The term surface loading is used both in connection with trickling filters and with secondary clarifiers.

Trickling filter: the surface load is the quotient of the amount of water per unit of time and the surface of the trickling filter. Typical values are 0.5 - 1.5 m3 / m2 h. With the surface load, the rinsing effect in the trickling filter can be influenced (clogging problems in the upper layers). For this purpose, the effluent of the trickling filter or the secondary clarification is recycled (via back-pumping). The recycling can also take place via the primary treatment. Disadvantages are the correspondingly higher energy costs and the higher hydraulic load to secondary clarification and possibly primary treatment.

Final clarification: In biological wastewater treatment, most of the wastewater ingredients are converted into biological sludge. This sludge is separated in the final clarification from the cleaned wastewater. The decisive operating parameter for secondary clarification is the surface load at which the feed quantity (feed including return sludge) is related to the surface of the final sedimentation tank. The better the sludge settling properties, the more the quantity of return activated sludge (RAS) can be reduced. On the one hand, that lowers the hydraulic load to the final clarification (and thus leads to better discharge values) and, on the other hand, reduces the energy costs for the return sludge production.


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