TDS), chemical oxygen demand (COD), biochemical oxygen demand (BOD), total hardness (TH) and iron contents. The results showed that the filtration process reduced the pH, turbidity, TH and TSS to acceptable limit and TDS to some extent. On the other hand it had negligible effect on COD and BOD. The treated water was ranked as low-grade water and was found to be suitable for use in the laundry unit for the first rinse only. Based on the obtained results large-scale treatment process was designed for the waste water. It was a low cost treatment system consisting of sedimentation and filtration. It was found that the total capital and operating cost per year of the system was US$ 1343. The daily saving of water was 1.80 m3 with a payback period of only 1 year in one case and half year in the second case. In the first case the price of fresh water was taken as US$ 2.2/m3 and in the second case as US$ 4.4/m3.