Liquid purification or separation – Processes – Treatment by living organism
Patent
1997-07-14
1999-11-02
Upton, Christopher
Liquid purification or separation
Processes
Treatment by living organism
210631, 210717, 210724, 210912, C02F 152, C02F 328
Patent
active
059763728
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a method of treating a biomass comprising liquid manure from animal husbandry containing heavy metals.
Liquid manure is a waste product from animal husbandry, which is rich in urea, organic matter, carbohydrates, fats, fibers, e.g. cellulose fibers, proteins, nutrient salts, and which also contains a certain amount of heavy metals. Liquid manure is widely used as a fertilizer for agricultural areas.
WO 94/03403 discloses a method of removing heavy metals from sludges, especially sewage sludges, 1) wherein the sludge is treated with sulphuric acid to solubilize the heavy metals, 2) wherein the resulting mixture is separated in a solid fraction and an aqueous fraction, 3) wherein the aqueous fraction is treated with hydrogen sulphide gas to precipitate heavy metal sulphides, 4) wherein the heavy metal sulphides are separated off and removed, and 5) wherein the aqueous fractions of step 2) and 4) are biologically reduced to convert sulphate into hydrogen sulphide.
EP-A1-80,981 discloses a method of treating sewage-water or raw-water so as to produce an aqueous concentrated solution of phosphorus and so as to recover heavy metal compounds, 1) wherein the water to be treated is subjected to a microbial aerobic purifying step in which the biological material is further separated by precipitation, 2) wherein the separated aqueous phase is subjected to a chemical treatment step in which iron is added to precipitate phosphorus, and in which a fraction containing solid matter including a major part of the iron and the heavy metals is separated, 3) wherein the separated solid matter fraction is treated with hydrogen sulphide under anaerobic conditions to form metal sulphides thus dissolving metal-bound phosphorus, the hydrogen sulphide being generated from biological decomposition of the biological material separated off in step 1), 4) wherein the metal sulphide fraction is separated from the aqueous phosphorus-containing fraction, and 5) wherein metal sulphide fraction is treated with e.g. a strong acid to form metal salts and hydrogen sulphide, the metal salts subsequently being recovered.
As a result of the use of liquid manure for fertilizing agricultural areas as mentioned above, the heavy metals of the liquid manure are introduced into the environment and thus into the food chain, which is highly undesirable due to the high toxicity of the heavy metals.
The problem of disposal also relates to a number of other heavy metal containing waste materials, such as organic waste materials from households and industry and sludge from waste water purification plants as well as ashes from the incineration thereof.
Waste materials having a relatively high content of heavy metals are often disposed of by deposition at a deposition site. Such a disposal is expensive and unsatisfactory from an environmental point of view, since it does not solve the problem, it just postphones it.
Waste materials having a relatively low content of heavy metals, such as sludge from waste water purification plants, are often disposed of by distribution on agricultural areas. This procedure means that heavy metals are introduced into the environment, which is undesirable for the same reasons as explained above in connection with the use of liquid manure for manuring purposes.
In recent years there has been an increasing focus on the issue of whether distribution of sludge from waste water purification plants on agricultural areas should be allowed, and in some countries such a practise is not used.
The object of the invention is to provide a method of the type stated in the preamble of claim 1, which is simple and does not require complicated process plants.
A further object of the invention is to provide a method, which utilizes the hydrogen sulphide gas formed in the microbial degradation of liquid manure, which hydrogen sulphide gas is undesirable due to its toxicity, and which may only be emitted to the atmosphere in very low amounts.
Also, hydrogen sulphide is undesirable due to its extremely high corrosive
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