First flush is the initial

surface runoff Surface runoff (also known as overland flow or terrestrial runoff) is the unconfined flow of water over the ground surface, in contrast to ''channel runoff'' (or ''stream flow''). It occurs when excess rainwater, stormwater, meltwater, or other ...

of a rainstorm. During this phase,

water pollution Water pollution (or aquatic pollution) is the contamination of Body of water, water bodies, with a negative impact on their uses. It is usually a result of human activities. Water bodies include lakes, rivers, oceans, aquifers, reservoirs and ...

entering

storm drain A storm drain, storm sewer (United Kingdom, U.S. and Canada), highway drain, surface water drain/sewer (United Kingdom), or stormwater drain (Australia and New Zealand) is infrastructure designed to drain excess rain and ground water from i ...

s in areas with high proportions of

impervious surface Impervious surfaces are mainly artificial structures—such as pavements (roads, sidewalks, driveways and parking lots, as well as industrial areas such as airports, ports and logistics and distribution centres, all of which use considerable ...

s is typically more concentrated compared to the remainder of the storm. Consequently, these high concentrations of

urban runoff Urban runoff is surface runoff of rainwater, landscape irrigation, and car washing created by urbanization. Impervious surfaces (roads, parking lots and sidewalks) are constructed during land development. During rain, storms, and other Precipitati ...

result in high levels of

pollutant A pollutant or novel entity is a substance or energy introduced into the environment that has undesired effect, or adversely affects the usefulness of a resource. These can be both naturally forming (i.e. minerals or extracted compounds like oi ...

s discharged from storm sewers to

surface water Surface water is water located on top of land, forming terrestrial (surrounding by land on all sides) waterbodies, and may also be referred to as ''blue water'', opposed to the seawater and waterbodies like the ocean. The vast majority of surfac ...

s.Alex Maestre and Robert Pitt; Center for Watershed Protection (2005
"The National Stormwater Quality Database, Version 1.1: A Compilation and Analysis of NPDES Stormwater Monitoring Information."
Report prepared for U.S. Environmental Protection Agency (EPA), Washington, DC. September 4, 2005.

First flush effect

The term "first flush effect" refers to rapid changes in

water quality Water quality refers to the chemical, physical, and biological characteristics of water based on the standards of its usage. It is most frequently used by reference to a set of standards against which compliance, generally achieved through tr ...

( pollutant concentration or load) that occur after early season rains.

Soil Soil, also commonly referred to as earth, is a mixture of organic matter, minerals, gases, water, and organisms that together support the life of plants and soil organisms. Some scientific definitions distinguish dirt from ''soil'' by re ...

and vegetation particles wash into streams;

sediment Sediment is a solid material that is transported to a new location where it is deposited. It occurs naturally and, through the processes of weathering and erosion, is broken down and subsequently sediment transport, transported by the action of ...

s and other accumulated organic particles on the river bed are re-suspended, and dissolved substances from soil and shallow groundwater can be flushed into streams. Recent research has shown that this effect has not been observed in relatively pervious areas. The term is often also used to address the first flood after a dry period, which is supposed to contain higher concentrations than a subsequent one. This is referred to as "first flush flood." There are various definitions of the first flush phenomenon.

First foul flush

Storm water runoff in a

combined sewer A combined sewer is a type of gravity sewer with a system of pipes, tunnels, pump stations etc. to transport sewage and surface runoff, urban runoff together to a sewage treatment plant or disposal site. This means that during rain events, the se ...

produces a first foul flush with a suspension of accumulated sanitary solids from the sewer in addition to pollutants from surface runoff. Inflow may produce a foul flush effect in

sanitary sewer A sanitary sewer is an underground pipe or tunnel system for transporting sewage from houses and commercial buildings (but not stormwater) to a sewage treatment plant or disposal. Sanitary sewers are a type of gravity sewer and are part of ...

s if flows peak during wet weather. As flow rates increase above average, a relatively small percentage of the total flow contains a disproportionately large percentage of the total pollutant mass associated with overall flow volume through the peak flow event. Sewer solids deposition during low flow periods and subsequent resuspension during peak flow events is the major pollutant source for the first-flush combined-sewer overflow (CSO) phenomenon. Sanitary sewage solids can either go through the system or settle out in

laminar flow Laminar flow () is the property of fluid particles in fluid dynamics to follow smooth paths in layers, with each layer moving smoothly past the adjacent layers with little or no mixing. At low velocities, the fluid tends to flow without lateral m ...

portions of the sewer to be available for washout during peak flows. The

wetted perimeter Wetting is the ability of a liquid to displace gas to maintain contact with a solid surface science, surface, resulting from intermolecular interactions when the two are brought together. These interactions occur in the presence of either a ga ...

of sewers may also be colonized by

biofilm A biofilm is a Syntrophy, syntrophic Microbial consortium, community of microorganisms in which cell (biology), cells cell adhesion, stick to each other and often also to a surface. These adherent cells become embedded within a slimy ext ...

nourished by soluble sanitary wastes. Hydraulic design is the underlying reason for solids deposition in sewers. Combined sewers sized for peak runoff events expected once a decade can carry up to 1,000 times the average sanitary flow. Less dramatically oversized sewers are common in new developments and near the upstream end of collection systems.

Suspended solids Suspended solids refers to small solid particles which remain in suspension in water as a colloid or due to motion of the water. Suspended solids can be removed by sedimentation if their size or density is comparatively large, or by filtration ...

may accumulate when low-flow fluid velocities generate insufficient

turbulence In fluid dynamics, turbulence or turbulent flow is fluid motion characterized by chaotic changes in pressure and flow velocity. It is in contrast to laminar flow, which occurs when a fluid flows in parallel layers with no disruption between ...

. Solids deposition is greatest where velocities are low during dry weather. In large combined sewers it may be impossible to attain sanitary sewage velocities generating sufficient turbulence to keep solids suspended during dry weather. Biofilm and previously deposited solids may be scoured and re-entrained during peak flow turbulence. The high pollution load in wastewater at the beginning of a runoff event occurs when increased flow rate erodes accumulated sewer sediment. Erosion of sediments in sewers can release pollutants in concentrations exceeding levels found in contributing sources. The initial highly polluting foul flush is released at the start of wet weather flow during speedy erosion of a weak layer of highly concentrated surficial sediment bed-load. When conditions favor dry-weather solids deposition, the first foul flush may contain as much as 30 percent of the annual total suspended solids discharged to a combined sewer system. Combined sewer suspended solids concentrations of several thousand milligrams per liter (mg/L) may be observed during the first foul flush. Pollutant concentration levels are influenced by the age and condition of the collection system and the amount of infiltration/inflow in comparison to the sanitary flow. Pollutant concentration peaks depend on size and slope of the piping system, time interval between storms, and solids accumulation in the collection system. Steeper sewer gradients and pipe bottom shapes that maintain high velocity flow during low-flow conditions will reduce sediment accumulation in sewers; and periodic sewer flushing of individual lines during dry weather may move accumulated solids to the

wastewater treatment plant Wastewater treatment is a process which removes and eliminates contaminants from wastewater. It thus converts it into an effluent that can be returned to the water cycle. Once back in the water cycle, the effluent creates an acceptable impact on ...

before stormwater runoff causes simultaneous peak flow in the entire collection system.

Related terms

Because the reference of the first flush is not always clear, the terms "concentration-based first flush" (CBFF) and "mass-based first flush" (MBFF) have been introduced. Apart from this definition, there are a number of rating parameters in literature to determine the occurrence of a first flush.

Rainwater harvesting

In the context of

rainwater harvesting Rainwater harvesting (RWH) is the collection and storage of rain, rather than allowing it to run off. Rainwater is collected from a roof-like surface and redirected to a Rainwater tank, tank, cistern, deep pit (well, shaft, or borehole), Aquifer s ...

, a first flush diverter is a simple device that is designed to protect a storage

cistern A cistern (; , ; ) is a waterproof receptacle for holding liquids, usually water. Cisterns are often built to catch and store rainwater. To prevent leakage, the interior of the cistern is often lined with hydraulic plaster. Cisterns are disti ...

from contamination by first flush runoff. This leads to a higher quality of water captured, and less silting of the cistern over time in dusty areas. The diverted first flush water is used for irrigation or other purposes in a fashion similar to

greywater Greywater (or grey water, sullage, also spelled gray water in the United States) refers to domestic wastewater generated in households or office buildings from streams without fecal contamination, i.e., all streams except for the wastewater fro ...

. Although many commercial versions are available, these devices are frequently constructed of spare pipe when the cistern is initially installed or thereafter. See ''Texas Manual on Rainwater Harvesting'' for calculations on sizing.

References

External links

- New South Wales Environment Protection Authority, Australia {{Sewerage Environmental engineering Hydrology Water pollution