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SPDES Permitting of Water Treatment Chemicals (WTCs)

Many industrial, municipal, and private/commercial/institutional (PCI) facilities use and discharge WTCs to improve treatment processes or to protect equipment. Water treatment chemicals include biocides, coagulants, conditioners, corrosion inhibitors, defoamers, flocculants, scale inhibitors, sequestrants, and settling agents. They can contain ingredients that may be present in the discharge at toxic levels and might not otherwise be reported on the permit application as potentially present in the discharge.

Water Quality Issues Associated with WTC use

WTCs often contain ingredients which have the potential to impact water quality. In response, the Department has developed ambient water quality criteria for a number of these chemicals; however, there are many more of these chemicals which have not been evaluated. An additional challenge is that there are few approved analytical methods available to verify the presence of many WTCs. For surface water discharges, aquatic toxicity testing data can serve as a partial surrogate for the lack of ambient water quality criteria and analytical methods.

Use of Multiple WTCs

Some permittees simultaneously use several WTCs. It is possible that the WTCs may be considered to have acceptable toxicity when considered individually but when considered together there may be sufficient justification to require a SPDES permit modification to add whole effluent toxicity testing.

Trade Secret Information

Some WTC manufacturers may be reluctant to reveal product formulations to the permittee. The WTC Authorization Request has been designed so that the review can be accomplished by the permittee and manufacturer each completing and submitting separate sections of the form. In accordance with 6 NYCRR 616.7 and 750-1.23, in order for the Department to consider product formulations to be excepted from disclosure under paragraph (d) of subdivision (2) of section 87 of the New York State Public Officers Law, a written request, stating the reason for exception, must be made at the time of the WTC notification.

WTC Use Requirements


In accordance with 6 NYCRR Parts 750-2.6(a) & 750-2.9(a)(1), new or increased use and discharge of a WTC requires prior Department review and authorization. At a minimum, the permittee must notify the Department in writing of its intent to modify WTC use by submitting a completed WTC Authorization Request for each proposed WTC change. The Department will review the submitted information and determine if a SPDES permit modification is necessary or whether WTC authorization may proceed without a formal permit modification. The majority of WTC authorizations do not require SPDES permit modification. See the WTC Authorization Request Instructions for more details and to obtain a copy of the form.

Note that there is no list of "authorized" WTCs that are exempt from Department review. Other than the exceptions noted in Additional Information, WTC discharges require case-by-case review and authorization because of the unique combination of variables at each facility (e.g., usage rate, outfall flow, receiving waterbody flow and classification, presence of other discharges, etc.). Note also that there is no "library" of characteristics for each WTC that would obviate the need for case-by-case submission of ingredient and toxicity information. WTC manufacturers change product names and formulations from time-to-time which makes it impractical to maintain an accurate library.

WTC Authorization Request and Review Training (PDF)


Permittees are required to maintain a logbook of all WTC use, noting for each WTC the date, time, exact location, and amount of each dosage, and the name of the individual applying or measuring the chemical. The logbook must also document that adequate process controls are in place to ensure that excessive levels of WTCs are not used and subsequently discharged through outfalls. The permittee shall retain the logbook for a period of at least 5 years from the date of WTC use in accordance with 6 NYCRR Part 750-2.5(c)(1).

Annual Reporting

Permittees that use and discharge WTCs are required to submit an annual summary using the WTC Annual Report Form (PDF), each year that they use and discharge WTCs. The permittee must summarize all WTC use for the prior calendar year, January 1 through December 31, and attach it either to the December Discharge Monitoring Report (DMR) or annual monitoring report required by the SPDES permit.

WTC Authorization Request Instructions

WTC Authorization Request (PDF)
The WTC Notification Form shall be submitted to as a pdf. All requested information must be provided, including exact chemical composition and toxicity information for the whole WTC product (i.e. not the individual ingredients). Safety Data Sheets (SDS) should be submitted with the notification form.

Toxicity Information

For surface water discharges, aquatic toxicity testing data on the full WTC formulation (whole product) is required to complete the WTC Authorization Request. Toxicity data for the individual WTC ingredients is unacceptable. Submission of acute (48- or 96-hour LC50 or EC50) and/or chronic (7-day NOEC or IC25) test results for at least one vertebrate and one invertebrate species from the approved lists are required. NOECs estimated from acute 48- or 96-hr tests do not qualify as chronic test data and may not be reported as such. All reported test data must represent tests conducted in accordance with current EPA Whole Effluet Toxicity (WET) Methods at 40 CFR Part 136.3 or equivalent method (both links leave DEC's website). If the discharge is to freshwater, then only freshwater species should be reported on the form. Likewise, for discharges to marine waters, only marine species should be reported.
If the discharge is to groundwater, then toxicity data is not necessary.

Following is the list of acceptable species. Toxicity data for non-aquatic species (e.g., mice, rats, rabbits), dermal studies, etc. will not be accepted.

Freshwater Invertebrates:

  1. Ceriodaphnia dubia (daphnid water flea) - recommended
  2. Daphnia magna (daphnid water flea)
  3. Daphnia pulex (daphnid water flea)

Freshwater Vertebrates:

  1. Pimephales promelas (fathead minnow) - recommended
  2. Oncorhynchus mykiss (rainbow trout)
  3. Salvelinus fontinalis (brook trout)
  4. Lepomis macrochirus (bluegill)

Estuarine/Marine Invertebrates:

  1. Americamysis bahia (mysid shrimp) - recommended

Estuarine/Marine Vertebrates:

  1. Cyprinodon variegatus (sheepshead minnow) - recommended
  2. Menidia beryllina (inland silverside)
  3. Menidia menidia (Atlantic silverside)
  4. Menidia peninsulae (tidewater silverside)

Further information concerning WET testing is contained in TOGS 1.3.2 (PDF) and on the WET webpage.
Applicants should screen WTCs for toxicity before submitting a WTC Authorization Request to avoid making requests which are unlikely to be authorized. The toxic-in-effluent calculation is simple to perform. The toxic-in-receiving-water calculation is also simple, but it requires that receiving stream dilution be known. This dilution value can often be found in the most recent SPDES permit fact sheet.

WTC Review

Toxicity Review

Toxic-in-Effluent Calculation: If only acute data is available, apply a safety factor of 100 to the lowest value and compare this to the effluent concentration. If acceptable chronic data is provided, apply a safety factor of 10 to the lowest value. The safety factor accounts for possible differences between test species and species that were not tested but might be more sensitive. If the WTC effluent concentration is less than the calculated toxicity, then the WTC "passes" the toxic-in-effluent screening. If the WTC "fails" the toxic-in-effluent calculation, then it must be further evaluated to predict if it could cause toxicity after dilution in the receiving water.
Toxic-in-Receiving-Water Calculation: If the WTC is predicted to be toxic-in-effluent, then a second toxicity evaluation is performed by considering available receiving water dilution. If the WTC "passes" the toxic-in-receiving-water screening, the Department will continue the WTC review. If the WTC "fails" the toxic-in-receiving-water screening, it may not be authorized for discharge.

Example toxicity screening:

Chemical ABC is proposed to be continuously used at a maximum of 10 lbs/d and discharged through a 0.50 MGD (avg) outfall. The calculated effluent concentration is 2.4 mg/L. Acute and chronic toxicity data are provided for acceptable vertebrate and invertebrate species. The lowest toxicity concentration is 20 mg/L for the most sensitive (chronic) species reported. Applying the safety factor of 10, the "toxic threshold" is predicted to be 2.0 mg/L and the toxic-in-effluent screening "fails" (2.4 mg/L effluent > 2.0 mg/L toxic threshold). Further toxicity review is required. The receiving stream low flow is 0.19 MGD resulting in a calculated in-stream concentration of 1.7 mg/L. The toxic-in-receiving-water screening "passes" since the predicted effluent concentration, in-stream, is less than the toxic threshold. The Department will continue the WTC review.

Water Quality Criteria Evaluation

The WTC ingredients must also be compared to the criteria in 6 NYCRR Part 703 and TOGS 1.1.1 to determine if water quality based effluent limits may be necessary. Additionally, any applicable TMDLs or water quality problems are considered during this review. Effluent limits may be established depending on discharge levels and the availability of acceptable analytical methods.

Review Decision

The review decision will be communicated back to the applicant by email. As with any other wastewater discharge, the Department may implement additional requirements including, but not limited to, best management practices, whole effluent toxicity testing, or other monitoring or conditional requirements.
If a WTC "fails" due to toxicity the applicant may be able to reduce the dosage to a level which is acceptable for both toxicity and their treatment needs and modify their request. Overdosing of WTCs is one of the most common triggers of Toxicity Identification/Reduction Evaluations (TI/REs). Otherwise, the applicant can work with the WTC supplier to identify an alternative chemical which is less toxic to use.

Additional Information

Special Case WTC Ingredients

  • Biocides - Many WTCs are also biocides and must be registered for use in New York State. Otherwise they may not be authorized for use and discharge.
  • Bromine - This includes bromine chloride, sodium bromide, and brominated hydantoins. These compounds have toxicity comparable to chlorine and their use will typically require a SPDES permit modification to include effluent limits.
  • Chlorine - Chlorine is very toxic to aquatic life and its use will require SPDES permit limit.
  • Dimethyldithiocarbamates - These compounds have been used as precipitation aids and possibly for other purposes. Dimethyldithiocarbamates are very toxic to aquatic life and can combine to form, or break down into, a number of other very toxic chemicals. Analytical methods do not exist to sufficiently assess these discharges. Permittees should seek less toxic alternatives.
  • Microorganisms - Microorganisms may be added to assist the function of biological treatment systems and for other purposes. The Department does not routinely review the acceptability of discharging microorganisms other than pathogens. The use of pathogenic, dangerous, or genetically engineered organisms is prohibited. Otherwise, a proposed use may proceed without Department review so long as it will not cause or contribute to noncompliance.
  • Nutrients - Nitrogen (N) and phosphorus (P) compounds may be added to biological wastewater treatment systems which are deficient in nutrients to maintain proper function. Discretionary use of such WTCs in other systems is generally discouraged tributary to waters with N or P TMDLs, impairments, or related water quality concerns. A permittee may be required to demonstrate that the use and discharge of such WTCs to waters with an N or P problem is necessary and that no acceptable alternatives exist. In some cases, permits should be modified to incorporate appropriate requirements to control these substances. If the permit already adequately regulates this pollutant, then further review is unnecessary and revised use may proceed.

Do all WTC uses require submission of a WTC Authorization Request?

WTC use does not require Department review when the chemical is used in a "closed system" where it is not expected to be present in the discharge to waters of the State.

Some WTCs are used by facilities which discharge to municipal sewage collection systems. While Department authorization of such discharges is not required, the facility should follow the authorization requirements of their sewer authority.
As noted above, use and discharge of certain microorganisms and nutrients may not require submission of a WTC Authorization Request. If the existing SPDES permit has the appropriate limits for the chemical ingredients and no limit modification is sought by the permittee then use can proceed without submission of a form.

  • Example: A facility uses chlorine and has determined that it should increase the chlorine dose to meet anti-fouling requirements. If the facility has an existing chlorine limit in its SPDES permit and can achieve this limit at the higher chlorine dose, then a form need not be submitted. Important note: the total residual chlorine limit (TRC) season must match the chlorine use. If the permit contains a TRC limit for the summer only, but chlorine is used year-round for antifouling, a permittee-initiated modification is required to extend the limit year-round prior to use of chlorine outside the summer season.
  • Example: A facility wishes to switch from one manufacturer of ferric chloride to another. If the WTC composition is the same, the toxicity is the same, and the dosage will not increase, a new form does not need to be submitted; however, the permittee should send a letter or email informing DEC of the change in manufacturer. This can cause a question during the annual WTC report later down the road.


A WTC form is required when dye is being used to identify flows to an outfall even though the dye is not being used for treatment. The WTC review process is the mechanism through which DEC can review and authorize or deny chemical additions to flows which will ultimately be discharged by the facility to Waters of the State. There are countless chemicals for which no water quality standards exist, so the WTC review process allows DEC to evaluate aquatic toxicity on a case-by-case basis when there is no water quality standard to protect the receiving waters. Most chemicals DEC reviews through this process are in fact used for treatment of waste and process waters. Nonetheless, this process also captures chemical additions like dyes which may have toxicity when released into receiving waters. Please note that adding dyes to surface waters for celebratory purposes is strictly prohibited.

Use of acids and bases

While pH limits control for the toxicity and water quality standards for most acids and bases, DEC requires submission of WTC Authorization Requests for acid and base use to confirm there are no applicable water quality concerns for components of the WTC. Acids and bases should also be reported on the annual report form as well.

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