Conditioning process water as potable alternative
Surface water offers a realistic alternative to potable water for many industrial needs. Christian Stark, head of marketing at Hans Huber, outlines the key design features of a typical preliminary treatment process.
While potable water reserves are running short, careful dealing with the valuable raw material is of great importance.
For many industrial processes it is actually unnecessary to use potable water. The often high water demand could be covered from other sources.
Instead of potable water, surface water may be used as an alternative for production, cooling or transportation water.
Such surface water, which is taken from rivers or lakes, will have to undergo a multi-stage preliminary treatment to meet the requirements in specific fields. The quality of surface water depends on the suspended solids and the concentration of contained dissolved material. This also determines the scope of preliminary treatment.
A typical multi-stage conditioning plant could be characterised by the following design features:
- The pumping station or branch channel for extraction features an inlet protected by coarse screens with bar spacing of more than 20mm to retain debris such as tree branches;
- Mechanical preliminary treatment performed using fine screens, step screens or wedge section screens to separate coarse material larger than 1-3mm, such as leaves. To reduce residual water content the combination with an integrated or separate screening press is possible;
- Addition of precipitants (iron or aluminium salts) allows for chemical conversion of water components into suspended material. It also allows for the precipitation of colloidal dissolved material and cracking of emulsified oils and greases;
- Neutralisation of the water by pH regulator controls acid or leach dosing pumps;
- A lamella separator allows for sedimentation of suspended solids (macro flocs) in a compact unit requiring only minimum space. This is applied when inlet solid contents are greater than 150mg/l. With high flow volumes (more than 500m/h) the lamella packs are integrated into settling tanks using scraping systems;
- Micro filtration screens, cloth filtration plants and sand filters are applied for inlet solids concentrations less than 150mg/l. The filter is able to reduce the outlet values to below 5mg/l.
If additional improvement of water quality is required, UV sterilisation plants, and ozonisation and chlorination plants for germ elimination can be used.
The sludge produced during water conditioning, in different composition and concentration, can be additionally dewatered in screw presses. This is important when an as low as possible water content is required for advanced treatment or disposal of the residual material.