.... 1%

~ Sludge dewatering and disposal in the and paper industry Herman R. Amberg

In the last 10 years the pulp and paper industry, like other 50% are generally associated with the manufacture of coated industries in the U. S., has made tremendous strides in reducing board, deinked pulp and paper, and integrated and non-inte- its biochemical oxygen demand (BOD) and total suspended grated fine coated papers. Primary sludges are low in heavy solids (TSS) discharges. Today, over 95% of the U. S. pulp and metals and can readily be used as agricultural soil amendments. paper capacity is receiving primary and secondary treatment. Ease of dewatering depends on the pulping process, additives, Primary treatment plants effectively remove more than 90% of and the length of the fraction. Sludges derived suspended solids and 20% of applied BOD, whereas secondary from the groundwood pulping process are generally very difficult treatment systems have been designed to remove an additional to dewater because most of the material passes through a 200- 80% to 90% of BOD. mesh screen. In general, sludges with a high proportion of fines There are about 368 mills that discharge directly. Of these, (material passing 150- to 200-mesh screens) are very difficult 156 are equipped with aerated stabilization basins, 68 use ac- to dewater. tivated sludge treatment, and 144 use a variety of treatment There are about 100 high-rate biological treatment systems systems. The major BOD removal in the industry is accom- throughout the industry that generate excess biological sludges plished by aerated stabilizationbasin (ASB) and activated sludge requiring separation and disposal. The most significant char- treatment (AST) systems. These two treatment systems treat acteristic of these sludges is their hydrous nature which makes 8.14 billion (2. I5 billion gal) of wastewater containing 2740 them very difficult to dewater. Again, as would be expected, t/d (6.04 million Ibs/d) of BOD. these sludges are low in heavy metals and ash and contain anywhere from 6 to 10% nitrogen and 2 to 5% phosphorous. Because sludge characteristics vary, it is difficult to generalize about dewatering and disposal practices. In addition, land avail- The most widely used dewatering processes ability and proximity to densely populated areas add to ultimate in the industry today are vacuum filters, disposal problems. belt filter presses, and centrifuges. This presentation is an attempt to summarize several systems commonly used in the pulp and paper industry to dewater and dispose of primary and secondary sludges produced by the var- An disadvantage of this high degree of treatment is the large ious treatment systems. The examples cited are taken from amount of both primary and secondary sludges generated. It is Crown Zellerbach mills, which are representative of modem estimated that the U. S. pulp and paper industry produces about industry practices. 1.9 million t/a (2,l million ton/yr of primary sludge). Secondary or biological sludge is generated at a rate ranging from 0.2 to PROCEDURES 0.5 g solids/g BOD destroyed. The 68 activated sludge plants All pulp tonnages reported in this paper are based on a 90% produce 95 560 t/a (105 437 ton/yr) of biological sludge and dry fiber and 10%moisture content. Paper production is reported the 156 ASB plants produce 120 OOo t/a (132 055 ton/yr). A- at an off-machine moisture content Of 5%. Sludge tonnage figures though the biological solids produced in the ASB units are not ax 011 MI avei: dry basis an6 wherever tonnages are used they cositinuously removed, they must ultimately be removed. The are metric tons followed by the common engineering units in sludge disposal problem is being delayed until such time that parentheses (short tons). deposition interferes with performance and sludge removal be- comes necessary. RESULTS The rate at which sludge is produced places limitations on the use of simple disposal systems such as lagooning of low- Ch" Zellerbach Corporation sludge dewatering and disposal solids sludges or landfilling of dewatered sludges. The industry systems. Mill A. This mill produces about 544 t/d (600 ton/ uses a wide variety of dewatering and disposal systems, and day) of linerboard from waste corrugated boxes. About 635 many are innovative. t/d (700 ton/day) of waste boxes are used to maintain this op The Primary sludges produced by the different production eration. Changes in sludge disposal have taken place at this mill subcategories of the U. S. pulp and paper industry vary widely since 1973, which brought to light some of the problems as- in respect to organic and ash content. Although the predominant sociated with land application disposal systems. All of the meth- constituent in most sludges is cellulose fiber, ash content can ods in this section use land application of the fibrous and bio- VarY from 5 to 60%. Sludges with ash contents of more than logical solids removed by the wastewater treatment facilities.

Journal WPCF, Volume 56, Number 8 Phase Z (1973-1977). From 1973 through 1977, so’me 9.1 with the primary and secondary sludges and the mixture is t/d ( 10 ton/day) of primary sludge were produced. This material dewatered to a solids content of about 19%. The combined (3% solids content) was pumped to a 6.9-ha (1 7-acre) field ad- dewatered sludge is trucked to a composting operation run by jacent to the mill property which had previously been used as an outside contractor. The mill pays the hauling costs and the an almond orchard. The soil was a sandy loam with good drain- contractor sells the compost produced in bulk to fanners in the age characteristics. area. Dewatering costs for the combined sludge are about $39.461 The sludge was pumped to the field and spread in a 76- to t ($35.80/ton) and hauling costs to the composting area are 102-mm (3- to 4-in.) layer, solar dried for 5 to 8 days and then $17.09/t ($15.50/ton). rototilled. No fertilizer was added to the soil-sludge mixture Although the present system is very costly to operate, it seems and decomposition of sludge could undoubtedly have been en- to be less costly than other available disposal systems such as hanced by the addition of nitrogen and phosphorous. The sludge, incineration. In addition, the final product provides a positive which was added at a rate of 516 t/ha-a (230 tonJyr/acre) value because it is an excellent soil conditioner. did not decompose rapidly enough to keep up with the appli- Direct land application of dewatered sludge as previously cation rate and there was a gradual increase in volume of the practiced at this mill could probably be made to work without soil-sludge mixture during the 4-year period the system of natural the problems described, but considerably more land would be decomposition was used. After about 2 years of sludge appli- required than the 6.9 ha (17 acres) available to the mill. For cation, soil volume had increased about 0.3 m (1 ft). In 1975, example, at an application rate of 9 1 t/a (100 tonlyr), 63.5 ha about 12 600 m3 (16 500 cu yd) of soil were removed and in (157 acres) of land would be required. This amount of land is 1976 another 7 030 m3 (9 200 cu yd). The total quantity removed not available. was 19 650 m3 (25 700 cu yd). Removal costs at that time were Mill B. Crown Zellerbach’s Mill B produces 907 t/d (loo0 $1.82/m3 ($1.391~~yd). ton/day) of refiner groundwood pulp and fully bleached kraft Odors were another problem associated with the project. The pulp, which is converted on four paper machines to 871 t/d land disposal system was located adjacent to a residential area (960 tonlday) of paper. and during the rainy season it was impossible to keep the area Primary treatment was installed in 1967 and secondary treat- dry. Odors that resulted from anaerobic conditions were a con- ment in 1975. Initially, two solid bowl centrifuges were used stant source of complaints from neighboring property owners. to dewater the primary sludge. Because of the abrasive name It was also difficult to get heavy equipment on the land for the of the sludge (35% ash), excessive wear of the moving parts of rototilling operation. the centrifuge was a continuing problem which resulted in very Although this disposal method was relatively inexpensive, high operating and maintenance costs. When the centrifuges there were a number of operational problems, particularly during started to wear, solids capture efficiency deteriorated rapidly. the rainy season, which precluded its use on a continuous basis. Frequent costly rebuilding of the machines was necessary to Phase II (1977-1981). To cope with some of the land ap maintain operational efficiency. plication problems, the sludge was dewatered prior to land ap In 1975, when the activated sludge plant was built, the cen- plication. Another problem was the addition of waste-activated trifuges were replaced with three coil spring vacuum filters, each sludge from the activated sludge plant which went into operation with a diameter of 3.5 m (1 1.5 ft) and a face width of 4.9 m in 1977. A 2-m (80-in.) belt filter press was installed to dewater ( 16 ft). The units were designed to handle primary and secondary 18 to 23 t/d (20 to 25 tonlday) of primary and secondary sludge. sludge at a rate of about 24.4 kg/mz - h (5 lb/hrJsq A). When A 3: 1 mixture of primary and secondary sludge was fed to the mixtures of primary and secondary sludge were dewatered, a press, and a cake with a solids content of 15 to 20% and an cake of low solids content was produced (13%). This wet cake ash content of 19% was produced. The dewatered sludge was caused landfill operational problems; heavy equipment could trucked to the application area and rototilled into the soil. not be used on the landfill to spread the sludge. Although this improved the general operational conditions, The primary and secondary sludges were separately dewatered it did not completely eliminate the problems. During periods to prolong the life of the landfill area and to eliminate the of heavy rain, it was impossible to get heavy equipment on the operational problems. A 2-m belt press was installed in 1979 land for rototilling. Ponding produced odors followed by com- to dewater the activated sludge and the coil filters were used plaints from the adjacent residents. Although this disposal system for primary sludge. The coil filters treat 17.3 t/d (19.1 tonlday) had its shortcomings, it was used from 1977 to 1981 and was of primary sludge (solids content 2% to 3%) and produce a cake very economical. For example, the 1979 disposal costs averaged with 20% solids content. The primary sludge can now be pneu- about $12/t ($1 l/ton). Costs in 1982 dollars were calculated to matically conveyed to landfill and it is sufficiently dry to take be $15.08/t ($13.68/ton). . heavy equipment at the landfill. Phase III (1981-present). A major change was made in the The belt press receives 9.8 t/d (10.8 tonlday of secondary mill operation in 198 1 which complicated the dewatering pro- sludge (solids content 1.5% to 2%). With the aid of high cess. “Crill,” very small particle-size material removed from the additions, this sludge is dewatered to a solids content of about waste paper pulping process, was added to the primary and 14%.The secondary sludge is landfilled in a separate area. Mois- secondary sludge. This material generally passes a 200-mesh ture content is too high to accommodate landfill equipment. screen and its dewatering characteristics are quite similar to In 1983 and 1984 the primary and secondary sludges will be those of activated sludge. dewatered on the coil filters to a 13% solids content and trucked A flotation save-all was installed to thicken the crill to a 7% to a hardwood plantation that the company has started 5 km solids content. Sixteen t (18 ton) of crill were treated each day. (3 miles) from the mill. The sludge will be stored in a sludge Another belt press was installed which had a design capacity pit and sprayed on the plantation after dilution to 1 to 3% somewhat greater than the original. The thickened crill is mixed solids content. This means of disposal will be used in 1983 and

August 1984 963 1984 on a trial basis. If successful, a pipeline will be installed and the combined sludge will be pumped to the plantation for spray application. The estimated cost for this operation is $50.08/ t ($45.42/ton) assuming a sludge production of 9 747 t/a (10 744 tonlyr). Mill C. Mill C produces 1 13 t/d ( 125 tonlday) of groundwood pulp and 363 t/d (400 ton/day) of coated magazine and book papers. The treatment system consists of a primary clarifier and an aerated stabilkation basin. Primary treatment became operational in 1968. The initial dewatering equipment consisted of two large solid bowl cen- trifuges followed by two V-type presses. The dewatered sludge was incinerated in a waste boiler. The high-ash, hydrous sludge was very difficult to dewater. For example, solids capture efficiency for the centrifugesaveraged about 75% and cake solids content was 15%. The V-press increased the solids content to Figure 1-Air entrained incinerator. about 20%. In 1972, an aerated stabilization basin was installed and the centrifuges were replaced by a coil spring vacuum filter, 3.5 m Initial dewatering takes place on two coil spring vacuum filters, in diameter by 4.9 m face length (1 1.5 fi X 16 ft). The primary each 3.5 m in diameter by 3.0 m in face length (1 1.5 fi X 10 underflow had a 3% to 4% solids content and 45% ash content. ft). These units dewater the sludge to 18% solids content at a The ash was predominantly coating clay lost in the papermaking loading rate of about 39 kg/m2 - h (8 Ib/hr/sq ft). Overall solids process and fiber fractionation studies conducted on the sludge recovery is more than 85%. indicated that most of the material would pass a 200-mesh The air entrained dryer-incinerator shown in Figure 1, starts screen. with a V-type press which removes liquid to increase the solids The coil filter dewatered 27.6 t/d (30.4 tonlday of sludge to content to 40%. From the top of the V-press, the cake is dis- a solids content of 17% to 20%. The V-press increased solids charged to a screw conveyor which transports it to a high-speed content to between 30% and 35%. Because of the high moisture hammermill to break up the cake for improved drying and and ash content, burning dewatered sludge presented serious incineration. The sludge then drops into the hot gas stream problems in the efficient operation of the wood waste fired boiler. from the incinerator and is carried up through the drier. Dried To ensure proper combustion and compliance with air emission sludge and conveying gas are separated in a cyclone. The gas standards, oil had to be fired as supplementary fuel when sludge is drawn through the entire process by a large induced-drafi was added to an already high moisture content wood waste. fan. Water is sprayed into the fan to scrub out particulate matter Ash disposal presented another problem. Incineration costs in- and the water containing the particulate matter is separated creased with the price of oil, and another less costly disposal from the gas in a cyclone. method was sought. It was estimated that sludge burning in the Dried sludge at a solids content of 70 to 85% is then air- power boiler resulted in an operating cost or penalty of about conveyed to the incinerator where it enters centrifugally at the $291/t ($264/ton) of sludge burned. top and is burned. Natural gas is burned in the lower part of In 1982, a contractor agreed to use the sludge in his compohng the incinerator to bring the unit to operating temperature during operation. The sludge (20% solids content) was trucked to two start-up, and to furnish supplementary heat when the fuel value composting sites about 32 and 48 km (20 and 30 miles) from of the sludge is low. Ash is cooled with a water spray and is the mill. The compost is sold to fmers in the area. transferred by conveyor belt to an adjacent landfill area. In 1983, a 1.5-m belt filter press was installed to replace the The sludge contains about 20% to 40% ash which has con- vacuum filter and V-press. The sludge is now dewatered to a tributed to erosion of high velocity areas of the drier, cyclone, solids content of 35% and trucked to the compost sites. The and ducts. Flatbacks were installed at elbows and places ofwear. Wiper blades on the air-lock valve required frequent operating cost, exclusive of depreciation and interest on in- rotary replacement. of hardened alloy tips flurer hammers vestment, was $7!.70/t ($65.05/ton) of sludge fo: 1982. De- Use on reduced maintenance cost. Downtime on the incinerator has watering costs were calculated to be about %39.68/t (36.001ton) been taken for slag removal, modifications to the ash discharge, of sludge. and replacement of the refractory. The latter wears out by erosion Mill D. Crown Zellerbach’s Mill D produces about 998 t/d in areas of high velocity. (1 100 ton/day) of pulp and paper. Chemical pulp is produced The other major maintenance item has been the wet fan by the sulfate and sulfite processes and all pulp is converted to dynamic scrubber and its cyclone separator following the fan. a wide variety of paper products on 15 paper machines. The original equipment was made of mild steel, and the fan The treatment system consistsof primary clarification followed housing and separator body were mild steel coated with as- by two aerated stabilization basins operated in series. The pri- phaltum. Because of erosion and corrosion, these items were mav treats 45.4 t/d (50 ton/day) of fibrous sludge.-, which replaced with stainless steel. This greatly reduced maintenance at a solids costs. system. A Typical annual operating and maintenance costs, in 1983 which was dollars, are presented in Table 1. Annual operating costs for 1, is shown in f %re 1. sludge dewatering, drying, and incineration are $345 100 or

Journal WPCF, Volume 56, Number 8 Industrial Wastes

Table 1-Annual operating and maintenance costs for of sludge at a solids content of 3 to 4% are pumped to a 3.5m sludge dewatering, drying, and incineration at Crown X 4.3m coil spring filter (1 1.5 ft X 14 ft). Polymer is added to Zellerbach's Mill D (costs in 1983 dollars). condition the sludge. The filter cake is discharged at a 15 to ~ 20% solids and drops into a V-press where it is dewatered to a item Doilars/year X of total solids content of about 33%. The dewatered sludge is then con- veyed to a fluffer which discharges to an upflow oil-fired drier. Operating labor 71 OOO 20.6 The gas-entrained sludge is conveyed to a cyclone and then air- Repair labor 53300 15.4 conveyed at a solids content of 54% to the wood waste steam Repair materials 54900 15.9 Misc. supplies 900 0.3 boiler for incineration. The dewatering equipment is located in Electricity 68OOO 19.7 the boiler room and is controlled by the boiler operators. Natural gas 97 OOO -28.1 Although very little, if any, waste-activated sludge is added Total direct 0 & M costs 345 100 100.0 to the dewatering and drying system, polymer conditioningaids are used to dewater the primary sludge. Polymer costs during 1982 averaged about $6.'44/t ($5.84/ton) of sludge dewatered. $26.42/t ($23.97/ton) of sludge destroyed. Approximately 24% The sludge contains less than 10% ash, and is not overly of the total operating and maintenance cost is associated with abrasive to the conveying system and drier. The drier has op dewatering, and 76% is associated with drying and incineration. erated for 10 years and is still in good working order. No serious Only primary sludge is dewatered, so are not required. operational problems have been encountered in burning the After 12 years of operation, during which considerabledown- material in the steam boiler and because the material has been time for repairs was taken, the system has been shut down and partially dried, it is incinerated without any steam penalties in the dewatered sludge is now conveyed to a landfill area adjacent the power house. to the treatment plant. The drier and incinerator are completely The sludge dewatering, drying, and conveying system was worn out and would have to be replaced. A Gecision as to installed in 1972 at a capital cost of $703 OOO. A breakdown ultimate disposal has not been made. However, because several of the annual operating and maintenance costs for 1982 are years of landfill capacity are available, the mill will probably presented in Table 2. Total annual operating and maintenance use this less expensive alternative for as long as possible. costs in 1982 were $3 16 489 or $54.85/t ($49.76/ton) of sludge Mill E. Many mills have at least one boiler fired with wood burned. The major operating cost item is fuel oil to fire the residues or coal, and these boilers can be used to bum the excess drier, which amounted to $22,18/t ($20.67/ton) of sludge dried sludge. Crown Zellerbach operates two such systems. A de- from a solids content of about 33% to a solids content of 54%. scription of the system at Mill E demonstrates the effectiveness The sludge dewatering and drying systems account for about of this disposal system. 43% of the total treatment plant operating and maintenance Mill E produces refiner and themal mechanical groundwood costs. pulp and about 38 1 t/d (420 tonlday) of newsprint and directory Other miscellaneous operations. Crown Zellerbach operates papers on two paper machines. The primary system was installed several other plants that dewater and dispose of primary sludges. in 1972 which included facilities for dewatering and drying the Two of these operations use waste paper to make tissue and sludge. The dried sludge was conveyed and burned in a wood toweling and have deinking operations. Deinking results in a waste-fired boiler. Secondary treatment (activated sludge) was high-ash sludge with a large percentage of very fine particulate installed in 1978. In 1982 the dewatering and drying system material which makes dewatering quite difficult. Both of these received 15.4 t/d (17 tonlday) of primary sludge and 0.4 t/d mills use belt presses which are capable of producing a cake of (0.4 tonlday) of waste activated sludge. about 27% solids content. For example, Mill F, which has a A schematic flow sheet of the sludge dewatering and drying very low deinking capability, produces 7.2 t/d (7.9 tonlday) of system is shown in Figure 2. About 16.1 t/d (17.7 tonlday) primary sludge containing 30 to 35% ash. Cake solids from the belt press in 1982 averaged 27%. Operating and maintenance costs were $40.37/Mg ($36.62/ton) of sludge dewatered and hauling costs to a landfill area 10 km (6 miles) from the mill ,,,, THICKENER /"\ II

Table 2-Annual operating and maintenance costs for r sludge dewatering, drying, and burning at Crown Zel- lerbach's Mill E (costs in 1982 dollars).

Item Doliars/year % of total

Operating labor 21 900 6.9 Repair labor 18 256 5.8 Repair materials 46 815 14.8 Misc. supplies & expenses 40 178 12.7 CONVEYOR AIR FAN Electricity 20 709 6.5 Polymer 37 146 11.7 Fuel oil 131 485 -41 -5 Total 0 & M costs 316 489 99.9 Figure 2-Dry sludgeburn with hot fuel.

August 1984 965 were $18,34/t ($16.64/ton). Total sludge dewatering and disposal Table 4-Loading and performance data for coil spri costs averaged $58.7 l/t ($53.26/ton). vacuum filters in dewatering primary and second The other wastepaper operation presently produces about sludges. Data from Crown Zellerbach mills. 100 t/d (90 ton/day) of deinked pulp and 141 t/d (128 ton/ day) of paper on two paper machines. The mill has a primary Average SIu d g e, system and is in the process of installing secondary treatment. loading rate % solids Twenty-eight t/d (3 1 ton/day) of primary sludge are produced and dewatered by centrifuge to a solids content of 13%. The Ib/hr/ % Solids ash content of the sludge averaged 35% in 1982. The centrifuge Type of sludge Kg/m2.h sq ft Feed Cake recovery was replaced by a belt press in 1983 and cake solids are now expected to be about 28%. Total costs for dewatering, hauling, Primary sludge and landfill disposal in 1982 averaged about $45.20/t ($41.011 Integrated pulp ton) of sludge. Dewatering and disposal costs are expected to paper 39 8 3.5 22 >95 significantly increase when waste activated sludge is added to Groundwood 29 6 3.0 20 85-95 the primary sludge because the combination of sludges win be Deink, tissue 29 6 4.0 25 85-95 very difficult to dewater. Combined sludge' P:S-2 15 3 2.5 15 85-90 DISCUSSION P:S-3 20 4 2.5 18 85-95

Minimizing sludge disposal costs centers on the dewatering (a) Primary and waste activated sludge. operation. This is particularly critical if final disposal is destruc- tion in an incinerator or a power boiler. The more water that is removed mechanically prior to thermal drying and combus- The belt press is a recent development and most units have tion, the lower the overall disposal cost. For example, at a solids been installed in the last 5 years. content of 20% the energy penalty is about $38.58/t ($35/ton) Crown Zellerbach's primary mills initially used solid bowl of sludge burned and this is reduced to $13.23/t ($12.00/ton) centrifuges for dewatering and these were later replaced by coil when sludge is dewatered to a solids content of 34%. spring vacuum filters. Operational performance of the coil filters Rosenfeld et al.' reported on field trials in which various has generally been satisfactory; maintenance costs have been amounts of dewatered sludge were added to wood waste and low and loadings and solids capture efficiencies high. Typical then fired in a commercial wood-waste boiler. In one trial, sludge loading and performance data are presented in Table 4. Loading, at a 37% solids content and 20% ash content was added to the cake solids, and solids capture efficiency are significantly de- wood waste to constitute 22% of the fuel mixture. The thermal creased when secondary sludge is added to the primary sludge. efficiency of the boiler was reduced 7.7%. In addition, the amount of polymer required for dewatering The trend in the last 10 years has been toward more efficient increases significantly as the amount of secondary sludge in the dewatering equipment in the pulp and paper industry. Solid combined sludge mixture increases. A V-press following the bowl centrifuges were almost exclusively used for dewatering filter can be expected to produce a sludge with a solids content some 10 to 15 years ago, and many of these units have now of 32% for primary sludge and a 23 to 25% solids content for been replaced. An industry survey conducted by the National combined primary and secondary sludges. Council for Air and Stream Improvement (NCASI) is sum- Data from four operational belt presses at Crown Zellerbach marized in Table 3 which shows a breakdown of the dewatering mills are shown in Table 5. In refemng to Table 5, it can readily technologies used in the industry. Vacuum filters and belt presses be Seen that dewatering activated sludge is very difficult, even seem to be the most widely used types ofdewatering equipment. with relatively high polymer addition. Experience indicates that solids contents between 12 and 16% can be expected which Table 3-A breakdown of dewatering technology in the is still a very wet cake. Polymer costs for dewatering secondary U. S. pulp and paper industry.' sludge are very high. The example in Table 5 shows that polymer costs for Press B are $49.6O/t of sludge dewatered ($45.00/ton). Dewatering Number of mills % of Some additional performance data from belt presses dewa- technology using iechndogy ioiai teikig various combinations of primary and secondary sludges are presented in Table 6. Again, the impact of secondary sludge Vacuum filters 71 29.0 on overall press performance is clearly demonstrated. A mixture Pond or lagoon 67 27.3 of primary and secondary sludges is generally much easier to Belt presses 35 14.3 dewater and solids contents between I6 and 25% can be expected Centrifuges 23 9.4 with optimum polymer dose rates. Primary sludges can be de- Screw presses 16 6.5 V-presses 8 3.3 watered to solids contents of 25 to 35% at low polymer Flotation 5 2.0 addition rates. Hewitt et aL2 showed by pilot plant studies that Filter presses 4 1.6 a diaphragm press can achieve solids concentrations of about Heat drying 1 0.4 45%, but the capacity of these units is very low and the capital Other misc. presses -15 -6.1 and operating costs are very high. With few exceptions, press Total 245 99.9 operation and maintenance at pulp and paper mills using dia- phragm presses were more complex than originally envisioned. Data supplied by National Council for Air and Stream Improvement. Adequate allowance must be made for down time. Present ex-

966 Journal WPCF, Volume 56, Number 8 .. Industrial Wastes

Table 5-Belt press performance review at four different Crown Zellerbach installations.

TSS, Belt Throughput Polymer cost Mill production press Sludge. Feed Cake Capture tid sit

Linerboard from wastepaper A P:S = 2.5 2.0 18 95 14-23 19.84 TMP groundwood B S 1.9 13 86 9-12 49.60 Wastepaper deinking C P 3.0 30 98 27-45 7.72 Wastepaper deinking & tissue from waste C P 2.0 27 95 >20 5.51 Groundwood pulp & coated paper grades D P 3.0 35 - 27 -

a P = primary sludge; s = secondary sludge.

perience, exclusive of start-up operations, indicates that an al- posting, sale, and land spreading. However, these practices ap lowance of one shift per day is required for maintenance. plied to only about 13% of the sludge reported. The remaining It has been shown that dewatering can be improved by adding 87% was landfilled. Incineration represented the major man- long found in primary sludge. It must, however, be noted agement alternative reported and accounted for 11% of the that fiber recovery for reuse makes the sludge more difficult to total. dewater. Fiber fractionation of the primary sludge indicates that A summary of the data on sludge disposal by landfill in 1979 a very high percentage of the sludge will pass a 200-mesh screen. is presented in Table 7. A total of 5679 t/d (6260 tonlday) of This material does very little to improve the dewatering char- sludge is landfilled by 167 mills in the U. S. Of the total sludge acteristics of combined primary-secondary sludge. Ground bark landfilled, only 5.4% was secondary sludge and the remainder or coal can be added to these difficult sludges to improve their was primary or a mixture of primary and secondary. dewatering characteristics. This is being done at several U. S. A breakdown of disposal methods other than landfilling used and Canadian mills. in 1979 is shown in Table 8. Incineration in power boilers was Centrifugation of secondary sludge is practiced at several mills the most widely used method and accounted for 655 t/d (722 using either a disc-type high-speed centrifuge similar to those ton/day). Four separate incinerators were in use in 1979 for used in the fermentation industry or a solid bowl centrifuge. burning primary sludge. Although only one mill reported a Pilot trials reported by Reilly3 showed that solid bowl centrif- composting operation in 1979, there are several other mills ugation with polymer additions could produce a cake with 1 1% practicing composting today. Land application is beginning to solids at solids recovery efficiencies in excess of 99%. Because be used more frequently and is expected to increase in the next of the low ash content and the absence of grit, secondary sludge 5 to 10 years. dewatering has reasonable associated maintenance costs. The practice of landfilling can be expected to gradually de- Although sludge dewatering is a difficult and costly operation, crease as available land becomes more scarce and as regulatory it represents only part of the overall disposal problem. Ultimate requirements become more restrictive. In general, industry de- disposal of sludge is becoming more difficult as public concern watering and disposal costs for sludge will increase and represent over secondary environmental impacts such as odor, ground- a high proportion of total waste treatment plant operating and water contamination, and aesthetics restricts the selection of a maintenance costs. The industry is just now beginning to develop politically acceptable disposal system. Although the industry its long-range options which are based on sludge utilization. today uses a variety of disposal systems, landfill disposal is by The reason for this shift is probably the increasing difficulties far the most widely used. encountered with siting new land disposal facilities. A 1979 survey ofthe U. S. pulp and paper industry conducted A fact often overlooked is that about 40 to 45% of total by the National Council for Air and Stream Improvement4 wastewater treatment costs are associated with the sludge de- found that mills in 22 states reported use of incineration, com- watering and disposal. The operating and maintenance costs, exclusive of depreciation and interest on investment, for sludge Table 6-Performance data for belt filter presses de- watering primary and secondary sludges. Table 7-Summary of data on sludge disposal by landfill Actual Output, metric by U. S. pulp and paper industry (1979 NCASl survey). Sludge Polymer cost cake solids tonslmeter ratio $/metric ton YO width No. of mills practicing landfill disposal ~~ ~ Primary sludge 110 Primary Secondary sludge 9 sludge 5-1 1 25-35 10-20 Mixed sludge 53 P:S-2.0 22-33 20-25 8-15 P:S-1.5 28-39 18-25 7-1 5 Total dry weight of sludge landfilled P:S-1 .o 33-44 16-20 5-10 Primary, t/d 3021 Secondary Secondary, t/d 308 sludge 33-100 13-1 6 4-8 Mixed, tld 2350

August 1984 967 Incineration in power boilers 20 1 11 393 3 259 Incineration in separate incinerators 4 0 0 93 0 0 Composting 0 0 1 0 0 4 Sale to others 2 0 2 15 0 22 Land application 7 7 1 77 40 20

dewatering and disposal at eight Crown Zellerbach mills are three-effect falling-film evaporator train for water removal. The shown in Table 9. Disposal costs vary widely from a low of patented Carver-Greenfield process is designed to maintain flu- $24.33/t of sludge ($22.07/ton) to $7 1.72/t ($65.06/ton). The idization of solids throughout the evaporation process and flu- average cost for the eight mills under consideration was $5 1.OS/ idization is maintained by mixing six parts of tallow to each t ($46.31/ton). The sludges handled by the eight systems vary part of sludge solids. After water removal, the mixture of oil from easily dewatered primary sludge to the more difficult pri- and biological solids is pumped to a centrifuge and a screw mary and mixed primary secondary. Mills H and D handle press for oil extraction. The final product is sold as an animal sludges that are relatively easy to dewater. The average disposal feed and contains about 16%tallow and 1% moisture. Although cost for these sludges is $33.47/t ($30.36/ton). The low disposal the system successfully dewaters a very difficult sludge, capital cost of $24.33/t ($22.07/ton) for Mill H does not include a and operating costs are substantial and unless a market can be penalty for loss of boiler efficiency and if this penalty were found for the product, the process seems to have limited ap- included a more realistic cost would probably be about $40/t plication in the pulp and paper industry. ($36/ton). The remaining mills, which handle sludges that are Several installations in the U. S. pulp and paper industry use more difficult to dewater, experienced an average disposal cost wet oxidation or thermal conditioning of sludge to improve of $55.91/t ($50.72/ton) and a range of $45.21 to $71.72/t dewatering efficiency and reduce the quantity of sludge that ($41.01 to $65.06/ton). must be landfilled. At one mill, where thermal sludge condi- An innovative process for dewatering and drying waste-ac- tioning is used preceding a belt press, activated sludge and tivated sludge has recently been installed at a Pacific Northwest primary sludge are processed and dewatered to a solids content mill and was described by Evans? This mill produces more of about 40% with a solids capture of 97%: Polymer addition than 18 t/d (20 ton/d) animal feed supplement with a protein rates are minimal. Savings of approximately $2900/day in sludge content of 45%. The waste sludge (2% solids content) is pumped conditioning, dewatering, and landfilling costs are claimed over to five belt filter presses for dewatering to a solids content of the former polymer-conditioning operation. Since start-up of 12%. High polymer doses are required for effective dewatering the system, the sludge volume for disposal was reduced by 60 and high solids capture efficiency. The partially dewatered ac- to 70%. tivated sludge is mixed with rendered tallow and pumped to a Although the introduction of novel disposal systems using new technologies can be expected to steadily increase in the future, landfill and incineration will probably remain the two Table 9-Some typical operating and maintenance costs' most widely used disposal methods for the remainder of the for sludge disposal by Crown Zellerbach pulp and paper decade. mills.

Type of Diaposal SUMMARY AND CONCLUSIONS Mill sludgeb Disposal system cost. $It

H P Dewatering + incineration in power 24.33 Over 95% of the U. S. pulp and paper industry capacity boiler receives primary and secondary treatment. Overall BOQ and D P Dewatering, drying, incineration in power 42.61 TSS removal exceeds 90%. One drawback to this high degree boiler of treatment is the large amount of primary and secondary sludges generated; the industry produces an estimated 2.1 million C P Dewatering, trucking to compost site 71.72 t/a (2.3 million ton/yr). The conclusions of an industry survey E P Dewatering, dlying, incineration in power 59.18 of the most common sludge dewatering techniques and disposal boiler systems are presented below. F P Dewatering, trucking to landfill 58.72 The key to minimizing sludge disposal costs is the dewa- G P Dewatering, trucking to landfill 45.21 tering technology used. The more water that can be removed A P + S Dewatering. trucking to compost site 56.56 prior to final disposal, the lower the overall disposal cost. The most common mechanical dewatering technologies B P + S Dewatering, trucking to land application 50.09 used by the industry include vacuum filters, belt presses, and 'Costs do not include depreciation and interest on capital. solid bowl centrifuges. There is a recent trend toward use of P = primary sludge; and S = secondary sludge. belt presses which can dewater secondary sludges to 12 to 14%

968 Journal WPCF, Volume 56, Number 8 I.. Industrial Wastes solids, primary sludges 35% solids, and primarylsecondary sludge ACKNOWLEDGMENTS mixtures to 16 to 25% solids (depending on the ratio of primary to secondary sludge). Credits. Statistical data on the U. S. pulp and paper industry Polymer costs for mechanical dewatering vary widely with were supplied by Mr. William Gillespie of the National Council sludge characteristics. For example, one mill using belt presses for Air and Stream Improvement, Inc. Cost data were compiled to dewater waste-activated sludge spent $49.6O/t ($45.00/ton) by Steven R. Young and James G. Coma of the Environmental of sludge dewatered to a solids content of 1396, whereas the Services Division of Crown Zellerbach Corporation. This paper polymer costs for mills dewatering combinations of primary was presented at the Water Pollution Control Federation 56th and secondary sludges can vary from zero to $2O.OO/t ($181 Annual Conference in Atlanta, Ga. on October 2-6, 1983. ton). Those mills dewatering only primary sludge by belt presses Author. Herman R. Amberg is director of Environmental can expect polymer costs below $10/t ($9/ton). Services, Crown Zellerbach Corporation,904 N. W. Drake Street, About 87% of pulp and paper mill sludges produced are Camas, WA 98607. landfilled and 13%are disposed of by incineration, composting, sale, and land application. Incineration represents the major management alternative to landfill disposal and accounts for REFERENCES 1 1% of the sludge produced. Total dewatering and disposal costs for pulp and paper mill I. Rosenfeld, A. S., et al., “Pulp and Paper Mill Sludge Disposal by sludges vary widely, but they can account for about 40 to 45% Combustion.” TAPPI, 56, 97 (1973). of total wastewater treatment plants costs. Typical dewatering 2. Hewitt, T. R., and Ellis, R., “Bear Island Dewatering 1009b TMP and disposal costs for the eight mills surveyed vary from Sludge, Buming in Boiler.” Pub and Paper, 57, 96 (1983). $24.33/t ($22.07/ton) to $7 1.72lt ($65.06/ton) of sludge, and 3. Reilly, M. T., and Krepps, W. E., “A Case Study-Trials With a Mobile Unit Demonstrate Centrifugation of Secondary Sludge.” average $5 1.OW ($46.3 Ilton). TAPPI, 65, 83 (1982). Although a number of innovative systems are being used 4. National Council for Air and Stream Improvement, Survey Data by the U. S. pulp and paper industry today, the most widely (unpublished). used disposal systems for the next decade will be landfill, in- 5. Evans, John, C. W., “Rayonier Produces Animal Feed From Sec- cineration, and land application. New technology will be grad- ondary Sludge at Sulfite Mill.” Pulp and Paper, 57, 124 (1983). ually implemented as landfill disposal requirements become 6. “TAPPI Annual Meeting, Atlanta, Ga.-Summaries of Meeting more stringent. Papers.” TAPPI, 66,25 (1983).

August 1984 969