The following points highlight the ten main factors affecting the construction of facultative pond. The factors are: 1. Site Selection 2. Pretreatment 3. Construction in Stages 4. Multiple Units 5. Pond Shape 6. Embankment 7. Pond Bottom 8. Inlet Structure 9. Outlet Structure 10. Pond Interconnections.
Factor # 1. Site Selection:
Facultative pond sites should be located as far away as practicable (at least 200 m) from habitations or from any area likely to be built up within a reasonable future period. If practicable the pond should be located such that the direction of the prevailing wind is towards un-inhabited areas. The pond location should be downhill of groundwater supply source to avoid their chemical or bacterial pollution. Special attention is required in this regard and in porous soils and in fissured rock formations.
ADVERTISEMENTS:
The pond site should not be liable to flooding. Wherever possible, the elevation of the site should permit the pond to discharge the effluent by gravity to the receiving streams. The site should preferably allow an unobstructed sweep of wind across the pond, open to the sun and not shaded by trees. Advantages should be taken of natural depressions while locating the ponds.
Factor # 2. Pretreatment:
It is desirable to provide medium screens and grit removal devices before facultative ponds.
Factor # 3. Construction in Stages:
In cases where the design flow will materialize only in the course of time, it is important to design facultative ponds in multiple units and construct the units in stages. Otherwise the small flows in the initial years may not be able to maintain satisfactory water levels in the ponds and objectionable weed growths and mosquito breeding may develop in the installations.
Construction in stages will also reduce initial costs and help in planning future stages based on the performance data of first stage.
Factor # 4. Multiple Units:
When the required area of pond exceeds 0.5 ha, multiple units should be provided. The multiple units may be either in parallel or in series or in parallel-series system.
Multiple units in parallel facilitate maintenance as any one unit can be taken out of operation temporarily for dislodging or repairs without upsetting the entire treatment process. The parallel system also provides better distribution of settled solids.
ADVERTISEMENTS:
Multiple units in series enable better BOD and coliform removal and reduced algal concentration in the effluent. The series system implies a high organic or BOD loading in the primary units and to avoid anaerobic conditions in these units, they should have 65 to 70% of the total surface area requirements.
A parallel-series system possesses the advantages of both parallel and series operations. A convenient arrangement for this system consists of three units of equal area of which two are in parallel and serve as primary ponds and the third serves as secondary pond in series.
Factor # 5. Pond Shape:
It is not necessary that the pond shape should be of any particular type. It may be round, square or rectangular or polygonal depending on the site contours. The shape of the pond should be such that there are no narrow or elongated portions.
Generally rectangular ponds with length not exceeding three times the width are preferred. Maximum pond length of 750 m is generally adopted. The corners of the pond should always be rounded to minimize accumulation of floating matter and to avoid dead pockets.
Factor # 6. Embankment:
ADVERTISEMENTS:
Ponds are usually constructed partly in excavation and partly in embankment. The volume of cutting and the volume of embankment should be balanced to the maximum extent possible in order to economize construction costs.
Embankment materials usually consist of material excavated from the pond site. The material should be fairly impervious and free from vegetation and debris. The embankment should be compacted sufficiently. The top width of the embankment should be atleast 1.5 m to facilitate inspection and maintenance. In case access to vehicles is desired for maintenance purposes, the minimum width should be 3 m.
Embankment slopes should be designed based on the nature of soil, height of embankment and protection proposed against erosion. Outer slopes are generally 2 to 2.5 horizontal to 1 vertical. Inner slopes are made 1 to 1.5 horizontal to 1 vertical when the face is fully pitched and 2 to 3 horizontal to 1 vertical when the face is unprotected.
The outer faces of the embankments should be protected against erosion by turfing. The inner faces should preferably be completely pitched to eliminate problems of erosion and growth of marginal vegetation. Pitching may be rough stone revetment or with plain concrete slabs or flag stones with adequate gravel packing.
ADVERTISEMENTS:
When complete pitching is not possible, at least partial pitching from a height 0.3 m above water line to 0.3 m below water line is necessary and the face above the line of pitching should be turfed to the top of embankment to check erosion due to wave action and also to avoid mosquito nuisance and grass growth at the water edge.
A minimum free board of 0.6 m should be provided. In larger ponds the free board should be designed for the probable wave heights and should be equal to 1.5 times the wave height.
Factor # 7. Pond Bottom:
The pond bottom should be level, with finished elevations not more than 0.10 m from the average elevation. The bottom should be cleared of all vegetation and debris. The soil formation of the bottom should be relatively impervious to avoid excessive liquid losses due to seepage. Where the soil is loose, it should be well compacted. Gravel and fractured rock areas must be avoided.
Factor # 8. Inlet Structure:
(a) Inlet Arrangement:
The incoming sewer line brings the sewage to the inlet chamber either by gravity, if contours permit or by a pressure line from a pumping station. From the inlet chamber the flow is carried well into the pond by means of a gravity inlet pipe laid at a constant slope on supports at intervals and terminating in a bend facing downwards over a concrete splash pad.
(b) Pretreatment:
The sewer line arriving at the pond is generally made to terminate in an inlet chamber where coarse screens are provided. Grit chamber may be provided prior to the pond in certain cases where large quantities of grit are expected.
(c) Measurement of Flow:
Suitable flow measurement device such as V-notch may be provided.
(d) Inlet Chamber:
The inlet chamber should be of suitable size. It should accommodate coarse screens and flow measuring devices, where necessary. The level of the gravity inlet pipe at the inlet chamber should be kept at least 30 cm above the operational level of the pond to ensure free fall. A suitable stilling chamber may be provided where flow measurements are to be made.
(e) Discharge Splash Pad:
Directly below the inlet pipe bend a concrete platform is provided as a splash pad to prevent erosion of pond bottom. Such a pad is made of size 1.2 m x 1.2 m square or twice the diameter of the inlet pipe whichever is larger.
(f) Point of Discharge:
The point of discharge should be such as to avoid short circuiting. Influent line to rectangular or oval shaped pond should discharge at 1/3 the length of pond but not more than 15 m for small ponds and not more than 20 m for large ponds.
(g) Number of Inlets:
In order to ensure uniform distribution of sewage multiple inlets should be provided along the inlet side of the pond at the rate of one inlet for every 0.5 to 1.0 hectare of pond area.
Factor # 9. Outlet Structure:
The outlet may be in the form of pipe or a suitable length of weir. In the former case vertical tees are provided, and in the latter case hanging baffles projecting 15 to 30 cm above water level and extending about 25 cm into the pond are provided to small ponds only single outlet may be provided, but for larger ponds it is desirable to provide multiple outlets which may be provided at the same rate as for inlets, i.e., one for every 0.5 ha pond area.
Factor # 10. Pond Interconnections:
Pond interconnections are required when ponds are designed in multiple units in series. These interconnections should be such that the effluent from one unit withdrawn from the aerobic zone can be introduced at the bottom of the next unit. Simple interconnections may be formed by pipes laid through the separating embankments as shown in Fig. 15.12.
At their upstream ends, the interconnecting pipes should be submerged about 0.25 m below the water level. The downstream ends may be provided with bends facing downward, to avoid short circuiting by thermal stratification, care is being taken to prevent erosion of the embankment.
