Methods for removing pollutants from water and waste water and for reducing sludge resistance to dewatering

A method for treating water or waste water to remove pollutants therefrom includes treating fly ash comprising aluminum, iron and silica with an aqueous base, recovering a base-treated, solid fly ash, washing the base-treated solid fly ash with water, recovering a rinsed, base-treated fly ash solid, reacting the rinsed, base-treated fly ash solid with mineral acid, and recovering an aqueous, solubilized acidic fly ash leachate, then mixing the aqueous, acidic solubilized fly ash leachate with water or waste water to remove pollutants therefrom, or with sludge to reduce its resistant to dewatering.

This invention relates to methods for treating water and waste water with a 
derivative of fly ash to remove pollutants from them, and to methods for 
reducing the resistance of sludge to dewatering. 
Fly ash is a particulate solid produced in great quantities in the United 
States from the combustion of coal. Fly ash commonly contains as its 
principal components silica, iron and aluminum, often together with lesser 
amounts of other metals, sulfur and carbon. Unmodified fly ash has been 
used for removing pollutants from water and waste water, but has proved 
ineffective. 
This invention provides methods for removing pollutants from water and 
waste water comprising treatment with aqueous acidic, solubilized fly ash. 
This aqueous, acidic, solubilized fly ash also reduces the resistance of 
sludge to dewatering when mixed therewith. To make such aqueous acidic 
solubilized fly ash, our method comprises treating fly ash with an aqueous 
base such as sodium hydroxide or potassium hydroxide at a temperature and 
for a time sufficient to break the physical bonds between the silica and 
the metals bound to the silica, and recovering from this aqueous base 
treatment a solid, base-washed fly ash, and an aqueous caustic wash 
containing some suspended aluminum, iron and silicon. The base-treated 
solid fly ash can then be washed with water, and filtered, by vacuum or 
otherwise, to obtain a substantially base-free, base-treated fly ash solid 
and a second aqueous caustic wash containing some suspended aluminum, iron 
and silica. 
The base-treated, solid fly ash is then reacted with an aqueous mineral 
acid, such as aqueous hydrochloric acid, for a time and at a temperature 
sufficient to solubilize a substantial portion of the silica, iron and 
aluminum in the base-treated solid fly ash. The acid-treated fly ash is 
then filtered to recover solubilized, acidic fly ash leachate, and an 
unsolubilized, acidic fly ash aqueous slurry. The aqueous slurry can be 
washed with water and filtered for recovery of an acidic, unsolubilized 
fly ash residue and an acidic aqueous supernatant containing some 
solubilized aluminum, iron and silicon. The solubilized, aqueous acidic 
fly ash leachate is highly effective in coagulating and flocculating water 
and waste water pollutants, such as color bodies, turbidity, and 
solubilized solids. The leachate is also effective in reducing the 
chemical and biological oxygen demands of polluted waters and waste 
waters. 
The acidic leachate, the aqueous, acid-treated fly ash solid slurry 
recovered as a by-product in manufacture of the leachate, and the acidic 
solid fly ash residue separated from this slurry by washing with water and 
filtering are all effective as agents for reducing the resistance of 
sludge to dewatering. 
The quantities of the acidic, aqueous, solubilized fly ash leachate 
sufficient to coagulate and flocculate water and waste water pollutants 
varies with the quantities of aluminum, iron and silica in the leachate 
and with the nature and quantity of the pollutants in the water and waste 
water to be treated. The nature and quantity of other metals in the 
leachate also affect the quantities needed, particularly of such metals as 
calcium, which tend to increase the solubility of the leachate in water. 
Surprisingly small quantities of leachate are required to remove 
pollutants from waste water or to reduce the resistance of sludge to 
dewatering by comparison to the quantities of such conventional coagulants 
as ferric chloride and aluminum sulfate required to achieve the same 
results. 
To prepare the leachates of this invention, the preferred embodiments of 
our process first treat raw fly ash with an aqueous solution containing a 
base such as sodium or potassium hydroxide with the concentration of the 
base in water in the range of about 10% to about 30% by weight and with 
the solution pH in the range of about 11.5 to about 13.5. The treatment 
takes place at a temperature in the range of about 90.degree. C. to about 
135.degree. C. or higher for a time in the range of about 0.5 to about 2.5 
hours, or for a time and at a temperature sufficient to break the physical 
bonding between the silica and the metals in the fly ash. Examples of this 
step appear in U.S. Pat. No. 4,130,627, issued Dec. 19, 1978, entitled, 
"Process for Recovering Mineral Values from Fly Ash." 
After treatment with aqueous base is complete, the base-treated fly ash 
solids are separated from the aqueous caustic decant which contains some 
suspended aluminum, iron and silica. Solid, base-treated fly ash is then 
preferably washed with water, and separated by vacuum filtration or 
otherwise from the aqueous wash to form a base-treated, washed solid fly 
ash residue. 
The basic, solid fly ash residue is then reacted with aqueous mineral acid 
for a time and at a temperature sufficient to solubilize a substantial 
portion of its aluminum, iron and silica. Preferably, this mineral acid 
treatment takes place for a time in the range of about 0.5 to about 2.5 
hours, at a temperature in the range of about 70.degree. C. to about 
90.degree. C. and at a pH in the range of about 1 to 2.5. In preferred 
embodiment, the acid concentration in the media is in the range of about 
10% to about 20% by weight. U.S. Pat. No. 4,130,627 contains additional 
details of this treatment. 
After mineral acid treatment is complete, the undissolved fly ash solids 
are separated from the solubilized fly ash solids, preferably by vacuum 
filtration, to form an acidic, base-treated and acid-reacted solid fly ash 
residue and an acidic, aqueous, fly ash leachate comprising substantial 
amounts of solubilized aluminum, iron and silica. This acid leachate is 
highly effective for coagulating and flocculating impurities in water and 
in waste water, and in reducing the resistance of sludge to dewatering. An 
acidic, solid, base-treated and acid-reacted unsolubilized fly ash slurry 
forms as a by-produce of leachate manufacture, and can be washed and 
separated into solid and liquid, preferably by vacuum filtration. The 
resulting acidic, unsolubilized fly ash solid residue is also effective in 
reducing the resistance of sludge to dewatering.

EXAMPLES 
We obtained a fly ash that contains about 15.3% by weight aluminum, about 
20.5% silicon by weight, and about 5.1% by weight iron, together with 
small amounts of such metals as strontium, manganese, titanium, calcium, 
potassium, magnesium and sodium and small amounts of carbon and sulfur. We 
treated each of five 100-gram samples of this fly ash with 800 milliliters 
of water containing 15% sodium hydroxide by weight for 90 minutes at 
90.degree. C., and then set the container aside to allow for gravity 
separation of the liquid from fly ash. We decanted the basic wash liquid, 
and analyzed the liquid for its aluminum, iron and silica content. We 
treated the solid, basic fly ash residue with water, and separated the 
basic fly ash solid from the resulting caustic wash liquid by vacuum 
filtration. Again, we analyzed both the caustic wash and the residue for 
aluminum, iron and silica content. 
We reacted the base-treated fly ash solids with 800 milliliters of water 
containing 15% hydrochloric acid for 90 minutes at 90.degree. C. We 
separated unreacted, now-acidic fly ash solids from the solubilized, 
acidic, aqueous leachate by vacuum filtration, and analyzed the leachate 
for its aluminum, iron and silica content. We washed the unsolubilized, 
acidic fly ash residue with water, and again separated liquid from solid 
by vacuum filtration. We analyzed the dry acidic fly ash residue and the 
acidic wash liquid for silica, aluminum and iron. Before subjecting the 
leachate to vacuum filtration, however, we recovered a quantity of acidic 
fly ash slurry. 
On average, the aqueous caustic decant contained 486 milligrams per liter 
of aluminum, 7.6 milligrams per liter of iron, and 6922 milligrams per 
liter of silicon. The caustic wash contained 222 milligrams per liter of 
aluminum, 2 milligrams of iron per liter and 4374 milligrams per liter of 
silicon. The base-treated, washed fly ash solid contained 12.4% aluminum, 
5.4% iron and 11.8% silica, with all percentages by weight. The acidic, 
aqueous fly ash leachate contained 8920 milligrams per liter of aluminum, 
3784 milligrams per liter or iron, and 300 milligrams per liter of silica. 
The base-treated, acid-reacted, unsolubilized fly ash residue recovered at 
the end of our process contained 10.2% aluminum, 2.6% iron, and 20.4% 
silica, all by weight, and constituted about 78.5% of the weight of the 
raw fly ash. 
To demonstrate the effectiveness of the acid leachate as a coagulant and 
flocculant for water and waste water, we first prepared a kaolin solution 
in water by mixing 17.5 grams of kaolin with 482.5 milliliters of 
deionized water at low speed for five minutes in a blender to obtain a 
3.5% aqueous solution of kaolin. We also obtained water samples from the 
Wolf River in Tennessee, and waste water from the grit chamber of the 
north waste water treatment plant in Memphis, Tenn. We analyzed both the 
Wolf River water and the waste water for pH, color, turbidity, total 
suspended solids and chemical and biological oxygen demands. 
We then added measured quantities of the acid leachate to five milliliters 
of the test water blended with 900 milliliters of deionized water. We 
adjusted pH as necessary, and mixed the samples at 100 rpm for five 
minutes. Thereafter, we mixed the samples at a slower speed for 20 minutes 
to simulate flocculation, then set each container aside for a 30-minute 
sedimentation period. We decanted the supernatant liquids from each 
sample, and analyzed for water quality. Sludge solids were either 
discarded or evaluated for dewaterability by vacuum filtration. 
For comparison purposes, we ran a series of similar tests on the same water 
and waste water samples using well-known, commercially-accepted 
coagulants/flocculants, namely ferric chloride, aluminum sulfate, and 
mixtures of ferric chloride and aluminum sulfate. Again, we adjusted the 
pH as necessary. In all cases, we tested for the quantity needed to 
achieve a supernatant water quality of 30 milligrams per liter or less of 
total suspended solids (for waste water) and 10 Formazin turbidity units 
(for river water and for kaolin-containing water). 
To demonstrate the effectiveness of the acid leachate, of the acidic fly 
ash slurry and of the acidic fly ash residue in reducing the resistance of 
sludge to dewatering, we mixed 500 milliliters of the sludge with each 
treating agent for three minutes at medium speed, then determined the pH 
and temperature of the sludge. We put 25-milliliter samples of treated 
sludge samples into a filter apparatus, and allowed the treated sludge to 
drain by gravity for two minutes. We then imposed a vacuum at 15 inches of 
mercury on the drained sludge samples and measured the volume of filtrate 
collected over seven minutes or until the vacuum broke because of cracks 
developing in the dried sludge. We recorded the wet and dry weights of the 
sludge samples after drying them in a 103.degree. C. oven to obtain dry 
weight measurements. Percent sludge cake solids were calculated and 
recorded. 
Our treatment of kaolin-containing, deionized water demonstrated that the 
supernatant quantity of water treated with our acid leachate was as good 
as or better than water treated with ferric chloride alone or aluminum 
sulfate alone. Using Nalco Chemical Company's, "Water Clarification 
Procedures," we determined that the sample equivalence, which is the ratio 
of the quantity of acid leachate required to reduce the turbidity of the 
water sample to 10 Formazin units divided by the quantity of standard 
required to achieve the same results, showed that the sample equivalence 
of our acid leachate to ferric chloride was 0.18 and, to aluminum sulfate, 
0.90. 
Our treatment of Wolf River water demonstrated that the acid leachate 
performed as well as a ferric chloride/aluminum sulfate-containing 
solution we prepared containing iron and aluminum in the same ratio as the 
acid leachate. Conventional, commercially-acceptable water and waste water 
coagulation/flocculation treatments do not use ferric chloride and 
aluminum sulfate in combination. However, our comparative results prove 
that our acid leachate performs as well as the prepared solutions. The 
only disadvantage of our acid leachate in treating the kaolin-containing 
water and the Wolf River water was that the acid leachate required pH 
adjustment, as by addition of lime. Even so, the cost of our acid leachate 
is far below the cost of either aluminum sulfate or ferric chloride. 
Moreover, our methods help with the waste disposal problems of fly ash by 
converting some of the fly ash to effective coagulation and flocculation 
agents for removing pollutants from water and waste water. 
Our acid leachate produces outstanding and surprising results as a 
coagulant/flocculant in treatment of waste water. The sample equivalence 
as compared to iron and aluminum were 0.05 for iron, and 0.60 for 
aluminum, when we compared our acid leachate with ferric chloride and 
aluminum sulfate. For these tests, we chose as our target the reduction of 
total solubilized solids in the waste water to 30 milligrams per liter. To 
achieve this target, our acid leachate required 6.3 milligrams per liter 
of iron and 14.9 milligrams per liter of aluminum, and a pH of 5. By 
contrast, a ferric chloride containing solution required 115 milligrams 
per liter of iron at a pH of 6, and an aluminum sulfate solution required 
25 milligrams per liter of aluminum at a pH of 5.4 to achieve the same 
results. Apparently, an unexpected synergy among the silica, aluminum and 
iron in our acid leachate is at least partially the reason for these 
outstanding results. Moreover, at comparable optimum levels of treatment, 
our acid leachate is just as effective as ferric chloride and aluminum 
sulfate in reducing turbidity, removing color bodies and in reducing the 
chemical oxygen demand of waste water. 
Again, one apparent disadvantage of our acid leachate is the need to adjust 
its pH. However, the caustic decant obtained from our treatment of fly ash 
with aqueous base can be used for this purpose instead of lime, reducing 
the cost of pH adjustment. 
The results of our sludge-conditioning tests were also surprising. As 
compared to untreated fly ash, our acid leachate was at least 33% better 
in reducing the resistance of sludge to dewatering. By comparison to 
ferric chloride alone, a commercially-acceptable sludge conditioner, our 
tests show that far smaller quantities of our acid leachate, after pH 
adjustment through addition of lime, are needed to reduce resistance of 
the sludge to vacuum treatment. At the optimum levels for our acid 
leachate and for ferric chloride, dewatering of the sludge after treatment 
with our acid leachate produces dried sludge containing less water than 
sludge treated with ferric chloride. 
Our tests results are more fully explained in a thesis entitled, "Recovery 
of Water and Wastewater Treatment Chemicals from Fly Ash," by Janet S. 
Condra, published Aug. 12, 1982. We incorporate that thesis in this 
specification by reference.