The depth of water while using ultra violet ray treatment for disinfection should be 10cm.

Explanation:
Ultra violet (UV) ray treatment is commonly used for disinfecting water. UV rays have the ability to kill or inactivate harmful bacteria, viruses, and other microorganisms present in the water. However, the effectiveness of UV treatment depends on various factors, including the depth of the water being treated.

Importance of the Depth of Water:
The depth of water plays a crucial role in the efficiency of UV disinfection. The depth affects the exposure time of water to UV rays, which in turn determines the effectiveness of disinfection. The depth of water should be carefully considered to ensure that all microorganisms in the water are adequately exposed to UV rays for a sufficient amount of time.

Optimum Depth of Water:
The optimum depth of water for UV disinfection is typically recommended to be around 10cm. This depth is considered ideal for achieving maximum disinfection efficiency. When the water depth is 10cm, the UV rays can penetrate and reach all parts of the water, ensuring that all microorganisms are exposed to the disinfecting effect of UV radiation.

Factors Affecting the Depth:
The depth of water required for UV disinfection may vary depending on several factors, including:
1. UV Dose: The required UV dose for effective disinfection is typically measured in millijoules per square centimeter (mJ/cm²). The depth of water should be sufficient to ensure that the required UV dose is delivered to all microorganisms in the water.
2. UV Transmittance: The transparency of water to UV rays, also known as UV transmittance, can vary. Water with lower UV transmittance may require a greater depth to ensure adequate UV exposure.
3. Flow Rate: If the water is flowing through a UV disinfection system, the depth of water should be adjusted to accommodate the flow rate and ensure that all water is adequately treated.

Conclusion:
In conclusion, the depth of water while using UV ray treatment for disinfection should be 10cm. This depth ensures that all microorganisms in the water are exposed to UV rays for a sufficient amount of time, resulting in effective disinfection. However, it is important to consider other factors such as UV dose, UV transmittance, and flow rate when determining the optimal depth of water for UV disinfection.