Wastewater examination and sampling

Necessity of examination of wastewater

1. To know the characteristics and strength (BOD, COD, solids, pH, nitrogen, bacteria) of the wastewater.
2. To design the treatment plant and choose the process.
3. To control and monitor the operation and efficiency of the units.
4. To decide the degree of treatment necessary before disposal into streams, or reuse in irrigation.
5. To check compliance with effluent standards and laws.
6. To study pollution of the receiving water and the effect on the public health and the environment.
7. To trace infiltration and industrial wastes in sewers, and to detect toxic wastes.

Sampling of wastewater

A sample must truly represent the wastewater, because the flow and strength change hourly.

Types of samples

• Grab (catch) sample: a single sample collected at one place and one time; used for pH, DO, temperature, chlorine, and where the flow is uniform.
• Composite sample: many small samples are collected at equal intervals (hourly) over 24 hours and mixed in proportion to the flow at that time. It gives the average quality and is used for BOD, solids, etc.
• Integrated sample (several points at one time) is used for wide channels.

Procedure and precautions

1. Select the sampling point: at an inlet works, a place with good mixing (flume, drop, manhole) and not at a deposit or a dead pool; sample at mid-depth and mid-stream.
2. Use clean polyethylene or glass bottles, rinse them 2-3 times with the wastewater, and fill completely (for BOD and DO, no air gap).
3. Take the sample quantity of 1-2 litres (more for composite).
4. Label with place, date, time, temperature and name of collector.
5. Preserve the sample at about 4°C (in ice), and analyse BOD within 6 hours (24 hours at the latest); add acid or other preservative for some tests as per Standard Methods.
6. Note site data (flow, weather, colour, odour) at the time.