I'd start with discussing why EHS feels you need to characterize the waste monthly. Per the rules, you only need to do it when you first generate the waste and only need to recharacterize it when the process changes or you have reason to believe the characterization has changed.
All facilities must determine if the waste they generate meets is a hazardous waste or not. This is
necessary when a waste is first generated and must be re-evaluated if changes are made that may
change the nature or composition of the waste. The waste must be re-evaluated if the materials used
in the process change, the process generating the waste is changed, or operational changes are made
that may change the composition and nature of the waste (e.g. cross contamination from material
overspray or even a change in storage temperatures that can result in a change in the nature or
composition of the waste). "
You can also note that "Process Knowledge is a legitimate characterization technique.
2.4.2.b Information Used to Make the Waste Determination
Waste can be characterized using the generator's knowledge or by testing a representative sample.
Process knowledge may be used in making a listed or characteristic waste determination. Information
used for making a listed waste determination may include the waste origin, composition, the process
producing the waste, feedstock, and other reliable and relevant information.. Information on the SDS or
other supplier and manufacturer literature may be useful when you have unused product needing
disposal. Knowledge that may be used in making a determination that the waste exhibits one or more
characteristics of a hazardous waste includes process knowledge; feedstocks and other process inputs;
knowledge of products, by-products, and intermediates produced by the manufacturing process;
chemical or physical characterization of the wastes; information on the chemical and physical
properties of the chemicals used or produced by the process or otherwise contained in the waste; or
other reliable and relevant information about the properties of the waste or its constituents. An SDS
often provides information about the flashpoint, pH, and if a discarded product is a hazardous waste or
contains hazardous constituents. Note, however, that an SDS is not completely reliable for determining
if a used material is a hazardous waste because it does not include information about contaminants
that might be in the waste from use. Since the SDS is designed to meet occupational safety
requirements, it also may not include all hazardous constituents requiring evaluation under the
environmental regulations. A waste stream may be presumed (by knowledge) to contain certain
constituents above regulatory thresholds for compliance purposes; however, testing may be required to
adequately document a hazardous or non-hazardous waste determination."