Liquid purification or separation – Processes – Making an insoluble substance or accreting suspended...
Patent
1996-05-17
1997-10-07
McCarthy, Neil
Liquid purification or separation
Processes
Making an insoluble substance or accreting suspended...
210770, C02F 1100, C02F 152
Patent
active
056744021
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a method for the treatment of sludge obtained from such a water treatment process wherein an aluminium or iron chemical is used as a coagulant, so as to reduce the amount of sludge to be disposed or dumped and to recover aluminium or iron in a useful form. The invention particularly deals with sludge obtained from a drinking water purification process.
Dumping of waste water sludge and drinking water sludge is a major problem in water purification plants. 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.
Aluminium and iron are the most commonly used coagulant chemicals in water purification. When e.g. aluminium sulphate or ferric sulphate is dissolved in water positively charged metal-ions are generated which attract impurities in the water such as colloids, humus and suspended particles which are all negatively charged. At the same time metal hydroxide flocs are formed by hydrolysis. The impurities are captured by the flocs and a sludge consisting of metal hydroxide and impurities is formed. Practically all coagulant added in water purification remains in the sludge. The coagulant content of a sludge is typically 100-200 kg metal/tn dry solids.
The patent publication U.S. Pat. No. 3,959,133 discloses such a method for recovering aluminium from a sludge containing aluminium hydroxide. The sludge is first acidulated with sulphuric acid. When sulphuric acid is added to a sludge containing Al(OH).sub.3 aluminium sulphate, in a soluble form, will be formed. Then the sludge is conditioned with an inert additive to facilitate the subsequent filtering. The filtrate is then separated from the remaining sludge. The alum in the filtrate is returned to the water treatment system. The obvious drawback with this method is that all acid soluble impurities of the solution are also returned to the coagulation stage which leads to a gradual enrichment of metallic impurities in the recycle. Also part of the insoluble organic impurities which cannot be removed by filtering are also returned to the coagulation stage.
The patent publication EP 0 087 268 A3 discloses a process for treating sludge produced by a water treatment process. The process comprises adding acid to the sludge to produce a acidulated sludge. The sludge is then filtered to yield a recovered coagulant filtrate. This is then recycled so as to effect flocculation in untreated raw water. The coagulants mentioned in this publication are aluminium sulphate and ferric sulphate. It is typical of this process that acid is added only in an amount which is sufficient to convert only part (40%) of the coagulant to an acid soluble form. In this way the amount of impurities in the recycle can be reduced. However, this method does not solve the problem of impurities. There still remains soluble impurities in the recycle. The only way to further reduce the impurity level is to use less acid. This in turn leads to a situation where more coagulant will be discarded and the economy of the method is consequently reduced.
The dry solids content of a drinking water sludge is typically only about 0.2%. Therefore the water content of the sludge is often reduced before the dissolution stage. It is possible to raise the dry solids content to about 1.0% by sedimentation. By adding a polyelectrolyte and performing a mechanical dewatering, the dry solids content of the drinking water sludge can be further increased to a level of 10-15%. In the so-called "dry method" the mechanically dewatered sludge is dried and finally incinerated at temperatures between 400.degree. C.-600.degree. C. so that all organic material i.e. humus can be removed. In the case of aluminium chemicals, aluminium can by adding sulphuric acid to the incinerated residue be
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Masides et al., "A Feasibility Study of Alum Recovery in Wastewater Treatment Plants", Wat. Res., vol. 22, No. 4, pp. 399-405, 1988 (abstract and article).
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Jokinen Simo
Nilsson Rolf
Chong Suet M.
Kemira Kemi AB
McCarthy Neil
Nath Gary M.
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