Floating new ideas for nutrient-neutral home-building

02 Aug Floating new ideas for nutrient-neutral home-building

Pollution laws protecting valuable but vulnerable conservation sites are blocking vital housing projects. Can we ease the problem? In many cases yes, with detailed assessments, special design measures, but also innovations like highly-efficient floating wetlands.

English house-building is in a quandary. We need thousands of extra homes. However, new residential developments can also release damaging levels of nutrients into local water networks.

Why does this matter? When large amounts of nitrogen and phosphorous enter key wetlands, as nutrients they can trigger algae blooms and plant growth spurts that threaten biodiversity and put highly-protected species at risk.

Natural England – the Government’s natural environment advisor sponsored by Defra – is tackling the problem through the concept of nutrient-neutrality where total run-off and emissions from new residential sites – during construction and long-term occupancy – must be no higher than before work started.

Homes of the brave

This can be good for local conservation but have negative consequences. One, unfortunately, is limiting new home-building opportunities in England – effectively a moratorium.

Fortunately, Enzygo (https://enzygo.com/) has been helping to develop a sustainable answer – with an innovative ‘solution in its tail’ – that could open up pollution-locked sites across the country.

To help other developers, I would like to outline a ‘blind-case study’ in rural southeast England that, subject to planning permission, will start on site in 2022.

Because the project is potentially complicated, I have described it in a series of steps, starting with several opening points.

Size, sources, pathways, receptors and special measures

– Broad scope and fine detail – the first is that I want to emphasise the size and geographical footprint of this type of proposal, but also the detailed issues involved in delivering it successfully. Our comprehensive experience is important here (https://enzygo.com/services/hydrology/).

– Advanced modelling – my second point is that our design work for large proposals uses a comprehensive pollution “source – pathway – receptor” model that brings these many issues together.

The source: –

The landowner in this specific case has provided enough arable farmland to develop a 100-home high-quality housing project, but importantly also open spaces and new wetlands to store and naturally process all sources of surface water run-off.

This ‘source’ will eventually be home to some 240 residents, with recreational and sports areas. By then, almost 12 ha of agricultural fields will have been transformed into urban land, open space and floating nutrient treatment wetland, the latter developed by the specialist Australian SPEL Corporation (https://spel.com.au/floating-wetlands-stormwater/).

The pathway: –

The link between source and receptor is ordinary local watercourses that link into a main river flowing through a wide catchment. Our work shows that the pathway will not be affected by the development.

The receptor: –

However, the potentially impacted area to be safeguarded is a complex series of habitats that include wetlands with a globally-recognised Ramsar conservation site which provides key breeding and wintering habitats for waterfowl.

The receptor also includes SAC (Special Areas of Conservation) noted for an Annex II species of Desmoulin’s whorl snail, SPA (Special Protection Areas) important for wintering birds, and a SSSI (Site of Special Scientific Interest) with a wide range of habitats rich in fauna and flora.

The site managed by Natural England combines shallow ponds, an artificial lake created in the 1970s, plus other complex water links. The river and its tributaries are the main source of incoming water.

Strict receptor requirements – crucially, there can be no fragmentation of existing habitats or species at the receptor. Species density must not change. No adverse impacts – water quality or quantity, climate change impacts, or water levels changes – are allowed. Conservation sites must be free of noise pollution; light pollution; recreational pressure and disturbance from new residents.

Being practical to be certain

My fourth point is that to guarantee the receptor suffers no pollution impacts during construction or residential occupancy, strict Embedded Design Measures will be implemented at the ‘source’.

These include a Surface Water Management Strategy involving SuDS (sustainable drainage systems) with, although it may sound improbable – a floating wetland in an artificially-created lake.

But before describing the significance of these points, I must show why limiting pollution is so important.

Bad fertilisers

Nitrogen and phosphorous in several forms are artificially-introduced fertilisers that can cause severe “eutrophication” in fragile wetlands – putting fish, shellfish, invertebrates and the birds mentioned above at severe risk.

To prevent this, our environmental design work begins with a detailed site assessment of the type, size and effects of pollution – and how we can reduce impacts before taking more drastic steps.

Farms – fertilisers, slurries and manure-based run-off are a primary source of both nitrogen and phosphorous. Because this specific development is on previously arable ground, agricultural run-off will be negligible as high levels of fertiliser inputs are replaced by lesser inputs from residential and other development.

Homes – housing developments are also significant sources via bathroom and kitchen foul waste to sewer which, by law, must be processed at a local wastewater treatment works (WwTW). However, WwTW outflows are returned to local water catchments.

In some cases the WwTW outflow is released into an adjacent catchment which does not flow into designated sites, making the solution more straightforward. But where, as in this case, the WwTW outflow goes into the same catchment, this must be assessed.

Other pollution sources are garden fertiliser, the home-composting of organic waste, plus pet waste washed off by rainwater. We can take steps to stop or reduce these.

Letting nature work in our favour

With all potential ‘total nitrogen’ and ‘total phosphorous’ sources minimised, including broken sewer pipes, we can move on to active sustainable solutions like SuDS (Sustainable Drainage Systems) that replicate natural treatment processes.

SuDS in general work well where ground can be profiled into vegetated swales, reed beds and retention ponds (or lakes) to slow down, store and clean run off and rainwater while it seeps away. The beauty of SuDS is that no manmade energy is needed, but solutions are robust, attractive, biodiversity-friendly and create local amenities.

SuDS and innovative floating treatment solutions

In this case, SuDS in the form of an excavated lake will be used solely for water treatment, flood attenuation being carried out in smaller storages adjacent to the developed site. The main form of treatment, with the capacity to reduce nitrogen and phosphorous levels to well within an acceptable receptor envelope, will be pioneering floating wetland technology.

This works on the same principle as conventional reed beds and vegetation where plants and microbes clean pollution through natural bio-mediation processes. Floating wetlands maximise contact between water and root systems and biofilm networks, turning pollutants into safe biomass and gases.

Importantly, while a minimum depth of water is required to maximise treatment efficiency, allowing the treatment wetland to float means the risk of drowning out the vegetation is minimised, and unlike a conventional ground based wetland, the flow of water is unimpeded.

We carry out many SUDS projects as a company. If you would like more information about these projects and floating wetland solutions, please talk to me directly ([email protected]).

Total Phosphorous and Total Nitrogen

Phosphorous comes in several forms: – orthophosphate (reactive); polyphosphate (inorganic); and organic phosphate from biological processes and human effluent. Similarly, nitrogen can be organic or inorganic, or come from vehicle exhausts, detergents and soil erosion.

This makes calculating Total Phosphorous (TP) and Total Nitrogen (TN) sources difficult, which is why an investigation due for completion in March 2022 is being carried out by the Water Industry National Environment Programme (WINEP).

Developers are not being asked to solve this problem on this project. But they must prove beyond reasonable scientific doubt that, firstly, there is no functional pathway for TP and TN to pass from source to receptor, and, secondly, receptor site ‘qualifying features’ will not be affected adversely. If this cannot be shown, ‘specific mitigation’ must be provided.

Embedded design measures

To ensure that TP and TN from the source site are minimised, Embedded Design Measures are being built into the project over and above basic nutrient-neutral calculations. Detailing how these work in practice could be helpful.

– Construction phase

Construction work in seven main stages will cover: – enabling works – haul routes, compound areas and clearing vegetation; site preparation – topsoil stripping and land re-profiling; excavation and utility diversions – drainage; internal road construction – car parks and footways; building 100 homes; fitting out homes; and landscaping.

Embedded design measures during construction will include a Construction Environmental Management Plan (CEMP) showing how supply chain environmental issues will be managed.

Aspects addressed will include: – reducing spillage or leakage risks; handling spills; avoiding pollution releases from pipelines; managing run off and dust; managing contaminated land; maintaining water quality and site drainage; safeguarding trees and hedgerows; minimising noise.

They also cover: – complying with best practice pollution prevention guidelines; managing recycling and waste disposal stipulated by regulations; highlighting issues of ecological importance and sensitivity in staff inductions; providing secure fencing; limiting night time lighting for bats and birds.

Meeting specific standards – CEMP will also include: – meeting best practice pollution prevention guidelines; consulting the Environment Agency or Internal Drainage Board about temporary or permanent works affecting watercourse beds, banks or floodplains; obtaining permits to operate pumps; monitoring the quality and flow rates of watercourse flows through sites.

Caveats cover other aspects of storage, spillage, drainage, refuelling, hazardous liquids, outlets to sewer, portable toilets, concrete laying, haul road cleaning, water sprays, de-silting, fencing and debris netting.

– In perpetuity residential occupation

If our calculations are correct – and they are – no further measures will be needed to contain TP and TN when residents move on site.

However, on the precautionary principle, ‘source’ embedded design measures in a Landscape Plan and Ecology and Landscape Management Plan will cover areas of open space and soft landscaping.

Water management – measures to control water quality and quantity will include: – removing accumulated sediments; creating a variety of SUDS habitats; managing grasslands, retention ponds and swales; grass-cutting and removing scrub to help diminutive species; open area planting to support wildlife.

No fertilisers –fertilisers will not be used in the open spaces except for dealing with invasive species and plan diseases. Structural buffers can be used to control surface water flows and minimise soil erosion.

Controlled vegetation – bankside vegetation will be reduced annually by a third on a rotational basis to control vigorous plants. Pond blanket weed will be thinned by two-thirds.

Dog bins + improved soil quality – waste bins will be emptied regularly. Arable farming has depleted soil quality on much of the site. New grassland, trees, scrub, hedgerows and water bodies will improve soil quality, stability, and the capture and store of phosphorous, carbon and other nutrients.

No home waste composting – another important measure will be providing green garden waste bins to discourage waste composting or burning.

A Surface Water Management Strategy designed around SUDS will handle rainwater runoff from undrained land and hardstanding areas – using domestic water butts, soakaways and rain-gardens.

Finally, a Foul Water Drainage Strategy to cut personal water consumption to 105 litres/day will include: – high-efficiency low flow taps and showerheads; double or low flush toilets; tank-less hot water heaters; water-efficient dishwashers and washing machines; plus rainwater harvesting.

Hopes, aspirations and tangible benefits

Clearly, many issues are involved here with substantial benefits. I hope I have shown the magnitude of schemes on this scale and the detailed measures involved.

If you would like to discuss any of the issues raised here, please feel free to contact me directly.