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Beating the cistern

technical

Building designers have become used to the water-flush lavatory as the normal method of dealing with human waste, but is it necessarily the best solution? To evaluate the problems and alternatives it is necessary to reconsider what human waste is.

Human waste

Urine is not strictly a waste material. It contains proteins that are needed by our bodies but cannot be stored. Urine is sterile, typically contains 70 per cent of the nutrients voided by humans and is, in effect, a concentrated liquid fertiliser. When placed in water, dissolved nutrients are expensive to remove. When placed not in water but in a carbon-rich environment, the nutrients are oxidised into stable, organic compounds.

Faeces comprise the material that the body cannot digest. They are usually fibrous, high in carbon, low in nutrients, and can contain protozoa, parasites, enteroviruses and bacteria. When faeces are placed in water, air is excluded which leads to slow anaerobic decomposition. The process becomes very smelly due to the production of hydrogen sulphide, an acidic gas, and methane, a potent greenhouse gas. When not placed in water, they are broken down by oxygen-breathing bacteria and soil creatures which produce carbon dioxide - still a greenhouse gas but 20 times less potent than methane.

Breaking the nutrient cycle

Nitrates and phosphates can be removed with tertiary treatment, but the extra expense can not be justified for most treatment works. Natural nutrient recovery is declining due to the draining of wetlands, allowing too much nutrient into the sea, which results in algal blooms. Consumption of these algae by oxygen-requiring bacteria reduces the oxygen levels in the water, harming higher forms of aquatic life. Once in the sea, nutrients are lost to land-dwelling creatures for geological time periods.

Valuing water as a resource

Ever since the first water-based lavatory, which had a continuous water flow, the government has passed laws to reduce the amount of water used. Even so, the wc flush uses 40 per cent of the total water consumption of the average household. Other concerns are the contamination of sewage sludge and water with industrial heavy-metal wastes.

Possible alternatives

The wc therefore has three distinct problems: it uses a large quantity of valuable drinking water, it wastes nutrients, and harms aquatic eco- systems. There are several alternative systems which address these problems to varying extents.

Composting waterless wcs. There is a wide range available, designed for holiday homes, other homes and large public facilities. They are the most environmentally desirable systems. The problems associated with these are the high cost of installation, user acceptability and the perception of maintenance and cleaning as a 'dirty job'

A one-litre flush composting wc. The flush gives the user the impression that they are using a conventional lavatory

Urine-separating-style lavatories, where the sterile and nutrient rich urine is never mixed with the potentially diseased and nutrient-poor faecal material. The separated urine can be diluted and used directly as a fertiliser or passed through a carbon-rich compost bed over the winter months.

Waterless male urinal systems, using a urinal coating and a perfumed oil-filled trap with quarterly maintenance. Again, the urine can be taken for direct land use. There is also a range of systems which require a water flush as a transport mechanism but are more environmentally desirable than a conventional system

Reed beds which can treat sewage and grey waste water. These require a large area of land and specialist maintenance and cropping for nutrient recycling

An aqua solar system which can treat sewage and grey water in a complete ecological system within a greenhouse. This system also requires specialist maintenance and cropping for nutrient recycling

Septic tank or small-scale packaged treatment plants can give nutrient recovery back to the land via leach fields, but may need tanker emptying

Anaerobic digesters treat sludge, recycle nutrients and produce methane which can be used as a power source but are expensive for small systems

Greywater- or rainwater-flush wcs and low-water flush wcs. These reduce the overall water use but will not affect nutrient loss or nutrient pollution problems unless linked to a treatment system which recovers nutrients on to planted land.

The future

Water is becoming a valued resource as the costs of its treatment, storage and distribution are exposed to harsh market economics. Environmental degradation of our aquatic habitats is now better understood, giving rise to a new awareness of the value of clean water amenities for wildlife and leisure.

This changing attitude to 'value' means that the automatic provision of water-flush lavatories and large-scale sewer systems to every building must be questioned. In many urban situations there are still few effective alternatives to existing arrangements. Where appropriate, however, building users must be drawn into the design discussion to assess what is valued. Capital availability, specialist maintenance ability and user acceptance must all be considered. The time for re-evaluation has arrived if we are to protect our dwindling water supplies and reduce environmental degradation of our rivers and seas. Brian Mark is a partner in Fulcrum Consulting

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