WATER

Summary

This page expands on the topics raised in the ‘Water Consumption and Contamination‘ section of the Sustainability page. It shows how water consumption would be minimised though Rain Harvesting and how using Integrated Constructed Wetlands (ICW) the site would return clean water to the natural environment.

Integrated Constructed Wetlands

Even after treatment by domestic or municiple Sewage Treatment plants (STPs) our waste water still contains:

High Levels of nutrients (nitrates etc).
Pharaceutical by products (e.g. anti-depressants, birth control hormones and bathroom cabinet medicines such as Ibuprofen, aspirin and paracetamol)
Many different chemicals (e.g. shampoo, fire retardants, pesticides and other industrial by products).

Levels of all these need to be reduced before the water is output back into the environment. Historically this has not been done and has, for example, resulted in algae blooms polluting rivers and coastal seas killing of the aquatic life. More worrying still many of the chemicals found in our waste water are what are called ‘endocrine disrupting chemicals’ (EDCs). These are now believed to be the major contributing factor in the current human infertility crisis where, for example, male sperm counts are now over 50% lower than they were in the 1970s. This has major implications for the human race but as yet is not getting sufficient publicity and action.

There are many plants that will help clear up waste (this is a process called phytoremediation) Sunflowers, Barley and Willow are but a few. Some will actually process and clean the waste and others will just accumulate it to aid removal (for example, Willow will remove heavy metal waste from soil and its wood can then be removed taking the metals away with it). The aquatic plants in the ponds of an Integrated Constructed Wetland, however, naturally process the waste water reducing and removing nutrients and chemicals. This is a gradual process where the water is piped between a number of ponds allowing nature to take its course.

The following site plan shows how the output of the dwelling’s Sewage Treatment Plants (STPs) on the conceptual site is piped into the Integrated Constructed Wetland (ICW) at the bottom right.

A worked example of this mechanism exists on the River Ingol in Norfolk . There an integrated Constructed Wetland (ICW) was used to reduce the nutrient output from a municipal sewage treatment plant that processed the waste from 6000 people.

The Ingol ICW reduced nutrients flowing into the River Ingol by 72% for nitrate, 69% for phosphate and by 53% for Dissolved Organic Carbon.

Natural England Nitrate Neutrality Calculations

The following calculations use the figures and processes defined in the Natural England Advice for LPAs to calculate the Nitrogen load for the conceptual development. References to this document are provided in brackets below as shown here ‘(section n.nn)’.

Assumptions

The size of the site = 15.87 acres = 6.42 Hectares
The following areas represent the approximate division of the field by usage for nitrate calculation purposes based on the site plan (taken from the CAD package):
- Allotments = 0.449 hectares.
- Wetland = 0.277 hectares.
- Suitable Alternative Natural Greenspace (SANG) = 3.437 hectares (the amenity area plus all the other space not used).
- Development area = 2.257 hectares (this includes the Access Ways and parking, the courtyard gardens and the earth sheltered roofs – this is a ‘worst case’ calculation as all these are permeable surfaces).
There would be an estimated population of 144 people living in the community (based on 60 dwellings at 2.4 average occupancy – the Office for National Statistics average figure given). Commercial outlet population is excluded as no overnight accommodation is provided (as per section 4.5)
On-site waste water treatment works (WwTW) or Sewage Treatment Plants (STP) have similar levels of nutrient output to municipal works.

Nitrogen Load Calculations

Step (1) – the total Nitrogen (N) output per year after Sewage Treatment (via STP) is calculated as follows:

Figures used:

110 litres per person per day (section 4.19).
STP total nitrogen limit = 27 mg/litre (section 4.24).
Water consumption a day = 144 x 110 – 15,840 litres.

The total nitrogen discharged after treatment = 27 x 0.9 x 15,840 = 384,912 mg/N/day. This works out per year in kilogrammes (kg) = 384,912×365/1,000,000 = 140.49 kg/N/year.

Step (2) – Calculate current load to offset existing nitrogen (N):

Figures used:

Field size = 6.42 hectare (ha).
Field is used for ‘general cropping’ which = 25.4 kg/ha.

Therefore the current land use nitrogen load = 25.4 x 6.42 = 163.07 kg.

Step (3) – Calculate load for proposed new land use with development:

The following figures apply to the different land use types:

SANG = 5 kg/ha/year (section 4.43)
Allotments = 26.9 kg/ha/year (section 4.45)
Urban development = 14.3 kg/ha/year (section 4.41)

The nitrogen load If we treat Earth sheltered homes as ‘urban’ (the worst case scenario – it reality they will perform better) is:

Nitrogen load from development = 2.257 x 14.3 = 32.28 kg/N/year
From SANG = 3.437 x 5 = 17.19 kg/N/year.
From allotments = 0.449 x 26.9 = 12.08 kg/N/year.

TOTAL Nitrogen = 61.55 kg/N/year.

Step (4) – Calculate net change in nitrogen load for the land under development:

The TN from development (step 1) + Load for new use (step 3) – load for old use (step 2) = 140.49 + 61.55 – 163.07 = 38.97 kg/N/year.

Therefore the Nitrogen Load Budget required including a 20% buffer = 46.76 kg/N/year.

Nitrate Mitigation

Mitigation through Integrated Constructed Wetland (Appendix 4 page 28) states that 1 hectare should be sufficient for 50 hectares of development. That is a 2% land allocation. This blueprint has a 4.3% allocation.

Another method of calculation is based on a median removal rate for ICWs of 93g/m2/year (Appendix 4 page 27). The proposed ICW is 0.277 hectare in size = 2770 m2. This ICW would therefore be capable of removing 257.6 kg/N/year from the site.

Regardless of which mitigation calculation method is used this ICW provision provides 215% to 550% of the required capacity using the worst case calculation scenarios.

Rain Harvesting

Rainwater harvesting needs to be used to maximum effect to minimise the cost of treating the water for the purpose it is required. For example, we often use expensive, drinking quality, tap to flush our toilets, water our gardens, wash our cars etc. In this scenario the extra cost of making the water drinkable is wasteful.

Rainwater from clean surfaces like, for example, PV panels and greenhouse roofs can be made clean enough to cook and drink with a little treatment. Rainwater collected from ditches around the site can be used with some treatment for washing, bathing and toilet flushing and with no treatment it can be used for irrigation. More details can be found on the Hockerton Housing Project video page and on the Freewater UK site.

The South of England gets 800-900mm of rainwater a year. If this was collected from building roofs it would provide at least 30%-35% of the water required and the additional home infrastructure would pay for itself in the 10-20 year timeframe in reduced water bills. If we assume that an average house has a 50m² roof area with a 0.85m rainfall = 42.5 m³ or 42500 litres of rainwater a year. Southern Water estimates that a 3 person household uses 370 litres a day so this rainfall would provide 115 days of water or 31% for the annual requirements. At a cost of about £1.40/m³ it would save £60 a year on fresh water bills plus a further saving in waste water charges if this water wasn’t all returned to the sewerage system. Southern Water charges wastewater at 92.5% of the consumed drinking water at £2.45 /m³ which is a saving of another £96 a year (total £156). Rainwater Harvesting systems retail cost is in the order to £2000-£4000 depending on the size of house.