There are GPs where household or decentralized systems are not feasible for the following reasons
- No space near houses due to densely located housing structure;
- Hard strata in habitations;
- High water table areas; and
- Water logged conditions around houses.
In such situations, it is necessary to opt for centralized systems for grey water management. A centralized grey water management system has the following essential components:
- An improvised bathing cubicle or similar structure at the household with a nhani trap/P trap
- A silt chamber at the household
- Transport of grey water from individual households to the final treatment unit with intermediate silt chambers
- A final treatment unit
Among these 1 and 2 have been discussed and detailed earlier (please refer xx).
A centralized waste water management system has two important components 1) Transport of waste water & 2) The final treatment unit. In many villages, while a transport system in the form of drains (howsoever erroneous & ill maintained) exists, treatment units are seldom found. The waste water is either left in the open, diverted & used in agriculture without any treatment or in most cases directed to a water body mainly a river.
Transport of Grey Water
It is advisable to convey grey water from individual houses to the treatment unit through a pipeline.
The advantages of a pipeline over a conventional drain
- More economical than a drain.
- Minimum resistance;
- Ease in implementation – laying of a pipeline is easier than constructing a drain;.
- No clogging on account of indiscriminate dumping of garbage in the drains;
- Maintenance easier than that of a drain; and
Specifications of the Pipeline
A) Diameter of pipe
1. Laterals: from houses to the main pipeline – 75 to 100 mm diameter PVC or SWR pipe;
2. Mains: up to the treatment unit – 150 mm diameter PVC pipe;
B) Laying of pipeline
The pipeline should be laid at a shallow depths. If old drains exist, pipelines can be laid in these drains and the drains closed. However, if these drains are at the roadside & are also meant for carrying storm water, these should be maintained & pipelines for grey water should be laid separately. It is advisable to have a contour survey of the village and pipeline laid accordingly to facilitate gravity flow. This will ensure a proper flow of grey water to the treatment unit and eliminate pumping stations and added costs
C) Silt chambers
1. Silt chamber should be provided at every household. (Refer Figures 9 and 10); and
2. Intercepting chambers on the main pipelines (Refer Figures 15) should be located at all junctions, at turns, curves and also on the straight pipeline at every 75 to 100 m depending upon the ground situations.
D) How about existing drains
Drains may or may not exist in many villages and these may be presently carrying none or all of the waste water. While planning for a waste water transport anew there can be several options for these drains
- If the locations of these drains are convenient & these are not usable for storm water (rain water), pipelines can be laid through the same drains & these drains can be closed
- If the drains are along the roadsides and used for storm water, then leave the drains for storm water and lay the pipes separately. In this situation, the drains will be dry for all the year round except rainy season & this can be a wonderful example of a successful LWM.
- If the drains are scientifically prepared & scrupulously maintained, (although this is rarely found) these can be used for transport of waste water
Final Treatment Unit
This is an important component of a centralized system. This can be of various types such as a Grey water stabilization pond or a Reed bed system or an Anaerobic baffled reactor, etc. The choice of the technology will depend upon: 1) quantity of grey water to be treated; 2) distance from habitation to the treatment unit; 3) affordability; and 4) land and human resources available with the GP.
1) Grey Water Stabilization Pond / Sullage Stabilization Pond
This is a series of basins or ponds located at a suitable site away from the human habitation where grey water is treated. The treatment is natural and involves:
- Sedimentation or settling of solids in the wastewater, and
- Degradation process involving bacteria, algae, sunlight and oxygen which degrades the organics and utilizes the nutrients in grey water and increases its usability
Essential Components of the System
The system has three basic units called ponds, placed in series and characterized by their function such as:
- Anaerobic pond – one number
- Facultative pond – one number
- Aerobic pond or maturation pond – one or more in number depending upon the impurities in the grey water
1. Anaerobic Pond
This is the first in the series. It receives the raw grey water and functions as a settling and degradation unit.
- Separation of solids and inert matter settling at the bottom
- Anaerobic digestion of organic matter in the settled solids
Hydraulic Retention Time (HRT): 2 days
Depth -2.5 to 3 m, Length and Width or the diameter to be adjusted accordingly to obtain the necessary volume to maintain the required HRT
2. Facultative Pond
This is the second pond in the series. It receives partially treated grey water from the anaerobic pond. In this pond, aerobic as well as anaerobic digestion takes place. Hence this is called facultative pond
- Aerobic digestion takes place at the upper layer
- Anaerobic digestion takes place at the lower layers
HRT: 5 days
To facilitate maximum aeration, depth of this pond is kept shallower & surface area is increased. Depth: 1.2 to 1.5 m, length and width or the diameter to be adjusted accordingly to obtain the necessary volume to maintain the required HRT
3. Aerobic Pond/Maturation Pond
This is the third pond in the series. It receives treated wastewater from the facultative pond. In this pond, mainly aerobic digestion takes place which helps in improving quality of the grey water.
- Aerobic digestion takes place at the upper layer
- Oxygenation also takes place which further helps in improving the quality of the wastewater
HRT: 5 days
To facilitate the maximum aeration depth of this pond, its depth is also kept at a minimum and the surface area is increased
Depth: 1.2 to 1.5 m, length and width to be adjusted accordingly to obtain the necessary volume to maintain the required HRT
If the waste water is highly polluted, the number of maturation ponds can be increased to two or three.
- Anaerobic pond: this can be circular or rectangular in shape to facilitate better sedimentation. It can be brick-lined for better stability
- Facultative and maturation ponds: these should have a trapezoidal shape to facilitate maximum aeration. This shape can also be achieved by proper excavation and earth work. These ponds may or may not be lined depending upon the soil structure. Wherever possible, rubble pitching also helps to achieve stability;
- Size, dimensions and inter-pond connections: (please refer figure 16);
- In facultative and maturation ponds, the width to length ratio should be preferably 1: 3; and
- Primary screening: bar screens can be installed before the anaerobic pond to screen out the solids (debris/inert matter/plastic, etc.) carried with the grey water.
End Use of the Treated Water
The treated water from the grey water stabilization pond can be used for agriculture or pisiculture (Fish rearing) depending upon the purity of water. The various parameters for such reuse are (CPCB, 2015)
- BOD – ≤10 milligram/litre (mg/l)
- COD - ≤ 50 mg/l
- Faecal coliform content (MPN/100 millilitre (ml)) ≤ 100
Chlorination of Treated Water
At the end of the maturation pond, a pot chlorinator can be installed to achieve the bacterial discharge standards. However, care should be taken to adjust and maintain the standard chlorination dosing.
Operation and Maintenance
- Cleaning of intercepting chambers on the pipeline and bar screens before anaerobic ponds;
- Maintenance of embankments of the ponds;
- Periodical desludging of anaerobic pond and filter/screen, if any;
- Removal of floating scum from all ponds;
- Post-monsoon maintenance of the entire system; and
- Periodical anti-mosquito spraying, as and when required.
Table 2: Volume of grey water stabilization pond for different sizes
|No. of Houses (Approx)||Grey water generation per day||Capacity (Effective Volume) of Anaerobic Pond||Capacity (Effective Volume) of Facultative Pond||Capacity (Effective Volume) of Maturation Pond|
|50-60||15,000 l||30,000 l (30 cum)||75,000 l (75 cum)||75,000 l (75 cum)|
|100-120||30,000 l||60,000 l (60 cum)||1,50,000 l (150 cum)||1,50,000 l (150 cum)|
|200-240||60,000 l||1,20,000 l (120 cum)||3,00,000 l (300 cum)||3,00,000 l (300 cum)|
|300-360||90,000 l||1,80,000 l (180 cum)||4,50,000 l (450 cum)||4,50,000 l (450 cum)|
Table 3: Dimensions of grey water stabilization ponds for different capacities
|Grey Water (l per day)||Anaerobic Pond||Facultative Pond||Maturation Pond|
|Length (m)||Width (m)||Depth (m)||Length (m)||Width (m)||Depth (m)||Length (m)||Width (m)||Depth (m)|
Note: Depth of all ponds includes a freeboard of 0.3 m.
Waste water Stabilization Ponds
Stabilization Pond in stone pitching
Stabilization Pond in stone masonry
Screens at the entrance of pond system
Maintenance of Stabilization Pond