ABSTRACT
Four main designs are used for equalization ( Fig. 20.1 ). In-line equalization is most widely used with all the fl ow passing through the balancing tank resulting in signifi cant fl ow and load (organic, toxicity, etc.) dampening ( Fig. 20.1a ). Alternating equalization uses two separate tanks so that while one is fi lling the other is being discharged after the maximum storage period for equalization ( Fig. 20.1b ). Where there are a number of compatible process streams these can be fed simultaneously into a single balancing tank which must be mixed to ensure full equalization ( Fig. 20.1c ). Where settleable solids are present or where the wastewater is readily degradable then mixing is required to prevent settlement, septicity or both ( Fig. 20.1d ). Where suffi cient mixing is available the tank also acts as a pre-aeration unit resulting in improved settling characteristics due to pre-fl occulation of particles as well as some biochemical oxygen demand (BOD) removal. Side-line equalization is used only periodically when the fl ow exceeds the normal design fl ow rate. Excess fl ow is diverted into a balancing tank and then allowed to discharge slowly back into the main process stream at a controlled rate ( Fig. 20.1e ). Pumping costs are signifi cantly cheaper with side-line systems as only a small portion of the total fl ow is diverted. Equalization is carried out after preliminary treatment but prior to primary sedimentation. Equalization tanks can be a major source of odour generation, especially if volatile compounds are being stored and mixed. In such cases tanks should be covered and the air space continuously vented through a scrubber or biofi lter. Enclosed tanks can be extremely hazardous to workers and may even result in the build-up of explosive or fl ammable gases, so care must be taken (Section 13.6).
