1

MEMORANDUM

TO: Tina Ndoh, U.S. EPA/OAQPS/SPPD – Minerals and Manufacturing Group

FROM: Eastern Research Group, Inc.

DATE: June 4, 2014

SUBJECT: Comparison of 40 CFR Part 60, subpart U (Standards of Performance for the 

Phosphate Fertilizer Industry: Superphosphoric Acid Plants) and 40 CFR Part 63, 

subpart AA (National Emission Standards for Hazardous Air Pollution from 

Phosphoric Acid Manufacturing Plants)

This document presents a general comparison between the applicability, definitions, emission 

standards, monitoring and operating requirements, and testing and compliance requirements of 

NSPS 40 CFR Part 60, subpart U, and NESHAP 40 CFR Part 63, subpart AA (hereafter referred 

to as subpart U, and subpart AA, respectively). Subpart U was last amended on October 17, 2000 

and subpart AA was last amended on April 20, 2006. This document does not encompass 

recordkeeping or reporting requirements: each subpart references the respective general 

provisions.

1.0 PROCESS DESCRIPTION FOR PRODUCING SUPERPHOSPHORIC ACID

Superphosphoric acid (SPA) is produced by further concentrating merchant grade phosphoric 

acid (54 percent P2O5) to approximately 70 percent P2O5.

1

The acid is usually concentrated using 

vacuum evaporation.

2.0 COMPARISON OF NSPS 40 CFR PART 60, SUBPART U TO NESHAP 40 CFR 

PART 63, SUBPART AA

The following sections provide a brief comparison of subpart U to subpart AA standards for 

phosphoric acid manufacturing.

2.1. Applicability

Both subpart U and subpart AA apply to SPA operations; however, subpart AA also applies to

wet-process phosphoric acid (WPPA) process lines and purified acid process lines. While NSPS 

40 CFR Part 60, subpart T applies to WPPA plants, there does not appear to be any NSPS 

standards applicable to purified acid process lines. Because subpart U only applies to SPA plants, 

the remaining sections of this document do not discuss WPPA or purified acid process lines. The 

 

1 Both subpart U and subpart AA specify the concentration of P2O5 in SPA as greater than 66 percent by weight.
2

applicability of each subpart to SPA is presented in Table 2.1. Subpart U references design 

capacity, while subpart AA applies to major sources.

Table 2.1. Applicability Provisions in 40 CFR Part 60, Subpart U and 40 CFR Part 63, Subpart AA

40 CFR Part 60, Subpart U 40 CFR Part 63, Subpart AA

Each SPA plant[a] having a design capacity of more 

than 15 tons P2O5 feed per calendar day [§60.210(a)]

Each phosphoric acid manufacturing plant 

[§63.600(a)], including the following new or 

existing affected sources at a phosphoric acid 

manufacturing plant:

Each evaporative cooling tower [§63.600(b)(2)]

Each phosphate rock dryer [§63.600(b)(3)]

Each phosphate rock calciner [§63.600(b)(4)]

Each superphosphoric acid process line[a]

[§63.600(b)(5)]

Does not apply to a new or existing phosphoric 

acid manufacturing plant this is not a major source 

[§63.600(c)]

No R&D exemption Includes R&D exemption [§63.600(d)]

No SSM exemption Includes SSM exemption [§63.600 (e)]

[a] For both subpart U and subpart AA, requirements apply to evaporators, hot wells, acid sumps, and cooling tanks.

[b] 40 CFR Part 60, subpart T applies to WPPA plants.

2.2. Definitions

Subpart U defines an SPA plant. Subpart AA defines an SPA process line. However, the 

definition of an SPA plant in subpart U specifically states that an SPA plant is a facility that 

concentrates acid “for eventual consumption as a fertilizer.” Subpart AA does not include this 

specific condition in the definition of an SPA process line. Subpart AA also includes several 

additional definitions related to SPA manufacturing. Refer to Table 2.2 for details.

Table 2.2. Definitions in 40 CFR Part 60, Subpart U and 40 CFR Part 63, Subpart AA

40 CFR Part 60, Subpart U 40 CFR Part 63, Subpart AA

SPA plant: any facility which concentrates WPPA 

to 66 percent or greater P2O5 content by weight for 

eventual consumption as a fertilizer. [§60.211(a)]

SPA process line: any process line which 

concentrates WPPA to 66 percent or greater P2O5

content by weight. [§63.601]

Definitions in Subpart AA related to SPA

manufacturing not in Subpart U

Evaporative cooling tower: an open water 

recirculating device that uses fans or natural draft 

to draw or force ambient air through the device to 

remove heat from process water by direct contact. 

[§63.601]
3

2.3. Standards

Subpart U contains a single standard for total fluorides from an SPA plant. Subpart AA contains 

total fluoride standards for new and existing SPA process lines. Separate standards apply to

vacuum evaporation and submerged combustion processes at existing sources.

2 As presented in 

Table 2.3, subpart AA also includes standards for evaporative cooling towers.

Table 2.3. Standards in 40 CFR Part 60, Subpart U and 40 CFR Part 63, Subpart AA

HAP/Pollutant 40 CFR Part 60, 

Subpart U

40 CFR Part 63, Subpart AA

Existing Sources New Sources

Total Fluorides (lb/ton 

of equivalent P2O5 feed)

Affected Facility: 

0.010 [§63.212(a)]

SPA Process Line

Vacuum Evap. Process:

0.010 [§63.602(b)(1)]

Submerged Combustion:

0.20 [§63.602(b)(2)]

SPA Process Line:

0.00870 [§63.603(b)]

None Specified NA

Prohibition on introduction of wet scrubber liquid 

effluent into any evaporative cooling tower. 

[§63.602(e) and §63.603(e)]

2.4. Monitoring and Operating Requirements

The most substantial differences between subpart U and subpart AA in regards to monitoring and 

compliance is related to establishing and adhering to parameter monitoring ranges for wet 

scrubbing systems. Both subparts include requirements to operate a continuous parameter 

monitoring system to record pressure drop, but only subpart AA includes a requirement to 

monitor and record the flow rate of scrubbing liquid. Subpart AA also includes specific 

requirements that facilities must operate within the establish parameters: such requirements are

not included in subpart U. Refer to Table 2.4 for details.

Table 2.4. Monitoring and Operating Requirements in 

40 CFR Part 60, Subpart U and 40 CFR Part 63, Subpart AA

40 CFR Part 60, Subpart U 40 CFR Part 63, Subpart AA

P2O5 Feed Accounting –SPA Plant/Process Line

 Install, calibrate, maintain, and operate a 

monitoring device which can be used to 

determine the mass flow of phos-bearing 

feed to process within an accuracy of ±5% 

over operating range. [§60.213(a)]

 Maintain a daily record of eq. P2O5 feed 

according to procedures in subpart.

[§60.213(b)]

 Install, calibrate, maintain, and operate a monitoring 

device which can be used to determine and 

permanently record the mass flow of phos-bearing 

feed to process within an accuracy of ±5% over 

operating range. [§63.605(a)(1)]

 Maintain a daily record of eq. P2O5 feed according to 

procedures in subpart. [§63.605(b)(1)]

 

2 Based on industry responses to the April 2010 Section 114, only one manufacturer is permitted to use a submerged 

combustion process for the production of SPA, although that process was indefinitely shutdown on June 1, 2006.
4

Table 2.4. Monitoring and Operating Requirements in 

40 CFR Part 60, Subpart U and 40 CFR Part 63, Subpart AA

40 CFR Part 60, Subpart U 40 CFR Part 63, Subpart AA

Scrubbing Systems

Install, calibrate, maintain, and operate a 

monitoring device which continuously 

measures and permanently records the total 

pressure drop across the process scrubbing 

system (accuracy of ± 5% over operating 

range). [§60.213(c)]

Install, calibrate, maintain, and operate a monitoring 

system which continuously measures and permanently 

records:

A) the pressure drop across [§63.605(c)(1)], and

B) the flow rate of scrubbing liquid to [§63.605(c)(2)],

Each scrubber in the process scrubbing system in 

15-min block averages (accuracy of ± 5% over operating 

range). [§63.605(c)(1) and (c)(2)]

Establish allowable ranges, maintain daily averages 

within the established ranges, for pressure drop and 

flow rate. [§63.604 and §63.605(d)]

2.5. Testing and Compliance

The testing and compliance requirements vary in frequency and method between subpart U and 

subpart AA. Subpart U only requires an initial performance test, whereas subpart AA requires 

initial and annual testing. Subpart AA also allows facilities a broader choice of test methods than 

subpart U in determining P2O5 content of feed.

Table 2.5. Testing and Compliance in

40 CFR Part 60, Subpart U and 40 CFR Part 63, Subpart AA

40 CFR Part 60, Subpart U 40 CFR Part 63, Subpart AA

Frequency of Performance Test

Within 60 days of max production rate, but no 

later than 180 days after initial startup [§60.8]

Existing Sources: On or before applicable compliance 

date and then annually. [§63.606(a)(1)]

New Sources: Within 180 days of compliance date and 

then annually. [§63.606(a)(2)]

Sampling and Analysis Methods

Total Fluorides: Method 13A or 13B with at 

least a 60 minute sampling time and 

0.85 dscm (30 dscf) sample volume.

[§60.214(b)(2)]

P2O5 Content of Feed: The Association of 

Official Analytical Chemists (AOAC) Method 

9. [§60.214(b)(3)(ii)]

Total Fluorides: Method 13A or 13B with at least a

60 minute sampling time and 0.85 dscm (30 dscf) 

sample volume. [§63.606(c)(2)]

P2O5 Content of Feed: Various methods from the 

Association of Florida Phosphate Chemists, Seventh 

Edition 1991. [§63.606(c)(3)(ii)]