Physical characteristics; decomposition of wastewater

Physical characteristics of wastewater

1. Colour: fresh sewage is light brownish-grey. As it becomes stale (septic), it turns dark grey and finally black because of sulphides. Colour tells the age and condition of the wastewater.
2. Odour: fresh sewage has a faint, musty smell. Septic sewage smells of hydrogen sulphide (rotten egg) and other gases like mercaptans. It is measured by the threshold odour number.
3. Temperature: usually 3-5°C higher than the water supply (about 20-25°C in Nepal). Temperature affects bacterial activity, DO solubility and the rate of decomposition. It is measured with a thermometer.
4. Turbidity: due to suspended and colloidal matter (clay, silt, organic matter); high in raw sewage. Measured in NTU with a nephelometer.
5. Solids: total solids are 500-1000 mg/L (suspended, dissolved, settleable).
6. Specific gravity is about 1.0 (nearly that of water).

Decomposition of wastewater

Organic matter in wastewater is broken down by bacteria. There are two ways.

1. Aerobic decomposition (in the presence of free oxygen) Aerobic bacteria oxidise organic matter using dissolved oxygen:

$$\text{Organic matter}+O_2\xrightarrow{\text{aerobic bacteria}}CO_2+H_2O+NO_3^-+SO_4^{2-}+\text{new cells}$$

Nitrogen goes to nitrates, sulphur to sulphates, and carbon to CO₂. It is rapid, odourless and the products are stable. It occurs in streams, oxidation ponds, trickling filters and activated sludge.

2. Anaerobic decomposition (in the absence of oxygen) When oxygen is exhausted, anaerobic bacteria work in two stages:

• Acid formation: complex organic matter is changed to organic acids and alcohols.
• Methane formation: these acids are converted to methane, carbon dioxide, ammonia and hydrogen sulphide:

$$\text{Organic matter}\xrightarrow{\text{anaerobic bacteria}}CH_4+CO_2+NH_3+H_2S+\text{new cells}$$

It is slow, and gives foul smell and a black colour, but methane is useful as biogas. It happens in septic tanks, sludge digesters and the bottom deposits of ponds.

3. Facultative decomposition: facultative bacteria work either with or without oxygen (as in facultative ponds).