Virginia Administrative Code › Title 9 - ENVIRONMENT › Agency 25 - STATE WATER CONTROL BOARD › Chapter 790 - VIRGINIA POLLUTANT DISCHARGE ELIMINATION SYSTEM (VPDES) PERMIT REGULATION › Part III - Manual of Practice for Sewerage Systems and Treatment Works › Article 9 - Natural Treatment › Section 9VAC25-790-890 - Constructed wetlands

Current through Register Vol. 40, No. 24, July 15, 2024
A. Design.
					 These unit operations typically consist of inundated or saturated media
					 supporting flora and fauna typically found in natural wetlands. Two basic
					 designs are referred to as submerged flow systems (SFS) and free water surface
					 systems (FWS). Terms that are also considered synonymous with these systems
					 include (i) rock-plant filters; (ii) marsh-reed filters; (iii) microbial
					 rock-plant filters; and (iv) artificial wetland bio-reactors.
1. The design of constructed wetlands is
					 considered nonconventional technology. Design loading values shall be
					 established in accordance with the type of treatment proposed, established
					 performance data, and site specific features. The use of indigenous wetland
					 flora is recommended provided that those species proposed have been evaluated
					 as suitable for such use by technical experts qualified to make such
					 judgements. Certain flora and fauna may be restricted for use in constructed
					 wetlands.
2. All constructed
					 wetlands shall be preceded by pretreatment of sewage, established as at least
					 equivalent to primary treatment in accordance with this chapter and standards
					 contained in this chapter. Constructed wetlands may be preceded by secondary or
					 better treatment when used for effluent polishing, nutrient reduction, or
					 advanced treatment.
3. The design
					 of individual constructed wetlands shall provide the appropriate features
					 specified for pond treatment systems in accordance with this chapter. Required
					 detention times may vary from one day to 20 days or more, in accordance with
					 the type of pretreatment and the issued permit or certificate effluent
					 limitations.
4. The following
					 factors shall be considered in the selection of the design hydraulic and
					 organic loadings: strength of the influent sewage, effectiveness of primary or
					 secondary treatment, type of media, ambient wastewater temperature for winter
					 conditions, and treatment efficiency required.
5. For design flows of 0.1 mgd or more, the
					 treatment system shall be divided into multiple units that can be operated
					 separately. Each unit shall have the ability to be sufficiently drained for
					 operational maintenance. Design considerations may include parallel treatment
					 streams or trains that can be operated independently of each other.
6. The constructed wetland units shall be
					 designed to operate with plug flow type hydraulics. A proper length to width
					 ratio to achieve this condition should be considered in the design of each
					 system. The inlet design shall provide for proper distribution of the
					 influent.
7. All treatment units
					 shall be provided with outlets that can withdraw flow at various depths (a
					 minimum of three). FWS outlets shall be submerged and be able to exclude
					 floating detrital material and scum.
8. The design shall allow for each unit to be
					 taken out of service at any time and its flows routed to another unit. The
					 treatment system must be capable of treating the daily average flow with the
					 largest unit out of service.
9. All
					 FWS systems shall be situated so as to minimize the adverse effects of the
					 prevailing winds.
10. All systems
					 should maintain a minimum slope along the bottom of at least 0.075% to
					 facilitate draining.
11.
					 Constructed wetland design should allow inlet and outlet depth levels to be
					 raised and lowered in order to (i) vary water levels within the unit basin;
					 (ii) provide the ability to flood the media surface when necessary; and (iii)
					 to drain the unit basin sufficiently for maintenance.