A. The project design, construction plans and specifications are required in sequence. Plans and specifications for
projects involving a pumping station with greater than two horsepower pump(s) will bear the stamp and signature of a professional engineer, registered in Delaware, responsible for the design on each sheet. Plans and specifications for
gravity collection systems and small grinder pump stations with pump(s) of two horsepower or less will bear the stamp
and signature of a professional engineer or professional surveyor registered in Delaware responsible for the design on
each sheet.
B. The project design report shall be prepared presenting the following information :
(1) Present area served, as well as future areas to be served, including population data, above the point of connection
with the county sewer system.
(2) Quantity and quality of existing sewage and changes in the characteristics anticipated in the future at the connection point. If such records are not available, flow determinations and sewage analysis shall be made and such information
included in the report.
(3) Maximum, minimum, average and design flow rates being considered in design for the proposed development.
(4) Terrain data in sufficient detail to establish general topographical features of present and future areas to be served.
(5) Minimum and maximum sewer grades and velocities proposed.
(6) Lift stations existing and/or required, the operating characteristics of the stations at minimum, maximum, average and design flows (both present and future) with an analysis of the pump operations.
(7) Effect of proposed system expansion on the existing system.
(8) Amount of infiltration anticipated and its hydraulic effect on the proposed or existing system.
(9) The engineering drawings submitted shall be prepared in accordance with the following :
(a) Maps and plans shall be reproduced on paper not greater than 24 inches by 36 inches in size. However, where variations are necessary, they shall be uniform as to the size and all plans and specifications shall be securely bound.
(b) Plans and profiles for sanitary sewers, insofar as practical, shall be prepared using the following scales :
Horizontal
Vertical
1 inch = 30 feet
1 inch = 3 feet
1 inch = 40 feet
1 inch = 4 feet
1 inch = 50 feet
1 inch = 5 feet
1 inch = 100 feet
1 inch = 10 feet
(c) The size, grade and type of pipe material shall be shown. The hydraulic gradient, losses at structures, etc., should be shown on all profiles of sewer lines.
(d) The location and structural features of the sewers, including manholes, to be installed shall be shown on plans and profiles. A separate sheet shall be provided showing the detailed design of appurtenances.
(e) The plans and specifications for lift stations shall include details as to the installation of pumps, valves, controls, ventilation facilities, construction of stairways, and all related/associated appurtenances.
(10) Industrial waste and individual treatment facilities are not considered in this article. Those facilities will be dealt with separately by the Public Works Director.
D. Specifications should be submitted at the same time as the construction plans. The construction plan will include, in general, the information contained in the project design report with changes in the plans of development.
Gravity Sewer System Design...
A. Design Capacities...
(1) In general, sewer lines shall be designed for the estimated contributory population to be served in the future, plus adequate allowance for infiltration, institutional and industrial flows. Provisions shall be made to extend sewer service
to all adjoining properties.
(2) The maximum hourly quantity of domestic sewage, maximum hourly quantity of domestic sewage and waste from industrial plants and groundwater infiltration shall be considered in determining the capacities of sanitary sewers.
(3) The design of extensions to existing sanitary sewage collection systems shall be based on data if adequate records
have been kept. In the event that adequate records are not available, or when new systems are being established, the
design criteria shall not be less than 300 GPD/equivalent dwelling unit (EDU) or 200 gpd/mobile home.
(4) Laterals and minor sewers shall be designed, when flowing full, assuming flow equivalent to three times the average
daily flow. Main trunk, interceptor and outfall sewers shall be based on the following equation :
Q MAX / Q AVG = ( 20 + E ) / ( 5 + E )
Where ;
Q MAX : Maximum rate of sewage flow
Q AVG : Average daily sewage flow
E : The number of equivalent dwelling units in thousands
(5) Main trunk, interceptor and outfall sewers shall be designed at minimum slope and maximum depth.
B. Design Details...
(1) No sewers other than building sewers shall be less than eight inches in diameter. Building sewer laterals located in
the sewer easement or public road right-of-way shall be a minimum of six-inch diameter with a vertical cleanout riser of four-inch minimum diameter located at the property line. Cleanout covers shall be cast iron or brass.
(2) All sewers shall be designed and constructed with hydraulic slopes sufficient to give mean velocities when flowing
full or half-full of not less than 2.0 feet per second.
(3) In general, grades which will produce velocities in excess of 15 feet per second will be considered as unacceptable.
(4) Sewers 24 inches or less must be laid in straight alignment with uniform grade between manholes. For larger pipes,
where horizontal curvature must be utilized to serve a particular area, the smallest radius of curvature shall be 200
feet, with the Public Works Director's approval.
(5) Sewers shall be designed deep enough so as to prevent freezing and pipe failure. Three feet is the minimum cover depth for normal design conditions.
(6) Manholes shall be placed at the points of all changes in alignment, grade or size of sewer, intersection of sewers, junction of streets, and at a distance not greater than 400 feet for sewers 15 inches or less in diameter. Not more than
500 feet spacing shall be allowed for pipes 18 inches or greater in diameter.
(7) Manholes should be designed large enough to provide easy access to the sewer. The clearance opposite the steps should
be sufficient for a man to pass through without difficulty.
(8) No manhole shall be located in gutters and ditches. Manholes in the low spots in the roads should be provided with watertight covers. No drop manhole or lamp holes shall be used without authorization of the Kent County Department of
Public Works, Engineering Division. All manholes shall be provided with inserts/bowls to mitigate inflow.
(9) A drop manhole should be provided for a sewer entering a manhole at an elevation of 24 inches or more above the manhole invert. If the drop is less than 24 inches, the invert should be filleted to prevent the deposit of solids. Drop manholes should be constructed with an inside drop pipe and utilize a minimum of a five-foot diameter manhole. A "T" shall be used
at the top of the drop pipe with the drop pipe connected to the branch and the end provided with a removable cap. The drop pipe shall be secured to the interior wall of the manhole at three-foot intervals and the flow directed into the flow
channel with a suitable elbow.
(10) Watertight manholes are required at all times.
(11) Manholes shall be of the precast concrete or poured-in-place concrete type. Manholes shall be waterproofed on
the exterior with a bituminous coating. Inlet and outlet pipes shall be joined to the manhole with a gasketed flexible
watertight connection or any watertight connection arrangement that allows differential settlement of the pipe and
manhole wall to take place. Connections to existing manholes shall be made utilizing a hole saw and a tycite adapter
with A-lok gaskets or link seals.
(12) The use of inverted siphons shall be kept to a minimum; but, where they must be used, they shall consist of at least
two lines, one of which will be of a size of not less than eight inches in diameter. The minimum velocity used in their design shall be three feet per second considering the average dry weather flow. Under the minimum dry weather flow conditions, the independent operation of one of the lines shall provide the minimum velocity of three feet per second.
Where the above conditions cannot be met, some other means shall be provided.
(13) Clearances between sewers and other utilities, both existing and proposed, should be designed as follows :
(a) Sewers crossing water mains. Sewers shall have a minimum clearance of 18 inches measured from the outside of the water main to the outside of the sewer. This shall be the case whether the water main is above or below the sewer. Where possible the sewer shall be beneath the water main. The crossing shall be arranged so that the sewer joints will be equidistance and as far as possible from the water main joints. Where a water main crosses under a sewer, adequate structural support shall
be provided.
(b) Sewer parallel to water main. Sewers shall be kept at least 10 feet horizontally from any water main as measured from
the outside of the water main to the outside of the sewer. In cases where it is not practical to maintain a ten-foot separation the Kent County Department of Public Works, Engineering Division, may allow deviation on a case-by-case basis if supported by data from the design engineer or surveyor. Such deviation may allow closer separation, provided that the water main is in a separate trench or undisturbed earth shelf located on one side of the sewer and at an elevation so the bottom
of the water main is at least 18 inches above the top of the sewer.
(c) If sewer and water main cannot be built to specified clearances, the sewer shall be designed and constructed equal to
the water pipe and shall be pressure tested to assure watertightness prior to backfilling.
(d) Crossing other utilities. Sewers shall have a minimum of twelve-inch clearance from drains, electric lines, gas mains, etc.
(e) Concrete encasement shall be considered where the required safe supporting strength cannot be obtained by other bedding methods.
C. Flow Measurement...
A Parshall flume with electronic transducer type flowmeter shall be provided on gravity discharges from a contract user
or from a sanitary district into the county conveyance system at a point where it discharges into a different sanitary district, a municipal system or a county pumping station. A flowmeter may be waived by the Public Works Director for
contract users if water billing or potable water flow measurement is provided and the user and county mutually agree
to use potable water flow for wastewater billing purposes. The flowmeter shall provide flow totalization and digital instantaneous flow rate in gpm. If the flume is located in a metering pit the floor shall drain to the flume. The
totalizer shall be located above ground in a suitable enclosure to protect it from the weather.
Sewage Pumping Station Design...
A. Pumping Station and Its Facilities...
Pumping stations must be turned over to the County immediately after final acceptance. They must adhere to the following design criteria :
(1) Selection of a site. In the selection of a site for a pumping station, consideration shall be given to its accessibility and its potential nuisance aspects. The station shall not be subject to flooding and it shall be accessible during all types of weather with mobile equipment. Stations shall be located at the lowest elevation including adjoining areas. Adequate provisions must be provided for continuous operation, and for emergency treatment or storage of the sewage in the event of
an unanticipated breakdown of the pumping station facilities or power failures.
(2) Type of pumping station. In general, all of the pumping stations should be of the wet well/dry well or submersible type. Suction lift pumping stations may be approved for certain installations. Grinder pump stations shall be reserved primarily for individual residential units with some exceptions made with the approval of the Public Works Director. Ejector stations are unacceptable.
(3) Wet-pit and dry-pit separation. Wet-pit and dry-pit shall be separated by a watertight and gastight wall with separate entrances provided to each. Equipment requiring regular or routine inspection and maintenance shall not be located in the wet-pit unless the top of the wet-pit is completely open or provided with a grate cover.
(4) Sump pump. A separate sump pump shall be provided for removal of leakage or other water from the dry well floor. The
sump pump shall be capable of delivering 20 gpm. The pump shall discharge into the wet-pit with a check and a gate valve. Water and bibcock for cleaning wet wells must be made available.
(5) Stairways. All underground dry-pits, and wet-pits where bar screens or other equipment requiring inspection and maintenance are located, should be provided with stairways. Removable ladders may be provided in small stations where it is impractical to install stairways, but manhole steps will not be acceptable.
(6) Heating and ventilating. Ventilation shall be provided for all pumping stations, including both wet and dry-pits. Mechanical ventilation shall be provided for all dry-pits where equipment is located. The ventilating system and lights
shall be turned on by a switch located near the entrance. The ventilation system of the wet-pit and dry-pit of the
station should be entirely separate and conform with the Ten State Standards. All openings for pipes or electric
cables should be caulked gastight. All pumping stations, except those entirely below ground, should be supplied with automatically controlled heaters to prevent freezing in cold weather.
(7) Lighting. Lights and switches will be properly provided for the stations. A vaportight light fixture with lamp guard
and a one-hundred-watt incandescent bulb shall be installed in the wet-pit. Adequate lighting shall be provided for convenience and safety of operation. The lighting shall provide illumination for all areas in the station and control
panels.
B. Pumping Design Criteria...
(1) Type of pumps acceptable. Except where grinder pumps are used in order to minimize clogging, open impeller or non-clog-type pumps capable of passing spheres of at least three inches in diameter shall be provided. Inspection and clean-out plates on the casing of the pump or hand hole in the first fitting connected to the suction on the pump shall be provided for clearing stoppages.
(2) Number of pumps required. At least two pumps shall be provided at all pumping stations. If no more than two pumps are provided, both should have the same capacity and each shall be capable of pumping the maximum flow with the standby pump
out of service. It is desirable for three or more pumps to be provided which are designed for actual flow conditions and
of such capacity that with any one pump out of service, the remaining will be of sufficient capacity to handle maximum
flows. Sewage pumps having suction lifts of a maximum of 15 feet will be approved only where the pumps are self-priming
and adequate maintenance provisions are included in the design. Suction lifts for pumps dependent on foot valves to maintain their prime will be approved only when preceded by adequate primary settling facilities.
(3) Pump suction connection to wet-pit. Each pump shall have an individual flow intake and suction line. Wet well design should be such as to avoid turbulence near the intake. Intake piping shall be a minimum of four inches and shall be as straight and short as possible. The submergence required above a flared inlet is approximately as follows :
Velocity at diameter D ( feet / sec )
Required submergence ( feet )
2
1
5
2
7
3
11
7
15
14
(4) Piping and valves.
(a) Full-closing shutoff valves shall be placed on the suction line of each pump except on submersible and vacuum-primed pumps. Suitable shutoff and check valves shall be placed on the discharge line of each pump. The check valve shall be
located between the shutoff valve and the pump. Check valves shall not be placed on the vertical portion of discharge
piping. Valves shall be capable of withstanding normal pressure and water hammer. Valves shall not be located in the wet well. Valves shall be located in the dry-pit, above ground structure or valve pit. Valve pits shall be drained to the wet well with a flapper provided to prevent sewage from entering the pit during surcharged wet well conditions.
(b) In the event that all pumps are inoperable and must be pulled for service a shutoff valve shall be provided downstream
of the discharge header and a blind flange provided on the discharge header or between this shutoff valve and the individual pump shutoff valves.
(5) Motors and controls. Three-phase, two hundred forty volt service is recommended for use at stations requiring less
than 40 horsepower motors, where the stations have higher horsepower requirements, four hundred and eighty volt service
is required.
(6) System analysis. The design engineer shall provide a hydraulic system analysis for each pumping station. It shall
include the calculation of the system-head curves and the use of these curves in conjunction with the characteristic
curves of available pumps.
C. Operation Schedule and Control...
(1) Wet-pit operation schedule. The wet-pit must be large enough so that excessive pump starting and stopping will not take place, which may result in overheating of motors and controls. The design engineer shall use the criteria for average and peak flow outlined in § 180-18A to determine the influent peak flow. For constant speed pumps the minimum time interval at peak flow between pump stop and lead pump start shall be five minutes. All stations shall have "soft starts." Stations with peak flows in excess of 350 gpm shall be provided with variable speed pumps and variable frequency drive controls. Except with suction lift pumps the high-water level should never be below the top of the pump casing and the impeller to ensure
that the pump will prime. The low level setting should not be less than six inches above the suction pipe.
(2) Pump operational control mechanisms. Control systems shall be of the encapsulated float type, electronic transducer
type or the flow measuring type. Float tube type and bubbler type controls are not acceptable. These devices shall be
located so that they will not be affected by flow currents created by the entering sewage or pump suctions. Provision shall also be made to prevent floating material in the wet well from interfering with the operation of the controls. The pump control unit shall automatically alter the pumps in use. The pump control cabinet shall be provided with "on-off-automatic" switches and a green run light for each pump. Control cabinets for submersible pumps shall be provided with a red seal failure light for each pump indicating shaft seal failure or potential shaft seal failure.
D. Screening or Comminution...
Except for grinder pumps screening or comminution of raw sewage shall be required prior to the raw sewage being pumped. On pumping stations up to 50,000 gpd as a minimum a screening basket shall be provided on the influent line with a rail and hoist system. On pumping stations up to one MGD as a minimum a comminutor shall be provided. On stations in excess of one
MGD a mechanically cleaned bar screen with openings not exceeding 3/4 inches shall be provided. A bypass channel with a manually cleaned bar screen with openings not exceeding 21/2 inches shall be provided where comminutors and mechanically cleaned screens are utilized to provide screening while maintenance is performed on the mechanical screen or comminutor. Manually cleaned screens should be provided with stairway access, adequate lighting and ventilation, and convenient and adequate means for removing screenings. If there is not adequate room in the wet-pit for a comminutor, a separate comminutor station should be used.
E. Flow Measurement...
A magnetic flow meter shall be provided at all pumping stations pumping sewage from a contract user or from a sanitary district into the county conveyance system at a point where it discharges into a different sanitary district, a municipal system, a county pumping station or the County Treatment Plant. A flow meter may be waived by the Public Works Director for contract users if water billing or potable water flow measurement is provided and the user and county mutually agree to use potable water flow for wastewater billing purposes. The flow meter shall provide flow totalization and digital instantaneous flow rate in gpm. A seven-day circular chart recorder shall be provided on stations pumping an average of 100,000 gpd or greater. The magnetic flow meter primary device shall not be located in the wet well. The primary device shall be located
in the dry pit, above ground structure, valve pit or separate metering pit. If a separate metering pit is utilized it shall be drained to the wet well with a flapper provided to prevent sewage from entering the pit during surcharged wet well conditions. The primary device shall be located a minimum of 3 straight upstream pipe diameters from any bend, valve,
"T" or other turbulence producing fitting. The primary device shall be located where the pipe remains full at all times
and shall not be located on a vertical downward flowing pipe. The primary device shall be provided with a bypass. Shutoff valves shall be provided on the bypass and upstream and downstream of the primary device.
F. Emergency Power Supply...
(1) All pumping stations shall be provided with emergency power supply unless the design engineer or surveyor can
demonstrate the following conditions are met: the station is a grinder pump station hooked to an individual residence or small business; the residence or business is served potable water by means of an on-site well which will preclude the generation of sewage in the event of a power outage; the quantity of water stored in tanks and site plumbing is less than
the volume of the wet well above the lead pump on level. Portable generators and manual transfer switches and electric outlets will be accepted for stations with a capacity of less than 50 gpm. Larger stations shall be provided with on-site emergency power generation equipment and an automatic transfer switch. All on-site emergency power generation equipment
shall be provided with diesel, natural gas or liquidized propane gas engines, according to the standard required by the
State of Delaware Department of Natural Resources and Environmental Control. Generator fuel storage tanks shall be above ground and meet all EPA and DNREC requirements.
(2) Alarm system. The alarm shall be activated in cases of power failure, pump failure or any cause of pump station malfunction. The alarms for stations requiring emergency power shall be connected to the county's Wastewater Treatment
Plant. A remote telemetry unit with communications adaptable to the county's existing SCADA system.
G. Force Main Design Criteria...
(1) Except for grinder pump stations the minimum force main size shall be four inches. The following values of "C"
are recommended for use in the Hazen-Williams equation for computing the friction losses in a force main for design conditions :
C = 100 for unlined cast iron pipe, ductile iron and steel pipe: 24 inches and larger
C = 96 for unlined cast iron, ductile iron and steel pipe: 12 inches and larger
C = 89 for unlined cast iron, ductile iron and steel pipe: four inches and larger
C = 120 for cement-lined cast iron pipe, reinforced concrete pipe and asbestos-cement pressure pipe and various types of plastic pipe
C = 140 for plastic pipe
(2) For friction loss in pipe fittings the standard coefficients (k) are to be used as a multiplier of the velocity head evaluated at the downstream velocity.
(3) The minimum velocity shall not be less than 2.0 feet per second for force main design. In general force main velocities shall not exceed 5.5 feet per second for force main design. If the total dynamic head at the pump discharge exceeds 100 feet a larger diameter force main will be used, provided that a velocity of 2.0 feet per second can be maintained. Air release valves shall be provided on lines at all high points in the profile. Blowoff valves shall be provided at all low points in the profile. Grinder pump force mains shall be laid on a rising profile as proceeding downstream. Magnetic tracer tape shall be provided for all force mains and collection lines. Clearance between force mains and water mains shall be the same as specified for gravity sewer lines in § 180-18B(13). Where force mains are constructed of material which might cause the
force main to be confused with potable water mains, the force main should be appropriately identified.
Grease Interceptor and Trap Design Criteria...
Plumbing Connections...
Grease interceptors or traps shall be located in the food service facility’s lateral sewer line between all fixtures that
may introduce grease into the sewer system and the connection to the County’s wastewater collection system. Such fixtures shall include, but not be limited to, sinks, dishwashers, garbage disposals, automatic hood wash units, floor drains in
food preparation and storage areas. Wastewater from sanitary facilities and other similar fixtures shall not be introduced into grease interceptors or traps.
Grease Traps...
Grease traps shall conform to the standards in the Plumbing and Drainage Institute (PDI) Standards G101. They shall be installed in strict accordance with manufacturers instructions, and equipped with a removable cover that can be opened for inspection, sampling and maintenance. The capacity and flow through rate shall meet PDI and DNREC requirements. A flow control device shall be installed to control the rate of flow through the unit, and the trap shall be properly vented.
Grease Interceptors...
Grease interceptors shall have a minimum of two compartments and shall be capable of separation and retention of grease
and storage of settled solids. They shall conform to the Delaware Plumbing Code and certified by the PDI. They shall be
sized in accordance with the Delaware Plumbing Code and the PDI G101 standards. The minimum capacity shall be 1,000 gallons and maximum shall be 3,000 gallons. Where sufficient capacity is not possible in a single unit, installation of interceptors in series is required.
