By Bryan Grotz
How can we place a value on water? It is the most used substance on earth and, of course, without it we’d have no concrete. But if it is such an abundant resource, what is the incentive to conserve its use?
Actually it is fresh, clean water that holds the most value. Whenever clean water touches anything in the plant, though, the water is not clean anymore; its status drops a notch or two to “process water.” And by law, all manufacturing facilities must collect this process water and discharge it in a manner prescribed by the EPA and state agencies. So then it becomes a prudent matter to reuse process water as much as possible before discharging it.
The harvesting of rainwater also makes sense, and rainwater and process water can both be used for water-dependent chores that don’t require clean water, such as coring and wash-down. This practice not only can save a precast plant some money on production costs, it can also distinguish it as an environmental steward – and a strong advocate for LEED points.
Process water characteristics
Process water is characterized as caustic, high in dissolved and total suspended solids (TSS), and high in pH (with a typical pH range between 11 and 12).1 Process water is typically discharged into the municipal or county sewer system after being treated to meet local water quality requirements. Some precast concrete plants combine process water and stormwater into one treatment system, while others direct the two streams into separate systems. Federal and local laws usually require discharge water to have lower TSS and pH (≥10) levels than process water.
Treatment of process water often includes a concrete grit separator or a holding tank with a series of baffles to allow solids to settle out. While in the holding tank, suspended solids will settle to the tank’s bottom, and the water will come into equilibrium with the air (including carbon dioxide, or CO2) around it, allowing the pH to decrease.
The pH and TSS continue to decrease as water is pumped into a second holding tank. In the second holding tank, process water can be dosed with sulfuric or hydrochloric acid for even lower pH. A lower pH can also be achieved by mixing more CO2 into the process water through the use of a bubbler or with the addition of dry ice. A number of factors affect the pH of process water, so it is wise to consult a chemical engineer when designing a plant-customized water treatment system.
It’s clear that costs are involved in meeting requirements for discharging water. However, using clean drinking water supplied by local utilities for process water only contaminates water that would otherwise be safe to drink, and the producer incurs additional costs in the time, money and energy spent purifying process water to acceptable standards. Since every precast plant must use, treat and discharge water, why not take advantage of recycled process water?
Recycling process water: going the “extra mile”
Concrete is made with potable water. Municipal or drinking water is favored, because it has consistent quality regulated by law. But now, efforts to reuse process water from homes and industries alike have increased, including precast industry initiatives to recycle process water. Treated process water remains nonpotable, but it has significant advantages in manufacturing.
Using treated process water rather than discharging it (and continuing to deplete the drinking water supply) requires that a producer take the added initiative to install a water recycling system. When it does, the costs of discharging process water diminish. That’s good news for the producer and the environment.
Treating process water often means additional reduction of pH and TSS to produce water adequate for production. Treated process water can be reused to produce concrete as long it meets ASTM requirements. ASTM C1602 states that, when making concrete with nonpotable water, the compressive strength must remain within 90% of the control mix with set times not > 1 hr. or > 1½ hr. of control concrete.2
This ASTM standard also lists optional chemical concentrations for mixes using recycled process water along with a mixture comparison guide of mixes made with potable and nonpotable water.
To bring process water to a quality that yields a good mix, some precasters use a mixture of treated process water, stormwater and municipal water. Additional water can dilute high TSS concentrations and lower pH to better meet mix requirements. Some precasters have even noted a better product finish as a result of using recycled water. In particular, they report better-looking surfaces with fewer bug holes. Improved surface appearance is most likely due to the increased level of fines (from TSS remaining in gray water) in the mix.
Are you ignoring on-site assets?
Recycled water can also be used in other ways, not just in concrete mixes. In the precast industry, collected stormwater or treated process water will often be suitable for toilets and irrigation. Rainwater harvesting systems recycle gray water3 from bathing, sinks or washing machines. By taking advantage of free rainwater and gray water, one can store useable water without relying on drinking water sources. The stored water is doubly important for regions subject to severe droughts.
Collected rainwater needs little to no treatment as long it is used within a short time, because if it sits too long in a recycling system, it can develop odors from decaying organic matter.
Downspouts and swales (longitudinal site depressions) are used to collect rainwater and direct it to a holding tank. (A surface swale directs water to the holding tank after natural “filtering” by surface vegetation.) Stored rainwater is then pumped from the tank as needed. Fountains can be added to the system for minimizing the buildup of organic decay and odors and for landscape watering.
Harvesting rainwater saves money and public drinking water from being literally flushed away. Efficient use of rainwater and process water decreases the volume of polluted water entering overtaxed combined sewer systems (CSS). The discharge of contaminated water into lakes and streams from heavy rains is a serious legal concern for large cities where EPA mandates now demand expensive CSS reconstruction.
Establish your plant as a LEEDer
Reusing process water and conserving and protecting water resources allow facilities to earn LEED (Leadership in Energy and Environmental Design) points (Pilot Credit 10: Sustainable Wastewater Management). LEED points are a green bonus that markets your plant as a good environmental steward to informed specifiers and customers. LEED certification means that your facility is reaping the economic benefits of lower energy costs, tax rebates, new equipment rebates, zoning allowances and other federal, state and utility business incentives.
As new EPA rulings and LEED specifications require environmentally friendly technologies in a project’s material selection, precast concrete producers will find it makes good business sense to adopt recycling systems earlier than later. Currently, precast concrete facilities are advancing green production processes and benefiting economically from more cost-effective water recycling measures. These green initiatives not only save production costs in the long term, but elevate your plant as the go-to producer for today’s specifiers and owners.
Bryan Grotz, a 2012 intern with NPCA, is majoring in environmental engineering at Colorado State University.
1 See the article “Muddy Waters” in the September-October 2012 issue of Precast Inc. for a description of pH balance in process water.
2 ASTM C1602, “Standard Specification for Mixing Water Used in the Production of Hydraulic Cement Concrete.”
3 Gray water is the recycling of ‘waste’ water that is generated in homes and commercial buildings through the use of water for laundry, bathing or manufacturing processes. Gray water differs from black water, which is discharged from toilets and garbage disposals into sewage systems. Gray water can be reused for a variety of purposes. Visit www.ecolife.com/define/grey-water.html