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Zone of Mixing Approval

A. Legal Authority

  • Clean Water Act, as amended, 33 U.S.C. 1342
  • Title 40 CFR, § 122
  • Chapter 342D, Part III, HRS, Water Pollution Control
  • Title 11, Chapter 54, HAR, Department of Health, Water Quality Standards


B. Purpose

The Zone of Mixing (ZOM) works “hand in glove” with the NPDES permit by providing a form of variance to allow an effluent discharge to be assimilated into state marine waters under certain conditions, so as not to violate water quality standards (§11- 54-9 HAR). A ZOM is a limited area around the outfall of a fixed point source that provides for the initial dilution of waste effluent upon its entry into the receiving waters. ZOMs are accepted as being necessary for the assimilation of certain discharges, including aquaculture discharges, which have received the best degree of treatment or control. It is the objective of these limited zones to provide for a practical means to manage the placement and manner of discharges or emissions so as to achieve the highest attainable level of ambient water quality or otherwise to achieve the minimum environmental impact from dilution and dispersion from substances which may be considered pollutants.

C. Applicability to Aquaculture

ZOM approval is required for aquaculture facilities that discharge effluent into a location where receiving water quality standards for that area would be violated by the effluent without further dilution or treatment prior to discharge. Specific water quality standards for the state and the classification of receiving waters are found in Title 11, Chapter 54 of the Department of Health Administrative Rules. In general terms, ZOMs are not allowed for discharges into waters categorized as pristine or protected (Class 1 Inland Waters, Class AA Marine Waters, and Class 1 Marine Bottom Ecosystems). Descriptions of the classifications are found in §11-54-2, through §11-54-7, HAR. ZOMs may be allowed for discharges into other waters (e.g., Class A marine waters) consistent with the standards and requirements set forth in the administrative rules. A ZOM approval is normally processed in conjunction with an NPDES permit.

Approval of a ZOM can only be made if:

  • The operation which is making the discharge is in the public interest;
  • The discharge does not substantially endanger human health or safety;
  • Compliance with the existing water quality standards from which a zone of mixing is sought would produce a hardship without equal or greater benefits to the public;
  • The basic standards of all waters are not violated; and
  • The discharge has/will receive the best degree of treatment or control.

Every ZOM approved and established must be monitored, including benthic (bottom) biological communities, on an ongoing basis by the successful applicant and the results reported to DOH and if required, to DLNR.

D. Administering Agency

The Department of Health, Clean Water Branch, is responsible for administering the ZOM approval. The Department may approve the establishment of a ZOM after review and concurrence by the Regional Office of the U.S. Environmental Protection Agency.

Clean Water Branch
Environmental Management Division
Department of Health
State of Hawaiʻi
919 Ala Moana Blvd., Room 301
Honolulu, Hawaiʻi 96814-4920
Phone: 808-586-4309
Fax: 808-586-4352
Web site: (see web site for Neighbor Island Branch Office information: Kauai; Maui; Hilo, Hawaiʻi; and Kona, Hawaiʻi)

E. Information Requirements

The application for a ZOM approval must be made on the DOH CWB-ZOM form found on the CWB website. The application should be made concurrent with any application for an NPDES permit. Categories of information on the form include the following:

  • Owner information;
  • Owner type;
  • Operator and facility information;
  • Receiving state water(s) information;
  • Period of discharge;
  • Note if attached to the NPDES application;
  • Pollution control measures presently applied prior to discharge;
  • Present waste water discharge;
  • Source and quantity of discharge;
  • Physical quality of discharge;
  • Specific water quality criteria parameters;
  • Description of present conditions and use of receiving water;
  • Proposed ZOM boundaries map; and
  • Best degree of treatment.

F. Public Participation

A public hearing is required prior to approval of a ZOM in the county where the source is located. If an NPDES permit is being considered in conjunction with a ZOM, the public notification and comment process established for the NPDES permit can be substituted for the hearing requirement.

G. Process Time

The processing time for a Zone of Mixing approval varies with the complexity of the project and the processing time for the NPDES permit. For minor project proposals, processing the NPDES/ZOM package can be expected to take from three to four months from the acceptance of a completed application. A ZOM approval is issued for a period of time up to five years and is tied to the approval period for a NPDES permit. Permits are renewable and transferable.

H. Sequence of Filing

An applicant should consult with the DOH CWB early in the planning process to consider applicable water quality standards for the proposed site and effluent disposal alternatives to determine approval requirements and facility design alternatives. Generally, for aquaculture projects, an approved NPDES/ZOM permit package must be in place before the yearly production reaches 100,000 pounds or the biomass on site reaches 100,000 pounds.

I. Cost

There is no filling fee for the ZOM. Public Notice and Hearing fees will vary. The cost of preparing the application and the necessary supplemental information varies with the scope of the project. When established, ZOMs are generally conditioned upon the applicant conducting certain monitoring tests to ensure compliance. The cost to conduct such monitoring can be significant.

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Department of Agriculture – Aquaculture and Livestock Services
99-941 Halawa Valley Street
Aiea, Hawaiʻi 96701-5602
Phone: 808-483-7130
Fax: 808-483-7110
Web site:


Pacific Region Aquaculture and Coastal Resource Hub
2525 Correa Road, HIG 238
Honolulu, HI 96822
Phone: (808) 956-7031

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