Guide for Reporting to the National Pollutant Release Inventory 2000: appendix 7

Appendix 7: Examples of How to Estimate Releases

Examples of how to estimate releases of substances with alternate reporting thresholds are explained fully in the companion document, Supplementary Guide for Reporting to the National Pollutant Release Inventory - Alternate Thresholds - 2000.

Direct Measurement (Code M)

This estimate method is based on measured concentrations of the substance in a waste stream and the volume/flow rate of that stream.

Example
An electroplating facility discharges its wastewater to a nearby body of water. The electroplater is required to monitor this discharge once a month for various parameters, including release of total chromium. What is the annual release of total chromium to the wastewater by this electroplater?

Step 1
Gather wastewater flow and concentration data from the monitoring results done in compliance with the municipal by-law for wastewater discharges. Analytical results for total chromium for the year are presented in the table below.

Step 2
Calculate the mass loading for those days on which a chromium analysis was performed. This is done by multiplying the daily flow by the measured chromium concentration.

Step 3
Calculate annual releases.

Based on an average daily release of 1.22 kg over the year and 250 days of discharge during the year, the yearly total chromium discharged to water is:

1.22 kg/d x 250 d/yr = 305 kg/yr = 0.305 t/yr (or 0.31 t/yr after rounding)

Mass Balance Calculations (Code C)

A mass balance is an accounting of the quantity of a substance going in and out of an entire facility, process, or piece of equipment. Releases can then be calculated as the difference between input and output. Accumulation or depletion of the substance in the equipment should be accounted for in the calculation.

Example
In the example presented earlier, the same electroplating facility operates a vapour degreaser.

Suppose that 14 t of trichloroethylene are used as a degreasing agent. Spent solvent and sludge that accumulate on the bottom of the degreaser are collected in drums for shipment to an off-site solvent reclaimer. Thirteen drums of solvent were sent to the reclaimer during the past year.

A known volume of representative sample taken from the drums is weighed, allowed to evaporate, and reweighed. From this, it is determined that the density of the sludge is 1.03 kg/L and that the trichloroethylene concentration in the sludge shipped to the reclaimer is 30%.

Step 1
The entire 14 t of solvent is released from the facility either as an air emission or as a transfer in the sludge. If the quantity of spent solvent shipped to the reclaimer is known, then the quantity transferred can be calculated based on the volume of sludge and the density of the sludge as shown below:

Volume of trichloroethylene to reclaimer

= 13 drums x 210 L/drum = 2730 L

Mass of trichloroethylene to reclaimer:

= volume of sludge x density of sludge x % trichloroethylene in sludge
= 2730 L x 1.03 kg/L x 0.30
= 844 kg
= 0.84 tonnes

Step 2
The quantity of trichloroethylene emitted to air can then be calculated by mass balance by subtracting the quantity shipped in sludge to the reclaimer from the quantity purchased:

14 t (purchased) - 0.84 t (to reclaimer) = 13.16 t (or 13 t after rounding)

Emission Factor (Code E)

An emission factor is based on average measured emissions from several similar processes. Emission factors usually express releases as a ratio of quantity released to process or equipment throughput.

Example
Suppose the electroplater previously mentioned has no information about the spent solvent and sludge accumulating on the bottom of the degreaser.

Step 1
In this case, the emission factor is found in a U.S. Environmental Protection Agency Publication entitled "Toxic Air Pollutant Emission Factors - A Compilation for Selected Air Toxic Compounds and Sources" (Bibliography no. 77). For an open-top vapour degreaser without emission control equipment using trichloroethylene (TCE), the emission factor is given as 0.93 t/t TCE used.

Step 2
Calculate the annual releases to air from the vapour degreaser as follows:

14 tonnes x 0.93 tonne/tonne = 13 tonnes (after rounding) (TCE used x emission factor (TCE released/tonne used) = TCE released)

When emission control devices are used, atmospheric releases are estimated by multiplying the "uncontrolled" emission by the quantity (1 - C/100), where C is the control device efficiency.

Engineering Calculations (Code O)

This estimate method is based on physical/chemical properties (e.g., vapour pressure) of the substance and mathematical relationships (e.g., ideal gas law).

Example
In this example, rinse water from a copper-plating unit is treated separately from other process wastewater. Sodium hydroxide is added to precipitate the copper (Cu) in the wastewater. The precipitate formed from this reaction is removed as sludge from the facility's central clarification unit. Purchasing and inventory records indicate that 0.9 t of sodium hydroxide were used for precipitating copper last year. The quantity of copper precipitated represents the quantity of copper released from this source as solid waste.

Step 1
For each mole of copper (Cu) present in the rinse water, two moles of sodium hydroxide (NaOH) must be added to precipitate the copper according to the following reaction:

Cu⁺⁺ + 2(NaOH) Cu(OH)₂ + 2Na⁺

Scientific literature indicates that this reaction would be complete at a pH of 7.7. Sodium hydroxide is added until a pH of 8 is maintained in the reaction mixture to ensure complete precipitation. It is also known that:

Molecular Weight of Cu = 63.5 tonnes/tonne-mole
Molecular Weight of NaOH = 40 tonnes/tonne-mole

Step 2
Calculate the quantity of copper released in the wastewater treatment sludge, as follows:

The estimation method is valid only if the NaOH reacts only with the Cu present in the wastewater.