Challenge

H+E designed and built a plant to treat wastewater from a process plant that uses a mixture of highly concentrated acids (including hydrofluoric and nitric acids), and also contains a range of soluble metals.

This highly sensitive application demanded that all equipment within the overall plant be designed and manufactured to a particularly high standard, with a target 40-year minimum design life.

It also included the supply of documentation to a very high standard.

Solution

The wastewater is first neutralised with calcium hydroxide in order to precipitate the fluoride as calcium fluoride. The pH of the incoming wastewater can be very low (less than 1) so the neutralisation is done in two separate steps.

The solids produced are flocculated before being separated from the wastewater in a tube settler, which is a highly efficient lamella clarifier.

The clarified wastewater then flows into a TOVEKO continuous gravity sand filter to remove any final traces of fine solids that were not removed in the tube settler.

At this stage, the wastewater stream may need to be cooled, as it can sometimes be too warm to discharge directly to sewer. Quality is monitored continuously before the treated stream is mixed with various other non-toxic wastewater streams being discharged from the site.

The final, mixed wastewater stream then has its quality monitored again before being discharged to foul sewer.

The sludge formed by the settling solids is transferred to a separate holding tank, which automatically removes excess water while the sludge settles further in quiescent conditions. The sludge is finally pumped from the tank for final dewatering, producing a solid that can be disposed offsite.

The plant’s control system is unusually sophisticated, and features a far greater degree and depth of continuous control and monitoring than is normal for this type of wastewater.

System pre-assembly

The plant as a whole was manufactured and pre-assembled to a very high level and was extensively inspected and factory-tested prior to being delivered to site.

Pre-assembly has been an integral part of H+E's culture for a long time, mainly because it enables quality control and because testing in the factories is much more efficient than doing so on site.