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COMMONWEALTH OF MASSACHUSETTS EXECUTIVE OFFICE OF ENVIRONMENTAL AFFAIRS DEPARTM

COMMONWEALTH OF MASSACHUSETTS EXECUTIVE OFFICE OF ENVIRONMENTAL AFFAIRS DEPARTMENT OF ENVIRONMENTAL PROTECTION ONE WINTER STREET, BOSTON, MA 02108 617-292-5500 JANE SWIFT BOB DURAND Governor Secretary LAUREN A. LISS Commissioner TITLE 5 RECIRCULATING SAND FILTERS DESIGN GUIDANCE Effective Date: June 24, 2002 (updated April 2006) Policy # : BRP/BWP/WPeP/G02-3 Program Applicability: BRP/DWM/Watershed Permitting/ Title 5 Program Supersedes: Recirculating Sand Filters (RSF) Design Guide Regulation Reference: 310 CMR 15.202 Approved By:____ [signed] ____________________ Cynthia Giles, Assistant Commissioner, Bureau of Resource Protection I. PURPOSE This document has been prepared by the Department of Environmental Protection to serve as a guide to the design of “Recirculating Sand Filters” (RSF). This is not intended to be a comprehensive design manual nor is it intended to restrict professional judgment in the design but rather reflects accepted design criteria for a typical RSF. This guidance also does not apply to proprietary sand filter systems with certifications or approvals from the Department. Deviations from the design guidelines presented in this document can be allowed by DEP if scientifically and technically valid. The design criteria described herein generally apply to larger systems (i.e. greater than 2000 gallons per day). Accordingly, changes in the design parameters may be necessary and desirable when designing for smaller systems. II. INTRODUCTION In recent years, a variety of environmental, economic and technological factors have conjoined to make enhanced treatment of sanitary sewage a viable alternative to the conventional on-site septic tank and soil absorption system. These alternative systems can treat sanitary sewage to produce effluent meeting or bettering secondary standards of 30 milligrams per liter (mg/L) biochemical oxygen demand (BOD5) and 30 mg/L total suspended solids (TSS). In addition, some of these systems will nitrify and denitrify, to This information is available in alternate format by calling our ADA Coordinator at (617) 574-6872. DEP on the World Wide Web: http://www.state.ma.us/dep Printed on Recycled Paper TITLE 5 RECIRCULATING SAND FILTER DESIGN GUIDANCE Page 2 produce an effluent with a total nitrogen concentration of 25 mg/L or less. While many enhanced treatment systems rely on proprietary technology, the RSF can provide a non- proprietary enhanced treatment alternative. Title 5, 310 CMR 15.000, requires the use of RSF, or equivalent alternative technology, for facilities with design flows from 2,000 to less than 10,000 gallons per day (GPD) when located in nitrogen sensitive areas. RSF are Certified for General Use for enhanced nitrogen removal and approved for Remedial Use under Title 5’s approval process for alternative systems (310 CMR 15.280 et seq.). Title 5 recognizes the denitrifying capability of the RSF. For facilities with a design flow up to 10,000 GPD, 310 CMR 15.2 17 allows an increase in wastewater loading to 550 GPD per acre with the use of an RSF or equivalent alternative technology. III. PROCESS SUMMARY A typical RSF system consists of a septic tank, may include an equalization tank prior to the recirculation tank and a sand filter followed by a soil absorption system (SAS). The septic tank provides primary treatment for the influent. The septic tank effluent may first flow to an equalization tank and then to the recirculation tank. Both tanks would also receive recirculated wastewater that has already been treated through the sand filter. In time controlled small doses, the mixture of septic tank effluent and sand filter filtrate is applied to a sand filter bed. The wastewater is evenly distributed over the media bed by a pressure distribution system. The sand filter unit should be split so that 80 percent of the effluent is returned to the recirculation tank and 20 percent to the equalization tank (if an equalization tank is included), with separate collection chambers and underdrains. As the wastewater trickles downward through the media, biological treatment on the surface of the media particles reduces BOD5, TSS, and bacteria and nitrifies the filtrate. The filtrate is than collected at the bottom of the sand filter, and is returned by gravity to the recirculation tank and equalization tank. If an equalization tank is not required, the 20 percent flow is returned to the septic tank. This highly oxygenated filtrate mixes with anoxic effluent from the septic tank in the equalization or septic tanks and recirculation tank. A portion of the recirculation tank mixture is discharged to the SAS, the remainder is recirculated to the sand filter where the cycle is repeated. The contents of the recirculation tank should be discharged to the SAS by a timer controlled pump equipped with a low level shutoff located at the two-thirds full depth of the recirculation tank. In accordance with 310 CMR 15.202(4)(f), all RSF systems serving a facility with a design flow of 2,000 GPD or greater must discharge effluent to the SAS using pressure distribution. Sand filter design is based on several important factors: TITLE 5 RECIRCULATING SAND FILTER DESIGN GUIDANCE Page 3 o The recirculation ratio (RR); o BOD5 concentration of the sewage; and o The time frame over which flow is generated in the facility. Changing any of these factors may alter the size of the recirculation tank or the surface area of the sand filter. The RR is defined as the ratio of the total flow through the sand filter to the forward (average design) wastewater flow. Typical RR values for an RSF system range between 3 and 5:1. The RR not only determines the amount of wastewater to be pumped to the RSF, but reduces or eliminates odors by mixing a large volume of treated, highly oxygenated filtrate in the recirculation tank with septic tank effluent before the mixture is applied to the sand filter. The time frame in which flow is generated, e.g.- schools, 6 hours; or restaurants, 8 to 12 hours, are examples of compressed flows. A compressed flow is a flow rate that will discharge the average daily design flow volume to the septic tank in less than 24 hours. A compressed flow produces a higher flow rate than the design flows listed in 310 CMR 15.203. This discharge of the design flow to the septic tank in a much shorter time interval than 24 hours (12 hours or less) must be taken into account by utilizing a properly designed equalization tank. The size of the equalization tank should be determined through a hydraulic analysis of the system that distributes the compressed flow over a 20 to 24-hour time period and eliminates surcharging of the sand filter. Flow equalization should be provided when the total daily flow (TDF) occurs over a period of twelve hours or less and should be sized based on a hydraulic analysis of hourly flows. The minimum size equalization tank should be 0.5 times TDF. The equalization tank mixture is pumped to the recirculation tank at a rate that equalizes total flow over a 24-hour period. A timer with a high-level override control should operate the pumps in the equalization tank. IV. DESIGN PROCEDURE The design of the RSF system requires, at a minimum, the following steps: 1. Determine the average daily flow by taking into account flows that are generated in less than 24 hours, compressed flows, and wastewater strength based on influent analysis. The organic loading should not exceed 0.005 lbs BOD5 per square foot or 5 lbs per 1,000 square feet of filter area. Sampling of existing nonresidential wastewater systems prior to design is recommended. For new facilities, wastewater characteristics such as BOD5, TSS, Nitrogen, etc. should be estimated on the basis of the best available comparative information. Organic loading, rather than hydraulic loading, may be the determining factor on nonresidential systems. TITLE 5 RECIRCULATING SAND FILTER DESIGN GUIDANCE Page 4 2. Determine the appropriate recirculation ratio, RR. Generally, this ratio ranges between 3:1 and 5:1. However, certain designs may require different recirculation ratios. 3. Design for compressed flows generated over less than a 24 hour period, generally 12 hours or less, by utilizing a flow equalization tank. 4. Size the sand filter based on the appropriate loading rate, either hydraulic, 3 to 5 GPD/SF or organic limit, 0.005 lbs BOD5/SF. When the organic loading rate is the criteria that control the sizing of the filter a hydraulic loading rate of less than 3 GPD/SF may be required. 5. Size the recirculating pumps based on the recirculation ratio, average daily flow, desired pump cycle time, and the sand filter pressure distribution system design. 6. Size the SAS according to the loading rates and other pertinent design criteria in accordance with 310 CMR 15.000. 7. For systems designed to nitrify and reduce nitrogen, the alkalinity of the influent shall be determined. If the alkalinity is low the design should include a method for adding and monitoring alkalinity to the system. Approximately 8 milligrams of alkalinity are consumed for every milligram of ammonia oxidized. V. INFLUENT CRITERIA A. Pre-treatment: Wastewater must receive initial treatment in a septic tank. Effluent from the septic tank shall discharge to an appropriately sized equalization tank, if required, prior to the recirculation tank that discharges to the recirculating sand filter. All units shall be uploads/s1/ equalisation-guide.pdf