The Hard-Hofsteig UWTP Hybrid™ Process, Case Study Austria

The Hard-Hofsteig used water treatment plant was built in its original form in the 1970s. However, population growth and increasing demand for more extensive nutrient removal then led to plant expansion. The first plant enlargement was carried out in 1997, but for the second, which was planned for completion between 2012 and 2014, insufficient space was available. Therefore, in order to solve this problem, a two-stage hybrid concept was selected. This is a patented, two-stage activated sludge process for extensive nutrient removal, which consists of a high-loaded first stage and a low-loaded second stage, with a combination of two sludge cycles for enhanced nitrogen removal. This case study considers the upgrade of the Hard-Hofsteig UWTP, which doubled its capacity through the employment of the hybrid concept without any alteration of the plant footprint. In fact, plant capacity was increased from 170,000 PE to 270,000 PE at very low cost, and operational data analyses indicate stable performance.