In the mechanical pre-treatment step solids are separated. This includes both, sediment particles and floating particles such as fats and grease. Different technologies are being used for this separation, depending on the plant size and the particles in the wastewater.
For smaller units, a multi-chamber pit is recommended. The first chamber represents the main part of the total volume. In this chamber, the primary sludge is separated. Primary sludge consists mainly of faeces, cellulose, food rests. In addition, secondary sludge that is already processed is stored as an innoculum. By cultivating different bacteria conditions in the additional chambers a further increase in efficiency can be achieved. The individual chambers are connected via diving walls so that floating sludge cannot pass through. In 3-chamber-pits, a reduction of the organic load of about one third can be achieved.
For bigger plants, more advanced pre-treatment steps are used, such as an Emscher Tank. Additional reaction tanks in the pre-treatment area are required if an up-flow denitrification (see also biological step) is foreseen. With these tanks a further reduction of the organic load can be achieved.
The pre-treatment step can be extended on request by elements such as screening units, rotating drum sieves, grit chambers and grease traps.
For dimensioning, the retention time of the raw wastewater in the pre-treatment step should be less than 1.5 – 2.0 hours.
For smaller units, System S&P® relies on traditional and economical multi-chamber pits in prefab construction. For bigger System S&P®wastewater treatment plants, additional rotating drum sieves are used.
If treatment plants are fed with mixed sewage, additional steps have to be taken into consideration for the treatment of the rain and storm water parts of the inflow.
Depending on the size and the flowrate, mixed systems can be easily treated by simple adaptations of the System S&P® treatment plant when ponds are available. Already existing non-aerated wastewater ponds can be extended or easy-to-built tanks or ponds can be used.
Separate channels for storm water and wastewater are recommended for bigger sized plants. In this way, storm water can be buffered and easily released from floating particles, such as grease or oils from the roads. These can be separated for example in a grit chamber.
Following the current state-of-the-art design of System S&P® wastewater treatment plants in combination with separate inflow channels, an economic solution can be realised for both kinds of mixed sewage treatment systems.