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Technologies For Grey Water Management in Neighborhood
(Semi-Centralized)

            In situations where the houses are densely located and there is no space for an individual leach pit or kitchen garden, or the soil structure does not favor a leach pit (rocky or impermeable strata), grey water management in neighborhood can be a desirable option. Under this reuse of grey water in common kitchen garden and / or community leach pit for a group of houses may be the desired option.


1) Community Leach Pit for a Group of Houses

Description

This is a brick-lined pit constructed at a convenient place for a group of houses. The number of houses to be connected should be calculated based on the grey water discharged from each house and the space available for the community leach pit. Grey water from the houses (kitchen wastewater, bathing water, washing water, etc.) should be carried to this pit.


Essential Components of the System

  1. Nhani trap or P trap at every house
  2. Primary silt chamber at every house
  3. Pipeline (3 to 4 inch diameter PVC or SWR pipe) laid preferably underground but at a shallower depth
  4. Secondary silt chamber near the leach pit
  5. A leach pit of specific dimensions

Significance of a Community Leach Pit

  • Dry environment: the entire wastewater from a household is absorbed by this underground structure. This results in dry premises and streets;
  • Freedom from the mosquito nuisance;
  • Odour-free environment;
  • Recharge of ground water: the wastewater enters the natural ecosystem and recharges the ground water;
  • A low-cost option compared to a drain + centralized treatment unit; and
  • O&M costs comparatively lower than the drain + centralized treatment system.

Site Selection

  • The pit should be located at a place which provided the required gradient for gravity flow of wastewater from households to the leach pit;
  • It should be 3 m away from any ground water source and 1 m from wall of the house.
  • It should be as near as possible from the source of wastewater (to save on the pipeline).

Design Considerations

  • Shape: preferably circular (cylindrical) to reduce cost, yet durable and maximizes volume;
  • Construction: honeycomb masonry in alternate layers to facilitate seepage of water;
  • Bottom of the pit: not to be cemented or concreted also to facilitate seepage of water;
  • Inlet pipe: 75 to 100 mm diameter PVC or SWR pipe to be connected to the pit keeping 300 mm freeboard; and
  • Volume: Effective volume (volume up to pipe level) should be kept equivalent to double the daily discharge of grey water from the house. e.g. If ten houses are connected to a community leach pit with a daily average discharge of 300 lit per day, the effective volume of the said leach pit should be 6 cum to accommodate 6000 lit of grey water (300 lit x 10 houses=3000 lit x 2 = 6000 lit. One cum of volume accommodates 1000 lit of water.)

Step-by-step Construction

  1. Excavation: dig a pit of the required dimensions
  2. Construction: construct the pit with 225 mm thick brick work with honeycombing in alternate layers. Brick masonry to be done in 1:8 cement mortar
  3. Piping: connect the pipe at a height keeping 300 mm freeboard. The pipe should protrude 100 mm inside the pit
  4. Corbelling: the diameter of community leach pit is always larger. Hence the top portion should have corbelling to reduce the diameter of opening at the top to about 1 m (please refer figure 15)
  5. Cover: 50 mm thick RCC cover cast in two halves/ferrocement slabs/flag stone
  6. The cover to be properly sealed to avoid vector entry
  7. The cover to be covered with a soil layer of about 300 mm

Connection with Household Wastewater

As described in previous paragraphs, the household wastewater must pass through a nhani trap or a P trap to avoid vector entry and odour nuisance. The nhani trap should also be covered with a suitable screen to screen out solids from the wastewater.


  1. If impurities such as ash/dirt are expected, a silt chamber (see Figure 15) at a suitable point is advisable
  2. The pipe from the nhani trap/silt chamber should directly go to the leach pit
  3. It is always advisable to lay the pipe underground and from the periphery of the courtyard to avoid accidental damage

Operation and Maintenance

A) Cleaning of the Nhani trap and silt chambers (at households as well as near the leach pit) should be done weekly / fortnightly or as required.
B) Removal of occasional blockages from the pipes as & when required.
C) De-sludging of the leach pit should be done once in two years. If the silt chamber is provided and is periodically cleaned, then the pit does not require any de-sludging for five to six years or more.


2) Reuse in Plantations/Orchards

Like kitchen gardens at individual household, wastewater from a group of houses can also be utilized in community kitchen gardens.


Significance of Reuse of household Wastewater in common plantations.


In situations where the geophysical conditions are not congenial or the houses are densely located and there is no space for a kitchen garden or even for a leach pit, the waste water from a group of houses can be channelized and utilized for growing vegetation at a distant place.


Requirements


  1. Adequate space near the habitation / lane. If at all such space is not available in the vicinity, this water can be conveyed to the required distance through a pipeline.
  2. Silt chamber at individual household to minimize the load of impurities from the grey water. (This chamber will be cleaned by the house owner)
  3. Secondary silt chamber at the plantation site where the end use is planned.
  4. Pipeline of required specification to convey the grey water from houses to the plantation site
  5. Some arrangement at the GP level for management of the system. This task can be assigned either to Self Help Groups (SHGs), youth groups or even to some private entrepreneurs.

Design Considerations


The design of plantation will differ with every habitation. Careful planning is essential. The following factors should be taken into consideration while planning for a plantation.
1. Space available for plantation
2. Quantity of grey water
3. Plant species intended to be grown


Suggested Usage


1. Vegetable garden for commercial purpose
2. Orchards (fruit trees) for commercial purpose
3. Existing park/garden in the village
4. Roadside trees/other vegetation


Operation and Maintenance


A) Cleaning of Nhani trap & silt chambers (at households as well at the plantation) should be done weekly / fortnightly or as required
B) Removal of occasional blockages from the pipes as & when required.
C) Deposition of inert matter /organic matter /salts on the soil surface should be removed after harvesting of crops; and
D) In the rainy season, when grey water is not required it should be diverted to some other system to avoid stagnation and vector breeding.