The rapid growth of world population - doubling every 20 years and currently heading for 7 billion people - means significantly increased global water use and resultant decrease in availability on a per capita basis. Couple this with the fact that Brazil, Russia, India and China (collectively known as the BRIC countries), and countries in Southeast Asia such as Malaysia, Indonesia, Thailand and Vietnam have both very young populations - all wanting the trappings of Western life - and are rapidly building up their own water-demanding industries and infrastructures. It is becoming clearer day by day that there soon will not be adequate readily available fresh water for all of us on this planet!
As a response to the unsustainable climate and resources position within which we find ourselves, in recent years, various national and international “green” organizations have sprung up. These organizations, focused on promoting conservation and sustainability, have developed best practices such as “green” water management programs for water-based heat-transfer systems. Also, the U.S. Department of Energy provides free Internet-based software for industrial water and energy assessments, optimization and savings.
Fresh water is a limited resource and, after the needs of agriculture, aquaculture and commercial, institutional and domestic users have been met, the remaining balance available for utility power generation, mining and general industry is reducing year-on-year. It also is getting pricier to obtain and to discharge, as discharge standards continue to get tougher. Thus, it makes both environmental and economic sense for industry at large to continue to find innovative ways to:
- Reduce water intake.
- Recycle and reuse water for other purposes.
- Seek to limit discharges.
This “green” approach to industrial water reuse is especially pertinent in the United States as we have less than five percent of the world’s population, yet we use 22 percent of its fresh water. To be fair, the U.S. power generation industry has been in the forefront of developing water usage minimization programs for many years, as have some other industries. However, it remains a fact that many industries have poor water conservation records and fail to employ even the most basic of water reuse technologies, apart from the now ubiquitous cooling towers and evaporative condensers.
Even with recirculating cooling systems, which may represent up to 50 percent of the water intake for a facility, the “green” picture is poor: Less than one percent of cooling systems employ recycled or reuse water as their primary source of makeup. Yet water savings and reuse also generally mean energy savings and other efficiencies - and these can represent significant dollar improvements to the bottom line. So consider how “greening” your water management can also improve your profitability.
Where to Start with Industrial Water Recycling?
Water analyses, mass-balance assessments (i.e., an understanding of all water flows from water-in to water-out) and similar evaluations for energy consumption are basic requirements. Minimum water utility consumption can be targeted, leading to process network design/retrofit and a resulting water cascade table, reflecting how water can be cleaned up or otherwise modified at a process discharge point and reused elsewhere rather than flowing to a central wastewater treatment point. Pinch technology/analysis software is used to identify the minimum process energy consumption required by calculating feasible thermodynamic energy targets (or minimum energy consumption), and achieving them by optimizing heat recovery systems, energy supply methods and process operating conditions.
Reduce, reuse, recycle are not just buzzwords: They are tools of an effective industrial water management and treatment program that can help processors improve their bottom line.
The key to any water recycling and reuse project is to have available a tool chest of water treatment technologies, from which certain tools can be withdrawn and employed in a specific configuration. There are many technologies today, and various ways in which they can be configured and used together to produce a desired result. For instance, for water reuse as cooling system makeup, some older technologies such as multimedia filtration and cold lime water softening still have a place in conjunction with newer technologies such as membranes.
Specifically, water treatment tools to use for reusing water can include the following:
Alkalinity removal via use of lime softening, sulfuric acid treatment or ion-exchange. Examples include a strong/weak acid cation (SAC/WAC) in the hydrogen form, plus degassing, or strong base anion (SBC) in the chloride form.
Dirt, grit, sand, slime and algae removal using a sand filter, multimedia filter, inclined plate clarifier, or conventional coagulation, sedimentation and clarification techniques with chemical coagulants.
Disinfection by UV, ozone, chloramines, chlorine dioxide, bleach or superoxidation using UV/peroxide to produce hydroxyl radicals.
Fats, oils, greases and hydrocarbon removal using coalescers such as API separators, corrugated plate interceptors (CPI), or induced/dissolved air flotation (IAF/DAF) units. A secondary filtration system also may be required using bentonite or organoclay.
Hardness and silica removal through hot/cold lime-soda softening or ion-exchange.
Heavy metal removal by use of coagulation and alkaline precipitation, or by carbamates.
Iron and manganese removal by aeration, coagulants, manganese greensand or permanganate techniques, or manganese oxide filters.
- VOCs, gases, ammonia, chlorine, color, turbidity, BOD and COD removal using various combinations of air scouring, activated carbon filters, redox or traditional aerobic-activated sludge methods, plus coagulants and flocculants.
For wide-spectrum contaminant removal, in addition to the above, various membrane technologies may be employed. These methods include microfiltration, nanofiltration, electro-deionization (EDI), reverse osmosis (RO), ultrafiltration (UF), vibrating UF systems, or membrane bio-reactors (MBR).
A benefit of MBR is its ability to accept a high loading compared to convention aerobic-activated sludge/clarification processes, resulting in a smaller footprint. However, designing an MBR also requires a good understanding of aerobic digestion processes. Additionally, a fundamental consideration is that all membrane technologies ultimately foul, and the membranes will need cleaning. So, effective pretreatment and a good quality clean-in-place (CIP) system must form part of an overall membrane system design (table 1).
Reusing Water Streams for Process Cooling System Makeup
Generally, cooling systems are accepting a diverse range of waters and - provided the water is reasonably consistent in quality - many industrial streams can be recovered economically and reused as tower makeup. Suitable candidates may include:
Waters used for washing, cleaning, dying, rinsing, melting, quenching, stripping, scrubbing, desalting, plating, surface coatings, fermentation, dust control, process liquors, steam heating and drying, cooking, pasteurization and domestic purposes.
Excess water that results from alcohol and spirit distillation, sugar/fish-meal/orange juice evaporator condensates, boiler blowdown, chemical manufacturing, recovery of fibers and chemicals, straining, filtration, drainage, storm water storage and other process operations.
Water from high-pressure boiler blowdown, softener and filter rinsing, steam condensate, RO-reject water, acid wash neutralization, electronics wash waters, produced water from oil and gas production, and refinery sour water stripper bottoms.
Waters from equalization (EQ) tanks and holding ponds, run-off, mine drainage, and tailings from mining operations and irrigation.
- Water from treated municipal wastewater.
An important consideration is determining what level of contaminants can be tolerated in reuse water for process cooling makeup.
Generally, the purer the better; however; RO or fully softened reuse waters tend to be very corrosive, and the presence of some calcium is to be encouraged. Additionally, most applications need adequately high cycles of concentration (COC) in the recirculating cooling water to effectively save water and justify the project. COC is the ratio of concentration of total dissolved solids (TDS) in recirculating water to that of the makeup water. It often is reported as a ratio of their respective chloride concentrations. COC relates to the increase in TDS over time in the recirculating water because as evaporation of pure water takes place from the tower - providing the cooling effect - and the pure water is replaced by makeup water containing additional TDS. If the user can identify a minimum COC (say, four to six times), the water treater then can place a limit of contaminants in the reuse water, so as to not create an uncontrollable problem in the cooling system when cycled up.