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    paint gun image - quantifying waste from purge solvent

    Industrial waste generated from solvents used in painting and coating processes contributes to EPA VOC emissions due to the chemical composition of commonly used solvents. Effective solvent waste tracking is necessary for accurate industrial waste reporting and to ensure compliance with permit regulations. Recording and quantifying the chemicals that come from multiple sources or cleaning processes is a major challenge for facility managers in their industrial waste tracking practices. As a facility manager, you will need an effective way of calculating how much of your solvent mixtures are contributing to your VOC emissions and how much are collected as waste.

    This article will outline:

    • Processes that produce solvent waste and VOC emissions.
    • Calculations for waste allocation and speciation for waste recording within your facility.
    • Ways to reduce air pollution from solvents and stay within permitted VOC limits.

    More in-depth information about this topic is available in our purge solvent emissions eBook. Click here to download a free copy.

    Uses of Solvents in Industry

    Industrial solvent waste is generated from processes such as spraying and coating of automobile bodies, paint mixing operations, and cleaning processes such as purging or flushing. Purging or flushing emissions are frequently over-reported during emission reporting. This happens if waste credit is not taken based on the amount of solvent waste collected at the end of the process. The mixture collected after flushing paint equipment is a mixture of both paint and purging solvent. It is difficult to quantify the ratio of paint to solvent in the waste mixture.

    How Do I Calculate VOC Emissions from Purging Solvents in Waste?


    When calculating VOC emissions and credits, you must consider the materials and chemicals present before and after the coating process. The simplest model for calculating emissions is the mass balance approach. It assumes that everything that enters the system, must be accounted for. If the used material is not collected as waste or stays with the finished product, then you must assume that it is emitted into the atmosphere. To effectively apply the mass balance approach, you will need to know the composition of the paint or coating and how much of it you use in each time frame. This will help in your emission calculations.

    Before choosing a calculation method, you must consider the following questions:

    • Is a chemical analysis available? If yes, how often is the analytical taken?
    • Is VOC content and/or percent solid known?
    • What percentage of the waste collected is water or some sort of non-chemical (i.e. Rags, Personal Protective Equipment (PPE), etc.)
    • Is the waste generated from a single source or multiple sources?
    • Should air emission credits be applied for this process.

    The questions above can be overwhelming and difficult to consider during your calculations. ERA has developed easy to follow calculations that allow seamless recording of all contributing factors.

    ERA’s Waste Module

    ERA’s industrial waste management and tracking software provides simple and effective calculations that account for all waste tracking and calculations, including ‘complex’ sources and waste streams such as purge solvents and cleaning processes. The two methods of calculating the chemical composition and air emission credits for waste include calculating by quantity or by product.

    Calculating by Quantity


    If you have no waste profile or analysis or if the analysis is taken infrequently, and your exact waste composition is unknown, we recommend calculating your credits and emissions based on the quantity and ratio of materials used which contribute to the waste stream. This model considers the chemical makeup of all materials used and quantifies the composition of the waste as a ratio or weighted average of the contributing materials used in the given time frame.

    For each waste stream, understanding which process(es) generate waste is the first requirement for the calculation by quantity. If your processes are consistent throughout the year, using the same materials and quantities, then you can use a fixed distribution estimate to quantify the ratio of each process and the contribution to the waste stream. When you have flexible processes with varying material usage quantities, then you can use the floating distribution model to automatically calculate the amount each process contributes to the waste based on the quantity of materials used in a time frame.

    Because processes and waste generation can vary, there are several methods that can be utilized to determine the total contribution from each process to the waste generated. In the sections below, we will review methods that are available in the ERA software and the context in which they may be useful. The first method that will be evaluated is calculating by usage only.

    Distribution by Usage Only

    When distributing waste based on usage only, the calculation is based solely on the amount of usage from each source in each period. This ratio can be used to estimate the amount of waste produced by each source or process by applying these percentages to the amount of waste collected.


    Once the amount of waste allocated to each sources is known, the system calculates the composition of the waste and the air emission credits based on a ratio (weighted average) of the composition of all materials used.

    This method is primarily useful if you have several processes which have similar materials used (i.e. multiple paint sources) and each contributed to the generated waste at an equal rate. 

    Download Now:  Quantifying Air Emissions From Purge Solvent Sources eBook

    Calculating by Usage and Other Parameters

    The next few methods are used when there are different conditions. If you have several processes which have different materials used and do not contribute to the generated waste at an equal rate (i.e., Purge solvent), then other methods can be used to determine how much of each source would contribute to the waste stream. In purging processes, for example, typically a high VOC solvent is used to clean spray equipment, and the resulting waste stream is a mixture of solvent and some paint that is collected in the cleaning process. This can be challenging to quantify the total amount of waste collected and properly calculate air emission credits for each source and with the correct VOC and chemical composition.

    To learn more about the different calculation methodologies, download our free purge solvent calculations eBook, including: ‘Usage and Maximum recovery’ ‘percentage solid waste.’

    Usage and Max Recovery

    For processes such as purge solvent, if a recovery rate for the solvent is known or tested, then a maximum recovery rate can be applied to the distribution calculation. This can be useful particularly if the amount of waste collected is greater than the amount of solvent used in the period being evaluated.


    You can define a maximum percentage distribution that can be allocated to a given source or sources based on your process knowledge or test records. A detailed example is provided in the eBook version of the document.

    Usage and % Solid Waste

    For processes such as purge solvent, if you have test data to quantify the % solids in the waste and if a recovery rate is unknown, then the % solids can be used to logically calculate the amount of paint vs. solvent in the waste. Purge processes use solvent which contains only volatile chemicals; therefore, non-volatiles (solids) must be coming from paint materials that are collected as part of the purging process. This is represented in the diagram shown here.



    Calculating by Product


    If you have your waste profiled by a company or have an SDS for expired materials, you can use the test record to analyze the composition of the waste. This method is useful when the exact composition of the waste is known and is tested regularly. You can use the known VOC content and composition of the waste profile to calculate the emissions and credits of each source. For example, in the image below, the emission potential and the information from the waste profile are both used to calculate the emissions

    One Source

    Get the Purge Solvents Free Guide

    These calculations are all explained in more depth on our free Purge Solvents Guide PDF. Get your free copy below.
    Download Now:  Quantifying Air Emissions From Purge Solvent Sources eBook

    How Can ERA’s Waste Module Help You?

    ERA's Hazardous Waste Module can effectively complete all the above-mentioned calculations for you and output a report that shows the distribution of waste per month such as recycling, incineration, and reuse. It also helps you track your progress and meet your waste-related goals by recording the amount of waste produced and the amount of waste recycled while providing information required by EPA VOC regulations.


    This Blog was Co-Authored By: 



    Kundai Mufara
    Post by Kundai Mufara
    April 19, 2021
    Kundai Mufara is a Science Content and Technical Writer at ERA.