Chemical and Biological Phenomena Observed with Sewage Sludges in Simulated Soil Trenches

Chemical and biological observations were made on sewage sludge in a simulated trenching system in the greenhouse. Limed and unlimed raw and digested sludges were placed in simulated trenches in soil profile boxes in which corn (Zea mays L.) was sown. During the 160-day growth period, roots penetrated entrenched digested sludge and root growth proliferated throughout the sludge. Root penetration into the raw sludges was severely restricted. Gas analysis showed that anaerobic conditions prevailed for extensive periods during the 160 days in raw sludge. Methane and CO, levels reached 45 and 25%, respectively. The levels of CH, and CO2 were <3 and 21%, respectively, in the digested sludge boxes. Nitrate nitrogen, adjacent to and beneath the sludge, was generally higher with digested than with raw sludge. Zinc and copper did not move from the sludge into the surrounding soil. The increase of these metals in corn leaves was relatively low, reaching only 131 /ig/g Zn and 5.9 /ig/g Cu as compared with 79 /ig/g Zn and 3.8 /ig/g Cu in the controls. Although low levels of fecal coliforms survived in the sludge, none were found in the soil surrounding the sludge. Total coliform numbers in the sludge after 160 days were negatively correlated with NH,-N concentrations, suggesting that NH,, generated on dissociation on NH4*, may be important in reduction of human pathogens. Additional Index Words: waste, sludge entrenchment, methane, carbon dioxide, nitrate, ammonia, zinc, copper, total coliforms, fecal coliforms. Sludge production in the United States is presently estimated at 5 million dry metric tons per year. Waste water treatment is expected to become more complete and advanced, and by 1985 sludge production is estimated to exceed 10 million dry metric tons. Environmental restrictions placed upon ocean dumping, the high costs of energy for incineration, and the increasing cost of land for landfills have caused municipalities to seek sewage waste disposal methods that will accommodate high application rates. A land application system should be able to accommodate large amounts of sludge without causing environmental pollution, while utilizing sludge for its organic matter content and plant nutrients. In 1972, a field experiment was initiated in which undigested and digested sludges were placed in trenches and covered with overburden (12). This concept offered an approach to disposal of large quantities of sewage sludge in an environmentally acceptable manner resembling landfilling, while improving marginal soils and making nutrients available for plants. Possible benefits 'Contribution from the Agric. Environ. Qual. Inst., SEA-USDA, Beltsville, MD 20705. This research was supported in part by the U.S. Environ. Prot. Agency, under Interagency Agreement EPA-IAG-D40510. Received 22 Mar. 1978. 3 Plant Physiologist, Soil Scientist, Soil Microbiologist, Plant Physiologist, Soil Microbiologist, and Research Chemist, respectively. J. Environ. Qnal., Vol. 7, no. 4,1978 477 included odor control, destruction of pathogens, improvement of water retention and transmission characteristics of soil, and enhancement of nutrient and water relations for crop production. Concurrent with the field studies, a greenhouse experiment was initiated to provide data on soil aeration, root proliferation, nitrogen transformations, metal movement, and pathogen survival under simulated field conditions. MATERIALS AND METHODS