At the present time businesses, govements, conservation bodies and members of the public are becoming increasingly aware and concerned about the environmental impacts associated with the discharge of silty (sediment rich) waters into our inland water ways. It is becoming apparent that a key driver to improving the quality and bio-diversity of our inland water ways is a need to reduce the amount of sediment that is discharged into the aquatic system. Increasingly more stringent targets relating to the quality of discharges into the environment are being enforced, backed up by increased levels of fines and penalties handed out by the Courts.
In early 2018, SEPA made changes to the CAR Licensing Regime and the EA revised their Regulatory Position Statement relating the temporary discharge of surface water from construction sites.
Following these changes there is now a greater emphasis for construction sites to:
- Adequately pre-plan their works to minimize the volume of “dirty” water which will be generated,
- Minimize the duration of the works,
- Progressively re-instate the site,
- Provide an adequately sized system for the control and treatment of “dirty” water prior to discharge into the environment,
- Ensure that any waters discharged from site are of the required quality (compliant with discharge conditions) and do not harm the environment.
In the press over the recent months there have been a number of articles highlighting both the impacts of silty waters containing only small amounts (<30 mg/l) of sediment (1 mg/l = 1 part per million), the need for improved farming practices to reduce the potential for soil to be eroded from agricultural land and reports of large fine/penalties being handed out by the UK’s environmental regulator (EA, NRW, SEPA).
The message being sent out is clear “The Polluter Pays”.
Failure to ensure that a company’s activities are undertaken in a manner which ensure compliance with the law and protects the environment will under the current climate have severe financial implications.
Updated Environmental Guidance
The Environment Agency (EA) updated position statement on the Temporary Dewatering from Excavations to Surface Water and Scottish Environmental Protection Agency (SEPA) changes to the CAR Licensing have increased the requirements associated with the discharge of excess water from construction sites. For the construction industry this means a greater need to conduct their works in a more environmentally acceptable manner.
The updated information issued by the EA and SEPA have a common theme; they have made it clear that they expect, require, and will enforce the need for construction companies to:
- Plan their works in advance;
- Minimise the volume of silt laden water which will be generated;
- Prepare appropriate schemes for the treatment of excess water prior to discharge into the environment.
- Have a high degree of certainty that the methods proposed will achieve the required discharge water quality under a wide range of weather conditions. A “suck it and see” approach to the implementation of abatement solutions will not be tolerated..
As part of the pre-commencement planning which now must be undertaken it is necessary to identify appropriate treatment solutions along with their sizing, taking into account the volumes of water which can be treated. In addition contingency plans to deal with emergency situations, such as, extreme wet weather need to be developed.
In planning site works it is necessary to take account of the treatment system hierarchy, a representation of the treatment hierarchy applicable to a construction site is presented below.
However, in practice at any site effective treatment which protects the environment will be achieved using technologies from across a range of Treatment Tiers. Planning site works in accordance with the treatment hierarchy encourages a company to focus on identifying a range of different technologies appropriate to individual parts of the site, rather than seeing the site as a whole.
Tier 1 (The First Line of Defence)
For any site the first choice should be adopt a Tier 1 solution.
Tier 1 solutions consist of options to:
- Prevent problems from occurring (ie keep clean water clean).
- Eliminate the problem.
- Adopt measures to prevent excess dirty water from migrating from the site and impact on a receiving water body (receptor). Reduce the amount of water which will be generated.
- Phased stripping of the working area and phased reinstatement of the area can have huge positive impact (reduction) of both the volumes of water to be treated and duration (longevity) that the site can negatively impact the environment.
- Diversion ditches and diversion banks which prevent the run-off from up gradient areas gaining access into the working area; minimising the volumes of dirty water which will be created and small earth banks which prevent the discharge of overland flow water into a receptor; are commonly used physical methods which minimise the volumes of dirty water that may be discharged.
Appropriate planning of the works can greatly reduce the volumes of dirty water which will be created, reducing the emphasis for utilising treatment solutions from a higher Tier.
Tier 2 (The Second Line of Defence)
Once all options to reduce/eliminate/minimise the impacts of the site works have been exhausted the site should consider implementing a Tier 2 (Erosion Control) solution.
Erosion control measures are characterised by treatment solutions which minimise the velocity at which water travels across an exposed surface. By reducing the flow velocity the potential for soil particles to be eroded (creating dirty water) is reduced and the potential for the subsequent suspended solids to drop out of suspension is increased.
On a construction site the most commonly used erosion control measures would include:
- Silt fences
- Check dams
- Temporary reinstatement of exposed surfaces by either compaction, use of grass seeds, mulches or specialist erosion geotextiles.
Tier 2 treatment solutions are commonly used to treat intermittent, small volumes of water locally to their point of production.
Frequently erosion control measures will be used in conjunction with sedimentation control and or active control treatment solutions.
Tier 3 (The Third Line of Defence)
Sedimentation control involves the adoption of treatment solutions which are designed to recover suspended particles from surface water run-off immediately prior to its release into the environment. The most commonly used sedimentation controls used on construction sites are:
Treatment solutions such as these use the ability of the water to soak into the ground or the ability of suspended solids to settle out of suspension. They are particularly applicable for use on sites where the soil has a particle size of medium silt and/or greater. On soil types such as silty clays, clays or on saturated ground they may prove to be ineffective.
Due to their limitations Tier 3 treatment solutions are frequently employed to treat intermittent, low to medium flow rates where the soil types are of a medium to coarse particle size.
Tier 4 (The Final Line of Defence)
Active treatment solutions form the final line of defence. Characteristically these measures are used to treat concentrated flow rates at locations with limited space available for treatment.
The most commonly used active treatment solution used on a construction site would be the use of pre-treatment chemicals (coagulant and/or flocculant) to permit the recovery of the suspended solids within either a settlement lagoon or lamella clarifier.
Due to their active nature Tier 4 Treatment systems can produce on a continuous basis, a high treated water quality of a consistent nature (low variability), they may have fail safes incorporated into their design to prevent overdosing of treatment chemicals and/or the discharge of out of specification water.
Tier 4 Treatment systems are frequently employed on sites that have a high environmental sensitivity or sites which have the potential to have a large environmental impact.
Importance of site size and flow rate
Whilst the use and ideas set out in the Treatment Hierarchy are accepted and frequently applied to the planning of and operation of a construction site, a key consideration which is often over looked is the size of the construction area and the flow rate that needs to be treated.
For small sites and small flow rates it can be practical to extensively use erosion control methods to minimise impact. At greater flow rates the dominate methodology will be sedimentation control and for larger flow rates/sites the use of active treatment will dominate. This relationship between site size, flow rate and control methodology.
Flow Rate Considerations
The amount of run-off generated from any construction site is principally a function of 4 factors:
The steeper the sites gradient, the faster the water will run-off, giving it less time to soak into the ground.
• Soil Type
The more permeable the ground conditions the greater the percentage of the storm which will soak into the ground, reducing the volume available for generating run-off.
• Annual Rainfall
The amount of rainfall which annuals falls across the UK is hugely variable, as a general trend the western sides of the country experience much greater rainfall volumes than the eastern side. The greater the rainfall the greater the amount of run-off which will be generated.
• Drainage Area
The greater the area drained by the site, the greater the volume of water which will be generated.
Estimating Flow Rates
Useful guidance on calculating flow rates which may be generated has been prepared by CIRIA in their publication C648 entitled “Control of Water Pollution from Linear Construction Projects”.
The CIRIA calculation methodology provides a quick and simple method of calculating peaks from a drainage area. The calculated peak flow can be used as part of the assessment of which technology to use and estimating quantifying the systems treatment capacity.
An additional consideration when selecting treatment control measures is to consider the environmental sensitivity of the site and the surrounding area. The greater the environmental sensitivity the higher the quality of the water that will need to be discharged from the site. Additionally, in environmental sensitive areas the discharge water will need to be more consistent (have a lower variability).
In terms of environmental sensitivity there are a number of features, which need to be considered. For example:
- The receiving water classification under the Water Framework Directive.
- The conservation status of the receiving water course and areas of land around and downstream of the discharge point.
- The volume of the receiving water course relative to the discharge rate.
- The existing uses of the water body, eg public water abstraction
Treatment technology from a higher Treatment Tier will produce a higher quality treated water which is less variable in its composition.
Pre-planning of construction activities is essential to ensure that appropriate environmental protection measures/treatment systems of the correct size are fully implemented in advance of the construction works commencing.
Source: Watery News 15th May 2018