Case Study: Small Scale Cooling Towers Bleed-Off (blow down) in Water Recycle System at a Local Hospital

Water in cooling tower gets concentrated due to evaporation of water for heat rejection. Apart from treatment against bacteria growth, every cooling tower condenser water system needs blow down to control the water quality to minimize scaling and corrosion. Approximately 7–10% of total water consumed by cooling tower will be bleed away with a typical cycle of concentration (COC) of 4–6 subject to make-up water quality. A nano recycle unit is installed at local Hospital in Singapore to recover 65–70% bleed-off water and recycle the permeate back to cooling tower as make-up water. The blow down recycle unit started to operate in mid Jan 2023 with the key performance indexes and observations of the recycle unit after 2 years of operation showed that the overflow constitutes more 21.4% blow down water loss, which was not recoverable; the recovery efficiency of the recycle unit is 65.3% at 1.61 kWhr per m ³ of permeate; COC improved from 12.7 to 34.6; during the first 6 months of monitoring, the minimal removing efficiency of the nano filter is above 90%, especially for hardness removal. For lowering conductivity and TDS, the nano filter performance is in-line with industry expectation (80–85%), at 81.7% and 82.7% respectively. Payback period after considering operation cost (S$ 0.76 per m ³ of permeate), maintenance cost (S$1,200 /year) and 6% discount rate is about 10 years. A net saving of S$ 1,100 p.a. can be expected after breakeven period at present prevailing water and electricity tariffs.