Flow Equalization

Flow equalization is the process of controlling hydraulic velocity, or flow rate, through a wastewater treatment system.  The equalization of flow prevents short term, high volumes of incoming flow, called surges, from forcing solids and organic material out of the treatment process.  Flow equalization also controls the flow through each stage of the treatment system, allowing adequate time for the physical, biological and chemical processes to take place.

Published in 1974, the USEPA TECHNOLOGY TRANSFER REPORT states “The cyclic nature of wastewater flows in terms of volume and strength is well recognized.”  It goes on to say “improved efficiency, reliability and control are possible when physical, biological and chemical processes are operated at or near uniform conditions.  For this reason, flow equalization is employed.”  Since the mid-1970’s, flow equalization has been widely used for commercial, municipal and industrial wastewater treatment systems, both in the design of new facilities and upgrades to existing systems.

This technology has only recently begun to be used in residential treatment systems.  The flow patterns of residential treatment systems are intermittent and variable in nature, generating frequent hydraulic and organic surges.  These surges can result in large quantities of solids being washed out of the system.  The SEPTIC SYSTEM OWNER’S GUIDE, published in 1999 by the University of Minnesota Extension Service, states “for complete and uniform treatment of wastes, the system needs time to work.  The ideal situation would be to have wastewater enter the system as evenly as possible throughout the day and week.”  The GUIDE continues to explain that when a surge occurs “suspended solids are carried into the soil treatment system, clogging soil pores and preventing adequate treatment.”  In 1970, the National Sanitation Foundation developed a model of daily residential flow patterns for use in testing onsite treatment systems.  This model flow pattern, which is still in use today, consists of three periods of concentrated flow, alternating with varied periods of no flow.  The pattern was purposely chosen to reflect the most severe flow rate fluctuations that are typical of residential wastewater treatment systems.  In 1982, a separate test procedure was completed to include stress sequences.  These stresses consist of prolonged no flow periods combined with surge flows several times the daily loading rate.  These stress sequences were added to reflect the less frequent but more harmful variations in flow that residential systems may very well experience.  A residential treatment system that can reduce these surges and properly process the wastewater will consistently have higher quality effluent and longer operational life.

When flow equalization is incorporated into a residential treatment system, numerous benefits are produced:

• In the case of a septic tank or pretreatment tank, gravity separation of solids is greatly enhanced.  This prevents short-circuiting and eliminates excess solids from being carried downstream into the secondary treatment facility or disposal system.
• In the case of a secondary biological or chemical treatment system, elimination of hydraulic surges guarantees adequate process retention time and a much higher degree of treatment.
• Clarifiers following secondary treatment will have greater solids separation and improved effluent quality.  If a filtration device is used, solids loading to the filtration device will be reduced, resulting in longer filter life and higher effluent quality.
• The operation of a downstream sand filter, media filter or constructed wetland is enhanced by more consistent loading, the equalization of surge flows and the removal of excess solids.
• All types of effluent disposal systems, including tile fields, mounds, irrigation systems, etc., will operate longer and more efficiently because organic and hydraulic surges are eliminated and system overloading is prevented.

These benefits clearly demonstrate the important role flow equalization can play in wastewater treatment.  Incorporating flow equalization into residential onsite treatment systems makes any system perform better and prevents premature failure.  Hydraulic surges are produced everyday by bathtubs, dishwashers, clothes washers, shower facilities and a variety of water using appliances.  When these surges occur, a residential treatment system without flow equalization is compromised and often will not provide adequate treatment.  Flow equalization allows residential systems to deliver the treatment they were designed to provide.  Refer to the Norweco Technical Bulletin COMMERCIAL, MUNICIPAL, INDUSTRIAL AND RESIDENTIAL FLOW EQUALIZATION FOR WASTEWATER TREATMENT SYSTEMS for detailed information on flow equalization.