Abstract: Apparatus for the treatment of a substance such as a liquid includes a treatment pit (10) which has a plurality of chambers (1 to 8). Walls (24, 26, 28, 30, 32, 34) separate the chambers. Apertures (36) are located in non-adjacent walls (24, 28, 32) near the base of the pit (10). Further apertures (38) are located in non-adjacent walls (30, 34, 40), near the top of the pit (10). The liquid is introduced into the first chamber (1), and reagents added to the liquid. The water and reagent mix then flows through apertures (36) in the wall (24) between the first (1) and second (2) chambers into the second (2) chamber. Further reagents may be added in the second (2) chamber. The water and reagent mix then flows through apertures (38) in the wall (26) between the second (2) and third (3) chambers into the third (3) chamber. This process continues until the water reaches the final (8) chamber, from which it is discharged.