High-loaded activated sludge – improvement of an srt controlled system.

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The activated sludge process was invented in 1914, and celebrates its 100 year anniversary in 2014. In general, the first 80 years the process only included the breakdown of organic material, i.e. COD and BOD. In the 90s, enhanced nitrogen removal (nitrification-denitrification) was incorporated into the process, increasing the solid retention time, aerial footprint and oxygen demand, and decreasing the sludge production. Since nitrogen removal was implemented, the concern about the environmental impact of wastewater treatment has increased. Energy efficiency, biogas production, nutrient recovery and introduction of new nitrogen removal processes have increased the attention of the high-loaded activated sludge process, without nitrogen removal, again.

Sjölunda Wastewater Treatment Plant (WWTP) in Malmö treats domestic and industrial wastewater (350,000 population equivalents). In ten years, the plant has to increase its treatment capacity due to increased load and more stringent effluent demands without increasing, or even decreasing, its environmental impact. The plant has a high-loaded activated sludge plant (solids retention time (SRT) of ca 2 days) for COD removal only and the nitrogen removal occurs in following trickling filters (nitrification) and moving-bed biofilm reactors (denitrification). The plant has recently implemented an operational strategy in one activated sludge line that includes online control of SRT which determines amount of excess sludge withdrawal. This control has to be evaluated and modelled and compared to lines which are not controlled by SRT.

Aim

The aim of this master thesis is, in full-scale at Sjölunda WWTP, to evaluate and compare the SRT control strategy to a manually controlled excess sludge withdrawal. The hypotheses are:
• A low SRT will increase the biogas production, but will at the same time deteriorate effluent quality.
• A too low SRT gives foaming problems.
• Different SRTs affects the settling properties.

Method

Existing on-line measurements and an extended grab sample campaign will be used to evaluate the operation of an SRT controlled line and a reference line. A model of the high-loaded activated sludge plant is created in the modelling tool WEST®. Dynamic simulation requires measurement campaigns of characterisation of the influent and effluent. Some kinetic parameters can be estimated by oxygen uptake rate (OUR) tests. The methane potential of the produced sludge will also be examined

Examiner

Professor Karin Jönsson, Lund University.

Supervisors

Supervisor: Åsa Davidsson, Lund University. Co-supervisor: David Gustavsson, VA SYD.

Interested?