In both rich and poor countries, farmers use animal manure to maintain the fertility of the soil. This is a classic example of a closed cycle. But what do we do with the valuable nutrients in human faeces?
The Environmental Technology department of Wageningen UR develops methods for recovering raw materials such as phosphorus, nitrogen and potassium from water treatment. These are valuable nutrients which often end up in domestic wastewater via our food and toilets. Grietje Zeeman, professor of New Sanitation, calculated that recovering phosphates from black water and kitchen waste could satisfy a quarter of the present worldwide artificial phosphorus fertiliser use.
The supplies of mined phosphorus, an important component in artificial fertilise, will be exhausted by the end of this century at the current pace of extraction. Moreover, we are highly dependent on extraction in China, Morocco and the USA. This concept is being translated into practice In the Noorderhoek neighbourhood of Sneek (NL). Faeces and urine (black water) are collected by vacuum toilets and transported, together with kitchen waste, to an anaerobic treatment plant for the production of biogas, which is then used to heat houses in Sneek.
The purified water that comes out of the anaerobic treatment plant still contains a great deal of phosphate and nitrogen in the form of ammonia. Adding magnesium transforms the phosphate and a part of the ammonia into struvite, which can be used to fertilise fields. This recreates a cycle that was broken in the last century for reasons of hygiene.
Gains in water & energy
Current sanitation and sewerage systems use a lot of water to bring a relatively small amount of waste to the treatment plant. In the Netherlands, ca. 130 litres (20 flushes) is used per person per day, while the corresponding figure in the US and Canada is three to four times higher. The vacuum toilets in Sneek only use one litre per flush, instead of the usual six to eight litres, which allows a considerable decrease in the use of water and in the volume of wastewater. Especially in areas where water is scarce, such savings are essential. Grey water (bath, shower, laundry and kitchen water) can be used for irrigation after treatment. The second problem of traditional water treatment is energy use, particularly the energy used to blow air into reactors which supply purifying bacteria with oxygen.
The installation in Sneek is a net producer of energy, however, as it uses anaerobic (oxygen-free) treatment in an up flow anaerobic sludge bed (ASB) reactor, a technique developed by Wageningen UR. UASB reactors are used around the world to treat concentrated industrial waste water. The challenge is to separate black and grey water at the source with water-saving toilets and thus treat household waste water on a large scale while saving water and energy.
Zeeman would also like to harvest nitrogen from black water. Nitrogen is an important nutrient for cultivated and other crops and therefore one of the main components in fertiliser. Despite the significant water savings achieved with vacuum toilets, the nitrogen concentration in the black water is still too low for efficient recovery. As the nitrogen from black water is converted to N2 using a low-energy Anammox technique in Sneek, the N2 disappears into the air, therefore not directly contributing to any fertiliser.
Another of Zeeman’s goals is to develop a (hyper) thermophilic anaerobic treatment technique which would also provide disinfected fertiliser in addition to biogas. A condition for this is further flush water reduction, as this would allow the temperature in the reactor to be increased from the current 25-30 degrees to a safe 60-70 degrees (rendering most pathogens harmless) with the same amount of energy. The fertiliser and water can then be reused in agriculture. Of course, legislation must be adapted to allow this. Conventional sewage sludge in the Netherlands is taken to incinerators, where the nutrients are lost, not allowing the cycle to be closed.
To apply it in agriculture, the sludge must comply with established limits on heavy metals. The sludge from anaerobic black water treatment contains far fewer heavy metals than conventional sewage sludge as it contains no rain or grey water. According to the standards enforced in the Netherlands, the latter contains too much copper and zinc in particular. Ongoing studies will show whether these heavy metals only come from food. If this is the case, Zeeman feels that regulations should be adapted to allow the cycle of manure to manure via crops and food to be closed in a neat way. "
Wageningen UR and Netherlands Institute of Ecology (NIOO-KNAW) work together to study the possibilities of using algae after the anaerobic treatment of black water. Algae can process all phosphorus from faeces and capture some three quarters of the nitrogen, with the help of sunlight and absorption of the greenhouse gas CO2. Once harvested, the algae can serve as a source of oil, which can in turn be used as biofuel, as a raw material for biobased chemicals, or directly as a fertiliser in agricultural use.
Recovery of nitrogen and phosphate from urine
A pilot for the recovery of nitrogen from urine will be started in 2013 using microbial fuel cell technology (MFC), developed incollaboration with Wetsus, Centre of exellence for sustainable water technology. MFC allows ammonia to be recovered from human urine, in combination with struvite precipitation for phosphate recovery. Unlike existing techniques, this process generates energy rather than consuming it. The technology is also interesting for the recovery of ammonia from anaerobically treated black water.
Removing pharmaceutical residues and pathogens from the water
Treated wastewater still contains components of pharmaceuticals andorganic compounds from shampoos, scrubs, conditioners and, cleaning agents. After being discharged, these can be harmful to fish, frogs and waterfleas and can also reach human drinking water. Another problem are pathogenic viruses and bacteria (such as the infamous MRSA or Ehec). New chemical and biological treatment techniques are therefore being developed to remove these substances and pathogens at a later stage of treatment. There are also studies to examine whether these can be made harmless in ground and surface water through a natural purification process. The aim is to reuse water while keeping the environment, drinking water and agricultural products clean.