Ultimate Residue Sewage Disposal

ULTIMATE RESIDUE DISPOSAL

• Sludge (Wet/Dry/Incinerated) is disposed by the following methods:

(i) Spreading on farm land

(ii) Dumping

(iii) Land filling

(v) Incineration

 (iv) Sludge lagooning

(vi) Disposal in water or sea.

(i) Spreading on Farm Land

• Dewatered sludge is disposed by spreading over farm lands and after drying the soil is ploughed.

• Wet dewatered sludge can be injected directly into soil by digging shallow trenches, 1918w bru 50 to 90 cm width, 0.3 to 0.4 m deep and 1 to 1.5 m C/C spacing. After the water evaporates, the trenches are covered with dry earth.

• After one month, the land is ploughed for cultivation.

• The sludge is a source of nutrients, nitrogen and phosphate and improves soil fertility.

• The sludge humus increases the water holding capacity of soil.

• Sludge acts as soil conditioner and reduces soil erosion.

(ii) Dumping

• The stabilised sludge, digested sludge, clean grit, incinerator residue that will not cause pollution or nuisance can be disposed in abandoned mines or quarry.

• The sludge may also be dumped into sea, where cities are located near sea shores and where such wind direction prevail which will discharge the sludge deep into sea.

(iii) Land Filling

• Sanitary landfills can be used to dispose off the sludge, grease, grit and solids both stabilised and not stabilised.

• To reduce transportation cost, it is better to dewater the residues.

• Sludge waste is deposited in designated area of landfills, compacted with tractor/ roller and covered with 30 cm soil.

• Safety measures have to be taken to prevent any pollution of surface or ground waters

(iv) Sludge Lagooning

• A lagoon is a shallow earthern basin into which untreated or digested sludge is deposited.

• The untreated sludge undergoes anaerobic or aerobic decomposition and may release objectionable odours. So the lagoons should be located away from towns.

• The depth of lagoon is 0.5 to 1.5 m, detention time is 1 to 2 months.

• Under drainage system is provided at the bottom.

• It works on fill and draw basis.

• After stabilisation or evaporation, the lagoon contents are dugout to half the volume and used as manure.

•  Lagoons are used for storage + digestion + dewatering + disposal in isolated locations where the soil is not porous and there is no chance of ground water contamination.

• Cost is less, but cause ugly sight, odours and mosquito breeding.

• Used as emergency storage when digesters are under repair.

(v) Disposal by Incineration

• The dewatered wet sludge may be disposed by burning at very high temperatures inside incinerators.

Types of Incinerators

 (i) Multi-hearth furnace.

(ii) Flash-type of furnace.

(iii) Fluid-bed furnace.

• The end product is ash which is harmless and can be disposed in low-lying areas, sea, water, abandoned mines etc.

(vi) Disposal in Water or Sea

• Not commonly adopted.

• Used where large volume of water is available for dilution.

• Sludge is barged into deep sea for proper dilution and dispersion.

• The outfall/disposal end should be carefully designed to prevent coastal pollution and navigation problems.