"The Federal Water Polution Control Amendments of 1972" (PL92-500) and other Federal public laws and regulations reflect the national objectives and public interest in reduction of environmental pollution. In particular this law establishes specific restrictions on discharge of pollutants into the nation's waters by 1985.
To meet the future requirements of law in regard to discharge of sewage wastewater effluents into public waters, it appears that costly tertiary treatment will be needed to remove substantially all solids prior to discharge of the clarified effluent. Additional costs will be incurred for disposal of the resulting sludges.
As wastewaters and most of their solids content are of potential value for irrigation, fertilization and soil conditioning, several studies have been published regarding these opportunities and the related problems.
An excellent review of this subject is the paper "Land Disposal . . . What's the Realistic View" by authors H. L. Michel, P. H. Gilbert and H. K. Cread, published in Water and Wastes Engineering, June 1974. This refers to a comprehensive study program completed for the U.S. Army Corps of Engineers by PBQ&D, Inc., Engineers relative to the San Francisco Metropolitan Bay Area.
The potentials of this opportunity are evident from the data published for untreated combined municipal and industrial wastewaters. Expressed in volumes per 1,000,000 persons the daily wastewater flow includes approximately 175 million gallons containing 645 tons of solids including about 87.5 tons of nitrogen plus large quantities of other fertilizer nutrients and soil building materials. Substantial economic and ecologic value potentials are apparent particularly in situations where an urban metropolitan area is adjacent to a water deficit agricultural area. Agricultural fertilizer potential values are substantial. The nitrogen content is of special importance because natural gas is the principal resource used in its manufacture. For instance about 40,000 cubic feet of natural gas is required to manufacture one ton of fertilizer nitrogen, according to Commercial Fertilizers, May 1975, published by the Statistical Reporting Service of the U.S. Department of Agriculture.
The study referred to also shows that this typical wastewater flow also contains 3.6 mg/ml or over 5000 pounds each day of heavy metals, principally cadmium, copper, molybdenum, nickel and zinc. These metals are potentially toxic, can accumulate in the soil and can be taken up by growing plants.
Conventional primary and secondary treatment is effective in removing up to 90% of putrescrible and other suspended solids. The discharged effluent may then be used for agricultural irrigation; however, such use is limited by the fact that about 60% of gross heavy metals content of the raw wastewater influent is still present in the discharged effluent.
Such treatment produces substantial quantities of sludge as a by-product containing a large content of heavy metals. The opportunities and limitations regarding agricultural uses of sludge are summarized in the following reports published by the Council for Agricultural Science and Technology (CAST), Iowa State University, Ames, Iowa: "Utilization of Animal Manures and Sewage Sludges in Food and Fibre Production", Report 41, Feb. 1975, and "Application of Sewage Sludge to Croplands: Appraisal of Potential Hazards of the Heavy Metals to Plants and Animals", Report 64, Nov. 1976. Heavy metals pollution, particularly by cadmium, is a major limitation. Costs of transportation and incorporation into the ground are major considerations.