Method of recovering iron and/or phosphorus from sludge of waste water treatment plants
Method of recovering iron and/or phosphorus from sludge of waste water treatment plants, said sludge being obtained after precipitation by iron salts, wherein said method comprises separating sludge from waste water and submitting said sludge to a lactic fermentation to release a liquid phase where iron and phosphorus are dissolved. Lactic fermentation is performed with addition of a co-substrate rich in carbon, preferably rich in carbohydrate, in one single step of biological acidification or a sequencing biological acidification in two steps by first releasing phosphorus from the PAO contained in the sludge. Iron can then be recovered by means of a cationic exchange resin. Phosphorus can be recovered as struvite or calcium phosphate from the remaining solution substantially free from iron ions, after a precipitation step in presence of a magnesium or calcium source and a pH above 7.
1. Method of recovering iron, phosphorus, or both from sludge of waste water treatment plants, said sludge previously obtained by precipitation by iron salts, said method comprising the steps of:
separating sludge from waste water; and
submitting said sludge to a lactic fermentation to release a liquid phase, where iron and phosphorus are dissolved.
2. Method according to claim 1 , wherein the step of dissolving of iron and phosphorus into said liquid phase comprises submitting said sludge to lactic fermentation with substrates that contain biodegradable carbon.
3. Method according to claim 2 , wherein said substrates include carbohydrates, leading, during lactic fermentation, to organic acids production and acidification of the medium to a pH value equal or below 5.
4. Method according to claim 3 , wherein said substrates containing carbohydrates are chosen among saccharose, glucose, agricultural industrial or urban organic wastes, crops and crop residues or parts or extracts thereof.
5. Method according to claim 2 , wherein said lactic fermentation step is an anaerobic or anoxic fermentation step, carried on at a temperature comprised between 20° C. and 60° C.
6. Method according to claim 2 , wherein said substrates are introduced in the lactic fermentation step at a concentration above 0.2 g COD/g VSS.
7. Method according to claim 2 , wherein the lactic fermentation step is followed by a liquid/solid separation step, the liquid fraction recovered after said separation step being put into contact with a material able to fix or to separate iron ions from phosphate ions.
8. Method according to claim 7 , wherein the cationic exchange resin or the adsorbing material is regenerated by an eluent solution, eluting the iron ions in a form that can be reused in the waste water treatment plant.
9. Method according to claim 8 , wherein the cationic exchange resin or the adsorbing material is regenerated by an hydrochloric acid solution, eluting the iron ions in the form of a ferric chloride solution.
10. Method according to claim 9 , wherein said method includes recycling iron in the form of ferric salts in waste water treatment plants.
11. Method according to claim 9 being used in waste water treatment plants which include a flocculation step, a dephosphatation step, or both, by ferric chloride.
12. Method according to claim 7 , for recovering phosphorus, wherein the lactic fermentation process is performed in the following sequential steps:
i) a pre-fermentation step where sludge is pre-fermented with said substrates at a low COD charge, leading to organic acids production and a pH about 6, to release phosphorus from the sludge,
ii) a lactic fermentation segment where said pre-fermented sludge is further fermented with said substrates at a higher COD charge, leading to more organic acids production and to a pH about 4, obtaining phosphorus dissolution in the liquid phase, and said fermentation process is followed by:
iii) said liquid/solid separation step for obtaining a liquid fraction, named first liquid solution
iv) an iron fixation step where said first liquid solution is contacted with a cationic exchange resin or with an adsorbent material, obtaining a second liquid solution substantially free from iron ions,
v) and a precipitation step by providing a magnesium source or a calcium source and increasing pH above 7 to precipitate phosphorus from said second liquid solution.
13. Method according to claim 7 , for recovering phosphorus, wherein the lactic fermentation process is performed:
a) under a single lactic fermentation phase where said sludge is fermented with substrates at a COD charge equal or above 0.3 g COD/g VSS, leading to organic acids production and to a pH decrease to a pH value equal or below 5, obtaining iron and phosphorus dissolved in the liquid phase, and said lactic fermentation process is followed by:
b) said liquid/solid separation step for obtaining a liquid fraction, named first liquid solution,
c) an iron fixation step where said first solution is contacted with a cationic exchange resin or with an adsorbent material, obtaining a second liquid solution, substantially free from iron ions,
d) and a precipitation step by providing a magnesium source or a calcium source and increasing pH above 7 to precipitate phosphorus from said second liquid solution.