IL35333A - Sewage treatment process - Google Patents

Description

Sewage treatment process ?' D.N. 4440 This invention relates to the heat treatment of sewage sludge to improve its dewatering properties.

Heat treatment of sewage sludge to improve its de-watering characteristics is a well-known process which has been practiced commercially for many years. See, e.g., U.S, Patents Nos. 1, 116 ,953; 2 ,075,224; 2 , 131, 711; 2 ,277, 718; 2,847,379; and 3,155,611. The conventional process Involves heating sludge at temperatures of 100° up to 180° C. for 30 minutes to several hours or days. Pilot plant treatments at slightly higher temperatures and shorter times have been reported. See Harrison, John "Heat Syneresis of Sewage Sludges," Water and Sewage Works, May, 1968, pages 217-220> and references cited therein. However, the approach taken by the prior art generally was to heat at lower temperatures by a batch process for whatever length of time was required to achieve the desired dewaterability, thereby avoiding the more expensive equipment required for a continuous process and the problems associated therewith.

More recently continuous heat treatments have been employed to improve thermal efficiency and increase processing capacity. In general these systems have also employed prolonged holding times, i.e., 30 minutes or more.

Although the prior art heat treatment processes the supernatant liquid. The color is only nominally affected by biological treatment and thus affects the color of the sewage plant effluent. This is significant because color and BOD value are criteria for measuring water quality and the high color and BOD value of the supernatant have been of concern to sanitary engineers considering the heat treatment of sludge to increase the sludge handling capacity of a sewage treatment plant.

Prom data obtained by heating sludge at holding times varying between 10 minutes and 2 hours at various tem-: peratures, it appeared that color formation was directly pro- : portlonal to temperature, i.e., the higher the temperature ■.t to which the sludge was heated to improve its dewatering properties, the higher the color value of the resultant super- natant liquid. Heating a mixture of primary and activated sludge for about 30 minutes at 150° C, gave an APHA color value of 1200; at 180° C, a color value of about 3000; at 190° C, a color value of about 4100; at 200° C, a color value of about 5000; and at 210° C, a color value of about 6200. Thus, if low color value in the supernatant liquid was desired, treatment temperatures above 180° C„ were clearly contraindicated.

Regression analysis of a series of pilot plant heat treatment runs on primary and waste activated sludge in which holding times were varied from 10 minutes to 70 minutes and reactor temperatures were varied from 171° to 210° C. gave the following results.

Variable Regression Coeffieient Standard Error Time + 18.4 + 12.0 NS Temperature + 50.5** + 7 - 1 C Raw Sludge #0D + 26.2 + 33. NS ** Significant to 99$ confidence level NS Not a significant effect Prom this data it would be concluded that over the conventional range of operation only temperature significantly affected the filtrate color.

Similarly analysis of the filtration rates for these f-aup eea. runs showed the following effects: Variable Regression Coefficient Standard Error Time .0865 ± .069 NS Temperature .1332 ** ± .0417 Feed Solids 1.424** + .558 Again, temperature was found to be a significant factor affecting filtration rate. Holding time was found not signi icantly to affect either filtrate color or filter rate ■ in the region of conventional operation. Contrary to the above indicated effects of holding time and temperature, it has now been found that substantial improvement in sludge de-watering characteristics can be achieved with less color formation than heretofore possible by briefly heat treating the sludge at a substantially higher temperature than convention-ally employed.

According to this invention, preheated sewage sludge is briefly heated to a temperature between 190° C. and about 230° C. for a period,of time up to 240 seconds which is solids while producing less color and BOD in the supernatant liquid, and cooling the sludge before excessive color values develop in the supernatant liquid. Any sewage sludge can be employed, e.g., primary, digested, activated, preferably a ia mixture of activated and primary sludges. IndustraAl as well as domestic sewage sludges can be employed. Preferred sludges are those having very high specific resistance to filtration values, e.g., above 500 x 10? sec2/g., and for all practical purposes are unfilterable on a rotary vacuum filter. Also preferred are those whose supernatant liquid has an APHA color value of less than 1,000.

The invention will now be described with reference to the accompanying drawings, wherein: Figure 1 is a schematic flow diagram of a sewage sludge heat treatment system employing the process of this invention; Figure 2 is a graph showing the relationship of ··'; time and temperature to color formation and the dewatering characteristics of the sludge in the process of this inven- and in tion/prior art processes; and Figure 3 is a graph showing the relationship of time and temperature to the solubilization of the BOD of the sludge in the process of this invention and in conventional prior art processes.

As shown in Figure 1, a mixture of primary and secondary sludge A is pumped by a low pressure pump 1 through a grinder 2 to produce a homogenous mixture which is then pumped to a tank 2. f°r storage until used. A second low pressure pump 4 pumps a stream of the stored sludge, to a to operating pressure and pumps it through the tube side of a U-shaped counter current heat exchanger The flow rate of sludge processed by the system is controlled by a variable output pump. Optionally, a non-condensible gas (NCG) is added at this juncture to improve the heat exchange efficiency of the heat exchanger. The sludge is heated in the exchanger to a temperature a few degrees below the desired final heat treatment temperature. The heated sludge enters up-flow reaction 13_, where the sludge is maintained at final treatment temperature for the selected treatment time, which time is determined by the sludge pumping rate. The sludge is heated to final treatment temperature by injecting the requisite amount of steam into the reactor from boiler 21 .

The heat treated sludge C. flows from the reactor through overflow exit pipe 1. which is positioned below the top of reactor 13_, thus providing a chamber 16 in which gases accumulate and flow as a mixture with the heated sludge out of standpipe 1^.. If desired, the reactor can be bypassed so that only a portion of the sludge flows through reactor by partially closing valve 1£ and partially opening valve 19 .

The heated sludge then passes through the shell side of U-tube heat-exchanger £ and then through valve to a separator 22_ which separates the gaseous phase from the liquid phase. The gaseous phase is exhausted through pressure control valve 23_ which reduces its pressure to atmospheric pressure. A chemical agent, e.g. lime, alum, ferric chloride or others, can be added at the inlet of the thickening tank to further improve the characteristics of the heat treated sludge.

The cooled sludge is transferred from the separator control valve 24, to a thickening tank 2 where the insoluble solids therein are allowed to settle. The thickened sludge is then dewatered in a dewatering system 2£, typically a vacuum filtration system. The supernatant E from the thick-ening tank 2 by-passes the dewatering system 2J_, thereby increasing its capacity. The dewatered solids P from the heat treated, thickened sludge D are transferred to a solids disposal system 2£, e.g., an incinerator. The unbound liquid portion of the thickened sludge D, i.e., the supernatant E from the thickening tank and the separated liquid G from the dewatering system, is transferred to a temporary storage tank 31. A small stream of air can be passed through the tank to prevent the accumulation of unpleasant odors and/or the development of an anerobic condition therein, with the exhaust air passing into the aerators of the aerobic biological treatment system 22· All or a portion of the BOD rich effluent E and G from the heat treated sludge is then subjected to a high load aerobic biological treatment system 3 separate from the con-ventional activated sludge system S and the effluent or effluent and sludge solids from the high load biological treatment 22* which is now comparable in soluble BOD to conventional sewage, is then passed to the input side of a conventional activated sludge system ≤ Any portion of the effluent from storage tank 1 which is not subjected to a separate biological treatment is passed to the activated sludge system 25L during periods when the BOD of the sewage going to that system is below the mean level, thereby maintaining a more constant rat of slud formation the system is equipped with a solvent washing system which can be employed without shutting down the system. At periodi< intervals a solvent for the material fouling the heat exchanger surfaces, e.g., caustic and/or detergent, which is stored in solvent storage tank 3J1 is used to clean the heat exchangers. To do this, valves 11, 1£ and 4 are closed, thereby stopping the flow of sludge to the system, and valves 41, 4χ and 1£ are opened, thereby releasing solvent into the system. The solvent passes through the tube side of the heat exchanger £, through valve 1 , through the shell side of the heat exchanger £ and then back to storage tank 3_£ or through valve 43. where it can be discharged into the sewage treatment system. If desired in an alternative arrangement (not shown) the separator can be by-passed and the entire sludge-gas mix-ture passed through control valve 24 to thickening tank 25 where the gas will be vented through a scrubber.

As shown in Figure 2, the time during which the sludge is heated is inversely proportional to the selected temperature. For example, if the sludge is heated to 210° C, it should be maintained at that temperature for about 30 seconds to ensure adequately improved dewatering characteristics but not longer than about 180 seconds to avoid excessive color production in the supernatant liquid. The higher the temperature to which the sludge is heated, the narrower the range between the minimum and maximum heating times becomes until, for all practical purposes, one cannot stay within this range if "the sludge is heated to above 230° C. Below 190° C, the time required to satisfactorily improve dewater ing temperature of 200° to 225° C. for between 180 seconds and 15 seconds is preferred.

For the purposes of this invention, the dewatering properties of mixed primary and waste activated sludge are considered satisfactorily improved if its vacuum filtration rate (standard conditions) is at least 10 lbs. dry solids/ ft2/hr. Similarly the dewatering properties of digested and activated sludge are considered satisfactorily improved if filtration rates are at least 3 lbs. dry solids/ft2/hr. Su-pernatant color is considered satisfactory if below about 3000 APHA color units.

Figure 3 shows the relationship of BOD solubilization to the time and temperature of the heat treatment. As shown by the cross-hatched area, conventional heat treatments result in a 5-day BOD of the supernatant liquid greater than 6 g./l when a satisfactory improvement in filterability is achieved. Surprisingly, when following the process of this invention, the 5-day BOD of supernatant liquid is only about 2 to 6 g./l, which is highly significant improvement because the supernatant of heat treated sludge can increase the BOD loading of the activated sludge system by as much as 20 percent.

The times shown in Figures 2 and 3 are the times at which the sludge is maintained in the reactor at about the temperature shown on the graphs. This is determined by dividing the volume of the reactor by the flow rate of the stream of sludge passing through the reactor. Not included is the time during which the untreated sludge is being heated in the heat exchanger and the time required to cool the heat of the reactor relative to the volume of the heat exchanger, the more significant is this time. Generally, however, no significant increase in the color of the supernatant occurs after the sludge passes from the reactor to the heat ex-changer.

The solids concentration of the sludge is not critical but concentrations between 2 and 8 percent are preferred.

The sludge velocity through the heat exchangers is preferably between about 3 and 7 feet per second. Sludge velocity through the reactor is preferably between 5 and 10 feet per minute. The reactor temperature is maintained be-tween about 185° and 230° C, preferably between 190° and 210° C. The dwell time in the reactor is maintained between 240 180 and &66 seconds at 190° C. and between 30 and 180 seconds at 210° C. The most preferred reactor temperature is about 205° C. and the sludge is preferably maintained at that temperature at least 30 seconds but less than 240 seconds.

Because its presence significantly improves heat transfer coefficient and reduces clogging of the heat exchangers and reduces the odor of the heat treated sludge, a small amount of air, e.g., about 0.1 to 1 standard cubic feet per gallon sludge, is preferably mixed with the unheated sludge prior to its entering the heat exchangers. If only odor improvement is desired, the air can be injected into the reactor. Although odor is improved, it is not due to a re- ma duction in the Chemical Oxygen Dea«nd because this volume is too small to significantly affect the COD of the sludge, i.e., - or a mixture of the two can be used instead of air.

In operation, from about 200 to about 300 Btu's per gallon of sludge treated must be supplied thereto to ma-tain the selected reactor temperature. This additional heat can be supplied by steam injected directly into the reactor. The additional heat can also be supplied by indirect heat exchange with other hot fluids which are heated in a separate heater.

As described above, in a preferred embodiment the drainage liquor from the heat treated sludge can be maintain* in a tank to which air is supplied to aerate the liquid.

Inoculation of the aerated liquid with activated sludge or with a specific sludge forming organism which produces a readily settleable sludge, will result in high load biologica treatment system which can rapidly reduce the BOD of the liquor down to normal sewage levels or lower. For example, with a BOD of 4 lb. BOD/dayLb. MLVSS or lower a 90-95 percent reduction in BOD can be achieved i.e., from 3 g./l down to 0.3 g./l or lower. Such a system can be operated on a fill and draw principle in which the liquor is aerated in the presence of the sludge-producing organisms as it is received but without overflow and once a day is allowed to settle. The treated supernatant is decanted to the activated sludge system, preferably when the BOD of the sewage influent is lowest A portion of the accumulated solids is withdrawn and mixed with other sludge solids for heat treatment.

The high load biological treatment system can also be operated on a continuous flow basis with the average dwell rate adequate to achieve this reduction. The overflow is then transferred to the influent end of the activated sludg< system.

EXAMPLE 1 The following is an example of a sludge heat tres ment system which employs the process of this invention.

A mixture of primary sludge and thickened activated sludge containing 22 g./l insoluble solids is pumped at the rate of 3 gal./mln. through a heat exchanger and reactor as shown in Figure 1 having capacities of 2.5 gal. (tube side and 1.8 gal.., respectively, at a pressure of 315 psi (gauge), Sufficient steam was injected into the reactor to maintain it at 210° C. and provide the requisite 80° for the heat exchanger. The velocity of the sludge in the tubes of the heat exchanger was 3.5 ft./sec. and 16 ft./min. in the reactor, thus providing dwell time in the reactor of 30 seconds. The cooled sludge had a vacuum filtration rate of 15 lbs. dry solids/ft2/hr. Only a small portion of the insoluble solids was solubilized. The supernatant liquid had an APHA color value of 1200 and a BOD of 2.2 g./l. Conventionally heated sludge has a comparable filtration rate. However, the supernatant BOD and APHA color values are to 6 g./l and 3000-4000, respectively.

EXAMPLE 2 Under comparable conditions, pumping a mixture of primary and secondary sludge containing 40 g./l insoluble solids at a rate of 60 gal./min. through a heat exchanger ha ing a 0 gal. (tube side) and 30 gal. (shell side) capacity at ressure of 4 0 si and at a velocit of 6 ft . sec. and I tained by steam Injected therein at a rate which maintains the reactor temperature at 210° C. and a Δ¾ in the heat exchanger of 18° C, the reactor being of a capacity which provides a dwell time at that temperature of 30 seconds and ther 5 through the shell side of the heat exchanger produces a sludg having a vacuum filtration rate of 15 lbs. dry solids/ft2/hr. a supernatant BOD of 4.0 g./l and APHA color value of 2000.

That sludge heat treated conventionally has an effluent BOD of 7 g./l and an APHA color value of 4000.

EXAMPLE 3 In a run otherwise similar to Example 1, with a sludge flow at 150 gallons per hour, 1.25 standard cubic feet of air per gallon of sludge was mixed with the sludge prior to entering the heat exchanger. This amount of air is insuf- 15 ficient to significantly alter the BOD of the sludge by wet air oxidation, i.e., it reduces it by only about 1 to 4 percent. The sludge had a bulk velocity of 1.36 fps at the tube inlet end and 1.79 fps at the tube outlet end, compared with 0.93 and 1.01, respectively, in the absence of the air. The average heat transfer coefficient in the absence of air was 70 and 108.5 in the presence of air, an increase of 55 percent. Even at 400 gph, an increase in heat transfer coefficient from 106.5 to 142.5, a 34 percent increase, was achieved The odor of the heat treated sludge was substantially better than that of sludge heated in the same way in the absence of air.

In some instances, particularly with heavy sludges, settling can occur in heat exchangers which leads to blockage heat exchangers and prevents this. Surprisingly the increase in heat loss through the heat exchangers by the addition of the air is negligible.

Claims (12)

1. A process for reducing color formation and solubilization of BOD in the heat treatment of sewage sludge to improve its dewatering characteristics, which comprises rapidly heating sludge for up to about 240 seconds to a temperature between about 190° and about 230° C, the heating period being inversely proportional to the selected temperature and just sufficient to increase the filterability of the sludge to at least 10 lbs/ft2/hour in the case of primary sludge and 3 lbs/ft2 hour in the case of activated digested sludges, and cooling the sludge before excessive color values develop in the supernatant liquid.

2. A process according to claim 1, wherein the sludge is heated to a temperature between 200° and 225° C. for a period of time between about 180 seconds and 15 seconds.

3. A process according to claim 2, wherein the sludge is heated to about 210° for about 30 seconds.

4. A process according to any one of the preceding claims, wherein the process is continuous and a stream of said sludge is preheated under pressure from a temperature below 100° C. to a temperature above 150° C. by indirect co ntercurrent heat exchange with a continuous stream of heated sludge, the sludge is immediately thereafter heated to the temperature between about 190° C. and about 230° C, and Immediately thereafter the heated sludge is rapidly cooled to a temperature below 100° C. by indirect countercurrent heat exchange with a stream of unheated sludge being preheated.

5. storing the heat treated sludge and periodically returning the supernatant liquid portion of the stored sludge to the input end of a secondary biological sewage treatment system during a low load period of the day.

6. A process according to claim , wherein the secondary biological sewage treatment system is an activated sludge system.

7. O A process according to claim 6, wherein a stream of air is passed through the heat treated sludge during its storage period and then through the aeration system of the activated sludge system.

8. A process according to any one of claims I . to 7> wherein the sludge is heat treated in an elongate heat treatment zone through which the sludge travels vertically upwardly to an exit positioned below a gas filled area which acts as a liquid level regulator and pressure surge control, Q

9. A process according to any one of claims ij. to 8, wherein the sludge to be heat treated is preheated as a mixture with an amount from about 0.1 to 1 cubic foot of a non-condensible gas per gallon of sludge which increases substantially the efficiency of the heat exchange from the heat treated sludge to the unheated sludge.

10. A process according to claim 9, wherein the non-condensible gas is air,

11. A process according to any one of the preceding claims, wherein the sewage sludge is a mixture of primary and activated sludge.

12. A process for reducing color formation and solubilization of BOD in the heat treatment of sewage sludge substantiall as herein described with reference to the Exam les.