Short note: Sludge disposal methods

After treatment (digestion and dewatering) sludge must be disposed of in a safe way. The common methods are:

1. Dumping: the sludge is disposed of in a large water body (sea disposal or deep water) or on a low-lying open land. In the sea, it is carried by barge to deep water beyond the coast. It is cheap, but it may pollute the water, so it is now banned in many countries. On land, it is spread or dumped in a designated site; only digested and dried sludge should be used.
2. Land filling: dewatered sludge (cake) is buried in trenches or mixed with municipal solid waste in a sanitary landfill and covered daily with 15-30 cm of soil. The decomposition goes on slowly. It needs a site with low groundwater and a leachate collection system. The land can be reclaimed afterwards.
3. Lagooning: digested sludge is run into a shallow earthen basin (lagoon) 1-2 m deep, in which it is stored for 6-24 months. The solids settle and decompose further, the supernatant is drained and returned to the plant, and the dried sludge is removed. It needs a large land area and is suited to small plants in sparsely populated places; odour may occur and the lagoon needs lining to protect groundwater.

Other methods are use as manure/soil conditioner on agricultural land (after digestion and drying) and incineration. The choice depends on the sludge quality, land available, climate and cost.