We call it Earth, but it should be called the ‘Water Planet.’ Over three quarters of Earth is covered with water. The Earth contains 326 million cubic miles of water. (One cubic mile holds over 1 trillion gallons of water.) Truly, there is no shortage of water on the ‘Water Planet.
So why is there so much concern over water conservation? The problem is not one of water volume, but water quality and distribution. Approximately 97% of the world’s water supply is salty and unfit for direct human consumption. Only 3% of water is fresh, but 68% of that is locked up in the ice and glaciers in Antarctica and Greenland. Another 30% of freshwater is in the ground. Surface water sources, such as rivers, constitute only one ten thousandth of a percent of fresh water supplies. But river water is the primary source of what most people use.
Since water is necessary for life, just like the air that we breath, people should have the right to unlimited amounts of clean, sanitary drinking water. The world’s increasing population, however, is stressing our limited supply of fresh surface and groundwater resources to the point of exhaustion.
One way to attack this problem is to better manage the available fresh water supplies so as to improve water productivity. Water productivity is defined as the value of goods and services produced per unit of water used. In essence, effective water management strategies include finding ways to produce the same result by consuming fewer water and energy resources. This includes:
- Educating consumers on the value of fresh and recycled water supplies.
- Reducing the volume of higher-quality water used to generate goods and services.
- Finding opportunities to use lower-quality water in place of higher-quality water.
- Using regeneration technology to produce higher-quality water from lower-quality sources.
CHANGING ATTITUDES ABOUT WATER
The average person on the street judges water quality by how it looks, smells and tastes. We like the idea that we are drinking water that is clean and disease-free. Pure drinking water harvested from a mountain glacier has strong public appeal as compared to drinking recycled wastewater even if the water quality is identical.
Just as the thought of using recycling wastewater as a supply source for drinking water carries a strong “yuck factor,” the same prejudice exists in industry against using lower-quality, non-potable water for utility and process applications. Although the technology exists to produce excellent quality water from plant wastewater, workers tend to decide intuitively whether a technology is good or bad. Once they have passed judgment on the water source and treatment method, they tend to over-estimate the risks and downplay the benefits. Hence, many industrial water conservation opportunities are stopped or stalled by this logic-defying thought process.
Any city at the end of a river, such as the Mississippi for example, is using the recycled wastewater from cities located upstream. Likewise, Chicago obtains its drinking water from Lake Michigan, which includes the recycled wastewater from Milwaukee. We must think about water in terms of its quality at the point of use rather than where it came from in the recent past.
Industrial water conservation projects require corporate support from leadership and plant managers. This also includes providing training programs to educate personnel on water quality and treatment technology. Further, on-going promotion is required to keep water-saving projects on-target and out of the weeds.
IMPROVED WATER MANAGEMENT
The first step in the industrial water conservation process begins with determining where and how water is used in the plant. This involves the preparation of a water balance that identifies the various sources used and sinks. Representative water samples should also be analyzed to verify the water quality. This is equally as important as water quantity. Experience indicates that 70% of the success of water management efforts comes from this activity as compared to 30% coming from developing alternative treatment technology.
An accounting spreadsheet can be used to track the monthly freshwater withdrawals and wastewater discharge volumes. In the early stages of this process, it’s common to find that as little as 50% of fresh water withdrawals can be accounted for. Much of the unaccounted water use arises as a result of leaks and unintended overflows to drains. Leaking distribution piping can waste large volumes of water. Using pipeline leak and flow detection systems is helpful in finding and correcting these water wasters.
The accounting spreadsheet can be organized by the water application. Industrial water can be categorized as:
- Process uses such as rinsing, washing, or incorporation into the product
- Utility uses such as cooling towers and boilers
- Domestic uses such as drinking and cooking
- Sanitary uses such as restrooms
- Recycling uses such as steam condensate and recycled wastewater effluent.
Tracking where and how water is used will help identify opportunities to improve the water productivity within the plant.
The water balance may reveal opportunities to convert systems from “high use” to “low use.” A common approach here is to replace faucets, toilets and showerheads with low-flow versions. This may make a significant contribution to water savings in office buildings and motels, but in a large industrial manufacturing enterprise, this represents a small percentage of the total water demand.
Utility applications such as cooling towers and steam boilers represent a large percentage of water consumption. If some or all of the cooling load is handled by a once-thru system whereby the water is passed through the point of heat exchange only once prior to disposal, significant savings can be achieved by installing a cooling tower. Cooling towers are water conservation devices that save water by rejecting heat to the atmosphere by evaporative cooling and then recycling the water back to the heat exchanger or process.
If the plant is equipped with a cooling tower(s), optimum efficiency is achieved by operating the tower at maximum cycles of concentration (COC). Installing water meters on the makeup and bleed enable one to determine the COC by the ratio calculated from the makeup volume to the bleed volume. Alternatively, the ratio between the specific conductance of the tower water to the specific conductance of the makeup can be used. A practical COC target is 10, but this varies depending on the local water quality and could be more (or could be less).
Many cooling towers are operated using potable, fresh water makeup. Many opportunities exist to replace fresh water with water than can be reused from other systems such as the reject from reverse osmosis arrays and treated municipal or industrial wastewater. Reusing water from other processes for cooling tower makeup conserves on fresh water withdrawals and reduces wastewater volumes.
In arid parts of the country, rainwater harvesting is a viable option for reducing the demand on surface water supplies. Likewise, collecting stormwater runoff for utility use has merit. The collection system and treatment requirements for the removal of suspended solids, metals, oils and nitrogen must be considered prior to use.
The idea of treating wastewater for direct addition back into the potable water supply has not gained any traction in the U.S. Although technically feasible, as mentioned previously, the “yuck factor” is too great. This “toilet to tap” concept has a ways to go before gaining acceptance by the general public.
The indirect recycling of treated wastewater into the public drinking water supply, however, has gained some momentum in areas of the US and throughout the world. The treated wastewater is injected into the ground as a replacement. The barrier created by discharging treated wastewater back into the environment seems to offer the general public some peace of mind that the water has been some how naturally purified prior to consumption.
Industrial plant wastewater can be successfully treated and reused for many utility and process applications. This includes “low treatment” technologies such as sedimentation and filtration and “high treatment” methods such as ultrafiltration (UF), reverse osmosis (RO), and ultraviolet (UV) sanitation. These technologies produce water that is acceptable for use as cooling tower makeup and boiler feedwater.
Sedimentation and sand filtration followed by aerated biological treatment removes suspended solids from the waste along with reducing the biological oxygen demand (BOD) and chemical oxygen demand (COD). The water may still contain a high concentration of dissolved salts such as sodium, calcium and magnesium carbonates, sulfates and chlorides. Although not suitable for potable use, this water may be acceptable for utility applications such as backwashing filters, cooling tower makeup, toilet flushing, and wash-downs.
Further treatment by ultrafiltration (UF) and reverse osmosis (RO) greatly improves the water quality making it suitable for “high treatment” applications such as steam boiler makeup, process rinsing and various washing operations.
Ultrafiltration yields >85% recovery of the feedwater at fluxes of 23.5 to 44 gallons per square foot per day. Reverse osmosis, which is a higher level treatment method, recovers >70% of the feedwater at a flux of 11.8 to 14.7 gallons per square foot per day. Reverse osmosis is capable of achieving greater than 97% salt removal, which makes the reuse of treated wastewater feasible for most utility operations with little to minimal additional treatment.
The membrane separation process requires periodic cleaning of the UF and RO modules. Operating experience suggests that the anticipated cleaning frequency is every 2 to 3 months. Life expectancy of the membranes is projected to be 5 years for the UF modules and 2 years for the RO membranes.
Management evaluations and project approval is often dictated by financial considerations such as cost of capital and return on investment. The operating cost for an enhanced wastewater treatment process varies depending on the quality of the wastewater and the intended application of the treated waste. In general, pretreatment of the waste stream by sand filtration and aerated biological treatment followed by ultrafiltration (UF) carries a projected operating cost of $0.625 per 1000 gallons. Incorporating reverse osmosis (RO) into the treatment scheme results in a higher operating cost of $2.00 per 1000 gallons of product water. Given the average cost of fresh water withdrawals and waste disposal fees, many projects of this type offer a return on investment of 2 years or less. Again, these are general guidelines. Each project should be evaluated on its own merits.
Our fresh water supplies are wasted due to inefficiency and poor management. As a result, water productivity is negatively impacted as measured by the value of goods produced per unit volume of water consumed.
Implementing effective water management strategies addresses future and ongoing challenges of sustainability, depletion of energy reserves, water security and preservation of ecosystem health.
Water technologies and management practices currently exist to effectively capture, treat and reuse water. Further education and promotion is required to take full advantage of opportunities to preserve our fresh water supplies. This may require regulatory changes and certainly greater public acceptance. But effective water management is clearly the path to the future.