An apparatus for feeding substantially uniform amounts of a chemical into a liquid stream having an irregular flow pattern. In accordance with an embodiment of the present invention there is provided a holding tank having an inlet port for receiving a liquid stream, such as the effluent of a septic system. The tank has an aperture located at a relatively high position in a side wall thereof. A siphon means is provided and comprises an inverted U-shaped section of pipe passing through the aperture and having an inner leg which extends to a position near the bottom of the tank. The siphon means also has an outer leg which extends to a position substantially below the bottom of the tank. Chemical feeding means, preferably in the form of an erosion feeder, are coupled to the outer pipe leg of the siphon means. The feeding means contains a supply of the chemical additive and is operative to feed the chemical into the liquid discharged from the outer leg. In the preferred embodiment of the invention, undesirable dripping is minimized by coupling the outer leg to the chemical feed means via a pipe having a substantially smaller diameter than the diameter of the outer leg. In this embodiment, an erosion feeder is coupled to the outer leg by a coupling pipe and also by a relatively small diameter tubing. The erosion feeder is adapted to dissolve the chemical in the liquid which flows through the tubing and merge the resultant solution with the liquid which flows through the coupling pipe. In operation, the effluent will accumulate in the tank until a predetermined level is reached whereupon the siphon means will operate to cause a substantially uniform flow rate to the erosion feeder. In this manner, the erosion feeder can feed the chemical, typically in solid form, at a substantially uniform rate. When the tank has been discharged, the flow stops abruptly and no further chemicals are added. Thus it is seen that the system attains control of chemical feed without requiring a complex sensing or pumping system or any companion control system.