A large industrial complex in the Southwest evaluated their process wastewater streams to determine which streams had the most potential for reuse as cooling tower make-up water.
Cooling tower make-up water accounted for 43% of the water used at the complex. Make-up water replaces water lost mainly from blowdown, evaporation and drift as well as sidestream filter backwash (only about 2% of the total requirement).
The second part of the study examined which technologies would provide the most effective pretreatment. The study concluded that as much as 14% of the plant's water demand could be conserved.
The lessons learned by operators of this industrial complex are applicable to chemical plants, pulp and paper mills, refineries and other industrial manufacturing facilities considering wastewater reuse.
The evaluation selected eight main water streams for possible reuse: air-handling unit condensate, cooling tower side stream filter backwash, cooling tower blowdown, boiler blowdown, softener and regeneration backwash, wastewater and graywater and stormwater runoff. For each of these streams, the site identified the total volume, reuse potential, water quality and the treatment technology required for reuse.
In terms of water quality, each stream was analyzed for TDS (total dissolved solids), calcium, pH, TSS (total suspended solids), and NTU (nephelometric turbidity units).
Reuse potential considers the requirements for pretreatment, treatment, water recovery, space, capital and operating costs, cost-effectiveness ($/gallons/day reclaimed) and operating advantages/disadvantages.
The major conclusion of the study is that cooling tower water reuse from blowdown is the most cost-effective program. This is based upon the initial water quality as well as the reuse potential analysis.
The recommended solution is pretreatment with multimedia filtration for TSS removal and ion exchange (dealkalization softening) for TDS removal. The advantages are low capital and operating costs, less maintenance and higher water recovery. The photograph above shows the PEP solids recovery system (SRS) that is installed with a cooling tower. (Not shown are the polishing bag filters or further treatment).