As enforcement of regulations governing discharge from precast concrete facilities becomes more stringent, precasters need to update themselves on washout best practices and compliance methods.
By Evan Gurley
Disposal of wastewater from batching operations at precast concrete plants is a growing issue for the precast concrete industry. As today’s precasters know, they cannot do things the way their grandfathers did. With stricter quality control standards and environmental regulations, producers must rely on creative, cost-effective compliance methods. In an effort to comply with these standards and regulations, minimize waste and reduce operating costs, proactive precasters have developed methods for treating and/or recycling concrete process water.
What defines “process water”?
Simply put, stormwater comes from the sky; anything else is likely process water. Process water is often related to coring operations and washout, but may also include stormwater that collects a measureable amount of cementitious materials from areas surrounding the production facility. Stormwater, on the other hand, is water that has not come into contact with source material. When stormwater mixes with process water, it all becomes process water. Stormwater can add pollutants to the facility’s discharge flows. Therefore, it makes sense to separate process water from stormwater whenever possible.
Concrete washout or process water is a slurry containing metals potentially toxic to plants and animals. Process water contains dissolved solids including: sulfates and hydroxides from cement; grease; oil and form release agents from equipment; small quantities of other chemicals associated with concrete production; and derivatives from chemical admixtures.1
Process water is naturally corrosive with a high pH.2 pH is the numerical measure of acidity, with a scale ranging from 0 to 14 (see Figure 1). A high pH is said to be alkaline, while a low number is acidic. A pH of 7 is neutral. A safe pH range for aquatic life is between 6 and 9, and an acceptable range for plant life is between 6 and 7. Process water, however, is alkaline with a pH of around 12.
Who regulates wash water?
The U.S. Environmental Protection Agency (EPA) monitors and limits types and magnitudes of the waste products a site is allowed to discharge into U.S. waters. Local sanitary and sewer authorities set limits for pH, total suspended solids (TSS) and chemical composition for discharged process water.
Production plants adjacent to lakes and streams will need to obtain a National Pollutant Discharge Elimination Systems (NPDES)3 permit, which is usually administered by the state.
Some states have industry-specific permits that cover the discharge of stormwater and process water from precast plants. The ready-mix industry’s concrete wastewater was curtailed by revisions made to the Clean Water Act in 1987, changes that include runoff from ready-mix plant yards and construction sites.
In a perfect world, precast plants would use a zero-discharge manufacturing process (recycling all process water). Leading precasters in the industry are striving for this goal, but total recycling of concrete process materials is often cost-prohibitive. Larger organizations are often able to absorb this additional expense.
Four methods to meet process water regulations
1. Lower the pH. Numerous systems and methods are available to reduce the pH of process water. A liquid with a high pH can be neutralized or returned to the safe range in a number of ways. For example, adding hydrochloric acid or sulfuric acid to the process water will return a slurry mixture to a neutral or safe range. Use great caution when using acids, as it is easy to overshoot the neutral range to a low and unsafe pH.
pH neutralization can also involve the use of solid or compressed carbon dioxide (CO2) gas. Neutralized stormwater may be discharged to surface waters under the General Construction NPDES permit, but this water must be managed to prevent any discharge to surface waters. Advantages of bubbling CO2 include:
• Rapid neutralization of high pH water
• Cost effective and safer to handle than acid compounds
• Self-buffering, so overdosing to low pH levels is unlikely
• Readily available
Process water’s volume, temperature, neutralizing agent and required discharge level will all have to be taken into consideration when adjusting the pH, so consultation with a chemical engineer is advised. Whatever neutralization method is chosen, all equipment must be handled in accordance with OSHA.
2. Recycle process water by the book. In 1978, ASTM C94, “Standard Specification for Ready-Mixed Concrete,” was revised to permit the use of wash/process water as mixing water in concrete. The Portland Cement Association (PCA) also assents to the use of wash water for mixing concrete that meets a maximum of 50,000 ppm TSS. Using different stipulations, AASHTO M157, “Standard Specification for Ready-Mixed Concrete,” lists criteria for using process water in the design of a hydraulic cement concrete. While ASTM, PCA and AASHTO all allow the use of wash/process water in the design of hydraulic cement concrete, impurity limits must be met. ASTM C94 maximum limits are:
• Chloride (Cl): 500 ppm
• Sulfate (SO4): 3,000 ppm
• Alkalies (Na2O): 600 ppm
• Total solids: 50,000 ppm
Since the late ’70s, ASTM C94’s process water standard remained unchanged until ASTM Subcommittee C09.40 on ready-mixed concrete developed new standards and test methods for water used in the production of hydraulic cement concrete. ASTM C1602, “Standard Specification for Mixing Water Used in the Production of Hydraulic Cement Concrete,” was a standard developed by Subcommittee C09.40. ASTM C1602 summarizes this specification as follows:
“This specification covers mixing water used in the production of hydraulic cement concrete. It defines sources of water and provides requirements and testing frequencies for qualifying individual or combined water sources. Mixing water shall consist of: batch water, ice, water added by truck operator, free moisture on the aggregates, and water introduced in the form of admixtures. Potable and non-potable water is permitted to be used as mixing water in concrete. The following are concrete performance requirements for mixing water: compressive strength and time of set. Density of water shall be tested or monitored with a hydrometer. Optional chemical limits for combined mixing water are given for: chloride, sulfate, alkalis, and total solids.”
ASTM C1602 further states that when qualifying the use of process water, all required tests should be performed at the highest TSS in the total mixing water anticipated during production. TSS equal to or less than the level qualified by testing may be used in mix water. At a minimum, concrete compressive strength and set time should be evaluated and compared to a control mix using 100% potable or distilled water. Compressive strength should be within 90% of the control mix, and the set time should not decrease by more than one hour or increase by more than 1.5 hours.
Recent research performed at the National Ready Mixed Concrete Association’s Alfred H. Smith Research Laboratory concluded the following when using process water in the design of hydraulic cement concrete:
• Water demand will increase as TSS increase and the process water ages.
• Water demand increases due to the fineness of the hydrating cement particles.
• Initial set time increases due to the increase in the amount of hydrated cement and calcium hydroxide.
• Compressive strengths are similar to mixes made with potable water.
• Concrete durability is similar to mixes made with potable water.
It should be noted that not all DOTs have accepted the idea of using process water in mixes.4
Various automated commercial systems for recycling process water are available. Some precasters have opted to design and install their own systems, providing a cost-effective solution for handling/recycling process water. Design of a customized recycling system requires persistence and patience. It is vital to do your homework, purchase and maintain the right equipment and properly train employees, particularly for high TSS fluids.
3. Explore stabilizing admixtures. The use of stabilizing admixtures can circumvent the necessity to remove wash water from the batching system, allowing the water to be reused for mixing additional concrete. The amount of admixture added depends on the amount of wastewater in the mixer and on the schedule for reuse of this water. These admixtures momentarily stop the hydration process, putting the cement in a dormant state wherein the cement in the process water will not harden into concrete nor will it adhere to the inside of the mixer. The stabilized water is then calculated into the next mix of concrete. The downside to stabilizing admixtures is their additional cost.
4. Pretreat discharge water. Requirements for discharging process water into local sewers vary among authorities having jurisdiction. Often the requirements for discharge into local sanitary sewers are less stringent than those required by the EPA and often allow for higher pH limits, a less onerous requirement for the precast concrete industry.
Local sanitary sewer departments often set TSS limits and may even allow for a pH as high as 10 for the discharge of concrete process water. These relaxed requirements often can be met by utilizing one of the following systems:
• Settling ponds
• Detention/retention systems
• Mechanical reclaiming units
• Precast concrete grit separators
• Holding tanks
• Sloped pits that flow into larger settlement tanks and, if necessary, a dosing tank (for pH adjustment)
These systems vary in size depending on the amount and content of process waters. Some plants will divert all wash water, coring water and even stormwater into one system, while others may use a number of smaller systems throughout their production operations.
Compliance isn’t an option
Compliance with more restrictive environmental regulations for plant discharge streams is a difficult but necessary task. Precasters must stay up to speed on a vast assortment of rules, permits and regulations for any concrete manufacturing process outflow that might cause pollution. But finding that affordable long-term solution for handling your plant’s process water will ensure peace of mind should a surprise inspection of your facility occur – especially one that comes with an environmental inspector eager to list every instance of noncompliance.
What to do if you discharge process water:
Obtain appropriate NPDES permit coverage. Failure to do so can result in significant penalties. NPDES permit requirements may include:
• Regular discharge monitoring for pollutants, such as pH, TSS, total dissolved solids and other pollutants
• Monitoring and reporting results
• Requirements to treat to ensure limit compliance of process discharges, such as pH reduction and solids removal
• Inspections, recordkeeping and documentation requirements
• Strict regulatory oversight
Where can I find additional help?
• Your plant environmental officer
• National Precast Concrete Association
• Your state environmental protection agency’s water pollution control department
• U.S. EPA
Evan Gurley is technical services engineer with NPCA.
Portland Cement Association (PCA)
Environmentally Friendly Solutions for the Disposal of Concrete Wash Water from Ready-Mixed Concrete Operations.
1 The most common derivatives of chemical admixtures are: ethanolamine, diethanolamine, formaldehyde, K-naphthalene sulfonate, and benzene sulfonic acid.
2 pH, an abbreviation for “potential of hydrogen,” is a measure of the acidity or alkalinity of a solution.
3 EPA’s website, www.epa.gov, has pertinent information about the NPDES and/or your state’s requirements for the discharge of process water into local waters.
4 For more information on process water used in mix designs in your state, visit concretereclaiming.com/results-of-state-dot-washout-water-position/