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
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
When flow equalization is incorporated into a residential
treatment system, numerous benefits are produced:
1. 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.
2. 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.
3. 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.
4. 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.
5. 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.