Black water is wastewater from toilets or water contaminated with fecal matter. Since it contains pathogens which could endanger health, effluent should not be discharged in the open or in the open drains.
1) Opt for Toilet Types which do not Discharge any Black Water
This is the ideal way to overcome the problem of black water. Toilet types such as: 1) twin pit toilets; 2) composting toilets; and 3) Ecosan toilets do not discharge any black water. In these types, the black water is either discharged into the surrounding soil or is contained in a receptacle (tank). Thus, they are non-polluting toilets.
2) Management of Effluent from Septic Tanks
1) Leach Pit
The effluent from a septic tank can be discharged in a leach pit suitably constructed near the septic tank.
Selection of Site
- The leach pit should be located 10 m away from any ground water source
- There should be a safe distance of 2 m between the bottom of the leach pit and the ground water
- A safe distance of 1 m should be maintained between the leach pit and the house wall
Size of the Leach Pit
For a family size septic tank, a leach pit having an effective volume of 0.5 m3 is sufficient. In case of larger septic tanks, the volume should be increased accordingly.
The leach pit should be preferably round (cylindrical), constructed in single brick masonry. The bottom should not be cemented /concreted. The brick work should be done in honeycomb fashion in alternate layers. It should be covered with RCC/ferrocement lids.
Operation and Maintenance
The leach pit does not have any sizeable deposition of solids since only the liquid portion flows into it. It can be cleaned once in five to eight years. However, this periodicity depends upon proper cleaning of the septic tank at regular intervals.
2) Community Leach Pit
In case of houses having septic tanks but no space for a leach pit or in case of habitations with hard strata or high water table, the best option is the community leach pit.
For design and other details, please refer to the section on the community leach pit for grey water. The volume of such a leach pit can be calculated on the basis of the quantity of black water discharged from the houses connected.
3) Common Treatment Unit (Waste Stabilization Pond)
In situations such as hard strata, water logged conditions, high water table areas, waste stabilization ponds could be an option for treatment of black water.
If the waste stabilization pond for wastewater, including black water, is to be designed, the HRT of different ponds should be:
- Anaerobic Pond – 5 days
- Facultative Pond – 10 days
- Maturation Pond – 10 days
At the end of the system, it is advisable to have a chlorination unit with pot chlorination or a chlorine doser as per the situation. This ensures pathogen-free water from the treatment unit. Such water is safe for agricultural reuse.
Operation and Maintenance
All O&M measures applicable to grey water stabilization ponds should be followed. Periodical testing of water for BOD, COD and fecal coliform content is essential.