The problem with centralised effluent treatment is that combining two waste streams that require different treatment technologies leads to a cost of treating the combined streams which is virtually always more expensive than individual treatment of the separate streams. On the other hand, if two waste streams require exactly the same treatment it is sensible to combine them for treatment to obtain economies of scale. The design of effluent treatment systems should in the first instance segregate the streams for treatment and only combine them if it is appropriate. If this policy is followed then the effluent treatment system becomes distributed rather than centralised. Distributed effluent treatment can, in the appropriate circumstances, lead to significantly lower capital and operating costs when compared with centralised treatment. This paper presents a general methodology for the design of distributed effluent treatment systems. The methodology first sets targets for effluent flowrates through the treatment processes for minimum treatment costs. Simple design rules allow the development of a design to achieve the targets. The approach is general in that a centralised or distributed system could be obtained, whichever is appropriate.