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Stabilization Ponds - 3...

Introduction...

A stabilization pond is a large shallow excavation that receives sewage from a sewer system, the sewage so that biological processes can destroy most of the disease-causing organisms, and discharges the effluent as treated. Sometimes two or more ponds are constructed and connected by pipes. Constructing stabilization ponds requires the services of an experienced construction supervisor and surveyor. Construction involves assembling labor, materials, and tools; preparing the site; staking the pond, embankment, and pipe locations; excavating the pond; building embankments; laying pipes; and finishing embankments. This technical note describes how to sewage construct a stabilization pond. Read the entire technical note before beginning construction.

Useful Definitios...

EFFLUENT : Settled sewage
TREATED SEWAGE : The liquid that flows out of a stabilization pond or series of ponds; treated sewage is safer than settled sewage and may be used to irrigate crops not intented for human comsumption.

Materials Needed...

Before construction can begindetains , the project designer must provide :

1. Location map, or master sewer map.

"Figure 1 : Master Sewer Map with Site of Stabilization"...

2. Design drawing of the pond.

"Figure 2 : Pond Configuration"...

3. Design drawing of the inlet, outlet, and embankment.

"Figure 3 : Detail of inlet, Outlet and Embankments"...

4. Materials list.

"Table 1 : Sample Materials List for a Stabilization Pond"...

If more than one pond based is being constructed, the project designer must also provide :

5. Design drawing of the pond system layout.

"Figure 4 : Parallel System Layout"...

"Figure 5 : Series System Layout"...

6. Design drawings of all interpond piping.

"Figure 6 : Inter-Pond Piping"...

7. Materials list attachment.

"Table 2 : Sample Materials List Attachment for Interpond Piping"...

You will also need :

8. All labor, materials, and tools described in the materials list and materials list attachment.

Construction Steps...

Depending on local conditions, availability of materials, skills of workers, and so on, some construction steps will require only a few hours, while others may take a day or more. Read the construction steps and make a rough estimate of the time required for each step based on local conditions. You will then have an idea of when during the construction process specific workers, materials, and tools must be available. Draw up a work schedule similar to Table 3 showing construction steps.

"Table 3 : Sample Work Schedule for Constructing a Stabilization Pond"...

Preparing the Site...

1. Locate the site and temporarily mark it on the ground.
2. Assemble all labor, materials, and tools needed to begin construction.
3. Clear the pond and embankment site of all trees, bushes, stumps, brushwood, large rocks, and any other material not suitable for building the embankment. Haul this list material to a landfill or other disposal site.
4. Remove any trees upwind from the site for a distance of l00 - 200 m. This will create an unobstructed windpath, which will improve the efficiency of the pond after it is put into operation.
5. Remove topsoil or solid from the site and place it to one side. This will be used later to finish the embankment.

Staking Pond Site and Pipe Locations...

1. Set reference stakes 5 -10 m apart indicating the boundaries of the bottom of the pond. Find the elevation of each stake using a surveyor's level and rod from the base point used in constructing the sewer. See "Constructing Sewer Systems," SAN.2.C.4.
2. Measuring the distance and elevation from the reference stakes, set slope stakes indicating the points at which to begin building the embankment and excavating the pond.

"Figure 7 : Staking the Pond Site"...

3. Set stakes to indicate pipe locations. This will eliminate reexcavating portions of the embankment.

Excavating the Pond...

1. Begin excavating at the inside slope stakes. Dig at the slope specified by the project designer until the bottom elevation is reached. Check this elevation with a surveyor's level and rod.

"Figure 8 : Pond Excavation"...

2. Continue excavating along the bottom elevation of the pond. Use excavated soil to build up the embankments (see "Building Embankments").
3. Make the bottom of the pond as level and as uniformly compacted as possible. If there are soft spots or tree roots, dig them out, fill with moist soil, and compact.
4. Make the corners of the pond rounded.
5. Leave some excavated soil on the pond bottom if small dikes are to be built for the start of pond operation. See "Operating and Maintaining Stabilization Ponds," SAN.2.O.5.

Building Embankments...

1. Begin building embankments as the pond is excavated. Embankments must be well tamped, with sides sloped according to design specifications.
2. Leave gaps in the embankment at pipe locations as shown in Figure 9. It may also be convenient to leave one or more wide gaps for removal of excavated soil.

"Figure 9 : Excavated pond and Embankments"...

3. The top of the embankment must be level, well-tamped, and at least 1.0 m wide. The horizontal distance from the top of the embankment to the bottom of the pond must equal the design depth of the pond plus 1.0 m.

Laying Pipes...

1. Excavate trenches for pipes at the design depth and locations. The bottoms of the trenches should be well-tamped.
2. Build bases about 0.5 m high for the inlet pipes from concrete or stone. The purpose of the bases is to raise the inlet pipe above the bottom of the pond.
3. Build slabs for the outlet pipes from concrete or stone. The purpose of the slab is to support the outlet pipe and to prevent erosion due to the discharge of treated sewage. Build support slabs under all valve locations.
4. Lay sewer pipe and mortar together sections; Install valves.
5. Build the vertical outlet from sleeved sections of pipe. The height of the vertical outlet determines the depth of the pond. It must be equal to the design depth calculated by the project designer. The sleeved sections will allow the pond to be drained when necessary.
6. Build a protective screen around the vertical outlet . with creosote-treated wood posts and rust-proof wire screen. The screen should extend at least 0.3 m above and 0.3 m below the vertical outlet. It will prevent floating debris from entering the outlet pipe after the pond is put into operation.
7. Carefully fill in pipe trenches with moist soil and tamp.

Finishing Embankments...

1. Fill in any gaps in the embankment that were used for laying pipe or removing excavated soil. Thoroughly tamp the top and slopes and make them uniform with the existing embankment.
2. Line the entire inner race of the embankment slope with rocks and flat stones. This will prevent erosion due to wave action during pond operation. Rocks and stones should be smoothly graded to conform to the design slope of the embankment. Avoid using gravel and pebbles because this material tends to move downslope.
3. If topsoil or sod was initially removed from the site, use it now to cover the outside slope and top of the embankment. If no sod is available, plant grass seed. This will help prevent erosion of the embankment from wind and rain.
4. Excess soil excavated from the pond can be used to build small dams to divert surface water away from the pond. If not, it should be graded level or hauled away from the pond site.

"Figure 10 : Finished Embankment"...