Chapter 9. Pre-treatments (38 pages)

Introduction
1.Bar-screening, straining, comminution
1.1.Bar-screening
1.1.1.Utilisation conditions
1.1.2.Different screen types
1.2.Strainer
1.2.1.Utilisation conditions
1.2.2.Different types of strainer
1.3.Selection data
1.4.Comminution
1.5.By-product removal and treatment - bar screen waste
2.Grit removal
2.1.General
2.1.1.Object
2.1.2.Utilisation conditions
2.1.3.Quantity of grit - Characterisation
2.1.4.Grit treatment
2.2.Area of utilisation
2.2.1.Surface water grit removal
2.2.2.Grit removal from UWW
2.2.3.Grit removal from IWW
2.2.4.Sludge pre-treatment
2.3.Structures
2.3.1.Pre-grit remover or ungraded waste pit
2.3.2.Circular grit removers
2.3.3.Aerated rectangular grit removers
2.3.4.«Metallurgy» grit removers
2.4.Hydrocyclone
3.Processing cleaning waste: Arenis
3.1.Nature of sewerage network cleaning waste
3.2.End use-regulations
3.3.Processing cleaning waste
3.4.Treatment systems
3.4.1.Arenis low flow rate system
3.4.2.Arenis high flow rate system
4.Grease and oil removal
4.1.Products to be separated out
4.2.Grease removers
4.2.1.Utilisation conditions
4.2.2.Circular grit-grease remover
4.2.3.Rectangular grit-grease remover
4.2.4.Rectangular grease remover with aerators-mixers at the inlet
4.2.5.Grease removal performances
4.3.Oil removers
4.3.1.Utilisation conditions
4.3.2.Preliminary gravity oil removers
4.4.Oil and floating matter skimmers for tanks or lagoons
5.Specific grease treatment
5.1.Products to be treated
5.2.The Biomaster principle
5.2.1.Process
5.2.2.Advantages
5.3.Performances
5.4.Other processes
5.4.1.Anaerobic digestion
5.4.2.Composting
5.4.3.Incineration

Introduction

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;
straining;
comminution;
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 chapter 10 § 3.1. in the case of desludgers;
in chapter 22 § 1.4.3. 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 chap.10), 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 § 5.) and grit (see Arenis§ 3.) 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 house-hold waste) and re-usable grit.