Volume-moisture relation; sludge disposal

Volume-moisture relationship

Sludge consists of water and solids. If the amount of solids does not change, the volume of sludge is inversely proportional to the percentage of solids (i.e. $100-P$ where $P$ is the moisture content):

$$\frac{V_1}{V_2}=\frac{100-P_2}{100-P_1}=\frac{S_2}{S_1}$$

where $V_1,V_2$ are the volumes at moisture contents $P_1,P_2$ (%) and $S$ is the percentage solids. This assumes the specific gravity of the sludge stays the same (about 1.02, since solids are so dilute).

Example: sludge of 98% moisture dewatered to 90% moisture gives $V_2/V_1=\dfrac{100-98}{100-90}=0.2$, so the volume falls to one-fifth (80% reduction). A fall from 98% to 96% moisture halves the volume.

Because a small drop of moisture content gives a large drop in volume, thickening and dewatering are very effective in reducing handling and disposal cost.

Methods of sludge disposal

1. Dumping on land or in the sea.
2. Sanitary landfill (burial of dewatered sludge in trenches).
3. Lagooning (storage in shallow basins).
4. Use as manure or soil conditioner on land (after digestion and drying).
5. Incineration.

Most suitable method: land application of digested and dried sludge (with landfill as a back-up)

For a town in Nepal, I choose digested sludge, dried on sand drying beds, and used as manure/soil conditioner on agricultural or forest land, with surplus sent to a sanitary landfill. The justification is:

• Digested and dried sludge is stable, inoffensive and has few pathogens.
• It contains nitrogen, phosphorus and organic matter that improve soil, so it is a resource, not a waste.
• It is cheap: drying beds need little energy or skill, and the climate allows drying for much of the year.
• Landfilling or lagooning needs large land and gives odour risk, dumping in rivers is unacceptable, and incineration needs high capital and fuel cost.
• Heavy-metal content must be checked, and use must be avoided on crops eaten raw.