Sludge Treatment


Sludge refers to the residual, semi-solid material left from industrial wastewater, or sewage treatment processes. Waste water sludge is the mixture of waste water and settled solids. Depending upon the source it may be primary, secondary, excess activated sludge. 2 % of wastewater and almost 97% water.

Objective of Sludge Treatment :

  • To reduce the volume of the material to be handled by removal of liquid portion
  • To decompose the organic matter and inorganic compounds for reduction in the total solids

  Goals of Sludge Treatment

  Sludge Types

Primary Sludge

  • 3 to 8 % solids
  • About 70% organic material

Secondary Sludge

  • Wasted microbes and inert materials
  • 90% organic material

Tertiary Sludge

  • If secondary clarifier is used to remove phosphate, this sludge contain chemical precipitates

  Sludge Treatment Overview

  Sludge Thickening by Gravity

  Sludge stabilization (mass reduction)

Anaerobic Digestion

Hydrolysis Process – conversion of insoluble high molecular compounds (lignin, carbohydrates, fats) to lower molecular compounds

Acidogenesis Process – conversion of soluble lower molecular components of fatty acids, amino acids and sugars (monosaccharide) to lower molecular intermediate products (volatile acids, alcohol, ammonia and CO2)

Methanogenesis Process – conversion of volatile acids & intermediate products to final product of methane and CO2

Anaerobic Digester

  • Sludge held without aeration for 10-90 days
  • Process can be accelerated by heating to 35-40 degree C
  • These are called High Rate Digesters (10-15 days)
  • Advantages - low solids production and useable methane gas produced
  • Disadvantages - high capital costs and susceptibility to shocks and overloads

Aerobic Digestion

  Sludge Dewatering

Dewatering aims to reduce the water content further. The sludge can then be handled like a solid. Dewatering can be done mechanically using a filter press (employing pressure or vacuum), or a centrifuge, also be done using drying beds


Drying beds are the most popular methods, a drying bed consists of a 30 cm bed of sand with an under-drainage. Sludge is applied on the sand bed and is allowed to dry by evaporation and drainage of excess water over a period of several weeks depending on climatic conditions

Bacterial decomposition of the sludge takes place during the drying process while moisture content is sufficiently high. During the rainy season the process may take a longer time to complete.

  Tertiary Treatment/ Advance Treatment

The process is rather the most expensive one, in India it is generally not done due to high cost. Mainly two nutrients which we deal with are Nitrogen and Phosphorus removal

Nitrogen Removal

Nitrogen is removed through the biological oxidation of nitrogen from ammonia to nitrate (nitrification), followed by denitrification, the reduction of nitrate to nitrogen gas. Nitrogen gas is released to the atmosphere and thus removed from the water. Sometimes the conversion of toxic ammonia to nitrate alone is referred to as tertiary treatment.

Many sewage treatment plants use centrifugal pumps to transfer the nitrified mixed liquor from the aeration zone to the anoxic zone for denitrification. These pumps are often referred to as Internal Mixed Liquor Recycle (IMLR) pumps.

Phosphorus removal

Phosphorus can be removed biologically in a process called enhanced biological phosphorus removal. In this process, specific bacteria, called polyphosphate-accumulating organisms (PAOs), are selectively enriched and accumulate large quantities of phosphorus within their cells (up to 20 percent of their mass). When the biomass enriched in these bacteria is separated from the treated water, these biosolids have a high fertilizer value.

Phosphorus removal can also be achieved by chemical precipitation, usually with salts of iron (e.g. ferric chloride), aluminum (e.g. alum), or lime. This may lead to excessive sludge production as hydroxides precipitates and the added chemicals can be expensive. Once removed, phosphorus, in the form of a phosphate-rich sludge, may be stored in a land fill or resold for use in fertilizer.


Chlorination remains the most common form of waste water disinfection due to its low cost and long-term history of effectiveness. One disadvantage is that chlorination of residual organic material can generate chlorinated-organic compounds that may be carcinogenic or harmful to the environment. Residual chlorine or chloramines may also be capable of chlorinating organic material in the natural aquatic environment. Further, because residual chlorine is toxic to aquatic species, the treated effluent must also be chemically dechlorinated, adding to the complexity and cost of treatment.

Ultraviolet (UV) light can be used instead of chlorine, iodine, or other chemicals. Because no chemicals are used, the treated water has no adverse effect on organisms that later consume it, as may be the case with other methods. UV radiation causes damage to the genetic structure of bacteria, viruses, and other pathogens, making them incapable of reproduction. The key disadvantages of UV disinfection are the need for frequent lamp maintenance and replacement and the need for a highly treated effluent to ensure that the target microorganisms are not shielded from the UV radiation

Ozone O3 is generated by passing oxygen (O2) through a high  resulting in a third oxygen atom becoming attached and forming O3. Ozone is very unstable and reactive and oxidizes most organic material it comes in contact with, thereby destroying many pathogenic microorganisms. Ozone is considered to be safer than chlorine because, unlike chlorine which has to be stored on site (highly poisonous in the event of an accidental release), ozone is generated on-site as needed. Ozonation also produces fewer disinfection by-products than chlorination. A disadvantage of ozone disinfection is the high cost of the ozone generation equipment and the requirements for special operators