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Question in Full:

Has the hydrodynamic vortex system ever been applied to industrial cooling towers to clean-up the recirculating water, especially the biological debris? If not, what particle size is the unit designed to remove?

If not, secondly, do you have a recommended technology that is able to filter biologics out of cooling tower water. Large debris is not an issue to remove typically, but the smaller items (assume less than 10 microns) is an issue as they build-up in a nutrient rich cooling water system of my client's.

Thanks for your consideration of my question.

Answer from: Prof. Robert Andoh, Chief Technology Officer, Hydro International

Has the hydrodynamic vortex system ever been applied to industrial cooling towers to clean-up the recirculating water, especially the biological debris?

• I don’t believe it has been used in this application.

If not, what particle size is the unit designed to remove?

• The usual application range for the HDVS (without the addition of chemical coagulants and flocculant aids) is the settleable solids range typically down to the 10 – 20 micron range. When coagulants and flocculant aids are used, the HDVS can tackle colloidal particulates in the 1 micron or less range.

If not, secondly, do you have a recommended technology that is able to filter biologics out of cooling tower water.   Large debris is not an issue to remove typically, but the smaller items (assume less than 10 microns) is an issue as they build-up in a nutrient rich cooling water system of my client’s.

• Our technologies that may be applicable for the application include:

1. 1. HDVS (i.e. Hydrodynamic Vortex Separator) with the addition of coagulants and flocculant aids
2. Up-Flo® Filtration System – A high rate water filtration system attuned to intermittent flow operation

• We also have a high speed video imaging system (FloCAM®) that can be utilized to analyse particulate solids in water samples. If you were in a position to send samples to our Test Facility in Portland, Maine, we could assess the nature of the biological debris and advise further regarding appropriate choice of technologies to address the problem.

Question in Full:

There is one detail I had not provided, which was an oversight on my behalf - the reason for the kerb is that there is a footway/cycle track behind it (see section on the attached drawing).

I also have another question (which is really a separate question all together). Gully spacing design is to HA 102/00 and the storm return period is 5 years, while storm drainage systems are often designed with a 30 year return period. Is there a conflict here, both in the return periods and the practicality of a standard gully grate being able to cope with a 30 year storm.....even if it is cleaned out regularly!

(see also earlier question: 'When does a swale become a ditch?)

Answer from: David Schofield, Associate Director, Hydro Consultancy

This is the dichotomy and conflict within our industry for hydraulic modelling and design, which I’ve pitched in during presentations and is met by a lot of looking at the floor.  In places like Australia (which I’ve done), you actually design the openings in the surface located openings for surface water runoff, to capture large events.

Meanwhile in the UK, we (magically) inject all of these flows into the upstream chambers in our modelled systems, seemingly ignoring the flow capture capability of gullies, rainwater downpipes etc.  the end result is that during an extreme event (1 in 30, 1 in 100 year etc.) what actually happens is a lot of overspill (gutter flow “jumps” gullies or channels and spills over the top of many roof drainage systems). Which becomes overland flow.  Overland flows now can and should be modelled, but there is a discrepancy between the required hydraulic performance of the primary system and the capability of the secondary capture system that includes gullies.

The capture performance can be improved by adding modifications and ancillaries to gullies, but these still aren’t modelled in the design.

This issues might well create a conflict of opinion, depending on where your allegiance lies.  But the simple fact is it’s true and it was a major factor in the 07 flooding.  Even during moderately heavy rainfall, if you go out onto a road and examine gutter flow (sad, but I have done it many times,,,,!!), you’ll see how water doesn’t go where its meant to!

Question in Full:

When does a swale become a ditch??

I am looking at drainage options for highway runoff. I am widening an existing road which could double the impermeable area, infiltration is unlikely, a deep ditch running along the back of the verge, with weirs at intervals would attenuate the additional runoff before discharge to the adjacent river.

However, road gullies are ususally built with 600 mm cover, a 150mm pipe and the outfall to a ditch would typically be 150mm above the ditch invert to reduce the risk of silting. This means the ditch/swale invert is 900 mm below road level, plus the fall on the pipe. Does this make it too deep for a swale? If not then the side slopes associated with this depth will create a very wide feature.

Answer from: David Schofield, Associate Director, Hydro Consultancy

The key decisions are do you want a swale or a deeper open channel and do you really want to keep the gullies?  Swales are normally less than 600 mm deep.  However it is not unusual to have a channel (or ditch) in a SuDS design, which

is deeper than a traditional swale, but that may have safety implications when located immediately next to a highway – but usually this is only a consideration during those periods when the water is deep and this timeframe is usually a few hours at most.  Depths to invert can be controlled to some extent by using check dams and stepping the invert of the channel.

In the first instance what I’d look to do is lose the road gullies, which are not the most effective of devices (even though we have a rather large amount of them scattered around!) and drain the highway into the open structure.  Doing this should allow the invert level of the swale/ditch to be no more than 600mm below road surface level and may also allow the flow to run over a filter strip along the road edge thus enhancing treatment.  The filter strip has the added advantage of keeping the slope down into the swale at least a metre away from the road edge.  The inlets and sediment/trash control ancillaries are crucial components, but if you are going to go for a SuDS approach, why not maximise all the benefits of doing so?  And there will be no more gullies to empty along that stretch of highway.

Question in Full:

Would it be possible to use redundant coal and lead mines for the storage of water?

There is no one size fits all answer and while there are opportunities to do it, there are many reasons why it can't be done in many instances.

Answer from: Alan Corner, Associate Director, Hydro Consultancy

Would it be possible to use the redundant coal and lead mines for the storage of water? Here in Derbyshire there are miles of empty tunnels, and in some cases, clean water is being pumped 24/7 from below ground.
Why build more water storage facilities above ground?

Answer from: Alan Corner, Associate Director of Hydro Consultancy

Mine water can be quite polluted and the pumping is done to keep water levels stable to avoid problems with water coming out in non-preferred locations. Otherwise the best solution is to leave well alone.

There is no one size fits all answer and while there are opportunities to do it, there are many reasons why it can’t be done in many instances.  Generally in respect of this question I would say that the problems would definitely outweigh the benefits – unless you have a completely un-fractured, water tight tunnel that has had all contaminants removed from it.

Question in Full:

Are constructed rills - surface based linear drainage systems, considered to be SuDS solution? Does water quality improve on transfer?

They are often used in Sweden, Spain and Arabic countries - so why not here? I am thinking of sites where the land is contaminated and infiltration is not possible and excavation for buried drainage excessively expensive.

Answer from: Sue Illman, Managing Director, Illman Young

Rills are traditionally narrow shallow structures, and are primarily used for conveyance purposes within a SuDS scheme.  In this role their capacity to provide attenuation storage on their own is limited, as is their ability to provide improvements to water quality.  However, there is the opportunity to think rather more broadly about how constructed channels could actually achieve both storage and water quality improvements  where infiltration is not permitted due to ground contamination.

There would seem to be two options: firstly to endeavour to include bioremediation of the water quality by the use of proprietory cleansing products within the channel system (although this would not improve capacity); but secondly, it would probably be more effective to consider how channel systems could be used more creatively.  Deep and broad channels could be incorporated within a rill system to provide a sumps containing soil and planting areas, although these would need to have sufficient depth to ensure that they did not dry out during hot summer months.  Allowing the channels to pass through such planted structures would allow bioremediation to take place, as well as providing additional storage capacity.  The use of channels would enforce a formal shape to a layout, but the same result could be more naturalistic and have greater capacity through the use of pond liners buried well under the soil, creating detention ponds which could form attractive semi-wetlands which would also be more biodiverse and provide effective treatment of the water.

SuDS components generally need to be used in combination to fulfil a site’s engineering requirements, so whilst rills can indeed form very attractive features even in our less sunny climate, it is probably unlikely that they could be sufficient or indeed, cost effective, on their own.

Answer from:Brian D’Arcy, Independent environmental consultant

SuDS are systems that seek to mimic natural drainage characteristics in terms of hydrology, and the concept aspires to achievement of multiple benfits, in terms specifically of water quality, quantity and amenity.  The  provision of SUDS technology for a development or a site therefore can be by a single feature that provides all of the above characteristics, or a combination of units that together deliver the same functions; holistic management of storm water and associated environmental issues.  Combinations of techniques include flow control devices, stormwater storage units, and if pollutant removal is not provided elsewhere in the system, filtration features such as biofilters, grass filter strips or swales, etc.  There has been a presumption in favour of passive systems in order to reduce operational costs and maintenance, as part of the aspiration to sustainable technology at affordable cost.Constructed rills – surface based linear drainage systems, I assume means concrete drainage channels, as often seen on edges of motorways and trunk roads. These structures deliver any pollutants entrained in runoff directly to the receiving water, and therefore are not in themselves of any value for pollution control, and unless a flow device is fitted at the end of the linear drainge system (with storage upstream as a pre-requisite) are not going to deliver flow control either.  Typically they enhance rates of delivery of runoff from impervious areas – the opposite to a SuDS feature.  And amenity interest similarly is not part of their function.  But of course they can have a place in densely developed lengths of road where space is too limited for grass swales for example, just as sections of conventional pipe are often convenient and appropriate too, but as for the latter, some SuDS functionality would need to be added as well in other features, which is how costs are maximised for clients.  Alternatives are better than add-ons.

For the contaminated land situation noted in the background to the question, SuDS would normally need to be as shallow as possible (hence the question), but grass turf (for example) on top of an enlarged and more gently sloping channel (consistent with swale design), and with a flow control outlet.  Settlement may be an issue too with concrete sections of a channel, allowing infiltration to flush pollutants from the contaminated land. 

Recommended SuDS could be a shallow grass swale on an impervious membrane, perhaps draining to a second level feature away from the high risk contaminated ground, depending on site details and type of development.

Question in Full:

A Highways Agency roadside gulley discharges into a culvert crossing underneath a road. The culvert is blocked downstream of the gulley but underneath the road (possibly due to poor maintenance of the gulley) and water is flooding back into the field upstream as a result of the blockage.

Question. Does the Highways Agency have a statutory responsibility to ensure the cuvert is running freely under the road?

Answer from: Alan Corner, Associate Director, Hydro Consultancy

I assume from your comments in respect of the culvert that it is an open pipe extending from one side of the road to the other with no manholes constructed at the ends of the pipe.

There answer as to who is responsible depends upon ownership of the culvert. As there is a highway gully connected into the culvert there is a strong possibility that it is owned by and maintained by the Highway Authority. The Highway Authority could be the County Council, Borough Council or if it is a major A road or motorway then it could be a management agent on behalf of the Highways Agency.

A highway can also have other types of open culvert laid below and fully across them.
1) There are protected species animal/mammal underpasses which are often aligned close to hedgerows, but not always. It would be unusual to have a gully connected into one of these underpasses. These might be maintained by the Highway Authority.

2) There are watercourse/ditch flow continuation culverts. This would be laid at the same level and on the alignment of a ditch/watercourse that crosses the highway. Gullies or drainage pipes can be connected into these culverts and it would normally be owned and maintained by the Highway Authority.
The watercourse upstream and downstream of the road could be privately owned by the adjacent landowners and maintenance of the watercourse would then reside with them. Alternatively an Internal Drainage Board could be responsible for the watercourse. These watercourse owners might be responsible for the maintenance of the culvert below the road.

3) There are culverts to prevent surface water runoff from building up at the uphill side of a road or a road embankment. These would be located at a low point in the ground on the high side of the road. Normally the carriageway drainage would not be designed to be connected into this type of culvert as it is part of the land drainage network and is not part of the carriageway drainage. It would normally be maintained by someone other than the Highways Authority. In unusual circumstances the Highway Authority might have taken on responsibility for this type of culvert. It is always possible that an incorrect gully connection has been built, or some later remedial work, has resulted in the gully outlet being connected into the closest culvert.

4) There is a variation on the culvert detailed under 3 above. This would be where highway gullies or pipes discharge into a pipe, culvert or ditch on the uphill side of the road and these discharge into the culvert. This type of culvert would then form part of the carriageway drainage system and would be owned and maintained by the Highway Authority.

The above assumes that the drainage network and highway has been constructed in accordance with best practice. It is always possible that an incorrect gully connection has been built, or some later remedial work, has resulted in the gully outlet being connected into the closest culvert.

Answer from: Alex Stephenson, Director, UK Stormwater Division, Hydro International

Water Sensitive Urban Design (WSUD) is a concept which originated in Australia.  It’s an approach to urban water management that is sensitive to natural hydrological and ecological cycles, and begins with good urban planning practice.

WSUD offers a strategic, holistic approach to good urban design in the context of urban water management, liveability, ecosystems and landscapes.   It considers the management of potable water and wastewater treatment alongside stormwater management.

From the perspective of SUDS, it’s an important reminder that planning for stormwater treatment should also be about water conservation and about reducing wastewater conveyance and treatment.

In the UK to date, very few developments have truly considered water efficiency and SUDS as an integrated approach from the masterplanning stage.   Viewed from the WSUD perspective, this is a glaring missed opportunity.  Rainwater harvesting for example can simultaneously provide stormwater attenuation, whilst contributing to water efficiency and availability.

With its holistic approach, WSUD offers an attractive model for integrated urban design and development in future.

For more information download the SUDs in the Urban Landscape e-guide

Answer from: Prof. Robert Andoh, Chief Technology Officer, Hydro International

Bioretention and biofiltration was first pioneered over a decade ago in the US as part of the Low Impact Development (LID) movement.  Conventional Bioretention systems were larger, landscaped drainage systems, developed as means not only of dealing with the quantity of stormwater from developments, but also filtering runoff through an aerobic plant, soil and microbe complex to capture, remove and cycle pollutants.

Frustrated with the limitations of conventional systems, one of the pioneers of LID collaborated with a leading American concrete company to develop an engineered bioretention system, which works in a more compact space, with predictable soils, was more easy to link to drainage inlet and outlet systems, easier to design against flow control and treatment parameters and performed consistently whatever the site characteristics.

The resulting system, named Filterra®, looks like a normal tree box, with a plant or tree protruding through a decorative grating in a typical concrete slab at pavement level.  Stormwater runoff is channelled through a kerbside inlet into a concrete container underneath, where it is filtered through a 75 mm mulch layer and 500 – 1000 mm of a unique soil filter medium to an under drain system.

Unlike America, removing of oils, pollutants and sediments from surface water has generally been something of a ‘nice to have’ in the UK, but stricter legislative requirements for stormwater treatment are on their way through the European Water Framework Directive, the Water Environment and Water Services Act in Scotland and Flood and Water Management Act in England and Wales.

Answer from: Beryl Kemplen, Engineering Manager, Aylesbury Vale District Council.

Unless land has been bought for the construction of the road, highway authorities have dedicated highway rights to a depth necessary for maintenance of the highway, but they do not own the land.  The subsoil within the highway is owned by the adjacent landowners.  As a ditch cannot be used ‘to pass and re-pass’, it is not highway and therefore belongs to the adjacent owner, who retains riparian maintenance responsibilities.  Highway authorities do, however, have a right to discharge into these ditches.

Answer from: Beryl Kemplen, Engineering Manager, Aylesbury Vale District Council.

Answer from Beryl Kemplen

All watercourses, including ditches, are maintained by the ‘riparian’ owner,  that is whoever owns the land up to the top of the bank.  This is why it is important to keep them in public open space or have an agreement for them to be adopted by the local authority.  Otherwise, drainage of a whole development could depend on an individual private landowner carrying out routine maintenance.