Sewage sampling and BOD dilution method

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.

BOD determination in the laboratory (dilution method)

A diluted sample is incubated for 5 days at 20°C in the dark in a standard bottle, and the oxygen consumed by the microorganisms in oxidising the organic matter is measured by the dissolved oxygen (DO) before and after incubation.

Apparatus and reagents

BOD bottles (300 mL, glass-stoppered), incubator at 20 ± 1°C, pipettes, DO meter or Winkler titration set. Dilution water is prepared from distilled water, with 1 mL each per litre of phosphate buffer, $MgSO_4$, $CaCl_2$ and $FeCl_3$ solutions (nutrients for bacteria), and aerated to saturate with oxygen. Seed (settled domestic sewage) is added if the sample has no bacteria.

Procedure

1. Select dilutions. Sewage needs far more oxygen than water can hold, so it is diluted in such a way that, after 5 days, at least 2 mg/L of DO is used up and at least 1 mg/L of DO remains. Take 1-5% for strong wastes (raw sewage).
2. Prepare the diluted samples: pour a measured volume of sample (e.g. 5 mL) into each of the BOD bottles and fill to the neck with dilution water without trapping air bubbles. Make a blank with only dilution water.
3. Initial DO ($D_1$): measure the DO of one set of bottles immediately (Winkler azide method or probe).
4. Incubate the second set, with stoppers sealed by a water seal, for 5 days at 20°C in the dark (to stop algae making oxygen).
5. Final DO ($D_2$): after 5 days measure the DO of the incubated bottles.
6. Calculate

$$BOD_5=\frac{D_1-D_2}{P}\ \text{mg/L},\qquad P=\frac{\text{volume of sample}}{\text{volume of diluted mixture}}$$

For example, 5 mL of sample in a 300 mL bottle gives $P=5/300=0.0167$. With $D_1=9$ and $D_2=5$ mg/L, $BOD_5=4/0.0167=240$ mg/L. For seeded samples, a seed correction is applied, and the blank is used to check the dilution water (its depletion should be below 0.2 mg/L).

The ultimate BOD can be found by $BOD_t = L(1-10^{-kt})$ if $k$ is known.