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Anaerobic Treatment

Activated sludge treatment step takes place into aeration tanks, whose footprint shape has to be defined according to the aeration devices to be installed.

Wastewater load and temperature affect the feasibility of wastewater anaerobic treatment. Generally, COD concentration higher than 1550–2000 g m-3 and reactor temperature in the range of 25-35°C are needed.

The principal advantages of anaerobic treatment can be summarised as in the following:

  • Low running costs, mainly due to the lack of any aeration system

  • High methane rich biogas production

  • Low sludge production

  • Less nutrients required by the process

The most diffused types of anaerobic reactor are:

  • Anaerobic contact process;

  • UASB (Upflow Anaerobic Sludge Blanket)

  • Upflow and Downflow attached growth processes

  • Fluidized Bed Reactor

Basing on the OLR (Organic Loading Rate) the anaerobic processes can be classified in low rate (up to 5 kg COD m-3 d-1) and high rate reactors.

Start-up time to develop biomass inventory is essential, therefore seed sludge has to be chosen very carefully.

The anaerobic process is very sensitive and the main instability, at full scale plants, are due to wide variations of temperature, flow rate and organic loading rate.

The main process control parameters are listed below:

  • Chemical and biological wastewater characteristics

  • Temperature

  • Organic Loading Rate

  • Hydraulic Retention Time

  • Biogas production

As a general advice, the type of anaerobic process to be applied has to be defined very carefully according to the specific site characteristics, taking into account all the factors that could affect the process, evaluating their average values and their variability. Process monitoring and control have to be guaranteed in order to prevent and avoid operational problems.

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