LETTERS Sludge incineration
Dear Sir: Your PAT report entitled, “MHF’s solve a sludge disposal dilemma” (ES&T, May 1974, p 412) presents a more optimistic assessment of sludge incineration than is warranted, largely because of several omissions: 0 The bar graph on emission standards does not include nitrogen oxides, important .in smog formation. 0 No mention is made of the emission of heavy metals as a health hazard. Any sewage sludge, even from a totally residential area, contains several ppm Hg, for example. In an industrial area it might be an order of magnitude higher. At the high temperatures in the furnace, one can be rather certain of the escape of this mercury into the atmosphere. 0 The point is made that sludge containing only 30% water can sustain combustion without additional fuel being expended. How was it reduced initially to only 30% water except by the combustion of fuel? 0 The amounts of fuel required are certainly appreciable for furnaces of this type. At the Blue Plains plant,
which is being expanded to handle the sewage for the metropolitan area of Washington, D.C., the projected fuel requirement per year is over 15,000,000 gal of oil. Unfortunately, there is no easy answer to the sludge disposal problem, which should have received much more research funding in the past. However, any assessment of the problem in a publication sponsored by the ACS should be rigorously objective; this particular article fell far short of that goal. Patrick J. Hannan Naval Research Laboratory Washington, D.C.20375 Sludge incineration retort Dear Sir: Although it isn‘t specifically identified as such in the PAT report (€S&T, May 1974, p 412), I believe Mr. Hannan’s concerns about NOx will be alleviated by the fact that NOx emissions were the pollutants of most critical consideration for the planned incinerators mentioned in the article for Livermore, Calif. (see page 413, top left column). The San Francisco Bay Area Air Pollution Control District hearings developed
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the fact that NOx emissions from an alternate system of trucks to haul the sludge 10 mi to the border of the valley would be about equal to those systems utilizing advanced incineration methods. The Livermore plan for using incinerator methods was permitted on determining overall that incineration was an insignificant contributor to air pollution. NOx emissions were not included in the bar chart because they are not part of the BAAPCD Regulation 2 standard. However, they are, on a per capita basis, only about 1/15th that of the proposed standard for a 1977 automobile traveling the national average of 12 mi per day. Regarding heavy metals, the EPA Sludge Incineration Task Force (Report No. PB 211 323) found that in general ash samples normally showed a higher concentration of heavy metals (in oxide form) when compared to sludge feed. However, mercury was one of the exceptions not detectable in the ash sample. The report further stated, “Fortunately, the quantity of mercury involved is small . . . If the average Hg concentration in the sludge is 0.01 mg/g, 80 Ib of mercury would be expelled into the atmosphere over the U.S. This amount is insignificant, particularly in the light of an estimated 3000 tons of mercury which are discharged into the atmosphere from the burning of coal over the earth.” It will also be recognized that sludge incineration plants do not generate new sources of mercury and that there is a small mercury content in normal human waste. Also, a two-year survey in 1968 by Williston [Williston, Samuel H., “Mercury in the Atmosphere,” J. Geophys. Res., 73 (22), November 15, 19681 which has been included as a part of an HEW committee report [Environ. Res., 4 (1) March 19711 noted that the percentage of mercury associated with water in the soil varies in the same location at different times of the year, depending upon the moisture of the soils, with the presumption that as the soils dry this fraction of mercury is released into the atmosphere. Thus, mercury contained in sludge put on the land would also be volatilized into the atmosphere. Other sources indicate that it may well be that methylation of mercury occurs in soils; thus, the potential exists that such mercury may be converted into a more hazardous form than the vapor. According to the aforementioned HEW committee report, “Unless a large increase in the level of atmospheric mercury occurs, the health hazard from airborne mercury would be relatively insignificant compared to that from eating contaminated food” (ibid., p 53). Relative to autogenous combustion, sludge in a multiple hearth furnace, due to its countercurrent pro-
