Two identical reactors were built; each reactor was 14 cm in diameter and 100 cm in height, providing an empty bed volume of 15 l. A small amount of freeboard or headspace (2.8 litres) was provided at the top of the reactor. The reactors were constructed from PVC, a non-transparent material that prevents the growth of phototrophic organisms. The columns were built with considerations for process air and influent supplies, backwashing air and water requirement and sampling outlets.

The control reactor was filled with 10.9 l cascade rings (Glitsch UK) whilst the second reactor was only partially packed with 5.5 l cascade rings. The media were stationary and held in place by a rigid polypropylene mesh with 15 mm diameter holes placed at the top and bottom of the packing. Three ports were placed along the height of the reactors for sample collection.

A synthetic waste prepared in the laboratory was used to provide a consistent organic substrate for all loadings. The basic make-up of the influent organic strength material used in the study was whey powder, glucose and meat extract (Lab Lemco powder) which contributed approximately 38%, 33% and 29% of the total soluble COD content of the substrate respectively. In order to guarantee that organic carbon was the limiting nutrient, a COD:N: P ratio of 25:5:1 was adopted. Nitrogen component of the feed came from whey powder (24.7%), meat extract (63.7%), and ammonium-dihydrogenphosphate (11.6%). 1 l of the prepared mixture produces a concentrated feed around 40000 mg/l COD.