(a) Tributary Population.
(1) Sewerage systems which provide for a complete watershed shall be designed and sized taking into consideration the estimated tributary population for a period of fifty years hence. The entire watershed shall be assumed to be completely built-up according to present or predicted zoning ordinances, whichever requires the greater capacity.
(2) Sewerage systems which provide for only a part of a complete watershed shall be sized to provide for the entire watershed. Otherwise, provision shall be made for future increased capacity. Proper modification to allow for the characteristics (i.e. domestic, commercial and industrial wastes and ground water infiltration) of the area under consideration shall be made.
(3) Trunk and subtrunk sewers shall be designed on a basis of population density of not less than ten persons per acre. Design provisions in excess of this minimum shall be made where the engineer deems it necessary. Supporting data shall be included in the design analysis. Design analyses shall be provided for all trunk and subtrunk sewers and, when required by the County, for collecting sewers.
(b) Capacities.
(1) In determining the required capacities of sanitary sewers, the following factors shall be considered:
A. The maximum hourly quantity of domestic sewage;
B. Additional maximum sewage or waste from industrial plants and commercial areas; and
C. Ground water infiltration.
(2) New sewerage systems shall be designed on the basis of an average per capita flow of sewage from the equivalent population served of not less than 100 gallons per day. On this basis, lateral and submain sewers shall be designed with capacities, when running full, in accordance with the peak flows indicated in Figure 1, entitled “Design Flow for Sewers When Average Flow: 100 Gal./Cap./Day,” following this chapter.
(3) The 100 gallons per capita per day figure is assumed to cover normal infiltration, but an additional allowance shall be made where conditions are especially unfavorable. This figure is likewise considered sufficient to cover the flow from cellar floor drains, but is not sufficient to provide an allowance for flow from foundation drains, roof leaders or unpolluted cooling water, which are prohibited by law from discharging into sanitary sewerage systems. See Chapter 1064.
(4) Unless evidence is presented to prove a different flow from industry at ultimate development, the minimum allowance for industrial flow shall be determined by providing an equivalent population of forty persons per acre or one equivalent population per employee, whichever is greater, in the industrial area. The area includes the entire area zoned for industry, except public road, street and highway rights of way, flood plains on which construction is prohibited and “green zones” separating industrial from residential areas, on which construction is prohibited.
(5) The minimum allowance for flows from commercial areas shall be determined by providing an equivalent population of thirty persons per acre, or one-half the equivalent population per employee, whichever is greater, in the commercial area. The area includes the entire area zoned for commercial development, including off-street parking areas and landscaped areas, but excludes the rights of way of public roads, streets and highways, flood plains of streams on which construction is prohibited and “green zones,” 100 feet or more in width, separating commercial from residential areas, on which construction is prohibited.
(6) In cases where the above criteria are not applicable, an alternate design procedure may be submitted to the County for approval. A description of the procedure used and justification for the modifications for the proposed sewer design shall be included with the design analyses and plans submitted for approval.
(c) Sewer Locations. In general, sewers shall be located on legally established streets or rights of way and shall be equidistant from property lines or curb lines wherever possible. The horizontal distance between sewers and existing or projected water mains shall be not less than ten feet, except where the water mains are located at a higher elevation (a minimum of one and one-half feet above the top of the sewer) than the top of the sewer, in which case a minimum horizontal distance of six feet will be permissible.
(d) Manholes.
(1) Manholes for access to sewers shall be provided at all intersections with other sewers, at all points of change in alignment, at all changes in grade and at the terminal of the line. In addition, access manholes shall be provided at intervals not exceeding 400 feet on all sewers fifteen inches in diameter or less and not more than 500 feet apart on sewers eighteen inches in diameter or larger.
(2) Sewer manholes for sewers up to twenty-four inches in diameter shall be not less than four feet in inside diameter. Manholes for sewers larger than twenty-four inches and up to forty-eight inches shall have an inside diameter of not less than five feet.
(3) Manholes shall have eight-inch walls constructed of brick or block and shall be parged inside and outside with one-half inch cement mortar or shall be of precast concrete construction equivalent to or greater than that specified in A.S.T.M. C 478, the latest revision, and having gasket material in accordance with specification A.S.T.M. 361, the latest revision.
(4) Manhole frames and covers shall be gray cast iron and shall be designed for roadway traffic. The word “sewer” shall be cast in the cover.
(5) Manhole bases shall be of Class A reinforced concrete and shall be a minimum of nine inches thick.
(e) Minimum Sewer Size. No public sewer shall be less than eight inches in diameter.
(f) Sewer Connections. Connections to sewer lines eighteen inches in diameter or larger shall be made only at manholes.
(g) Hydraulic Design Criteria. The hydraulic design and determination of sewer sizes shall be based on the following conditions:
(1) Sewers shall have a uniform slope and straight alignment between manholes.
(2) At all junctions where a sewer of smaller diameter discharges into a larger one and at all locations where the sewer increases in size, the invert of the larger sewer shall be lowered so that the energy gradient of the sewer at the junction is at the same level. Generally, this condition will be met by placing the crowns of the two sewers at the same elevation.
(3) Sewers shall be designed to be free flowing with the hydraulic grade below the crown and with hydraulic slopes sufficient to provide an average velocity, when running full, of not less than 2.25 feet per second (f.p.s.). Computations of velocity of flow shall be based on the following values of “n,” as used in the Kutter or Manning formula for velocity of flow. For pipe sizes eight to twenty-four inches in diameter, laid in lengths up to five feet, “n” equals 0.013. For pipe sizes eight to twenty-seven inches in diameter, laid in lengths of six feet or longer, “n” equals 0.012. For pipe sizes thirty inches in diameter and up, laid in lengths of four feet or more with tongue and groove-type joints carefully made smooth, “n” equals 0.012.
(4) For sewage flow depth less than one-fourth full, allowance shall be made for increased value of “n” and in no case shall velocities of less than 1.3 f.p.s. be permitted. The improved velocities shall be accomplished by steeper grades and not by increasing the pipe diameter.
(5) The maximum permissible velocity at average flow (before applying the peak flow factor) shall be fifteen f.p.s. Suitable drop manholes shall be provided to break the steep slopes in order to limit the velocities in the connecting sewer pipes between manholes. Where drop manholes are impractical for the reduction of velocity, the sewer shall be of cast iron or other abrasion-resistant material.
(h) Structural Design. The structural design of sewers shall conform with the methods given in the A.S.C.E. Manual No. 37 for the “Design and Construction of Sanitary and Storm Sewers.” Sewers installed in streets shall generally have a minimum cover of six feet.
(i) Pipe Materials. The County reserves the right to select the type of material used in sewer installation from the following list of materials:
(1) Asbestos cement pipe. Asbestos cement sewer pipe shall be in conformity with A.S.T.M. Designation C428, the latest revision, and must also be autoclave cured in a manner to eliminate all free lime.
(2) Clay pipe. Vitrified clay pipe shall be in conformity with A.S.T.M. Designation C13, the latest revision, for standard strength clay sewer pipe, or A.S.T.M. Designation C200, the latest revision, for extra strength clay sewer pipe. All joints shall conform to A.S.T.M. Designation C425, the latest revision.
(3) Reinforced concrete pipe. Reinforced concrete sewer pipe shall be in conformity with A.S.T.M. Designation C76, the latest revision, for Class II, Class III, Class IV or Class V with the exception that absorption shall be limited to five percent. All joints shall be of the “O” ring type.
(4) Cast iron pipe. Cast iron pipe used for sewer installation shall be in conformity with American Standard Specifications for Cast Iron Pipe for Water (A.S.A. A-21.2, A-21.6 and A-21.8). Wall thickness shall be computed according to A.W.W.A. Manual C101. No cast iron soil pipe shall be used as a public sewer, but such pipe may be used for building sewers as required by Chapter 1064. (Adopting Ordinance)