Method for dewatering a slurry using a twin belt press with cationic amine salts

A method for dewatering a slurry of solid particles suspended in a liquid using a twin belt press dewatering system, comprising the following steps: PA1 feeding the slurry of solid particles into the dewatering system; PA1 feeding a cationic amine salt solution into the dewatering system at a point just prior to the mixing drum, the cationic amine salt being a latex copolymer of acrylamide and dimethylaminoethylmethacrylate sulfuric acid salt having a mole ratio in the range between about 30:70 to 70:30, preferably 53:47; PA1 mixing the cationic amine salt solution with the slurry of solid particles in the mixing drum; and PA1 feeding the cationic amine salt solution/slurry mixture to the twin belt press, whereby the slurry of solid particles is dewatered. The cationic amine salt solution having a strength of less than 1% and a polymer concentration of about 35%.

BACKGROUND OF THE INVENTION 
The invention dramatically improves the dewatering capability of a twin 
belt press by the addition of a high molecular weight cationic amine salt 
to a slurry of solids to be dewatered. The addition of a high molecular 
weight cationic salt to a feed slurry dramatically improves the capture of 
solids in the gravity drainage filtrate with much lower solids in the belt 
wash water and results in a dryer cake at discharge. 
Dewatering of slurries, such as waste activated sludge or aerobically 
digested sludge, is a process of liquid-solid separation wherein large 
quantities of liquid can be removed from a slurry by mechanical and 
chemical means. Dewatering does not, however, achieve 100% liquid-solids 
separation. Normally, moisture remains in a dewatered cake, and in many 
instances the filtrate contains solids from the slurry. Cake dryness and 
solids capture are typical indicators of the efficiency of a dewatering 
system, and are influenced by the mechanical device used, the chemical 
conditioning of the feed slurry and the characteristic of the slurry 
itself. 
Water in refuse or process slurries may be present in one of three forms: 
free water, capillary water, or intracellular water. Free water drains 
easily from the solid particles, since no adhesive or capillary forces are 
to be overcome. Capillar water can be separated from solids by overcoming 
the adhesive and capillary forces holding the water amongst the solid 
particles. Capillary water is typically transformed to free water by 
increasing the particle size through the use of polymer flocculants. 
Intracellular water, water contained inside cell walls, can not be removed 
unlsss all the cell walls are lysed or broken. The breaking of cell walls 
requires high mechanical forces, heat and/or chemical treatment. 
Therefore, the content of intracellular water sets the theoretical upper 
limit for cake dryness in slurries where large amounts of the solids have 
a biological origin. 
The twin belt press is one mechanical device which has been found to be 
particularly effective in dewatering slurries, such as waste activated 
sludge or aerobically digested sludge. Although the twin belt filter has 
been found to be most effective in removing free water from these 
slurries, it alone is not capable of overcoming the adhesive and capillary 
forces holding the capillary water in between the solids of the slurry. 
Due to the desirability of obtaining a much dryer cake at discharge, and 
reducing the solids permeating into the filtrate, attention has been given 
to various means for improving the capture of capillary water. 
It is well settled that the use of polymer flocculants can assist in 
overcoming the adhesive and capillary forces holding water between the 
solids of a slurry, and thereafter transform such capillary water into 
free water which easily drains from the solid particles in a dewatering 
device, such as a twin belt press. The use of polymer flocculants in the 
dewatering of minerals, such as coal, phosphates, slimes, tar sands, 
mineral tailings, bentonite, and other clay products is demonstrated in 
the following patents: U.S. Pat. Nos. 3,408,293 (Dajani et al.), 3,578,586 
(I. Gal et al.), 4,342,653 (Halverson), and 4,569,768 (McKinley); Japanese 
Patent Publication No. 49-10182; and European Patent Application No. 
81110828.1 (Braun et al.). U.S. Pat. No. 3,408,293, U.S. Pat. No. 
4,569,768, Japanese Patent Publication No. 49-10182, and European Patent 
Application No. 81110828.1 all demonstrate sequential adding of anionic 
polymers followed by cationic polymers to assist in the flocculation of 
mineral slurries prior to dewatering. The chemical addition of two 
polyelectrolytes is undesirable since it dramatically increases the cost 
of the dewatering process. 
U.S. Pat. No. 4,342,653 describes the flocculation of aqueous suspensions 
of solids, such as phosphates, slimes, tar sands, coal refuse, etc., by 
addition of a polymeric anionic flocculant. 
U.S. Pat. No. 3,578,586 proposes that it is preferable to treat slurries of 
coal or other minerals with a polyelectrolyte at various points throughout 
the dewatering system. The polelectrolytes having a molecular weight from 
10,000 to 10,000,000 and added in at least two steps. 
Ionic flocculants have also been added to slurries of sludge prior to 
dewatering with a mechanical device. The following patents all demonstrate 
the use of a two step approach using both anionic and cationic 
flocculating agents: U.S. Pat. Nos. 4,105,558 (Heinrich et al.), and 
4,479,879 (Hashimoto et al.); and Japanese Patent Application Nos. 
82/185578, 84/197287, 83/336544, 81/17903 and 81/18499. In particular, 
Japanese Patent Application No. 82/185578 adds polycations and amphoteric 
copolymers. Japanese Patent Application No. 83/236544 dewaters sludge by 
using inorganic coagulants and dewatering agents containing cationic 
organic polyeer coagulants, anionic organic polymer coagulants and acids. 
Japanese Patent Application No. 81/18499 mixes the sludge with a cationic 
polymer coagulant and a surfactant. 
There are various patents which describe the use of cationic polymer 
flocculants to assist in the dewatering of sewage sludge. U.S. Pat. No. 
3,531,404 proposes that sludge be mixed with a polyelectrolyte flocculant 
which has a high molecular weight and is cationic prior to dewatering the 
sludge. The following patents also disclose the use of a cationic polymer 
flocculants to assist in the dewatering of sludge: U.S. Pat. No. 4,358,381 
(Takeuchi et al.), and Japanese Patent Application Nos. 
84/6852,883/226535, 82/98698, 82/69471, 75/148242, 75/148241, 74/146371, 
80/43556,778/28602, 75/136280, and 75/136279. 
Japanese Patent Application No. 82/69471 demonstrates the use of a cationic 
polymer to assist in increasing the interlayer bond strength necessary for 
forming paper from pulp slurries. The cationic polymer being an 
acrylamide-dimethylaminoethylmethacrylate (DMAEM) sulfate copolymer. 
Japanese Patent Application No. 75/148242 discloses mixing a cationic 
amine salt having a 9:1 mole ratio and being a 30% aqueous solution with 
dimethylaminoethyl methacrylate methochloride polymer in a 86:14 ratio to 
provide a flocculant. 
Japanese Patent Application No. 75/148241 prepares a cationic flocculant of 
acrylamide and DMAEM sulfate which are copolymered in a 9:1,8:2,7:3,5:5, 
or 3:7 mole ratio as a 20% aqueous solution. Japanese Patent Application 
No. 74/146371 also prepares a cationic flocculant by copolymering 
acrylaiide with a salt or quaternary derivative of DMAEM in aqueous 
acetone. 
Although the aforementioned patents recognize that ionic flocculants may be 
used to assist in the dewatering of sludge, the present inventors have 
discovered that the particular cationic amine salt used in accordance with 
the process parameters of the present invention dramatically improves the 
capture of solids in the gravity drainage filtrate of a twin belt press 
with lower solids in the filtrate and a dryer cake at discharge. The 
cationic amine salt solution used in accordance with the process developed 
by the present inventors is much more cost effective than earlier 
flocculants, provides a cleaner filtrate requiring less recycle of total 
suspended solids therein, permits a wider polymer dosage, and allows for 
very fast floc formulation. Additional advantages of the present invention 
shall become apparent as described below. 
SUMMARY OF THE INVENTION 
It is the primary object of the present invention to improve the dewatering 
capability and the capture of sludge solids in the gravity drainage 
filtrate of a twin belt press, resulting in a much lower solids content in 
the belt wash water and a dryer cake at discharge. The method of the 
present invention provides for the dewatering of a slurry of solid 
particles suseended in a liquid, such as waste activated sludge or 
aerobically digested sludge, using a twin belt press dewatering system 
comprising the following steps: 
(a) Feeding a slurry of solid particles into the dewatering system; 
(b) Feeding a cationic amine salt solution into the dewatering system at a 
point just prior to either a mixin drum or an adjustable static mixer with 
a polymer injection ring, the cationic amine salt being a latex copolymer 
of acrylamide and dimethylaminoethylmethacrylate (DMAEM) sulfuric acid 
salt having a mole ratio in the range between about 30:70 to 70:30; 
(c) Mixing the cationic amine salt solution with the slurry of solid 
particles in either a mixing drum or an adjustable static mixer wherein 
flocculation occurs; and 
(d) Feeding the cationic amine salt solution/slurry mixture to the twin 
belt press, whereby the slurry of solid particles is dewatered. 
It is often preferable to concentrate the slurry of solid particles prior 
to dewatering by means of a thickener in order to have a slurry solution 
of between 1.5 to 2.5%. That is, the concentrated slrrry can be in a range 
between about 13,500 to 17,200 ppm. The slurry of solid particles can be 
fed to the dewatering system via a sludge pump at a feeding rate of 
between 50-120 gpm/meter of effective belt width. 
The mole ratio of acrylamide to DMAEM sulfuric acid salt is preferably 
53:47, wherein the percent polymer in the cationic amine salt solution is 
about 35%. The cationic amine salt solution is typically a concentration 
of 1%. However, it may be diluted by the addition of water to a range 
between about 0.25-1.0%, preferably diluted to a range between about 
0.25-0.5%. One to five volumes of water is satisfactory for diluting the 
cationic amine salt solution to the desired concentration level. The 
cationic amine salt solution may be diluted in a static mixer or mixing 
"T". The cationic amine salt solution is thereafter fed into the 
dewatering system at a point at least 2 feet before the mixing drum. Some 
machines are equipped with a polymer injection ring followed by an 
adjustable static mixer instead of a mixing drum. 
An additional object to the present invention is that the mixing of the 
cationic amine salt solution and the slurry of solid particles be 
conducted in a mixing drum or the like, at very slow speeds in the range 
between about 1 to 5 rpm. If the machine is equipped with a polymer 
injection ring and adjustable static mixer, then the mixer should be set 
at a minimum to low setting. Also, the twin belt press should be operated 
at slow speeds in the range between about 2 to 5 ft/min., whereby cake 
dryness is enhanced. 
Additionally, the cationic amine salt has a molecular weight in the range 
between 2,000,000 to 30,000,000. The use of this cationic amine salt 
solution is particularly helpful in dewatering slurries, such as 
biological primary sludge, biological secondary sludge, waste activated 
sludge, and aerobic digested sludge. 
The present invention may also include many additional features which shall 
be further described below.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention provides a novel method for dewatering slurries of 
solid particles suspended in a liquid, such as waste activated sludge or 
aerobically digested sludge, by mean of a twin belt press using a high 
molecular weight cationic amine salt flocculant, whereby solids capture 
and cake dryness are marketably improved. This unique method includes the 
following steps: feeding of a slurry of solid particles into a twin belt 
press dewatering system; feeding a cationic amine salt solution into the 
dewatering system at a point just prior to a mixing drum, the cationic 
amine salt being a latex copolymer of acrylamide and DMAEM sulfuric acid 
salt having a mole ratio in the range between about 30:70 to 70:30, 
peeferably 53:47; mixing the cationic amine salt solution with the slurry 
of solid particles in the mixing drum where flocculation occurs; and 
feeding the cationic amine salt solution/slurry mixture to a twin belt 
press, whereby the slurry is dewatered resulting in a dryer cake and 
increased solids capture from the filtrate. 
Belt filter presses employ single and/or double oppositely disposed moving 
belts to continuously dewater sludges through one or more phases of 
dewatering. The typical twin belt press includes the following three 
operational stages: 
(1) chemical conditioning of the feed slurry, 
(2) gravity drainage to a non-fluid consistency, and 
(3) compaction of the dewatered sludge 
Good chemical conditioning is the key to successful and consistent 
performance of the belt filtered press, as it is for other dewatering 
processes. Chemical conditioning of sludge with 1% or less solution of a 
cationic amine salt, has been found to be particularly effective. The salt 
being a latex copolymer of acrylamide and DMAEM sulfuric acid salt having 
a mole ratio of 53:47, wherein the percent polymer in the salt is about 
35%. 
After chemical conditioning, the readily drainable water is separated from 
the slurry by discharge of the conditioned slurry onto a moving belt 
filter in a gravity drainage section of the dewatering system. Typically, 
less than one minute is required for drainage depending on the 
characteristics of the sludge and the effectiveness of the chemical 
conditioning. Following drainage, the sludge will have been reduced in 
volume by about 50% and will have a solids concentration of between 6-10%. 
The formation of an even surface cake at this point is essential to 
successful operation of subsequent stages of the dewatering cycle, i.e., 
pressure and shear dewatering stages. The even surface prevents uneven 
belt tension and distortion while the relative rigidity of the mass of 
sludge allows further manipulation and allows for maximum speed through 
the twin belt press. The compression dewatering stage of the twin belt 
press begins as soon as the sludge is subjected to an increasing pressure, 
due either to the compression of the sludge between the carrying belt and 
cover belt or the application of a vacuum on the carrying belt. Pressures 
can be varied widely by alternate designs. During the compression 
dewatering stage, the sludge cake is squeezed between two belts and 
subjected to flexing in opposite directions (shearing) as it passes over 
the various rollers, causing increased water release and allowing greater 
compaction of the sludge. The advantage of the present invention is thtt 
preconditioning with the cationic amine salt solution permits additional 
free water to be filtered from the slurry resulting in a much dryer cake. 
There are different designs of the double belt filter on tee market. Common 
to all is the use of a pair of endless or seamed belts (upper and lower), 
usually constructed of a polyester wire mesh, which merge together and 
enclose the cake there-between. The enclosed cake then passes through a 
series of roller arrangements before the belts part to discharge the 
dewatered cake. Before the merging of the belts, the bulk of the free 
water is separated from the solids by gravity drainage through one of the 
belts. 
FIG. 1 demonstrates one example of a twin belt press. This mechanical 
device has an upper belt 1 and a lower belt 2 which are oppositely 
disposed. Rollers 3 are disposed on the inside of belts 1 and 2. 
In the chemical conditioning stage the sludge feed is mixed with cationic 
amine salts in mixer 4. The conditioned sludge is thereafter fed to belt 2 
where readily drainable water is separated by gravity from the sludge. 
During the gravity drainage stage the sludge volume is reduced by about 
50%. Thereafter, the partially dewatered sludge enters the compression 
dewatering stage where belts 1 and 2 converge about the sludge, liquid is 
squeezed out through the belts, and a substantially dry cake is formed. 
During the compression dewatering stage the sludge volume is reduced 
geometrically by a series of roller arrangements 3. The liquid is driven 
out through both belts because of the forced reduction of volume, 
resulting in a pressure in the sludge cake that is higher than the 
surrounding aatmospheric pressure. The cake is thereafter discharged from 
the twin belt press for further downstream treatment. 
Conditioning of the sludge with the particular cationic amine salt solution 
discovered by the present inventors results in greater solids capture and 
cake dryness. Conversion of capillary water into free water by 
flocculation of the solid particles in the sludge is enhanced by the 
conditioning of the sludge with the cationic amine salt solution of this 
invention. 
FIG. 2 is a flow diagram in accordance with the previously known twin belt 
press dewatering system. Referring to FIG. 2 sludge is introduced via 
sludge pump 10 into conduit 11 and a polymer solution is thereafter fed 
into conduit 11 to aid in the flocculation of the sludge. The polymer 
solution is normally injected into conduit 11 at a point very near the 
introduction of the sludge. This was for the purpose of allowing 
additional contact time between the polymer solution and the sludge since 
previously used polymers required more time for satisfactory flocculation. 
The treated sludge then traveled via conduit 11 to mixing drum 12 for 
further mixing of the sludge and polymer solution to enhance additional 
flocculation. Thereafter, the flocced sludge was fed via conduit 13 to a 
twin belt press 14 for dewatering of the sludge. Typical polymer solutions 
used in accordance with the flow sheet of FIG. 2 were latex copolymers of 
acrylamide and dimethylaminoethylmethacrylate methyl chloride quaternary 
with a mole ratio of 65:35, 55:45 and 45:55. The percent polymer in these 
polymer formulations typically being 35%. Another polymer solution used in 
accordance with FIG. 2 was Allied Chemical's C-310 which is quaternarized 
DMAEM latex emulsinn product. 
Referring to FIG. 3, the present inventors have discovered that the 
cationic amine salt solution of the present invention, not only results in 
producing a dryer cake, but also flocs extremely fast. Therefore, the flow 
sheet of a twin belt press dewatering system must be modified such that 
the fast floccing cationic amine salt solution is introduced along conduit 
20 at a point just before mixing drum 21. Waste activated sludge, 
aerobically digested sludge, biological primary sludge, or biological 
secondary sludge may be introduced into the twin belt dewatering system of 
FIG. 3 via sludge pump 22; however, prior to introduction of the sludge it 
is preferable to concentrate the sludge in a thickener to about 1.5 to 
2.5%, i.e., 13,500 to 17,200 ppm. Pump 22 feeds the sludge into the 
dewatering system via conduit 20 at a rate of between 50-120 gpm per meter 
of effective belt width. Just before the sludge feed enters mixing drum 21 
the cationic amine salt solution is introduced into conduit 20 via conduit 
23. 
The cationic amine salt is a latex copolymer of acrylamide and 
dimethylaminoethylmethacrylate (DMAEM) sulfuric acid salt having a mole 
ratio in the range between about 30:70 to 70:30; preferably a mole ratio 
of 53:47. It has a molecular weight in the range between about 2,000,000 
to 30,000,000. The percent of polymer in the cationic amine salt solution 
is typically about 35%. The copolymer is approximately a 1% solution which 
may be diluted prior to introduction into the dewatering system to about 
0.25-1.0%; preferably 0.25-0.5%. The cationic amine salt solution may be 
diluted by mixing a 1% copolymer solution with water in static mixer or 
mixing "T38 24. The copolymer solution is diluted with water in an amount 
between 1-5 volumes. 
The cationic amine salt solution enters the dewatering system via conduit 
20 at a point at least 2 feet before mixing drum 21. It is fed into 
conduit 20 at a rate of between 2-3 gpm. Since the cationic amine salt 
solution flocs very rapidly its injection at or near mixing drum 21 is 
very important in order to avoid clogging of the system. The sludge and 
cationic amine salt solution then travel together via conduit 20 to mixing 
drum 21 where preconditioning or flocculating of the sludge results due to 
mixing at slow speeds, typically in the range between about 1 to 5 rpm. If 
the dewatering system is equipped with a polymer injection ring and 
adjustable static mixer, then the mixer should be set at a minimum to low 
setting. The cationic amine salt solution acts to floc the solid particles 
of the sludge, thereby overcoming any adhesive or capillary forces holding 
water between the particles and increasing the amount of free water. 
The conditioned sludge is then fed via conduit 25 to twin belt press 26 for 
dewatering. In order to enhance dryness of the cake it is preferable that 
the twin belt press have a belt speed in the range between about 2 to 5 
ft/min. 
EXAMPLE 
A biological waste activated sludge from a mixed liquor suspended solid was 
concentratdd in a thickener to about 1.5% to 2.5% prior to being fed to a 
twin belt press dewatering system. Thereafter, a Stranco Polyblend was 
used as the make-down device into a mixer equipped cationic polymer feed 
tank. A first batch of cationic amine salt solution was made up at about 
1%. The next batch was made up at about 0.5% solution, and finally a post 
diluted batch of 0.25%. The cationic amine salt solution was a latex 
copolymer of acrylamide and DMAEM sulfuric acid salt with a mole ratio of 
53:47. The percent polymer in the formulation was 35%. The molecular 
weight was in the range of 2,000,000 to 30,000,000. 
Biological solids were fed into the dewatering system via a sludge pump at 
a rate of about 40 gpm and the total solid suspension range was from about 
13,500 to 17,200 ppm. A 0.5% solution of cationic amine salt was 
introduced for preconditioning of the sludge feed, the polymer flow rate 
being between 2-3 gpm. The belt wash flow rate was 50 gpm. A 0.5% solution 
of cationic amine salt resuleed in a gravity drainage having 10 ppm, belt 
wash water having 150 ppm and solids captured calculated to 98.7%. In 
contrast, Allied Chemical's C-310 resulted in a gravity drainage of 50 
ppm, belt wash water of 250 ppm and solids capture calculated to 97.6%. 
Thus, the cationic amine salt of the present invention improved the solids 
capture by approximately 1%. 
Anionic polymer was injected prior to the cationic amine salt solution but 
did not improve either cake dryness or cationic usage. 
Minimum conditioning of the sludge feed with the cationic amine salt 
solution was desirable as the floc formed quickly, and was subject to 
shear degredation with more mixing of the two elements. Thus, the best 
injection point was found to be just before the rotary mixing drum on the 
twin belt press dewatering system. It was also discovered that the slower 
the speed of the belt of the twin belt press, the better the cake dryness. 
The slippage at various slow belt speeds (a machine function of this 
earlier model A/A twin belt press drive) prevented a full investigation. 
This slippage was the result of a thick cake leaving the wedge zone and 
entering the high pressure section. The process according to the present 
invention operated better when the mixing drum speed was slower. It is 
important, however, that some minimum rpm be maintained in the mixing drum 
as inadequate polymer conditioning was regularly noted when the mixing 
drum was turned completely off. 
The cationic amine salt solution prepared above was injected approximately 
two feet upstream of the mixing drum. Conditioning of the waste activated 
sludge resulted in a dramatic reduction in gravity drainage solids and 
belt water solids recycle. Furthermore, condtioning with the cationic 
amine salt solution as prepared above resulted in an estimated 1% increase 
in cake dryness over Allied Chemical's C-310 quaternary DMAEM solution and 
approximately a 10% increase over anionic flocculants. It was also 
discovered that this cationic amine salt solution was very responsive to 
both primary and secondary sludges, thus having a wide operating window. 
Table 1 below sets forth the disposal cost of cake from a twin belt press. 
It is based on a feed rate of 40 gpm of 1.5% consistency sludge and a 
98.7% recovery. 
TABLE 1 
______________________________________ 
(DISPOSAL COST OF CAKE) 
Cake Water in Cake Disposal Cost @ $0.05/Lb. 
Solids 
Lbs/hr A H.sub.2 O/hr 
Solids Water Total 
______________________________________ 
12 2169 -- $14.80 $108.45 $123.25 
13 1979 190 $14.80 Change = 
$9.50 
14 1817 162 $14.80 Change = 
$8.10 
15 1676 141 $14.80 Change = 
$7.05 
16 1553 123 $14.80 Change = 
$6.15 
17 1444 109 $14.80 Change = 
$5.45 
Polymer Lbs/hr at 30 Lbs/ton = 4.4 Lbs. 
Polymer Lbs/hr at 40 Lbs/ton = 5.9 Lbs. 
Polymer Lbs/hr at 50 Lbs/ton = 7.4 Lbs. 
______________________________________ 
As is apparent from Table 1 above, a 1% improvement in cake solids capture 
results in a savings equal to or exceeding the cost of the polymers used 
in flocculating the feed. This results in extremely dramatic improvement 
in the operational process of the twin belt press dewatering system. Using 
the process and cationic amine salt solution of the present invention 
increases the solids capture of a dewatered sludge by at least 1%. No 
anionic polymer flocculant is required, thus reducing the cost associated 
with any of the prior art processes using both cationic and anionic 
solutions. Moreover, the process of the present invention resulted in 
dramatically improving the capture of solids in the gravity drainage 
filtrate with a much lower solids content in the belt wash water. This 
reduces potential problems associated with particle recycling and improves 
capture of the fines in the cake discharge. 
The slow belt speed of the twin belt press promoted the cake dryness and 
the dilution of the polymer to a 0.5% solution established the best 
flocculation at the lowest cost/time of dry solids and still resulted in 
enhancing cake dryness. 
While we have shown and described several embodiments in accordance with 
our invention, it is to be clearly understood that the same are 
susceptible to numerous changes and modifications apparent to one skilled 
in the art. Therefore, we do not wish to be limited to the details shown 
and described but intend to show all changes and modifications which come 
within the scope of the appended claims.