Urban wastewater sewers carry a wide range of matter that is often bulky, especially in the case of combined systems.
A pre-treatment is necessary in order to protect raw water lifting systems and pipelines against blockages as well as other treatment equipment against abrasion and, more generally, to remove anything that might interfere with subsequent treatment.
The following constitute pre-treatment operations (a water treatment plant can comprise one or more of these operations, depending on its importance and raw water quality:
- bar screening;
- grit removal ;
- grease removal, frequently combined with grit removal ;
- oil removal;
- by-product treatment: grit and grease;
- combined treatment of mains cleaning waste and of plant grit.
Similar operations can be carried out on:
- water taken from a river supply point;
- industrial wastewater.
On the other hand, sludge removal, required to protect certain settling tanks against a major inflow of suspended solids, is sometimes considered to be a pre-treatment whereas, in fact, it is really static settling; this aspect will be discussed:
- in static settling tanks in the case of desludgers;
- in clarification with regard to its incorporation in a treatment system.
We also need to highlight two major trends in modern plant:
- the need to have a pre-treatment that is not merely effective, but grouped as part of a compact system, and, therefore, easier to cover, ventilate and to provide odour control; in this sense, the Sedipac 3D or Densadeg 4D (see flocculators – settling tanks – flotation units), regrouping the grit removal-grease removal-settling functions in the same unit, with or without reagents, constitute the most appropriate devices;
- minimising by-products that have to be discharged outside the plant, leading to residual grease (see the Biomaster specific grease treatment) and grit (see Arenisprocessing cleaning waste: arenis) treatment systems, both of which allow us not just to treat, or even recycle, a plant’s residues, but also to import grease from the outside as well as mains cleaning waste which, in both cases, pose elimination problems that are far better resolved within a wastewater treatment plant than using other solutions put forward: shared infrastructure and operation: sludge, returns to the plant inlet…
This means that the only by-products that still have to be removed are those retained by the «dewatering » screen (similar to household waste) and re-usable grit.