Stormwater first flush pollution
Aim of this document
This document describes what first flush is, situations in which it is likely to occur and how it can be managed. It identifies factors and criteria that should be considered in the design of first flush stormwater pollution control systems.
Pollutants deposited on to exposed areas can be dislodged and entrained by the rainfall-runoff process. Usually the stormwater that initially runs off an area will be more polluted than the stormwater that runs off later, after the rainfall has 'cleansed' the catchment. The stormwater containing this high initial pollutant load is called the 'first flush'.
The existence of this first flush of pollutants provides an opportunity for controlling stormwater pollution from a broad range of land uses. First flush collection systems are employed to capture and isolate this most polluted runoff, with subsequent runoff being diverted directly to the stormwater system.
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The existence of first flush should not be assumed in all cases. Intensive monitoring of stormwater runoff from some (usually larger) catchments has failed to observe this phenomenon. Clearly the existence or non-existence of first flush is critical in the design of stormwater pollution controls.
First flush may not be observed for one or more of the following reasons:
- The drainage characteristics of the catchment may prevent it. Particularly in large catchments, initial runoff from the most distant parts of the catchment may not reach the catchment outlet for some time after a storm starts. This time lag is rarely an issue for smaller, individual premises.
- The pollutants may not be very mobile. Rainfall does not remove some pollutants, like oils and greases, as easily or as quickly as soluble materials and fine dusts. Bare soils or vegetated surfaces are generally not 'cleansed' as easily or effectively as sealed surfaces.
- Pollutant sources that are effectively continuous may exist within the catchment. First flush is generally seen only where the supply of pollutants is limited. Sediment generated from soil erosion, for example, will not give a first flush because the supply of soil particles is (for all practical purposes) unlimited. In cases like this, on-line, flow-through pollution controls will be needed. In urban catchments during large storms, continuous discharges from sewer overflows may mask any first flush associated with stormwater runoff.
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A broad range of pollutants can be found in stormwater runoff. The nature of these pollutants depends strongly on the land use and the activities carried out on the site or catchment.
Pollutants can come from atmospheric fallout, accidental spills, leakages, materials handling practices, or the application of chemicals (including fertilisers) or wastes to land.
The appropriateness of first flush containment depends primarily on the nature and source of the pollution, in terms of the drainage hydrology, pollutant mobility and pollutant supply.
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First flush is most readily observed on small catchments or individual premises, particularly if a high proportion of the catchment is impervious (such as paved surfaces and roads). In such cases, the first flush collection system can form an integral part of the stormwater pollution control system.
The first flush containment system also acts as an emergency backup if there is a chemical spill or similar incident. This reduces the risk of pollution and subsequent prosecution.
The following principles are a general guide to controlling stormwater pollution from individual premises.
- Minimise the availability of pollutants to be entrained by stormwater runoff.
Isolate areas that pose a clear pollution risk so they do not drain into the stormwater system. This can be carried out by roofing the area (for example, a vehicle wash bay) or by using bunding and drainage to a collection point for re-use, treatment or disposal (see below). Ideally, all potentially polluting activities at industrial premises should be done in covered or (less preferably) bunded areas.
- Install a first flush collection system and associated drainage works to capture the most polluted portion of the site's stormwater runoff.
More than one first flush pit may be needed, depending on the drainage needs of the site (for example, there may be multiple discharge points).
- Re-use or dispose of first flush water quickly and properly.
It is important that, after it rains, the stormwater captured in the first flush collection pit is promptly re-used or disposed of before the catchment becomes re-contaminated.
Explore every opportunity to re-use the collected first flush water, so that it does not have to be put into the environment and potable water sources are conserved. Alkaline first flush water has been successfully re-used at concrete batching plants for many years. More recently, collected stormwater has been employed at a hot-mix bitumen emulsion plant and intensive horticultural premises.
First flush systems installed as recirculation dams at intensive agricultural sites like market gardens and nurseries clearly have the potential to re-use nutrient-rich stormwater for irrigation.
If it is not practicable to re-use the water beneficially it will have to be disposed of. The most acceptable disposal means are land application (in accordance with the DEC's Environmental Guidelines: Use of Effluent by Irrigation (effguide.pdf 504 KB), or disposal into sewers in accordance with a Trade Waste Agreement with the local sewerage authority. Some pre-treatment may be necessary in each of these cases.
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The following points should be considered in the design of a first-flush system. Some of these are illustrated in the "Typical First Flush Collection Pit" diagram.
- Barriers such as 'speed humps' may be used to isolate the surface area that is to drain to the first flush system. This will prevent the relatively uncontaminated water from 'clean' areas entering the first-flush system, and thereby keep the required volume of the pit to a minimum.
To ensure contaminated first flush water is directed to the collection and subsequent clean runoff water is diverted to the stormwater system via the clean runoff bypass channel once the first flush collection pit is full, either (a) a low weir across the entrance to the clean runoff bypass channel; or (b) the inlet channel leading to the collection pit slopes towards the collection pit so that the bottom of this channel is lower than the bottom of the clean runoff bypass channel.
- There must be adequate separation distance between the collected first flush water and the bypass channel, to minimise entrainment of captured stormwater by bypass flows. This is particularly important where hazardous materials are involved, including acid and alkaline materials.
- The volume of the collection pit must be big enough to capture most of the pollutant load expected from the catchment. The volume will be a function of the nature of the catchment surface (pervious or impervious) and the nature of the pollutant(s) expected. Unfortunately limited information on the amount of rainfall necessary to cleanse polluted surfaces is available. However, Table 1 below may help.
- A marker should be provided on the wall of the tank to indicate the level to which collected water needs to be pumped down to to ensure the required capacity is available for the next rain event. Additional volume will be required if it is expected that contaminants will accumulate at the bottom of the pit. As a guide, an additional depth of 500 mm or an additional volume of 30% of the required capacity should be provided.
If a below ground tank or pit is not feasible, an alternative solution may be to use a collection sump with a pump and float switch together with an above ground tank storage tank. A method for removing any contaminants that accumulate at the bottom of the tank would need to be provided. An existing stormwater pit could be used as a collection sump providing the outlet to stormwater in the pit is a suitable distance above the bottom of the pit.
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Table 1: Design criteria for first flush containment systems
Examples of industries
Rainfall level to be contained
Substances easily mobilised, such as soluble materials, fine dusts and silts
Impervious: concrete, cement, bitumen
Concrete batching plants
Substances that are more difficult to mobilise, such as oil, grease and other non-volatile hydrocarbons
Impervious: concrete, cement, bitumen
Petrochemical plants, motor vehicle courtyards, chemical manufacturers, hot mix bitumen emulsion plants, roadways
All types of pollutant
Pervious surfaces (including natural ground surface) that are not as easily cleansed of deposited pollutants
Market gardens, nurseries
It is an offence under the section 120 of the Protection of the Environment Operations Act to pollute waters.
First flush systems play an important role in the control of stormwater pollution, particularly if the drainage area is small and large parts of the catchment are impervious.
Wherever possible, first flush water should be re-used on site as make-up water or similar. Discharging treated first flush water to sewers or stormwater, usually after pre-treating it, is a less preferable means of disposal.
For first flush systems to work properly they must be properly designed and installed, and captured stormwater must be removed quickly before the source catchment becomes recontaminated with pollutants.
Officers should be aware of these design criteria so they can incorporate them into development controls.
For Authorised Officers:
Domestic wastewater and septic systems
Page last updated: 26 February 2011