Concrete batch plants generate particulate matter (PM) emissions from aggregate handling, cement transfer, batch weighing, truck loading, and haul roads. The emissions are primarily fugitive (released from open operations rather than through a stack) and consist mainly of calcium carbonate, calcium silicate, and silica dust from the aggregate and cement. Without controls, a large ready-mix plant can emit 50 to 200 tons per year of PM, which triggers major source permitting and NSPS requirements under the Clean Air Act.
The good news is that concrete batch plant dust is well-characterized and highly controllable. Baghouses on cement storage silos capture 99%+ of transfer emissions. Water spray systems on aggregate stockpiles and conveyors reduce fugitive dust by 70% to 90%. Enclosures on batch mixers and truck loading chutes contain the dust for collection. A well-designed and well-maintained plant can operate at 5 to 15 tons per year of PM, well below major source thresholds. This guide covers the emission sources, AP-42 factors, control equipment, and permitting considerations.
Emission Sources at a Batch Plant
Concrete batch plants have five primary emission source categories: cement and supplementary cementitious material (SCM) handling, aggregate handling, batch mixer loading, truck loading and washout, and haul roads. Each category has different emission characteristics and different control strategies. The relative contribution of each source depends on the plant design, throughput, and control measures in place.
Cement and SCM handling (fly ash, slag, silica fume) generates the finest and most visible dust. Cement particles are typically 10 to 50 microns, with a significant fraction below 10 microns (PM10). Every transfer point — truck unloading to silo, silo to weigh hopper, weigh hopper to mixer — is an emission point. The displacement air from filling a cement silo with pneumatic delivery can exceed 2,000 CFM and carries a high concentration of cement dust. Without a silo vent filter, a single truck unload can create a visible plume for 20 to 30 minutes.
Aggregate handling generates coarser dust (mostly PM10 and larger) from loading, conveying, screening, and stockpile management. Wet aggregates produce very little dust. Dry aggregates, especially manufactured sand and crushed stone, can emit significant PM when dropped from conveyors or loaders. The emission rate depends on the moisture content, particle size distribution, wind speed, and drop height. AP-42 Section 11.19.2 provides emission factors for each handling operation.
Haul roads are often the largest single emission source at a batch plant. Unpaved roads with truck traffic generate PM from tire contact, vehicle wake turbulence, and wind erosion. AP-42 Section 13.2.2 estimates unpaved road emissions based on vehicle weight, speed, surface silt content, and moisture content. A single 40-ton ready-mix truck making 20 round trips per day on a 500-foot unpaved road can emit several pounds of PM10 per day without dust suppression.
E = k × (s/12)^a × (W/3)^b
Where E = emission factor (lb/VMT)
k = particle size multiplier (1.5 for PM10, 0.15 for PM2.5)
s = surface silt content (%), W = vehicle weight (tons)
a = 0.9, b = 0.45
Concrete Batch Plant Dust Emissions Calculator
Estimate particulate matter emissions from concrete batch plant operations using EPA AP-42 Chapter 11.12 factors. Calculate PM10 and PM2.5 from material handling, truck loading, and storage piles with control efficiency.
Baghouse Design and Performance
Baghouses (fabric filter dust collectors) are the primary control device for cement and SCM transfer emissions. A properly sized and maintained baghouse achieves 99% to 99.9% PM removal efficiency, reducing silo vent emissions from pounds per hour to grams per hour. The baghouse collects cement dust on filter bags (typically polyester or aramid fabric) and periodically cleans the bags using reverse air or pulse-jet compressed air to drop the collected dust into a hopper for return to the silo.
Sizing a baghouse requires knowing the maximum airflow (CFM) and selecting an appropriate air-to-cloth ratio (the ratio of airflow to total bag surface area). For cement dust, the recommended air-to-cloth ratio is 4:1 to 6:1 for pulse-jet cleaning (CFM per square foot of bag area). A silo that receives pneumatic cement deliveries at 2,000 CFM displacement air needs a minimum of 333 to 500 square feet of bag area. Standard industrial baghouse units with 100 to 200 bags of 6-inch diameter by 8-foot length provide 250 to 500 square feet of filter area.
Baghouse performance monitoring is typically based on pressure drop across the bags. Normal operating pressure drop is 2 to 6 inches of water column (in. w.c.). Below 2 inches suggests the bags are too clean (possible bag leak bypassing filtration) or the fan is not pulling enough air. Above 6 inches indicates excessive dust loading, inadequate cleaning, or blinded bags that need replacement. Most permits require a pressure drop gauge or differential pressure transmitter with alarm capability.
Bag failure is the leading cause of opacity exceedances from batch plant silos. A single torn bag in a 100-bag baghouse can increase outlet PM emissions tenfold because the air takes the path of least resistance through the tear. Weekly visual inspection of the bags during cleaning cycles, combined with a triboelectric leak detector on the outlet duct, catches failures before they become regulatory problems. Replace bags on a preventive schedule (typically every 2 to 4 years) rather than waiting for failure.
Permit Thresholds and Regulatory Requirements
Concrete batch plants are regulated under the Clean Air Act at multiple levels. The first threshold is whether the plant is a "major source" or a "minor source" (also called an "area source"). A major source emits or has the potential to emit 100 tons per year or more of any criteria pollutant (250 tons per year in attainment areas for PSD purposes) or 10 tons per year of any single hazardous air pollutant (HAP) or 25 tons per year of combined HAPs. An uncontrolled concrete batch plant can exceed the 100 ton/year PM threshold, making it a major source.
Most concrete batch plants avoid major source status by installing controls (baghouses, water sprays, paving) and accepting enforceable emission limits in a synthetic minor permit (also called a "permit by rule" or "general permit" in some states). The permit limits the plant's potential to emit below the major source threshold by requiring specific control measures, limiting production throughput, or both. These limits are enforceable and must be demonstrated through recordkeeping.
NSPS Subpart OOO (40 CFR 60, Subpart OOO) applies to nonmetallic mineral processing plants, including concrete batch plants that process aggregate. The standard limits PM emissions from affected facilities (crushers, screens, conveyors, storage bins) to 0.014 gr/dscf for new sources and sets opacity limits. Not all equipment at a batch plant triggers NSPS, but aggregate processing equipment typically does if the plant was constructed or modified after August 31, 1983.
NESHAP Subpart XXXXXX (40 CFR 63, Subpart XXXXXX, the "6X" rule) applies to concrete batch plants that are area sources of HAP. The primary HAP concern is crystalline silica (quartz) in aggregate and cement. The 6X rule requires good management practices including enclosed conveying of cement, baghouse venting of cement storage silos, and water spray or enclosure of aggregate handling. Compliance is demonstrated through management practice standards rather than emission testing.
Major source (Title V): ≥100 TPY of any criteria pollutant
Major HAP source: ≥10 TPY single HAP or ≥25 TPY combined HAPs
PSD major source (attainment area): ≥250 TPY
NSPS Subpart OOO: Applies to aggregate processing equipment
NESHAP 6X: Management practices for area source HAP
Dust Control Best Practices
The hierarchy of dust control at a concrete batch plant starts with prevention (minimize dust generation), then containment (enclose emission sources), then collection (baghouses and filters), and finally suppression (water sprays and chemical binders). The most effective plants use all four strategies at different points in the process.
Prevention measures include maintaining aggregate moisture content above 3% to 4% (wet aggregate produces negligible dust), reducing drop heights from conveyors and loaders (halving the drop height reduces emissions by roughly 30%), using telescoping chutes on truck loading systems, and paving haul roads (paving reduces haul road emissions by 90% or more compared to unpaved surfaces). Each of these measures is low-cost relative to the emission reduction achieved.
Enclosures on batch mixers, truck loading points, and conveyor transfer points contain dust for capture by a local exhaust ventilation system connected to a baghouse. The enclosure does not need to be airtight, but it must maintain negative pressure so that air flows into the enclosure rather than dust flowing out. Design the exhaust rate to maintain 100 to 200 feet per minute face velocity at any openings in the enclosure. This velocity is enough to capture fugitive dust without creating excessive airflow through the baghouse.
Water spray suppression on aggregate stockpiles, conveyor belts, and unpaved roads reduces PM emissions by 50% to 90% depending on application rate and frequency. The effectiveness of water suppression depends on maintaining surface moisture, which requires re-application every 30 to 60 minutes in dry weather. Chemical suppressants (polymer emulsions, calcium chloride, lignin sulfonate) provide longer-lasting suppression (4 to 12 hours) at higher material cost. Many permits specify a minimum water application frequency or a chemical suppressant program as an enforceable condition.