Technology shift pays off

The municipal water utility company Vejen Forsyning A/S now sees a 20% reduction in the volume of dewatered sludge for disposal after modernisation of their wastewater treatment plant in Vejen and installation of an Alfa Laval Aldec G3 decanter centrifuge  – a clear win both economically and environmentally. The fully automatic decanter dewaters biological wastewater sludge to a dry solids content approaching 24%, representing an improvement of 6-8% over the old belt press.

Vejen’s wastewater treatment plant has a maximum capacity to process wastewater from a population equivalent (PE) of 24,000  and is currently processing for a PE to 21,000. In 2014 it became clear that the facility’s old belt press was in a state of such disrepair that it would either have to be replaced or undergo major renovation. Vejen Forsyning therefore decided to invest in new dewatering equipment. In the project’s tender documents Vejen Forsyning were open to consider a range of technologies, as their other treatment plants had already seen benefits from using other equipment, such as decanters, for dewatering of sludge.

Tough demands for improved dewatering to reduce disposal costs

Vejen Forsyning pays considerable costs in the post-processing and removal of the 2,400 tonnes of excess sludge (at only 16% dry solids) accumulated annually after biological wastewater treatment. 60-70% of the excess sludge is dewatered using a belt press, after which the dry solids suitable for agricultural use is taken to farming sites. The remaining 30-40% of non-dewatered sludge is pumped out to the treatment plant’s drying beds for mineralisation. After some years, the residual product is spread on agricultural farm land.

The desire to reduce residual sludge volumes (and hence also disposal costs) was therefore a considerable driving factor behind the decision to invest in new dewatering equipment that would be able to achieve a considerably higher degree of dry solids than the 16-18% that the old belt press was able to deliver. The tender documents therefore set significant performance demands on the new dewatering equipment: a high level of dry solids in the dewatered sludge with low polymer consumption and, above all, a fully-automated process with minimal need for manual operation and maintenance. A new machine was specified to dewater 35 m³/hour with 353 kg dry solids/hour and produce sludge with a dry solids level of 23%, using 8 kg of polymer per 1 tonne of dry solids and with a filtrate quality level below 1,000 mg/l.

A winner on all supplier requirements

Stjernholm A/S, one of Alfa Laval’s business partners in the marketing of decanters on the Danish market, was already enjoying a good working relationship with Vejen Forsyning and was one of several suppliers invited to propose a new dewatering solution.

Stjernholm offered Alfa Laval’s newest and most effective decanter centrifuge, the Aldec G3 decanter centrifuge, which, given its unique technical capacity, can guarantee a very high level of dry solids compared to similar equipment on the market.

The tendering round revealed that the Alfa Laval Aldec G3  decanter was not only a good solution, but also the most economically advantageous. It also won on other critical selection criteria such as operating cost and functionality in addition to the quality of dry solids and filtrate. Vejen Forsyning therefore decided on the proposed Aldec G3-75 decanter, which was installed in March 2015.

Actual performance exceeds expectations

The initial tests and first month’s operational results with the new Aldec G3 decanter centrifuge confirmed that all requirements regarding dewatering performance, polymer consumption and filtrate quality were met. A dry solids content of nearly 24% exceeds the required 23% originally specified by the client. In comparison with the 16-18% that could previously be achieved, Vejen Forsyning now saves 20% on costs compared to the volume of residual sludge that previously had to be disposed of. This also means savings in operating expenditure.