ABSTRACT
The microbial fi lm takes 3 – 4 weeks to become established during the summer and up to 2 months in the winter. Only then has the fi lter reached its maximum purifi cation
capacity, although nitrifi cation will take longer to become established. Unlike many industrial wastewaters, it is not necessary to seed domestic wastewaters treated by percolating fi lters as all the necessary micro-organisms are present in the sewage itself, with the macro-invertebrate grazers either fl ying onto the fi lter or entering the sewer via storm water run-off and subsequently colonizing it. The micro-organisms quickly form a fi lm over the available medium. However, the fi lm only develops on the surfaces that are receiving a constant supply of nutrients, so the effectiveness of media to redistribute the wastewater within the fi lter, to prevent channelling and to promote maximum wetting of the medium is an important factor affecting performance. The fi lm is a complex community of bacteria, fungi, protozoa and other mesofauna, plus a wide diversity of macro-invertebrates, such as enchytraeids and lumbricid worms, dipteran fl y larvae and a host of other groups which all actively graze the fi lm ( Figs 15.2 and 16.3 ) ( Table 16.1 ).
