PRACTICAL, AVAILABLE TECHNOLOGY
MHFs solve a sludge disposal dilemma Envirotech's multiple hearth furnaces have been ordered .for more than half of all units specified for municipal systems last year For many years disposal of municipal sewage sludge has been regard: ed as more of a nuisance than a problem. Today, wastewater treatment planners are having to deal with the reality of ultimate sludge disposal. Due to the higher treatment standards coming into effect under the new water law, P.L. 92-500, the problem will grow in proportion to the added sludge volumes from new plants. While some people regard sludge as still a largely unsolved problem, there is a growing confidence in and commitment to incineration processes as the best available sludge disposal systems. I n everyday practice, consulting engineers are designing and specifying modern multiple hearth furnace systems at a record pace. I n calendar year 1973 alone, there were 30 incineration projects bid with a total value of $52-75 million. The rapid growth in the acceptance of sludge incineration by some practicing engineers, while others still regard it as inadequate technology, indicates that there is a great deal of misunderstanding about the advanced state of the ert and practical application of multiple hearth systems. Several examples of recent incineration installations and projects under design by Envirotech's BSP Oivision (Brisbane. Calif.) will help to dispel the uncertainty surrounding incineration. These examples reinforce the fact that this sludge processing system will meet requirements of the future and is available today. Systems approach The many advantages of a total systems approach to sludge handling are evident in the facilities being designed for Granite City, 111. The heart of the system is a third generation multiple hearth furnace (MHF) operated in an energy-conserving mode by recovering waste heat for sludge heat treatment. R. C. Haller, vice-president of the consulting engineering firm of M. W. Inc. (Indianapolis, Ind.), says that the closed loop energy conservation system begins with gravity thickening 412
EnvironmentalScience &Technology
, M. W. Inc.'s Haller Closing the energy loop
of the combined primary and wasteactivated sludge. The thickened sludge is then thermally conditioned in one of two 6000-gph ESP Heat Treatment Systems. The effects of thermal conditioning at 250 psig and 380°F allow the sludge to be mechanically dewatered without chemicals to approximately 30% solids prior to incineration. The sludge at 30% solids is autocombustibie, needing no oth'er fuel for complete burning. The Granite City plant. a 23-mgd secondary treatment facility is scheduled to go on stream in late 1975. At design, an estimated 11,000 Ib/hr of dewatered sludge will be reduced to 1000 Ib/hr of inert ash via an 18-ft diameter, seven-hearth BSP multiple hearth furnace. Except during start-up and standby operation, no auxiliary combustion fuel will be required. Exhaust gases from the furnace will pass through a waste heat recovery heat exchange/boiler wherein ampie steam will be produced for operating the sludge thermal conditioning systems. Finally, the exhaust gases will be cleansed and cooled via an ARCO/Envirotech scrubber. Any air pollutants? A new 35-mgd regional wastewater treatment plant in Palo Alto, Calif., has two multiple hearth furnaces to oxidize the combined primary, secondary sludge. Ronald Doty of the City of Palo Alto, says that the Envirotech furnaces have been operating since December 1972, well within the tight regulations of the Bay Area Air Pollution Control District (BAAPCD). John Jenks. of the consulting and design engineering firm of Jenks Adamson (Palo Alto) says, "These furnaces are equipped with impingement scrubbers and a hybrid afterburner in the top hearth. When the system was nearing final steps, in 1972, new and more stringent standards'were adopted by the BAAPCD. Design modifications were made to accommodate the new standards." I n Contra Costa County, Calif., the
consulting engineering firm of Brown & Caldwell was engaged by local authorities for the design of an advanced wastewater treatment system to meet water quality standards as well as recapturing waste heat from a furnace to provide steam for heat and power elsewhere in the wastewater plant. Again two multiple hearth furnaces are included in the design for co-combustion of organic sludge and calcination of lime sludge. Bill Loftin of Livermore says that when the above application was first made in June 1971 to obtain a permit to construct the facility from the BAAPCD, the permit was denied. "The Board was forced to take this action since the plant would be located in the Livermore Valley, already designated a critical air area for oxidants, and therefore no new sources of any air emissions could be built," he says. Following this denial, the application was resubmitted along with statements on the overall favorable impact of a water reclamation plant and the relatively insignificant emissions to result from the furnaces. Recognizing the importance of the project, the BAAPCD Board voted to amend their regulations to allow construction of "insignificant sources," endorsing the insignificance of the furnace emissions by granting the permit for construction. In comparison to other emission sources in the valley, the furnaces under the worst conditions would be only a tiny fraction of those due to automobiles and about equal to the minimum emissions from alternate systems of trucks hauling the sludge 10 miles to the border of the valley. After all this, approval of the project was unfortunately delayed because of limited federal funding. The plant will be under construction within the next year, as soon as federal funding is received. What to do with the ash
Sometimes incineration is not regarded as an end to the sludge problem. Some say the problem has only been reduced; you still have the ash to dispose of. Unlike biological sludge, however, the ash is sterile and harmless. Liquid sludge disposal practices reveal that in many cases sludge drying beds or sludge piles are carelessly tended, with little regard for the potentially dangerous pathogens which may be present. Cases of possible groundwater contamination were common with few disposal sites being maintained to the degree required for solid waste sanitary landfill sites, for example. On the contrary, sludge ash is free of bacterial contamination and re-
Closed energy loop, sludge handling systems incorporates heat treatment, incineration, and heat recovery
duced 90% in volume from the original wet sludge. I t also is potentially beneficial to the environment. It is useful as a soil 'conditioner, and in Japan it is marketed as quasifertilizer for its phosphate content. The energy factor
Now that the energy crisis is everywhere apparent, there has been concern that sludge incineration may be draining away precious energy resources and that limited fuel availability may make furnaces white elephants. Make no mistake, multiple hearth furnace systems can be designed to be self-sustaining if the sludge cake is dried to 30-35% solids. Once this condition is reached, the sludge becomes a fuel resource in itself. The resulting heat can be recovered for processes such as heat treatment, or space heating. Other sludge handling and disposal systems, designed with a closed loop energy cycle to convert fuel, are being designed for the ChesapeakeElizabeth wastewater treatment plant (Va.) and the Western Branch plant (Prince Georges County, M d . ) , Some auxiliary fuel is needed for start-up and standby operation. A wide variety of fuels may be used including digester gas, natural gas, heating oil, or at the Virginia plant site, one of five under the Hampton Roads Sanitation District, No. 6 fuel oil. Fuels with high sulfur contents can be used without creating an SOn emissions problem. The high efficien-
cy ARC0 wet scrubbing systems have been shown to remove up to 90% of SO2 created in the combustion cycle, according to test results of Envirotech's BSP Division. In some cases, the fuel value of the sludge may be supplemented by mixing in waste and heavy grade oils or even pulverized coal. A recent report from the EPA detailed the electrical energy demands of all the unit processes in a wastewater treatment. Analyses of flowsheets in such systems reaffirm that energy demands of incineration are a minor contributor to total plant usage of electricity. For example, in a 100mgd activated sludge plant which would conveniently serve a population of 1 million, MHF incineration accounted for only 4 % of the total requirement of electricity. I f digestion is used with the activated sludge plant, the requirement drops to 2.4% of the total electricity requirement. Incineration has several other clean advantages over other methods of sludge disposal: The sludge problem is handled on site without having to transport potentially dangerous material to outlying and usually reluctant rural communities. 0 The pollution resulting from truck or barge operation is eliminated, 0 The energy demands of pumping are avoided. 0 Only small areas of urban land are required in contrast to large drying beds. Volume 8, Number 5, May 1974
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