LU81671A1 - PROCESS FOR THE TREATMENT OF RESIDUAL SLUDGE - Google Patents

Description

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kZ5k GL / DPL

The subject of the present invention is a new process for treating residual sludge, more particularly decantation sludge from domestic and / or industrial water treatment plants, with a view to facilitating their final conditioning, such as their filtration, their transport , their possible incineration and their final disposal or spreading.

The traditional wastewater treatment sector involves the stages of screenings, grit removal and oil removal, primary decantation, biological purification by activated sludge and finally secondary decantation. The activated sludge extracted from the secondary decanters is partly recycled to the biological treatment tanks, while the excess is sent to the primary decanters.

The sludge extracted from the primary settling tanks * - called primary sludge - can be conditioned as such, with a view to their elimination, but in general they undergo an anaerobic digestion treatment beforehand with methane formation.

After settling in the primary settling tanks respectively in the digesters, this sludge still contains a very large amount of water (2 - 5% of dry matter for the primary sludge; about 6% on average in dry matter for the sludge digested).

Their final conditioning will therefore require, $ i t t

In order to improve the filterability of this sludge, it has been proposed to treat it with aggregating agents for suspended particles, in particular with ferrous sulfate - hydrogen peroxide systems; ferric chloride - lime; ferrous sulfate - lime.

Each of these reagent systems however has drawbacks either by insufficient drying results and / or an excessive increase in the weight of the filter doughs (in particular FeSO ^ -CaCOH) ^), or by the price of the reagents * (in particular H ^ C ^, FeCl ^) necessary to achieve an interesting dryness of the filter cakes.

In accordance with the present invention, an improved treatment of waste sludge is now proposed, which eliminates the main drawbacks of known methods, and which moreover allows remarkable and unexpected savings on the overall cost of the complete packaging of the waste sludge.

V

According to the new process of the invention, for the treatment of residual sludge in order to facilitate its final conditioning, the sludge is treated with a combination of ferrous salts, an oxidizing agent and lime.

According to the invention, this oxidizing agent is preferably hydrogen peroxide and the iron salt used is preferably ferrous sulfate.

According to an additional characteristic of the invention, it is possible to use:.

• 1 - between 15 and 50% by weight of ferrous sulfate (FeS04.7H20); - between 0.5 and 5% by weight of hydrogen peroxide (^ O ^); - Between 5 and 30% by weight of lime / Ca (OH) 2_7 î these percentages being expressed relative to the weight of the dry matter of the hoe.

In a particularly suitable manner, it is possible to use, in accordance with the invention: - approximately 20 to 30% by weight of ferrous sulphate (FeS04.7H20); - approximately from 1 to 2.55% by weight of hydrogen peroxide (H ^ O ^) î - approximately from 10 to 20% by weight of lime / Ca (0H) 2_7 ï these percentages being expressed relative to the weight of the material hoe dryer.

In certain embodiments of the invention, it may be preferable to define the quantities of reagents used as a function of their respective molar ratios. In this case, in accordance with an additional characteristic of the invention, the molar ratio of the \ quantities used of sulfate ferrous (FeSO ^ * 7 ^ 0) and hydrogen peroxide (^ 0 ^) is between 1: 0.25 and 1: 1, and the molar ratio of the amounts used of ferrous sulfate (FeSO ^. 7H20) and lime / “Ca (0H) 2_7 is between 1: 1 and 1: 4.

According to a particular embodiment of the invention, it may be advantageous to use lime in the form of milk of lime and / or ferrous sulphate and hydrogen peroxide in the form of aqueous solutions.

In accordance with a preferred characteristic of the invention, the reagents are added to the hoe J - J. m * V> Λ .n Jl a t \ 4 Λ V> "V. rt VJ * T “* î J il -î λ VJ 1 A _" 1 -O f _! - ... IL · ["* added first, then the oxidizing agent and finally lime.

According to the preferred embodiment of the invention, the hoe is carefully mixed during and / or after the addition of each reagent.

Other interesting characteristics and properties of the invention will appear from the following description of two examples of embodiments * thereof, given purely by way of illustration, and from an example of comparative calculations of the costs of the total packaging of the hoes.

EXAMPLE 1

Treatment of primary hoes 300 ml of a primary hoe, with an average dry matter content of 3.5 / °, from the primary settling tanks of the WASMUëL treatment plant (MONS region in BELGIUM) are placed in a container fitted with a JAR-TEST type stirring mobile, rotating at 150 rpm.

Ferrous sulfate is injected in the form of a 10? O solution of heptahydrate (FeSO ^ .T ^ O). After 30 seconds the hydrogen peroxide is added in a solution of 1 to 6 to the medium and stirring is maintained for 60 seconds. Then the lime is added in the form of milk of lime at 10? A and the mixing is still ensured for 30 seconds.

The treated bope is then poured over Buchner filters, and the vacuum is kept constant at a value of approximately 600 mm Hg, until cracking

The specific resistance to filtration of the mud is calculated from the readings during the experiment of the volumes of filtrate collected according to the method described by P.F. NOTEBAERT, D.A. WILMS & A.A. VAN HAUTE in Water Research, vol. 9, pp. 667 to 673 1973.

The specific resistance to filtration (for a vacuum of 600 mm Hg) is reduced by

initial value, for untreated primary sludge, i P

! of 180. 10 m / kg, up to a value between 1.5 and 2'1012 m / Kg for the sludge treated by:

Fe S0 ^. 7H20 25 ° S by weight,

Ca (OH) 2 20c / o by weight, H202 2% by weight; * these percentages being expressed relative to the weight of dry matter in the sludge.

For comparison, Table I shows the amounts of respective reagents necessary to achieve the same reduction in filtration resistance, by known methods.

TABLE I

Quantities of reagents necessary to lower the filtration resistance of a primary sludge, from an initial value 12 of approximately 180, 10 m / kg to a value comprised between 1 d 5 and 2. 10 ^ m / kg.

it, --------------------------- ^ -------------------- --l · -------- II H202 JFeSO ^ 7H20J Ca (0H) 2 d FeCl ^ {System \% by weight \% by weight]% by weight \% in po} ti I ---- ----------- 1 ------------ l ---------- 1 ----------- J.- -------- iii | \ H202 + FeSO, | 5 | 40 î - *! (known) j! dd d --------------- {_----------- d ---------- r -------- --T --------- iCa (0H) ~ + FECEU | ί - i 25 i 7.5! (known) 3 | dd d --------------- 1 ---------- 4 ---------- d ---------- -j --------- jCa (0H) 2 + FeS04! j 30 | 30 | j (known) j! ! ! i --------------- 1 ----------- j ---------- j ---------- -j --------- | FeS04 + H202 + jj | j i Ca (0H) 2 j 2! 25! 20 | i (according to the invention-ι! ί i j tion)! ! | | i --------------- i ----------- 1 ---------- 1 ---------- .i --------- EXAMPLE 2

Digested sludge treatment

The procedure described in Example 1 is applied to a digested sludge with an average dry matter content of 6%, obtained from the anaerobic digesters of the WASMUëL treatment plant. The specific resistance to filtration is reduced from an initial value, for untreated digested sludge, of approximately 100.1 m / kg up to a value between 2 and 2.5 12 r 10 m / kg for sludge treated with FeS0 ^. 7H20 30% by weight;

Ca (OH) 2 10; en by weight; H202 1,250 by weight; these percentages being expressed relative to the weight of dry matter of the sludge.

/ I "By way of comparison, Table II shows the amounts of respective reagents necessary to achieve the same reduction in filtration resistance by known methods.

TABLE II

Quantities of reagents needed to lower the filtration resistance of a digested sludge, an i? initial value of around 100.10 m / kg at a value between 2 and 2.5 1012m / kg.

j Th? 0 system? iFeSO, .7H2o! Ca (0H) 2 FeCl, i \ ° / at -weight]% by weight \% by weight% by ^ weight! ______________ i _____ 1 ---- 1 ----------- 1 ---- ------------------- 1 1 i | H20 + FeS04 j 4! ? ° I “j (known) j} j 1 -------------- ΐ ---------- 1 ----------- T ------------------------ I Ca (0H) 2 + FeCl3 j - j! 20 5.

i Î222SH! _______ 1__________1___________1________________________! 1 1 i j Ca (OH) + FeS0, | - [30 j 20 1 (known) iii i -------------- 1 ---------- j ----------- j- ----------------------- 1 1 1 i FeS04 + H20 + i 1,2 i 30} 10 i ca (° H> 2 iii! (According to the invention! i | tion) | j | EXAMPLE 5

Comparative calculations of the costs of the total conditioning of primary and digested sludge, treated by the process according to the invention and treated by known processes.

Three stages of sludge conditioning are taken into account in this comparative calculation, namely: 1- treatment in order to improve the filterability (process described in examples I and II and corresponding known methods); 2- transporting the hoes to an incineration plant; 3- the incineration stage itself.

The adopted model assumes several treatment plants, serving an area of 80 km radius, treating the effluents of 1,000,000 population equivalents at the rate of 80 grams of solid matter per day and per inhabitant.

The various treatment plants incinerate their sludge in an incineration facility located not far from the region's production barycenter. The sludge conveyance distance will be on average 60 to 65 km, since the large stations are not in the immediate vicinity of the furnace and the actual distance to be covered is greater than the distance as the crow flies.

Since the basic data is the dry matter flow rate of the water treated in the stations and the systems tested restore different quantities of cake, the calculations are carried out not on the unit weight of cake, but on t that of initial dry matter . So that's the ton of. dry matter supplied to treatment plants and subjected to conditioning treatment by the four systems which is taken into account.

On the other hand, it is considered that the digested sludge contains 40% of organic matter and the primary sludge 59%. The figures are each time modified to take account of the modification of this percentage after conditioning due to the reagents used and the small loss by solubilization, decomposition, etc.

With regard to the respective amounts of reagents required for the different processes, the comparative calculations were based on the experimental values shown in Tables I and II.

The prices of the reagents used are for H202: 42FB / kg of H202 100 #

FeCl ^: 14FB / kg CA (0H) 2: 1.5FB / kg

FeS0 ^. 7H20: 0.8FB / kg, or 1.45FB / kg for pure FeSO ^.

The cost of transport by truck of 25 tonnes, for a distance of 60 to 65 km, is estimated at 170FB / t, * including loading and unloading.

The estimation of transport costs takes into account the weights of the treated sludge, which differ from one process to another.

Tables III and IV show the experimental values of the sludge weights (per tonne of dry matter supplied to the stations), obtained by treating "primary (Table III) and digested sludge (Table IV), under the conditions of Examples 1 and 2 , for the process according to the invention and the known processes.

These same tables also show the weights of the ash obtained after incineration of the various treated sludges.

TABLE III

Weight of primary sludge treated under the conditions of Table I, and weight of ash obtained by incineration of this sludge, (per tonne of initial dry matter)

{Sludge Weight System | Weight of ash treatment | processed (kg) | kg ii ---------------- 1 -------------------- 1 ---------- ------------

I I

H202 + FeS04! 5,771! 420 (known) J {---------------- j -------------------- 1 ------ ----------------

I I

Ca (0H) 2 + FeCl3 | 6378 {590 (known) i j ---------------- L____________________I______________________

I I

1 I

Ca (0H) 2 + FeS04 j 7 222 j 710 (known) ii —----- t -------------------- 1 ------ ----------------

I I

FeS0, + H? 0 + i 5 381 j 540

Ca (0H) 2 d d (according to 11inven-ι i tiorO ___________ [___________________ j______________________

TABLE IV

Weight of the digested sludge, treated under the conditions of Table II, and weight of the ash obtained by incineration of this sludge (per tonne of initial dry matter).

t -11- Γ T j ----------------------: i System <3.6 i Weight of "sludge i Weight of ash j treatment | treated (kg) ! kg I Ί j jH202 + FeS0 ^ I 6 125 | 600] (known) J! I -------------- 1 _______________________ j __________________; ___ JCa (0H) 2 + FeCl3i 6743 i 800 i ( known) it j -------------- j -----------------------! ------- --------------- ii | Ca (0H) 2 + FeS04! 7,500 j 800

j (known) j J

not a word-----------------------;---------- ------------ II j | FeS0 ^ + H20 +! 6 343 d 710! Ca (0H) 2 | j I (according to the in!! jvention) | i

For the third stage, incineration, the cost estimate is "based on the thermal balance of a stepped hearth furnace, brought to the temperature of 15 ° C, assumed supply temperature of the sludge. The fumes exit at 460 ° C, ash at 350 ° C. The air is preheated to 165 ° C.

We considered the possibility of the alternative use of two types of fuel, gas and heavy fuel. The prices used for the calculation are as follows: - for gas (Slochteren type): 2.6F3 / Nm ^, or 3.5FB / kg - for heavy fuel: 3945 Fb / t (SHELL reference, zone 2 1-7-79) *

For the disposal of ash after incineration, a final discharge cost of 400 FB per tonne of ash is allowed (see Tables II and IV).

The results of these various calculations are summarized in Tables V and VI, which give the costs of the reagents necessary for the respective treatments, of transporting the sludge to the place of incineration, of fuel required for incineration (fuel and natural gas) , and elimination of incineration ash, for the overall conditioning of primary (table V) and digested (table VI) sludge using the method according to the invention, compared to known methods.

TABLE V

Total cost of conditioning primary sludge per tonne of dry matter supplied to treatment plants I -------- T -------------------- T- ----------- 1 ------------- 1 ------------- I System i itransport treatment i Incineration} Disposal {Cost total Sde trajé' Assets j} Fuel - Gas {ash J Fuel gas j tly | j | j 1 I --------- h ----------- 1 ---------- h ------------ 1- ------------ 1 -------------

I I I I 1 I

iH2 ° 2 * ί 2,420 | 981 il022 886 j 168 I 4591 4445 ÎFeSO. ! i! ii I —-— | ---------- H --------- i ----------- + ----------- + ------------ | Ca (0H) 2 + i> | 425 | 1 084 | 1262 1096 j 236 j 4007 3841 | Feci5! ί! ! ! j i i i i i jCa (0H) 2 + j 59o ji 228 jl594 1386 j 284 j 3796 3588 i 4 i i i i i

{------ lll I I

iFeSO ^ t j i 340 | 915 | 836 728 J 216 J 3307 3199 h2o * ί! ί ί ί

I U j I I I I

jCa (OH) 2! ! ί! ί {______ t -------------- 1 --------- + ------------ 1 --------- ---- 1 ------------- li

Total cost of packaging digested sludge, per ton of dry matter supplied to treatment plants.

TABLE VI

J System J treatment | transport {Incineration {Disposal {Total cost J of reagents {i Fuel Gas i ash ash i Fuel gas i {{| {{I ---------- (-------------} ----------- j ---------- --- 1 ------------ j ------------! S 11 11! H202 + {1920 j 1041 I 1584 13761 240 j 4785 4571)! FeS04! ! ! il j Ca (0H) 2+} 1000! 1146 i 1811 1572 "320 i 4277 403Σ

iFeCl3 I! ! I I

j --------- ~ | ------------- 1 ---------- Ί ---------—--- -i ------------- i · ------------ i Ca (0H) 2+ d | 12? 5 d 2103 1827j 32Q j 4238 3g6, | FeS04 d d | {dd | ------------- d ----------- j ------------- d ------- -----} ------------

FeSO, +! 894! 1078 | 1610 1396! 284 j 3866 365Î H2 ° + Ca! j i {{j (0H) 2 I ί ί i ί

It is therefore found that the treatment process according to the invention allows an overall saving: - of 21% (primary sludge) respectively 11% (digested sludge) compared to the process using the lime-ferric chloride system, "of 3956 (primary sludge ) respectively 24% (digested sludge) compared to the process using the ferrous sulfate - hydrogen peroxide system, and - by 12% (primary sludge) respectively 8% (digested sludge) compared to the treatment process using the lime-ferrous sulfate system .

Of course, the invention is not limited to the embodiments and details described above, and many modifications can be made to it.

by those skilled in the art, without departing from the scope of the invention as defined in the claims which follow. I

Ί I 1! i «ί ί t | i r i * -15-

Claims (9)

1. A method of treating residual sludge in order to facilitate its final conditioning, characterized in that it is treated with a combination of ferrous salts, an oxidizing agent and lime. 2. Method according to claim 1, characterized in that the oxidizing agent is hydrogen peroxide. 3- A method according to claim 1, characterized in that ferrous sulfate is used as the ferrous salt. k. Process according to claims 2 and 3 "4 characterized in that one uses: - between 15 and 50> 0 by weight of ferrous sulfate (FeSO ^, 7H20); - between 0.5 and 3% by weight of hydrogen peroxide (H ^ O ^); - between 5 and 30% by weight of lime / Ca (0H) 2_7; these percentages being expressed relative to the weight of the dry matter of the sludge. 5. Method according to claims 2 and 3 "characterized in that one uses: - about 20 to 30% by weight of ferrous sulfate (FeS04.7H20); - approximately 1 to 2.5 $ by weight of hydrogen peroxide (H202); - About 10 to 20% by weight of lime (Ca (OH) 2) j these percentages being expressed relative to the weight of the dry matter of the mud. 6. Method according to claims 2 and 3 "characterized in that the molar ratio of the amounts used of ferrous sulfate (FeS047H20) and hydrogen peroxide (K202) is between 1: 0.25 and 1: 1, and the ratio molar of the quantities used of ferrous sulphate * (FeSO ^ HgO) and lime (CaÇOH ^) is between 1: 1 and 1: 4. 7. Method according to either of the preceding claims, characterized in that lime is used in the form of milk of lime. 8. Method according to one or other of Claims 4 to 7, characterized in that ferrous sulphate and hydrogen peroxide are used in the form of aqueous solutions. 9. Method according to one or other of the preceding claims, characterized in that I add ferrous salt first to the mud, then the oxidizing agent and finally the lime. 10. Method according to claim 9, characterized in that the mud is mixed thoroughly during and / or after the addition of each reagent. -17- '