Sludge digestion and disposal methods

SLUDGE DIGESTION AND DISPOSAL

• The sludge which is deposited in a primary sedimentation tank is called raw sludge and the sludge which is deposited in a secondary clarifier is called secondary sludge
• Raw sludge is odourous and contains higly putescible organic matter while secondary sludge also contains organic matter, it is a little less objectionable
• The sludge withdrawn from the bottom of the sedimentation basin must be stabilised before final disposal
• Another problem posed by sludge is its high moisture content as sludge containing very high moisture content becomes bulky and difficult to handle
• Complete moisture can be removed only by special treatment as water is held very tightly in the sludge
• If sludge volume is reduced by half, if its moisture content is reduced to 96% from 98%

SLUDGE DIGESTION

•  In order to avoid emnation of foul odour from the sludge, it is stabilised by decomposing organic matter under controlled anaerobic conditions
• The process of stabilization is called sludge digestion
• During the process of sludge digestion, the sludge gets broken into
• Digested sludge which is stable humus like solid matter with reduced moisture content. It is free of pathogenic bacteria that are killed in the digestion process
• Supernatent liquor that contains liquified and finely divided solid matter having high BOD
• Gases like methane, carbondioxide and trace inert gases like Nitrogen and Hydrogen Sulphide are evolved
• The digested sludge is dewatered, dried and used as fertiliser. The gases evolved are used as fuel to drive gas engines.

 Stages in sludge digestion process

The three stages found in biological action during the natural process of sludge digestion are

• Acid fermentation
• Acid regression and
• Alkaline regression

During the first stage of acid fermentation or acid production the sewage-sludge begins to acted upon by anaerobic and facultative bacteria called acid formers. These organisms solubilize organic solids through hydrolysis. Soluble products are then fermented to organic acids and organic alcohols of low molecular weight. Gases like CH4, CO2 and H2S are evolved. Intensive acid production make the sludge highly acidic. Extremely foul odours are evolved during this stage which continues for 15 days. BOD of the sludge increases during this stage

In the acid regression stage, the volatile organic acids and nitrogenous compounds of the first stage are attacked by bacteria to form acid carbonates and ammonia compounds. Small amounts of CO2 and H2S are also released. The decomposed sludge has a very foul odour and the decomposed sludge entraps the gases of decomposition, becomes foamy and forms scum. This stage continues for a period of about three months causing the BOD of the sludge to remain high

Finally, in the alkaline fermentation stage, resistant materials like proteins and organic acids are attacked and broken by anaerobic bacteria called methane formers into simple substances like ammonia, organic acids and gases. In this stage, liquid separates out from the solids and the digested sludge is formed. This sludge is granular and stable and does not give offensive odours. This digested sludge is collected at the bottom of the digestion tank and is called ripened sludge. Digested sludge is alkaline in nature. Large volumes of methane gas along with small amounts of CO2 and N2 are evolved during this stage. This stage extends for a period of about one month.

The important factors affecting the process of sludge digestion are

• Temperature
• pH value
• Seeding with digested sludge and
• Mixing and stirring of raw sludge with digested sludge

Disposal of digested sludge
The digested sludge from the digestion tank contains a lot of water requiring that it be dried or dewatered before further disposal by burning or dumping

Dewatering, Drying and disposal of sludge by sludge drying beds

• Sludge drying beds are open beds of land, 45 to 60 cm deep, consisting of about 30 to 45 cm thick graded layers of gravel or crushed stone of size 15 cm at bottom to 1.25 cm at the top.
• A layer of 10 to 15 cm thick coarse sand lies on top
• Open jointed under-drain pipes are laid below the gravel layer
• The sewage sludge from the digestion tank is brought and spread over the top of the drying beds through distribution troughs having openings at a distance of 2m
• A portion of moisture drains through the bed while most of it is evaporated
• It takes about two weeks to two months to dry the sludge depending on the weather and condition of the bed.