Wastewater treatment: how to choose the most suitable technology?

The most efficient wastewater treatment solution for every industry

In order to make progress in terms of sustainability, it is necessary for the producers of industries that generate wastewater to be environmentally aware, so that they invest in treating it appropriately. At the same time, Salher promotes solutions to return this treated water to natural watercourses (river, sea or underground) and, when possible, to reuse it for cleaning, irrigation and toilet flushing in the industries themselves.

Salher analyses the wastewater generated by all types of industries and studies the most appropriate configuration, which will depend, among other factors, on the pollutants in each type of industry:

• Agri-food industry: water from the dairy industry, water from the beverage production industry and water from the food industry.
• Chemical and energy industries.

Even though each sector has its own particularities, the water generated by the agri-food industry is characterised by high biodegradability, making it highly suitable to treat it with one of the many biological systems. Salher also designs and manufactures all kinds of preliminary elements for water conditioning, such as conventional pre-treatments (screens, screw screens or rotary screens) and advanced primary treatments, such as its DAF systems.

These dissolved air flotation systems, manufactured by Salher, are so effective in eliminating a large part of the contamination of organic matter in suspension and in oils and grease that, in some cases, depending on the required quality of the final discharge, subsequent treatments will be eliminated.

For example, when an agri-food industry with medium pollutant loads discharges to a municipal sewer in a city like Madrid, an adequate pre-treatment and advanced primary treatment such as Salher’s DAF systems would be enough. However, if it is discharged into a river or the sea, in addition to the DAF it would be necessary to incorporate biological treatment.

Other types of industries, such as metallurgy or the automotive industry, generate waste that is not highly biodegradable. For this reason, it is not possible to resort to biological treatment for these waters and it would be necessary to resort to physical-chemical treatment.

Salher works with the different technologies available on the market for biological treatment:

• Activated sludge technology:. There are many variants of activated sludge, but the most widespread for small and medium-sized plants is the low-loaded activated sludge, or activated sludge with extended aeration.
• MBBR technology: Also called moving bed technology, where the bacterial population is attached to a plastic support material to develop its growth.
• Bio-membrane technology: MBR systems achieve optimum water quality for reuse.

The choice of each technology will depend on the factors mentioned above, although if there are no major restrictions, Salher recommends aerobic biological systems, that is, with oxygen supply. These are effective, simple, do not generate odours or toxic gases and are more controlled processes in terms of performance.

The reuse of wastewater from an industry will be justified when the customer requires, according to local regulations, a high quality of liquid waste removal and when it is feasible to treat this water within its production process. In general, treated wastewater that has passed through a basic reuse system cannot be used again in the production processes of that industry.

However, this treated wastewater can be discharged unpolluted or be reused for industrial flushing and cleaning or for garden irrigation and toilet flushing with relatively low costs. In the event that the industry in question wishes to reuse this water for industrial purposes, much more specific technologies would have to be applied to achieve the quality required by the regulations. In any case, Salher will study the most suitable option for each project, selecting from among its reuse technologies:

1. Filtration: Saher works with ring filters and multimedia filters (sand, flint-anthracite and activated carbon).
2. Ultrafiltration.
3. Nanofiltration, ideal for reusing water in cooling towers.
4. Reverse osmosis: This has the advantage that, if the water to be treated has dissolved salts, chlorides, phosphates or nitrates, it will retain them. It is used when the water has to retain particles to eliminate the concentration of dissolved salts or conductivity.

Urban water treatment and reuse in times of drought

In order to treat domestic water, municipalities have sewage and wastewater treatment networks that will depend on the number of equivalent inhabitants, their BOD5 contribution (the biological oxygen demand for five days) and the local regulations in force. For small communities of up to 50 inhabitants, Salher designs and manufactures decantation-digestion systems: conventional septic tanks, under the UNE-EN 12566 standard, with the possibility of adding biological filters to increase their performance.