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 6 - Biological Treatment › Section 9VAC25-790-670 - Attached growth processes

Current through Register Vol. 40, No. 24, July 15, 2024
A. The
					 contactor, or media filled reactor, utilized for attached growth biological
					 processes shall be preceded by primary clarification equipped with scum and
					 grease collecting devices. Other pretreatment facilities equivalent to primary
					 clarification may be proposed for evaluation by the department. The media shall
					 provide sufficient surface area to support the attached biological growth
					 necessary to achieve the desired performance standard. Recirculation of treated
					 wastewater back to the contactor influent should be provided to maintain design
					 loadings.
B. Trickling filters.
					 Biological contactors called trickling filters shall be designed so as to
					 provide either the reduction in biochemical oxygen demand required by the
					 issued certificate or permit, or the treatment necessary to properly condition
					 the sewage for subsequent treatment. This section provides performance criteria
					 to achieve final effluent limits to meet federal secondary equivalency
					 requirements for trickling filters. Such biological contactors may be designed
					 to achieve higher degrees of treatment or used in conjunction with other unit
					 operations. Where the design intent is to achieve other than secondary
					 equivalency levels, the proposed design parameters shall be thoroughly reviewed
					 during the preliminary engineering conference.
1. The hydraulic loading used for design of
					 standard rate trickling filters shall be between two and four million gallons
					 per acre per day with an organic loading between 400 and 800 pounds of
					 BOD5 per acre foot per day.
2. The hydraulic loading used for design of
					 high-rate filters shall be between 10 and 30 million gallons per acre per day
					 with an organic loading between 1,200 and 3,300 pounds
					 BOD5 per acre foot per day.
3. Other design loadings that are based on
					 pilot studies and related to design and performance parameters through rational
					 design equations or models will be evaluated by the department.
4. The performance of biological contactors
					 can be detrimentally affected by diurnal loading conditions. The volume of
					 media as determined from either pilot plant studies or from acceptable design
					 equations shall be based upon the design peak hourly organic loading rate
					 rather than the average rate. An alternative for reducing the design peak flow
					 would involve provision of adequate flow equalization prior to the
					 contactor.
5. Consideration should
					 be given to the use of two-stage biological contactors in series operation
					 where single stage reactors may not accomplish the required removals. Expected
					 treatment efficiencies shall be calculated and documented.