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Effective April 28, 2015 Chapter Four Subsurface Sewage Treatment System Regulations Table III — Minimum Septic Tank Capacity Number of Bedrooms Tank 1 Tank 2 Two or less 1,000 500 Three 1,000 1,000 Four to Five 1,500 1,000 Six to Seven 2,000 1,000 Eight to Nine 2,500 1,250 Ten or More Septic tank shall be sized as an MSTS with the second tank in series being at least 50 percent of the capacity of the first tank Multiple Family Dwelling containing two or more units Size shall be the sum of the individual dwelling unit requirements. 13.6 Common tanks. For systems serving ten (10) or fewer dwellings with a common septic tank, the liquid capacity must be determined by adding the capacities for each dwelling as determined in this section. 13.7 Septic tank capacity for multiple dwellings. For systems serving more than ten (10) dwellings with a common septic tank, the requirements of subpart (1) and (2) apply: (1) Total septic tank capacity for common tanks serving multiple dwellings under gravity flow to common tanks is determined by multiplying the design flow by 3.0; or (2) Total septic tank capacity for common tanks serving multiple dwellings under pressure flow to common tanks is determined by multiplying the design flow by 4.0. (3) Total septic tank liquid capacity for systems employing individual tanks at each dwelling discharging into a collection system must be determined: (A) By a Minnesota licensed professional engineer; or (B) According to the Prescriptive Designs and Design Guidance for Advanced Designers, incorporated by reference under part 7080.1550, subpart 2. 13.8 Septic tank capacity for other establishments. Total septic tank liquid capacity for other establishments with domestic strength waste as defined in Section 2.2 (32), is determined by multiplying the design flow by 3.0 if receiving sewage under gravity flow, by multiplying the design flow by 4.0 if receiving sewage under pressure flow, or in accordance with Section 13.12 (Prior to other treatment devices). Additional design considerations, such as equalization tanks, additional capacity, grease interceptors, or secondary treatment, are required for influent concentrations that exceed the levels identified in Section 5.2 (2)(A). 13.9 Holding Tank Capacity for Other Establishments. Holding tanks serving Other Establishments must provide storage of at least five times the design flow. 13.10 Effluent Filters Required An effluent screen with an alarm must be installed on the outlet of the last tank in series. 13.11 Tanks connected in series. Septic tanks must be connected in series. Each tank or compartment must contain at least 25 percent of the required total liquid capacity. The first tank must be equal to or larger than any subsequent tank in the series. 13.12 Prior to other treatment devices. Septic tank liquid capacity prior to other treatment devices must accord with the manufacturer's requirements, accepted engineering principles, or as identified in the product registration recommended standards and criteria. 49 Washington County Development Code