KTH / Research projects at Water Engineering and Resource management Research Group
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Interactions between wastewater and sludge treatment

Operation and intensification of wastewater treatment plants

Participating persons:

Bengt Hultman, Professor
Erik Levlin, PhD
Elzbieta Plaza, Assoc. professor
Jozef Trela, PhD
Monica Löwén, researche engineer

Project period: 1997-2004

Finansing: VA-forsk and others.


Possibilities to use sludge as a product for different purposes is dependent on the interest from other sectors as industry (use of apatite as raw material for the phosphate industry), energy sector (use of biogas), agricultural and food sector (acceptance for use of sludge on agricultural land), transport sector (use of methane gas for vehicle fuel) och waste sector (combined treatment of sludge and wastes for product recovery). Sludge as a raw material is influenced by factors as consumer behavior, connections of industries and design of the wastewater system (combined or separate system, sorting of waste streams inside houses etc).

Product recovery from sludge gives rise to a large number of side-effects due to changes of sludge composition (as the inorganic fraction), that side-streams may be formed with high concentration of organic materials, ammonium and phosphate and that separate treatment can be made to produce for example nitrification bacteria which may be seeded into the activated sludge process for the main stream and phosphate compounds. A significant emphasis was laid on description of these side-effects as a scientific basis to evaluate different methods for product recovery. The project performance was connected with works by three research students (Puhua Li, Liisa Piirtola and Jozef Trela). Studies were done in parallel on product recovery as master thesis works with special emphasis on phosphorus recovery from sludge.

The project showed that several methods are possible to improve the operation and decrease operational costs if increased consideration is given to interactions between wastewater and sludge treatment. Different examples are:

  • Recycling of sludge with a high fraction of inorganic material to the activated sludge process to improve the sludge sedimentation properties
  • Supply of organic materials (for instance produced by hydrolysis of sludge) to improve the denitrification efficiency
  • Seeding of nitrification bacteria from a separate step with a high influent ammonium concentration (as supernatant from dewatering of digested sludge) to the activated sludge process for the main stream
  • Use of biological phosphorus removal technology to later facilitate the phosphorus recovery from sludge
  • Mechanical, biological and chemical treatment of excess sludge to decrease sludge production and foaming and to increase biogas production in anaerobic digestion (Hultman and Levlin, 2003).


The work have been reported in VA-Forsk 2004-15.


Page created by Erik Levlin