Liquid purification or separation – Processes – Making an insoluble substance or accreting suspended...
Patent
1997-08-18
1999-03-30
Hruskoci, Peter A.
Liquid purification or separation
Processes
Making an insoluble substance or accreting suspended...
210713, 210722, 210724, 210726, 210912, 210913, 423132, 4231503, 423306, C02F 164
Patent
active
058884041
DESCRIPTION:
BRIEF SUMMARY
This application is a 371 of PCT/FI95/00717 filed Dec. 29, 1995, and claims priority thereof.
The invention relates to a method for treating waste water sludge comprising at least one metal, especially iron and possibly aluminium, originating from a waste water treatment coagulant, and phosphorus and heavy metals in order to recover said at least one metal and phosphorus and to discharge said heavy metals. In particular, the sludge is from a waste water purification process where waste water is chemically-precipitated i.e. by using coagulants containing iron and possibly aluminium.
Dumping of the waste water sludge from a waste water purification plant is a major problem. This is partly due to the heavy metal content of the sludge. It is difficult to find suitable places for the waste and as standards rise landfilling is becoming more and more expensive. From this perspective the idea of recycling the waste water sludge is becoming increasingly important. Recycling involves treatment of the sludge to recover coagulant chemicals, particularly iron and aluminium, used in the water purification plant, and phosphorus.
The first stage in a waste water purification process of the above kind is precipitation using Fe and possibly Al chemicals and sedimentation which yield a chemical sludge i.e. the precipitation sludge. One possible treatment procedure is first to dewater the sludge to a dry solids content of 15-25% and then to compost, incinerate or transport the dewatered sludge to a dump.
Another possible procedure is to acidify the precipitation sludge to dissolve metals. Insoluble substances are removed by filtering. The dissolved metals and phosphorus in the filtrate are precipitated and a sludge, which will be called a metal sludge, is obtained. The metal sludge contains the iron and aluminium of the used coagulant and, in addition, phosphorus and heavy metals. The procedure can also be performed at an elevated temperature to improve the filterability i.e. the dewatering properties of the sludge. The sludge to be treated can be a pre-precipitation sludge, a simultaneous precipitation sludge, post-precipitation sludge or a mixture thereof.
One additional alternative for treating the sludge is hydrolysis where the purpose is to hydrolyse organic material of the sludge into short-chained compounds to be utilized in later stages of the waste water treatment process, especially as carbon source in the denitrification stage. During hydrolysis, the metals of the raw sludge dissolve in the hydrolysate solution. In the so-called thermal acid hydrolysis the temperature is 150.degree.-160.degree. C., and pH<2, preferably 1-1.5. After the hydrolysis, the insoluble part i.e. the organic sludge is separated, the sludge containing primarily fibres and possibly insoluble silicate minerals. The pH of the obtained solution is raised above the neutral level using a base so that the dissolved metals precipitate as hydroxides and phosphates. The precipitated sludge, which will hereinafter be called metal sludge is then separated. The metal sludge contains iron and aluminium and also phosphorus and heavy metals.
The metal sludge can be dissolved in sulphuric acid or possibly in hydrochloric acid and the insoluble substances can be separated by filtering.
Acidification nor hydrolysis of sludge is not commonly used in waste water purification. One reason is poor profitability. An additional problem is the metal sludge, which has no use. The metal sludge contains heavy metals, which makes the sludge a harmful waste for the environment.
The applicant has earlier proposed (PCT/FI94/00376) a method for recovering the coagulants and phosphorus from the acidic filtrate obtained by dissolving metal sludge in sulphuric acid. The acidic filtrate typically contains at least 1% by weight of Fe. This method involves an extraction step, in which Fe and Al are separated, followed by two successive precipitation stages for separating heavy metals and phosphorus. Although this process has given promising results so far, extraction may turn ou
REFERENCES:
patent: 4671882 (1987-06-01), Douglas et al.
patent: 4765911 (1988-08-01), Rasmussen
patent: 4954168 (1990-09-01), Crnojevich et al.
Scott et al., Enviro. Sci. & Tech. 9:849-855 (1975).
Wilhelm et al., Chemical Abstracts 109:338 (134448e) (1988).
Hruskoci Peter A.
Kemira Kemi AB
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