Process water has a negative impact on the environment, but you can do something about it
Precast concrete manufacturers by nature are problem solvers, whether they are providing an innovative and cost-effective alternative for a large-scale cast-in-place project or redesigning production operations to minimize waste. As today’s precasters are finding out, they cannot always do things the same way their grandfathers – or even their fathers – did them. With increasing quality control standards and environmental regulations, precasters are once again relying on their own ingenuity to provide cost-effective solutions for today’s challenges.
In an effort to meet increasing environmental regulations, minimize waste and reduce operating costs, a number of precast concrete manufacturers have developed innovative methods for recycling and/or treating concrete process water. Process water is often related to washout and coring operations but may also include stormwater, which collects a measurable amount of cementitious material from areas surrounding the production facility.
Characteristics of concrete process water
Concrete process water is caustic and typically has a high pH value ranging between 11 and 12. It contains dissolved solids including sulfates and hydroxides from cement, oil and grease from equipment and derivatives from chemical admixtures. As an example of how this impacts the environment, the Canadian Department of Fisheries and Oceans says that the effects of high pH on fish may include death; damage to outer surfaces such as gills, eyes and skin; and the inability to dispose of metabolic wastes. In addition, the high content of total suspended solids (TSS) typically found in process water may clog a fish’s gills, destroy natural breeding areas, affect a fish’s ability to feed and create an overall imbalance in the surrounding environmental habitat.
The U.S. Environmental Protection Agency (EPA) monitors and limits the nature and magnitude of waste products a site is allowed to discharge into the waters of the United States, while local sanitary and sewer authorities set the limits for total suspended solids, pH and chemical composition for process water discharged into local sanitary sewer systems.
Every precaster should be familiar with requirements governing the discharge of process water. Precast concrete manufacturers near surface waters will likely be required to obtain a stormwater National Pollutant Discharge Elimination Systems (NPDES) permit. In most cases the NPDES permit program is administered by an authorized state agency. Visit the EPA’s Web site at www.epa.gov for more information about the NPDES or your state’s requirements for the discharge of process water into local waters.
A number of authorized states have industry-specific permits. For example, New Jersey has a general permit for the Concrete Products Manufacturing Industry (Permit No. NJ0108456). This permit covers the discharge of stormwater and process water from precast plants. In addition, this permit contains language requiring manufacturers to optimize the recycling of all byproducts and/or waste materials generated during the manufacturing of concrete products, including but not limited to excess concrete, concrete washout wastewater, concrete debris and aggregate by reusing the materials in concrete production.
As a whole, the industry should investigate ways to minimize waste by recycling materials and ultimately strive for a zero-discharge manufacturing process. Unfortunately the recycling of concrete materials often appears to be cost-prohibitive, but a handful of proactive precasters are leading the way to a cleaner environment with their efforts in pretreating process water and, in some cases, striving for zero-discharge manufacturing by completely recycling all process water.
Pretreatment for discharge into local sewers
The requirements for discharging process water into local sewers vary among authorities having jurisdiction. Russell Ellis, regional environmental manager for Hanson Building Products America, indicated that the requirements for discharge into local sanitary sewers are often less stringent than those required by the EPA and often allow for higher pH limits, which is helpful within this industry.
Local sanitary sewer departments often set limits on the amount of total suspended solids and may even allow for a pH as high as 10 for the discharge of concrete process water. Such requirements often can be met by utilizing a large precast concrete grit separator or a holding tank with a series of baffles and sufficient retention time that allows for the required amount of settlement. Systems will often vary in size depending on the process waters that are diverted into them. 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.
For easier maintenance and sludge removal, some plants utilize a sloped pit for initial settlement, which allows for easy access with a front-end loader. The process water is then pumped or allowed to flow into a larger settlement tank and, if necessary, into a dosing tank where the pH is adjusted to meet the required discharge limits. Process water can be diluted with water from other sources or stormwater, but this water may also have an elevated pH.
Lowering the pH
Various systems and methods are available for reducing the pH of process water depending on how much time and money you are willing to spend. The most common method is the addition of an acid, typically sulfuric acid or hydrochloric acid. Alternatively, you can bubble gaseous carbon dioxide through the process water or add chunks of dry ice (frozen carbon dioxide). Always remember to wear appropriate personal protective equipment and to take applicable safety precautions when handling harmful chemicals. The pH of the process water, volume, temperature, neutralizing acid and required discharge level will all have to be taken into consideration when adjusting the pH, so it is best to consult a local chemical engineer when designing your system. Automated monitoring and adjustment systems are available and may be justifiable when considering the human factor.
Oldcastle Precast Inc. (DBA Rotondo Precast) in Telford, Pa., utilizes two 1,000-gallon tanks and an automated pH monitoring and adjustment system for treating process water. Bob Dando, Rotondo’s safety director, indicated that the plant recycles as much of the treated water as possible when cleaning production equipment with a pressure washing. The remaining water is discharged into the municipal sewage system with an adjusted pH of 8.
Although Rotondo is one of the few companies leading the way in process water treatment, the ultimate goal is a zero-discharge facility. Local sanitary and sewer departments often charge additional fees for discharging a high annual volume of waste in addition to the costs associated with the handling and safe disposal of concrete sludge and slurry from the settlement tanks.
Recycling concrete process water
The unwritten rule when evaluating the suitability of water for use in concrete has typically been, “If you can drink it, you can make concrete with it.” Many within the industry still have this perception and it is the reason why some customers may not allow the use of process water for batching, even though industry evaluations led to a 1978 revision in ASTM standards. ASTM C94, “Standard Specification for Ready Mixed Concrete,” permits the use of process water as mixing water in concrete. The requirements within ASTM C94 remained the same until recently when ASTM subcommittee C09.40 on Ready-Mixed Concrete developed two new standards on the requirements and accompanying testing 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,” defines acceptable sources of water for batching concrete and provides requirements and testing frequencies for qualifying individual or combined water sources. In addition it contains optional chemical limits for combined mixing water, setting maximum values for chlorides, sulfates, alkalies and total solids by mass.
When qualifying the use of process water, perform all required tests at the highest solids content in the total mixing water anticipated during production. This way, mixing water containing total solids equal to or less than the level qualified by testing may be used. At a minimum the compressive strength and set time should be evaluated and compared to a control mix using 100 percent potable water or distilled water. According to ASTM C1602, the compressive strength should be within 90 percent of the control mix, and the set time should not decrease by more than one hour or increase by more than one and a half hours.
Recent research conducted at the National Ready Mixed Concrete Association’s Alfred H. Smith Research Laboratory sheds new light on the properties of concrete containing process water. The solids content and age of the recycled process water was varied to reflect typical real-world applications. Additional water was added at the time of batching to obtain a target slump value, thus the water-cementitious ratio varied between batches.
Research has shown that when targeting a specific slump, the water demand will increase as the solids content increases and the process water ages. Hydrating cement particles will become finer, resulting in increased water demand. In addition, as the age and total solids content increases, so does the amount of hydrated cement and calcium hydroxide, both of which have been found to accelerate the initial set time of concrete. This poses somewhat of a challenge for the ready-mixed concrete industry, while the precast concrete industry can use this to their advantage in meeting accelerated production schedules.
The compressive strength and durability of concrete containing process water is similar to that of concrete produced with potable water. Compressive strength variations in the study mentioned above reflect that the water-cementitious ratio was varied to achieve a consistent slump. A similar study conducted at the University of Toronto maintained a constant water-cementitious ratio and indicated that similar strengths were easily obtained regardless of the amount of solids present.
When using recycled process water in the production of concrete, it is important to measure and monitor the total amount of solids entering the mix. Many engineers, especially state Departments of Transportation, may set limits on the allowable amount of total solids in addition to requiring compliance with the optional limits presented in ASTM C1602.
Recycling process water
There are a number of automated commercial systems available designed for recycling concrete materials and process water. These systems have established their presence within the ready-mixed concrete industry, and the manufacturers have begun to target the precast industry. A number of precasters have opted instead to design and install their own systems, providing a cost-effective solution to the handling and recycling of concrete process water.
Before designing a system, take a step back to determine the overall goals and requirements of the system. Your customer may set a limit on the total amount of solids allowed in the concrete mix design, so recycling slurry may not be an option. Some precasters may run all coring and sawing water into the system in addition to washout water. The water sources and target solids content for the actual batching water will all have to be taken into consideration when designing a process water recycling system.
Wilbert Precast Inc. in Spokane, Wash., set out to minimize the amount of waste and costs associated with cleaning and coring operations. Wilbert Precast utilizes a 2,100-gallon tank for collecting process water. All production and batching equipment is washed over a sloped ramp with a weir that leads into the holding tank. The weir is designed to collect larger aggregates that can be recycled or discarded. A float within the tank monitors the process water level. When the level begins to drop, a spray nozzle is activated, supplying fresh water to the coring machine trench drain, which leads directly into the holding tank.
The holding tank has a mechanical agitator that keeps the solids suspended within the slurry. The corners of the tank are rounded to minimize buildup and ensure consistency throughout. Water can then be drawn from the slurry tank, on-site boiler and municipal water source in varying proportions to ensure the desired fresh and hardened properties are achieved.
Darrin Cary, quality control manager at Wilbert Precast, indicated that his company originally began using slurry water in conventional concrete, but has since switched to self-consolidating concrete. Cary indicated that the high level of fines within the mix allow for a superior-looking finished product with minimal bug holes and imperfections.
Designing a customized recycling system requires patience and persistence. It is extremely important to do your homework and purchase the right equipment, in addition to spending the time to properly train employees. You must ensure that the pumps and equipment are maintained and designed to properly handle fluids with high solid contents. Running slurry water through stock batching equipment will likely burn out pumps and clog nozzles and meters designed for clarified water.
Increasing environmental regulations will likely limit the means and methods for future disposal of byproducts associated with the manufacture of precast concrete. Today’s precaster must look to the future, have an open mind and be willing to investigate and embrace new technologies and methods, especially when it comes to preserving our environment and natural resources.