Removing COD from steel industry wastewater

At DESOTEC, we pride ourselves on our problem-solving approach and the adaptability of our mobile filtration solutions. The tertiary wastewater filtration set-up we installed at a Spanish steel plant looks very different from what we’d originally envisaged! That’s because we had to adjust it to overcome unforeseen issues with other technology. By drawing on our expertise and experience, we can meet such challenges head-on and install filtration systems that work. This keeps our customers on the right side of the authorities, protecting their business and our natural environment.

The problem

A steel factory in Spain has an on-site wastewater treatment plant (WWTP). When chemical oxygen demand (COD) discharge limits were lowered to 220 mg/ litre, this WWTP was no longer sufficient. 

At the same time, changes in production meant that COD levels in the wastewater had risen to around 300 mg/ litre: roughly 40% higher than was permitted by the tightened regulations. The flow rate was 70 m³/ h and production at the plant is 24/7, making this a huge project. 

The company had already been in discussions with DESOTEC about a different issue. It now turned to us to install filters as a polishing step after the WWTP to meet this new and significant challenge. 

The solution

We began by analysing a water sample, which determined that the best-performing type of activated carbon would be our Organosorb®10-AA product. 

The COD in the sample was 246 mg/ l – somewhat lower than the maximum reported by the customer. 

Due to the high flow rate and 24/7 production, we knew that the consumption of activated carbon would be high. 

We proposed installing a sand filter between the WWTP and DESOTEC’s Mobicon filter to remove suspended solids in the water and conserve the carbon. Our tests suggested that after it was saturated, this carbon would be suitable for reactivation – helping to boost the sustainability of our solution. 

Once the set-up was in place, we discovered the high flow rate was proving too much for the sand filter. This is where our flexibility really paid off – we quickly adapted the solution by adding a second sand filter, meaning we could easily alternate between the two. 

The flow rate has been reduced to 0-40 m³/ h, and the wastewater is being processed in batches rather than as a continuous flow. 

The results

COD levels are now well within the permitted levels, and there are no issues with the authorities. The set-up, which began as a pilot test, is now considered permanent. 

A welcomed side effect has been that this system is also reducing fluorine levels in the treated wastewater, for which a separate team within the company had been seeking a solution. 

These fluorines come from inorganic components and are not per- and polyfluoroalkyl substances (PFAS), which would require a different treatment regime. The system may yet be tweaked to reduce fluorine levels further. 

The project is smaller than envisaged, and the carbon consumption is lower than foreseen. In 16 months, the Mobicon filters have been exchanged twice: it’s a simple process with no disruption at the plant. 

Safe and sustainable waste handling is an integral part of DESOTEC’s full-service solution. We transport saturated carbon in closed filter units away from customers’ sites and take it to our facilities. Adsorbed molecules are destroyed in our furnaces, and the carbon reactivated wherever possible, driving down costs and boosting sustainability.